Air and Space Museum


Well-Known Member
I went to the new museum at Dulles this past weekend. It is amazing there! I was there with a few fellow pilots, and we had a question about the SR-71's engines.

It is a ram jet correct? It uses that cone in front to compress the air instead of fans? If it uses ram air to build compression, how does it generate thrust at low airspeeds? It must have some sort of fan, right? Do the afterburners produce most of the thrust at low airspeeds?


I wrote a paper on the engines of the SR71, but I'll be darned if I remember anything about it. I do believe the cones on the engines are used to keep the airflow laminar and reduce drag at high airspeeds (very high). I do not think it has anything to do with the actual compression/combustion/etc. Remember that when you go past Mach1, aerodynamics change totally...the pointy cones are there to reduce drag.
I keep meaning to get up there. How was the new museum? I've heard they have a little mini atc tower there to watch the traffic at Dulles.
It is an awesome museum! It is amazing how many different planes they have, some of which are suspended in various maneuvers. Everything from ultralights to the Space Shuttle Endeavor.

I was a little skeptical until I actually got there. It was worth the drive out to Dulles.

I beleive the SR-71's engine's are normal turbojet engines. The cone slides in and out depending on the speed of the aircraft to regulate the volume and flow rate of the air coming into the engine. I read a book on the Lockheed Martin Skunk works that made the SR-71, it was written by Ben Rich, the guy who designed the intake system. Check out the book if you want an indepth explanation.
It's a fairly short flight, bus ride and subway ride away for me. I try to get to the Air & Space Museum about twice a year just for fun, and now they have that shuttle that goes from the mall to Dulles for the annex. Now if I can just get my schedule and my wife's schedule to match up, we can make the trip.....
The SR-71 has "regular" axial-flow turbine engines.

The cones on the inlet of the engines are used to generate an oblique shock wave that reflects off the lip of the inlet. Oblique shocks slow the flow and compresses the air going into the engine more efficiently than the normal shock that would form without the cone. The catch is that the angle of the shock wave and required inlet area changes with different mach numbers, but diameter of the inlet doesn't. The solution was to move the cones in and out on a speed schedule; retract the cones at low speed to increase the inlet area and stick 'em out for higher speeds to decrease the inlet area.
I went to the Dulles part of the air & space museum a little before Christmas. It was awesome. My friend video taped the whole thing. I’m not sure if you are normally allowed to photograph and tape or if that was just because it had recently opened up. In any case, as my friends and I were watching the video later, one of them noted, “I looked like a kid in a candy store.” Also, the parking was free at that point, but I think that was only because it had just opened up. I didn’t get to go up into the replica control tower. I’ll bet it was cool. You could only go part way into the room where the shuttle was, most of it was roped off. I also noticed that different sections of the shuttles leading edges were missing on its wings. I think the leading edges that were used for testing after the shuttle disaster must have come from the air & space shuttle. Other than that I could not imagine why it would be missing parts of its leading edges. Maybe someone else has more information?
Some interesting theories on the cone but we are missing the key point of supersonic air inlets, to slow down the air to subsonic speeds.

Turbine compressors do not react very well to supersonic shock waves. The F-15 uses a variable angle baffle to reduce the airspeed as well.

Ideally we want the air to be mach .94 or less.

At certain conditions the J-58s bypass the turbines and become pure afterburner, because it is no longer possible to slow the air down below .94.

The speed this happens is "near mach 3."

More info at
I went with a group of 10 airline pilots to the Udvar-Hazy museum last week. It was phenomenal. If you have the chance to go, don't miss it.
Yeaha, the missing leading edges on the shuttle were used in the disaster investigation and will be replaced eventually. That shuttle has been on dulles airport property since the 80's and has been a graveyard for spare parts for other shuttles. I was up in the tower before xmas as well, and the 30 minute line was well worth it. there is a navigation room half way up the tower with stuff about ils's, atc, vor's, ndb's and radar services. Its pretty cool. The tower gives you a great look at 1 r/l and is 400 ft high and right in the middle of the two. the day i was there they were arriving 1 r/l and we had some great looks at heavies inbound. defintely worth the trip.