Archives for the month of: November, 2025

India’s Kaveri Engine: The Long ‘Atmanirbhar’ Path

November 26, 2025 //

The History

Any mention of the Kaveri Engine brings up visions of repeatedly missed deadlines, out of capabilities vision, underperformance & underinvestment. If a performance manager were to look at the Kaveri’s progress report, he/she will look to set up clear and realistic goals, realistic timelines & performance milestones with adequate investment across funding, human capital & materials.

*The Kaveri Engine on display. Pic Source : Wikipedia*

Project Kaveri was launched in 1986 under the oversight of India’s GTRE (Gas Turbine Research Establishment) to develop an indigenous turbofan to power India’s then under development LCA (Light Combat Aircraft) which would become the Tejas.

GTRE already had experience working on turbojets dating back over 20 years. In 1956 (the same year as the first flight of the B-58 Hustler) the Indian Government decided to develop inhouse a Mach 2 capable multirole aircraft and appointed German, Kurt Tank ( responsible for the creation of many important German aircraft during WW2 including the FW190) as design lead. From the get go the project was heavily constrained by a limited industrial base (under British Rule the nascent Indian Industry was not allowed to grow) and even more limited funding, and developing an inhouse engine looked out of reach. The Indian Government made the first compromise here by looking outside India to develop/purchase the engines for the aircraft.

The original vision of the HF-24 Marut was envisioned to be a Mach 2 aircraft and needed capable engines. Finally the project settled on the Bristol Siddley Orpheus 703 engines already in use on aircraft such as the Gnat and Hunter for over a decade. The engine was non afterburning and developed 4.850 pounds of thrust each (the Marut had two). The limited power of the engines barely got the Marut upto Mach 0.95 and was considered inadequate by the Indian Air Force (IAF). Furthermore the Indian Government had turned down a $17.5 Mn proposal by Rolls Royce to improve the performance of the engine as they believed it expensive. Similar conversations with the USSR & Egypt produced no results. The Marut would serve its entire operational life with underpowered engines limiting capability. The aircraft retired from service in 1990.

*The HF-24 Marut on display in Bangalore. Pic Source: Wikipedia*

By the early 1970s GTRE decided to improve the Orpheus engine performance by developing afterburners and the engine was certified in 1973-74 with between 5,700-6,500 pounds of thrust with afterburners at a temperature of 1,700°K. Unfortunately the engine design did not match the Marut’s and could not be integrated with the aircraft.

The 1974 Pokhran tests resulted in sanctions against India and finding replacement engine parts got to near impossible.These actions further incentivised GTRE to once again go back to the drawing boards and ungraded the afterburners to 2,000°K and reworked the engine’s subsonic compressor stages with a new transonic design that further improved the aging engine’s dry thrust. Unfortunately all of these efforts went in vain as the engine failed to integrate with the Marut (another airframe could have been developed). The plug was pulled on further engine development right about 1973-4 and the hardfought experience gained by a fledgling aerospace industry was put on hold for another four years.

The Kaveri History

In 1977 GTRE, still smarting under international sanctions, had developed an afterburning turbojet prototype, the GTX37-14U. The experience gained from working on the Orpheus 703 was of great help. It was the very first engine developed indigenously in India. The technology demonstrator was to showcase GTRE’s capabilities. A turbofan variant, the GTX37-14UB was constructed to further enhance GTRE’s capabilities. The GTX37-14U developed 14,550 pounds of thrust with afterburner and the GTX37-14UB developed over 20,000 pounds of thrust. Remember both the engines were proof of concept and should be viewed as such. The GTX37-14UB did have a large diameter frontal area considered ineffective for a fighter aircraft. Having said that India produced both an afterburning turbojet and turbofan by the time the project was completed in 1981. Both engines were stand alone projects (highlights the silos the various defense establishments operated in).Gas Turbine Research Establishment

A jet engine consists of the following stages, the intake, the compressors, the combustion chamber, the turbines and the afterburner. The intake is a critical element of any supersonic aircraft as air that is flowing at supersonic speeds needs to be slowed down to subsonic speeds for ingestion into the engine compressor, the intake includes elements of both the airframe and the engine and a fantastic example is the J-58 engine used in the Blackbird series of aircraft (https://theaviationevangelist.com/2025/11/21/the-blackbird-family-aircraft/ ). The compressors need to be optimized to take the air coming off the inlet, compress and fire it in the engine core (the Kaveri’s core is called Kabini) the combusted air is then blown through the turbines followed by the afterburner. Maintaining optimal pressure through the entire entire engine length is critical to the engine’s efficiency (will speak of materials later).

The Kaveri’s ‘Kabini’ core is a connecting element from India’s foundational GTX37-U engine to the current Kaveri GTX35-VS. Other than the core everything needed to be rethought as per the IAF’s changing needs(it is important to scope any project and not move the goalposts after scoping). The target thrust requirement for the Kaveri was 81kN or 18,210 pounds of thrust and multiple high altitude test runs in Russia on IL-76 Test Beds never produced a thrust of more than 15,800 pounds or 70.4kN. The K1 iteration of the engine’s dry thrust was never more than 49-51kN or 11,000 pounds.In addition the engine had a target weight of 1,100 kg or 2,450 pounds and the weight of the K1 engine never went below 1,423 kg or 3,139 pounds. The engine was off on both thrust and weight targets. In 2008 the engine was delinked from the LCA / Tejas program and iterations continued. By 2009 after years of weight saving efforts the weight came down to 1,235kg or 2,723 pounds.and a thrust of 65kN or 14,612 pounds of thrust with afterburner. As of 2024 the engine weight has been brought down to 1,180 kg or 2,601 pounds. The engine weight is still considerably heavier than the GE F404 engine used in the LCA/Tejas at 1,036kg or 2,284 pounds and 84.5kN / 19,000 pounds of thrust. The Kaveri has a ways to go. It can be said the delinking of the Kaveri program from the LCA/Tejas was a body blow as funding and Government interest was inconsistent(the IAF kept looking outside India for their immediate strategic needs). A derivative of the Kaveri in only dry form was tested in 2024, again in Russia and it developed 49-51 kN or approx 11,200 pounds of thrust.This engine will be used in India’s UCAV Ghatak program. DRDO Ghatak – Wikipedia . As per Wing Commdr R.K Narang, India’s aerospace independence is based on 4 pillars: a fighter aircraft, a transport aircraft , a UCAV & an engine. True independence can only be attained by having your own versatile multirole capable engine, and we now realize the importance of the Kaveri Engine. 107 – Kaveri, Naval Fighter, AMCA and Supercruise: Can India Build a Truly Indigenous Air Power?

Before we proceed into analyzing the improvement areas of the Kaveri program, it’s best we take a look at a couple of examples. The first is ISRO (Indian Space Research Organization) and their radical change post the Cryogenic Engine story of the 1990s and SAC (Strategic Air Command) in the USA, both of whom have created a vibrant ecosystem that encourages innovation and excellence, and yes works on improving failures.

Note: In Nov 2024 Bahmos Aerospace reported the development of an afterburner for the dry Kaveri engine increasing thrust from 50kN to 80kN (11,240 pounds & 17,985 pounds), almost at the thrust requirements of the Tejas. Certification of the same is expected in 2032 and is currently undergoing tests in Russia. GTRE GTX-35VS Kaveri – Wikipedia

*The afterburner nozzle developed by Brahmos Aerospace. pic Source: IRDW*

The Examples

In the early 1990s India’s ISRO (Indian Space Research Organization) and Russia’s Glavkosmos had almost reached a deal for the purchase of Cryogenic engines for India’s GSLV (Geosynchronous Satellite Launch Vehicle). After going through multiple negotiations ISRO had reached the decision that the Russian cryogenic engine was the right engine at the right price. The United States citing violation under the Missile Control Technology Regime (MTCR) imposed sanctions on both ISRO & Glavkosmos (ISRO had earlier rejected the American proposal). The recently formed Russia (after the fall of the USSR) was under financial strain, could not stand up to the sanctions and backed out of the deal, leaving ISRO’s GSLV without engines.

*India’s cryogenic engine. Pic Source : Reddit user*

A cryogenic engine is a rocket engine that uses liquefied oxygen and hydrogen stored at very low temperatures. The engines provide the highest thrust per unit of propellent mass as compared to other engines. These engines use specialized materials, handle extremely volatile liquid gasses under high pressure and sophisticated engineering to prevent propellent from boiling off under heat. All this needed to be developed inhouse now. There were fake controversies being generated against involved scientists ex: Nambi Narayan (who has been exonerated and compensated).

ISRO created an ecosystem that included companies like Godrej Aerospace & MTAR Technologies. Godrej established a vacuum brazing facility , a complex method of joining metal components at very low pressure (vacuum) in a furnace. Such a furnace eliminates gasses while joining metal sections of varying size , strengthening joints and eliminating oxidation. A critical property when operating under high pressure. MTAR Technologies created a turbopump. In the cryogenic engine the turbopump is a critical component to feed high pressure fluid into the combustion chamber. Such technologies are niche and needed to be developed in house.Developing such a high cost engine had high funding costs and this is where the public-private partnership (PPP) paid dividends. The government needed to be consistent while private entities developed the complex components.ISRO decided to step back from the day to process and instead became a technology incubator.

