4 ms·

> H2 is dangerously explosive. Does it matter for an unmanned vehicle? Such a thing could be inflated at a safe distance. In some cases, being able to self des

by gnode 7y ago

> H2 is dangerously explosive. Does it matter for an unmanned vehicle? Such a thing could be inflated at a safe distance. In some cases, being able to self destruct is a safety benefit (to prevent crashes when control is lost) and a security benefit (to prevent analysis of captured aircraft). Particularly UAVs are popular in military / spying operations.

cr0sh 7y ago
I'd argue that hydrogen is actually among the best, with helium of course being second. Helium wins out on the inertness, but has far lower lifting capacity, and of course has a supply issue. Hydrogen wins on lifting capacity, supply is not an issue at all - but it has a weird image problem, thanks to one singular accident - which was arguably not caused by the lifting gas! Prior to that accident, hydrogen had a fairly great record with airships, and it was used extensively for ballooning as well without many incidents. Hydrogen by itself isn't flammable - you need another particular gas involved for that to be an issue. You also need a source of ignition.
- evgen 7y ago
  The other problem with hydrogen is that it is hard to contain. Leakage and permeability of the envelope would probably end up being a bigger engineering concern than would flammability.
  - m0zg 7y ago
    And yet another problem with hydrogen is that unlike helium it is reactive, including with some metals. And those metals tend to become brittle fall apart after a while: https://en.wikipedia.org/wiki/Hydrogen_embrittlement https://en.wikipedia.org/wiki/Hydrogen_embrittlement
  - philipkglass 7y ago
    http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/Mylar_Chemical_Properties.pdf http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/... See Figure 2, Gas Permeability of Mylar vs. Temperature Helium permeates Mylar significantly faster than hydrogen over the full temperature range tested. It looks like the helium permeation rate is nearly double that of hydrogen near room temperature. Surprising! I would have guessed that lighter gases permeate faster. Note on the same figure that oxygen permeates faster than nitrogen even though it's heavier. There is more tabular data here: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690004779.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/196900... It confirms the same trend. Helium permeates Mylar faster than hydrogen.
- upofadown 7y ago
  Hydrogen is only something like 8% better for lifting than helium because they are both much lighter than air: * https://en.wikipedia.org/wiki/Lighter_than_air#Hydrogen_and_helium https://en.wikipedia.org/wiki/Lighter_than_air#Hydrogen_and_... Agreed that the danger of hydrogen is less than perceived. In an accident it goes up which is normally where the people are not. When it burns it doesn't create much radiant heat. Outdoors in the daytime firefighters have to use something like a broom to find hydrogen fires. Otherwise you could just walk right by one ... which is an issue if you instead walk right into the fire.

by gnode 7y ago

cr0sh 7y ago

I'd argue that hydrogen is actually among the best, with helium of course being second. Helium wins out on the inertness, but has far lower lifting capacity, and of course has a supply issue. Hydrogen wins on lifting capacity, supply is not an issue at all - but it has a weird image problem, thanks to one singular accident - which was arguably not caused by the lifting gas! Prior to that accident, hydrogen had a fairly great record with airships, and it was used extensively for ballooning as well without many incidents. Hydrogen by itself isn't flammable - you need another particular gas involved for that to be an issue. You also need a source of ignition.

evgen 7y ago
The other problem with hydrogen is that it is hard to contain. Leakage and permeability of the envelope would probably end up being a bigger engineering concern than would flammability.
- m0zg 7y ago
  And yet another problem with hydrogen is that unlike helium it is reactive, including with some metals. And those metals tend to become brittle fall apart after a while: https://en.wikipedia.org/wiki/Hydrogen_embrittlement https://en.wikipedia.org/wiki/Hydrogen_embrittlement
- philipkglass 7y ago
  http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/Mylar_Chemical_Properties.pdf http://usa.dupontteijinfilms.com/wp-content/uploads/2017/01/... See Figure 2, Gas Permeability of Mylar vs. Temperature Helium permeates Mylar significantly faster than hydrogen over the full temperature range tested. It looks like the helium permeation rate is nearly double that of hydrogen near room temperature. Surprising! I would have guessed that lighter gases permeate faster. Note on the same figure that oxygen permeates faster than nitrogen even though it's heavier. There is more tabular data here: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19690004779.pdf https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/196900... It confirms the same trend. Helium permeates Mylar faster than hydrogen.
upofadown 7y ago
Hydrogen is only something like 8% better for lifting than helium because they are both much lighter than air: * https://en.wikipedia.org/wiki/Lighter_than_air#Hydrogen_and_helium https://en.wikipedia.org/wiki/Lighter_than_air#Hydrogen_and_... Agreed that the danger of hydrogen is less than perceived. In an accident it goes up which is normally where the people are not. When it burns it doesn't create much radiant heat. Outdoors in the daytime firefighters have to use something like a broom to find hydrogen fires. Otherwise you could just walk right by one ... which is an issue if you instead walk right into the fire.