Rendezvous too fast, can't reach target.

[lizarddan]

Hi all

I've played KSP for maybe 10-20 hours overall. I have a grasp on the fundamentals, can reliably get into orbit, etc. However, one of my few attempts at orbiting went sour when I ran out of fuel. Now, Kerman has been patient for the 9 hours or so he's been up there - so I decided to take on the challenge of getting him down. Since I'm in career mode, parts are somewhat scarce, I don't have RCS yet. But, I managed to make a pretty robust ship using liquid and solid fuel engines. The total Delta Force is 5420m/s, the beginning stage's TWR is 2.7.

Orbit inclination, setting the rendezvous intersection and even making the orbit IDENTICAL is easy. The hard part is that Mr. Kerman (in a Low Kerban Orbit of around 70km) is going 4,000m/s. I don't have enough fuel in the world to brake enough for this guy, how would I handle this? Optimize launch time? Wait for him to come around the bend? Should I eject kerman from his lightning-probe and make him stay still?

Taking all suggestions!

Thanks all!

Edited January 21, 2014 by lizarddan

[DisarmingBaton5]

The hard part is that Mr. Kerman (in a Low Kerban Orbit of around 70km) is going 4,000m/s.

Is that target relative velocity? In that case you are screwed...

Screenshots would help.

[Bengsch85]

Orbital velocity at 70km kerbin orbit is roughly 2300 m/s and if you match orbits they relative speed should be 0 because the orbital velocities are identical. The only thing I can think of is your 2 ships are orbiting in opposite directions?

[Taki117]

It is ver likely your ships are orbiting in opposite directions, meaning that instead of your relative speed being the difference of your two ships it is actually the addition of them. what you need to do is figure out which way ship A is going (west to east or vice versa) and launch that direction. (if ship A is goint east to west launch west and vice versa)

[lizarddan]

Screen shots ahoy!

I'm sorry I didn't screen cap when I had it in exact orbit, kind of a silly thing to not do. But this one got close, yet... so far :|.

The ships are going the same direction, east.

Orbital Inclination always ends up at 0 degrees on the mark, my intercepts stay constant. I'm at a loss!

Thanks for the quick reply, by the way, guys . Much appreciated!

Edited January 21, 2014 by lizarddan

[zeppelinmage]

What you need to do is close distance. If you're ahead of it, make your orbit slightly wider. Then timewarp until you're close enough. If the target is ahead, then you need to lower your orbit a little and catch up.

Then make sure "target" is in your navball and reduce that speed to zero. Rendesvous from there is via RCS approach.

[lizarddan]

So, smaller orbit = faster velocity and a larger orbit = slower?

That'd explain a lot if so. However, it seems when I achieve a lower orbit then my fellow, the apsis shrink without a burn. Most likely because he's already so close to Kerbin's atmosphere! I'll have to make both apsis minutely smaller and play the waiting game.

Is there a calculated approach to this? I.e a way to know when they'll reach a certain destination given the speed?

[zeppelinmage]

So, smaller orbit = faster velocity and a larger orbit = slower?

Yes.

Pretty sure Kerbal alarm Clock will help.

What I usually do is just see how much the Intercept Distance changes between orbits, then do a quick division to estimate how many orbits I need to timewarp through. It's rough though, so be careful not to timewarp too much.

Generally, as soon as my intercept distance is under 5 km, I begin to match orbit. Reducing the target relative velocity to 0, etc.

[Bean]

There are addons to help, also Kerbin's atmosphere ends at around 69-70km, so any lower as you will be aerobraking.

[Mako]

One thing you might want to try is having your trapped kerbal get out of the cockpit at apoapsis and then push against the front of his spaceship using his EVA suit. Since you're already so close to entering the atmosphere, you'll probably be able to slow his ship down enough to drop his periapsis into the atmosphere. This way you won't even need to have a second ship rendezvous. Be sure to point the ship prograde before getting out and pushing. Also, make sure to get back in after pushing to check if the periapsis has dropped enough. Then, just sit back and let the atmosphere do all the work.

If you do want to attempt the rendezvous, make the second ship's apoapsis higher by a decent amount by burning prograde at it's periapsis and then wait/timewarp until your closest approach becomes very small. Make sure you set the first ship as your target, and when you reach your closest approach time burn so that your relative velocity to the first ship is zero. Then it's a matter of burning towards the first ship so you approach it and then burning again to make your relative velocity zero when you get close. It may take a few tries to really get the hang of it. That's why I mentioned the first method of getting out and pushing. Rendezvous are made a whole lot easier with RCS.

Edited January 21, 2014 by Mako
Auto-correct typo.

[Streetwind]

The confusing thing about orbits is that the concepts of "faster" and "slower" are upside down.

If you increase your speed, then the orbit gets wider. And a wider orbit takes longer to go around the planet. That means the faster you fly, the slower you are for rendezvous purposes.

If you want a ship that is behind you to catch up with you for a rendezvous, you need to accelerate to widen your orbit. If you want to catch up to something that's ahead of you, you need to decelerate for a tighter curve around the planet.

