What is the most fuel efficient way to rendezvous with a space station

[leocrumb]

I am fine with rendezvousing (that word looks weird). I currently do it fine going from a smaller orbit intersecting it with one burn and then doing the retrograde target prograde thing. It works fine but u often use quite a bit of fuel slowing down to the target speed when u close distance. I was wondering if there is a more efficient way keeping a more constant speed. rather than stop start which is very inefficient.

How do you guys do a fuel efficient rendezvous?

Edited July 1, 2015 by leocrumb

[Frank_G]

Going directly up to the station without prior circularization burn is the most efficient way, i think. I also keep the rocket engines burning at low thrust for the whole ascent - so no unpowered coasting phase - to not loose precious speed to gravity and drag. I found this method to also be very useful, when piloting SSTOs to orbit.

[Feradose]

Direct rendezvous. If your station is on Keostationary orbit, you can smack your modules to your stations with minimum maneuvering.

[leocrumb]

Going directly up to the station without prior circularization burn is the most efficient way, i think. I also keep the rocket engines burning at low thrust for the whole ascent - so no unpowered coasting phase - to not loose precious speed to gravity and drag. I found this method to also be very useful, when piloting SSTOs to orbit.

But what about the final approach bit. There must be a more efficient way than stop start speed up slow down, prograde retrograde turning around. What do I aim at to keep the speed more constant?

[imaspacestation]

But what about the final approach bit. There must be a more efficient way than stop start speed up slow down, prograde retrograde turning around. What do I aim at to keep the speed more constant?

If you aim it well (will take lots of practice!), you will arrive within just a few km from your target. From there the approach is simple, but of course you will have to do that whole "stop start speed up slow down, prograde retrograde turning around" thing that you mentioned, but you really only need to do two burns to get close enough. Aim at the target, burn, turn around, burn again to come to a relative stop once close enough. From there it's docking time.

If you are having issues with the "turning around" part because your ship is big and unwieldy, then you can do the approach using RCS only, that way you can fire it retrograde to slow down without turning around.

[TheXRuler]

Well, the most efficient way to rendevousz that I find is still practical is to raise your Apoapsis to the target orbit, then rais your Periapsis just outside the atmoshpere. Next, set your target as target (duh ^^) and create a manouvre node at apoapsis, note that it is important that you actually intersect the target orbit at ap.

Now comes the interesting part, if you left click on your manouvre node you can manipulate it as usual, but if you right click after you left click the markers dissapear and you are left with two little circles at the bottom left and right and the x at the top left (assuming this has not changed in 1.0.x). The little circles enable you to look one (or more) orbits ahead (right button) or go back (left button [note that you can't look back in time ]). Cycle through the orbits until you get the closest encounter within a few orbits. then adjust the pro/retrograde components of your manouvre and watch the magic happen as your encounter distance changes. Then simply kill your relative velocity at closest approach.

By using this method you should be able to get an encounter within a couple of hundred meters at the first try. If you do it right (and time your rel. velocity negation burn just right) you can get within 50m without faffing around. From then on it is all just RCS work.

Also one of the big advantages is that launch timing is not as essential as it is when doing it differently.

Let me know if this was any help or if you want some more detailed explanations.

Edited June 30, 2015 by TheXRuler

[Rhomphaia]

But what about the final approach bit. There must be a more efficient way than stop start speed up slow down, prograde retrograde turning around. What do I aim at to keep the speed more constant?

Rather than stopping and starting when you have a close approach.

as you close in on your final approach, point at the target relative retrograde marker. then imagine a line between this marker and the anti-target marker then point yourself the opposite direction along this line (rolling the spacecraft so the anti-target marker appears directly above or below the retrograde marker can help visualize this)

burn a bit and you will notice the retrograde marker gets pushed towards the anti target marker. When they line up, you are heading right for it, as long as you weren't too far out when you started.

[Snark]

Going directly up to the station without prior circularization burn is the most efficient way, i think.

+1 to this (and the other posters who recommend the same thing).

But what about the final approach bit. There must be a more efficient way than stop start speed up slow down, prograde retrograde turning around. What do I aim at to keep the speed more constant?

I'm a bit confused by this statement. What do you mean by "stop start speed up slow down prograde retrograde turning around"? There's none of this, if done properly.

For brevity in the following, I'll use "TRVR" for "target relative velocity retrograde" and "TBR" for "target bearing retrograde" when referring to navball markers.

1. On the launchpad, wait until your target is in the right spot (typically about 5 degrees or so above the western horizon, for something in LKO).

