Hmmmm.... I detect another reason that SpaceX is interested in methane as a fuel.A quick bit of googling shows (that with proper orientation of shading and radiation coolers of the sun and the earth) that temperatures capable of keeping both O2 and CH4 liquid in LEO.Not true for H2: you'd always have gas boiling off in LEO: keeping things at 20K is hard to do....
It depends on the altitude of the depot. A 500 km altitude depot wouldn't need a lot of reboosting. That said, the choice between hypergolics and LOX/LH2 won't be made on the basis of reboost requirements, but more likely on the basis of Isp and TRL. It does seem likely the boil-off vs reboost argument could influence the optimal altitude for a LOX/LH2 LEO depot once you decided to build one. I suspect the optimal altitude for a LOX/LH2 would be lower than that for a hypergolics depot. Then again, I'm not convinced LEO hypergolics depots would be all that useful, while LOX/LH2 ones certainly would be.
Certainly LOX/LH2 or LOX/CH4 depots are likely to be very useful, particularly once one can supply them from somewhere other than the surface of our planet...
One problem with higher depot altitudes is that they impose a higher delta-V penalty on the depot architecture. Raising apogees (so long as the line of apsides is oriented right) doesn't really cost you anything on net, but raising the perigee (to circularize at a higher altitude for the depot) does have a net cost delta-V wise. It's not a ginormous penalty for most high-performance expendable launch vehicle, but it is a non-negligible one that needs to be factored in.
Does anybody know what are the actual numbers comparing the Isp of warm gas H2 and hypergols?
Quote from: JohnFornaro on 05/11/2013 01:58 pmDoes anybody know what are the actual numbers comparing the Isp of warm gas H2 and hypergols?Download the Rocket Propulsion Analysis Lite tool and you can generate numbers like this to your heart's content: http://www.propulsion-analysis.com/downloads.htm . Its default thermodynamic database seems to only include H2 (gas) down to 200K, so you either need to extend that database yourself or limit the expansion ratio to keep exhaust temperature above that.
What about a BYOH (Bring Your Own Hydrogen) LOX only depot?Long duration with minimal or no boil off in LEO. Methane and hydrogen are significantly lighter and could be more easily included in the IMLEO mass of the main mission. And heck, if we get cheap oxygen off of the moon or an asteroid we got an infrastructure to handle it already. Oxygen is easier to get to liquid phase using relatively low energy systems.It's the heaviest part of the fuel regime. Optimize for that problem. Might as well haul hypergolics up as well to the same depot. There are tons of mission scenarios that could benefit greatly from having a ready supply of one or both of these two materials in LEO, L2 or even in Mars orbit.... Just remember to BYOH.
Quote from: SpunkyEnigma on 05/14/2013 05:19 amWhat about a BYOH (Bring Your Own Hydrogen) LOX only depot?Long duration with minimal or no boil off in LEO. Methane and hydrogen are significantly lighter and could be more easily included in the IMLEO mass of the main mission. And heck, if we get cheap oxygen off of the moon or an asteroid we got an infrastructure to handle it already. Oxygen is easier to get to liquid phase using relatively low energy systems.It's the heaviest part of the fuel regime. Optimize for that problem. Might as well haul hypergolics up as well to the same depot. There are tons of mission scenarios that could benefit greatly from having a ready supply of one or both of these two materials in LEO, L2 or even in Mars orbit.... Just remember to BYOH.Of course, there will always be a need for hypergolics in the cis-lunar arena. Kinda like there will always be a need for lawn mowers in the cis-yard arena.The main problem with BYOH, assuming you mean to bring it from Earth, is that the tanks have to be so physically large, and once you bring 'em up to your O2 Depot, you still have to deal with boil-off. I'm not sure what you think the savings are.
The trick of course is getting quick rendezvous and quick loading of LOX into the spacecraft to limit H2 boiloff.
Bring your own hydrogen can be made to work if necessary, and could be part of early cryo depots. And because LH2 is such an excellent coolant, it might even be useful for the LOX in a kerolox depot. Then again, I keep wondering about cryocoolers, which have flown in space. I can understand why you wouldn't want to put one on your transfer stage, but not why you would want to keep it off a depot. And they're available today.
Quote from: mmeijeri on 05/21/2012 08:17 pm My main issue is starting with existing technology, not hypergolics per se. And we already have, with regard to ISS. (Sorry, I'm repeating myself.) Maybe the thread title should be "Hypergolic Depots and Cryogenic Depots."
My main issue is starting with existing technology, not hypergolics per se.
If your propellant depot is at the edge of Earth's gravity well, you have the option of refueling but you'll need more propellant for the same trip using chemical because you can't use the Oberth effect.
I would suggest a focusing on an oxidizer depot in LEO. If you're using LH2/LOX the bulk of the mass to be launched is going to be the LOX, while the LH2 could be launched in vast quantities in a single launch. Hydrocarbons are also typically much lighter than the oxidizer they're burned with, though apart from methane they are not cryogenic and fairly easy to store.
If your propellant depot is at the edge of Earth's gravity well, you have the option of refueling but you'll need more propellant for the same trip using chemical because you can't use the Oberth effect.I would suggest a focusing on an oxidizer depot in LEO. If you're using LH2/LOX the bulk of the mass to be launched is going to be the LOX, while the LH2 could be launched in vast quantities in a single launch. Hydrocarbons are also typically much lighter than the oxidizer they're burned with, though apart from methane they are not cryogenic and fairly easy to store. ...