ISRO Scientists learned from failures and this became a consistent theme through the culture of the entire organization. A brilliant example of this is Chandrayaan 2 in 2019. The mission was to land the Vikram rover on the Moon, however due to issues that occurred during the landing sequence (15 minutes of terror) the spacecraft impacted hard on the Moon’s surface and Vikram lost communication. The Team at ISRO went through a root cause analysis and rectified the errors that caused the hard landing, built impact tolerances across the lander and the rover. In 2023 Chandrayaan 3 successfully landed on the moon.ISRO opts for ‘failure-based’ design for Chandrayaan-3 – Rediff.com Today India’s ISRO is known for commercially launching satellites for other countries at the most reasonable cost. At the heart of this story is India’s cryogenic engine, 100% Indian. The long road to cryogenic technology – The Hindu .

*Chandrayaan 3 and river. Pic Source : NASA Science*

The SAC in the United States has created a similar ecosystem of contractors who have amassed formidable experience across multiple disciplines of aerospace. For every great aircraft in operation, they have a competition between two finalists who are chosen after multiple rounds of iterations. Examples: The F-35 Lightning had a competitor in the Boeing X-32 at the experimental stage. The F-22 Raptor had a competitor in the YF-23. There is competition between Pratt & Whitney and General electric for the engines. The ecosystem is an extremely competitive technology incubator with SAC and even NASA playing the technology and funding facilitator.

GTRE Improvement Areas

India’s HAL and GTRE have so far been assembly partners to their international partners and this needs to change. The change needs to be based on four pillars: Human Capital Development (HCD), Materials, Funding & Testing Facilities. But first the improvement areas for the Kaveri Program.

The scoping of the Kaveri engine was never realistic from the very beginning. The parameters set down at the get go were the GE-404 turbofan and GE has over a century of experience manufacturing multiple engine types across speed regimes. The more important question is what goes into manufacturing a high performance jet engine. The answer immediately goes back to the four pillars mentioned above.

At the very head of HCD is company culture. GTRE culture is said to be too orthodox, monopolistic and institutional to manage cutting edge advances. They will do well to take a leaf from ISRO’s book and work on ‘ failure based’ design. Having said that, much like a Phoenix rising from the ashes and ISRO after 1994, GTRE can still develop its Human Capital using standard project development methodologies.

Every Project has a critical path and each critical path has milestones. The management of these milestones needs a special kind of project leader, one with deep knowledge of jet engines, governmental affairs & Human Capital Management, if we go back to the Marut failure Kurt Tank the German Engineer was informally accused of a rigid design stance, lack of engagement with the Government, lack of co-ordination across the project critical path & frequent infighting between his German & Indian counterparts. The project manager needs to balance all the abovementioned dynamics, leaning too much in any one direction will deliver below optimal results.

Human Capital needs to feel well compensated and appreciated, especially when they are working and creating something special like the Kaveri Engine. Moreover they need direction from an effective project manager who looks at every failure as an opportunity for a fresh and better improved iteration.Motivation is key when it comes to Teams and they need to feel every failure is one step back, but two steps forward. Losing Human Capital to other countries without a fight is criminal and our Indian ecosystem has to devise a way to keep and nurture our capital. GTRE has to focus on this instead of building the engine itself. The enabler.

Specialized equipment such as the Kaveri engine need special niche materials that are capable of withstanding extreme thermal variations and pressure reliably. Some examples are single crystal turbine blades made of nickel based superalloys such as inconel and rene. Such alloys lack grain boundaries found in traditional metals and their construction renders the blades resistant to thermal fatigue that causes blade deformation. This was a major stumbling block until 2021 when Defense Metallurgical Research Laboratory (DMRL, with great pride I say my Uncle retired as a Director there in the late 1980s) delivered 60 single crystal high pressure turbine blades for HALs indigenous helicopter engine.

*The single crystal blades delivered by DMRL. Pic Source DMRL*

In 2025 Lucknow based PTC Industries received an order to manufacture the same for the Kaveri Derivative engine to be used in the Ghatak USAV . While such capabilities are scaling up, the vision of an afterburning Kaveri Engine capable of delivering upto 120 kN thrust 27,000 pounds of thrust should always be kept in the minds of the GTRE enablers. PTC Industries Receives Purchase Order from GTRE for Single Crystal ‘Ready-to-Fit’ Turbine Blades.

*The single crystal blades delivered by PTC Industries. Pic Source : PTC Industries*

Building an afterburning jet engine needs specialized test facilities such as a high altitude wind tunnel and flying test beds. So far GTRE has been dependent on Russia for wind tunnels and their IL-76 test bed for high altitude testing. India already has IL-76s. It’s a matter of modifying one aircraft that can be used for such testing. Being dependent on other countries means negotiating prices (which are never on your side) and waiting for a window of testing as per the other country’s schedule. Furthermore the lack of validation facilities means the incomplete prototype might end up on the test bed and might require going back to the drawing boards after testing. In the United States companies such as Boom Supersonic who are building the non afterburning Symphony engine for their Overture airliner use additive manufacturing (3D printing) to manufacture prototype parts at a rapid pace in their 3D printer farms. This after using CFD (Computerized Fluid Dynamics) to rapidly iterate the right design. The Boom Overture : A Return to Supersonic | theaviationevangelist Such processes speed up the iteration, design improvements & validation by a factor of ten. In house testing and validation facilities are a must.

The cost of developing a new supersonic capable engine with afterburners costs anywhere between $2-5 Bn. In the case of the Kaveri engine the initial tranche of funding dating back to 1989 was $53 Mn. By 2009 the Kaveri project cost approx $595 Mn , and from the project’s perspective had severely overrun projected costs. A much larger perspective is China’s total expenditure of over $42 Bn developing a range of aero engines for their range of aircraft. Funding is extremely important for such projects and the Government & DRDO along with stake holders such as the IAF, Indian Navy & Military need to be clear on their priorities and reserve a significant portion of their expenditure to indigenous R&D instead of looking outside. The last 30 years the IAF has consistently looked outside, be it the Su-30 or the Rafale and to go further back the Mirage 3000 and Jaguar aircraft.

The Future

The Kaveri derivative on the Ghatak UCAV should be viewed through the prism of incremental improvements. Furthermore the engine’s development needs to be viewed as a National ‘Atmanirbhar’ priority and developing the four pillars i.e HCD, Materials Technology Development, consistent funding (including potential cost overruns) & testing infrastructure have to be nurtured and built (common infrastructure is always good and saves costs). On the testing infrastructure Boom Supersonic as an example has consistently spent millions of dollars on inhouse equipment to cut down development lead time. The Indian Navy’s aircraft carrier project is a brilliant example of ‘Atmanirbhar’.

*The indigenously constructed INS Vikrant. Pic Source: Wikipedia*

PPP under the GTRE & DRDO’s oversight is the right way forward with cost and profit split. ISRO has already shown the path here as has the Indian Navy with their carriers (they do not yet have a Marine variant of the Tejas fighter yet).

Persistent Indigenous Innovation is the only path to ‘Atmanirbhar’.

Late Edit : Coincidentally at the same time as publishing this article , Defense Minister Rajnath Singh has announced a comprehensive development contract with Safran for the AMCA engine. While this is great for immediate to interim strategic needs, the Kaveri project has to be seen through. True mastery of the jet engine can only then be had.

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Tags 3D printing, additive manufacturing, afterburner, AMCA, Atmanirbhar, blades, BOOMSupersonic, CFD, Chandrayaan 2, Chandrayaan 3, combustor, compressor, Computational Fluid Dynamics, critical path, Defense & Metallurgical Research Laboratories, Design, DMRL, DRDO, Engine, funding, goal posts, GTRE, HAL, Human Capital, in one, India Space Research Organization, INS Vikrant, intake, ISRO, iteration, Jet, Kaveri, Kurt Tank, Marat, Materials, Moon, nickel, PPP, priorities, project management, PTC Industries, Rajnath Singh, Rene, SAC, Safran, scope, single crystal blade, Strategic, thermal resistance, thrust, turbine, turbojet, Vikram Rover

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The Amazing Blackbirds

November 21, 2025 //

SAMs , Satellites & Unseen Speed

The very first SAMs were the German V2’s from WW2. While their value was being understood they still had a long way to go as on the range & accuracy parameters.

The US began developing its missiles from the late 40s onwards and by the mid 50s had batteries of Nike Ajax missiles to guard against Soviet bomber attacks. By 1955 the Soviets themselves had the S-25 Berkut system and the famous S-75 Dvina came into being by 1957 having range, speed & accuracy.

The Soviet Sputnik launch of 1957 started off a whole new Cold War race and it was dominance from space. However satellites were still in their infancy and the CORONA & GAMBIT missions were still between 5 and 10 years away. Missile technology had a head start over satellite tech.

By 1956 the recently released U-2, Dragon Lady was already being painted regularly by Soviet defense systems, however the U2’s cruising altitude of 70,000 feet was still thought to be out of range of Soviet missile systems, even at its subsonic speed. Gary Power’s being shot down in 1960 only reinforced the need for speed, altitude and agility, the need for a Blackbird (which was already in development).

However even before the 1960 incident a fresh thought went through the US Armed services and it was speed. The recently launched B-58 Hustler had shown that Mach 2 was possible (if a little dangerous) and the various wings of the armed forces and CIA began to look at Mach 3 as the speed benchmark, stealth was not yet in the picture.

