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A while ago, I asked a similar question here, but I think it was confusing, so nobody answered.

Two questions:

  • This one is a bit subjective: Do you think that lengthy (+5y) multi-flyby trajectories heading to either Mercury or the outer planets (Galileo, Cassini, MESSENGER, BepiColombo) will ever be considered for spacecraft carrying humans onboard? or will this destinations not be worth pursuing until there are cheaper methods that allow for vastly faster direct trajectories and deceleration upon arrival?

  • If yes, then, is it possible for the passengers to board the spacecraft on the last possible Earth flyby, in order to shorten the flight time, or are the orbital parameters inadequate to allow for a Soyuz-ISS kind of rendezvous?

Thank you very much.

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    $\begingroup$ As long as a return to Earth from a multi-flyby trajectory is far beyond our capabilities, a manned mission is impossible. $\endgroup$ – Uwe Apr 10 at 11:14
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    $\begingroup$ It is interesting to note that, during the Space Shuttle program, Hohmann transfer orbits (which are the most efficient way, in terms of propellant consumption, to transfer from one orbit to a different, designated orbit) were typically not used (after the OMS-2 circularisation burn - which was generally Hohmann-like, if not pure Hohmann) for planned rendezvous burns, as they tended to take too long. $\endgroup$ – Digger Apr 10 at 16:54
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    $\begingroup$ @Uwe Or rather, politically awkward. :) $\endgroup$ – Asteroids With Wings Apr 11 at 13:20
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Given the mass costs in terms of consumables and the risk and support costs of keeping humans in space for longer, it seems unlikely that the multiple Earth-Venus flybys used by a lot of robot probes to get out to Jupiter or in to Mercury will ever be a sensible choice for humans. A Jupiter flyby on the way to Saturn is probably a no-brainer apart from maybe the radiation issues, if the planets are in the right place, but everything else adds years to the journey to save relatively small amounts of delta-V.

What might make sense is to launch a large vehicle full of supplies on a complex trajectory and then boost a relatively small vehicle with the humans on board to rendezvous with it on its last Earth flyby. On the other hand, the risks of this are huge. You get one opportunity to launch the humans, and if you miss the rendezvous in some way they are dead -- the whole point is that their vehicle will not have fuel to return. If you miss the launch window (or at least the window to leave parking orbit) the whole mission is scrubbed.

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    $\begingroup$ You do get the chance to assess the health of the vehicle that has done all the flybys before you launch the crew or commit them by spending too much propellant to get them home to the mission. It is the government's assessment of the public's risk assessment. I think in the Apollo days that would be acceptable, but not now. +1 $\endgroup$ – Ross Millikan Apr 11 at 2:27
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Mission design is very hard because it has so many different constraints. Finding the path to solar escape with minimum delta v within a given calendar window is well defined and will probably have the solution you suggest. Then someone will say that is too long for the astronauts to be on the whole mission and suggest the last pass rendezvous that you suggest. Then someone else will say the risk is too high because if they miss the rendezvous they will be stranded in space. I don't guarantee that this scenario will play out, but it gives an idea of the problems, especially for human flight. You need to commit to a mission profile early in the design phase. It gives a soft limit to the mass of the spacecraft-you might be able to improve the rockets to raise the allowable mass. You haven't designed the spacecraft yet, so the allowable mass is an input to that design. Can you design a spacecraft that will accomplish what you want within the mass limit? That is why it is hard.

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