Um.. thanks a little? Plugging those numbers in gave me 4.0 years, but that is relying on my algebra and zero inefficiency. Isn't 40kW just the energy that goes in?
I had a go at estimating the difference of the SEP tug could make.Found these numbers (they may not be latest)SEP tugTotal: 15.5tPropellant: 11.8tISP: ~3000(I remember something about the mission taking 6 years, so Im assuming that is how long to exhaust fuel.)
Yes I was thinking about the other direction, how small can we go while not being unrealistic.
Building a human-scale station out of 3-5T modules launchable to anywhere in the Earth system via F9R is much harder, for the class of missions which don't do any assembly in LEO and rely on purely chemical propulsion. It's also more expensive at the outset to launch three F9R missions than one FH mission, without taking into account the complexity of assembly.
Making a lunar ferry and lander from an already flight-proven upper stage and using depots is a very viable strategy. Masten Space System has been doing some work on a centaur-derived lander called XEUS.I thought I'd add a couple notes on orbits:-Nodal precession makes it impossible to permanently align the orbit of a LEO depot with the orbit of the moon. So using a permanent LEO depot as a staging point for cis-lunar missions means you will have to eat a delta-v penalty for the inclination change. Since you can do it near apoapse, i.e. include it in the lunar capture burn, the delta-v cost isn't too high but doing that will limit the number of launch windows to two per month. This can be circumvented entirely by using temporary staging points where the transfer stage is being refilled directly by tankers launched from the ground.
NASA can already do Gemini-style missions without a depot. Orion on D4H, 28-45t stage on Falcon heavy (can be small enough for a D4H, too, if hydrolox), rendezvous with a pre-launched lander at EML2.
I don't think RobotBeat is arguing against depots. You could just keep the human part nice and simple: go directly to the moon and meet your lander (which is effectively your depot) there. If you don't meet it, you still have everything to get home.For Apollo I think the entire lander (ascent + descent) was under 15 tons, and with space storable propellant.I think we could probably do a really nice mission with two FH if you exploited SEP to get your lander there ahead of time. I bet that has already been looked at in another thread somewhere.
Quote from: Nilof on 07/12/2014 08:47 amMaking a lunar ferry and lander from an already flight-proven upper stage and using depots is a very viable strategy. Masten Space System has been doing some work on a centaur-derived lander called XEUS.I thought I'd add a couple notes on orbits:-Nodal precession makes it impossible to permanently align the orbit of a LEO depot with the orbit of the moon. So using a permanent LEO depot as a staging point for cis-lunar missions means you will have to eat a delta-v penalty for the inclination change. Since you can do it near apoapse, i.e. include it in the lunar capture burn, the delta-v cost isn't too high but doing that will limit the number of launch windows to two per month. This can be circumvented entirely by using temporary staging points where the transfer stage is being refilled directly by tankers launched from the ground.If the goal is the poles, you want a transfer orbit inclined with regard to the moon's orbit. I imagine transfer orbits coplanar with the earth's equator.. No nodal precession for an equatorial LEO depot. And equatorial orbits are easier to reach from earth's surface as well.As you mention, a lunar transfer orbit's apogee is quite slow. So an ~20º direction change at apogee isn't a big hit on delta V. The big disadvantage is loss of anytime return. Lunar polar orbits would have two per month launch opportunities, as you say.
I don't think RobotBeat is arguing against depots. You could just keep the human part nice and simple: go directly to the moon and meet your lander (which is effectively your depot) there. If you don't meet it, you still have everything to get home.
For Apollo I think the entire lander (ascent + descent) was under 15 tons, and with space storable propellant.
"Start in an equatorial low earth orbit at 300 kilometer altitude."Hmm, that's sort of a "spherical cow in vacuum" sort of thing. How about from an orbit that's reachable from KSC (or Boca Chica :-) )? Cheers, Martin