The WS-110A or what would become the XB-70 Bomber already underway was in trouble even before it got off the ground as it was believed Soviet SAMs could take down a large bomber with no stealth capabilities, the XB-70 would become an experimental aircraft as an attempt to just their cost of over $1.5 Bn for two vehicles or $750 Mn a pop! (Read here about the XB-70)

(https://theaviationevangelist.com/2025/10/10/xb-70-valkyrie-the-grand-daddy-of-supersonic/ ).

At around the same time and in parallel to the XB-70 program the CIA went to Lockheed to develop a Mach 3 capable reconnaissance aircraft that flew at over 90,000 feet (considered untouchable by SAMs) and would be difficult to detect by Radar. Lockheed with previous experience developing the U2 Dragon lady which had a service ceiling of 70,000 feet albeit at subsonic speed looked the right outfit to build such a plane.

This is the story of the Blackbird Family of aircraft and it all started with the A-12 Oxcart, an ironic name considering the A-12 was the exact opposite of an Oxcart.

*A pencil sketch of the Blackbird by my Daughter from a few years ago. Hung in my office.*

Project Archangel

In Apr ’58 Kelly Johnson the legendary head of Skunk Works said ‘ I recall having long discussions with (CIA Deputy Director for Plans) Richard M. Bissell Jr. over the subject of whether there should be a follow-on to the U-2 aircraft. We agreed … that there should be one more round before satellites would make aircraft reconnaissance obsolete for covert reconnaissance ‘.

The CIA’s hunt for a U-2 successor was called Project Gusto and by 1958 the two finalists were Convair with their Kingfish and Lockheed with their Archangel. Convair’s Kingfish had a lower cross section than Lockheed’s A-3 concept. Both companies were asked to refine their designs and here is where Lockheed pulled into the lead.

*The A-11 that would be modified to become the A-12. Pic Source : Wikipedia*

Following the A-3, (the A stands for Archangel) Lockheed’s iterations A-4 to A-6 used Blended Body Fuselage (BSF) designs along with turboramjet (more on this later) & rocket propellant, but they fell well short of the range requirements. Iterations A-7 to A-9 used a single J58 engine (just the turbojet) with two Marquat XPJ-59 Ramjets that used J-150 fuel, a highly classified type of fuel the JP stands for Jet Propellent and was expected to improve range, however still well short. The A-10 used two GE J93 turbojets (same as the XB-70 Valkyrie) with underwing inlets for better range, however the iterations continued to fly short of the required parameters. Iterations on the A-11 Lockheed added twin inward canted fins that were angled inwards at 15° made of composite materials, other leading edge surfaces featured composites as well, together the improvements went a long way towards improving RCS ( Radar Cross Section) of the aircraft. To add to these design improvements the wings were extended through chines that went right upto the cockpit and the bottom of the aircraft flattened with the wings blended into the fuselage, the improvements won Lockheed a $96.6 Mn contract to construct a dozen A-12s. The dozen airframes would extend to 18 if you include the three airframes used for the YF-12 , one trainer and two M-21 aircraft. Project Archangel / A-12 was underway.

*The A-12 design: Pic Source : Wikipedia*

The J-58 Engine

While the A-12 was an amazing aircraft design that is yet to be replicated almost 70 years on, it is the insane engineering that went into the engines of the aircraft that needs to be spoken of first.

The J-58 Turboramjet!

The external dimensions of the engine were a length of 17.10’ a diameter of 4.9’ and weight of 6,000 pounds might feel puny by today’s standards, the engineering that went into them is unique.Pratt & Whitney JT-11 Mach 3+ jet engine (J58) . (strongly recommend a watch). The engine generated 30,000 pounds of thrust with afterburners and had 8 compressor stages.Pratt & Whitney J58 (JT11D-20) Turbojet Engine | National Air and Space Museum

Sometime between 1956-58 the US Navy approached P&W to develop a Mach 3 capable engine for their planned Martin P6M Jet Seaplane. P&W had begun testing their prototype when the Navy realized the costs involved did not justify an aircraft when their main weapons were ships, submarines & missiles. The Navy pulled out by 1958ish. The CIA, which already had Lockheed in advanced design stages of their project Archangel/A-12 had obviously heard of this engineering marvel and approached P&W to continue development on the J-58…and the rest is history.

*The J-58 engine. Note the three pipes heading towards the afterburner. The plate right upfront on the side is the hydraulic computer. Pic Source : Air & Space*

Coming back to the turboramjet, a couple of definitions.

Turbojet definition : In a turbojet all the air that goes in the front is sent through the compression stages, fired up in the combustion chamber and the resulting exhaust gases generate thrust.

Ramjet definition : A ramjet is a type of engine that uses the forward motion of the aircraft to compress air and fire it up. Such an engine has no moving parts and aircraft using such engines need to be launched off another transport aircraft generally.

So why did the J-58 need both?

The J-58 was optimized for Mach 3.2 cruise and such high speed generates heat in the excess of 750°F which would melt the internals of the J-58 turbojet. A solution was required and here lies the engine’s unique feature, the frontal spike and six tubes running (three on each side) from the stage four compressor straight back to the afterburner section (a type of bypass).

The J-58’s variable geometry spike is where over 50% of the engine’s thrust is generated, but first another bit of information. At Mach 3.2 the compression at the engine’s inlet was almost as high as the thrust generated out the back, the engine would be in a neutral state of thrust, and in some cases negative (this is where the inlet management is critical). The pressure recovery on the J-58 is at 88% showing it is highly efficient at Mach 3.2.

The spike moves front and back by 26”. Right up to Mach 1.6 the spike stays in the full front position and the engine operates as a normal turbojet. At Mach 1.6 the engine begins moving back 1.6” for every increase in speed by Mach 0.1. The spike itself moves backwards into a conical receptacle and the backward movement of 1.6” for Mach 0.1 increase in speed maintains the ‘normal’ just behind the throat of the spike receptacle. The normal is the point where dynamic pressure switches to static pressure, and the movement of the normal is carefully calibrated by the spike to maintain optimal thrust across the speed Mach 1.6 – Mach 3.2 range.

At approx Mach 2.2 sensors detect that airflow and temperature are right to begin turboramjet operation by opening up the compressor bleed bypass valves, these valves are placed at the fourth compressor stage, and direct ram air through the tubes direct to the afterburners. The air is approx 400°F and helps keep the combustion chamber and turbines relatively cooler and within thermal limits. The afterburner fires more efficiently as a result of the cooler air.

*A schematic showing the various engine regimes. Pic Source: Wikipedia*

At Mach 3.2 the engine’s spike aligns the shockwave with the engine’s nacelle perfectly. The engine has a series of doors that maintain optimal pressure through the entire length. The cowl bleed doors is a porous strip on the inlet’s inner surface, the purpose is to bleed off excess boundary layer air and prevent an unstart at high speed. Further back the engine has suck in doors, these doors open up at low speeds (below Mach 0.5) such as the beginning of a take off roll to feed the engine with more air and aid low speed thrust generation. Furthermore at low speeds right at the afterburners are tertiary doors that automatically open and close as per ambient pressure relative to the exhaust gases, these doors let in additional air as required. The spike itself has a porous strip that manages slow moving boundary layer air. At low speeds the engines are extremely air hungry and this creates a low pressure area at the engine nacelle, the strip pulls in the air into the centre body and vents it out through centre bleed louvres. The air reverses direction at approx Mach 1.5 the air inside the spike centrebody duct reverses.

There is a story of a SR-71 pilot who decided to speed check his bird and got up to Mach 3.4 before he swallowed his own shockwave, flaming out both engines at 80,000 feet! He recovered one at 65,000 feet and the other at 25,000 feet. There was of course a discreet rap on the knuckles!! This story does highlight the fine balance within the engine and how it was optimized for Mach 3.2.

A look at the engine shows a tremendous amount of plumbing, not all of it is air, oil or fuel!. On each side of the engine nacelle is a hydraulic computer, yes hydraulic! The plumbing you observe is the computers optimizing engine operation. One of the computers is to manage the afterburner and the other is for the engine. The J-58s were created when computers were in their infancy and a solid state system was required that could withstand high temperatures and work optimally, hydraulic computers were the option.

The operating temperatures expand the engine by 6” in length and 2.5” in diameter and this sort of expansion and contraction needs exotic metals. The very front of the engine at the nacelle is titanium, the rest of the engine is made of iron nickel alloys such as Waspalloy, Inconel & Astrology. All the metal in the engine is directionally solidified so the metal expansion is directional and can be managed. The plumbing on the engine is made of steel 321 and 347 and there are over 600 pieces of plumbing on the engine.

The oil used in the engine is synthetic, made of polyphenyl ether and is stable at 650°F. The oil is maintained at 400°F by routing through a fuel oil cooler, a heat exchanger where the oil contacts with the cooler fuel heating it up and cooling itself before the fuel is routed into the engine.

The complex system was started by two V8 Buick Hellcat motors which were a petrolhead’s delight, apparently the crew blew through most of the Buick motors that salvage yards across the United States had with them. The two motors would spool up to 6000 rpm and the crank interfaced with a gearbox at the bottom of the engine and needed to retract as the aircraft engines got to 3000 rpm, the J58s fired up at 4000. The crew got so carried away with revving the Hellcats that they delayed retraction blowing their engines up! Once the Hellcat stock was run through the crew moved to Chevy 454 cu.in engines, but they were not the same.