This works until you have visual contact with your target, at most two or three kilometers apart. Then you can start gently steering your ship directly towards your target, because at that point variation in your orbits is negligible and the mere act of steering towards the target helps further lessening the difference.

[SRV Ron]

You have to make those changes in tiny amounts once you have a visual. Gradually close that distance since quicker maneuvers will just use more fuel.

Note, both ships were launched separately, then one was used to sync orbits with the other. This without the benefit of translation thrusters on either ship.

Edited January 21, 2014 by SRV Ron

[Rickenbacker]

What most posters are missing, though, is that the OPs ships are orbiting in opposite directions.

You can either raise your apoapsis a decent amount (a few 100 km or so) and try to reverse the direction of your orbit at apoapsis. If you don't have enough Delta-V for that, just launch another ship in the right direction, then look up the guides on rendezvous.

[JiWint]

I'm afraid the targets orbit is vice-versa-wise (like clockwise)

[Padishar]

No it isn't, he says they are orbiting the same way. The issue is that the objects are currently on different sides of the planet so the magnitude of the relative velocity is very high. He just needs to lift one of them into a higher orbit (preferably circular and at least 30km higher) to let the other one catch up. Once they are closer, experiment with a maneuver node that pulls the orbit back down to the target and adjust the node so that intercept markers match up and show a nice small number. If you can't get an intersect moving the node to any point on your orbit then just timewarp a bit more to change the relative positions before trying again. After a while your head will click with orbital mechanics and you will be able to predict where the ships need to be to get a decent intercept from different orbit heights etc.

You may also be able to get an acceptable intercept from the same orbit by just doing a highly eccentric orbit. Create a node at periapsis to push your apoapsis nice and high. It will take your craft much longer to complete the orbit and get back to periapsis so if you lift your apoapsis to the right height you will meet up with your target after one complete elliptical orbit. Just experiment with the node and watch for intercept markers appearing...

Pad.

Edited January 21, 2014 by Padishar

[radonek]

You may also be able to get an acceptable intercept from the same orbit by just doing a highly eccentric orbit. Create a node at periapsis to push your apoapsis nice and high. It will take your craft much longer to complete the orbit and get back to periapsis so if you lift your apoapsis to the right height you will meet up with your target after one complete elliptical orbit. Just experiment with the node and watch for intercept markers appearing...

I would not recomend that. It sure is easier to set up, but intercept tends to occur at bigger relative velocities (you are basicly falling from higher up) and thus have much smaller interception window. Not much problem for experienced pilot with well equiped ship, but I don't think either of these apply here :-)

IMO your first method is better, correction burns have bigger time window and don't require much TWR. Which is important – final approach without RCS will need very precise thrust control, easier with smaller engine. Thinking about that, ship should be as small as possible anyway so that it could flip prograde/retrograde quickly.

Another thing that pops to mind is electricity – rotating a ship drains power and if you have only entry-level ship with small onborad battery and no solar array, you will be empty fast. So, first thing to do would be to turn off SAS in stranded craft to save power.

[Smidge204]

No it isn't, he says they are orbiting the same way. The issue is that the objects are currently on different sides of the planet so the magnitude of the relative velocity is very high.

Two objects in the same, circular orbits going in the same direction will have a relative velocity of zero. In a nearly circular orbit, it would be nearly zero. Since escape velocity for Kerbin is ~3400m/s, the only way for you to get 4000m/s relative velocity is for one of the orbits to be an escape trajectory or very near to it.

Or they're orbiting in opposite directions.

I mean, look at the screenshot:

Nothing in that situation should be traveling anywhere near to 4000m/s. Not in a stock game, anyway... (no mention if he's using any mods.) If you're going 4000m/s at 70km you are on your way to interplanetary space and that is clearly not what that screenshot shows.

If, however, one ship is going east at ~2300m/s and the other going west at ~2300m/s, then 4300m/s relative velocity is easily within reason depending the exact phasing. That's why it's easy to conclude the orbits are opposing.

Now to be helpful: If the orbits really are that low, get out and push. It will not take much at all to lower your Pe into the atmosphere and then you can just wait it out while each orbit decays a little bit more than the last.

=Smidge=

[Aphobius]

Smidge as you can see one ship is going to the right and one ship is going to the left in the picture, the relative velocity is probably measured in a cartesian coordinate system (xyz). So it's very normal and logical for the relative velocity to be around 4000m/s in that situation.

You are not supposed to use the relative velocity when you are so far away from your target, look for some rendezvous tutorials or something.

[Smidge204]

Smidge as you can see one ship is going to the right and one ship is going to the left in the picture

There is not enough information in that image to tell which way they're going. All we know for certain from that screenshot is that the orbits are low, very close to the same and very close to circular, and that the two objects are approaching or receding at 4367 m/s.

=Smidge=

[Padishar]

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.

Specifically, this bit:

Two objects in the same, circular orbits going in the same direction will have a relative velocity of zero.

Is only true if the objects are at the same point in the orbit so they are travelling in the same direction. Two objects travelling in different directions can never have a relative velocity of zero. Two objects on exactly opposite sides of the same orbit will have a relative velocity of double the orbital speed, approx 4600m/s.

