5 ms·

I thought the true top speed of the SR-71 is either highly classified or truly “unknown”. How does the article justify saying the 72 will be roughly 2x the spe

by cdolan 5y ago

I thought the true top speed of the SR-71 is either highly classified or truly “unknown”. How does the article justify saying the 72 will be roughly 2x the speed of the 71?

Arnt 5y ago
It was seen by Soviet pilots and radar (it was poorly visible but not invisible). As to top speed, from what I've heard that's a matter of how much wear and tear you're willing to incur.
- cdolan 5y ago
  So in theory, could be Mach 4-6 already in an SR-71?
  - Arnt 5y ago
    If the SF71 was like many other military aircraft, then it could be flown far beyond the long-life envelope. Overclocked, if you will, getting extreme performance for an hour at a cost of years' wear on the engine and body. You can see how this approach might make sense for an SR-71 where one might sensibly want to sacrifice the aircraft to get some pictures out. You can also see how this complicates defining "top speed".
noisy_boy 5y ago
You don't have to know the actual speed to say, "whatever the (classified) speed was, we will double it".
- cdolan 5y ago
  X * 2 = (Mach) 7 That’s the formula expressed by the article, is it not?
  - noisy_boy 5y ago
    Ah I missed that - you are correct.
HPsquared 5y ago
It depends on your definition of "top speed": on the Blackbird and other supersonic aircraft, higher speeds mean higher temperatures. It's possible to sprint for a short time and absorb some of the excess heat, but have a lower limit for continuous operation. The shorter the duration, the higher the speed.
baybal2 5y ago
There is very big, fundamental problem of flying at mach 4+ anywhere in the atmosphere: heat from aerodynamic friction > heat dissipated. All, and everything that flew so fast before relied on evaporative cooling, and under the evaporate, I mean the solid parts of the spacecraft/airplane/missile. For a reusable craft, it will mean big disposable heat shields.
- HPsquared 5y ago
  The Blackbird used fuel as a heat sink. This sounds like something affected by scaling laws, such that a larger craft would have more mass available for heat absorption.
  - baybal2 5y ago
    Yes, fuel as coolant worked at around mach 3, but at mach 4+ the kerosene itself may not not be energetic enough to both sink heat, and produce more energy than the energy of incoming airflow. Supercooling the fuel may work to some extend. Having the entire aircraft being a flying liquid hydrogen tank is another least improbable option.
    - HPsquared 5y ago
      I suppose heat flux is still a limiting factor that's independent of scale. Though things like liquid evaporative cooling (by "sweating") also become more practical with larger scale (more mass available). Not sure how big it would have to be though...
    - masklinn 5y ago
      > Yes, fuel as coolant worked at around mach 3, but at mach 4+ the kerosene itself may not not be energetic enough to both sink heat, and produce more energy than the energy of incoming airflow. The blackbird did not use kerosene, it used JP7, specially designed for high range of operation as well as being usable for cooling and hydraulics. > JP-7 is unusual in that it is not a conventional distillate fuel, but is created from special blending stocks in order to have very low (<3%) concentration of highly volatile components like benzene or toluene, and almost no sulfur, oxygen, and nitrogen impurities. JP-7 is famously hard to ignite, the blackbird needed TEB shots to start ignition, and was initially started by driving the engines with a starter cart composed of paired V8s (later replaced with a smaller and simpler compressed-air starter). The Waverider used the same fuel, and fuel-cooling, and reached Mach 5.1.
geocrasher 5y ago
It's accepted that the top speed of the SR-71 was likely around Mach 3.5. The Mach 3 records were just for show, and pilots have said that even at Mach 3 when they pushed the throttle forward, it just went. This article also says that the SR-72 will use "a dual-mode engine that combines turbine and ramjet technologies." The SR-71 already did that.
- masklinn 5y ago
  > It's accepted that the top speed of the SR-71 was likely around Mach 3.5. The Mach 3 records were just for show, and pilots have said that even at Mach 3 when they pushed the throttle forward, it just went. FWIW the official record is Mach 3.3, Air & Space/Smithsonian reported that it'd been clocked at 3.4 by USAF, and Brian Schul claimed to have exceeded 3.5 during evasive manoeuvres. Mach 3 was never claimed to be anywhere its limits. Hell, the design / cruise speed was 3.2.