At Mach 3.2 over 50% of the engine’s thrust was created at the inlet and an additional 28% at the afterburner. This left just about 20% of thrust needed from the turbojet! While the first A-12s flew with less effective J-75 engines, once they cutover by 1963-64 to the J-58, the blackbirds never went back to anything else.

The Design

The external dimensions of the A-12 Oxcart (the foundational Blackbird) was a length of 101.7’, wingspan of 55.7’ and a height of 18.6’. The MTOW of the aircraft was 124,600 pounds.

A front view of the aircraft showed off a flattish underbelly with blended in wings at the fuselage. A sharp angular cockpit at the very front and twin tail canted in at 15° each. The flow of the wing’s leading edge was interrupted by heavily integrated engines on each wing right in the centre.

A *front view of the SR-71 note the canted fins, the flattish underbelly and the blended wing fuselage. Pic Source: Reddit User*

The nose of the A-12 looks more conventional than the Blackbirds that followed. While it slopes up towards the angular cockpit windows in a more or less conical manner, the bottom is more flattened to merge with the rest of the flattish underbelly. This sort of contouring is necessary to manage shockwaves and keep the aircraft aerodynamically optimized.

While Blended Wing Bodies have existed since the early days of flight, they had never been used practically. The blackbirds are not traditional BWBs (as we know them since the 1990s) in the new sense they are what is called a Blended Wing Fuselage. Read here (https://theaviationevangelist.com/2025/09/19/the-flying-wing-part-two-the-blended-wing-body/ ).

The chines that begin on each side of the cockpit at a sharp angle of approx 70-80° and swept back towards the delta wing were an integral part of the BWF serving multiple functions. The first was stealth (yes the A-12 is the very first purpose built stealth aircraft https://theaviationevangelist.com/2025/10/22/the-theory-of-stealth/ ). The specially designed edges with their composite materials reflected radar waves away from the source and reduced the aircraft’s RCS to about 10m2 or a largish bird, a big improvement of over 90% over the RCS signatures of preceding aircraft Reducing the A-12 Blackbird’s Cross Section. The second purpose was the chines served had a critical to the aerodynamics of the aircraft and that was to generate lift. They worked to generate approx 17-20% of total aircraft lift in two ways. The first was the creation of vortices over the chines, inner wing and fuselage, delta wings with a sharp leading edge sweep, at high Angles of Attack (AoA) rely on vortex lift . The second is the blended and flattish underbelly works as a lifting body and contributes towards the 17-20% lift. This means the load is off the wings and more evenly distributed which is critical at high Mach numbers. The reason the chines were terminated at the cockpit i.s.o going right to the nose like the SR-71 was the A-12 was a single pilot aircraft and the chines terminating at the cockpit saved weight and were optimized for higher speeds at altitudes of up to 95,000 feet.

The chines blended into a delta wing with a leading edge sweep angle of 60°. The edge of the wing was interrupted in the middle by the engine nacelle.Close observation of the leading edge and the engine shows up a gap on both sides of the engine, this was to accommodate the 2.5” expansion of the diameter of the engine and boundary layer control. On the trailing edge the gap is more pronounced as this was the business end of the engine with the hot exhaust gases. Other than this the wing was fairly standard in the front view profile! A top view of the wing shows a second chime that comes off the outboard engine cowling on both wings blending back into the leading edge, these chines increase the aspect ratio of the swept back delta improving lift.

The trailing edges of each wing had a pair of elevons, one inboard and one outboard of the engine. In tailless delta wings the elevons serve the purpose of the elevators and ailerons. When they move together they control pitch and when they work opposite to each other, they control roll on the aircraft.

Further back is a pair of twin fins each canted in 15° as mentioned earlier, the canting is part of the aircraft’s stealth and the original fins were made of composite (because of they non reflective properties), however most of the aircraft in the entire Blackbird fleet used titanium fins with composite accents.

The entire Blackbird was a flying fuel tank. Fuel was stored in six tanks throughout the body and wings including the chines. The fuel was burned in a specific sequence as the center gravity moved significantly rearwards at higher speed numbers. The Blackbirds famously had wet wings. That is the skin of the wings and body of the aircraft was the fuel tank itself. In the interests of saving weight and the fact the titanium skin of the aircraft was heat resistant, the fuel was stored directly. The thermal expansion in flight meant the panels had gaps on ground and there were thresholds by area of the aircraft as to the number of fuel drops falling per minute that was acceptable. The same gaps sealed in the air as the metal expanded.

*Acceptable fuel leak range by zone of the aircraft. Pic Source : Reddit User*

The aircraft had a tricycle landing with the main gear having three wheels in parallel. The main nose gear had a single two wheel bogie. The Goodrich tires were infused with aluminium for thermal resistance and were inflated with nitrogen, a non combustible inert gas for safety.

Most of the aircraft was constructed of titanium because of its thermal resistance, however titanium is extremely hard to work with and a specialized set of rigs and tools had to be created to work with the metal. At the time the Blackbirds were being constructed the Soviet Union was the largest exporter of titanium and the CIA procured the required titanium through a series of shell companies making the final buyers (the CIA) untraceable.

The wings of the aircraft had corrugation on the top and bottom prompting jokes that Kelly Johnson was building a Mach 3 Ford Trimotor (an early airliner). The corrugation was to aid thermal dissipation and while there was a drag penalty at lower speeds which was powered through, at Mach 3 and over 80,000 feet the drag was minimal.

The aircraft was painted black with iron ball paint. The paint helps with stealth by converting radar waves to heat and dissipating it. Furthermore according to Kirchoof’s Law of Thermal radiation a good absorber of thermal radiation is also a good emitter, means that the black iron ball paint is the right color to repel heat by emitting it!

With a first flight in April 1962 the A-12 quickly demonstrated its capabilities even with the less capable J-75 engines. The USAF which was initially part of project GUSTO quickly realized this was an aircraft that was the answer to its need for a high speed aircraft. They put out the requirements for RS-71 (Reconnaissance Strike) by approx 1963, it was President Johnson who called the aircraft SR-71 erroneously and the name stuck. Furthermore the A-12 needed to be kept classified (which it was until 1990) and the USAF’s requirements for a high speed aircraft made a great cover story in 1964 when the SR-71 and YF-12 projects were announced. The M-21 Tagboard was never officially announced during its active life. TheYF-12 and the M-21 aircraft had approx same dimensions as the A-12 Oxcart while the SR-71 was longer and bigger. The M-21 aircraft had a pylon on top between the two fins to fit a D-21 drone on it. Of the two prototypes built, one crashed in 1966 when the the D-21 drone collided with the fins after separating, the plane crashed while the pilot survived, the M-21 was cancelled immediately after this and the surviving prototype is at the Museum of Flight in Seattle. Lockheed M-21 (Blackbird) | The Museum of Flight .

*The M-21 with the D-21 drone. Pic Source : Wikipedia*

The YF-12 took spots 7-9 on the A-12 Oxcart assembly line and was a Mach 3 interceptor prototype. It was to be a replacement to the F-106 Delta Dart, however severe cost cuts in view of the Vietnam War resulted in the program being scrapped. The main modifications was cutting the A-12’s nose chines to accommodate radar and infrared tracking equipment. The chines of the YF-12 show a clear indentation. Today of the three aircraft constructed only one survives at the USAF Museum in Dayton Ohio, it flew with NASA until 1979 after the YF-12 program was cancelled in 1967.

*The YF-12 Interceptor. Note the truncated chines. Pic Source: Wikipedia*

The YF-12 with modified chines to accommodate the radar equipment. Pic Source : Wikipedia

The Lockheed SR-71 Blackbird is a fairy tale of an aircraft, it has been immortalized in movies, books, articles like this and forum across social media with a huge fan following even 60 years after its first flight. Where the A-12 was heavily classified decades after its operation, the SR-71 was heavily publicized (to cover the A-12) and this is why the SR-71 is considered the most famous of the Blackbirds. Lockheed SR-71 Blackbird | Military Wiki

The SR-71 was to have a two man crew as against the A-12 single pilot. And where the A-12 carried a high resolution camera system the SR-71 carried a sensor array that included Side Looking Radar (SLR) and Electronic Intelligence Systems (ELINT). Where the A-12 was about covert photography for the CIA (the aircraft was disguised in USAF markings) the SR-71 was more about strategic reconnaissance (SR) for the USAF. To accommodate the radar installations, the chines were extended to the nose in the manner we know so well. The chine extensions on the SR-71 had the same lifting and stealth properties of the A-12, where lifting contribution remained at the same 17-20% as the A-12, the RCS was slightly higher than the A-12 but not by much (it was the larger bulk).

In case you are wondering why the A-12 on the USS Intrepid has the chines right to the nose tip, it’s because it was used as a radar object when understanding the stealth characteristics of the SR-71.

*A front view of the A-12 at the USS Intrepid. Pic Source: Wikipedia*

The SR-71 was longer than the A-12 by six feet to accommodate the second crew member and had a length of 107.5’. The wingspan and height of the aircraft was identical to the A-12.The dry weight of the aircraft was 6 tons heavier than the A-12 and MTOW was 22 tons heavier than the A-12. The additional bulk and mass made the SR-71 slower than the A-12 whose max speed was Mach 3.35 vs the SR-71s Mach 3.2. The SR-71s service ceiling was 85,000 feet vs the A-12s 95,000 feet. The range of SR-71 was 3250 miles vs the A-12s range at 2500 miles.When we see a comparison of the numbers we realize the A-12 Oxcart is just not celebrated enough.

