SR71 Max speed?

[Exhaustgases]

The Space Shuttle was a rocket powered glider that reentered the atmosphere from low Earth orbit.

No jet engine is capable of operating at 300,000 ft. There is all but zero air available to sustain combustion. It could not happen.

Rocket powered? No power when its gliding at Mach 25.

How do you know a jet engine won't operate in an area of the upper atmosphere where ultra high speed meteor's will start to burn up (25,000 to 160,000 mph)?
If there is enough air up there to cause that then at the correct speed there would be enough for a "properly" designed air breathing engine to work. Another plus of the high upper atmosphere is that it is very cold, that is an advantage for air density, and vehicle cooling if done correctly.

 

[billt460]

Rocket powered? No power when its gliding at Mach 25.

How do you think it gets up to Mach 25? And it's not "gliding" at Mach 25, it's in orbit.

How do you know a jet engine won't operate in an area of the upper atmosphere where ultra high speed meteor's will start to burn up (25,000 to 160,000 mph)?
If there is enough air up there to cause that then at the correct speed there would be enough for a "properly" designed air breathing engine to work. Another plus of the high upper atmosphere is that it is very cold, that is an advantage for air density, and vehicle cooling if done correctly.

You're out there where the busses don't run. If a jet engine could run at those altitudes they would have had one on the X-15. They didn't because it couldn't. The highest altitude it ever achieved was 354,200 ft. And to accomplish that it had to be dropped from a B-52 at over 40,000 ft.

And it had reaction control thrusters on it to control pitch and yaw at that altitude, just like the Space Shuttle does........ Because there is no air to operate the control surfaces at that altitude.

One half of the atmosphere is contained from the Earth's surface to approximately 18,000 ft.. The other 50% is contained from 18,000 ft. to 100,000 ft. The Karman Line, (where space begins), is at 62 miles. Or roughly 327,000 ft.

"In summary, the mass of Earth's atmosphere is distributed approximately as follows:

50% is below 5.6 km (18,000 ft). 90% is below 16 km (52,000 ft). 99.99997% is below 100 km (62 mi; 330,000 ft), the Kármán line".

https://en.wikipedia.org/wiki/Atmos...In summary, the mass of,ft), the Kármán line.

 

[billt460]

Below shows the hypersonic breathing corridor. Without LOX or rocket assistance, the limit is around 140,000 feet (40 km) at around Mach 15.

In 2004 NASA's X-43 achieved a record Mach 9.6 at 109,000 feet, which if you compare below, is a good match for the aforementioned corridor.

 

[Astro14]

Rocket powered? No power when its gliding at Mach 25.

How do you know a jet engine won't operate in an area of the upper atmosphere where ultra high speed meteor's will start to burn up (25,000 to 160,000 mph)?
If there is enough air up there to cause that then at the correct speed there would be enough for a "properly" designed air breathing engine to work. Another plus of the high upper atmosphere is that it is very cold, that is an advantage for air density, and vehicle cooling if done correctly.

How do I know that jet engines don’t work at that speed or altitude?

Physics - Let’s start with speed. The rise in temperature at that speed (total air temperature - see here: https://en.wikipedia.org/wiki/Total_air_temperature) means that the engine will be so hot inside as to melt the turbine blades and casing. If you were to measure the temperature at each point in the engine, flow path, the faster you go, the higher all of those temperatures are, including in the inlet, then the compressor, then the combustion chamber, and then, necessarily, at the turbine.

The MiG-25 could go Mach 2.8, but would melt the turbines when it hit that speed. If it were to go faster, and the temperature was higher, those blades would still melt.

Now let’s talk altitude. Airliners cruise at approximately 35,000 feet. At that altitude, the indicated airspeed is around half of the true airspeed. Indicated airspeed, or the measure of dynamic pressure, is what gives the airplane enough lift to fly.

At 60,000 feet, indicated airspeed is less than 1/4 of true airspeed. 60,000 feet was about the limit of where Concorde could fly, because even though it’s going Mach 2, the amount of dynamic pressure over the wings to create lift is close to the minimum needed to keep the airplane in the air. The indicated airspeed is low, and requires flying at higher AOA to create lift, and higher AOA increases induced drag.

When you get to 80,000 feet, indicated airspeed is very low. There isn’t enough air up there to keep an airplane in the air unless it’s going very fast. When cruising at nearly Mach 3, the SR 71 saw an indicated airspeed of around 200 kn. That’s barely enough airspeed to generate the lift to keep the airplane in flight.

If you were to go much higher than that, the indicated airspeed drops below the stall speed of the airplane. Sure, you could make an airplane with much bigger wings, but then you get drag rise - tremendous increase in drag as the wings would be big enough to be outside of the shock cone of supersonic flight.

So, drag limits how big the wings can be.

