3 ms·

Seems like the gases are getting compressed either way and it's just different ways of wording the same effect. As for it being reversible or not, is it not jus

Seems like the gases are getting compressed either way and it's just different ways of wording the same effect. As for it being reversible or not, is it not just a matter of whether the energy was actually transferred somewhere? Like you could technically undo the shock the same as you could depressurise air in a pump no? I don't really know what I'm talking about though, fyi.

pfdietz 3y ago
Gas is being compressed, but that doesn't mean the heating is from compression. There's a gas heater on the market that works by using rapidly moving vanes to induce shock waves in the gas. The outflow has the nearly the same pressure as the inflow, but the gas has been heated, potentially to a temperature higher than could be achieved by resistive heating elements. EDIT: I mistated this; see below for link. Consider also that once the shock heated air around the reentry vehicle has expanded back to ambient pressure, it will be hotter than it initially was.
- kragen 3y ago
  resistive heating elements made of carborundum can heat air to 1625° https://www.kanthal.com/en/products/furnace-products/electric-heating-elements/silicon-carbide-heating-elements/ https://www.kanthal.com/en/products/furnace-products/electri... and molybdenum disilicide to 1800° https://zircarceramics.com/wp-content/uploads/2017/02/Design-of-Low-Mass-Furnaces-for-High-Temperatures.pdf https://zircarceramics.com/wp-content/uploads/2017/02/Design.... while graphite heating elements can go to 2200° https://apps.dtic.mil/sti/tr/pdf/ADA329681.pdf https://apps.dtic.mil/sti/tr/pdf/ADA329681.pdf but not in air. what are these vanes made of?
  - Tuna-Fish 3y ago
    The point is that the shock isn't the air hitting the vanes, it's the air hitting other air. Similar to re-entry heating: the specific kinetic energy of the returning capsule is many times greater than would be required to melt and vaporize any material. So why do things survive re-entry? Because most of the energy is dissipated in the bow shock, significant distance away from from the capsule, where air gets heated to temperatures higher than the surface of the sun when other air slams into it. The purpose of the heatshield is to protect from radiative heating from the bow shock, not convective heating. Ablative heatshields do not work because ablation consumes energy which removes heat (again, there is sufficient energy going around to ablate the entire craft), but because they place a shade (made of ablated carbon particles) between the bow shock and the craft, which shields it from the radiative heating.
    - pfdietz 3y ago
      > The purpose of the heatshield is to protect from radiative heating from the bow shock, not convective heating. In this case the entry regime was such that convective heating far outweighed radiative heating.
      - kragen 3y ago
        i'm interested to hear more about these heaters. do you know what they are called or what they are made of?
        pfdietz 3y ago
        See link in a comment below.
        kragen 3y ago
        got it, thanks! https://news.ycombinator.com/item?id=39540283 https://news.ycombinator.com/item?id=39540283 https://coolbrook.com/wp-content/uploads/2023/06/REPRINT-2023-04-LR-Coolbrook-Breakthrough-tech-2.5p-MB.pdf https://coolbrook.com/wp-content/uploads/2023/06/REPRINT-202...
    - kragen 3y ago
      hmm, that's a good point; so if you run coolant through the vanes they can operate without damage while producing temperatures that would vaporize them?
- ballenf 3y ago
  Do you have a link or company/product name? Sounds fascinating.
  - pfdietz 3y ago
    https://coolbrook.com/wp-content/uploads/2023/06/REPRINT-2023-04-LR-Coolbrook-Breakthrough-tech-2.5p-MB.pdf https://coolbrook.com/wp-content/uploads/2023/06/REPRINT-202... I mistated slightly: the gas is accelerated to supersonic speed then slowed in a diffuser, where shock waves heat it.
    - kragen 3y ago
      thank you, this is great! it sounds like they're only targeting 1700°, though, which is a temperature that exotic resistive heating elements can reach. it's too bad they didn't include any kind of diagram
BryanLegend 3y ago
The gasses get so hot they give off a lot of black body radiation. Heat shield is mostly beat up with infrared. When the gases decompress they'll be a lot cooler, just like your AC.
- pfdietz 3y ago
  I believe for entry from LEO, and particularly for small RVs like this one, convective heating is orders of magnitude higher than radiative heating. https://ntrs.nasa.gov/api/citations/20140012475/downloads/20140012475.pdf https://ntrs.nasa.gov/api/citations/20140012475/downloads/20... (see slide 7)
  - BryanLegend 3y ago
    From Slide 7: "Radiation dominates convection at ~11.5km/s for 1m radius • Radiation dominates convection at ~10km/s for 5m radius"
    - pfdietz 3y ago
      Since this is reentering from LEO, it's maybe 8km/s (and dropping). And the radius of curvature on the nose of that shield was maybe 20 cm?

