4 ms·

Is the SR-71 constrained by its engine or airframe? I know I’ve read that if the plane went any faster something would start melting and the plane would be lost

by mikece 7y ago

Is the SR-71 constrained by its engine or airframe? I know I’ve read that if the plane went any faster something would start melting and the plane would be lost but I don’t recall if it was the airframe or the engine.

starpilot 7y ago
Presumably both, to a large degree, since it's probably a pretty optimized aircraft. The engine & inlet are designed to compress the air using the shock patterns that develop at certain Mach numbers. Beyond that Mach range, it won't work. Likewise they probably didn't bother designing the airframe to work at speeds in which the engine couldn't.
- cbanek 7y ago
  The SR-71 is also high altitude, so if there is less air (pressure), that can change things like: how much lift you need or can generate, the outside temperature, how much air can be used by the engine, heating due to air friction, etc.
- JustSomeNobody 7y ago
  > Presumably both, to a large degree, since it's probably a pretty optimized aircraft. And all that was done without a computer.
deepnotderp 7y ago
Both propulsion (scramjet+ramjet TWR, reliability, startup, etc.) and aerodynamic heating are major problems for hypersonic flight.
dexen 7y ago
The aircraft was actively cooled[1], so I suppose they could always force a bit more cooling if needed. My guess is on engines - and not necessarily thermal limitation, probably rather inlet speed. Funnily enough, having read the Sled Driver[2], I don't remember any clear explainer whether the engine or the airframe was thermally limited. Perhaps it was there and I forgot by now? [edit] Wikipedia to the rescue, "Maximum flight speed was limited by the temperature of the air entering the engine compressor, which was not certified for temperatures above 800 °F (430 °C)." [3] The inlet air was necessarily heated as side effect of being slowed down to subsonic speeds, necessitated by the nature of the turbine engine. -- [1] by circulating the JP-7 fuel through critical sections, before sending it off to the engines [2] https://www.goodreads.com/book/show/1349004.Sled_Driver https://www.goodreads.com/book/show/1349004.Sled_Driver [3] https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#cite_ref-64 https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#cite_...
tastygreenapple 7y ago
In SR-71 Revealed, a book written by an SR-71 pilot and squadron commander, the author addressed this and to the best of his knowledge, the USAF and Lockheed both don't know the true top speed of the SR-71. (Doing this from memory so I may get something wrong here), the SR-71 usually cruised at mach 3.2, it could accelerate to mach 3.3 when fired upon, and could go to mach 3.4 if the pilot felt it was critical to preserve the crew/airframe however the plane needed to return to base after getting out of harms way and needed to be inspected before returning to the flight line. A lockheed test pilot apparently got the SR-71 to mach 3.5 (or maybe Mach 3.6?) but said it started to get scary. If this is accurate, it's probably the case that no one knows what fails first on the SR-71.
- redis_mlc 7y ago
  The SR-71 had a moving engine inlet spike, so that would be an operating limit based on temperature and speed. Fun fact: the SR-71 flew its entire mission in radio silence, including multiple aerial refuelings, to avoid pesky Russian trawlers, etc. tracking it. Generally aluminum is good to Mach 2.2, then you go to steel or titanium for either leading edges or the entire fuselage. Which makes the F-5 all that more impressive, as its fuselage was designed for Mach 2.0, near the limit of aluminum, even theough the engines limited it to Mach 1.3 to Mach 1.6. (The F-5 was so small that it was optically stealthy - adversaries couldn't see it outside 5 miles.) Titanium was used as a last resort in the F14 to reduce weight in the central fuselage box, which meant building the world's largest titanium casting plant at the time. Your tax dollars at work.
mikeytown2 7y ago
I've heard rumors that the mach number dial had a 4 on it; But the engine is rated to 3.6, anything past that would bottom out the inlet spikes and then you'd get an unstart.

by mikece 7y ago

starpilot 7y ago

Presumably both, to a large degree, since it's probably a pretty optimized aircraft. The engine & inlet are designed to compress the air using the shock patterns that develop at certain Mach numbers. Beyond that Mach range, it won't work. Likewise they probably didn't bother designing the airframe to work at speeds in which the engine couldn't.

cbanek 7y ago
The SR-71 is also high altitude, so if there is less air (pressure), that can change things like: how much lift you need or can generate, the outside temperature, how much air can be used by the engine, heating due to air friction, etc.
JustSomeNobody 7y ago
> Presumably both, to a large degree, since it's probably a pretty optimized aircraft. And all that was done without a computer.

deepnotderp 7y ago

Both propulsion (scramjet+ramjet TWR, reliability, startup, etc.) and aerodynamic heating are major problems for hypersonic flight.

dexen 7y ago

The aircraft was actively cooled[1], so I suppose they could always force a bit more cooling if needed. My guess is on engines - and not necessarily thermal limitation, probably rather inlet speed. Funnily enough, having read the Sled Driver[2], I don't remember any clear explainer whether the engine or the airframe was thermally limited. Perhaps it was there and I forgot by now? [edit] Wikipedia to the rescue, "Maximum flight speed was limited by the temperature of the air entering the engine compressor, which was not certified for temperatures above 800 °F (430 °C)." [3] The inlet air was necessarily heated as side effect of being slowed down to subsonic speeds, necessitated by the nature of the turbine engine. -- [1] by circulating the JP-7 fuel through critical sections, before sending it off to the engines [2] https://www.goodreads.com/book/show/1349004.Sled_Driver https://www.goodreads.com/book/show/1349004.Sled_Driver [3] https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#cite_ref-64 https://en.wikipedia.org/wiki/Lockheed_SR-71_Blackbird#cite_...

tastygreenapple 7y ago

In SR-71 Revealed, a book written by an SR-71 pilot and squadron commander, the author addressed this and to the best of his knowledge, the USAF and Lockheed both don't know the true top speed of the SR-71. (Doing this from memory so I may get something wrong here), the SR-71 usually cruised at mach 3.2, it could accelerate to mach 3.3 when fired upon, and could go to mach 3.4 if the pilot felt it was critical to preserve the crew/airframe however the plane needed to return to base after getting out of harms way and needed to be inspected before returning to the flight line. A lockheed test pilot apparently got the SR-71 to mach 3.5 (or maybe Mach 3.6?) but said it started to get scary. If this is accurate, it's probably the case that no one knows what fails first on the SR-71.

redis_mlc 7y ago
The SR-71 had a moving engine inlet spike, so that would be an operating limit based on temperature and speed. Fun fact: the SR-71 flew its entire mission in radio silence, including multiple aerial refuelings, to avoid pesky Russian trawlers, etc. tracking it. Generally aluminum is good to Mach 2.2, then you go to steel or titanium for either leading edges or the entire fuselage. Which makes the F-5 all that more impressive, as its fuselage was designed for Mach 2.0, near the limit of aluminum, even theough the engines limited it to Mach 1.3 to Mach 1.6. (The F-5 was so small that it was optically stealthy - adversaries couldn't see it outside 5 miles.) Titanium was used as a last resort in the F14 to reduce weight in the central fuselage box, which meant building the world's largest titanium casting plant at the time. Your tax dollars at work.

mikeytown2 7y ago

I've heard rumors that the mach number dial had a 4 on it; But the engine is rated to 3.6, anything past that would bottom out the inlet spikes and then you'd get an unstart.