I noticed during the DIRECT presentation at today’s HSF public meeting, that they were asked why they would need an HLV if they had depots.Â Now, I didn’t hear the exact question, since I had a phone call come in just a few minutes earlier (ironically enough from one of my friends in the depot community), but I think the questioner was asking about drylaunching a lunar stack on an EELV and then tanking up at a depot.Â The reply given was that while an EELV could loft the mass, it couldn’t handle the volume.
Now, I have a lot of respect for the DIRECT guys, and have to give them kudos for even mentioning depots, but the fact is that this argument for heavy lift isn’t anywhere near as solid as it appears on the surface.Â The fact is that there are many ways you could use dry-launch/propellant depot techniques to do ESAS-sized missions without the need of a big fairing or an HLV.Â It may be true that you can’t cram the current Altair conceptual design into a Delta-IV fairing, however there are several legitimate alternatives out there to the current Altair concept, that can do the same job without requiring anything bigger than the already massive 5m fairings that come with existing EELVs.Â And it’s important to remember that Altair is still in the early conceptual phase, where even fairly significant changes don’t really cost that much yet.
I want to keep this post brief, so I’ll just list a few options out there:
- Horizontal landers: I don’t have the latest numbers, but the most recent numbers I’ve found show the LSAM descent stage holding only a little more propellant than a Centaur stage.Â A horizontal lander of the type described by ULA in their papers could easily fit within the fairing of an EELV.Â Even more so if based on 5m diameter tanking.
- Crasher stage landers: It’s possible to split a lander up such that the descent burn is mostly done by a “crasher stage”, which is dropped shortly before the final landing burn (with shortly possibly being over a minute before).Â This means that your actual lander stage can be a lot smaller and more compact.
- EDS TLI burn: There’s nothing that says the LOI burn has to be done by the LSAM.Â An EDS that’s big enough to do the TLI burn can still be fit within an EELV fairing, especially if using a 5m diameter stage like the Common Upper Stage.Â With the lander descent stage only having to do the actual descent burn, it can be a lot more tightly packed.
- LLO depots: If you have a depot in lunar orbit that is regularly topped off, you can tank up the lander stage after doing the LOI burn and before doing the landing.Â This allows for less tankage, since you don’t have to size the lander for both LOI and lunar descent.Â Alternately, if you have a reusable lunar lander (ie an SSTO lander designed to work with depots), you can send that lander independently from LEO to to the lunar vicinity.
- L2 rendezvous: Having the CEV separate from the lunar stack prior to LOI, and then perform its own powered swingby maneuver greatly reduces the size of the lunar stack that needs to perform the LOI burn.Â At this point having either the EDS or the lander do the burn allows for a much smaller lander.
And the list could go on and on.Â Basically, so long as you don’t stick to “black aluminum” lander and transfer stage strategies, you can actually use depots to enable ESAS-equivalent landings without needing HLVs or big payload fairings.Â There may be other arguments for big fairings (Mars reentry shields if we can’t get hypercones or rocket decelerators to do the trick, other large payloads, who knows), but this isn’t it.
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