Rendezvous too fast, can't reach target.

[lizarddan]

So, I'm getting closer.

I managed to make target velocity 0ms, with fuel to spare. However, when I burned retrograde, my orbit went away completely and I ended up going towards kerbin.

In the tutorials I see, when they burn retrograde their orbits match up with the target. Since my orbit is originally bigger than the target's, should I be burning prograde? Thanks all .

[sdj64]

To rendezvous in the first place you can't be on the same orbit, because the ships are on opposite sides of Kerbin right now. In an orbit that low you might want to raise the apoapsis higher (which will make you take more time to go around) and once the other ship has nearly caught up, lower your apoapsis back down to get an intercept.

If you got target velocity 0 m/s and you can see your target ship or at least the marker for it, turn towards it and burn that way. On the target navball you get two sets of icons: The pink is direction towards target (the pink circle with dot) and away (the 3 pink lines with dot.) and also the standard pro/retrograde markers but these are for your target velocity, not orbital velocity (as Radonek said).

You want to try to line up the prograde marker with the towards target marker and you will go closer to your target. When you get close enough for the Kerbal to EVA across, turn around and burn retrograde until your speed relative to target is zero.

[zeppelinmage]

I highly suggest this video. Following his advice is how I was able to learn how to rendezvous/dock first time.

[lizarddan]

Quick question.

To find the radius of an orbit that isn't circular... Let's say the apoapsis is 100,000 and the periapsis is 60,000, how would I calculate that? I found the formula for the radius of an ellipses here: http://www.mathopenref.com/coordgeneralellipse.html

It's been awhile since I've done this type of algebra and I'm at a loss. Heh, thanks all! Still trying to get Kerman down....

[iBeej]

The main thing you need to remember is orbital mechanics 101. Your speed is directly relative to your altitude. So it's literally (physics) impossible for you to "catch" your target while at the exact same orbit and eccentricity. As soon as you burn and increase speed, you will increase your altitude. For example, burning prograde at Periapsis will increase your Apoapsis altitude, and vice versa.

So how do you "catch" your target? The simplest explanation I have given folks on this one is imagining your orbit like a circular race track. We all know that a race car cutting on the "inside" of the track while both cars are going the same speed results in a "gain". So if you want to catch up, you gotta cut. If you want to slow down, you gotta move to the outside lane.

Once you do this, and you begin to close in on your target, remember to fix your eccentricity so both Periapsis and Apoapsis are relatively close to target orbit. The rest is simply minor maneuvers with your target when you're within range. And by range I mean, 2km at most if you don't want to be ping ponging back and forth between prograde and retrograde all night. So to make it easy on yourself, make sure you get your intercept markers to within 2km before fixing your eccentricity.

It takes practice, but once you figure it out, it's like a light bulb. And riding a bike. You will ask yourself why you ever thought it was hard after all of this.

Good luck!

[iBeej]

In the tutorials I see, when they burn retrograde their orbits match up with the target. Since my orbit is originally bigger than the target's, should I be burning prograde? Thanks all .

No. First, remember this rule. It's always most efficient to perform your prograde or retrograde burns at Apoapsis and Periapsis.

Let's say your orbit is LARGER than your target orbit at your orbit's Apoapsis. The quickest and most efficient way to match your orbit there, is to perform a RETROGRADE burn at your Periapsis. This will reduce the Apoapsis Altitude. To increase the Apoapsis altitude, you burn PROGRADE at your Periapsis. This is true whether you are at Periapsis or Apoapsis. Burning at Periapsis will effect Apoapsis. And burning at Apoapsis will effect Periapsis. You will need to adjust BOTH Periapsis and Apoapsis at their OPPOSITES.

Right now, you're probably thinking, "But if I burn retrograde at my Periapsis, it will lower my Periapsis and I will fall to the planet!" And while that's an "intuitive" thought, it's wrong and it doesn't work that way. In fact, if you are PRECISELY at Periapsis and burn retrograde, your altitude AT Periapsis will barely change at all. This is due to "Precession" thanks to our friend gravity. It takes gravity time to pull us down to the planet. And since we are already at velocity, the effect of burning retrograde doesn't happen until later in the orbit. (At Apoapsis). I'm telling you all of this because it's important to understand these core concepts if you want to be successful.

Just remember that retrograde slows your speed. Hence lowers your altitude. And prograde increases your speed, hence raises your altitude. But much later in your orbit.

WARNING: Be careful with your use of burning toward the purple target marker at any distance greater than 2km from your target. This is the quickest way to screw up your orbit unless you are close enough.

Does that make sense?

