A reasonably functional depot (with sun-shade, solar power, active-cooling, safe haven etc) will have reasonably large mass and volume. Think something SkyLab size here. While it would be possible to launch one in several pieces and then launch crew to assemble it, a single 50t+ HLV with a big payload fairing would make the whole process (& depot design) a lot simpler.
Next consider the need to launch several depots, plus replacements over time. Possibly 8 HLV launches compared to how many EELV size launches? Plus expensive crew assembly missions. I'd rather send the crews exploring.
Then consider the in-space tugs to move propellant from one depot to the next. Yes, an EELV could launch a complete expendable tanker spacecraft each time, but it would be more efficient to just launch full tanks. Then have a small fleet of high effciency (SEP?) tugs take them to the depots. Those tugs would tend to be rather large, again benefitting from HLV launch.
Throw in the requirements of crew trans-habs, Mars EDL vehicles, lunar landers and you begin to see how moderate HLV, (50t+, big payload fairings), complements a propellant depot architecture.
150t+ HLV? No. 50t to 100t? Yes.
Quote from: kkattula on 08/08/2009 04:30 pmA reasonably functional depot (with sun-shade, solar power, active-cooling, safe haven etc) will have reasonably large mass and volume. Think something SkyLab size here. While it would be possible to launch one in several pieces and then launch crew to assemble it, a single 50t+ HLV with a big payload fairing would make the whole process (& depot design) a lot simpler.A dry fuel functioning cryogenic fuel depot in the 35-40t range (J-246 capacity to EML-1) will have the capability to store about 5 times its dry mass or up to 200t. That's enough for several lunar sorties or interplanetary missions from your EML-1 space station + fuel depot. That means, your depot can be launched with a single HLV launch to EML-1.Once you have advanced propulsion technology online like VASIMR (or other electrical propulsion) you can add another depot for a different type of fuel (argon for VASIMR).That being said, I don't see why you need 8 HLVs or a Skylab-sized depot in any event.
Why would any depot be big? A Centaur weighs 2 tonnes empty and can house 20 tonnes of LOX-LH2.In space tankage weighs peanuts.
And if you are using NTR, why not use ammonia?
Quote from: meiza on 08/09/2009 05:01 pmWhy would any depot be big? A Centaur weighs 2 tonnes empty and can house 20 tonnes of LOX-LH2.In space tankage weighs peanuts.MMOD protection might be fairly heavy. And even 10% is a sizeable percentage of total fueled mass.
I wonder, how corrosive is anhydrous NH3 with Al-2195 alloy?Could ammonia be stored in typical propellant tanks for long periods of time?
In space tankage weighs peanuts.
The Mars DRM assumes 65mT chunks, and with the delta-v numbers above that means less than 65mT in propellant. A depot that contains such amounts of propellant can fit easily within an EELV fairing.Note that the DRM also assumes NTR. With L1 staging you don't need the nukes (and you can reuse the ITV). And even if you do want them, L1 will likely give a lot less political problems than LEO. In that case you would not want to use an Earth swingby obviously. And if you are using NTR, why not use ammonia? Much higher density, and it avoids those allegedly difficult cryogenic depots.
Ammonia has lower Isp than LH2, reducing the advantage of going NTR.
Would multiple depots really be the most economical investment for a single flag planting mission to the moon and a second flag planting mission to Mars? If NASA only launches one or two missions per year, is a depot arcitecture too expensive to maintain?If, as I suspect, Depots are most effective when used often (missions in bulk), is there any indication what-so-ever that NASA and Congress would support a program that involves 6-12 missions per year?
While I find the concept of small EELVs launching small rockets that can 'fill-er-up' at depots located at multiple Lagrange points fascinating, isn't this the same basic fallacy that caused the Shuttle to fail to meet projected cost and performance goals? Overesimating demand and building an expensive but underused infrastructure?
Would multiple depots really be the most economical investment for a single flag planting mission to the moon and a second flag planting mission to Mars? If NASA only launches one or two missions per year, is a depot arcitecture too expensive to maintain?
If, as I suspect, Depots are most effective when used often (missions in bulk), is there any indication what-so-ever that NASA and Congress would support a program that involves 6-12 missions per year?
Don't get me wrong, I would love to see a permenant manned base on the moon. I would love to see a skylab over Mars scouting locations for Martian outposts. Depots might help make that more affordable. I just feel like I am very much in the minority when it comes to spending money on space and don't want to spend the limited budget creating depots that will be used 1-2 times in a decade before joining the unused Saturn V Launcher, unused 2nd Skylab, retired early Shuttles and unused Ares I as another waste of potential and valuable resources by a fickled vision for space.
How many 65t chunks? The every two years launch window is not that large. Are you going to re-fill the depot in a week? What if something goes wrong? Surely you want enough propellant in the depot for the whole mission, before you launch the high value payloads? Otherwise it's not a depot, just propellant transfer.
Would multiple depots really be the most economical investment for a single flag planting mission to the moon and a second flag planting mission to Mars?
Sure, but from L1/SEL-2 you don't need nukes or even cryogenics. Isp of 600s is still very good performance. I'm not in favour of nukes on the critical path, but I am in favour of developing NTR as a later upgrade. Similarly I'm not in favour of putting cryogenic depots on the critical path, but definitely in favour of developing them as later upgrades. In this case even more so because of political considerations. I'm even more strongly opposed to HLV, not just on the critical path, but even as a later upgrade, although it would be good to keep the option as a backup. EELV Phase 2 could be that option.I would assign the following priorities:1) depots (initially hypergolic ones)2) ISRU3) SEP4) manned surface bases5) NTR (using ammonia if payload fairing size is an issue)
QuoteAnd if you are using NTR, why not use ammonia?And if you use ammonia, why not use arcjet thruster and solar electric propulsion ? Ammonia NTR = 600 s ISP. Ammonia arcjet / SEP = 1000 s ISP. The more I look at ammonia, the more I like it. To my knowledge, its the only fuel which is, altogether (!)- hypergolic (with H2O2) - "kerosene" (with LOX = ISP is similar) - Thermal propulsion (either solar or nuclear) - electric propulsion (if arcjet thrusters are used) Talk about a versatile propellant ! It is also dense, midly cryogenic (-34°C), easy to ISRU (nitrogen and hydrogen are abundant in solar system bodies). Last neat thing with NH3: it may replace gasoline in cars. At least it is much easier to handle than LH2. And you benefit from the fertilizer networks and infrastrctures. "Ammonia economy" anybody ? What a noble mission for human spaceflight: bring ammonia back to Earth, to fuel the World economy...