Why does the J-58 engine inlet spike retract at high speeds on the SR-71?

[rbogie]

I've been reading about the SR-71 and the J58 engine and I'm confused about what I've read about the inlet spike. The spike would start out extended 3 feet but as speed (Mach) went up it would retract, up to 26 inches when at Mach3.2. This confuses me as I thought you'd always want the shockwave to be at the lip of the engine inlet.
As Mach goes up the shockwave angle increases so that in order to have the shockwave at the lip of the inlet, I'd think you would have to Extend the spike, not retract it.

Maybe the lip of the inlet on the aircraft is still some distance away from the compressor face, Maybe its okay to have the initial shockwave in that inlet, because it will have reflection between the inlet wall and the back side of the spike. So many reflections that the shockwave will have 2 or more decelerations before the air becomes subsonic--at the compressor face.

Anybody know about this or know where I can see Schlieren photos of this interaction?

 

[Astronuc]

Some info
http://www.wvi.com/~sr71webmaster/j-58~1.htm

SR-71 Flight Manual » Section I (Description and Operation including Engine)
http://www.sr-71.org/blackbird/manual/1/ (see page 33)
SR-71 Flight Manual
http://www.sr-71.org/blackbird/manual/

Somewhere I found what you're looking for, but I can't seem to find it at the moment.

It might be available in some archives at Dryden. I'll keep looking.

Update: I found some notes showing shock patterns. Now I just have to track down the source, which I think was Lockheed. The article was "Case Studies in Engineering: the SR-71 Blackbird"

This site used the same source: http://www.456fis.org/YF-12A_SR-71_ENGINE.htm

See also
http://www.scribd.com/doc/18029452/NASA-SR71-Blackbird-Challenges-and-Lessons-Learned-2009

 

[rbogie]

Thanks a lot for the excellent collection of data.

I took another look at the set of pictures on Wikipedia that shows the engine/spike at different Mach values and when you look very closely you can see that at Mach 3.2 with the spike retracted 26 inches, the shockwave is just barely attached at the front lip of the nacelle. Which makes sense in order to have subsonic air entering the inlet.

What doesn't make sense is why the spike is extended at all the lower Machs instead of being slightly more retracted and still trying to have the shockwave just touching the lip of the nacelle. I would think that having the spike too extended is also a bit bad in that having the shockwave way in front of the nacelle would allow enough distance for the air to develop some eddies and probably drag before being ingested by the engine. But I'm probably thinking too localized and not knowing enough about the bigger picture, ie the shock waves being thrown off by the nose and canopy and what their interaction was with the shockwave at the engine spike.

Thanks

 

[Astronuc]

I believe the goal is to ensure that the shockwave off the spike doesn't coincide with the shockwave of the nacelle/intake.

I haven't seen the details of the shock off the nose to know how it affects the engines.Update (July 28, 2017): I found a video that nicely describes the SR-71 and J-58


Update (August 4, 2017):
Richard Graham - SR-71 J58 Engine Tour

 

[TERR0RBYTE]

This one stumped me also.

I think the spike is extended at lower speeds as the inlet needs to be bigger to take in more air as it is less compressed. (mostly for the turbofan engine anyhow).

 

[mugaliens]

The position of the spike also controls what percentage of the air is sent through the turbine, what percentage is routed around the turbine directly to the afterburner, and what percentage is dumped.

At full speed, 80% of the air bypasses the turbine.