# Is it possible to connect to a docking port on a tumbling or rotating spacecraft?

Situation 1. You want to dock to a docking port on the outside rim of a revolving centrifuge wheel spacecraft.

It seems to me that this is an intercept problem. You know the tangential velocity at the outside rim. You know the phase angle of the docking port as a function of time. You start your engines at a computed moment in time, such that you begin moving towards the spacecraft at the same velocity as the tangential velocity, and where the intercept between your spacecraft and the target will happen at exactly the same instant the docking port has revolved around and is exactly where you will be at closest approach.

So for a single instant in time, the docking port and your spacecraft are going the same velocity and are in the same place. You grab it with big honking magnets and let the docking port yank your whole spacecraft into matching the acceleration of the space station. You would need a really well built docking port, a lightweight spacecraft, and some monster shock absorbers.

1. The whole spacecraft is tumbling on multiple axes, having undergone a mishap. You want to grab some piece of structure that is sticking out far from the tumbling mess. It seems like this is a more complex version of problem #1, and the same solution would work. You just have to have a more complex computer model, and you set up an intercept to where your spacecraft arrives at the rendevouz with the same velocity as the tip of the structure will have at the rendevouz time, predicted in the future. Note this problem becomes completely impossible if the wrecked spacecraft is undergoing angular velocity change because it is still venting gas at unpredictable rates.

If you read the histories of Gemini VIII, youll see the astronauts had a difficult enough timetrying to undock while tumbling.

Couldn’t you, with multiple jets, move in a tightening arc shaped like a 9, such that at the moment of contact, your arc has tightened into a circle with the same circumference as the rotating station? If so, this would cut way down on the shock absorber needs.

In any case, I believe this scenario, or one very like it, formed an exciting scene in Nemesis Games, the fifth book in the Expanse space opera series. The series does pretty well with space physics, AFAICT.

Situation 1: This would work in a science-fiction setting, with everyone congratulating the pilot at the end. But I have a hard time imagining a real space agency plotting a trajectory for a capsule to graze the outer surface of the ring just so, with a tolerance of a few cm and some fraction of a second, so that the station will grab the capsule and pull it forcibly into a circular motion, and then its center of mass will shift just a little bit, etc. I’m sure even the infallible people at SpaceX would perform some sort of failure mode analysis and rule this out as too dangerous.

Yes. But you would need unrealistic amounts of fuel if the station is spinning fast enough for there to be 1 G of experienced centripetal “gravity” for the occupants.

If it’s a chemical rocket, you’ll burn through too large a percent of your rocket in fuel mass.

A nuclear rocket with high ISP could use a reasonable amount of fuel if you had amazing performance. The problem is every form of proposed nuclear rocket - fission, fusion, antimatter, about 5 different variants on each - is going to emit lethal amounts of gamma rays and neutrons out the back of the rocket when the engine is running. In the case of many of the fission variants, the radiation continues even after the burn.

You can shield the occupants of the spacecraft doing the docking but not the people on the space station.

Notably, in the James Corey books, they have figured out a way to get fusion to work incredibly well, letting them fly on fusion exhaust for days at multiple gravities. While this is unrealistic in itself (their burn times are too long even if their rocket exhaust is several % of the speed of light), the radiation from a drive like that is going to be really lethal to anyone exposed directly to the light from the drive.

Agree completely. Just wondering if it can be done at all. Ironically, this is a maneuver that is virtually certain to end catastrophically and fail if you attempt it in Kerbal Space program or Orbiter.

One thing to consider is that after you have docked. your craft becomes essentially part of the whole system, altering the centre of mass. There could be some nasty shear forces and wobble effects as a result.

I think any docking to a rotating target is going to on the axis of spin as in 2001. That will be complicated enough.

Docking to the outer ring of a rotating station I’m sure could be managed with a light robotic craft but is going to be very wasteful of delta-V. Maintaining a circular or spiral trajectory requires continuous thrusting, and docking is a slow process. As Mangetout points out, you then have an out of balance station. Your docked spacecraft is now at the ‘bottom’ of your wobbling, simulated gravity station.

There is a persistent rotation mod for KSP.