Indicated airspeed (or, the drop off in air density) limits how high the airplane can fly before it will stall from a lack of lift.

Engines limit how fast the airplane can go.

If it were so easy to “properly” design the engine, or the airplane, it would have been done.

 

[Exhaustgases]

The SR-71 air intakes to the engines slow the air down so no problems with turbines or compressor blades melting. Remember at high altitude very high Mach is like very low Mach at low altitudes as far as frictional heating.
Then there is the super cold temperatures at very high altitude.

Cool chart, black bird engines, part turbine, part ram, part scram jet.

Wow then it must go faster than Mach 15 cool. But then are we allowed to see the true air breather engine chart?

 

[Astro14]

The SR-71 air intakes to the engines slow the air down. Remember at high altitude very high Mach is like very low Mach at low altitudes.
Then there is the super cold temperatures at very high altitude.

You’re ignoring the stagnation of the airflow.

The problem is at that high speed when you slow the air down, the temperature goes way up.

In fact, the SR 71 is temperature limited in the inlet. And that temperature peaks out somewhere around 3.2 to 3.33 Mach on a standard day.

Clearly atmospherics affect the temperature to give an altitude on a given day. It’s just meteorology.

The fact that the air is 65 below zero, gives it a slightly lower starting point, but you still end up with temperatures as well over 400° in the inlet. And then the necessary compression, which adds temperature, prior to the combustion chamber, as well as the temperature added by combustion and the next thing you know you’re melting the turbine.

Your ability to ignore so much of the physics of Flight, to reach these conclusions, is truly fascinating.

 

[Exhaustgases]

Isn't that why the engines have the bypass system? Scram jets live on that super sonic heated air.

 

[Astro14]

Isn't that why the engines have the bypass system? Scram jets live on that super sonic heated air.

There is a limit to what can be bypassed - the engine still has all the temperature limits for what goes through it.

Read the Blackbird flight manual.

There are total air temperature limits. The F-14 had a total air temperature limit of 388F, measured on the fuselage before the inlet.

For the SR 71 the maximum compressor inlet temperature, measured just after the inlet spike, is 417C. That’s hot. Aluminum softens about there. The faster you go, the higher that temperature gets as a result of stagnation. It’s physics, you can’t get around it by bypassing the air because the air that comes into the engine inlet at about Mach 3.3 will reach that temperature. If the air is really cold that day, say 10 or 20 C below normal, then yeah you can get another maybe .1 Mach out of the airplane and stay under that CIT.

Similarly, there are limits on the exhaust gas temperature. Those are measured just after the turbine. Again, it’s physics, the faster you go the more that temperature goes up. The compressor inlet temperature goes up and the temperature goes up through the entire gas path through the engine. Eventually, you reach a materials limit.

The whole engine bypass system in the J58 served to keep the compression ratio down to a workable limit. The air is compressed by the spike in the inlet, then again in the compressor section, and then again as it’s burned in the combustion section. At high speed, that compression ratio keeps going up because of all of the inlet compression. But there is a limit to how much the air can be compressed. It’s kind of a mechanical thing, and the J58 itself had a relatively low compression ratio compared to contemporaries like the J 79. Very high speed, the air would simply choke out the engine because of that compression ratio, so by bypassing a significant amount of air around the normal gas path, you were able to lower the compression ratio through the compressor, combustor, and turbine. Then that bypass air was burned and heated in the after burner. which added thrust, while keeping the compression ratio internal to the engine, low enough that it would still run.

The GE F110 in the F14 had a higher compression ratio, and more thrust than the TF 30. But the airframe had the same top speed with both motors. Because the faster you went, the higher compression ratio in that GE engine, started to limit the amount of air it could move, while the TF-30, with its slightly lower compression ratio, actually continued to increase thrust as the airplane went faster. The fastest I’ve ever been in the F-14 was in a TF 30 equipped airplane. Less thrust at sea level, and at low speed, but high speed, pretty good thrust.

Look, heat is a big deal in this discussion - if the outside of the airplane was heated to over 600°F at that speed, the same thing is happening inside the engine. The outside of the airplane is heated because of this phenomenon, the stagnation temperature/total air temperature, and it raises the temperature of the whole airplane. The SR 71 is famous for this, being made of titanium alloy so that it could withstand that heat, and famously the airplane stretches in length by about a foot when it’s a cruise speed.

But the same heating that happens on the outside of the airplane is happening throughout the engine inlet, and the engine itself. And eventually reach a temperature limit of materials. What makes the airplane so impressive, is that they were able to go significantly faster than anything else built.

Ever.

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

 

[billt460]

Astro, how do engine pressure ratios play out in supersonic flight? Are they much different than they are on subsonic airliners or transports?

 

[Exhaustgases]

At say 200,000 feet how fast does the plane have to go to reach 600F of skin temp?

What is keeping hypersonic rockets from burning up?