pfdietz 3y ago

Gas is being compressed, but that doesn't mean the heating is from compression. There's a gas heater on the market that works by using rapidly moving vanes to induce shock waves in the gas. The outflow has the nearly the same pressure as the inflow, but the gas has been heated, potentially to a temperature higher than could be achieved by resistive heating elements. EDIT: I mistated this; see below for link. Consider also that once the shock heated air around the reentry vehicle has expanded back to ambient pressure, it will be hotter than it initially was.

kragen 3y ago
resistive heating elements made of carborundum can heat air to 1625° https://www.kanthal.com/en/products/furnace-products/electric-heating-elements/silicon-carbide-heating-elements/ https://www.kanthal.com/en/products/furnace-products/electri... and molybdenum disilicide to 1800° https://zircarceramics.com/wp-content/uploads/2017/02/Design-of-Low-Mass-Furnaces-for-High-Temperatures.pdf https://zircarceramics.com/wp-content/uploads/2017/02/Design.... while graphite heating elements can go to 2200° https://apps.dtic.mil/sti/tr/pdf/ADA329681.pdf https://apps.dtic.mil/sti/tr/pdf/ADA329681.pdf but not in air. what are these vanes made of?
- Tuna-Fish 3y ago
  The point is that the shock isn't the air hitting the vanes, it's the air hitting other air. Similar to re-entry heating: the specific kinetic energy of the returning capsule is many times greater than would be required to melt and vaporize any material. So why do things survive re-entry? Because most of the energy is dissipated in the bow shock, significant distance away from from the capsule, where air gets heated to temperatures higher than the surface of the sun when other air slams into it. The purpose of the heatshield is to protect from radiative heating from the bow shock, not convective heating. Ablative heatshields do not work because ablation consumes energy which removes heat (again, there is sufficient energy going around to ablate the entire craft), but because they place a shade (made of ablated carbon particles) between the bow shock and the craft, which shields it from the radiative heating.
  - pfdietz 3y ago
    > The purpose of the heatshield is to protect from radiative heating from the bow shock, not convective heating. In this case the entry regime was such that convective heating far outweighed radiative heating.
    - kragen 3y ago
      i'm interested to hear more about these heaters. do you know what they are called or what they are made of?
      - pfdietz 3y ago
        See link in a comment below.
        kragen 3y ago
        got it, thanks! https://news.ycombinator.com/item?id=39540283 https://news.ycombinator.com/item?id=39540283 https://coolbrook.com/wp-content/uploads/2023/06/REPRINT-2023-04-LR-Coolbrook-Breakthrough-tech-2.5p-MB.pdf https://coolbrook.com/wp-content/uploads/2023/06/REPRINT-202...
  - kragen 3y ago
    hmm, that's a good point; so if you run coolant through the vanes they can operate without damage while producing temperatures that would vaporize them?
ballenf 3y ago
Do you have a link or company/product name? Sounds fascinating.
- pfdietz 3y ago
  https://coolbrook.com/wp-content/uploads/2023/06/REPRINT-2023-04-LR-Coolbrook-Breakthrough-tech-2.5p-MB.pdf https://coolbrook.com/wp-content/uploads/2023/06/REPRINT-202... I mistated slightly: the gas is accelerated to supersonic speed then slowed in a diffuser, where shock waves heat it.
  - kragen 3y ago
    thank you, this is great! it sounds like they're only targeting 1700°, though, which is a temperature that exotic resistive heating elements can reach. it's too bad they didn't include any kind of diagram

BryanLegend 3y ago

The gasses get so hot they give off a lot of black body radiation. Heat shield is mostly beat up with infrared. When the gases decompress they'll be a lot cooler, just like your AC.

pfdietz 3y ago
I believe for entry from LEO, and particularly for small RVs like this one, convective heating is orders of magnitude higher than radiative heating. https://ntrs.nasa.gov/api/citations/20140012475/downloads/20140012475.pdf https://ntrs.nasa.gov/api/citations/20140012475/downloads/20... (see slide 7)
- BryanLegend 3y ago
  From Slide 7: "Radiation dominates convection at ~11.5km/s for 1m radius • Radiation dominates convection at ~10km/s for 5m radius"
  - pfdietz 3y ago
    Since this is reentering from LEO, it's maybe 8km/s (and dropping). And the radius of curvature on the nose of that shield was maybe 20 cm?