There were Gemini and Soyuz missions (and possibly others) that involved direct (i.e. first orbit) rendezvous,
but no vehicle flying to ISS attempts this. Is this because a launch anomaly would put ISS at too much risk? It was not attempted for the Apollo/Soyuz Test Project either.So: are there issues with orbital mechanics that make prompt rendezvous particularly difficult?
I had wondered about this very question. Although there are significant problems flying a KSC to ISS docking profile in a day or less, there is a great potential to have a vehicle that is lighter, less complex, and less costly. It would be interesting to get a detailed analysis of the pro's and con's.
Quote from: CitabriaFlyer on 07/09/2010 04:52 amI had wondered about this very question. Although there are significant problems flying a KSC to ISS docking profile in a day or less, there is a great potential to have a vehicle that is lighter, less complex, and less costly. It would be interesting to get a detailed analysis of the pro's and con's.It would be heavier since it would require more propellant and larger thrusters. And it still will have the plume issues with the target.
It's not really "first-orbit rendezvous" if you use a tug. [...] the whole point of first-orbit rendezvous is get to the station before using many consumables.
If the tug has enough power & life-support [...], it's a mini space station...
For that reason, it's probably worth pointing out that while first-orbit *rendezvous* with ISS is possible, first-orbit *docking* or *berthing* is not.ISS requires that visiting vehicles receive a "go" to proceed inside both the Approach Ellipsoid (2x1x1 km semi-axes) and the Keep Out Sphere (200 m radius). Prior to the "go", the no-burn trajectories must be "free-drift safe" for 24 hours for the AE, and some shorter period (sorry don't have my references handy) for the KOS. This precludes a visiting vehicle targeting a direct intercept trajectory on the first orbit, as was done on Gemini 11 and Apollo 14-17. It would *not* preclude, say, a first-orbit *rendezvous* where the TPI burn targets a point just outside the AE, such that a no-burn trajectory with 3-sigma dispersions would not intersect the AE within 24 hours. But such a trajectory could not support a first-orbit docking or berthing unless the subsequent closing rates were high, and as Jim points out, that will result in plume impingement (among other issues).
This is where it's handy to have an orbital tug and hab module.Then the launch/reentry vehicle could target a launch day rendezvous with the tug, which would maneuver the capsule and hab to the ISS over the next two days. The hab would provide extended power, ECLSS and hygiene.Like Soyuz, but with a reusable hab that stays on orbit.
Quote from: Jorge on 07/10/2010 03:41 amFor that reason, it's probably worth pointing out that while first-orbit *rendezvous* with ISS is possible, first-orbit *docking* or *berthing* is not.ISS requires that visiting vehicles receive a "go" to proceed inside both the Approach Ellipsoid (2x1x1 km semi-axes) and the Keep Out Sphere (200 m radius). Prior to the "go", the no-burn trajectories must be "free-drift safe" for 24 hours for the AE, and some shorter period (sorry don't have my references handy) for the KOS. This precludes a visiting vehicle targeting a direct intercept trajectory on the first orbit, as was done on Gemini 11 and Apollo 14-17. It would *not* preclude, say, a first-orbit *rendezvous* where the TPI burn targets a point just outside the AE, such that a no-burn trajectory with 3-sigma dispersions would not intersect the AE within 24 hours. But such a trajectory could not support a first-orbit docking or berthing unless the subsequent closing rates were high, and as Jim points out, that will result in plume impingement (among other issues).Soyuz and Progress would seem to violate these flight rules.
All other visiting vehicles must conform to them, even Orion.
Quote from: Jorge on 07/26/2010 12:14 amAll other visiting vehicles must conform to them, even Orion.What do you mean by "even" Orion?
But why should NASA vehicles be given special treatment?