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Re: SR-71 top speed

Unread postPosted: 10 Oct 2018, 18:24
by foxhound_31
I remember hearing stories about the SR-71 reaching speeds of Mach 4.5 if not straight out Mach 5... :roll:
But on a more serious note, wouldn't it be possible to reach a ground speed of Mach 4, should the plane fly Mach 3.8 while being "carried" by a high speed upper atmosphere wind current?

It would work the same (in the opposite way) as an An-2 or WW1 fighter being able to fly "backwards" compared to the ground while taking advantage of their 35 kph stalling speed and of a 50+kph frontal wind...

Re: SR-71 top speed

Unread postPosted: 10 Oct 2018, 20:16
by jetblast16
The SR-71 was as fast as its limiting factors. Compressor inlet temperature and inlet shock wave management were of primary concern.

Re: SR-71 top speed

Unread postPosted: 10 Oct 2018, 21:05
by sprstdlyscottsmn
jetblast16 wrote:The SR-71 was as fast as its limiting factors. Compressor inlet temperature and inlet shock wave management were of primary concern.

It wasn't "Thrust", that much is for sure. It was a matter of the engine no longer being able to breathe or burning itself up.

Re: SR-71 top speed

Unread postPosted: 13 Oct 2018, 05:59
by Gums
Salute!

Actually, Spurts, I think it WAS the engine.that was the limit, same as the 104.

You can read it yourself in the Dash One. A really neat buncha info here:

https://www.sr-71.org/blackbird/

Gums sends...

Re: SR-71 top speed

Unread postPosted: 13 Oct 2018, 12:21
by sprstdlyscottsmn
Over 1,000 pages? Not something to read on the phone before coffee. I'll have a look a bit later. Thanks Gums.

Re: SR-71 top speed

Unread postPosted: 13 Oct 2018, 23:36
by aaam
Two other things that need to be considered:

First, the ability of the fuel to act as a heat sink for the aircraft skin and second the shock wave off the nose. As the airspeed increases, the angle of the shock wave gets narrower. At a certain point it will start to impinge on the wingtips, with disastrous consequences

Re: SR-71 top speed

Unread postPosted: 17 Oct 2018, 16:10
by Gums
Salute!

c'mon, AAAM, we call them oblique shockwaves, and that's what you get once above the mach. No problems with the oblique things interacting with other parts of the plane. In fact, some planes of the era and maybe today have "diamond" airfoils that exploit the pressure diferential of the oblique shockwaves. Spurts might chip in here.

The "normal" shockwave ( 90 deg from flight path) is found in the intake, so the air is subsonic entering the compressor section. The SR-71 Dash One ref I provided has a good section on how the intake and motor divert some of the incoming air, and that feature gave rise to many folks calling the J-58 a "turbo-ramjet". It wasn't, and our original Prat tech rep at Hill helped me a lot setting up academic sessions on the F100 engine. He had come from the J-58 shop and filled us in on that motor over a few beers.

The motor could have been a dual cycle turbo and ram if they had a bypass like most turbofans - annular, and the compression of the air might have been hot enuf for true ramjet operation using the bypass duct. Back in the day we had Bomarc drones for practice intercepts, and they ran at 3+ mach, so there may have been a way to have a combined cycle motor, but Lockheed and Pratt kept is simple(?, heh heh). Then there were temp and materials to worry about, as AAAM suggested.

Gums sends...

Re: SR-71 top speed

Unread postPosted: 17 Oct 2018, 21:34
by aaam
As explained to me (I don't have the background to work the point out myself), as the Blackbird goes faster, the angle of the shockwave relative to the aircraft grows more acute. At a certain point it would impinge on the widest part of the aircraft itself. I included that bit as a consideration to take into account why the SR couldn't hit those M4-5 airspeeds that some people were tossing out.

Re: SR-71 top speed

Unread postPosted: 17 Oct 2018, 22:54
by sprstdlyscottsmn
You are correct that the shockwave angle relative to the aircraft longitudinal axis decreases as Mach increases. That is now the whole story however. The angle of the shockwave is not a simple trig equation based on speed. The angle or the object itself matters. Think of the "mach angle" as the theoretical minimum "shock angle" that a "thin spike" would make, while a blunt object would make a significantly wider shockwave.

Re: SR-71 top speed

Unread postPosted: 18 Oct 2018, 16:50
by vilters
Simple answer :
The Top speed is "unknown" because nobody ever took the bird to its Top Speed.

Design and test speed was achieved at less then full power.

It was flown faster then designed during certain "escape" ops.

It was never flown at its "true mechanically possible top speed".

Re: SR-71 top speed

Unread postPosted: 18 Oct 2018, 18:25
by sprstdlyscottsmn
vilters wrote:Simple answer :
The Top speed is "unknown" because nobody ever took the bird to its Top Speed.

...

It was never flown at its "true mechanically possible top speed".

That's one way to put it. No pilot seems to have ever stopped accelerating while at full power, for safety reasons.

Re: SR-71 top speed

Unread postPosted: 20 Oct 2018, 00:20
by aaam
sprstdlyscottsmn wrote:You are correct that the shockwave angle relative to the aircraft longitudinal axis decreases as Mach increases. That is now the whole story however. The angle of the shockwave is not a simple trig equation based on speed. The angle or the object itself matters. Think of the "mach angle" as the theoretical minimum "shock angle" that a "thin spike" would make, while a blunt object would make a significantly wider shockwave.


Understood and agree. What I was trying to illustrate with the two considerations I was discussing is that are a number of considerations beyond just thrust vs. drag that determine an aircraft's top speed. I was just giving two more that needed to be taken into account. In this latter case, even if other considerations didn't come into play, the SR was not designed to function with the shock wave impinging on the wingtips. As jet blast16 said: "The SR-71 was as fast as its limiting factors".