1) Really Slow Route - SEP flight
Ideally what's desired is a chemical propulsive capture into the high orbit and then a gradual series of aerobraking leading to either an LEO docking or landing on Earth. Something safe yet fuel-efficient.
If you do propulsive capture you might as well leave the transfer vehicle in cis-lunar space. Use a smaller vehicle to return the crew.
Can anyone verify if ~0.75 km/s sounds about right for Earth capture? I've seen charts saying it could even be as low as 0.4.
Two remarks:1) Is there any reason you are not considering direct entry and landing of the crew, rather than capturing into an orbit first? (Guessing you want to reuse the transfer vehicle?)
2) How often would you get an opportunity to make use of lunar gravity assists? How constraining on mission planning would this be?
Quote from: Welsh Dragon on 04/21/2017 09:21 AMTwo remarks:1) Is there any reason you are not considering direct entry and landing of the crew, rather than capturing into an orbit first? (Guessing you want to reuse the transfer vehicle?) As I said before not so much discussing the vehicle but rather the conditions any vehicle may have to deal with. There are currently 3 main ways we could handle arriving at Earth (or technically most other planets): Electric/Ion Drive, Aerocapture, and Chemical rocketry. Chemical rocketry is the most straightforward of the bunch although ion drive the most efficent and aerocapture the-best-of-both-if-you-can-stand-the-heat. If you know about the physics of anyone of these I encourage contributing what you know of the math of their arrival physics.Quote from: Welsh Dragon on 04/21/2017 09:21 AM2) How often would you get an opportunity to make use of lunar gravity assists? How constraining on mission planning would this be?I wish I knew, although I heard lunar gravity assists go hand-in-hand with the Obereth Effect which requires velocity differences and tends to require routes favorable to chemical impulse or aerocapture. The only other bit of physics I know of regarding planetary bodies (such as our dear Luna) is that to slow down you must cross in front (so their gravity pulls on you and they absorb your velocity) and approach from behind for vice versa (so their gravity speeds you up and you boost off their velocity). I am unaware how large an effect the Moon could have, which is another reason I setup this thread to nail down such quandaries.
This paper would probably be useful: https://ntrs.nasa.gov/search.jsp?R=19800062349Max excess velocity that can be scrubbed with a lunar flyby is 1.9 km/s, or 2.2 km/s with a double flyby.