Quote from: neilh on 06/02/2010 09:56 pmQuote from: 2552 on 06/02/2010 09:30 pmThey also seem to be leaning towards a LOX/methane upper stage/depot (page 16).I'm not sure how much I agree with it, but here's the current rationale (presumably subject to change as the RFI proceeds) on preferring LOX/LCH4 over LOX/LH2:http://www.nasa.gov/pdf/458814main_FTD_CRYOGENICPropellantSTorageAndTransferMission.pdfQuoteCryogen Options Review: Why Methane (vs. Hydrogen)• A early LOX/Methane demo offers advantages:– Enables methane-based systems and mitigates risks for LH2 systems.– Allows direct comparison of active vs. passive cooling.– Leverages recent investments in LO2/LCH4 cryo fluid management– Leverages recent investments in pressure-fed engines– Breaks the barrier for long-duration cryo systems.• A LOX/Hydrogen demo in foreseeable future is possible.– Low cryo cooler TRL implies shorter mission duration.– No accurate gauging method for unsettled propellants.• Due to similarity of LOX and LCH4 properties (e.g. temp, density, etc.), the same components may be qualified and used for ground test and flight hardware.I still don't follow the desires for methane. Every EDS supporting EM-L1, LLO, NEO, LMO, Lunar surface, Mars Surface (you get the picture) is base lined to use LH2 thanks to the much higher ISP. We also have 50 years experience with handling LH2 on orbit, with existing flight hardware but relatively little experience with methane. So what about cryo-cooler TRL, just use passive thermal protection and deal with the modest boil-off, performance wise you are still way ahead.
Quote from: 2552 on 06/02/2010 09:30 pmThey also seem to be leaning towards a LOX/methane upper stage/depot (page 16).I'm not sure how much I agree with it, but here's the current rationale (presumably subject to change as the RFI proceeds) on preferring LOX/LCH4 over LOX/LH2:http://www.nasa.gov/pdf/458814main_FTD_CRYOGENICPropellantSTorageAndTransferMission.pdfQuoteCryogen Options Review: Why Methane (vs. Hydrogen)• A early LOX/Methane demo offers advantages:– Enables methane-based systems and mitigates risks for LH2 systems.– Allows direct comparison of active vs. passive cooling.– Leverages recent investments in LO2/LCH4 cryo fluid management– Leverages recent investments in pressure-fed engines– Breaks the barrier for long-duration cryo systems.• A LOX/Hydrogen demo in foreseeable future is possible.– Low cryo cooler TRL implies shorter mission duration.– No accurate gauging method for unsettled propellants.• Due to similarity of LOX and LCH4 properties (e.g. temp, density, etc.), the same components may be qualified and used for ground test and flight hardware.
They also seem to be leaning towards a LOX/methane upper stage/depot (page 16).
Cryogen Options Review: Why Methane (vs. Hydrogen)• A early LOX/Methane demo offers advantages:– Enables methane-based systems and mitigates risks for LH2 systems.– Allows direct comparison of active vs. passive cooling.– Leverages recent investments in LO2/LCH4 cryo fluid management– Leverages recent investments in pressure-fed engines– Breaks the barrier for long-duration cryo systems.• A LOX/Hydrogen demo in foreseeable future is possible.– Low cryo cooler TRL implies shorter mission duration.– No accurate gauging method for unsettled propellants.• Due to similarity of LOX and LCH4 properties (e.g. temp, density, etc.), the same components may be qualified and used for ground test and flight hardware.
It isn’t so much for EDS stages, it is more for in space propulsion in general. We know how to build LH2 rockets. However we don’t know how to build Methane ones and a methane engine could be useful for mar s landers or just a better in space propulsion system in general. Right now we can use hypergolic for long term propulsion (i.e. propellant for missions that last months or years). However hypergolic do not deliver the same performance as cryogenics (LH2 or Methane). LH2 is the hardest propellant to store in space. They haven’t totally ruled it out, but they think that it is going to be much too hard for a quick cheap mission to advance our confidence in propellant transfer technology. In theory a lox\methane rocket would offer better ISP than a hypergolic one. It would also give confidence to plans like Zurbin’s that require generating methane on mars. However the reality is a little murky due to the additional mass a lox methane rocket might need (insulation…although at mars distance the problem will be keeping the lox and methane from freezing in space).
As for ISRU the H2 for cracking the CO2 needs to come from somewhere. Is it from Mars water ice? Then you have H2 and O2. Hard to store in large quantities on the Martian surface. It isn't clear what the Mars ascent propellants should be H2, methane, pentane, or other higher order more storable hydrocarbon. Jumping to methane today seems a stretch.
NASA want to develop multi-mw electric thrusters but not the power source ( nuclear or solar ) to power them quite odd dont you think so ?
Quote from: isa_guy on 06/03/2010 04:12 pmNASA want to develop multi-mw electric thrusters but not the power source ( nuclear or solar ) to power them quite odd dont you think so ? Not odd. We already have solar panels which could do the job (150-200+W/kg) and larger solar arrays using thin-film cells could be much lighter (potentially 1000W/kg or even more) but would require more money.
