Rendezvous help

[thevegimobil]

Me again... I am using erkles clamps as a basic docking mechanism... I have launched Apollo style missions where the lem is behind the cm and turn and dock ect... That works fine.... But when I use mechjeb to time orbit to rendezvous it is always miles off I don't even get close.... Any help or other plugins would be so much appreciated ... Then I can get going on my stations! Thanks

[draeath]

Do you expend any significant amount of fuel in the turn/dock maneuver?

I wonder if MechJeb doesn't realize the previous stage is still connected (eg after you disconnect then re-dock, it might only be considering the CM's mass?)

[thevegimobil]

nope ive used infinate fuel to try and get close and get nothing...

[bsalis]

Read Orbital Rendezvous Made Easy and watch the vid there as well.

[Frederf]

Mechjeb's rendezvous algorithm works by using a known LPA. The LPA is calculated based on the previous launch. It also only works equatorial plane and I think only eastward at that. You haven't said how you're using the rendezvous (what ship on what body) but I'm assuming this is a post-munar landing rendezvous with the CSM?

Try quicksaving on the surface, doing a munar liftoff, noting what the LPA turns out to be in Mechjeb. Then manually enter this number for your second try quickload.

Alternately look up the transit time for a Hohmann transfer from surface to destination and figure what that is in fractions of an orbit that is for your destination craft. The should be very close to your LPA. To find my burn time for my last Kerbin-Minimus injection I did the following:

1. Find the transit time from parking orbit to Minimus orbit.

2. Find the angular orbit speed of my parking orbit and of Minimus

3. Find the orbit angle Minimus traveled in the time from #1. This is my LPA.

4. Find my effective orbit speed by subtracting Minimus's.

5. Find the time after opposition to achieve LPA from #4 #3.

6. Calculate the orbit angle of Minimus when it appears to come over Kerbin's disk.

7. Use #6 and #4 to find time since opposition to sighting.

8. Subtract time since opposition to sighting from time after opposition to burn to get time from sighting to burn.

9. Observe Minimus emerge from behind Kerbin, start timer.

10. At #8 time later, begin injection burn.

I intersected with Minimus's rather small SOI quite well using this method and all it took was looking out the window which was surprisingly precise time-wise. The higher your parking orbit the lower your orbit angle rate which will enhance precision.

[thevegimobil]

Hmmm I make a note of the target lpa when I launch it then put that number into the autolaunch in mechjeb but its still miles off

Edited September 28, 2012 by thevegimobil

[felixar90]

Hmmm I make a note of the target lpa when I launch it then put that number into the autolaunch in mechjeb but its still miles off

"Miles off" is pretty normal. Timing your launch with mechjeb will usually get you 5~20 km away from your target... Depending how close you are, you can either directly close in to you target by alternating between burning toward TGT+ and RVEL- (when 10km>distance)

For longer distances use normal rendezvous manoeuvres, if you are in advance on your target, accelerate on a bigger orbit to let it catch you. if you are late, decelerate on a smaller orbit to close on it.

You can use very small amount of fuel and have the manoeuvre take 3 days, or use more fuel and catch you target faster.

[thevegimobil]

Ok thanks I just don't feel natural using the rcs to get close... Always loose control... Can you quickly run through the new commands on the smart a.s.s on mechjeb when you have a target selected?

[felixar90]

Yes. And if your command module produce enough torque on your ship, turn the rcs off on large ships, mechjeb tend to waste a lot of rcs fuel trying to orient the ship perfectly. But yea, once you have a target selected in the rendezvous module, just use the smart A.S.S. (but like I said, you must be in the vicinity of your target to use TGT+ and RVEL-)

For longer distance, if you ship is behind the target, make your pe and ap about 5 km lower that your target's pe and ap.

if you ship is in front of your target, make your orbit 5 km higher that your target's.

When the distance on the render-vous module is at it's lowest (around 5 km) and you can se on the orbital map that you are directly over or under your target,

place your ship in RVEL-, and burn until the relative velocity in around 0, then burn toward TGT+ until you got a good speed going (25~30ms) and immediately replace your ship in RVEL-

Each time you can tell you're going off-course, burn toward RVEL-, then give a little push toward TGT+.

Edited September 28, 2012 by felixar90

[thevegimobil]

I will try... Thanks tbh most helpful thing yet on rendezvous

[Beowolf]

Something that helped me a lot in my manual docking last night was the Rendezvous option in the Engineer plugin. It breaks your relative speed into forward, vertical and horizontal components (ORDA has this too, but it's in a harder-to-read form). Once I was within a few km, I made the rendezvous by thrusting forward a few seconds with RCS, then thrusting sideways to eliminate the vertical and horizontal velocities. Doing it this way made it my easiest approach ever.

To get close in the first place, here's what I did:

Target was in a 100 km equatorial Kerbin orbit. I let MechJeb try to time the launch, but had no LPA and just assumed the computer'd miss. So I set my orbit for 95km instead of 100.

As soon as the circularizing burn was finished, I checked the map to see how far off we were, and in which direction.

If the target's ahead of me in orbit, I'll catch up.

If the target had been behind me in orbit, I'd need to slow down so would have immediately thrust prograde to increase my apo to 105 km, wait for apo, then circularize.

Now I waited, letting the distance close.

I had previously used the rendezvous spreadsheet here: http://kerbalspaceprogram.com/forum/showthread.php/8915-Orbital-Rendezvous-Made-Easy

to calculate when to burn to reach rendezvous from my standard orbits.

So, from my 95km orbit, when distance to target reached 12.5km I boosted prograde until my apo reached 100km.

If I'd been at 105km, I'd have boosted retrograde at 13.1km to reduce my apo.

When I arrived at 100km, I circularized yet again. It was tempting to try to dock instead since I was only a few hundred meters away. But I've learned what happens when I get impatient during orbital maneuvers, as I'm sure we all have.

Finally, I switched to RCS and precision control, zeroed out my relative velocities, and thrust forward at 1m/s. Because the orbit isn't a straight line, vertical and horizontal velocities keep creeping in and have to be frequently zeroed out. It was slow, but put me dead on-target without ever losing control.

Oh, and one silly tool that was quite helpful in the final stages: First, note how to tell which side of your orbital stage is "up", so that when viewed from behind, the I,J,K,L RCS translation keys push you in the proper direction visually. Next, take a PostIt note, draw a big plus-sign in the middle, and label the ends with the I,J,K,L directions. Mark "top" on the note to match the rear view of your rocket. Now you have a visual reference you can stick on the screen, oriented the same way as your craft. No matter how you happen to be rolled, look at it from behind and you'll know which key to press to move in the direction you want. Trying to translate the rotations in my head was making me crazy,and ORDA insists on a specific orientation to work.

Eventually I imagine we'll be able to roll our camera view independent of the craft, but until then this helps me a lot.

[Supernovy]

I have found that doing rendezvous in IVA can help with the orientation thing. Works best with the Mk1 cockpit and RCS translation. I got within 6 metres and less than 1 m/s relative velocity to my orbital craft without plugins this way.

Also, it looks cool.