Actually, why not just purge with gaseous hydrogen on the ground anyway. For first stages, if the gas can't be flared off at the exit bell without damaging the rocket, then the rocket isn't durable enough to be launched in the first place. There must be ways to ensure reasonably even/stable combustion (maybe a flow of oxygen gas or something).
I think Martijn's idea with nitrogen followed by warm hydrogen is pretty workable for an upper stage. All the purges on the ground can be done with nitrogen. Any further purges before chilldown (warm hydrogen then liquid hydrogen) should occur sufficiently high up that the ambient environment is a vacuum.I'm pretty sure that there's some way to make this work for upper stage without any safety concerns.
The issue is that "warm" GH2 is still colder than solid N2.
Quote from: madscientist197 on 04/26/2010 12:48 amI think Martijn's idea with nitrogen followed by warm hydrogen is pretty workable for an upper stage. All the purges on the ground can be done with nitrogen. Any further purges before chilldown (warm hydrogen then liquid hydrogen) should occur sufficiently high up that the ambient environment is a vacuum.I'm pretty sure that there's some way to make this work for upper stage without any safety concerns.The issue is that "warm" GH2 is still colder than solid N2.
Quote from: Jim on 04/26/2010 10:49 amQuote from: madscientist197 on 04/26/2010 12:48 amI think Martijn's idea with nitrogen followed by warm hydrogen is pretty workable for an upper stage. All the purges on the ground can be done with nitrogen. Any further purges before chilldown (warm hydrogen then liquid hydrogen) should occur sufficiently high up that the ambient environment is a vacuum.I'm pretty sure that there's some way to make this work for upper stage without any safety concerns.The issue is that "warm" GH2 is still colder than solid N2. I can understand that for LH2, but surely GH2 can be any temperature you like above the boiling point?Or is it an issue of heating the LH2 once it's been boiled?cheers, Martin
Inevitably you will have gas lines near the LH2 tanks that would otherwise freeze up (by conduction or radiation) with residual nitrogen
The complications in flight and added risk on the ground during development would probably negate the benefits of LH2 vs. methane or other classic upper stage fuels.-Alex
The 8.4-m diameter core is derived from the Space Shuttle’s external tank and powered by five RD-180 engines.