Overture
On January 28 2025 the BOOM XB -1 demonstrator aircraft broke the speed of sound three times and achieved ‘Boomless ‘ supersonic cruise hitting speeds of Mach 1.1 . The Mojave supersonic corridor used by XB-1 was the same as the one used by the X-15 program. The XB-1 was flying in the footprints of history.
Boomless cruise was a concept first pioneered by NASA with their Quiet Supersonic Transport (QueSST) program in the 1990s. Lockheed Martin is in advanced stages of building the experimental X-59 aircraft to demonstrate low boom technology.
Before the X-59 program Northrop Grumman showed this is possible with a modified T-38/F-5E Tiger dating back to the 1950s. The F-5E Tiger had a reshaped nose and fuselage in an early attempt to control the sonic boom that follows supersonic flight.
BOOM supersonic is now using data from the tests to design the BOOM Overture. While airframe aerodymanics are key to achieving sustained Boomless supersonic cruise, there is one aspect that is just as important and that is the engines.
SST History
To suitably know the present we first need to understand and acknowledge the past. Aviation history has two SSTs to use as a reference point. The first is the legendary Concorde and the second, the fabled TU-144 both developed in the 1960s , a decade that gave us several aircraft that helped shape the future of air travel. The B747 (Jumbo) & the B737 among them, not to mention the grand daddy of High Supersonic (Mach 2-5) aircraft technology the XB-70 Valkyrie and the B-58 Hustler (first flight 1956) with which the Overture shares some similarity.
This piece focuses on supersonic transport engines and their evolution.
Turbojet v/s Turbofan
Before we dive into the BOOM Symphony we first need to understand the difference between a Turbojet and a Turbofan. What separates the two is a concept called Bypass. Turbojets have what is called a bypass ratio of 0. This means that all the air that goes in through the engine nacelle also goes through the compression stages of the jet before being pushed out the back. Hot exhaust gasses can be loud because of exhaust gas velocity and in several cases the use of afterburners at various stages of flight be it take-off, climb or cruise.
The turbofan was an evolution of the Turbojet and has a bypass ratio of greater than 0. Engineers soon discovered that not all air needs to pass through the engine combustion chamber all the time. Infact, this is not only inefficient and noisy, but leads to high operating temperatures and this often resulted in structural failures both on the engine and airframe.
Turbofans can be generally divided into three broad categories, low(LBR), medium (MBR) and high (HBR) bypass ratio. The TU-144 used a LBR engine, the Symphony is a MBR and most subsonic airliners of today use HBR. Turbofans may also use afterburners.
What are Afterburners?
Afterburners are an extra combustion chamber located at the end of the turbine sections of a Jet engine. Afterburners inject and combust additional fuel into the hot exhaust gasses passing out of the turbine section of a jet engine.They generate incremental thrust in engines that use them.
The Concorde’s Olympus 593 engines increased thrust from 32,000lbs to 38,050lbs with afterburners. On the TU-144s initial NK-144 Kuznetsov engines the thrust increased from 22,500lbs to 33,000 lbs. The later RD-36-51 Kolesov engines increased from 20,000lbs to 44,115lbs. (These engines came in too late in the TU-144 program and could do little to avoid the closure of the program. In any case the Kolesov engines too used afterburners during cruise, which meant the TU-144 was always short on range).
Afterburners might increase thrust substantially but the engines get thirty and range gets impacted negatively.
The Concorde & TU-144
The TU-144 won the first to flight race against the Concorde by 61 days.
The placement and size (diameter) of the engine types was dictated by aerodynamic performance across the speed spectrum from zero to Mach 2+.
The choice for both these aircraft were engines embedded under the wings in squarish shaped pods. Since the nacelle diameter could not be large (due to supersonic aerodynamic concerns) the width on the Concorde was 1.8m, and the TU-144 was 1.7m per engine and the total width under each wing was 3.7 & 3.5m respectively The length of each of the pods were 9.2m on the Concorde and 8.5m on the TU-144. The height of the pods on both the aircraft was 2m. From these figures we see the engines were similar in size , the magic was what happened inside.
The Olympus 593 engineers of the Concorde opted for a Turbojet and the NK-144 engineers of the TU-144 opted for a very low bypass Turbofan ( BPR 0.6:1 ). Very low bypass turbofans have operating characteristics very similar to a Turbojet.
One of the key pieces inside a jet engine is its compressor. Every compressor has low pressure and high pressure stages each driven by coaxial shafts (twin spool)that drive the various stages of compression as air is driven through the engine and ignited.
The Concorde’s Olympus 593 had seven low pressure and seven high pressure compression stages. Alternatively the the TU-144’s, NK-144 had 6 low pressure and 7 high pressure compression stages. The overall compression ratio of the Olympus 593 was 15:1 and the NK-144 was 11:1 . These numbers look low by today’s standards but history acknowledges these were the limitations of the engines given the materials and technology of the time.
Compressors operate in a very narrow range. An engine compressor on the Concorde spun between 3,500 – 7,000 r.p.m and that on the TU – 144 spun between 3,000 – 7,000 r.p.m. These were the compressor rotations across the entire speed range . Maintaining a stable flow of air through the compressor at all times was one of the secrets of maintaining stable supersonic flight. Disruptions can result in either a compressor stall that is loss of compression and engine performance or Compressor Surge where airflow reverses direction, leading to a flame out.
