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

November 26, 2025 //
0

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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The Boom XB-1 – The Little Plane That Could

October 9, 2025 //
2

Intro

On Feb 10th 2025 the Boom XB-1 completed her 13th   and final flight. Baby Boom got to 36,514 feet in altitude, went supersonic all the way to Mach 1.18, flew for 41 minutes and was captured in vivid schlieren images going supersonic. While all these are stunning achievements, there are several standouts. The first is boomless cruise, the XB-1 went supersonic with no audible sonic boom and the second was this aircraft was almost directly responsible for having the 52 year old supersonic over land ban in the United States overturned and finally the Boom XB-1 is the very first privately funded aircraft to go supersonic. This is the story of ‘The Little Plane That Could’.

A Schlieren image of the XB-1 going supersonic. Pic Source : Boom Supersonic website

The Idea

The idea of the ‘Baby Boom’ as a technology demonstrator finds its underpinnings in 2013. Their CEO Blake Scholl was always interested in supersonic flight post a visit to the British Airways Concorde G-BOAG at the Museum of Flight in Seattle and had set up a Google alert if anyone ever came up with the idea of making a supersonic airliner, nothing ever came about. 

A X post of mine with Blake’s reply saying that’s where supersonic started for him.

In 2013 after yet another transatlantic flight delay, he pondered if this was something do-able. Like most ‘cowboy entrepreneurs’ he just decided to do it and Boom was born in 2014 with seed funding of approx $1-2 million and 10 driven engineers.

By 2016 the team at Boom had its first iteration of what the Boom Overture airliner would look like. It was a trijet design with two engines underwing and the third buried inside the tail. The aircraft looked very similar to Concorde being a tailless delta and was designed to carry 65-88 passengers and cruise at Mach 2.2. The aircraft renderings were unveiled at the Paris Air Show and the Boom revealed a letter of intent from Virgin Atlantic (that is a story by itself and speaks volumes of Richard Branson & Blake himself). This triggered a Series A funding of $50 million . The media highlighted the idea of ‘democratization of supersonic air travel’. 

The first iteration of the three engined Boom Overture. Pic Source: Flight Global

Developing a full scale airworthy Overture is a $8bn venture. There are designs to be proven, manufacturing processes to be validated and of course the need to prove that a small start up with almost no background in aerospace can build a supersonic aircraft.

It’s around this time a pivotal realization emerges within the Boom Team. They needed a ‘Falcon 1 moment’, they needed a demonstrator.

The Demonstrator

Small aerospace companies such as Boom have no track record but are extremely ambitious about their disruptive technologies and need a proving platform. Successful subscale (a percentage of full size) demonstrators provide the foundation their technologies and capabilities need at a fraction of the cost (Boom spent $13 million on the XB-1 & $156 million on the project, including early Overture development costs).

The Falcon-1 moment that Blake was referring to was the fourth launch of Space X’s first little rocket. After three failures between 2006-08 and on the verge of bankruptcy Space X had enough money for one more launch. The fourth of course proved private rocket launches were viable and the rest is history.

The XB-1 was a 1:3 scale prototype of the full scale Overture (first iteration of the Overture was a trijet which changed to four engines in 2022. The XB-1 was ready to start testing by 2020 itself, and hence stayed a trijet) and the project goals were to prove privately funded supersonic flight was possible by validating aerodynamic design & highlighting Boom’s manufacturing capabilities.

A mockup of the first iteration of the XB-1 at the Wing over Rockies Museum: Pic Source: Wing Over Rockies

The Design

The XB-1 had two designs. The first iteration had a length of 68 feet and a wingspan of 17 feet. The final iteration had a length of 62.6 feet and a wingspan of 21 feet. The increased wing span of the slender delta on the latter gives the aircraft an improved aspect ratio (even though the wing had a shorter chord at the root, the original had the wing chines right up to the cockpit, while in the latter the wings chines end behind the cockpit and the aircraft was shorter by over 5 feet) which in turn improves aerodynamic handling especially at lower speeds. The wing itself is an ogival delta, where the leading edge has a S shaped curve on the leading edge to manage vortices as lifting devices.

