Launch for rendezvous on moons etc.

[Jackissimus]

Hi!

How do you launch for rendezvous on some other planet than Kerbin? I know that on Kerbin you should aim for about 400km distance from your target and then launch. Is there some general method how to calculate such distance on other bodies/different orbits? I even tried it with mechjeb and I often end up tens of kilometers away from my target.

And now that I think of it ... since these are nonatmospheric bodies, it really depends on the acceleration of my rocket doesn't it? On Kerbin the speed is more or less fixed, but not so on moons etc., right? Anyway, this question might not have an answer, I think.

[Sauron]

I've never had a problem launching to rendezvous by eye on smaller bodies. If your instincts aren't up to this, you can aim to get it roughly right and put yourself in a slightly lower phasing orbit and make any corrections there. Eyeballing gets easier as the moon or planet gets smaller.

Alternatively, you could use math...

[Frederf]

You want your time to orbit to match the orbit of the target. For simple transfers like Hohmann this is well defined. For long-duration powered ascents the math is significantly more complex. For sanity's sake I suggest building fault tolerance into your mission profile. I'll give an example:

Active vehicle

altitude: 0 m

orbital velocity: 175 m/s (surface)

longitude: KSC

Target

altitude: 100km

orbital velocity: 2256 m/s

longitude: TBD

A Hohmann transfer from 0 to 100km takes 14m36s. The target is sweeping 11 deg/min. Your ascent will probably take a lot shorter time than this, but I don't know how to calculate it. I'm just going to assume 2000 m/s at 20 m/s/s and get 100 seconds. OK, 100 seconds is two minutes. Leading your target by 22 degrees should mean a 267 km separation (trigonometry) at launch. Because you can't be exact, aim for a 90 km orbit but launch one minute after the 267 km marker. You'll be behind and faster and should catch up within about two orbits.

[Jackissimus]

You want your time to orbit to match the orbit of the target. For simple transfers like Hohmann this is well defined. For long-duration powered ascents the math is significantly more complex. For sanity's sake I suggest building fault tolerance into your mission profile. I'll give an example:

Active vehicle

altitude: 0 m

orbital velocity: 175 m/s (surface)

longitude: KSC

Target

altitude: 100km

orbital velocity: 2256 m/s

longitude: TBD

A Hohmann transfer from 0 to 100km takes 14m36s. The target is sweeping 11 deg/min. Your ascent will probably take a lot shorter time than this, but I don't know how to calculate it. I'm just going to assume 2000 m/s at 20 m/s/s and get 100 seconds. OK, 100 seconds is two minutes. Leading your target by 22 degrees should mean a 267 km separation (trigonometry) at launch. Because you can't be exact, aim for a 90 km orbit but launch one minute after the 267 km marker. You'll be behind and faster and should catch up within about two orbits.

Seems like a nice approach. It made me think that maybe I could do it even simpler and for non-atmospheric bodies just assume that I am going basically just horizontally (if the station is in very low orbit) and I am basically trying to accelerate from 175m/s to 2256m/s with a fixed acceleration (MJ tells me the max acceleration of my rocket) and then calculate how long it will take me. It could roughly work.

I don't have much experience with eyeballing it, but I was expecting some surprises when launching from different moons and different gravities. I don't very much enjoy surprises

Anyway, thanks for replies!

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