  - geocrasher 5y ago
    I was generalizing, but your point backs mine up all the same. Thanks :)
numpad0 5y ago
There was a declassified manuals for one of Blackbird models from few years ago, and it wasn’t inconsistent with Mach 3.75 top speeds. Maybe they do 3.6669999 or 3.78925 but can’t be like it could go all the way to 22 flat out in a straight line.
VLM 5y ago
A couple comments on supersonic aircraft: There are no jet engines ever made that I'm aware of that can run at supersonic speeds. The fastest mass air flow rate I've ever personally heard of was some Soviet thing that didn't immediately self destruct around mach 0.98. The way supersonic aircraft generate thrust while supersonic is the inlet geometry does WEIRD things to turn supersonic cold low pressure air into low subsonic hot higher pressure air. Most jet engines are happiest with inlet air flowing around 200 to 400 mph. If you run a jet engine above some critical subsonic speed occasionally shock waves will impact the compressor and every time the compressor rotates the blades will hit those shockwaves, and they rotate pretty fast... So the MTBF for a SR71 compressor blade might be 100000 hours at a nice subsonic mach 3.25 but whack it with supersonic shock waves every rotation and at mach 3.5 the MTBF for a compressor blade might be one hour. The nozzles in front of the engine fully retract at mach 3.25 and I'd interpret that as the inlet system is not designed to run "much above mach 3.25". There's a story about a flight over Libya where the pilot pushed a SR71 to mach 3.5 and that engine was likely pulled and sent to scrap yard after that stunt. The other side of the problem with inlet geometry is the inlet temp gets very hot so you have to go ramjet to get most of the thrust because trying to generate significant thrust will melt the turbine blades off. So hypothetically if you got 10K pounds of thrust by increasing the temp of the air by 1500 degrees and the inlet was dumping 800 degree air at mach 3.25 into the engine, the combined exhaust temp would be 800+1500=2300 degrees which is getting hot for turbine blades (all numbers made up). So the second engineering limit on why the SR71 never flew at mach 9 or whatever is based on public engine designs and the temperature of mach 9 air when slowed to subsonic, there are no metals you can make a turbine out of. In fact at mach 6 or whatever its kinda mysterious how they keep the ramjet from melting itself. Must look like a rocket engine with cooling passages for fuel everywhere. So based on the published engine design (and is the published data trustworthy?) there's no way to bypass enough air to not melt the blades off the turbine above mach 3.something because nobody has high enough melting point metals. As with the compressor situation I'm sure a 3.25 rated turbine blade that has a MTBF of 100000 hours might survive an hour or two at 3.5, but the engine is scrap after that flight. The third supersonic comment is mach number is not constant with altitude so a pessimist person could say a mach 6 that flys high enough could very well indeed have a slower groundspeed than a sr-71 or even a 747. If the speed of sound at 120Kft is like 190 knots, then mach 6 at 120Kft would be a ground speed of merely 1140 knots. Famously the SR-71 could achieve ground speeds of 1800+ knots (supposedly...). So its simultaneously true that a SR72 might run twice the mach number but if it runs at a high enough altitude like my 120Kft example, its possibly only 2/3 the ground speed of a SR-71. The flight envelope of the SR-71 from the declassified manual is available; note it could not run supersonic below 20Kfeet or so. This also comes up in discussion of interceptor missiles; there's a huge difference in speed between a plane that can't go supersonic below 20Kft and a SA missile that can go mach 5 or whatever at ground level right off the rail. So its not as simple as "a mach 4 surface to air missile can never hit a mach 6 airplane" because mach 6 at 120Kft is much slower than mach 4 at ground level.

by cdolan 5y ago

I thought the true top speed of the SR-71 is either highly classified or truly “unknown”. How does the article justify saying the 72 will be roughly 2x the speed of the 71?