A front view of the XB-1 showing the nose curvature, merged canopy landing gear, underwing inlets & the leading edges of the wings. Pic Source: Boom Supersonic site

The XB-1 uses area ruling right through its entire length. Area Ruling a.k.a Whitcomb Rule is an aerodynamic principle that states the wave drag on an aircraft flying at transonic speeds (Mach 0.8-1.2) is minimized if the total cross sectional area of the aircraft changes smoothly and gradually along its length. An example is the manner in which the cockpit canopy tapers off where the wings begin, and a top view of the XB-1 shows off the horizontal stabilizer roots beginning where the wing trailing edge root ends. 

The underside of the XB-1 clearly shows the metal to the rear and the silhouette of the wings trailing edge root merging with the root of the stabilizer leading edge. Pic Source: Boom website

At the very tip of the nose is a ram air pressure sensor (works as a pitot tube). The ram air pressure tube reads the speed of the aircraft. The tube has two vanes attached to it , the alpha vane for the AoA of the aircraft and the beta vane to measure yaw (lateral directional movement relative to oncoming wind). Such a set up is used on experimental aircraft where data is still being gathered in real time as against an aircraft which already has a data history and is in clean air (undisturbed by the aircraft’s turbulence). Such a system is critical to validating the performance of the aircraft across the entire speed regime.

The nose of the XB-1 looks like a cone that has been flattened on the bottom. The contoured underside of the nose is critical to aircraft stability during take off and landing when the aircraft has a high Angle of Attack (AoA).The original iteration of the XB-1 nose was more conical but computational fluid dynamics (CFD) directed the nose to be flattened on the bottom. The nose design itself took about two years! The shape of the nose had a direct impact on the shape of the wing,the leading edges of the  wings of the final XB-1 are much further back on the fuselage than the original iteration as this combination gave the maximum stability at low speeds (more on this later). To sum up, having a perfectly conical nose means it sheds vortices in unpredictable directions to small changes in aircraft speed and attitude which interfere with the lifting surfaces and the vertical stabilizer. Behind the cockpit on the port side is the AC inlet that creates cold air via a turbocharger inside and bleed vanes behind.

The nose slopes up towards a canopy that is contoured into the curve of the upper nose cone. This gives the canopy a very merged look relative to the nose curve.  The nature of the curve of the canopy in tandem with the overall design of the aircraft means the pilot has very low visibility at landing and takeoff (slender delta wings have high AoA at low speeds). The Concorde had a dropping nose which is a relatively heavy mechanism, The XB-1 uses augmented reality, more on this later.The entire underbelly of the aircraft appears to be relatively flat and heavily contoured, all the way to the rear trijet configuration. The contours merge with the redesigned wings seamlessly. Right through the length of the underbelly of the aircraft  are a series of access panels that merge seamlessly with the surrounding fuselage contour. The panels are held in place by quarter turn fasteners such as those made by Howmet Aerospace. These panels give access to various avionics bays and fuel tanks (refueling happens off the top of the aircraft).

The landing gear on the aircraft have been optimized at the lowest possible cost. The first thing that strikes you about the nose gear is the heavily machined trunnion and pivot arm, they are made of titanium, these parts are mated to mechanisms salvaged for an F-4 & a T-38 to complete the gear. On the gear are two HD cameras and these are part of the augmented reality system that XB-1 uses to overcome the poor cockpit visibility mentioned earlier. They have a 12.5° down angle to compensate for aircraft attitude at low speeds. There are two cameras for redundancy and are spaced more than a standard bird wingspan apart, this ensures a birdstrike cannot take out both cameras at once. As the gear retracts, the strut first compresses to have a smaller profile as it sweeps up & forward into the fuselage.

