if i docked a craft in sub orbit to a craft in orbit of the same size...

[Penguinhero]

I'm wondering what would happen. I think that the peri would move halfway so if it had a peri of 100,000 it would move to 50,000 but knowing this is ksp how would the game handle it????

[Wjolcz]

To dock you need to match orbits. The Pe and Ap must be the same.

[geb]

If one craft is in orbit while another is suborbital, one is going much, much faster than the other.

Your docking ring isn't going to survive a collision at a hundred metres per second or more.

If you matched speeds before docking, then you have already moved the crafts into matching orbits.

[Jason_25]

The craft that is coming in to dock will assume the approximate orbit of the craft it is docking with.

[CodatheSpaceFox]

I'd guess it'd depend on the condition, for example, if you managed to match a relative speed to your target, you'd probably also be matching the orbit. Considering speed is half responsible for said orbit. If you managed to "crash" the two together against different, but acceptable conditions, the entire object would rely on the game's mechanics I figure, since it usually considers the bulk of two objects as one if they're connected. At least from other tests.

I'm curious on this now though, would they adjust respectively? would it even be possible considering the need to match speeds and connect without de-orbitting the ballistic craft? how much Jebadiah glue would you need to keep them together? so many variables.

[Xeldrak]

As said before - if one has sub-orbital speed and one has orbital speed....a rather expensive crash is what would happen

[Brotoro]

Parts.

Parts everywhere.

[Toshogu]

Because you are docking you have matched orbits. relative speed of both objects are the same. Nothing would happen immediatly. decay of obit would be slightly faster I think, but since the bigger mass already has the required acceleration for orbit and joining with the less mass object, energy from both gets put into new bigger vehicle. nothing really changes. bad explanation. but close enough.

[Evrion]

As other have stated, in game, docking must be done a very low speed to work, which means that the orbits will automatically have to be matched. This isn't something you have to keep track of while docking, but is something you can check out if you want.

Suppose: docking can be done at high speed. Then what?

In a physics sense, this would be a completely in-elastic collision. So momentum, and not energy, would be conserved. Momentum is mass x velocity. So this means that the velocity of the resulting object after docking would have a velocity that is a weighted (by mass) average of the two velocities coming in.

So: small ship in sub-orbital docking to a large station in orbital could work! If the station can survive the drop in speed given by the small ship. The effect of the drop in speed would primarily be seen on the opposite side of the planet, so best to do it at the stations periapsis

[Sma]

I can confirm this. I was testing my setup for the solar flare challenge on reddit and my 2nd ship was suborbital, and when I burned retrograde to let the target craft catch up, by the time the speeds were closely relative both ships were in the same orbit.

[Travisfv]

I know it wouldn't work, but I imagine if you were skilled enough (I'm talking like... smart person smart) it would be kinda crazy,

If at some point you managed to launch so that when you do meet the other craft, speed matches for a quick moment, and then capture.

Just imagine how cool it would be to launch and nail a target only a meter wide. But it is unfeasable so it's just my crazy imagination going crazy.

[leax256]

maybe the only way this woulb be possible is high gilly orbit.

[Mephane]

If at some point you managed to launch so that when you do meet the other craft, speed matches for a quick moment, and then capture.

If you have to objects close to each other with the same speed and direction, they are in the same orbit. There is physically no way to have two craft stand still relative to each other for a moment without them being in the same orbit. Thus reaching the situation you described is exactly the same as matching orbits.

[lammatt]

how can this question exist?

u dont think two crafts of a very high relative speed dock, (unless u think an inelastic collision a docking),do u?

well,... u can say u dock with a train when u jump a down the platform onto the rail

[Pingonaut]

Lots of lag and then nothingness.

[Bacterius]

Think of it this way - upon impact (or capture, if your craft can somehow survive a collision with a payload at orbital speeds without losing any parts), the momentum of the payload (mass * velocity) is transferred to the payload + target system, imparting a net change in velocity to the resulting object. That change in velocity will cause your object's orbit to change, possibly decaying into whatever body you're orbiting, or being ejected out of its sphere of influence altogether.

As an example, if you have a 100-ton station orbiting at, say, 6 km/second (taking Earth-like speeds here) and send a 1 kg payload at it at a speed of 8 km/second, then we have:

Momentum of target = 100 tons * 6 km/s = 600000000 kg * km/s (velocity towards the orbital tangent)

Momentum of payload = 1 kg * 8 km/s = 8000 kg * km/s (velocity perpendicular to the orbital tangent)

Velocity of payload + target after impact = sqrt(600000000^2 + 8000^2) / (100 * 1000 + 1) ~ 5999.94 km/s

That corresponds to a change in velocity of 0.06 km/s = 60 m/s, taking the target craft slightly off course off the orbital tangent. Consider how much you can change your orbit with that much delta-v. Now consider a larger payload (or multiple payloads) and the fact that surviving an orbital collision with a 1 kg payload is basically impossible, and you now know why this isn't done

Now if you meant not actually colliding, but having the payload come very close to the target (apoapsis near your station) and then "latching onto it", then something similar happens - the payload has no momentum at its apoapsis (its velocity is zero), yet when it docks with the target, the resulting payload + target system has a greater mass, and the same momentum as the target alone. As a result, its velocity will be lowered, causing the orbit to slowly decay.

Reusing the above example, we have a 100-ton station orbiting at, say, 6 km/second (taking Earth-like speeds here) and there's a 1 kg payload hovering at its apoasis "snatched" by the docking port, then:

Momentum of target = 600000000 kg * km/s

Momentum of payload = 0 kg * km/s

Velocity of payload + target = 600000000 / (100 * 1000 + 1) ~ 5999.94 km/s

A loss of 60 m/s of velocity (60 m/s delta-v spent towards decaying the orbit).

As an observation, in the first case (collision) the speed of the payload doesn't actually matter that much if it is much smaller than the target. From the point of view of the target, the main difference is that it gained mass for no additional momentum, leading to reduced velocity. The only way for velocity to the conserved is if the two momentum vectors add constructively and as such are precisely in the same direction, which is just another way of saying that the payload must be in the same orbit as the target

This is really the same thing as having a train going at constant velocity on rails (suppose no acceleration and no friction), then putting a car on the rails in front of it. If the car survives the crash, then the train (and the car in front of it) will be going slower. Add enough cars and your train will grind to a halt. Add a huge obstacle (with a mass far greater than the train) and the train will simply smash into it and stop, period. Similarly, if you shoot enough bullets at such a train, it will stop, eventually. It will take a lot of bullets but every bullet that bounces off will steal some momentum from the train, and every bullet that stays embedded in the train increases its mass (and hence lowers its speed), ultimately the train must stop!

Applied to an orbital situation, if it were possible to design materials to survive such high speed impact, your station would still need enough fuel to restore its original orbit after every payload is received, so at that point you may as well just use that fuel on the payloads themselves and get them into the proper orbit to spare yourself the collision (which is what we are doing). You won't save any fuel either way (conservation of momentum).

Edited July 20, 2013 by Bacterius

[Tw1]

You'd have to time it absolutely perfectly, so that your sub-obital craft's docking ports meat the other's at exactly apoapsis.

[leax256]

You'd have to time it absolutely perfectly, so that your sub-obital craft's docking ports meat the other's at exactly apoapsis.

you'd still have a huge difference in velosity, un les you were at gilly.