Question Rendezvous Help

[Chub777]

I already know how to get into orbit, align planes, sync orbits and dock, but when ever I meet up with another space craft, I'm usually ~50-500km away from the target. I also use up a lot of fuel to close the distance.

For example, I'm in a default DG and I just started picking up the XPDR signal from the ISS. The closest I will end up to the ISS is, say, 100 km. To close the distance and dock, I will need to fire the main engines towards the station. I burn and use up nearly 2 tons of fuel in total. Docking also uses up 1 ton for me. Is there a way to rendezvous with the station and dock without using that much fuel?

Thanks.:tiphat:

 

[IronRain]

You don't need to fire your engines towards the station but towards the velocity markers in the Docking HUD. I suggest to watch the tutorial made by Tex. He will explain it

http://www.orbiter-forum.com/showthread.php?t=2055

 

[Mister Mxyzptlk]

If you master sync orbit MFD, and catch up with ISS slowly and gradually, you hardly need to burn more than a few seconds to match ISS's orbit.The key is reducing DTmin to a minimum, preferably zero. Another useful tool is to use the IMFD "Velocity match" program as it does the burn for you and you can finish up with either the autodock program in the DG4 or standard docking procedure. PM me if you want detailed step by step instructions.

 

[C3PO]

Did you make sure that the planes are 100% aligned? A RInc of 0.1° can put you several 10s kilometers away.

 

[Urwumpe]

Did you make sure that the planes are 100% aligned? A RInc of 0.1° can put you several 10s kilometers away.

0.1° = Almost exactly 11 km off at the ISS orbit.

 

[C3PO]

0.1° = Almost exactly 11 km off at the ISS orbit.

Yes, but after burning towards target during a bad rendezvous you can end up far away. And you rarely see a rendezvous that's 0.1° off but perfect in every other aspect.
The reason I mentioned RInc is that, in my experience, this is the orbital property that beginners have the most problems "visualizing".

 

[Chub777]

RInc is usually 0.0° but DTmin drifts to 0.1.

 

[C3PO]

RInc is usually 0.0° but DTmin drifts to 0.1.

If both RInc and DTmin are 0.0, you will actually hit your target. You need to practise breaking. Time to break out the pocket calculator.

 

[Poscik]

If both RInc and DTmin are 0.0, you will actually hit your target. You need to practise breaking. Time to break out the pocket calculator.

You will not hit target at DTmin 0.0 in case when you want to intercept target at your vessel's apogee, and this apogee is for example 50km below station orbit. DTmin does not count orbit intersection. You have to set it up manually.

 

[C3PO]

You will not hit target at DTmin 0.0 in case when you want to intercept target at your vessel's apogee, and this apogee is for example 50km below station orbit. DTmin does not count orbit intersection. You have to set it up manually.

You should only use Ap modes for the initial transfer burn. For course corrections use the intersection modes. Small RCS bursts is enough to get you really close.

 

[Urwumpe]

More important: Learn to be imprecise.

Perfection costs more fuel than permitting some small deviations that slowly get reduced until you hit the target.

 

[Grover]

You should only use Ap modes for the initial transfer burn. For course corrections use the intersection modes. Small RCS bursts is enough to get you really close.

i never use Ap modes, only ever Ref1 mode, and ive always managed to get close to the ISS in one brake burn, my closest was about 70m, and the farthest was about 650m. just use IMFD velocity match, and start the burn when the time to intersection (with DtMin=0) is half the burn time

 

[C3PO]

More important: Learn to be imprecise.

Perfection costs more fuel than permitting some small deviations that slowly get reduced until you hit the target.

I'll have to disagree on that one. For now anyway. :lol:

Chub reported that he was ~50-500 km off target. You need to be a lot closer then that.
When you can be spot on target every time, you can start experimenting with the degree of accuracy required. Turn off non-spherical gravity so you can see how the RCS inputs change your orbit/approach.

Select a high catch-up orbit. The relative speed on arrival will be much more manageable. Practise stationkeeping and maneuvering around the ISS to get a "feel" for the relative velocity HUD markers.

To get really nice approaches use this MFD:
[ame="http://www.orbithangar.com/searchid.php?ID=1199"]Rendezvous MFD 050621[/ame]

 

[Tommy]

I think what he's saying is don't worry about being absolutely perfect when you are still a few orbits away. You's spend a lot of fuel "chasing" perfection - when it's better to make a tiny adjustment 1/2 orbit from rendezvous when accuracy is more attainable. This is even more the case if non-spherical gravity is enabled - DTMin will keep changing so there's no point in trying to get it at absolute zero when you still have a few orbits to go.

It's much like an interplanetary flight. There's little point to trying to get the PeA perfect when you are making the transfer burn - it's going to change during the flight. Making MCC's too soon is also wasteful.