Edited January 22, 2014 by iBeej
Grammar

[Smidge204]

The OP states clearly in post #5 that they are going the same way, east. Try reading up a bit about the difference between speed and velocity and then come back and retract your comments.

The only thing wrong with my comment, which I do apologize for, is somehow I got my head stuck in a non-inertial reference frame: If the reference frame is allowed to rotate with the ships, then their relative velocity is zero if they are in identical orbits. (Think two painted dots on a spinning disk - relative to the disk they are not moving, but relative to the table or other inertial reference, they are)

=Smidge=

[radonek]

So, you just need whole universe to rotate around Kerbin? I see, such triffles are easy to forget :-D

[RoboRay]

To simplify the explanations... the 4km/sec velocity difference is simply because the two ships are nearly on the opposite side of the planet from each other. If you shift one of their orbits so that they approach each other, when they are on the same size of the planet their relative velocity will be very low.

Edited January 22, 2014 by RoboRay

[Rickenbacker]

To simplify the explanations... the 4km/sec velocity difference is simply because the two ships are nearly on the opposite side of the planet from each other. If you shift one of their orbits so that they approach each other, when they are on the same size of the planet their relative velocity will be very low.

Oooh! That makes so much sense. You learn something new every day.

[tSk]

@lizarddan:

Your situation is exactly the same I was in during my first career, trying to learn how to do an orbital rendezvous to rescue Jeb from high orbit. Like you, I also had no RCS parts yet. In reality, it's a pretty simple procedure once you get the hang of it. Here's how to save your kerbal:

1. Once in orbit in the chase ship, target the stranded ship.
   
2. Maneuver and burn so that your orbit lines up roughly with your target's orbit, but is not identical to it. If the ships are in identical orbits, they'll never catch up with each other.
   
3. At either the ascending or descending node on your map, place a maneuver node and tweak the purple arrows until your orbital inclination matches the target's. Burn as required. The orbits should now be "flat" relative to each other. Congratulations, you've removed one whole dimension from the equation.
   
4. Find the marker for "intercept 1" on your map. Place a maneuver node at that location and tweak it (playing with prograde/retrograde should be all that's required) so that the markers for intercept 1 and target position at intercept 1 line up as close as possible. That's where you'll intercept your target. Note the separation; you want it to be a kilometer or less ideally. To save fuel, you may want to wait a few orbits before doing this, letting the ships get closer together.
   
5. Execute the maneuver.
   
6. Afterward, you should now be on course to intercept your target. When you reach the intercept point, you'll need to kill your relative velocity quickly or risk overshooting. Click the top of your navball where it gives your speed, until it reads "target". This shows you your velocity relative to your target, and changes the prograde and retrograde markers on your navball to correspond to the target ship. That last part is crucial.
   
7. Point your ship at the retrograde () marker and burn your main engine. By thrusting in the opposite direction of the target's motion relative to you, you reduce your relative velocity. Keep it up until your velocity reads as 0.0. If you overshoot, just turn the ship around and burn in the other direction. You are now stationary.
   
8. And now the fun part. Unless you've been very exact, it won't stay at 0.0 for long. The ships will gradually begin drifting apart unless their orbits are exactly the same. So it's time for a walk. Switch back to the other ship and say goodbye (for now) to your stranded vessel, then jump out and head for the chase ship using your jetpack.

Good luck!

Edited January 24, 2014 by tSk

[tSk]

I managed to make target velocity 0ms, with fuel to spare. However, when I burned retrograde, my orbit went away completely and I ended up going towards kerbin. In the tutorials I see, when they burn retrograde their orbits match up with the target.

I'm not sure but I think I see the problem. The difference, if I'm right, is that you're burning prograde relative to the planet and not relative to the target ship. That would explain why it has that effect.

Prograde is the direction you're traveling, relative to some other object, which is usually the planet you're orbiting; retrograde is the exact opposite direction. Thus, when you burn facing prograde, you add to your velocity, which causes you to gain altitude on the other side of your orbit. But burning retrograde has exactly the opposite effect.

What's happening in the tutorials, I think, is that the people you're watching are burning retrograde while their navballs are set to <i>target</i> mode. That means the retrograde marker on the navball doesn't show the opposite of the direction they're traveling, but the opposite of the direction their target is traveling. Burning in that direction therefore causes them to 'slow down' relative to the target ship - in other words, their orbits line up - which is normal procedure for a rendezvous.

If I'm right, your navball is set to 'orbit' or 'surface' mode. Therefore, your retrograde marker points in the opposite direction your ship is traveling relative to the planet, and by burning in that direction, you're slowing yourself down relative to the planet and losing altitude.