Project Nice Girl

Project Nice Girl was the face off between the A-12 & the SR-71. The costs of running multiple high cost projects for the various services was getting out of control and in the autumn of 1967 the A-12 & the SR-71 had a play off. While the A-12 had superior speed and altitude , it was hampered by cloud cover during the fly off and the high resolution panoramic cameras on the A-12 were beaten by the SR-71s sensors that could peer past the clouds and collect valuable accurate data. The dividing factor was beating the weather and the A-12 was retired in 1968, the project was only declassified in 1990 and the aircraft handed over to museums across the United States.

Summation

As satellites got better and were in a position to take over from the considerable duties the SR-71, the amazing bird saw its days numbered. Additionally astronomical sosts of keeping the birds in the air just did not make sense to keep them flying and the decision was taken to retire the program.

Over sixty years after it first flew the SR-71 and the Blackbird Family of Aircraft continue to inspire awe, several of the projects they were involved in continue to be classified and this is what contributes to their enduring legacy. Their speed and altitude records intact over 35 years after the last flight of a Blackbird.

The peak of innovation…

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Tags A-12, aerodynamics, aluminum dust, blackbird, Blended Wing Body, blended wing fuselage, boundary layer, Buick, BWB, bypass, chine, CIA, D-21, delta wing, Drone, engines, expansion, J-58, landing gear, Lockheed, M-21, NASA, nascelle, Navy, nitrogen, Radar Cross Section, ramjet, RCS, resistance, SR-71, stealth, thermal, thrust, tires, turbojet, turboramjet, USAF, V8, YF-12

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The Boom Overture : A Return to Supersonic

November 11, 2025 //

Social Media continues to be flooded with images, stories and statistics about the Concorde, a troubled engineering marvel that retired in 2003. The ‘Concorde’ feed highlights the love and awe the aircraft inspired. The retirement had a sense of finality about it, like a curtain being drawn, not on just the Concorde but on Supersonic Travel itself, the reason being the issues were not about the aircraft but about the accompanying noise pollution at take off ,landing and the shadow called the sonic boom.

Boom Supersonic a startup founded in 2014, but birthed in the mind at least 2-3 years prior decided to do something about. Their CEO Blake Scholl decided he was going to make a supersonic aircraft that not only went supersonic , but was quiet and without an accompanying sonic boom over land. This is the story of The Overture and is part three of the QueSST series which you can read here. https://theaviationevangelist.com/2025/10/09/the-boom-xb-1-the-little-plane-that-could/ & https://theaviationevangelist.com/2025/11/04/the-lockheed-x-59-quesst-pinocchio-swordfish/

The Overture First Iteration

The first iteration of the Overture was unveiled in Nov 2016 along with the first iteration of the XB-1 the one third scale technological demonstrator of the Overture, and both of them looked extremely similar to what the Concorde looked like! The difference being the technologies available in 2016 vs 1969 when the Concorde first flew.

Overcoming the laws of physics means any supersonic aircraft needs to be shaped a certain way, and this is where the Concorde design was lightyears ahead of its time. Concorde was the only successful supersonic jet and it made sense to look at the Concorde baseline, the original Overture was to be a trijet as was the original XB-1.

By 2018 the XB-1 subscale model was ready for wind tunnel tests and the first set of tests confirmed the predicted aerodynamic calibrations that were arrived at through CFD was off by 30%. Such a difference keeps magnifying as it goes up in scale to the full sized Overture. The Team at Boom had to go back to the drawing boards after almost 4 years of work and rework the design of the XB-1 which in turn would impact the Overture.

*The very first iteration of the the Overture & XB-1 : Pic Source : The Independent*

Repeated finetuning of the design resulted in the XB-1 that broke the sound barrier in early 2025 with no apparent sonic boom (uses a concept called Mach Cutoff), the jet itself was still in a trijet configuration but looked considerably different from the original when it rolled out on Oct 7,2020.

The tests (both CFD & wind tunnel) highlighted the design on the final XB-1 was not scalable to the Overture and there would have to be a complete rework on the fuselage, wings and engines of the Overture. Some of the highlighted issues were high take off and landing speeds due to the very low aspect ratio on the reworked XB-1’s wing, the very high angle of attack that Boom addressed with an augmented reality system, a trijet would not produce enough thrust practically to push the bigger Overture to supersonic.

In July 2022 Boom unveiled its significantly reworked Overture. The unveiling was done after considerable CFD testing followed by wind tunnel testing across five locations in the USA & Europe covering various flight regimes.The fuselage and wings looked extremely sculpted and the aircraft featured four underwing podded engines instead of the original three.

The Design

At transonic speeds (Mach 0.8-1.2) local air flows accelerate over and around the aircraft fuselage and wings can reach the speed of sound. The minimum speed at which this occurs varies from aircraft to aircraft and is known as critical Mach number. The shockwaves formed at these localized zones cause a sudden increase in drag is called wave drag. To mitigate the strength and number of shockwaves an aircraft’s cross sectional area needs to transition smoothly from front to rear. This is known as Whitcomb Area Rule of 1952 .The phenomenon was observed in various forms by multiple aerodynamicists before Whitcomb.

In the case of the Concorde the area rule was applied at Mach 2 and the rear fuselage was extended by 12.2’ on the production aircraft over the prototype and reduced wave drag by 1.8%. A similar concept was applied to the first iteration of the Overture and XB-1, the results we have already spoken of. The final iteration of the Overture extensively uses the area rule to maximum effect.

The external specs of the Boom Overture are a length of 201 feet ,a wing span of 106 feet and a height of 36 feet. The interior cabin is expected to be 79 feet in length with an aisle height of 6.5 feet, good enough for a tall person to walk through at full height. It will be capable of Maxh 1.7 at 60,000 feet cruising altitude and a max range of 4.250 NM approx 350NM more than the Concorde. https://apnews.com/press-release/pr-newswire/technology-airlines-climate-and-environment-7e88c34a01a4194c6f1e6b4760d2bb86

A front view of the nose is the first observation of the rule. Where subsonic aircraft bodies in general are circular to oblong in appearance the Overture’s nose and body behind has a distinct oval shape (left ↔️right) like an egg starting from a singular point the tip of the nose. Much like the final XB-1 the nose slopes upwards at a much higher angle from the nose tip than the bottom, like a cone that has been pushed down flattening the bottom. The oval shape of the nose permits the cabin to have the maximum permissible height allowing for passenger comfort as they walk through the aisle while at the same time minimizing aircraft front on cross section. The reason for the differing angles top and bottom of the nose tip is to control shockwaves. One of the main lessons learnt from the XB-1 was shockwaves tend to be unpredictable when the nose is a perfect cone and sometimes tend to blanket the vertical stabilizer, doing so prevents the occurrence and ensures a smooth flow over the nose and aft across the fuselage.

*A front view of the Boom Overture: Pic Source: Boom Media Assets*

The nose slopes up to the cockpit windshield, the cockpit is the widest and tallest part of the fuselage. Much like the XB-70 Valkyrie where the wasting is clearly visible as the fuselage narrows down towards the tail from the cockpit the Overture does the same. In Fact the black stripes that extend from the cockpit and run rearwards forming an incomplete loop around the widest part of the fuselage looks almost Jumbo Jetish from a top view. The fuselage belly is comparatively flat and a similar design is seen on the XB-1.

Where the Concorde had a drooping nose which was had heavy hinges and actuators, the Overture does away with the entire mechanism and instead has an augmented reality system tested on the XB-1. If the Overture’s system is like the XB-1’s it will have two 4K cameras that are mounted on the nose gear (so they can be retracted completely in flight), the cameras will be at least one large bird’s wingspan apart to build redundancy against bird strikes. The screen inside the cockpit will display a composite image along with airport markings etc if at ground level. The system is a huge weight saving over the Concorde of approx 1650 pounds.

The gull wings of the Overture have a complex geometry.

Boom – FlyBy – It’s About Time For a Bold New Era of Supersonic Flight . The modified delta planform has several special design tweaks to it. The wing appears to have a dihedral angle at the wing root and inboard section which transitions to an almost flat to anhedral angle at the outboard sections. A positive dihedral (approx 3-5° upward angle) helps with lateral stability and keeps the passenger cabin level at cruise. The flat to slight anhedral angle of approx 1° helps optimize supersonic wave drag while maintaining aileron command. The underside of the wings appear to be blended into the fuselage to soften shockwaves.

*A side view of the Overture. Note the wing architecture & staggered engines. Pic Source : Boom Media Assets*

A look at the leading edge from the top shows off a clear kink much like a cranked delta on the inboard form. The kink slows down the air over the wing even as the aircraft is supersonic. Imagine if you were running towards a fence that is perpendicular to you, when you hit the fence all of you hits it at once, now imagine the fence is at an angle and kinks slightly towards you, when you first run towards it the first bit goes much faster than the rest of the wing after the kink, even though you are running at the same speed and only a little bits of you hits the fence as you keep running, the same is good for the gull wing. The inboard kink generates a powerful vortex at high AoA over the inboard wing which generates lift at takeoff & landing. The vortex prevents air separation and stalling at high angles of attack.

