Quote from: Nilof on 01/10/2018 09:39 pm...No, the increased Isp is not just about reducing mass to IMLEO. It's about making your spacecraft physically capable of performing it's mission. Chemical propulsion has a maximum delta-v per stage, and if you go past that you are forced to introduce staging.For example, an Earth to Ceres or Earth to Mercury transfer is relatively straightforward with electric propulsion, but requires multiple expendable stages if done with chemical propulsion. Mars missions can be done without staging for near-minimum energy transfers only because of hard aerobraking. It is physically incapable of doing an emergency Earth to Mars or Mars to Earth transfer outside of transfer windows.Secondly, in the outer solar system, Beaming power from the inner solar system is very viable for ships with solar arrays in the square kilometer range and up. SEP with beamed power from the inner solar system will generally have significantly higher performance than any plausible NEP design. ...Square kilometer* arrays and up?(!!!) PPE at 40kW is the biggest SEP currently envisioned... by the time you are driving around on square kilometer solar arrays, chemical rocketry could be in the full colonization mode. SEP will have near zero role because it is so trivial in capacity. It is like parachutes on Mars... nice for the one tonne payload, but doesn't scale.* ISS has 2,500 sq meters generating about 100kw -- 0.25% of a single square kilometer. So, take ISS arrays times 400... 40 MegaWatts -- easy peasy. PPE is 40kW... so just strap 1,000 PPEs together and off you go.
...No, the increased Isp is not just about reducing mass to IMLEO. It's about making your spacecraft physically capable of performing it's mission. Chemical propulsion has a maximum delta-v per stage, and if you go past that you are forced to introduce staging.For example, an Earth to Ceres or Earth to Mercury transfer is relatively straightforward with electric propulsion, but requires multiple expendable stages if done with chemical propulsion. Mars missions can be done without staging for near-minimum energy transfers only because of hard aerobraking. It is physically incapable of doing an emergency Earth to Mars or Mars to Earth transfer outside of transfer windows.Secondly, in the outer solar system, Beaming power from the inner solar system is very viable for ships with solar arrays in the square kilometer range and up. SEP with beamed power from the inner solar system will generally have significantly higher performance than any plausible NEP design. ...
These SEP vehicles can act as a force multiplier for BFR. In cislunar space, a fleet of them enables one BFR launch to put > 100 tonnes in EML-1. I'd guess that's an improvement by a factor of around 4 compared to BFR refuelling in LEO and carrying the payload itself.For Mars one could imagine pushing chemical prop to EML-1 or HEO or into Mars orbit, thereby expanding the launch windows, reducing the transit time, and/or increasing the payload that can be carried.
It should be possible to accelerate fine regolith to a few 1000m/s somehow. E.g. a bucket full of regolith driven by a linear electric motor. Why wouldn't it work? It would probably benefit from being very long, but remember that we are talking about "settlement scale" here, not small space probes like dawn.
Quote from: Exastro on 01/13/2018 12:36 amThese SEP vehicles can act as a force multiplier for BFR. In cislunar space, a fleet of them enables one BFR launch to put > 100 tonnes in EML-1. I'd guess that's an improvement by a factor of around 4 compared to BFR refuelling in LEO and carrying the payload itself.For Mars one could imagine pushing chemical prop to EML-1 or HEO or into Mars orbit, thereby expanding the launch windows, reducing the transit time, and/or increasing the payload that can be carried. This is exactly right; CEP is an extra which will be most suitable for delivery of large masses which don't mind radiation and the use of which will relieve limited transport resources. SEP may also open up Mars orbital activities (smaller gravity well and no radiation belts).
How large of masses do you imagine? ...what you are probably doing is thinking in terms of tiny, 1-3t landed payloads that we've been struggling to get delivered as status quo, and electric tugs could deliver cargoes that are factors of several tens larger (10-30 t?). But then what do you do with them once they've spent two years in transit and another in aero-braking? Who carries them to the ground?BFR is supposed to be capable of landing 150t... three months or so after it departs Earth. If you were on Mars, which supply line would you prefer?