Killing Relative Velocity

[The Jedi Master]

I did an Apollo-style Mun mission, partially in honor of Apollo 11 and partially to learn docking. I got a pretty good intercept, but when it's time to kill relative velocity, the ships wizz right by each other. This MIGHT be because they are in opposite orbits, but the thing is, killing relative velocity is what ALWAYS gets me. This is supposed to be the EASY part of docking, darn it!

So, my question is: how do I kill relative velocity? Thanks in advance!

[Sathurn]

You must match orbits to kill relative velocity. Or kill relative velocity to match orbits

above your nvaball it will say either, orbit, surface or target. Click it until it say target and burn retrograde.

Edited July 20, 2013 by Sathurn

[Cheebsta]

You're probably boned if I am reading your post right, unless you accidently, or intentionally, built one of the sections with enough delta V to turn around and go the other way.

[sgt_flyer]

If you have an orbit which turn in the same direction of your target, and you manage to intercept your target, you need to :

Set your navball into target mode

Wait until you are under 2.3 km of your target

Burn towards the retrograde marker (while in target mode) until your relative velocity is 0. (at this point, the two orbits will be almost matched)

Once you have killed your relative velocities, start moving directly toward your target, the round pink marker. (but not too fast). During this time, try to keep your 'target' prograde marker and the round 'target' pink marker lined up, by making translation, and slow down more and more asyou get near your target.

You can try to practice orbital rendez-vous in low kerbin orbit too

[Tex_NL]

You're in opposite orbits and you expect you'll be able to kill relative velocity?

Jumping from one car to another while driving on the freeway is hard enough when the cars are driving side by side. What you're trying to do is jump between cars that are driving towards each other.

1: Launch your ship into a similar orbit.

2: Match planes

3: Increase/decrease Pe/Ap in such a way you get close to your target

4: Switch to target mode and burn at the retro marker when you get close. If your relative V is a few hundred m/s you might not be able to slow down in time.

[espm400]

~snip~

Jumping from one car to another while driving on the freeway is hard enough when the cars are driving side by side. What you're trying to do is jump between cars that are driving towards each other.

Is this part of your job, or just for kicks? Either way, sounds like fun...

[Skorpychan]

If they're orbiting in opposite directions, you are, in technical terms, screwed.

However, if you set up a maneuver node, you can probably use it to tweak your inclination 180 degrees to be going the other way. At least, I don't see why that wouldn't work.

[ChrisDaBox]

If they're orbiting in opposite directions, you are, in technical terms, screwed.

However, if you set up a maneuver node, you can probably use it to tweak your inclination 180 degrees to be going the other way. At least, I don't see why that wouldn't work.

Thats kind of impossible to do with one maneuver node :/ You're velocity would be too high by the time you got a 90 degree inclination change. You would reach kerbin escape.

[Dunbaratu]

I did an Apollo-style Mun mission, partially in honor of Apollo 11 and partially to learn docking. I got a pretty good intercept, but when it's time to kill relative velocity, the ships wizz right by each other. This MIGHT be because they are in opposite orbits, but the thing is, killing relative velocity is what ALWAYS gets me. This is supposed to be the EASY part of docking, darn it!

So, my question is: how do I kill relative velocity? Thanks in advance!

(Emphasis mine)..

Did I read you right.... you're trying to make rendezvous between objects in OPPOSITE orbits????? Trying to match their orbits as they pass each other by going OPPOSITE directions? Holy cow no wonder you can't kill the relative velocity enough. That's going to take you even MORE delta-V than it took to get up into orbit in the first place. You have to kill all the velocity you generated getting up, and THEN grow your velocity in the other direction just as far again.... No way ... Your mission is screwed if that's what you have to do.... UNLESS... is one of your two vessels in a highly eccentric orbit with a tall apoapsis? MAYBE if that's the case you can get that vessel to turn around at its apoapsis for a LITTLE bit less fuel than it would take to do it elsewhere in its orbit. Maybe if it's orbiting so high that its going less than 100 m/s at its peak you might be able to turn it around there... but probably not.

[Dunbaratu]

Thats kind of impossible to do with one maneuver node :/ You're velocity would be too high by the time you got a 90 degree inclination change. You would reach kerbin escape.

Also, the total delta-V needed is the same. Whether you turn one vessel completely around the opposite direction, or turn both of them 90 degrees to match each other, the total delta v summed between them is still the same. It's just that turning them both 90 degrees allows you to spend some fuel from each vessel instead of spending it all from one. But the amount of fuel needed is probably still going to be too much.

[Kimberly]

(Emphasis mine)..

Did I read you right.... you're trying to make rendezvous between objects in OPPOSITE orbits????? Trying to match their orbits as they pass each other by going OPPOSITE directions? Holy cow no wonder you can't kill the relative velocity enough. That's going to take you even MORE delta-V than it took to get up into orbit in the first place. You have to kill all the velocity you generated getting up, and THEN grow your velocity in the other direction just as far again.... No way ... Your mission is screwed if that's what you have to do.... UNLESS... is one of your two vessels in a highly eccentric orbit with a tall apoapsis? MAYBE if that's the case you can get that vessel to turn around at its apoapsis for a LITTLE bit less fuel than it would take to do it elsewhere in its orbit. Maybe if it's orbiting so high that its going less than 100 m/s at its peak you might be able to turn it around there... but probably not.

Matching their relative velocities should be pretty easy, actually. Just make sure the craft are heading straight at each other, and they'll work it out.

[Lazro]

Step 1: get an exact encounter

Step 2: EVA kerbals

Step 3: Fly past the hatch at 1000 m/s relative velocity and press F repeatedly

Step 4: Fly a rescue mission >_>

[Xeldrak]

Well, I have to turn one of your orbits around If your vessels don't have enough dv for this, you are screwed....

[maltesh]

Also, the total delta-V needed is the same. Whether you turn one vessel completely around the opposite direction, or turn both of them 90 degrees to match each other, the total delta v summed between them is still the same. It's just that turning them both 90 degrees allows you to spend some fuel from each vessel instead of spending it all from one. But the amount of fuel needed is probably still going to be too much.

Actually, that's incorrect.

The minimum delta-V necessary to do a ninety-degree directional change is 1.414x the speed of your spacecraft. Two separate 90-degree changes cost 2.818x the speed of your spacecraft (assuming your spacecraft is moving at the same speed in both locations.

The minimum delta-V necessary to do a 180-degree directional change is 2x the velocity of your spacecraft.

[MonkeyFuzz]

As a previous poster mentioned, learning to dock on the way to the Mun is probably not the best time to figure things out. Fly some Gemini-like docking missions in Kerbin orbit first, get a feel for approaches and translation between two craft. Only after you're comfortable doing would I recommend doing any kind of Apollo-style mission.

[MBobrik]

The minimum delta-V necessary to do a 180-degree directional change is 2x the velocity of your spacecraft.

Actually, it is only 83 % of your orbital velocity. boost to escape velocity minus arbitrarily small amount, wait till apoapsis, reverse, wait till periapsis, decelerate. you paid 2*(sqrt(2) - 1 + arbitrarily_small_amount) which is roughly 83 % of your orbital velocity.

Edited July 21, 2013 by MBobrik

[maltesh]

Indeed. Which is why I went with "Directional change", rather than "orbital plane change.". Probably should have further qualified as "instantaneous directional change,"