2. Do your usual liftoff and gravity turn.

3. Once you're above 30km or so, switch to map view and tune your up/down pitch a bit so that your trajectory kisses the target's orbit right at a perfect intersect (you can get it within a km or two without too much trouble).

4. Do the usual coast-upwards-towards-apoapsis.

5. As the target approaches, thrust in the TRVR direction to keep the closing speed manageable. Note that this is not going orbital-retrograde, it's only retrograde relative to the target. It's actually orbital-prograde, i.e. it's almost exactly the same burn you'd be doing if you just wanted to put yourself in orbit in the most efficient manner possible.

6. As the target approaches you, you can fine-tune your approach to keep your TRVR marker lined up on your TBR marker. You're going to be doing your thrust in the TRVR direction anyway; all you need to do is to adjust your direction slightly off of that to nudge the TRVR until it lines up with TBR. Should need minimal nudging, since they start off pretty close together.

7. Slow yourself to a stop (relative to the target) when you're just a few meters from it.

8. Rotate your craft to line up the docking ports, then a tiny nudge (0.2 m/s or so) to set them drifting towards each other, with a bit of lateral RCS correction if needed.

...The important thing is that in the above sequence, everything except step 8 is exactly what you would do to launch efficiently to orbit. You're not "wasting" anything by docking; all of your thrust is contributing efficiently to achieving orbit, just with the constraint that you want your orbit to exactly match the target's. The only docking "wastage" is in step 8, and that's negligible, it's just a few centimeters per second.

[moogoob]

I've only done a direct redezvous launch once, and it was by accident. They're tricky to time. Mostly I just bring a ton of fuel into orbit and launch into a smaller orbit for later intercept maneuvers. This takes longer, but is much simpler and easier on the brain. The "Big dumb rocket" solution, as it were.

[Empiro]

I am fine with rendezvousing (that word looks weird). I currently do it fine going from a smaller orbit intersecting it with one burn and then doing the retrograde target prograde thing. It works fine but u often use quite a bit of fuel slowing down to the target speed when u close distance. I was wondering if there is a more efficient way keeping a more constant speed. rather than stop start which is very inefficient. How do you guys do a fuel efficient rendezvous?

It sounds like you're already doing it fairly efficiently. The bigger the differences in your two orbits (for example, if you're in 100km orbit, and your target is at 2000km), the more fuel it will take to initiate the transfer and to slow down, because you're basically matching your target's orbit when you slow down relative to it.

However, one thing you might not be doing correctly is that when initiating your transfer, you want your orbit to tangent to your target's (i.e. touching it at only one point, either at your AP if you're going up to meet your target, or at your PE if you're going down). You don't want to cross your target's orbit and shoot past it. If you do, your relative speed will be much higher when you meet, and you'll need to spend much more fuel slowing down.

A direct launch to intercept as some have suggested is indeed more efficient, but only if you follow an efficient ascent profile. Even if done perfectly, you're not saving too much compared to getting into a slightly lower orbit first. It's only a few tens of m/s to adjust your orbit by 10km. If during your ascent, you have to adjust your orbit quite a bit to get an intercept (or if your orbit shoots past your target to get a close intercept), then you can very easily spend more fuel compared with simply going into a slightly lower orbit and waiting until the right window approaches.

Edited July 1, 2015 by Empiro

[Snark]

This is true only if you follow an efficient ascent profile. Even if done perfectly, you're not saving too much compared to getting into a lower orbit first, because it's only a few tens of m/s to adjust your orbit by 10km. If during your ascent, you have to adjust your orbit quite a bit to get an intercept (or if your orbit shoots past your target to get a close intercept), then you can very easily spend more fuel compared with simply going into a slightly lower orbit and waiting until the right window approaches.

True. However, with practice, I've found that it's not too hard to eyeball the position; I launch straight to intercept and come pretty close without having to do a lot of adjustment on the way up.

There's another cost associated with intercept besides funds, fuel, or what-have-you: patience. I find that with practice, the direct-ascent-to-dock takes about the same dV as circularize-at-slightly-lower-radius, and requires considerably less fiddling (i.e. amount of play time I have to spend setting up the intercept). Almost-same-dV, much-less-of-my-time, translates to a win for direct-intercept in my case. I realize others may have a different set of priorities.

Another factor that can influence the choice is what mods you're running. For example, I like to play with RemoteTech, which requires having direct line-of-sight links to control unmanned craft. Until I get around to setting up a planet-spanning relay network, there's value in direct-intercept because I can get there and dock before I'm over the horizon from KSC.

[ExaltedDuck]

Two words: harpoon winch. You're welcome.