A top view of the Overture. Note the kinks on the leading and trailing edges of the wings and the cropped wingtips. Pic Source : Boom Media Assets

The steep inboard sweep which is in the region of 70-75° transitions to a shallower sweep in the region of 50°, the sweep change happening at the kink or crank, the leading continues its transitionary sweep through to the wing tips The steep inboard geometry delays shock formation and reduces wave drag at Mach 1.7 while the shallower outboard sweep increases wing area which in turn boosts lift while at the same time delaying stall formation. The crank or kink creates a natural break between the inward vortex lift and the outboard attached flow, such geometry results in superior roll authority across the speed regime.

The leading edge further shows a thicker front view profile than the Concorde did, this helps generate more lift across the speed range while at the same time exhibiting heavy contouring. Where the Concorde had an S curved leading edge that was sharp and thin, the Overture has a more ‘traditional airfoil’ although there is nothing traditional about it. At the wing roots the wings tend to blend upwards (dihedral angle) into the tapering fuselage while they drop downwards (neutral to slight anhedral angle) and lower towards the cropped wingtips. Such a design naturally helps with managing roll and gives the wings the distinctive gullwing shape.

The Overture’s cropped wingtips represent an evolution over the Concorde’s pointed ogival delta tips. On the Concorde the tips maximized the wings aspect ratio (span to average chord ratio) while helping minimize wave drag, however they were vulnerable to flutter (vibrations at high speed) on the overture the cropped wingtips ensure the aircraft maximizes area ruling through the whole profile cross section, while details of the crop are not available, we can expect the wing to be about 10-15% more efficient in fuel burn per passenger. Since a large portion of the flight time will be in the subsonic/transonic regime, the cropped wings lessen induced drag aiding quieter takeoffs (the Overture aims to be below 75dB). At supersonic speeds the sharp tips of the Concorde amplified the sonic boom , while the Overture’s cropped wingtips combined with the gullwing design soften the boom signature.

The trailing edge of the wing has a very obtuse lambda (Λ) on it. The first function of the Λ is vortex control, if you look at it relative to the leading edge kink (the vortex generator) it is slightly outboard from it. The shape helps break up and weaken these vortices as they exit the wing surfaces by inducing geometric discontinuity. The trailing edge shape also acts a sonic boom diffuser by preventing the coalescence of shocks and softening the boom overland. The trailing edge Λ and the kink on the leading edge means the wing is also called a cranked arrow.

The Λ on trailing edge helps low speed handling and stall characteristics by promoting an earlier flow separation at the root encouraging an inboard to outboard stall pattern. The overall wing design should have a washout. The trailing edge Λ further reinforces the area rule ethos of the aircraft. The edge has an inboard flap inboard of the kink and an outboard flap that appears to begin exactly on the kink. On the outermost part of the edge is the aileron. https://boom-press-assets.s3.us-west-2.amazonaws.com/Newsroom-Media-Assets/Overture/Videos/Overture-Systems-Configuration.mp4

The four symphony engines of the Overture appear staggered and spaced out with the inboard engines about 5 feet in front of the outboard engines (no specifics available). The staggering enables the coke bottle design (area rule) and synergizes with the wing’s highly sculpted gull wing design to minimize shockwaves at Mach 1.7, increasing range. The offset also helps with yaw control in the case of asymmetric thrust and improves low speed handling as against the Concorde’s close engines placed further back on the aircraft fuselage.The engine setup looks like a B-58 Hustler from the 1950-60s. The Hustler itself was yet another troubled but genius engineering marvel, from a time when supersonic aerodynamics understanding was still in its infancy.

The wings transition towards the empennage of aircraft. Unlike the Concorde that had an ogival delta that used elevons (combination elevators and ailerons) the Overture has a traditional aircraft’s tail with vertical and horizontal stabilizers. The vertical stabilizer’s slanting leading edge appears to land on the top of the wasting fuselage at the same spot the wingtip’s trailing edge outboard corner finishes, respecting the area rule principle. With a height of approx 18-20 feet and an area of 450-500 sq ft the stabilizer has more area than the Concorde’s at 380 sq ft and height of 27 feet. The rudder on the vertical stabilizer enhances directional stability at high AoA and provides authority in an engine out situation.

The span of the horizontal stabilizers is approx 55-60’ and an area of approx 350-400 sq ft. The stabilizers are set at a slight anhedral angle of 3-4° (cannot confirm) work closely with the vertical stabilizer and provide the aircraft with pitch authority. The control surfaces on the horizontal stabilizers are actually at the very tail of the plane and are again placed in such a manner once again to respect area ruling. The stabilizer is trimmable to manage the angle of incidence for various speed regimes and centre of gravity shifts.

*An image showing the structure of the Overture. Pic Source : Boom Media Assets*

The landing gear of the Overture is a tricycle setup with the nose gear having two wheels which retracts into itself and forward into the fuselage to lessen its volume profile. The main landing gear features six wheels on each bogie , which is highly unusual for an aircraft weighing in at approx 415,000 pounds. So why take on the additional weight and space? Delta winged aircraft have a normal landing speed of approx 140-160 kts which is considerably faster than a normal subsonic airliner which 135 kts such a speed will require additional braking power plus the added redundancy in case of a blow out.

AoA at take off and landing has always been the central focus to the Concorde’s design. At take off the Concorde’s AoA was between 15-18° and landing approx 15-17°, such an angle was steep enough to entail the droop nose to manage visibility. The Overture with it’s wing and tail design is expected to have a take off AoA of approx 12-14° and a landing AoA of approx 11-13°. Visibility is managed by the augmented reality cameras spoken of earlier. For reference the 777 has a take off AoA of between 12-18° and touchdown of between 6-8°. The Overture aims to have an almost subsonic aircraft type of landing angle.

Design reference points: Boom – Overture & https://boom-press-assets.s3.us-west-2.amazonaws.com/Newsroom-Media-Assets/Overture/Videos/Overture-Systems-Configuration.mp4

The Engines

Engines are the key to any aircraft’s success and in the case of SST’s they assume an added importance. They need to be capable of speed, be quiet & efficient in addition to being 100% SAF (Sustainable Aviation Fuel) compatible. Such engines can be expensive to develop and need fresh though and innovation at each step of the design and construction process. To understand SAF read here. https://theaviationevangelist.com/2025/09/25/alternative-aviation-fuels/

By 2017 Boom was on the lookout for an engine partner and Rolls Royce looked the part with their previous experience developing Concorde’s Olympus Snecma 593 turbojets. The partnership with Boom looked like the natural step forward ushering in the sustainable supersonic era (the engines were to be 100% SAF compatible) and the partnership was announced with much fanfare in July 2020. After two years embedded with the Boom Team in Colorado as they narrowed down engine specs and characteristics and then the partnership fell through in September 2022 and by December 2022 Boom decided it was going to develop the Boom Symphony engines inhouse. The parting was cordial but stiff with RR saying that developing a supersonic engine was low on their priorities list and Boom stating they were appreciative of RR’s work.

The reality was RR was wary of another Concorde like disaster where they lost the equivalent of $1.32 Bn per aircraft and did not have the wherewithal to go through the development pains in the tough economic scenario the World was currently in (COVID). Boom for their side felt the engines offered, a low bypass Pearl 700 used in Bombardier Jets aiming for 30-35,000 pounds of thrust per engine with modifications such as the inlets and exhaust with chevrons would seriously compromise efficiency by approx 23-30% below Boom’s targets. Several options were studied but nothing came off. There was added pressure on RR with the worldview on emissions (supersonic aircraft burn 3X fuel compared to subsonic aircraft) and RR’s failed partnership with Airion developing the infinity engine which ended in 2021 with Airion folding up.

Further partnerships were explored by Boom with GE, Pratt & Whitney and even Safran but all of them declined to partner, this is when Boom decided they are going it alone.

*The Boom Symphony. Pic Source: Boom Media Assets*

The exterior dimensions of the Symphony engine are a length of 42 feet & height of 7 feet. The supersonic inlet is 12 feet in length with the variable geometry exhaust at 19 feet, the turbofan and sprint sore section at 11 feet.The Symphony is a medium bypass twin spool engine with 3 low pressure & 6 high pressure compressors with no afterburner developing 40,000 pounds of thrust per engine.The design of the engine is optimized to the Overture and is 100% SAF compatible.

All engines have three phases suck, bang & blow. The suck is done by the main frontal fan and compressors. The bang is in the sprint core and the blow is through the high & low pressure turbines just before the exhaust.

The inlet of any SST engine is where the magic happens. While the aircraft is supersonic the engines can only gulp in air at between 400-500 mph. The inlet is where the air is slowed down by use of shockwave creation. In the case of Concorde a series of ramps and bleed valves for excess air was used to slow the air down to approx 500 mph from Mach 2. The architecture of the Overture and Symphony is different where the engines are podded below the wings instead of being integrated into them in a cluster as on the Concorde. The Inlet of the Symphony is axis symmetric with a spike at the central axis (much like the Lockheed SR-71). The spike moves back and forth as per the speed of the aircraft and manages the inlet shockwave. In the case of the SR-71 the central spike moved back up to 26” at high supersonic flight. The Symphony will probably be up 18” (speculation).

Boom is currently in the advanced prototyping phase and last month they announced that 95% of all parts were done and have been moved to manufacture. Boom is making use of extensive 3D printing of parts at their printer farm (additive manufacturing) for a number of parts being used in the Symphony prototyping phase. The Sprint Core currently being tested has 193 3D printed parts. The alloy used is Haynes 282 a nickel based alloy that can withstand extreme heat and stress.Such an approach enables rapid prototyping & iteration. An example of the speed they work at is they prefer vertical integration (in-house manufacture) vs waiting upto six months for parts to be delivered and choose to spend a couple of million dollars on the required machine instead.

