Sr-71

[Brian Abraham]

Reading a book written by an SR-71 pilot and says "At Mach 3.2 cruise, over 80% of the thrust created comes from the inlet, 20% from the engine."

I realise at this speed the the engine is basically a ramjet and the flight manual quotes "At high altitude, military thrust is approx 28% of the maximum thrust available."

What I'd like is an explanation of (how? aerodynamics? mechanics of?) is the inlet providing 80% of the thrust.

Question for John Farley or those of his ilk?

Blue Skies,
Brian

[barit1]

I'm not sure I'd be able to reconcile those numbers, but essentially:

The inlet converts the considerable dynamic pressure of its fairly small airflow to static pressure at the engine inlet face.

The engine turbomachinery is running barely above idle, limited (probably) to a safe compressor exit temperature.

The afterburner/augmenter/reheat is working like a ramjet, accelerating the flow to a high exit velocity; the core engine is excess baggage at this point.

[forget]

In a previous life, around 1980 I suppose, I was flying out of Changi one night, seat 56J as usual. I got talking to the chap next to me who turned out to have flown the SR-71. There's no doubt he wasn't Walter Mitty - I knew just enough about the aircraft to accept he knew what he was talking about.

He said the weirdest thing about the aircraft was that, as you went though M?, the fuel flows just wound right down to next to nothing. We spent the next few hours trying to figure this out. I'll let you know when I've got it.

[18-Wheeler]

I've got the flight manual for one, and I think the fuel flow charts back that up - to a degree.
If they're short of fuel getting from A to B, the way to fix that is to push the throttles up and go faster. They're designed to be most efficient at M 3.2, but "At the discretion of the Commander M 3.3 is authorised".

[Onan the Clumsy]

I saw the course of a standard rate turn printed over a map of the US. It started up on the Canadian Border and finished up almost over Mexico :ooh:


Still, the space Shuttle over Dallas is what...five minutes from landing?

[Tarnished]

Is that five minutes in bits or still in one bit?

[barit1]

...as you went though M?, the fuel flows just wound right down to next to nothing.

18-Wheeler may have the answer in his FM, and I think it's that the FF indicator only shows core engine usage - not the afterburner.

As I said, at high Mach the engine rolls back near idle for temperature protection.

[mfaff]

The way to see this is not as a inlet/ engine/ reheat/ exhaust issue but as an overall powerplant concept.

The core engine (the turbine) is most efficent at lower airpseeds and the majority of the air coming into the powerplant goes thro the core and the combustion chambers to produce the thurst.
Here the intake+ turbine are needed to generate the thrust...

However as airpseed and altitude increase the 6 turbine bypass tubes open and begin to take a higher and higher percentage of airflow...it is this airflow which, being compressed by the intake geometry and then returned to the core flow upstream of the reheat ring which generates the majority of the thrust.. this is the ramjet part of the powerplant.. the core is still working conventionally but overall contributes much less. Effectively the intake and reheat are doing most of the work....and hte core very little.

So in effect the powerplant functions primarily as a conventional jet at lower speed ranges, and as a ramjet at higher speeds..by diverting airflow around the core turbine.

The reheat/ exhaust nozzle are the last piece of this puzzle...the reheat, used continuously in supersoinc flight acts as the sole burner for the bypassed air.. hence the ramjet tag..and the nozzle does it bit, regardless of what regime is dominant...

The reason why these figures can be calculated is that is relatively easy (!) to calculate the pressure increase as the airflow goes thro the intake and hence the thrust that generates..as well as calculate the fuel/ airflow through the engine.. again to work out how much thrust is there...the reheat/ exhaust is thus the remainder of the total...

What is interesting is that whilst the core only produces 25ish percent....the system is not self sustaining...the ramjet portion needs hte turbine section to be functioning..unlike a true ramjet which would not need the trubine once the right airspeed etc was attained.

[ionagh]

The inlet cones would certainly generate thrust but 80% seems optimistic.
The cones would slow down the intake air, ideally to subsonic speeds that the core turbojet could handle. Once hypersonic the cones retreat into the intakes and the shock wave from the back edge of the cone becomes trapped in the intakes and this generates very high pressures which are then bypassed around the core turbojet into the afterburner.
I guess they are talking about the pressure developed by trapping cone's shockwave inside the inlet as being equivalent to thrust?

OTOH I could be totally wrong.........

[ORAC]

SR-71 Engine (http://www.airandspacemagazine.com/ASM/Mag/Supp/FM99/oxcart.html)

"At Mach 3, the inlet itself produces 54% of total thrust through pressure recovery, the engine contributing only 17% and the ejector system 29%. The compression ratio at cruise is 40 to 1."

[Noah Zark.]

I am by by no means an expert, but there is on the web in an article on Concorde in which I read that at cruise it was being sucked along by the intakes in a similar proportion to being pushed by the exhausts.

[18-Wheeler]

Here's (http://aerostories.free.fr/technique/J58/J58_01/page8.html) another good link to info on the J58 engine.
Most interesting indeed.