September 21, 2018, ©. Leeham News: In the last Corner we looked at the heating of the aircraft when cruising at high Mach.
Now we will address the biggest problem for supersonic airliners, the engines and nacelles.
The subsonic airline engines and their nacelles are technology which has been perfected over the last six decades. The engines have morphed from straight jets to high bypass turbofans and the nacelles are lightweight designs with only the thrust reversers as movable parts.
This changes dramatically when we talk about engines for Mach 2 airliners. To understand why, we need to look into some fundamentals on how engine thrust is generated and the role of the nacelle.
A jet or turbofan engine is an air pump. It develops thrust by taking in air at the front and kicking it out faster at the back. The difference in the speed of the air entering the engine at the front and going out the back generates thrust:
Engine thrust = air mass flow * overspeed
If our engines would work internally like this (simply increasing the speed of the air) we wouldn’t have a big problem with engines for supersonic flight.
Unfortunately, for jet engines, there is “a candle in the wind” problem and a few more which complicate things.
The “candle in the wind” problem is the air can’t pass the engine’s combustor faster than Mach 0.2. Otherwise, the flame will blow out on the flame holders.
Additionally, fan and compressor stages don’t work well if the air passes them faster than Mach 0.5 (with Mach 0.5 axial velocity the blade tip airspeed is well over Mach 1. Any faster axial flow and the whole blade is supersonic, which spoils efficiency).
So for a Mach 2.2 SST the air molecules, hovering in the air at zero speed before entering the inlet, must accelerate to Mach 1.7 in the inlet (Mach 0.5 relative speed to the fan/compressor stages) and then to Mach 2.0 for the part which passes through the core.
Air weighs 1.2kg/m3 at ground level and about 0.2kg/m3 at SST cruise altitudes. With a Boom SST engine swallowing between 250m3 to 500m3 of air per second dependant on bypass ratio, a mass of 50kg to 100kg must be accelerated every second from zero to Mach 1.7 for the bypass flow and to Mach 2.0 for the core flow.
The acceleration of the air is done with a force generated by the inlet. The resultant opposing force on the nacelle and by it the airframe is called “Ram air drag” as it has the same direction as airframe drag.
Additional aircraft drag means more thrust to keep the aircraft speed. The thrust to compensate for the airframe drag and the Ram air drag is created by the engine burning fuel in the combustors to heat the air which is then ejected at high speed out the back (the heated air has increased pressure and this is used in a nozzle to convert pressure to airspeed).
We need to take the core air from Mach 2.0 and the bypass air from Mach 1.7 relative to the surrounding air to about Mach -1 (the minus showing the acceleration is in the negative flight direction) to get thrust when the air exits the engine.
Once again we are changing the speed of the air we have going through the engine. This time, the change in speed is Mach -2.5 (bypass air) to -2.8 (core air). The change of speed of the air weighing 0.2kg/m3 costs energy, energy which is taken from the fuel.
A normal airliner engine must also accelerate the air in the inlet and then brake it to zero and kick it out the back at about Mach -0.5. But now the bypass air, which is 90% of all air, is only speeded up from zero to Mach 0.35 (Boeing 787, Airbus A350 cruising at Mach 0.85) and the total change of speed before the air goes out the back is less than Mach -1.
The Ram air drag for a subsonic airliner’s turbofans is modest compared to an SST engine’s Ram air drag.
The process described above is the reason a Mach 2 SST engine consumes twice the fuel per produced thrust unit compared to a normal airliner engine if all processes have the same efficiency. This is also a challenge as we will see.
One of the challenges involves the acceleration of the air in the inlet to Mach 1.7 and then kicking it out the back at Mach -1 (all relative to the surrounding air). This is not so easy as we will see in the next Corner.