2023. 1. 4. 07:06ㆍ■ 우주 과학 건설/空中 航空機
Why Are Supersonic Engines So Complex? (msn.com)
Why Are Supersonic Engines So Complex?
Story by Dr. Omar Memon • Yesterday 8:31 a.m.
Specialized engines are required to power an aircraft that flies twice or even three times the speed of sound. While engines for a combat-type supersonic aircraft may be standard, there is much to learn about the complexity and performance of an SST engine.
Engine thrust
An aircraft engine creates thrust by taking the air through the inlet and pushing it out at an accelerated speed. The engine thrust equals the air mass flow times the air mass overspeed. It is fairly simple in n subsonic engines since the fan, compressor, and combustor can take inlet air at low subsonic speeds.
For supersonic speeds however, the inlet scenario becomes rather complex. The aircraft’s maximum speed determines the necessary behavior of air intake on an SST. The inlet fan and compressor stages are unsuitable to take the inlet air faster than Mach 0.5, at which the tip speed is well over Mach 1.
For a Mach 2.0 flight, the inlet air must be accelerated to Mach 1.5 before entering the fan and compressor. Similarly, the air in the combustor cannot exceed Mach 0.2 to prevent the flame-out. As such, the air in the core must be accelerated to Mach 1.8 to pass through the combustor. For a Concorde-sized SST, an air mass of 220 lb (100 kg) must be accelerated every second to Mach 1.5 for the inlet and Mach 1.8 for the core flow.
SST inlet design
The SST engine intakes must be designed to continuously accelerate the air mass at supersonic speeds. Fixed normal shock inlets can work for SSTs that run below Mach 1.5. However, variable multi-shock inlets, such as the ones on the Concorde, are required for supersonic speeds approaching Mach 2.0.
Variable inlets gradually increase the speed of the incoming air before it hits the fan and compressor. While variable inlets cater to supersonic cruise speeds, the air must continue to flow through (and gets spilled overboard) during an engine failure. Failure to achieve that can result in tremendous instability due to drag created by the inlet.
Comparing the two existing SST designs, the Aerion AS2, with its proposed Mach 1.4 speed, uses a fixed inlet to manage airflow acceleration. On the other hand, the Boom Supersonic design must manage the incoming air using a variable multi-shock inlet.
Inlet efficiency
Irrespective of the type of inlet, the generated by the inlet to accelerate the air results in an opposing force called Ram air drag. The engine thrust must counter the airframe and Ram air drag at supersonic speeds.
This means that the engine not only takes energy from fuel to accelerate the air but also to counter the additional drag. Typical Mach 2.0 SSTs consume more than twice the fuel per generated thrust unit compared to a subsonic engine with the same efficiency.
What do you think about the complexity of SST engines and their fuel consumption? Tell us in the comments section.
Want answers to more key questions in aviation? Check out the rest of our guides here.