We have 50 years of working with LH2 in space near zip with methane. I believe that a LH2 depot by 2015 is far easier to develop than a methane depot as long as we don't demand zero boil-off. The vent gas is useful for station keeping anyways.
It will pretty much always be about 10 times harder to cool liquid hydrogen than liquid oxygen or methane. This is just because it simply gets less and less efficient to remove heat the colder you get.
Quote from: Nancyloo on 06/03/2010 09:54 amWe have 50 years of working with LH2 in space near zip with methane. I believe that a LH2 depot by 2015 is far easier to develop than a methane depot as long as we don't demand zero boil-off. The vent gas is useful for station keeping anyways.Nothing wrong with having something else in your back pocket anyway. We have seen, too many times, systems that could benefit from R&D started years ago.Not dissing LH2, it's just not something I would have all the 'eggs in one basket' approach. If we solve LH2 storage as a multi-purpose propellant, that's great, but that may not help you everywhere you go.Anyway, all this is notional at this point.
Quote from: Robotbeat on 06/03/2010 04:46 pmIt will pretty much always be about 10 times harder to cool liquid hydrogen than liquid oxygen or methane. This is just because it simply gets less and less efficient to remove heat the colder you get.You make the assumption that one needs to actively cool the LH2. I don't know anyone working with hydrogen that proposes that. Hydrogen has 10 times the sensible heat capacity of oxygen and 4 times that of methane. With an efficient thermal design the heat of vaporization and vapor cooling is sufficient for propellant depots where one uses the vented hydrogen to provide cold gas station keeping. An actively cooled broad area system can be added to the MLI to intercept acreage heating at a moderate temperature as an added augmentation.
Quote from: Nancyloo on 06/03/2010 11:39 pmQuote from: Robotbeat on 06/03/2010 04:46 pmIt will pretty much always be about 10 times harder to cool liquid hydrogen than liquid oxygen or methane. This is just because it simply gets less and less efficient to remove heat the colder you get.You make the assumption that one needs to actively cool the LH2. I don't know anyone working with hydrogen that proposes that. Hydrogen has 10 times the sensible heat capacity of oxygen and 4 times that of methane. With an efficient thermal design the heat of vaporization and vapor cooling is sufficient for propellant depots where one uses the vented hydrogen to provide cold gas station keeping. An actively cooled broad area system can be added to the MLI to intercept acreage heating at a moderate temperature as an added augmentation.Robotbeat is making a safe assumption. The process is starting with hydrogen gas produced by the electrolysis of liquid water. That will require the hydrogen to be cooled by at least 250 degrees kelvin.Heat the super cooled ice up, melt it, perform electrolysis and liquefy the oxygen and hydrogen by refrigeration - that is going to be a high energy process. Even using heat exchangers.
That is accomplished here on Earth. To get people to the first Flexible path destination EM lagrange poitn you need LH2/LO2 in LEO. The LH2 can be launched already liquified, potentially even subcooled. No need for high power space rated cryocoolers, which don't currently exist. What you describe is an issue years (decades) down the road with ISRU.
Quote from: Nancyloo on 06/04/2010 12:36 amThat is accomplished here on Earth. To get people to the first Flexible path destination EM lagrange poitn you need LH2/LO2 in LEO. The LH2 can be launched already liquified, potentially even subcooled. No need for high power space rated cryocoolers, which don't currently exist. What you describe is an issue years (decades) down the road with ISRU.As it stands now LH2 is difficult to store for long periods. Meaning current upperstages can only contain enough LH2 to last for a few days not months or years. We need a cryogenic fuel that can be stored for months. Right now we lack the technology to do so. In theory LH2 storage is achievable, in reality it is harder to store than methane. NASA might change it’s mind and go for LH2 storage, but doubtful. Methane storage helps because you can use methane for propellant instead of LH2. Lox methane should give better ISP than hypergolics. The reality is more complicated. Hypergolic storage is known.
You are correct that cryo coolers are much easier at LCH4 temperatures than at LH2 temperatures. You are also correct that it is easier to store methane with zero boil-off than LH2.However there is no need for either of the above. Storing LH2 with modest boil-off is not particularly hard. This boil-off/vent GH2 is useful for station keeping anyways, so is not lost from a systems perspective. A depot could actually vent half of the hydrogen over the course of a year and still demonstrate substantial performance benefit over a zero boil-off methane system for even the smallest dV beyond LEO mission.
Quote from: Nancyloo on 06/04/2010 11:28 amYou are correct that cryo coolers are much easier at LCH4 temperatures than at LH2 temperatures. You are also correct that it is easier to store methane with zero boil-off than LH2.However there is no need for either of the above. Storing LH2 with modest boil-off is not particularly hard. This boil-off/vent GH2 is useful for station keeping anyways, so is not lost from a systems perspective. A depot could actually vent half of the hydrogen over the course of a year and still demonstrate substantial performance benefit over a zero boil-off methane system for even the smallest dV beyond LEO mission.It is not just the depots that need zero boil off technology, the spacecraft need it as well. In a 2 year return trip to Mars you can boil off a lot of hydrogen. The easiest way to account for the boil off could be to treat it as an extra stage with a tiny Isp.