Arnt 5y ago

It was seen by Soviet pilots and radar (it was poorly visible but not invisible). As to top speed, from what I've heard that's a matter of how much wear and tear you're willing to incur.

cdolan 5y ago
So in theory, could be Mach 4-6 already in an SR-71?
- Arnt 5y ago
  If the SF71 was like many other military aircraft, then it could be flown far beyond the long-life envelope. Overclocked, if you will, getting extreme performance for an hour at a cost of years' wear on the engine and body. You can see how this approach might make sense for an SR-71 where one might sensibly want to sacrifice the aircraft to get some pictures out. You can also see how this complicates defining "top speed".

noisy_boy 5y ago

You don't have to know the actual speed to say, "whatever the (classified) speed was, we will double it".

cdolan 5y ago
X * 2 = (Mach) 7 That’s the formula expressed by the article, is it not?
- noisy_boy 5y ago
  Ah I missed that - you are correct.

HPsquared 5y ago

It depends on your definition of "top speed": on the Blackbird and other supersonic aircraft, higher speeds mean higher temperatures. It's possible to sprint for a short time and absorb some of the excess heat, but have a lower limit for continuous operation. The shorter the duration, the higher the speed.

baybal2 5y ago

There is very big, fundamental problem of flying at mach 4+ anywhere in the atmosphere: heat from aerodynamic friction > heat dissipated. All, and everything that flew so fast before relied on evaporative cooling, and under the evaporate, I mean the solid parts of the spacecraft/airplane/missile. For a reusable craft, it will mean big disposable heat shields.

HPsquared 5y ago
The Blackbird used fuel as a heat sink. This sounds like something affected by scaling laws, such that a larger craft would have more mass available for heat absorption.
- baybal2 5y ago
  Yes, fuel as coolant worked at around mach 3, but at mach 4+ the kerosene itself may not not be energetic enough to both sink heat, and produce more energy than the energy of incoming airflow. Supercooling the fuel may work to some extend. Having the entire aircraft being a flying liquid hydrogen tank is another least improbable option.
  - HPsquared 5y ago
    I suppose heat flux is still a limiting factor that's independent of scale. Though things like liquid evaporative cooling (by "sweating") also become more practical with larger scale (more mass available). Not sure how big it would have to be though...
  - masklinn 5y ago
    > Yes, fuel as coolant worked at around mach 3, but at mach 4+ the kerosene itself may not not be energetic enough to both sink heat, and produce more energy than the energy of incoming airflow. The blackbird did not use kerosene, it used JP7, specially designed for high range of operation as well as being usable for cooling and hydraulics. > JP-7 is unusual in that it is not a conventional distillate fuel, but is created from special blending stocks in order to have very low (<3%) concentration of highly volatile components like benzene or toluene, and almost no sulfur, oxygen, and nitrogen impurities. JP-7 is famously hard to ignite, the blackbird needed TEB shots to start ignition, and was initially started by driving the engines with a starter cart composed of paired V8s (later replaced with a smaller and simpler compressed-air starter). The Waverider used the same fuel, and fuel-cooling, and reached Mach 5.1.

geocrasher 5y ago

It's accepted that the top speed of the SR-71 was likely around Mach 3.5. The Mach 3 records were just for show, and pilots have said that even at Mach 3 when they pushed the throttle forward, it just went. This article also says that the SR-72 will use "a dual-mode engine that combines turbine and ramjet technologies." The SR-71 already did that.

