3 ms·

I'm sceptical. You'd need a lot of insulation and possibly refrigeration to prevent excessive boil off on long flights, adding weight. Also you'd want the tanks

by jabl 7y ago

I'm sceptical. You'd need a lot of insulation and possibly refrigeration to prevent excessive boil off on long flights, adding weight. Also you'd want the tanks to be, if not spherical, at least cylindrical. Where would you put them? Rockets make do with minimal insulation since they can top off the tanks just before liftoff and the flight itself takes only minutes. And then there's also the thing that the climate forcing of flights is roughly a factor of 2.6 worse than the CO2 emissions alone, largely due to high altitude water vapor emissions. Switching to hydrogen would obviously not fix this.

ncmncm 7y ago
Practical aerogel production makes a huge difference in insulation effectiveness. Lifting body designs provide plenty of interior room for tankage. Spherical/cylindrical tankage is needed only to contain high pressure, but we don't need to contain pressure. The greater energy density of hydrogen means much less fuel burnt at altitude, vs. diesel. Water vapor is a grrenhouse gas, but condensed into clouds they reflect sunlight before it gets converted to IR. Not so easy to know the net effect, but the lifetime of water vapor is much less than CO2. Visible contrails suggest that condensation is preferred.
- jabl 7y ago
  > Spherical/cylindrical tankage is needed only to contain high pressure, but we don't need to contain pressure. You want the minimum surface area to volume, to minimize heat losses (and minimize the surface area that has to be insulated when weight matters such as in an aircraft). Spherical being the optimum, but obviously unpractical. If you look at tanks in current aircraft, typically in wings, they have huge surface area vs volume. Lifting body might be better, I'm not sure. > Not so easy to know the net effect Luckily we have researchers who have dedicated years, if not decades, to studying this exact problem. Still, not an easy problem, surely. Some quick searching turns up this overview: https://www.carbonbrief.org/explainer-challenge-tackling-aviations-non-co2-emissions https://www.carbonbrief.org/explainer-challenge-tackling-avi... (and it seems I was wrong, it's not water vapor that is the major factor)
  - ncmncm 7y ago
    Thank you, this provides a lot of detail I was not aware of. The NOx contribution argues for use of onboard LOX, which would then enable a much higher flight deck, increasing efficiency further on long-haul routes. I wonder what mechanism transports water vapor out of the upper atmosphere. Availability of aerogels radically alters the equations that have dictated tank shape for liquified gases.

by jabl 7y ago

ncmncm 7y ago

Practical aerogel production makes a huge difference in insulation effectiveness. Lifting body designs provide plenty of interior room for tankage. Spherical/cylindrical tankage is needed only to contain high pressure, but we don't need to contain pressure. The greater energy density of hydrogen means much less fuel burnt at altitude, vs. diesel. Water vapor is a grrenhouse gas, but condensed into clouds they reflect sunlight before it gets converted to IR. Not so easy to know the net effect, but the lifetime of water vapor is much less than CO2. Visible contrails suggest that condensation is preferred.

jabl 7y ago
> Spherical/cylindrical tankage is needed only to contain high pressure, but we don't need to contain pressure. You want the minimum surface area to volume, to minimize heat losses (and minimize the surface area that has to be insulated when weight matters such as in an aircraft). Spherical being the optimum, but obviously unpractical. If you look at tanks in current aircraft, typically in wings, they have huge surface area vs volume. Lifting body might be better, I'm not sure. > Not so easy to know the net effect Luckily we have researchers who have dedicated years, if not decades, to studying this exact problem. Still, not an easy problem, surely. Some quick searching turns up this overview: https://www.carbonbrief.org/explainer-challenge-tackling-aviations-non-co2-emissions https://www.carbonbrief.org/explainer-challenge-tackling-avi... (and it seems I was wrong, it's not water vapor that is the major factor)
- ncmncm 7y ago
  Thank you, this provides a lot of detail I was not aware of. The NOx contribution argues for use of onboard LOX, which would then enable a much higher flight deck, increasing efficiency further on long-haul routes. I wonder what mechanism transports water vapor out of the upper atmosphere. Availability of aerogels radically alters the equations that have dictated tank shape for liquified gases.