Quote from: TheRadicalModerate on 10/01/2022 06:13 pmI believe that laddering makes sense when extreme C3 outweighs the need for mission simplicity. However, for both lunar and Mars missions, that's simply not the case. In the case of an Artemis mission, you'll want to limit the LSS to a single refueling, simply to reduce risk. That requires limiting the maximum refueling energy to something that the LSS can reach with its launch prop. In the case of a Mars mission, with crew already on board, a single refueling in VLEO is more than adequate.E = 1/2*mv2The other type of high energy mission is sending a lot of mass. Laddering through a high energy Earth orbit significantly* increase the mass that can be sent to Mars in a single ship. Fewer ships tied up in transit and fewer ships that need refueling at Mars.I think we differ in that you think refueling is complex, risky and difficult. I think refueling will become routine and easy. Up to now almost all of the rendezvous and docking has been in human space flight. Not only are human pilots bad at this but the risk to humans has resulted in extreme conservatism and overly complex procedures. I expect SpaceX to try new procedures, blow up a few robots and end up with something far faster, safer and more reliable.* By a factor of 13 if you ladder through a trans-moon injection orbit and use aero braking from Mars transfer orbit down to low Martian orbit or the surface. This allows an absurd mass of almost 8300t of cargo to Mars in a single SS. This would allow for example a storm shelter with 10m of water shielding. (Fitting 10m of water in a 9m SS is left as an exercise, perhaps it would be better to use aluminum.) Laddering through GTO "only" increase the cargo by a factor of 4.75, which is still very useful.
I believe that laddering makes sense when extreme C3 outweighs the need for mission simplicity. However, for both lunar and Mars missions, that's simply not the case. In the case of an Artemis mission, you'll want to limit the LSS to a single refueling, simply to reduce risk. That requires limiting the maximum refueling energy to something that the LSS can reach with its launch prop. In the case of a Mars mission, with crew already on board, a single refueling in VLEO is more than adequate.
You need less propellant to go to Mars than land on the Moon, let alone do the ~7-8km/s mission that Starship HLS needs. So no 1200t depot required. 3.7-3.9km/s or so (depending on opportunity… 3.5km/s for TMI, 200-400m/s for landing) is about the minimum amount. 500t would even be enough.
Also, quit taking TheRadicalModerate’s figures for the Moon as gospel.Pretty annoying when people who post long pages about what “MUST” be required for lunar missions are taken as gospel because they post a lot, while making unqualified assumptions about dry mass that drive their conclusions.…yes, yes, glass houses and stones, but you shouldn’t take my prognostications as gospel either!
If you use overly conservative assumptions, your conclusions about what is optimal to do is going to be wrong. This is partly why NASA assumed a 3 stage lunar lander but SpaceX is using one stage. Conservative dry mass leaves you with a whole bunch of stages.
Quote from: Robotbeat on 10/02/2022 12:34 pmYou need less propellant to go to Mars than land on the Moon, let alone do the ~7-8km/s mission that Starship HLS needs. So no 1200t depot required. 3.7-3.9km/s or so (depending on opportunity… 3.5km/s for TMI, 200-400m/s for landing) is about the minimum amount. 500t would even be enough.Ah, so the key things that make this possible (correct me if I'm wrong) are a) The Mars trip is one-way b) using the Mars atmosphere for braking saves you a lot of delta-v c) Although the trip to Mars takes much longer, the delta-v to go from LEO to a minimum energy TMI isn't really very much more than what's required for TLI. Is that a fair summary?
If two fully fueled ships leave LEO docked with crossover plumbing, the attached tanker can empty in a very few minutes. Tanker with 1,500 tons of fuels and LSS with 1,200 tons of fuels at ignition. Tanker empty and detached at about 2,500 m/s while fully fueled LSS continues on to the Lunar mission. Tanker falls back from apogee reenters to base. Skimmed through a lot of this discussion without seeing this possibility suggested.
As an aside but still topical for this in-orbit refueling discussion: We are focusing here on good strategies for orbits, ΔV, proximity operations, and fuel settling, all of which is fine. But the risk that worries me is the risk of chopsticks landing accidents. Fireballs have bad optics. Falcon 9 boosters land superbly, but soon SH/SS refueling needs even higher precision landings.
But this requires engineering a nose-to-tail docking mechanism, which isn't straightforward. I suspect that there were good reasons why SpaceX gave up on tail-to-tail docking, and this would be worse. However, I also suspect that this would be easier to engineer than side-by-side burns with cross-feed.
My thought on the side to side docked and cross feed is along the lines of the heavy versions of Delta and Falcon with two cores instead of three. If it were three, the mass ratio would put it beyond escape and getting the boosters back would be a problem. Plus the extra dry mass of another core.
The tanker pressurant would be the same in either case to empty the tank of liquid whether it is in x minutes or 1/2x minutes.
It seems the tanker would not necessarily need a burn at apogee to put it on a reentry trajectory as long as the TLI trajectory was creating an HEEO with perigee at reentry altitude.
Quote from: redneck on 10/03/2022 01:15 amIf two fully fueled ships leave LEO docked with crossover plumbing, the attached tanker can empty in a very few minutes. Tanker with 1,500 tons of fuels and LSS with 1,200 tons of fuels at ignition. Tanker empty and detached at about 2,500 m/s while fully fueled LSS continues on to the Lunar mission. Tanker falls back from apogee reenters to base. Skimmed through a lot of this discussion without seeing this possibility suggested.How fast props can be transferred is an open question. IIUC, you're suggesting the two ships be locked together while thrusting. This is done with FH at launch but my gut says that this is different enough to need some serious testing. Not a deal breaker but it is something new in the mix.