masklinn 5y ago
> It's accepted that the top speed of the SR-71 was likely around Mach 3.5. The Mach 3 records were just for show, and pilots have said that even at Mach 3 when they pushed the throttle forward, it just went. FWIW the official record is Mach 3.3, Air & Space/Smithsonian reported that it'd been clocked at 3.4 by USAF, and Brian Schul claimed to have exceeded 3.5 during evasive manoeuvres. Mach 3 was never claimed to be anywhere its limits. Hell, the design / cruise speed was 3.2.
- geocrasher 5y ago
  I was generalizing, but your point backs mine up all the same. Thanks :)

numpad0 5y ago

There was a declassified manuals for one of Blackbird models from few years ago, and it wasn’t inconsistent with Mach 3.75 top speeds. Maybe they do 3.6669999 or 3.78925 but can’t be like it could go all the way to 22 flat out in a straight line.

VLM 5y ago

A couple comments on supersonic aircraft: There are no jet engines ever made that I'm aware of that can run at supersonic speeds. The fastest mass air flow rate I've ever personally heard of was some Soviet thing that didn't immediately self destruct around mach 0.98. The way supersonic aircraft generate thrust while supersonic is the inlet geometry does WEIRD things to turn supersonic cold low pressure air into low subsonic hot higher pressure air. Most jet engines are happiest with inlet air flowing around 200 to 400 mph. If you run a jet engine above some critical subsonic speed occasionally shock waves will impact the compressor and every time the compressor rotates the blades will hit those shockwaves, and they rotate pretty fast... So the MTBF for a SR71 compressor blade might be 100000 hours at a nice subsonic mach 3.25 but whack it with supersonic shock waves every rotation and at mach 3.5 the MTBF for a compressor blade might be one hour. The nozzles in front of the engine fully retract at mach 3.25 and I'd interpret that as the inlet system is not designed to run "much above mach 3.25". There's a story about a flight over Libya where the pilot pushed a SR71 to mach 3.5 and that engine was likely pulled and sent to scrap yard after that stunt. The other side of the problem with inlet geometry is the inlet temp gets very hot so you have to go ramjet to get most of the thrust because trying to generate significant thrust will melt the turbine blades off. So hypothetically if you got 10K pounds of thrust by increasing the temp of the air by 1500 degrees and the inlet was dumping 800 degree air at mach 3.25 into the engine, the combined exhaust temp would be 800+1500=2300 degrees which is getting hot for turbine blades (all numbers made up). So the second engineering limit on why the SR71 never flew at mach 9 or whatever is based on public engine designs and the temperature of mach 9 air when slowed to subsonic, there are no metals you can make a turbine out of. In fact at mach 6 or whatever its kinda mysterious how they keep the ramjet from melting itself. Must look like a rocket engine with cooling passages for fuel everywhere. So based on the published engine design (and is the published data trustworthy?) there's no way to bypass enough air to not melt the blades off the turbine above mach 3.something because nobody has high enough melting point metals. As with the compressor situation I'm sure a 3.25 rated turbine blade that has a MTBF of 100000 hours might survive an hour or two at 3.5, but the engine is scrap after that flight. The third supersonic comment is mach number is not constant with altitude so a pessimist person could say a mach 6 that flys high enough could very well indeed have a slower groundspeed than a sr-71 or even a 747. If the speed of sound at 120Kft is like 190 knots, then mach 6 at 120Kft would be a ground speed of merely 1140 knots. Famously the SR-71 could achieve ground speeds of 1800+ knots (supposedly...). So its simultaneously true that a SR72 might run twice the mach number but if it runs at a high enough altitude like my 120Kft example, its possibly only 2/3 the ground speed of a SR-71. The flight envelope of the SR-71 from the declassified manual is available; note it could not run supersonic below 20Kfeet or so. This also comes up in discussion of interceptor missiles; there's a huge difference in speed between a plane that can't go supersonic below 20Kft and a SA missile that can go mach 5 or whatever at ground level right off the rail. So its not as simple as "a mach 4 surface to air missile can never hit a mach 6 airplane" because mach 6 at 120Kft is much slower than mach 4 at ground level.