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Boyster

Matching Velocities undocked, possible in Kerbal?

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I was trying to completely match velocities side by side but i found even with precision controls on and low RCS twr it felt impossible.

I was wondering if its doable or its always gonna be a small difference.

I am guessing in real life its impossible and there is always a need for corrections, well except if they dock.

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Posted (edited)

It is physically impossible, I'm afraid - in real life and in KSP both.

What you're experiencing is called "microgravity". When you are side by side with another spacecraft, you are still not in the exact same position - your centers of mass are a few dozen meters or so apart. And that means that these two spacecraft are effectively in different orbits. The difference is extremely small, sure - but over time, orbital mechanics will pull the two spacecraft apart, or towards each other, or the like. A perfectly nulled out velocity difference will immediately start growing, however slowly, the moment you turn off your engines.

Edited by Streetwind

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Posted (edited)
9 minutes ago, Streetwind said:

It is physically impossible, I'm afraid - in real life and in KSP both.

What you're experiencing is called "microgravity". When you are side by side with another spacecraft, you are still not in the exact same position - your centers of mass are a few dozen meters or so apart. And that means that these two spacecraft are effectively in different orbits. The difference is extremely small, sure - but over time, orbital mechanics will pull the two spacecraft apart, or towards each other, or the like. A perfectly nulled out velocity difference will immediately start growing, however slowly, the moment you turn off your engines.

Could there be a device, like a magnet, or a certain rotation/mechanism that constantly cancels that difference with some distance between the two spacecrafts?

I mean, like in the near future, not a sci fi one lol.

Eitherway, tnx for the good explanation.

Edited by Boyster

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Posted (edited)
43 minutes ago, Boyster said:

Could there be a device, like a magnet, or a certain rotation/mechanism that constantly cancels that difference with some distance between the two spacecrafts?

A set of electromagnets on each ship can provide enough force to keep a couple of craft stationary wrt each other without using propellant, and without them physically touching -- over a distance of a few meters.

Alternately, you can use steerable solar sails on two craft to keep them stationary wrt each other at much larger distances with no propellant.

Edited by bewing

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You can put the crafts close to each other, then use a few RCS puffs to set the orbit periods to the same value. The crafts will move slowly, remaining close to each other. Be careful they don't get too close...

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It's possible - though virtually unmanageable - to do this in KSP, as long as you are happy for them to be "in front"/"behind" each other, rather than "side by side". As in, if they are side by side and one is closer - even microscopically closer - to the body you are orbiting, it won't work, but if they are in front/behind each other in their orbit, but otherwise in an identical orbit, they will stay the same distance apart. Think of two keostationary satellites for example - they are in the same orbit, above two unmoving spots over Kerbin, and must therefore stay exactly the same distance apart. As mentioned though, it's nigh impossible to achieve without cheating them into position.

5 hours ago, peteletroll said:

...set the orbit periods to the same value...

This is the slightly simplified version of the above, the craft will drift slowly towards and away from each other by the same amount each orbit, but never end up moving further apart. It's still almost impossible to do though, as the level of rounding on the orbital period figure will mean that even if you match them exactly, they will still be out by a tiny amount of decimal places, and so will eventually move away from each other.

 

This is why a lot of people, in the absence of station-keeping computers in KSP, fly their relay constellations out to where they need to be to show that they can get them there, and then replace the real ones with cheated-into-place versions which have *identical* orbital parameters.

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3 hours ago, eatU4myT said:

the level of rounding on the orbital period

KER can help here.

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4 hours ago, eatU4myT said:

This is why a lot of people, in the absence of station-keeping computers in KSP, fly their relay constellations out to where they need to be to show that they can get them there, and then replace the real ones with cheated-into-place versions which have *identical* orbital parameters.

Hmmm, maybe if your vessels had close enough orbital parameters an option could appear in tracking station to 100% synchronize them.

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actually you can syncrozine ships, not side by side (for reasons streetwind explained well) but one after the other. if one ship is ahead of the other, they can effectively be sharing the same orbit, and move exactlty at the same speed.

and if you really want them to be facing  side by side, just have them point in a direction other than prograde/retrograde

 

granted, from a practical side, it is impossible to completely cancel their velocity. eventually there is going to be a difference of 0.0001 m/s that was unaccounted for that will make them drift apart. but you can get close enough. one time i had to dock two particularly bothersome ships, i did put them in the same orbit, with an orbital time matching to the second. it would have taken at least several days for a difference to become apparent.

from a realistic point of view, it also is impossible to achieve perfectly, because there are always disturbances (solar wind, gravity of other planets, microscopic anysothropies of the local gravitational field) that are going to make your ships drift. but again, you can keep them in place for decades, at least. and any small form of propulsion would be enough for stationkeeping.

it is part of the reason why any problem with gravitation involving more than two bodies cannot be solved exactly.

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