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Old 19th Apr 2013, 13:59
  #143 (permalink)  
Join Date: Apr 2013
Location: Richmond, Virginia
Age: 65
Posts: 4
Wink "Inlet Thrust"

Hello All,
I have been reading and studying for many, many years the topic of "Supersonic Inlet Thrust. I do not hold a degree in Aeronautical Engineering or physics, but feel that I have my head around the physics of propulsion, at least to a degree.

I am not posting to irritate others or create a flame war, I only seek the truth, and there is MUCH confusion regarding the issue of how supersonic inlets work, even among the engineering staff at NASA. I know, because I have spoken with them and exchanged much mail and documents regarding this interesting issue.

First of all, in trying to get of grasp of this, one must have an understanding of supersonic and subsonic fluid flow. Supersonic fluid flow is not magic, but it behaves very differently than fluids at subsonic flow, and the transition from one to the other must be kept in mind too. To state that "Most of the thrust of the SR-71 or Concord" comes from the intake is a misunderstanding of jet propulsion. Yes, there is positive pressure recovery in the inlet, and yes it is greater than free stream static pressure. Jet engine propulsion creates thrust thru momentum change, accelerating gas out of the exhaust. The "positive pressure" in the inlet system or that acting on the rear faces of compressor blades is what is know as "PRESSURE THRUST". It is used to explain rocket and jet propulsion to high school students. It is a VERY INEFFICIENT way of producing thrust, and is avoided by engineeers.

If you look at rocket nozzle designs, you will see a different flow regions the relationship of "pressure thrust" to change in momentum. Engineers would like to convert all pressure thrust to a change in momentum, however some pressure thrust will always be created. It is NOT the lions share of thrust, even in the case of the SR-71 or the Concord. The supersonic inlet on the SR-71 is very efficient and it is thru that (at high mach speeds) it performs the lions share of compression, freeing up compressor stages of the J58 engine. At the 4th or 5th stage (can't remember which one), 6 bypass ducts pass the compressed air directly into the afterburner inlet, so a large portion of the airflow is acting as a ramjet engine. One could state that (at high mach) the inlet is responsible for the compression and ramjet action, BUT it is NOT producing the greatest portion of total thrust anymore than the Pistons in you car create the greatest portion of the horsepower generated.

This misconception has been perpetuated and is a misunderstanding of how the entire propulsion system works together. When some state that "The engine falls back in it's mounts while the intake system transmits forward thrust to the airframe", you should be willing and able to offer proof. DO the math, you will not find "positive pressure" in the inlet system "pushing" on the rear end of inlet components, or doors to amount to "75% or whatever number" of the total thrust. If you are careful to study ALL of the inlet system, you will find, again at high mach much of the "overpressure" air in the inlet is dumped overboard thru ducts and vents, as the inlet can take in more mass flow than the J58 can effectively deal with with some bypassed around the core and mixed in the complicated exhaust system again with it's blow-in doors and de-laval type noxzzle.

I am not posting to offend anyone, I like most everyone here am interested in the exact workings supersonic inlets, and propulsion. If, I have overlooked anything or failed to take into account forces not considered, please call me on it and educate me. The supersonic inlets and exhaust systems on the SR-71 are extremely interesting and work together, but it is not "Sucking itself thru the sky. I find it incredibly that all of this was conceived, worked out and tested in the 1950's with slipsticks.................
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