QuoteWhy do you insist on having the thrust of a J-2X?That's what it takes to boost the Ares V EDS into orbit. The stage will weigh something like 280 tonnes at ignition - more than an entire Delta IV Medium! RL10 only produces something like 10-11 tonnes of thrust. Imagine an EELV needing to be powered not long after liftoff by an RL10 cluster!
Why do you insist on having the thrust of a J-2X?
If not Ares V and EDS, then some other big stage will be needed for the lunar mission, even for a dual-launch architecture, and it will still need a lot of thrust to make it into LEO. If not 12 RL10s, then maybe 11 - and 22 or 24 total for a single dual launch mission (about as many RL10s as have flown in this entire *decade*). If a TLI-only stage is used, which would mean less thrust for the TLI burn, another ascent stage would still be needed, again with lots of thrust but requiring the launch vehicle to carry even more expensive engines on even more stages, etc..
DIRECT proposes using an EDS with 6 RL-10s and you can still send something like 70mT through TLI together. With L1 rendez vous you can even make do with a DHCSS, with LOR-LOR you would need a slightly larger stage.
The DIRECT literature I can find talks about an upper stage with *two* J-2X engines to power a stage that weighs about 100 tonnes *more* than the Ares V EDS. http://www.launchcomplexmodels.com/Direct/documents/DIRECT_Summary_v2.0.2.pdf
Why do we need such a big EDS? Because NASA chooses to do EOR-LOR instead of LOR-LOR
LOR-LOR means LSAM and CSM are each launched with their own EDS? Sure, that would allow for a smaller EDS configuration, but the combined dry mass of the two stages through TLI would be higher.
Isn't the better criticism of the CxP mission profile that the EDS has to lift the LSAM to LEO and is therefore sized larger than if it had been launched on its own?
EOR-LOR isn't the problem. The 1.5 launch is the problem.
Apollo 13 is Exhibit A in the case for EOR-LOR. If the crippled CSM had to coast to cislunar space and perform an LOI burn before docking with the LM, then the crew would have been lost. If the CSM and LM were to have docked in LEO for checkout before the TLI burn instead of after, then the failure would have occurred much closer to earth and the abort would have involved far less risk of crew loss.
If we had the booster to lift another single-launch lunar mission, it would still make sense to do EOR-LOR. If we use two launches, EOR-LOR is practically imperative for manned missions. We can't responsibly send the CSM beyond LEO without the spare life support vessel and propulsion stage provided by the LSAM.
The Orion doesn't have a backup on the return trip, I don't see how no backup is acceptable during return from the moon, but irresponsible on the way to the moon.
Yeah, I thought of that. You can get the redundancy on the departure trip at very little cost (some extra consumables to operate the LSAM) because all of that mass is going to lunar orbit anyway.
So it's irresponsible not to provide all those added crew safety contingencies. On the return trip, you'd have to push the ascent stage of the LSAM through TEI, so that's more of a performance vs. capability trade-off.
I was actually considering asking on the DIRECT thread about the feasibility of retaining the LSAM ascent stage through the bulk of the return trip. I'm sure the crew would appreciate the extra cabin space.
Any way you slice it, a more-or-less healthy CM is required for reentry. I can imagine some contingency plans involving an LEO rendezvous with either a LON rescue CSM or an ISS layover that could possibly save a crew that resorted to the LSAM lifeboat due to a CM failure that would preclude reentry.
Quote from: butters on 05/02/2009 04:58 pmYeah, I thought of that. You can get the redundancy on the departure trip at very little cost (some extra consumables to operate the LSAM) because all of that mass is going to lunar orbit anyway.I wouldn't say at little cost, because the JUS is very expensive to develop.
I've become fond of an architecture that uses hypergolic refueling at L1 anyway. L1 is such an excellent staging location and depots are so powerful that even with the low Isp of hypergolics, cargo missions are actually more efficient than current plans. And once boil-off mitigation is developed and cryo depots become operational, it would become even more efficient. And L1 is not only useful for trips to the moon, but also to other locations in near-Earth space.
You need the JUS to push both spacecraft through TLI anyway. It's just a question of whether Orion docks with Altair before or after TLI.
I'm only vaguely familiar with the practical usage of Lagrangian points as rendezvous locations for space exploration. This is something I should read more about.
And when I said cost, I meant payload efficiency, not development cost.
Quote from: mmeijeri on 05/02/2009 11:41 amDIRECT proposes using an EDS with 6 RL-10s and you can still send something like 70mT through TLI together. With L1 rendez vous you can even make do with a DHCSS, with LOR-LOR you would need a slightly larger stage.The DIRECT literature I can find talks about an upper stage with *two* J-2X engines to power a stage that weighs about 100 tonnes *more* than the Ares V EDS. http://www.launchcomplexmodels.com/Direct/documents/DIRECT_Summary_v2.0.2.pdf - Ed Kyle
One potential solution is to use a modified Altair ascent stage as a mission module.
And note that the Orion RCS is sized to be able to function as a backup for the TEI. Or at least it was at some point.
The ascent stage is extremely light ~3.3mT burnout. This is partly achieved by leaving the life support system behind in the lander. The crew can survive for some hours on stored consumables, then things get a bit sticky...
RCS & MPS share the same propellant, and similar Isp's. By routing prop through the RCS system, similar gross delta-V can be achieved. Since thrust is only a few percent of MPS, though, gravity losses will be much higher, and the thrusters will have to burn for a very long time.
Quote from: edkyle99 on 05/02/2009 02:24 pmThe DIRECT literature I can find talks about an upper stage with *two* J-2X engines to power a stage that weighs about 100 tonnes *more* than the Ares V EDS. http://www.launchcomplexmodels.com/Direct/documents/DIRECT_Summary_v2.0.2.pdfSince then they have switched to SSME as their baseline core engine, in part because it allows switching to RL-10 as an upper stage engine in case there isn't enough money to develop J-2X. The new configuration is called J-246.
'Kinda hard to discuss something that keeps changing.
Quote from: edkyle99 on 05/02/2009 08:42 pm'Kinda hard to discuss something that keeps changing.Good thing Constellation is so stable.
How, by the way, do six RL10B-2 engines fit? They need a lot of clearance for those big French nozzle extensions.
Ah, I had wondered how its dry mass could be so low. In that case, one of the required modifications would be to add full-size life support systems...
Every gram added to the LSAM ascent stage has a massive multiplier throughout the whole system. It's the most mass-critical part of the whole mission- you'd be better off building in the required margin in Orion, and then not taking it down to the surface and back.