The main landing gear continues the heavily machined theme and is capable of taking a hard landing load of 200,000 pounds and bouncing right back! The gear is a combination of three arms. The main strut is made of Aermet100, an ultra high strength steel alloy. The alloy has iron, cobalt and nickel as additives. The drag arm is grade 5 titanium as is the main shock absorber. The arms converge just above the wheels and radiate out into the bay. The twin main gear of the aircraft converge into the bay with the wheels laterally radiating out. The bay has two hydraulic reservoirs for redundancy, one on each side and an emergency DC pump as a backup. The XB-1 has no RAT, the DC pump is the back up. The landing gear bays are absolutely packed with different systems and include a generator control unit in there as well. All extremely well laid out. This gear was the second iteration.

Under each wing is an engine inlet (the third is on top of the fuselage with an S shaped duct to the third engine). The inlets look relatively small for a supersonic aircraft, what strikes you is the careful contouring of the inlet to slow supersonic air down to subsonic speeds for the engine compressors to work optimally. The front profile of the engine inlet is rectangular in 2D. The upper lip of the inlet starts about two feet in front of the lower lip. The sideview of the engine inlet shows a line running from the upper lip and converging with the vertical line running up from the lower lip at about 80% of the height of the inlet (there are no specs in the public domain).The inlet has no moving parts and the air is slowed by the inlet shockwave off the upper lip. The third engine of the trijet has a similar looking inlet, only thing is the geometry appears to run in reverse to the under wing inlets.The engine inlet geometry took about a year and half to finalize and hugely enhanced the engineering team’s capabilities.

The inlet architecture clearly visible and the main gear. Pic source: Boom site

A subtle design feature on the XB-1 is the gap between the underwing engine inlets and the wing. This gap is a boundary layer diverter to ensure the air the engines operate in clear air free of any turbulent vortices off the aircraft. While the diverters under the wings are thinner when compared to the center engine diverter on top of the fuselage as the boundary layer there is much thicker. The boundary layer itself is the layer of air that clings to the aircraft and is pulled with it. It is generally much thinner at the nose of the aircraft than the rear. In fact the boundary layer to the rear of the aircraft is so thick, the fasteners on the titanium engine bay covers do not need to be recessed and they actually protrude a bit with no additional drag.

The rudder on the vertical tail which is around six feet tall looks small but actually has about 20% higher authority than predicted by CFD. The all moving horizontal stabilizer (necessary for supersonic flight) is pivoted on a titanium torque tube that runs through the fuselage and connects to the stabilizer on the other side.This ensures the two horizontal stabilizers always move in unison. The horizontal stabilizer itself is positioned below the wings on the horizontal plane to avoid being blanketed by wing vortices.

The XB-1 has five fuel tanks running longitudinally down the length of the aircraft. The onboard flight control computer controls the centre of gravity by pumping fuel forwards or backwards depending on the acceleration or deceleration regime.

The Assembly

Industry 4.0 has revolutionized manufacturing by integrating digital technologies for efficient design, simulation and production. An example of this is computational fluid dynamics (CFD) where engineers ran thousands of simulations to optimize the aerodynamics of the XB-1 at a fraction of the time and cost. Traditionally the designs were validated in physical wind tunnel testing and reiterated upon, but now computers and artificial intelligence run the process end to end. It needs to be acknowledged that modern computing power and software have enabled Industry 4.0. 

Virtual prototyping has ensured a digital replica of the actual physical XB-1 is available at the touch of a keyboard. Generative design has ensured that algorithms can create complex component designs beyond human capability. End to end digitization is a digital information thread maintained from initial design to current status.

Boom has excelled by compartmentalizing the entire end-to-end creation of XB-1 by optimizing each component through the most suitable design and manufacturing process, and then bringing them together in perfect synergy across the whole aircraft.

A quick glance at the XB-1 immediately tells you the aircraft essentially has two colors. Most of the aircraft is white and towards the rear of the aircraft the engine bays are metal. All the white on the XB-1 is carbon composite and most of the metal on the aircraft is titanium.

The two dominant colors of the Boom XB-1. Pic Source : Boom Website

Boom has designed and constructed the composite parts of the XB-1 inhouse at a facility close to Denver, Colorado. The carbon fibre composites are used for most of the fuselage, wings and tail empennage. The material is lightweight, has very high tensile & flexural strength in addition to high impact resistance & a high strength to weight ratio. The heat resistance is very good. Additionally composites are easier to work with when creating complex aerodynamic shapes as compared to traditional materials such as aluminium.

