What are the steps between rocket launch and docking to the ISS?

Question

This question focus on space missions docking with the ISS. On this youtube video, the process to rendez-vous with the ISS is described. If I understand correctly, it consists in taking of, place the spacecraft into a stable but lower orbit, wait there for the good timing, and then perform a Hohmann transfer to the orbit the ISS is flying. The good timing is the one such that once on the ISS' orbit, the spacecraft is just few miles in front of the ISS.

Although the last step seems clear to me, the first ones don't. What happen between the spacecraft launch and the stable lower orbit from which the last Hohmann transfer will be performed? How is this orbit chosen? How to get there?

In short: What are the steps between the spacecraft launch and the spacecraft being in position to dock to the ISS?

Answer 1

What are the steps between the spacecraft launch and the spacecraft being in position to dock to the ISS?

This varies from vehicle to vehicle. Some vehicles dock to the ISS, others berth with the ISS. Some vehicles do it very quickly, others take days. Amongst those that dock, there are a number of docking ports on the ISS. The timing of the operation, whether the vehicle docks or berths, and location and orientation of the docking port or berthing box make for widely varying sets of steps. I'll look at things from the perspective of how the Japanese HTV berths with the ISS. Orbital's Cygnus and SpaceX's Dragon follow a very similar pattern.

The first phase of flight is launch. Typically the vehicle that will eventually dock or berth with the ISS is a passive payload during this initial phase; it's the launch vehicle that is in charge. This phase ends with orbit insertion.

The insertion orbit may be rather low, so low that the vehicle would reenter in short order without boosting to a higher orbit. This higher orbit is a phasing orbit, one whose altitude is lower than that of the ISS. This lower altitude means the orbital period is faster than than of the ISS. How much time is spent in this phasing orbit (or phasing orbits; some vehicles use more than one) depends on how far the ISS (the target vehicle) is ahead of the vehicle in question (the chaser vehicle). Some vehicles (e.g., the European ATV) might spend multiple weeks in their phasing orbit(s).

When the timing is right, the chaser will raise its altitude so that it is ideally a pre-planned distance behind the ISS. This is not a Hohmann transfer. It is instead a sequence of burns that target a specific point in space at a specific point in time. One approach to doing this is to use Lambert targeting, or some modification thereof. The start of this sequence of burns marks the start of the far-field rendezvous phase of flight.

When the vehicle reaches the aim point (typically hundreds of kilometers behind the ISS), it performs another burn that makes its perigee a bit smaller than that of the ISS. With time, this moves the vehicle toward the ISS. At some point, the vehicle comes within communications range of the ISS. This marks the transition from far-field to near-field rendezvous.

During far-field ops, the HTV (and Cygnus and Dragon) uses GPS to correct its navigated state. The ISS also uses GPS, but the distance between the two vehicles means that the ISS and the chaser may be using different sets of GPS satellites to estimate their state. Once the HTV (or Cygnus or Dragon) comes within in comm range, it switches to using relative GPS. This significantly increases the accuracy of the estimate of the relative state between the two vehicles.

Once the chaser comes within a prescribed distance of the ISS, it switches from near-field rendezvous to approach. In the case of HTV (and Cygnus and Dragon), the vehicle switches at this time from using relative GPS to visual navigation. GPS on the Earth doesn't work as well in cities as it does out in the country because of signals bouncing off of tall buildings (the multipath problem). Relative GPS doesn't work very well in the close vicinity of the ISS for the same reason.

During the approach phase, the chaser must follow a prescribed corridor that eventually bring it to the docking port or berthing box. The approach phase has a number of hold points where the chaser is to come to rest with respect to the ISS. Deviations outside of the corridor or failure to stop at a hold point are signs that the vehicle needs to transition to yet another mode, abort or retreat. There are multiple decision points along the way where the chaser (or the human controllers on the ground) will decide that the best thing to do is either abort or retreat.

The very last phase is of course the actual docking or berthing.

Answer 2

All space launches take a rocket on the earth and give it some altitude and a lot of sideways speed until it reaches orbit. The ways to do this are all pretty well known and not specific to Soyuz. You "just" have to plan your orbit and feed in the parameters to the system.

For an ISS docking, the initial orbit selected will be:

• In plane with the ISS orbit. Plane changes are terribly expensive for fuel, so the first requirement is to match that. This is done by letting the Earth turn until the launch site is brought underneath the orbital plane. Launch at that time in the right direction and the plane is selected.

• High enough that you have a few orbits to check out your equipment, make sure stuff is working, and start getting closer. Much of the equipment necessary to do this could not be tested during the launch. Now is your time to make sure it's working.

• Low enough that the craft will re-enter on its own. As a contingency against certain failures, a nominal launch orbit will decay in a few days, allowing the crew to return without the need for a re-entry burn.

That constrains the initial parking orbit. If this were earlier in the program, you'd now wait until phasing was correct. The Soyuz and ISS would be in the same plane, but could be at any distance along the orbit relative to the other craft. The Soyuz at a lower altitude would orbit faster. This would allow it to "catch up" and change the phase angle. Rendezvous burns would begin when phasing was correct. This wait meant that launch to dock took up to 3 days. Not too bad on a shuttle which had a lot of room, but quite annoying packed into a Soyuz.

For the last few years, an updated launch profile has been used for crewed launches. The ISS orbit is modified to place it at the correct phasing point at the predicted launch time. This takes a lot of planning and uses fuel on the ISS, so it is not done for cargo flights. When it works, the phasing step is complete at launch, and the total process goes from a few days to 6 hours.

The rest is just playing golf. You know (basically) where you are and (basically) where you want to be, but not with such precision that you can expect to swing and get it there in one shot. When the Soyuz was developed, it didn't have enough smarts to do orbital rendezvous by itself. Instead it would have to pass over ground stations to analyze the status and communicate new commands. So rather than something like flying a plane or driving a car where you have constant control, you're sending commands to do actions and then waiting to see how well you have done. The multiple approach burns get things nice and close in a way that reduces the chance of colliding with the station and doesn't require impossible precision in each burn.