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A long-range spacecraft would probably not have LH2 on board. It evaporates quite quickly unless you want to install cryo cooling systems and the power to run t

A long-range spacecraft would probably not have LH2 on board. It evaporates quite quickly unless you want to install cryo cooling systems and the power to run them.

curtis 11y ago
Boil-off is a bigger problem with LH2 than with liquid oxygen or liquid methane. However, I'm under the impression that it's a tractable problem out to months or maybe even years. Apparently the Spitzer Space Telescope maintained a supply of liquid helium for about five years [1][2]. A lot of Mars manned-mission proposals (starting with Zubrin's Mars Direct plan) depend on the use of a Sabatier reaction which would convert hydrogen brought from Earth into Methane and Oxygen. So the experts must think they can at least minimize boil-off over at least 8 or 9 months. [3] [1] https://en.wikipedia.org/wiki/Spitzer_Space_Telescope https://en.wikipedia.org/wiki/Spitzer_Space_Telescope [2] http://www.spitzer.caltech.edu/news/436-ssc2009-12-NASA-s-Spitzer-Begins-Warm-Mission http://www.spitzer.caltech.edu/news/436-ssc2009-12-NASA-s-Sp... [3] https://en.wikipedia.org/wiki/In-situ_resource_utilization#Mars https://en.wikipedia.org/wiki/In-situ_resource_utilization#M...
- jccooper 11y ago
  Mars Direct uses suspiciously low boil-off rates for LH2. Certainly well past current state of the art. Best achievable (not flown, but could be) is 0.1%/day, and Mars Direct wants 0.1%/month. You don't lose a lot at 0.1%/day, but it adds up.
  - curtis 11y ago
    At 0.1% per day, I get 30% boil-off in 1 year (0.999 ^ 365 ==> 0.694), which is expensive but probably still a net win. I don't think boil-off is a problem once you're on Mars because you can probably just use the hydrogen gas as feedstock into the Sabatier process. Which is not to say that Mars Direct isn't overly optimistic, possibly about many other things as well.
delibes 11y ago
That reminds me of the Arthur C Clarke story 2010, where he says they used ammonia instead to avoid the boil off.

A long-range spacecraft would probably not have LH2 on board. It evaporates quite quickly unless you want to install cryo cooling systems and the power to run them.

curtis 11y ago

Boil-off is a bigger problem with LH2 than with liquid oxygen or liquid methane. However, I'm under the impression that it's a tractable problem out to months or maybe even years. Apparently the Spitzer Space Telescope maintained a supply of liquid helium for about five years [1][2]. A lot of Mars manned-mission proposals (starting with Zubrin's Mars Direct plan) depend on the use of a Sabatier reaction which would convert hydrogen brought from Earth into Methane and Oxygen. So the experts must think they can at least minimize boil-off over at least 8 or 9 months. [3] [1] https://en.wikipedia.org/wiki/Spitzer_Space_Telescope https://en.wikipedia.org/wiki/Spitzer_Space_Telescope [2] http://www.spitzer.caltech.edu/news/436-ssc2009-12-NASA-s-Spitzer-Begins-Warm-Mission http://www.spitzer.caltech.edu/news/436-ssc2009-12-NASA-s-Sp... [3] https://en.wikipedia.org/wiki/In-situ_resource_utilization#Mars https://en.wikipedia.org/wiki/In-situ_resource_utilization#M...

jccooper 11y ago
Mars Direct uses suspiciously low boil-off rates for LH2. Certainly well past current state of the art. Best achievable (not flown, but could be) is 0.1%/day, and Mars Direct wants 0.1%/month. You don't lose a lot at 0.1%/day, but it adds up.
- curtis 11y ago
  At 0.1% per day, I get 30% boil-off in 1 year (0.999 ^ 365 ==> 0.694), which is expensive but probably still a net win. I don't think boil-off is a problem once you're on Mars because you can probably just use the hydrogen gas as feedstock into the Sabatier process. Which is not to say that Mars Direct isn't overly optimistic, possibly about many other things as well.

delibes 11y ago

That reminds me of the Arthur C Clarke story 2010, where he says they used ammonia instead to avoid the boil off.