 

[Urwumpe]

I still have to disagree - perfection is not about getting the exact numbers, but about learning how far you can leave the perfect path and how to stay on course. The MFD helps you seeing the perfect course, but it isn't it.

 

[C3PO]

i never use Ap modes, only ever Ref1 mode, and ive always managed to get close to the ISS in one brake burn, my closest was about 70m, and the farthest was about 650m. just use IMFD velocity match, and start the burn when the time to intersection (with DtMin=0) is half the burn time

I guess we have different temprament when it comes to approaching another craft. I never liked the 700+ m/s maneuver close to a target, even if it's relatively easy in a Deltaglider. It's just too unrealistic for my taste. I can put the DG on the docking line ~25-30 m from the port without any problem.
I usually never stop completely during docking. I just put the "bug" where I want to be next, and adjust the approach on the fly. The DG is so overpowered that you can do almost any maneuver you want/need. :thumbup:

I find it much more satisfying to plan the approach so that you end up on the docking line with >1 m/s DV using orbital mechanics, just before orbital sunrise.

---------- Post added at 06:11 PM ---------- Previous post was at 06:01 PM ----------

I still have to disagree - perfection is not about getting the exact numbers, but about learning how far you can leave the perfect path and how to stay on course. The MFD helps you seeing the perfect course, but it isn't it.

I don't really disagree with your point. But when it comes to rendezvous, you need to be closer than 100 km at "interception". Once you get a "better" understanding of the mechanics involved, you can try out how far off is close enough.
Chasing zeros several orbits ahead is useless, even with spherical gravity. Perturbations can still pull you off target by some margin.

 

[Urwumpe]

Yeah. also if you have a good strategy for the rendezvous, and know how to split the approach into multiple orbits, the perturbations are just a minor factor. Getting it all right at the first shot is usually the wrong, even though more glorious way. But nothing beats the glory of arriving.

 

[C3PO]

.... But nothing beats the glory of arriving.

Yeah, that's probably it. I like the view too much, so 5 m/s DV is better then 500 m/s. :lol:

 

[Grover]

I guess we have different temprament when it comes to approaching another craft. I never liked the 700+ m/s maneuver close to a target, even if it's relatively easy in a Deltaglider. It's just too unrealistic for my taste. I can put the DG on the docking line ~25-30 m from the port without any problem.
I usually never stop completely during docking. I just put the "bug" where I want to be next, and adjust the approach on the fly. The DG is so overpowered that you can do almost any maneuver you want/need. :thumbup:

I find it much more satisfying to plan the approach so that you end up on the docking line with >1 m/s DV using orbital mechanics, just before orbital sunrise.

i agree, it is MUCH easier when your approach velocity is below triple figures (not counting decimals ), i try to keep it below a 15-20 second thrust of Dv at all times, maybe a little more for low acceleration craft like the shuttle or Dragon spacecraft, especially if i can get them both to interception burn with loads of spare fuel, i try to make my interceptions rather easy:

1)choose a launch date where you will get to stable orbit no more than 30* (angle from Earth-s center) away from target
2)stay lower than target (IE dont raise orbit up to target) until you are about 10* ahead (do plane adjustment now)
3)THEN raise AP and stabilise at roughly the same altitude as target
4)raise or lower one side of the orbit to get DTMin to zero (use lots of entries on SyncMFD, and allow a few orbits before interception, if you are already close, you can get the best approach to move 5 places in under 5 seconds or burn time)
5)adjust DtMin once or twice per orbit, as necessary
6)when it comes time to stabilise orbits again, you should only have a little burn to do


its never failed me since i developed that technique, just dont get a very elliptical orbit because you were on the far side of earth, wait to "catch up" on a lower orbit first

 

[C3PO]

1)choose a launch date where you will get to stable orbit no more than 30* (angle from Earth-s center) away from target

Not a factor. You can always stay low for longer. It doesn't make that much difference in time, and NO difference in fuel consumption.

2)stay lower than target (IE dont raise orbit up to target) until you are about 10* ahead (do plane adjustment now)

Huge waste of fuel. If you get in front of your target, the only way to get back is to go to a higher sma (then your target's) and back down again for rendezvous.

5)adjust DtMin once or twice per orbit, as necessary

Completely unnecessary. You only need to be that accurate during the last ½-1½ orbit. (except for plane alignment. You might save some fuel doing plane corrections early, but not always)

You should gradually raise your orbit, but don't get above your target. You should be ~50 km below the target when you're 10-20° behind. (50 km below both ApD and PeD) From there you can even do a visual rendezvous without MFDs (using just docking HUD + COM/NAV-MFD)

If you're using a Deltaglider and don't care too much about fuel economy, you can come in screaming with your tail on fire and disregard all of the above.