Currently the Sprint core is being tested at Georgia Tech’s Combustion laboratory where the hot section is currently being put through its paces. Similarly each component of the engine will be tested independently, such an approach saves time and helps with iterations. Once testing is complete across all the engine components, they are integrated into the prototype engines, fired up and parameters checked.

Blake says the Symphony expects to generate thrust early 2026. Such a tight timeline places great pressure on the propulsion team.

Generating 40,000 pounds of thrust on-time is critical to Boom’s future funding (will speak about this).

Symphony Reference: Fact Sheet

The Superfactory, Construction, Assembly & Partners

The 180,000 sq ft Boom Superfactory has been constructed by BE&K building group and cost approx $100 Mn to construct. Boom Supersonic Overture Superfactory | BE&K Building Group . The superfactory is at Piedmont Triad International Airport in North Carolina on 65 acres of leased land, which incidentally will also host the factory producing JetZero’s Z4 Blended Wing Body Aircraft. As per a press release, Boom plans to invest $500 Mn in NC of which the building is $100 Mn, that leaves approx $400 Mn in terms of tooling yet to come in. Governor Cooper Announces Boom Will Manufacture Supersonic Aircraft in North Carolina Creating More Than 1,750 Jobs by 2030 . The Superfactory is where the final assembly of the Overture will take place and there is an entire ecosystem of partners involved in constructing the individual parts. Some of them are Aernova for the wings (they list Boom as a top innovation). https://www.aernnova.com/products/wings . Safran Landing Systems will be manufacturing the Overtures beautiful landing gear. https://boomsupersonic.com/press-release/boom-supersonic-and-safran-landing-systems-sign-supplier-agreement-for-overture . The Overture’s empannage is manufactured by Aciturri.There are several other key component suppliers who are part of the ecosystem to help make the Overture flight ready.Fact Sheet. All of them are currently design ready .

Most of the Overture will be carbon composite including the fuselage, wings & empennage. Titanium will probably be used in high stress areas such as the landing gear, engine bays and wing & stabilizer leading edges. The engine internals will have alloys such as Inconel in addition to Haynes 282 mentioned earlier. The Superfactory will have autoclaves (large ovens that cure the layered prepeg under pressure). To put a cost perspective autoclaves can cost up to $4 Mn a pop.

*The Overture inflight. Pic Source: Boom Media Assets*

Cash Runway

Developing the Overture is estimated to cost approx $8 Bn up from a previous estimation of $6 Bn. Boom has so far raised approx $700 Mn through 12 rounds of funding and the investors have shown patience through the iterations process. However Boom is still well short of the required number by a long way. An IPO might be a way forward, but it will not bridge the gap.

The last funding round in late 2024 was termed as a series A showing a reset within the company, the valuation down to $584 from peak valuations of $1Bn and even $6Bn after the Aug’24 funding. Looking at the volatility of the valuation, it is extremely important for Boom’s Symphony engines to generate 40,000 pounds of thrust in early 2026, this can very well pitch the valuation up skywards and open a round of extremely high funding, Boom should target at least $1 Bn or more (thrust is the single most important milestone from here on) raised after the thrust milestone.

In Nov ’23 Neom Investment Fund invested in Boom as a strategic investment. NIF is a subsidiary of Saudi Arabia’s Public Investment Fund (PIF) and is a key vehicle to Prince Mohd Bin Salman’s Vision 2030, an ambitious plan to diversify Saudi Arabia from its oil dependence. If PIF’s investment in Lucid Motors is any indicator where they have become a majority shareholder with over $8 Bn invested in a relatively short period. Boom has much to look forward to as they generate thrust and tool up their superfactory.

An IPO will probably be at either the Overture’s first supersonic flight or just as FAA certification progresses past 50%. Boom has been working very closely with the FAA at each step and only moves with each part after the FAA certifies it, this vastly cuts lead time. Some of the other aerospace startups like Joby & Archer Aviation have valuations that are at ± $10 Bn and Boom should target at least that much if not more.

Innovation at Work.

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Tags Aciturri, Aernova, autoclave, Boom, BOOMSupersonic, construction, drag, engines, funding, General Electric, Gull Wings, JetZero, landing gear, lift, Lucid, Neom Investment Fund, North Carolina, Overture, PIF, Pratt & Whitney, Rolls Royce, Sagan landing systems, shockwave, Sonic Boom, SR-71, superfactory, Symphony, tooling, USAF, Vision 2030, vortex, XB-1, XB-70 Valkyrie, Z4

Categories Uncategorized

The Lockheed X-59 QueSST: Pinocchio/Swordfish

November 4, 2025 //

Introduction

The X-59s first flight last week was a major step in NASA’s Quiet Supersonic Technology (QueSST) program. Every aircraft that flies supersonic is accompanied by the shadow of the sonic boom (read about it here : https://theaviationevangelist.com/2025/04/13/shockwave/ ). QueSST’s aim is to fly supersonic with no audible sonic boom. To achieve that there are two approaches. The first is to use atmospheric refraction (where the sonic boom makes a U-Turn back towards the sky) like what Boom Supersonic achieved with its XB-1 (read about it here : https://theaviationevangelist.com/2025/10/09/the-boom-xb-1-the-little-plane-that-could/ ) and the other is to engineer sonic booms that are weak and do not have the strength to reach Earth, and if they do, are very weak and barely audible. Enter the QueSST program.

*A subscale model of the X-59 in a wind tunnel test, showing shock wave formation points. Pic Source: NASA*

The QueSST Program

The 1973 supersonic over land ban was a bodyblow to the Concorde and civilian supersonic travel over land in the USA. While supersonic travel in general went into hibernation and Concorde motored on until 2003, the writing was on the wall. For supersonic travel to be profitable, aircraft needed to have the ability to travel supersonic over land and to achieve this, they needed to have no audible sonic boom.

NASA’s High Speed Research (HSR) program which ran between 1990-99 focused on a High Speed Civil Transport (HSCT) that would be environmentally acceptable. The program took into account fuel burn, emissions and of course noise pollution from sonic booms. The aircraft was to be Mach 2.4 capable and have a 300 passenger capacity. The tests involved tracking a Concorde in a U-2 spy plane to measure high altitude emissions and even had sonic boom mitigation technologies tested using a Lockheed SR-71 testbed. New engine nozzle technologies were tested to reduce takeoff and landing noise (on the Concorde the afterburners are responsible for the 110dB noise level). The HSR was probably the first time Computational Fluid Dynamics (CFD) was used to test new innovative designs.

A heavily modified T-38 Talon and a F-4 Phantom in addition to F-16XLs were used to validate these designs and collect a database of sonic boom signatures, which could be applied to future aircraft designs. While Schlieren photography had existed for over a hundred years, NASA innovated on the technique and created Air-to-Air Background Oriented Schlieren Technique (AirBOS) where a full sized aircraft could be photographed using the Sun as a background to see where the shockwave formation and their interaction with each other and ambient surroundings.

*An image of a T-38 flying in close formation with a second T-38. Note the shockwave interaction off the tail of the aircraft. Pic Source: NASA*

*Schlieren image of both T-38s in close formation. Note the shockwave interaction. Pic Source: NASA*

While the actual HSCT program itself ended in 1999 due to lack of funding and Boeing’s withdrawal from the program , the AirBOS series of the tests continued through the 2010s, of note were 2019 flights featuring two T-38s going supersonic at close formation and the shockwave interaction of the two planes.

Lockheed Martin was awarded the preliminary design contract for the X-59 by NASA in 2016.

The X-59 First Steps

The design parameters were to create a Mach 1.42 capable aircraft with a service ceiling of 55,000 feet and a supersonic perceived sonic boom of not more than 75dB, the equivalent of a car door being shut (https://ntrs.nasa.gov/api/citations/20220009074/downloads/QuesstMissOvuaX59Pres_050822r.pdf) . This puts the X-59’s perceived sound over 10 times quieter than the Concorde’s 110dB.

The preliminary design steps were to create a 9% subscale model that could be tested in a wind tunnel between Mach 0.3 – 1.6. Lockheed was finally awarded a $247.5 Mn contract to design and build the low boom X-59 in April 2018, the designation X-59 QueSST would follow in June 2018.

The X-59 Design

The design approach to the X-59 was to alter the aircraft’s shape & aerodynamics to prevent shockwaves from merging into a loud sonic sonic boom, instead the design was about dispersing them before they got stronger and keeping the sonic thump under 75dB. Such aircraft would be of a long and slender profile which distributes pressure disturbances over a longer axial distance (https://www3.nasa.gov/specials/Quesst/how-x59-designed.html )

*The X-59 dimensions: Note the symmetrical angles that will be spoken of as you move ahead. Pic Source: NASA*

The highlevel specs of the X-59 are an overall length of 99.7’ , a wingspan of 29.7’ and a height of 14’. The all moving stabilizers have a span of approx 15’. For a long aircraft the wheelbase is just 17.6’. With the center of gravity exactly above the main landing gear borrowed from an F-16. The aircraft is area rule compliant.

To prevent the formation of a N wave , the X-59’s nose is over a third of the X-59’s length at between 30-35 feet to canard, 38 feet to cockpit. The nose of the X-59 is treated as an independent structure before being mated to the fuselage and is manufactured by Swift Engineering.