Boom partnered with TenCate Advanced Composites (now Toray Group) to supply high temperature epoxy resins & prepregs for hot sections such as leading edges (temps as high as 307°F/153°C) and nose. Incidentally these are the same materials used on the Space X Falcon 9. Wing spar load testing was done in 2017.

For the Carbon composite Boom used the hand layup process where engineers manually layered carbon fibre prepreg sheets (pre-impregnated with TenCate’s epoxy resin) into molds to form the fuselage halves in addition to other components such as the wings. The sheet fibers are layered at 0°,45° & 90°, such layering gives the final product the longitudinal, shear / torsional & transverse strength we spoke of earlier. The process began in 2019, the fuselage halves were joined together in 2020. 

Post the layering the layered sheets with their molds, Boom used both autoclave & out of autoclave (OoA) curing using an inhouse oven for the curing of parts. The difference between an autoclave and an oven is the autoclave cures the composite sheets under heat & pressure while the oven heats the composites & their molds in a vacuum. 

During the post processing, the cured parts are checked for fit, surface quality & performance. Excess material is trimmed out and subjected to non destructive testing such as ultrasonic scans or tap tests to detect voids or delamination. Finishing involves surface preparation with primers and cleaners specific to the composite materials, followed by painting. The XB-1 was painted in high gloss white PPG Aerospace CA 9800 paint and added roughly 125 pounds to the aircraft while staying within the centre of gravity limits. The choice of white paint was prioritized by thermal management. The titanium sections were unpainted.

The titanium parts of the XB-1 are at critical high-temperature or high-strength areas of the aircraft . The XB-1 had a total of 21 titanium parts including engine bay covers, exhaust components and structural elements such as parts of the landing gear. Titanium has a high melting point and thermal stability which makes it ideal for such sections.

Boom partnered with VELO3D to produce the titanium parts using additive manufacturing (3D printing). VELO3D used their Sapphire 3D printing system which uses laser powder bed fusion (LPBF). In this process a laser fuses titanium powder (probably Ti-6Al-4V a.k.a titanium-aluminium-vanadium) layer upon layer on a digital CAD model. This method allows for complex geometries & reduced material waste for rapid prototyping which is ideal for XB-1s small batch, high precision needs. The precision & complexity enabled by the process is far superior to traditional processes such as using CNC machines. VELO3D’s expertise helped accelerate XB-1’s production timeline.

The post processing of the 3D printed titanium parts involved heat treating to enhance mechanical properties, surface finishing to remove any surface irregularities & non destructive testing using X-rays and ultrasonic testing. The finished titanium parts were integrated with the carbon composite fuselage.

The use of 3D titanium parts validated additive manufacturing processes for the Overture. In fact the XB-1 has a total of 193 parts on it that are 3D printed!

Most of the avionics used on the aircraft are standard off the shelf and a few that have been created specifically for the XB-1. The XB-1 sits midway between a fly by wire & a manual system. The reason for this is Boom needed the reliability and simplicity of a manual system for the experimental XB-1 at the same time they wanted to develop engineering expertise for a digital flight control system. The Overture will be a fly by wire aircraft. The engines on the XB-1 are General Electric J-85s with afterburners that produce approximately 4100 pounds of thrust each. The engines themselves date back to the 1950s. The XB-1 was rolled out in 2020, it was only much later (2022) that Boom decided to make its own Symphony engines which were unveiled at the Farnborough International Airshow.

The Overture

As of Jun’24 the Boom Superfactory was completed in North Carolina and currently the Superfactory is being tooled up. The factory will eventually produce 66 aircraft per year. 

Boom is currently prototyping its Symphony engine sprint core using surprise surprise 3D printing! Boom Symphony expects to produce thrust early 2026.

The Overture prototype should be ready by 2029.

The XB-1 ‘ The Little Plane that Could’….. 

The Overture. Note the similarity in wing design to the XB-1. The aircraft underwent a change in design in 2022. Pic Source. Boom website
The original mockup of the XB-1; Pic Source: Wings over Rockies

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