How Inlets Generate Thrust on Supersonic aircraft

Some time ago I read Skunk Works, a very good “engineering” read.

In the section on the SR-71, the author Ben Rich made a statement that has puzzled me ever since, something like: “Most of the engines thrust is developed by the intake”. I didn’t get it – surely an intake is a source of drag rather than thrust? I have since read the same statement about the Concorde and it’s inlets.

Lately I’ve been watching a lot of AgentJayZ Gas Turbine videos. This guy services gas turbines for a living and is kind enough to present a lot of intricate detail and answer questions from people. I find his presentation style and personality really engaging, and get a buzz out of his enthusiasm, love for his work, and willingness to share all sorts of geeky, intricate details.

So inspired by AgentJayZ I did some furious Googling and finally worked out why supersonic planes develop thrust from their inlets. I don’t feel it’s well explained elsewhere so here is my attempt:

  1. Gas turbine jet engines only work if the air is moving into the compressor at subsonic speeds. So the job of the inlet is to slow the air down from say Mach 2 to Mach 0.5.
  2. When you slow down a stream of air, the pressure increases. Like when you feel the wind pushing on your face on a bike. Imagine (don’t try) the pressure on your arm hanging out of a car window at 100 km/hr. Now imagine the pressure at 3000 km/hr. Lots. Around a 40 times increase for the inlets used in supersonic aircraft.
  3. So now we have this big box (the inlet chamber) full of high pressure air. Like a balloon this pressure is pushing equally on all sides of the box. Net thrust is zero.
  4. If we untie the balloon neck, the air can escape, and the balloon shoots off in the opposite direction.
  5. Back to the inlet on the supersonic aircraft. It has a big vacuum cleaner at the back – the compressor inlet of the gas turbine. It is sucking air out of the inlet as fast as it can. So – the air can get out, just like the balloon, and the inlet and the aircraft attached to it is thrust in the opposite direction. That’s how an inlet generates thrust.
  6. While there is also thrust from the gas turbine and it’s afterburner, turns out that pressure release in the inlet contributes the majority of the thrust. I don’t know why it’s the majority. Guess I need to do some more reading and get my gas equations on.

Another important point – the aircraft really does experience that extra thrust from the inlet – e.g. it’s transmitted to the aircraft by the engine mounts on the inlet, and the mounts must be designed with those loads in mind. This helps me understand the definition of “thrust from the inlet”.