mechjeb and rendezvous issues

[dpraptor]

Hi,

I installed Mechjeb because I could not dock my last part of my station. I did a practice run on two normal simple ships I used before. I used it to launch and orbit the 1st ship, and then set up a rendezvous with a 2nd. After getting into orbit I used the phasing step to get the 2 together. But the recommended orbit was 120 to match the 1st, but it put it at 123. I then set te Hohman transfer to find the closest approach, but the closest approach was 214km. I manually brought the orbits close and tried to do the hohman again, but it was still at 214km.

Is this a flaw in Mechjeb? Or do i still have to do that part manually. Would appreciate any help in understanding how tis works.

Thank you!

[tommygunyeah]

MechJeb isn't perfect, neither is anyone, just do some research and try your best to do it manually. It's sometimes a lot easier to do it yourself rather than let MechJeb do it for you.

[ThatBum]

Try "fine tune approach to target," which adjusts inclination. Hohman doesn't do inclination.

[dpraptor]

My inclination was 0.0. It seem like it has a lot of trouble getting rendezvous right. I had to do a lot of it manually. I just want help in the docking step = but i was trying mechjeb to see how it works. I think it has problems with larger ships, but once I finish he station, I will rarely use Mechjeb - I only use it when I am having way to many problems without it.

I read as much as I could - I searched mechjeb, but there is very little info on the latest version, and I could find little info about issues. I guess they were lost during the backup....

If anyone can give me someinfo about this - or someplace that has it I would appreciate it. I read the manual, but that was little help.

Thanks

[Pedonymous]

Ditch the mechjeb and play with the nodes until you get a proper rendezvous. C'mon son.

[TwoHedWlf]

Yup, I generally use mechjeb mostly as an upgraded ASAS. For holding heading or just circularizing orbit etc. I find the actual rendezvous module to not be very reliable.

[dpraptor]

Ditch the mechjeb and play with the nodes until you get a proper rendezvous. C'mon son.

I have never used mechjeb before - the only reason I am using it is because I tried so many times to dock a large piece to my station and failed so many times... I was sooo frustrated that i was ready to call it quits on doing that - Mechjeb is my last hope. I must have wasted at the least 12 or 13 hours and over 18,000 units of RCS fuel for one dock - one can only take so much!!! hahahah!

I would only use it again when i get stuck like that - I dont normally give up very easily!

[ThatBum]

Well, a major part of rendezvous is when you launch the thing you intend on getting to the target. It varies with different launch vehicles and ascent profiles, but I find a good time is when the target is just coming up over the horizon relative to the launchpad.

Also, have you tried MechJeb 2? Assuming that's not what you have now.

[Boris_T_Roach]

I use MJ for rendezvous all the time, but only as an assistant.

Generally I launch just before the target ship goes over KSP, and aim for an initial orbit with a AP the same altitude as the target and a PE about 10-15KM below that.(this also lets any debris pass behind the target should things go a bit 'wrong' )

Use MJ to align the orbits, then fire prograde or retrograde to get the closest approach down to within 1Km(or closer), match speed with target at closest approach, then look for a docking port to align with,usually by using the PAR- button and flying on the RCS with the translation controls.

[ThatBum]

Yeah, once you get into visual range, you kill relative velocity then simply point at the target and thrust forwards slowly. No fancy MechJeb maneuvers required once you get that close.

I don't quite trust MechJeb 2's rendezvous autopilot to run to completion, it gets really close to the target before coming to a complete stop. I'm a bit afraid it will blow off my station's solar panels with the reverse thrust. It would do to have a "kill relative velocity to target" button on MechJeb...perhaps it's already there and I'm an idiot. I can't check because I'm away from the gaming machine, and MechJeb feature discussion is for its own thread, anyway.

[Nibb31]

A Hohmann transfer only works if you have different orbits. When your orbits are at 120 and 123km, they are basically the same, so it will take ages to catch up.

What you want to do is to make a phasing orbit, typically 30 to 50km higher than your target (if the target is behind you) or lower (if your target is in front of you). MechJab can suggest a phasing orbit, but you can do it yourself.

When you get to something like a 60-45° phase angle to your target, then you can ask MechJeb for a Hohmann transfer.

[TwoHedWlf]

It would do to have a "kill relative velocity to target" button on MechJeb...perhaps it's already there and I'm an idiot.

It does, there's a relative velocity+ and - button. You have to do the throttle manually though.

[chainedlupine]

What Nibb said. To summarize the steps:

1) Align inclination between you and target orbit.

2) Get into a phasing orbit in order to catch up or slow down. (Either in a higher or lower altitude.)

3) When around 45-ish phase angle difference (ie: around 45 degrees behind your target), raise (or lower) your orbit to perform an intercept.

4) Once you are as close as you will get, match orbital speed by burning relative (what would be the prograde marker in target mode of the navball).

At that point you should be in a stable, matched orbit, within 1km of your target.

MJ2 seems to always wants to create a lower phasing orbit, which isn't always what I want. Say, I'm at 100km LKO and ask MJ2 to do the phasing orbit. It suggests 70km, which means I lose a warp level.

[dpraptor]

Hi All,

Thanks for the great help and all your replies. I can get to the target pretty well on my own, I was just playing with Mechjeb to see how it works. I figured out that the 2 ships were too far apart. Once I got them closer it worked just fine. I can see how this program can make y9ou lazy!! hahaha!! But it can be used when you have to do a lot of repetitious things like clearing debris or sending up multiple tankers for refueling. I was able to do this last dock with mechjeb,

I still don't know why I was so unable to dock, maybe because the station was getting pretty big, I figured the gravity would help pull me closer, but the damn ship kept moving away when I got real close, even when I had a perfect alignment and matched its speed.... So Mechjeb has its uses, I just don't like the idea of having it do things that take the fun away from the game.

But thanks for all the replies!

[TwoHedWlf]

I figured the gravity would help pull me closer, but the damn ship kept moving away when I got real close, even when I had a perfect alignment and matched its speed....

What gravity would pull you closer? Ships don't have any gravity. And unless you're directly ahead or behind of your target your orbit will be slightly different so you'll always drift around relative to it even if you start off with exactly 0 relative movement.