Some context. The good news is that the subcommittee has stated quite bluntly that superheavy lift is not necessary. It has also said depots should be part of any option the commission considers. The bad news is that Jeff Greason has said he is no longer sure 25mT is enough. General Lyles has also said depot technology is still immature.
It sounds as if there may need to be a bigger launcher. What should we argue for? Obvious choices include EELV Phase 1, J-130 and NSC. EELV Phase 1 is very heavy from the point of view of a depot enthusiast, but the good thing is it might also increase the payload capacity of the more reasonably sized Delta Medium. An SDLV would be wasteful, and all EELV might be better, but if we aim too high we may end up with nothing. The commission may recommend depots, but I think we can expect tremendous opposition from the congressional delegations from Alabama and Florida. Resigning ourselves to an SDLV may be the right thing to do.
Any thoughts on this alternative to LH2 - LOX storage: What if the fuel was stored as Stable Water? Then when the time comes to transfer the fuel the water could be broken into H2 and O2 via energy gathered from solar panels. This could reduce the amount of loss due to the "boil off" effect, and would be more stable and less prone to "unfortunate incidents" on orbit.
Team-Any thoughts on this alternative to LH2 - LOX storage: What if the fuel was stored as Stable Water? Then when the time comes to transfer the fuel the water could be broken into H2 and O2 via energy gathered from solar panels. This could reduce the amount of loss due to the "boil off" effect, and would be more stable and less prone to "unfortunate incidents" on orbit.Thoughts?Drew Montgomery AKA TOG.
I'd say fuel depots can realistically be a. for hypergolics only or b. for LOX only. Only in the very long run will we be able to build depots with LH2/LOX capability.
Can someone explain or provide a link as to why propellant depots are such a great idea. Why are they better and cheaper than having a large rocket. Thanks.
Quote from: yg1968 on 08/03/2009 08:06 pmCan someone explain or provide a link as to why propellant depots are such a great idea. Why are they better and cheaper than having a large rocket. Thanks. Just a couple of benefits:1. Costs. You get your fuel up to the depot with a number of smaller launch vehicles. High flight rates (>20 flights per year) reduces cost per launch significantly. On the other hand large rockets do have high fixed costs and only make sense at large flight rates (that you don't get if you have a 120mt vehicle).2. Flexibility. Your architecture basically involves your flight stack with an empty EDS going up to the depot and be fueled and then you go whereever you want to go. You can have an empty spacecraft (EDS and payload) in the 75mt range and after it's fueled you have a spacecraft with say 200mt ready to propel 60mt to a Mars trajectory.3. Creating a market for commercial rockets. You get fuel up to your depot constantly. A commercial provider with e.g. a 15mt to LEO rocket may be able to sell 20 launches per year to NASA for the fuel depot. That lowers this providers cost per launch quite significantly. That also means that the DoD and NASA can use these launchers for other payloads for quite a lower price.
The problem is that electrolyzing water (and then chilling it down to cryo temps) takes a nearly insane amount of energy. You'd be better off using the much smaller amount of energy it takes to actively cool the LH2/LOX.
Quote from: TOG on 08/03/2009 07:31 pmTeam-Any thoughts on this alternative to LH2 - LOX storage: What if the fuel was stored as Stable Water? Then when the time comes to transfer the fuel the water could be broken into H2 and O2 via energy gathered from solar panels. This could reduce the amount of loss due to the "boil off" effect, and would be more stable and less prone to "unfortunate incidents" on orbit.Thoughts?Drew Montgomery AKA TOG.Interesting idea, but not really viable.4.4 kilowatt-hours of electricity converts 1 liter of water into 1.59 liters of liquid hydrogen and 0.79 liters of liquid oxygen. With efficiency losses we can think about 7-8 kilowatt-hours of electricity required.For a metric ton of water converted into H2/O2 you would thus require about 8MWh. The peak output of the ISS' solar panels are 100KW. With these panels it would take 80 hours to convert one 1 metric ton. If you require say 50 metric tons of fuel for one mission, you end up having to wait 4000 hours or 160 days for your fuel depot to convert the water. And while it does so, you got the same boil-off problems you have if you bring up H2 and O2 separated in the first place.I'd say fuel depots can realistically be a. for hypergolics only or b. for LOX only. Only in the very long run will we be able to build depots with LH2/LOX capability.
Why is the choice of fuels at a depot between hypergolics and LH2? I would think that there are fuels that are more easily stored and transferred than LH2, and that also has a better Isp than hypergolics.Just curious why the other options have been discarded.
Hypergols such as the nitrogen tetroxide and monomethyl hydrazine combination the space shuttle burns to maneuver on orbit are considered highly reliable and easier to store than other propellants. But they offer lower performance than methane and other so-called green propellants and are highly caustic, requiring painstakingly careful -- and therefore expensive -- handling by workers on the ground who have to be extremely careful to avoid potentially lethal exposure.Scott Horowitz, NASA associate administrator for space exploration, said the decision to drop the methane-engine requirement from the CEV program came down to changing assumptions about the performance advantages and technical risk. There are no methane-fueled space propulsion systems in service today. Hypergolic systems, on the other hand, were used on board the Apollo command and service modules and the lunar landers.