Better launch calculations for orbital rendevous

[goldenpsp]

Hey all,

So in my current game I am often launching into LKO or even other bodies specifically to rendevous with an orbiting station. On Kerbin I have gotten fairly good at knowing about where in the stations orbit is best when to launch, but I'd love it if there was a more calculated way to figure it out for better consistency. Any tips/mods/ that can help with this? I guess it is similar to eyeballing a interplanetary burn vs using a porkchop plot to find when is best to go.

[Snark]

For interplanetary transfers, here's a really handy and easy-to-use tool:

http://ksp.olex.biz

For launching from surface to rendezvous with an orbiting station, I confess that I'm just eyeballing it. I generally have pretty good luck, I guess a year or so of practice with KSP is paying off.

[GoSlash27]

goldenpsp,

Actually, I do have a trick for that.

I keep my orbital station in an orbit that passes over KSC at the same times everyday.

The first time I launch a new type into the orbit, I note it's passage over KSC.

After that, I can use launch windows.

Altitude /transit time

101,306m/ 36 min (10 times per day)

147,314m/ 40 min (9 times per day)

201,693m/ 45 min (8 times per day)

Best,

-Slashy

[goldenpsp]

For interplanetary transfers, here's a really handy and easy-to-use tool:

http://ksp.olex.biz

For launching from surface to rendezvous with an orbiting station, I confess that I'm just eyeballing it. I generally have pretty good luck, I guess a year or so of practice with KSP is paying off.

Yea I know. I was just using that as the example that there is a tool to not have to "eyeball" an interplanetary transfer, but I was unsure if something like that existed for an orbital rendevous.

[Montag]

Mechjeb has a toggle to launch to rendezvous under ascent guidance. It has been getting me ~ 15 km of the target which I can close within 1 orbit with minimal dV.

[ajburges]

goldenpsp,

Actually, I do have a trick for that.

I keep my orbital station in an orbit that passes over KSC at the same times everyday.

The first time I launch a new type into the orbit, I note it's passage over KSC.

After that, I can use launch windows.

Altitude /transit time

101,306m/ 36 min (10 times per day)

147,314m/ 40 min (9 times per day)

201,693m/ 45 min (8 times per day)

Best,

-Slashy

Hmm that sounds pretty slick actually. How do you account for different ascent profiles? Do you just empirically measure time to circular orbit for each craft?

I personally just reckon required phase angles for my different craft from empirical data. For 70-90 km circular orbit, my rockets launch while target is â…›-â…“ over the sea to the West. At the same altitude, my space planes launch as the target is about to leave the gulf to the West of the before mentioned sea. I favor a long low throttle gravity turn above 50 km, so I know an approximate intercept early enough that I can either stick around an atmospheric orbit longer to advance phase more, or insert to a higher orbit faster to fall back. As long as I can get an encounter within a orbit and a half for less than 30 m/s, I'm happy.

Edited July 9, 2015 by ajburges

[GoSlash27]

Hmm that sounds pretty slick actually. How do you account for different ascent profiles? Do you just empirically measure time to circular orbit for each craft?

Pretty much. I find that the time- to- orbit is pretty regular for vertical lifters and a little more deviation in spaceplanes.

I'm essentially doing the same thing you are. It's just that since I'm basing it on time rather than landmarks, I can do it with a lot more precision. After all, the station's exact position is a function of the time of day if it's in a regular orbit.

This gives me the ability to launch directly into a rendezvous instead of taking an orbit to set up an intercept because I have figured out the required moment of launch down to the second.

Best,

-Slashy

Edited July 9, 2015 by GoSlash27

[Kar]

This will solve your problem: http://forum.kerbalspaceprogram.com/threads/83173

If you do not like using mods then: http://ksp.olex.biz/

Have fun!

[KSK]

Empirically I've found that a 200km orbit is very nice for easy rendezvous. Simply launch as your target vessel passes over KSC and go straight for a 200km altitude - no parking in a lower orbit required. You might need a radial burn to fine tune the intercept. Circularise as you approach the target and Bob's your father's brother.

One time I managed to launch straight into a 1.5 km closest approach - that was a good one.

[Padishar]

Quite some time ago I played around with the KER rendezvous tab trying to work out if there was an easy way to use the information provided to do this accurately (I was building a complex station needing 6 launches). The numbers do give you some useful help and I thought of another few readouts in KER that would be extra helpful but I don't remember much of the details. Hopefully I can pull some notes off my old laptop otherwise I'll try and recreate what I was doing and post a tutorial or something about it (and also implement the other KER readouts).

I started getting quite good with all the practice. On the 5th launch, I cut it way too close. When I got to Ap I needed nearly 400m/s to circularise but the station was only 6km behind me and I only had a TWR of about 1 and I had to shift my burn off prograde to make sure the station didn't hit me (I over-did it a bit to be safe and it missed me by just under 200m) and I finally matched speed with it about 2km in front of me...