In a lot of conversations I’ve heard over the past year or so regarding COTS and its implications, many people trying to downplay the role of COTS (and explain why Orion has to be so heavy that only the Stick can launch it safely) have tried to play up the long duration features of the CEV. Basically they say “oh that Dragon capsule sure sounds nifty (if it ever flies), but it’s only capable of barely getting people to LEO, it doesn’t have all the features for exploration that the CEV does”. After all, it’s an exploration vehicle–it says so in the name!
Which is all well and good until you hear about what Orion doesn’t have. It doesn’t have a toilet (because nothing says exploration quite like Depends!) though it does have a “WCS”, it doesn’t have an airlock (so you have to depressurize the whole module if you need to do an EVA), and while much bigger than the Apollo CM, it only has about 19m^3 of volume inside (equivalent to a cube about 8.75ft on each edge, or smaller than the bathroom in my apartment complex). To be fair, it’s a lot more capable than the Apollo CM, or Dragon all by itself.
But all of that got me thinking about an idea I had a while back–combining Dragon and a Sundancer module for use as a long-duration space transport. So, I ran the numbers. Interestingly enough, once you factor in the fact that the Orion SM has to provide about 100-300m/s of delta-V just to make it to orbit (even with the CLV delivering them to -11x100km now instead of -30x100km), the CEV only delivers about ~1380m/s of delta-V once on orbit. Now, once you factor in extra propellants and tankage on the Dragon/Sundancer stack to bring its total delta-V up to that level (ignoring how much propellant Sundancer itself has because I have no numbers for it), the total “Initial Mass in LEO” for both designs are almost identical: ~51.7klb. The difference is that the Dragon/Sundancer stack could be launched on Falcon IX’s or Atlas V 401s.
Well, there’s that difference as well as the fact that the Dragon/Sundancer combo also get you nearly 10 times the interior volume (about the size of my 2-bedroom apartment), a toilet, and (I think) an airlock. Oh, and the possibility of taking 7 people instead of just 4. If I were going on a multi-week or multi-month trip to an NEO, you can be sure what I’d want to ride in.
Now, to be fair, there would need to be some changes to the stock Dragon/Sundancer combo for this mission. Dragon would need a beefed up heat shield (which is already in their plans), Sundancer might need a slightly beefed up life support system (since it’s only designed for three people for six months, it may not have enough “throughput” capacity to handle more than three or four people, and it is unclear how long Dragon could provide enough throughput for extra people), and there would probably be some communications and navigation upgrades you’d want for using those vehicles outside of LEO.
But it is interesting that they come out to such similar masses.
Now, one last thing. While a Dragon + Sundancer could probably be used as a CEV replacement, and could probably be used for a lower price than an Orion + Ares I, that’s not the optimal way to use it. You’d preferably not want to toss away the Sundancer module after every mission if you could avoid it, which leads you to entirely different mission modes, and technologies like aerobraking. But I’ll go into that more, some other day.