[AR] Re: to the stars, soon

  • From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
  • To: Arocket List <arocket@xxxxxxxxxxxxx>
  • Date: Fri, 1 Apr 2016 14:57:12 -0400 (EDT)

On Fri, 1 Apr 2016, Norman Yarvin wrote:

Behind the intake ... you have air at quite high pressure...
Now, describing this situation as "the intake is the part that
produces the thrust" has always seemed completely ludicrous to me...

Indeed so. The "engine proper" has the job of producing the desired pressure distribution within the system; it's silly to claim that it's not the part that produces the thrust, even if some of that thrust is *delivered* via the pressure distribution on other parts of the hardware.

...Indeed, you can get more "thrust produced by the intake" this way than the engine actually is delivering to the airframe...

In fact, in general you *need* lots more thrust produced than is actually delivered to the airframe, because the intake also produces so much drag. The intake drag and the gross thrust are often several times the net (delivered to airframe) thrust, which is basically the difference between the two.

This fact shows up externally in the way a lot of thrust reversers work: they turn the exhaust jet only about 90deg, sometimes less, rather than the near-180deg that one might naively think would be needed. To produce lots of braking force, it suffices to remove most of the gross thrust, leaving the intake drag unopposed. (And then throttle up, to make the engine swallow as much air as possible.)

Henry

Other related posts:

  1. Home
  2. arocket
  3. Archive
  4. 04-2016