On the Concorde this was done with innovative use of a series of ramps and bleed valves.The initial TU-144 was much simpler and had immovable fixed inlets and this led to compressor stalls and surges which were difficult to reverse. Later TU-144 models had ramps similar to the Concorde.
The use of afterburners on both aircraft only added to compressor demands. The Concorde used afterburn for takeoff and climb only. The TU-144 used it all times.
The use of afterburn at take off and climb added to severe sound pollution. The Concorde takeoff decibels was approx 120 – 125 dB and reduced a bit during climb. Landing was approx 100dB . The TU-144 had a take off sound level getting toward 140dB and climb at 120dB, landing was similar to the Concorde at approx 100dB. These figures show both aircraft were loud, the TU-144 more than the Concorde. For comparison a very loud night club is rarely over 110dB. Anything over 130dB is painful.
Such sound was unacceptable even over 50 years ago and definitely not today. The Concorde which flew the daily route out of JFK to London / Paris had a specially designed route out of JFK to minimize noise impact on residential areas.
Enter the Symphony
Creating a supersonic (airliner) engine has many issues specific only to Supersonic travel.
The first is sound at take off, climb and cruise. The second is fuel burn. Supersonic travel burns three times the amount of fuel a normal sub sonic engines burn. ( BOOM plans to use Sustainable Aviation Fuel/SAF to reduce carbon footprint). The third is the biggest enemy, air resistance also known as drag. Lastly the use of afterburners needed to be avoided for reasons stated above.
The lessons learnt from Concorde/TU-144 was that having engines embedded in the wings/airframe created turbulence and drag related efficiencies ,there were other inefficiencies related to maintenance and safety that needed to be overcome.
BOOM has decided to go with engines slung below the wing much like the B-58 Hustler from 1956.
We are now in the realm of efficiency dictating form.
BOOM decided to go with a medium bypass ( MBR 1.5:1 ) turbofan for reasons of overall efficiency in the areas of noise, fuel consumption and eliminating afterburners. The Symphony will generate 35,000lbs of thrust ( compression ratio of 25:1)without Afterburn and this requires a great deal of optimization.
Initially BOOM decided on a three engine Overture, but this needed a much larger diameter turbofan which increased sound and reduced redundancy , BOOM decided to go with the traditional four turbofan layout . The current jets have 72in (6 ft) diameter engines. Rough back of the hand math shows the three jet config to be approx 96in (8ft) in diameter leading to inefficiencies mentioned earlier all round.
The engine design of an underslung engine is necessarily different from an embedded engine especially entry and exit.
The air nacelles are axis symmetric on the Symphony v/s levered ramps on earlier SSTs. The intake is engineered to manage airflow through the speed spectrum.
An excellent example of this kind of an intake is the system on the SR-71 a.k.a Blackbird/Habu. The inlet spike moved back 26 inches as speed increased to control the shockwave position and maintain compressor pressure. The engine had bypass doors and bleed slots to further manage internal pressure. In the case of the SR-71 80% of all thrust came from compression inside the intake. This clearly suggests a sophisticated yet simple design.
The Symphony has three low pressure (LP) and six high pressure (HP) compressor stages. Having much fewer LP stages is offset by having a bypass of 1.5:1 which neither the Concorde nor the TU-144 ( bypass 0.6:1 ) had. The HP stages work at optimizing the engine as it moves through sub, trans and supersonic speeds. Having fewer LP stages and more HP stages highlights technology improvement across aerodynamics and materials.
The 787 with chevron nozzles represents a fresh look at managing exhaust gasses. The chevron nozzles are effective in damping sound by managing the mixing of the hot exhaust gasses with the ambient cooler air. The Concorde with it’s variable geometry buckets showed how supersonic airliners can optimize exhaust gasses. In the case of the Symphony we can expect to see variable geometry chevron nozzles?
The last bit is Artificial Intelligence ( AI ). When Honda created the VTEC engine, it took the world by storm. It had effectively created two engines in one. One engine for pottering around the city and the second for zipping down expressways. When the iVTEC came out it added efficiency to this exciting mix and created the modern Honda engines of today. The i stands for intelligent, AI that manages engine requirements through the rev range. AI effectively developed fresh efficiencies.Honda has further used a twin scroll turbo (independent exhaust gas streams through one turbo two scrolls), effectively cutting turbo lag without the need for two turbos (compound not twin turbos) and still being relatively compact. The Acura TLX Type S is an exciting mix of VTEC technology and raw instantaneous power response . Much like what BOOM is attempting to do with the Symphony.
Postlude
BOOM appears to have learnt the lessons from Overture’s Supersonic siblings well. They are leveraging the efficiencies new materials and technologies to create a fresh new tomorrow.
During the course of the writing of the piece I once again realized what great aircraft the Concorde and the TU-144 were, a doff of the hat to the scores of engineers who built and worked these birds.
Disclaimer: This article has used images from multiple sources accessed through Google. No plagiarism intended. This article is for recreational / educational purposes only and of no monetary value.
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