This could be the answer. If we get to see this now, then they have had it for the last 50 or more years.

https://www.space.com/nasa-hypersonic-magnetohydrodynamic-control

 

[Random_Kiwi]

You’re ignoring the stagnation of the airflow.

The problem is at that high speed when you slow the air down, the temperature goes way up.

In fact, the SR 71 is temperature limited in the inlet. And that temperature peaks out somewhere around 3.2 to 3.33 Mach on a standard day.

Clearly atmospherics affect the temperature to give an altitude on a given day. It’s just meteorology.

The fact that the air is 65 below zero, gives it a slightly lower starting point, but you still end up with temperatures as well over 400° in the inlet. And then the necessary compression, which adds temperature, prior to the combustion chamber, as well as the temperature added by combustion and the next thing you know you’re melting the turbine.

Your ability to ignore so much of the physics of Flight, to reach these conclusions, is truly fascinating.

Is the temperature rising as the airflow slows because the air pressure in the inlet increases as the flow slows (effectively compression)? Or am I way off track with my thinking?

Sort of like when you fill a diving bottle with compressed air and it gets warm?

 

[Random_Kiwi]

There is a limit to what can be bypassed - the engine still has all the temperature limits for what goes through it.

Read the Blackbird flight manual.

There are total air temperature limits. The F-14 had a total air temperature limit of 388C, measured on the fuselage before the inlet.

For the SR 71 the maximum compressor inlet temperature, measured just after the inlet spike, is 417C. That’s hot. Aluminum softens about there. The faster you go, the higher that temperature gets as a result of stagnation. It’s physics, you can’t get around it by bypassing the air because the air that comes into the engine inlet at about Mach 3.3 will reach that temperature. If the air is really cold that day, say 10 or 20 C below normal, then yeah you can get another maybe .1 Mach out of the airplane and stay under that CIT.

Similarly, there are limits on the exhaust gas temperature. Those are measured just after the turbine. Again, it’s physics, the faster you go the more that temperature goes up. The compressor inlet temperature goes up and the temperature goes up through the entire gas path through the engine. Eventually, you reach a materials limit.

The whole engine bypass system in the J58 served to keep the compression ratio down to a workable limit. The air is compressed by the spike in the inlet, then again in the compressor section, and then again as it’s burned in the combustion section. At high speed, that compression ratio keeps going up because of all of the inlet compression. But there is a limit to how much the air can be compressed. It’s kind of a mechanical thing, and the J58 itself had a relatively low compression ratio compared to contemporaries like the J 79. Very high speed, the air would simply choke out the engine because of that compression ratio, so by bypassing a significant amount of air around the normal gas path, you were able to lower the compression ratio through the compressor, combustor, and turbine. Then that bypass air was burned and heated in the after burner. which added thrust, while keeping the compression ratio internal to the engine, low enough that it would still run.

The GE F110 in the F14 had a higher compression ratio, and more thrust than the TF 30. But the airframe had the same top speed with both motors. Because the faster you went, the higher compression ratio in that GE engine, started to limit the amount of air it could move, while the TF-30, with its slightly lower compression ratio, actually continued to increase thrust as the airplane went faster. The fastest I’ve ever been in the F-14 was in a TF 30 equipped airplane. Less thrust at sea level, and at low speed, but high speed, pretty good thrust.

Look, heat is a big deal in this discussion - if the outside of the airplane was heated to over 600°F at that speed, the same thing is happening inside the engine. The outside of the airplane is heated because of this phenomenon, the stagnation temperature/total air temperature, and it raises the temperature of the whole airplane. The SR 71 is famous for this, being made of titanium alloy so that it could withstand that heat, and famously the airplane stretches in length by about a foot when it’s a cruise speed.

But the same heating that happens on the outside of the airplane is happening throughout the engine inlet, and the engine itself. And eventually reach a temperature limit of materials. What makes the airplane so impressive, is that they were able to go significantly faster than anything else built.

Ever.

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

Ben Rich had quite a bit of discussion about heat on the SR-71 in his book, the skunk works. A very interesting Read.

 

[97prizm]

What makes you think it is capable of 6,000 MPH at, "half throttle"? That is ridiculous. Pete Knight still holds the ultimate aircraft speed record in the X-15 at a bit over 4,500 MPH.

And that aircraft was powered by the LR-99 rocket engine that burned a ton of fuel in 11 seconds. So to say the SR-71 could achieve 1,500 MPH faster is preposterous. Let alone fly at 300,000 feet doing it.

Lockheed claims to have an operational mach 6 vehicle when the CEO mentioned that "Ten years ago a vehicle like this would have turned to slag. Now with advancements this truly is possible. " so who knows.

 

[97prizm]

Is the temperature rising as the airflow slows because the air pressure in the inlet increases as the flow slows (effectively compression)? Or am I way off track with my thinking?

