September 28, 2018, ©. Leeham News: In the last Corner we started looking at the biggest challenge for an SST, the powerplant. We first discussed the most fundamental problem of the engine, the level of Ram drag for an SST engine.
Now we continue with the challenges of the Nacelle. This week we talk intakes.
The complexity of the SST intake is dependent on the maximum speed of the aircraft. As described last week an SST must invest considerable energy in accelerating the air which enters the intake to a speed close to the aircraft speed.
This energy investment can be used to raise the pressure of the air, in which case the investment in compression in the engine compressor will be less.
If the intake doesn’t manage to conserve the invested energy it will heat the air instead and some of the energy will be lost to spillage outside the intake. As air with a higher temperature is harder to compress, this is a double loss for the engine.
There has been quite some research into intake design. Figure 2 shows how different intake types recover the invested energy to pressure, based on flight speed.
As long as the speed stays below Mach 1.5 a fixed inlet will work. Once we want to fly at or over Mach 2 we need a variable multi-shock inlet, which gradually accelerates the incoming air.
The most demanding control environment of the Concorde was the variable multi-shock inlet. It required such precise control so the analogue computer types which were OK for the Fly-by-Wire flight controls weren’t precise enough for the inlets. The first digital computer on the Concorde was designed to control the inlets.
The precise control of the multi-shock inlet is only half the solution of an SST inlet. If the inlet doesn’t work or the engine surges, the air must continue to flow into the inlet or the drag created by an intake which doesn’t swallow air can risk the stability of the aircraft, Figure 3.
There is also the need for a larger intake area a take-off speed. Concorde had several additional movable inlet doors to cater for these and other functions.
It’s an old rule that each additional movable part in an inlet/nacelle creates its own problems and needs extensive testing to verify all the functions to the design team and to the Certification authorities.
The Boom Supersonic project will have to solve all the intake problems created by a Mach 2.2 inlet whereas the Arion AS2 can be designed with a fixed normal shock inlet as its cruise Mach is 1.4.
Next Corner we move to the back of the Nacelle to discuss the nozzle and thrust reverser problematic.