3 ms·

There is very big, fundamental problem of flying at mach 4+ anywhere in the atmosphere: heat from aerodynamic friction > heat dissipated. All, and everything t

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.
    - baybal2 5y ago
      > The Waverider used the same fuel, and fuel-cooling, and reached Mach 5.1. Not so much as coolant, but as a thermal mass reservoir, and only for a few minutes.
- fmajid 5y ago
  It also had body panels that had gaps at ordinary temperature (and leaked fuel), and the gap closed as the metal expanded with heat. Also, the US did not have enough titanium production capacity so bought it from the USSR using shell companies...
  - masklinn 5y ago
    Not the body panels, the fuel piping.
mdorazio 5y ago
You seem to be ignoring the space shuttle entirely, which handled the heat of re-entry at 17,000 mph and was still reusable. You have to make trade-offs on weight and materials (like heat tiles), but it's not impossible.
- nradov 5y ago
  The Space Shuttle wasn't reusable in any meaningful sense. It required a complete overhaul after every mission, including replacement of many damaged ceramic heat shield tiles. And it only flew at hypersonic speeds for a few minutes during re-entry.
- baybal2 5y ago
  Space Shuttle used expendable ablation, thermal barrier materials, and thermal mass of its structure. All of that was only to last for a few minutes of re-entry, not constant flight in such conditions.
  - mdorazio 5y ago
    I'm sorry, but you are either misinformed or we're talking about different things. The HRSI tiles on the shuttle were not ablative, were absolutely reused multiple times, and there's no indication that they couldn't withstand high temperatures for long durations. They were largely inspected and replaced due to impact damage from micrometeorites and ice, not due to thermal ablation. The biggest issues with the shuttle's thermal management approach are that it used an aluminum airframe and the tiles alone wouldn't be suitable for the leading edges of an atmospheric air breathing plane due to the higher dynamic pressures and probably higher heat involved. At mach 4 temps they would be fine. At mach 6+ you'd need something better on the leading edges combined with a temperature-resistant airframe. There's a good paper on the topic here [1]. [1] https://ntrs.nasa.gov/api/citations/20080017096/downloads/20080017096.pdf https://ntrs.nasa.gov/api/citations/20080017096/downloads/20...
    - baybal2 5y ago
      Thermal barriers will slowdown the rate at which heat gets into the aircraft, but it cannot keep it out it forever.
orbital-decay 5y ago
Ablative heat shields are reflective, not evaporative though. They generate a thin gas layer that reflects the heat from the compression wave.

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.
  - baybal2 5y ago
    > The Waverider used the same fuel, and fuel-cooling, and reached Mach 5.1. Not so much as coolant, but as a thermal mass reservoir, and only for a few minutes.
fmajid 5y ago
It also had body panels that had gaps at ordinary temperature (and leaked fuel), and the gap closed as the metal expanded with heat. Also, the US did not have enough titanium production capacity so bought it from the USSR using shell companies...
- masklinn 5y ago
  Not the body panels, the fuel piping.

mdorazio 5y ago

You seem to be ignoring the space shuttle entirely, which handled the heat of re-entry at 17,000 mph and was still reusable. You have to make trade-offs on weight and materials (like heat tiles), but it's not impossible.

nradov 5y ago
The Space Shuttle wasn't reusable in any meaningful sense. It required a complete overhaul after every mission, including replacement of many damaged ceramic heat shield tiles. And it only flew at hypersonic speeds for a few minutes during re-entry.
baybal2 5y ago
Space Shuttle used expendable ablation, thermal barrier materials, and thermal mass of its structure. All of that was only to last for a few minutes of re-entry, not constant flight in such conditions.
- mdorazio 5y ago
  I'm sorry, but you are either misinformed or we're talking about different things. The HRSI tiles on the shuttle were not ablative, were absolutely reused multiple times, and there's no indication that they couldn't withstand high temperatures for long durations. They were largely inspected and replaced due to impact damage from micrometeorites and ice, not due to thermal ablation. The biggest issues with the shuttle's thermal management approach are that it used an aluminum airframe and the tiles alone wouldn't be suitable for the leading edges of an atmospheric air breathing plane due to the higher dynamic pressures and probably higher heat involved. At mach 4 temps they would be fine. At mach 6+ you'd need something better on the leading edges combined with a temperature-resistant airframe. There's a good paper on the topic here [1]. [1] https://ntrs.nasa.gov/api/citations/20080017096/downloads/20080017096.pdf https://ntrs.nasa.gov/api/citations/20080017096/downloads/20...
  - baybal2 5y ago
    Thermal barriers will slowdown the rate at which heat gets into the aircraft, but it cannot keep it out it forever.

orbital-decay 5y ago

Ablative heat shields are reflective, not evaporative though. They generate a thin gas layer that reflects the heat from the compression wave.