Edited January 24, 2014 by tSk

[Alistone]

To simplify the explanations... the 4km/sec velocity difference is simply because the two ships are nearly on the opposite side of the planet from each other. If you shift one of their orbits so that they approach each other, when they are on the same size of the planet their relative velocity will be very low.

It can be reasoned that spokes on a bicycle wheel have zero velocity to eachother because they are in the same orbit (and physically attached such that they aren't moving relative to eachother). But as the bicycle wheel rolls over the surface, the top of the wheel has to travel twice as fast as the bike to get in front and the bottom of the wheel does not move relative to the surface.

Thus you can see that the top of the wheel seems to be going twice as fast as the bottom of the wheel relative to the ground as the bicycle rolls along even though relative to eachother the top and bottom of the wheel are not moving; they are always the same distance apart.

But it seems that the vectors of the target and the active ship are subtracted from eachother to calculate the "target velocity"; whereas I (and others) assumed it was calculated by the change in distance over time between the target and active ship.

Ships in circular orbits are like the bicycle spokes and have no change in distance between eachother. But (it seems to me) that is not what "target velocity" is calculating.

I would clarify that when he says "shift one of their orbits" he means change the phasing of their orbits so they are on the same side of the planet (and still sharing the same same normal or anti-normal direction).

But I better understand it now. Even if my distance to kerbin is constant (as I'm in a circular orbit) I still have velocity relative to kerbin (my orbital velocity). It is as though even in the same orbit going in the same direction the two ships can maintain the same distance from eachother but they still have relative velocity as they slide sideways with eachother just as I slide sideways above kerbin.

Constant distance does not mean zero relative velocity.

Edited January 25, 2014 by Alistone

[lizarddan]

Well, guys. I figured it out.

UNFORTUNATELY, my save was tragically deleted through some stupidity on my part. I'd been cleaning files in a frantic attempt to find a paper I had due in a course.

However, I did a tutorial, watched the same videos REPEATEDLY and discovered my issue.

Number 1 - I suddenly realized that the TARGET intercept was the one with the dotted line. Wow. When they said line up the intercepts, I thought they meant the purple and orange intercepts on the same orbit! How silly of me.

Number 2 - I knew about clicking to target velocity, but I think I forgot to do that on some occasions. I was able to successfully rendezvous and dock in a scenario when I corrected my mistakes.

What now? Well, I'm not one to ignore a challenge. I'm going to launch another Kerman at a 76km orbit without fuel, to replicate my last predicament and use a rescue probe to get him down. I'll also mark this as answered, as I received so many brilliant responses.

Thank you guys! I'll update this soon.

[Taki117]

Well, guys. I figured it out.

UNFORTUNATELY, my save was tragically deleted through some stupidity on my part. I'd been cleaning files in a frantic attempt to find a paper I had due in a course.

However, I did a tutorial, watched the same videos REPEATEDLY and discovered my issue.

Number 1 - I suddenly realized that the TARGET intercept was the one with the dotted line. Wow. When they said line up the intercepts, I thought they meant the purple and orange intercepts on the same orbit! How silly of me.

Number 2 - I knew about clicking to target velocity, but I think I forgot to do that on some occasions. I was able to successfully rendezvous and dock in a scenario when I corrected my mistakes.

What now? Well, I'm not one to ignore a challenge. I'm going to launch another Kerman at a 76km orbit without fuel, to replicate my last predicament and use a rescue probe to get him down. I'll also mark this as answered, as I received so many brilliant responses.

Thank you guys! I'll update this soon.

Well, that first one was the big one, So congrats, and I suggest instead of putting your ship into orbit without any fuel but instead launch it with enough fuel to de-orbit itself so that you don't have to de-orbit a craft with two command pods and have to worry about them getting larger than 2.5km from each other.

[tSk]

You can also use the TAC Fuel Balancer mod to dump all the ship's remaining fuel once you get it where you want it to be.

It's a great mod to have, anyway. It helps move fuel around the different tanks in your ship - something that you can do anyway, just with more clicking around than you have to do using the mod.

[Taki117]

You can also use the TAC Fuel Balancer mod to dump all the ship's remaining fuel once you get it where you want it to be.

It's a great mod to have, anyway. It helps move fuel around the different tanks in your ship - something that you can do anyway, just with more clicking around than you have to do using the mod.

I second this motion. It's a great mod to have, and really helps if you have either lots of fuel tanks (Like on some lander designs) or need to transfer large amounts of fuel around in a hurry.