Sort of like when you fill a diving bottle with compressed air and it gets warm?

Thats why the spikes would move in and out. From an article on the F-22 and YF-23 laminar flow is also a biggie. Keep in mind if on of the engines flames out the crew only has 10-12 shots of TEB (Tetraethylborane) which essentially explodes on contact with air.

 

[Astro14]

Ben Rich had quite a bit of discussion about heat on the SR-71 in his book, the skunk works. A very interesting Read.

A great book. Read it years ago - wonderful insights into the engineering.

 

[Astro14]

Astro, how do engine pressure ratios play out in supersonic flight? Are they much different than they are on subsonic airliners or transports?

Very different. The total compression in the system (from air molecule just in front of the airplane, to air molecule just behind) has an upper limit.

Subsonic airliners don’t have the inlet system to decelerate/compress the airflow, so the core jet engine can have a much higher compression ratio for efficient combustion. They then use a big fan (with bypass ratios of as much as 8:1 between fan and jet core) to gain fuel efficiency. Airliner engines make less thrust as they go faster because of the effect of that big fan section - fuel efficient, yes, but it can’t move the air much over 0.9 Mach, can’t have supersonic flow in the fan section, and thrust is only made when the exhaust velocity of the air is faster than the velocity of the airplane.

Basic momentum consideration - the airplane lost momentum when it picked up an air molecule at zero velocity. In order to gain momentum from eh engine thrust - that same molecule must be moving faster, away from the airplane, than the surrounding air.

Supersonic aircraft have compression in the inlet, so, the engine has a much lower limit on how much compression happens in the compressor section. For an airplane with efficient compression in the inlet - they make more thrust as they go faster. A lot happens in the afterburner - pressure/temperature increase, and a big velocity increase in the exhaust, leading to greater momentum transfer between airplane and the surrounding air.

 

[Astro14]

At say 200,000 feet how fast does the plane have to go to reach 600F of skin temp?

What is keeping hypersonic rockets from burning up?

This could be the answer. If we get to see this now, then they have had it for the last 50 or more years.

https://www.space.com/nasa-hypersonic-magnetohydrodynamic-control

About Mach 3 - it’s a total air temperature thing, but at 200,000 feet, you’re going to have so little air, that at Mach 3, the airplane’s wings can’t make lift. Air up there is really, really thin.

So, at sea level, the air is thick enough for an SR-71 to make lift, right?

At 80,000 feet, the air is about 4% as dense as sea level - You’ve lost 96% of the lift at the same speed, so the minimum speed for the SR-71 just to stay in the air (not stall) is around Mach 2.6.

At 200,000 feet, the air is about 100 times less dense than it is at 80,000 feet. So, flying speed (not stalling) is in the neighborhood of Mach 25. Which is about orbital velocity.

The math just doesn’t work for flying much above 80,000 for an SR-71. Even a fantasy engine (warp drive, whatever, doesn’t matter) and the airplane has to be going Mach 25 to generate lift and at Mach 25, the heating is incredible.

Think of the space shuttle re-entering the atmosphere at slightly below that speed - it is affected by the air (heating) but it is not really flying yet - it still needs reaction control systems to maintain control.

Even the X-15, which flew near Mach 6, needed reaction controls above 100,000 feet - wings, and other flight controls, just don’t work when the air is over a thousand time less dense than it is at sea level.

 

[The_Eric]

Astro, would you explain why, if there is so little air to fly in (lift), that at the speed that it would take, the heating would be bonkers? The heating isn't a function of drag in the atmosphere?

 

[Astro14]

Astro, would you explain why, if there is so little air to fly in (lift), that at the speed that it would take, the heating would be bonkers? The heating isn't a function of drag in the atmosphere?

Give this a read - while the mechanisms of heat transfer in the boundary layer still aren’t well understood, the heating was much higher than expected when missiles started going faster than Mach 2.

https://www.nasa.gov/wp-content/uploads/2023/04/sp-4232.pdf

The SR-71 gets up to 600F in air that is, according to the paper (at 85,000 feet, so it varies slightly from my numbers) 2% the density of air at sea level.

The “heat barrier” has long been discussed by engineers and researchers into high speed flight.

Even though the air is thin - it hits the airplane so fast, and stagnates, translating all that velocity (kinetic energy) into temperature (thermal energy) in the boundary layer - and because the air itself is so hot, and the boundary layer attached to the surface, the normal cooling mechanisms that we experience at low speed just don’t exist in high speed flight.

Rather famously, the X – 15 had external temperatures over 1200°F while flying at Mach 6. The outer structure of the airplane was built out of Inconel, a nickel chromium superalloy designed for very high temperatures. Much more heat resistant than steel, for example.

And on the Mach 6 flight at 100,000 feet, the Inconel skin was damaged by heat.

 

[The_Eric]

Thanks for the info and link!