A frontal view of the nose presents a flat almost duck beak-like profile with a tip in the middle. The top view of the nose tip looks like swept back wing leading edges. The highly sculpted surfaces leading off the nose leading edge look like the nose ramping upwards and back towards the cockpit and a relatively flattish profile leading back towards the rearset nose landing gear. The reason for the shape of the nose is having a conical shape like earstwhile Concorde leads to shockwaves going off in unpredictable directions and in some cases blanketing the vertical rudder. As we move further back the nose cross section transitions from the flat tip to an elliptical type of complex shape as it moves up towards the cockpit. The nose funnel cross section is approx 2 x 2 feet. Overall the nose with its length and continuous uninterrupted design ensures the first shockwave off the tip is soft and has nowhere to merge into propagating downwards and staying soft (below 75dB).

*A front view of the nose, note the flat profile. The nose camera is visible, along with the canards and the wings. Note the under wing sculpting. Pic Source: NASA*

Just before the cockpit at around the 30-35 foot mark aft of the nose tip are the X-59’s fixed canards. They serve two purposes, the first is shockwave separation , distribution and keeping it off the cockpit & eXternal Vision System (XVS, more on this later), the second is to provide forward lift. The X-59’s design is such that the wings are at mid fuselage and the single engine is right back under the vertical T tail, to compensate for the unbalanced lift (as the centre of gravity moves rearward) generated by the lifting surfaces towards the rear of the X-59, the aircraft needs fixed canards that are upward canted generating a dihedral angle. Such positioning helps the X-59 generate a nose up pitch in cruise (at low speeds with their 63° swept angle, they generate vortex lift at high AoA) and maintain balanced flight without generating drag. Furthermore the canards interact with the shockwaves coming off the nose and with the wings further back to ensure shockwave strength is at a minimum, this helps with sonic boom mitigation. The trailing edges of the canards meet the fuselage at 59° angle with the fuselage. Such angling helps with boom mitigation as well. The canard root chord is 10.2’, tip chord of 3.8’ and the span is 13’. The canards generate approx 15-18% of total lift. The canards are subtly blended into the forward fuselage.

The nose design of the X-59 is such that there can be no cockpit canopy bubble like most experimental aircraft. Instead the cockpit of the X-59 has one major similarity to Charles Lindberg’s, Spirit of St Louis, both of them do not have a forward windshield and both of them use side windows to help with external pilot vision. In the Spirit of St Louis , Lindberg had to look out the flat side windows, in the case of the X-59 the side windows are contoured and offer a truncated forward view with the vision line running parallel to the nose. What the X-59 has is the XVS. The XVS consists of two high resolution 4K cameras, one is on the upper nose just forward just forward of where the cockpit windshield would have been, the camera is fairinged to deflect shockwave formation. The bottom camera is just forward of the nose gear and approx 12-18 feet aft the nose tip. The feed from both cameras is processed by the XVS computer for real time stitching and augmentation and overlaid on the secondary cockpit display which includes augmented reality (AR such as runway lines, glide slope indicators & Traffic Collision Avoidance System (TCAS). The latency is >50ms and is designed to feel natural to the pilot.

Note the gap between the wings & canards, the cockpit side windows, the camera, landing gear from an F-16 and the engine cowling. Area Ruling clearly visible. Pic Source: NASA

The flat (0°)low aspect ratio (AR) wings of the X-59 are a double delta(such wings trade pure supersonic speeds for incremental low speed handling) with the inward leading edges of 76° and outward leading edge of 68.6°, the crank is at approx 50-55% outboard sweep. The leading edge of the wings starts about a foot behind the canard trailing edge root (approx 45’ from the nose tip), the geometry of the canard trailing edge is such that the canard tip appears to be at the same level (or close) to the wing leading edge. Such architecture is critical to boom splitting, while at the same time maintaining aerodynamic continuity. The leading edge of the wings starts exactly at the cockpit side windows. The wing root chord is 25’ and the wings feature a washout of approx 2-3° for tip stall mitigation.

The trailing edges of the wings have a pair of inboard flaps and the outboard ailerons (with restricted movement to avoid fouling up with the crank)are just beyond the trailing edge crank. The canards work in tandem with the ailerons by providing a small positive deflection and reducing mid body shock by 4%. We realize the X-59 is actually a cranked arrow (read here: https://theaviationevangelist.com/2025/10/02/lambda-wings-moving-wingtips-flying-wings-part-3/ ). The trailing edge crank is approx 50-55% of the span with an inboard sweep of 59° and an outboard sweep of 63°. We also observe matching canard angles (something stealth aircraft implement (https://theaviationevangelist.com/2025/10/22/the-theory-of-stealth/ ). The trailing edge allows inboard and outboard control surfaces to be decoupled. Area rule compliance is important for all transonic aircraft and the trailing edge crank ensures the same.

The lower wing root is seamlessly blended into the fuselage underbelly using area rule sculpting to avoid waist shock like what Concorde had. The 22’ of blending appears to look canoe-like and is 1.8’ at the deepest point between the wing and fuselage. The blending reduces underside pressure by 18%. The sculpting incorporates fuel tank #2.

The all moving horizontal stabilizer has a span of 26.2’ (almost as large as the wings) root chord 14.6’ and a tip chord of 5.9’. Here is where it gets interesting! The area of the stabilizers at 2582 ft is actually larger than the wings at 2152 ft!! The reason for this is at Mach1.4 and 55,000’ the stabilizers are responsible for pitch control. The leading edge 63° (we now have the canards and horizontal stabilizers & wings having similar angles). The trailing edge of the stabilizer has a forward sweep of 12.6°. Such a design delays tail shock and extends pressure recovery, the delay is 22ms v/s a neutral trailing edge. A front view of the aircraft highlights how the canards & wings are mounted at two distinct vertical heights relative to the fuselage waterline. Such a placement further prevents shockwaves merging by separating them vertically, thereby reducing boom strength. ( Carnards at 1.8’ above the wing and the horizontal stabilizers appear to run at a similar angle and height to the wing).

*A rear view of the X-59 showing the engine aft tray, horizontal stabilizers, wings with ailerons, vertical T tail. Pic Source: NASA*

The vertical tail assembly of the X-59 includes the engine fairing (the engine sits on top of the fuselage using area ruling) , the vertical tail above it and the T tail at the top. Behind the engine is the aft shelf that extends about 3-4’ under the engine exhaust to divert supersonic exhaust upwards, without it the jet’s mach diamonds will interact with the fuselage boundary layer amplifying sonic booms.

The vertical tail of the X-59 is 14’ tall (including the engine height) with a backward sweep of 59° and a pure tail height of 10.5’. The horizontal fins on the T tail appear to have a similar geometry to the canards albeit about half the size.

The Engine

The X-59 uses a single GE F-414 low bypass afterburning turbofan with a bypass ratio of 0.25:1. The same engine is used on the F-18 Super Hornet. As mentioned earlier the engine is mounted on top of the fuselage and below the vertical tail fin. The engine produces 14,000 pounds of thrust dry and 22,000 pounhds of thrust with full afterburner. The engine with a service ceiling of 60,000’ is capable of handling the X-59’s test parameters.

Materials & Construction

Over 85% of the aircraft is made of carbon fibre reinforced polymers (CFRP), working with such a material helps achieve the complex aerodynamic shaping of the X-59. The nose of the aircraft which weighs in at approx 300 pounds is a hollow CFRP cone. The inside of the nose has between 6-8 CFRP bulkheads that are each spaced about 4-6’ apart. Between the bulheads are between 8-12 stringers that run the full length of the nose cone. The nose is OOA (Out of Autoclave) cured to hold precise shape and is bolted to the fuselage with shear resistant bolts at flange joints that transfer loads to the fuselage. The nose is pressurised at 2-5psi with dry nitrogen. This increases panel stiffness by between 30-40% and helps boom consistency while preventing flutter and protecting avionics.

The fuselage is a central load bearing barrel that integrates 4 fuel tanks that hold 12,500 pounds of fuel, enabling the aircraft a range of 3,500nm. Furthermore the fuselage also integrates the cockpit and avionics. Titanium is used in the X-59 wherever it comes in direct contact with CFRP. The wingbox is aluminium and is bonded to CFRP skins via co-cured doublers. All the other bulkheads in the fuselage are CFRP (Toray 2510 prepeg). The forward engine firewall is titanium-aluminium-vanadium (Ti-6Al-4V), while the mid engine ring is Inconel, the aft nozzle is CFRP+Inconel liner.

Summation

The X-59 promises a fresh new look at supersonic flight and the data collected will be invaluable to future supersonic airliners. With a total of approx 50-100 flights planned over a 2-3 year period, the first 20-30 flights are expected to be subsonic and will test onboard systems. The next 20-40 flights will be transonic (over Mach 1.0) and will focus on acoustic measurements over remote areas such as Edwards AFB. The final 10-20 flights are planned over cities such as Galveston,TX and others to gather public perception.

The first planned supersonic flight is expected to be mid 2026….

Innovation is Key…

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Tags 4K, ailerons, aluminum, blending, camera, canards, CFRP, dihedral, elevons, F-16, F-4, horizontal stabilizers, HSCT, HSR, landing gear, Lockheed, Schlieren image, shockwave, Sonic Boom, T-38, T-tail, titanium, vanadium, X-59 QueSST

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The Boom Overture : A Return to Supersonic

The Lockheed X-59 QueSST: Pinocchio/Swordfish

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