Clever, I've never considered having the lander leave its return fuel in LLO.
Quote from: TrevorMonty on 11/23/2016 08:24 amClever, I've never considered having the lander leave its return fuel in LLO. I thought of this technique quite a long time ago. It was called Lunar orbit propellant transfer with the idea of transferring LOX produced on the Moon to the next craft coming in to land. I was a bit bummed when my paper got rejected by the BIS.
I love the idea of leaving propellant in orbit for the return flight. The problem with this is it only works in a constant chain of flights. Maybe the ITS would stay on the moon until the next mission. Like crew capsules on the ISS. I am not sure this is the best operation though. Conditions on the moon are harsh for a spaceship with the long days and long nights. Very different to LEO or deep space.Alternatively there could be a depot in lunar orbit. ITS fills it on the way down and takes the fuel back on the way up. The depot would not even need to be very large.
One important aspect is thrust levels while landing on the Moon. Here's the case using two of the vacuum Raptors which have 3.5 MN thrust each. I know Musk said that these are fixed, but for Lunar missions they could be made to swivel. I don't like the idea of switching from vacuum to inefficient sea level engines for the Lunar landing.
Quote from: Steven Pietrobon on 11/23/2016 08:28 amOne important aspect is thrust levels while landing on the Moon. Here's the case using two of the vacuum Raptors which have 3.5 MN thrust each. I know Musk said that these are fixed, but for Lunar missions they could be made to swivel. I don't like the idea of switching from vacuum to inefficient sea level engines for the Lunar landing.I don't see how gimballing can be implemented without making the nozzles a lot smaller. There isn't enough room for any movement. Its not necessary anyway, differential thrust plus RCS can provide sufficient control authority in all 3 axes with a smaller performance hit (which is how attitude control during TMI/TEI is planned, and most likely during Mars landing)
Space stations, tankers and propellant depots in lunar orbit will need station keeping. A set of ion thrusters should be able to supply this.
Quote from: brickmack on 11/23/2016 07:01 pmQuote from: Steven Pietrobon on 11/23/2016 08:28 amOne important aspect is thrust levels while landing on the Moon. Here's the case using two of the vacuum Raptors which have 3.5 MN thrust each. I know Musk said that these are fixed, but for Lunar missions they could be made to swivel. I don't like the idea of switching from vacuum to inefficient sea level engines for the Lunar landing.I don't see how gimballing can be implemented without making the nozzles a lot smaller. There isn't enough room for any movement. Its not necessary anyway, differential thrust plus RCS can provide sufficient control authority in all 3 axes with a smaller performance hit (which is how attitude control during TMI/TEI is planned, and most likely during Mars landing)This has been discussed elsewhere, but the lag inherent in differential thrust makes control a lot harder. Not undoable but gimballing makes avoiding overshoot a LOT easier.
Quote from: A_M_Swallow on 11/24/2016 04:11 amSpace stations, tankers and propellant depots in lunar orbit will need station keeping. A set of ion thrusters should be able to supply this.Too small for two thousand tonne tankers. Fully loaded ITS tanker is 5-6x ISS mass, and lunar orbits are unstable for most part. Fine for Orbital Outpost (maybe 100 tonnes) in high orbit or EML-1/2.
Might I suggest a simpler approach? A dedicated ITS lander with large on-board battery packs and which transits between the surface and a man-tended depot, where not only are gasses stored but battery maintenance and cargo transshipping occurs. Leave the solar arrays in orbit, too, or at the very least work out a schedule which will deliver batteries to the surface to allow overnight stays. Crews would travel uphill aboard a 'standard' ITS without Lunar landing capability, but also carrying cargo. Use the depot to offer a shirtsleeves cargo transfer facility without EVAs or manipulator arms, and build up a bespoke delivery service for third-party exploration and development.
I don't see how gimballing can be implemented without making the nozzles a lot smaller.
(estimates I've seen before being ~30 tons to the lunar surface with only refueling in LEO, ~350 tons with refueling in lunar orbit after ascent, even the first of which is well beyond what previous studies indicated necessary to build a base)
Quote from: AncientU on 11/24/2016 09:08 pmQuote from: A_M_Swallow on 11/24/2016 04:11 amSpace stations, tankers and propellant depots in lunar orbit will need station keeping. A set of ion thrusters should be able to supply this.Too small for two thousand tonne tankers. Fully loaded ITS tanker is 5-6x ISS mass, and lunar orbits are unstable for most part. Fine for Orbital Outpost (maybe 100 tonnes) in high orbit or EML-1/2.Steven Pietrobon only talked about leaving 105.1 t of propellant in lunar orbit.
Quote from: A_M_Swallow on 11/25/2016 12:38 amQuote from: AncientU on 11/24/2016 09:08 pmQuote from: A_M_Swallow on 11/24/2016 04:11 amSpace stations, tankers and propellant depots in lunar orbit will need station keeping. A set of ion thrusters should be able to supply this.Too small for two thousand tonne tankers. Fully loaded ITS tanker is 5-6x ISS mass, and lunar orbits are unstable for most part. Fine for Orbital Outpost (maybe 100 tonnes) in high orbit or EML-1/2.Steven Pietrobon only talked about leaving 105.1 t of propellant in lunar orbit.In OP, calculation with Lunar orbit refueling after ascent from surface delivered 180.6 tonnes, then 105.1 for the spaceship that supplied the on orbit fuel. (The second ship would need to be refueled like the first.)
Attached is my Pascal program used to work out the payload.
Quote from: brickmack on 11/25/2016 01:57 am(estimates I've seen before being ~30 tons to the lunar surface with only refueling in LEO, ~350 tons with refueling in lunar orbit after ascent, even the first of which is well beyond what previous studies indicated necessary to build a base)Is there a reference for that 30 t value? My calculations show that a Direct Lunar mission (using refueling in LEO) gets a cargo mass of -36.7 t, which means the scheme won't work since you need a have a payload with negative mass!