[Aphobius]

Red and green lines are ships velocities.

On the right there is a simple vector subtraction that results in the relative velocity (blue line).

EDIT: That subtraction might look a little strange, but I think it's more clear that way. We invert the red vector and we add it to the green vector.

Edited January 21, 2014 by Aphobius

[lizarddan]

Hey all, I'm very sorry about the confusion, I should have been way more descriptive.

First, I'd like to clear up that they are going the same direction, east. I used the same orbit techniques for both, at a heading of 90 degrees to 0, both ships are orbiting at the same heading.

Second, it's a vanilla game, no mods - I want to clear that up for a poster who asked if it was modified.

Third, I have read a LOT of tutorials on this, which is why I came here for help.

So, basically what I need to do is either widen or lower my orbit and timelapse until one catches up with the other? Any lower of an orbit and I'll be pulled into the atmosphere, so maybe I need to make a higher, slower, orbit and wait for the ship to catch up? Then I could make an intercept at the approximate point the ship is going to be, brake, and save Kerman?

Again, I'm sorry for all confusion. I appreciate the help so much guys . Even though it's confusing, I'm having fun. Solving this problem is entertaining and I find myself laughing when I end up shooting my Rescue ship into the water through too low an orbit and not enough fuel. Plan B will be push into the planet, but I'd really like to nail this orbital maneuver.

[Mako]

Good luck! I for one am interested to hear how it turns out. Having the Plan B of getting out and pushing is good, but getting the hang of the orbital manoeuvres necessary for rendezvous is far more rewarding.

[Padishar]

I think most of the confusion has been because people haven't properly read what you wrote. I've only been playing this since the end of November and I only did my first orbital rendezvous less than 2 weeks ago but I have been interested in the subject for many years so I picked it up pretty quickly. The first bit of general advice I would give is to always give yourself a bit of breathing room when launching to orbit. I generally always launch to at least 72 km and usually 80+.

Do you still have a reasonable amount of fuel in your rescue ship? For the first "solution" I presented you will need to make a prograde burn to raise your apoapsis to ~100km. Then, when you reach that point, again burn prograde until the orbit is circular at 100km. Then wait for the relative positions to get much closer (at the moment they are about 130 deg apart, you probably want Mr Kerban to be about 20 degrees behind the rescue ship but there is no harm in experimenting). Then set up a node to do a retrograde burn to get your periapsis back down to 70km. When you do this you should see the intercept markers on the projected path. If you see the top and bottom parts do not line up very well then move the node around your orbit until they line up as close as possible. You can then slightly adjust the burn in various directions to improve the intercept distance further (you should be able to get the closest approach within 0.2km quite easily if the orbits are the same inclination). Wait until you get to the node and do the burn in the usual way. After the burn, check the intercept markers again to check nothing went wrong. At this point, if you have RCS, you can adjust your path with that to get the intercept as close to 0 as you dare. With no RCS the corrections are a bit harder to work out but you can just do a small burst of burn in one direction and see what effect it has on the intercept. If it makes it worse then just turn around and burn the other way. Then switch the navball into target mode (if it isn't already) and wait until you are nearly at the intercept point. Point your ship at the retrograde marker on the navball and wait until you are nearly as close as you will get before burning until the target speed reads 0.0 (you may need to adjust your heading towards the end of the burn, just keep the ship pointing at the retrograde marker). If you are still quite a distance from the target then point directly at it and burn slightly to get a few m/s of speed. Then turn to retrograde again and wait until close before burning to a stop again.

Also, remember that quicksave is your friend though the only things that can really go wrong during this are you run out of fuel or you overshoot the intercept, drop into the atmosphere and don't correct it quickly enough (or possibly that your intercept is a bit too good and you manage to hit him, rather unlikely though)...

Pad.

[radonek]

So, basically what I need to do is either widen or lower my orbit...

Avoid excentric ( wide ) parking orbits when doing rendezvous. They work well, but are harder to plan with. On circular orbit, you just create maneuver node, push in minimal delta-v to get shallow intersection of orbits anywhere. Then you just drag that node around the circle until your intersection is where the other ship actually will be (both intersection marks point at same point). With noncircuklar orbit, you have to correct for excentricity by hand to get shallow intersection. Fiddly job at best. And yes, you want meet other craft in shallow angle. That way you spend most time in its vicinity, giving you more time to orient yourself and equalize speed.

and timelapse until one catches up with the other? Any lower of an orbit and I'll be pulled into the atmosphere, so maybe I need to make a higher, slower, orbit and wait for the ship to catch up? Then I could make an intercept at the approximate point the ship is going to be, brake, and save Kerman?

Basicly, yes. Except for the "approximate" part: when you set other ship as target in map view, you will see interception marks with precise position of both crafts. You can use this to plan very precise maneuvers (my best catch is <30 m from target craft). For your first try, and without RCS, anything less then 20km is "doable" and less then 5km is great.

BTW get rid of "breaking" meme, it just clouds your mind. You are making basic Hohmann transfer - you are waiting on faster/slower orbit to get your target where you want him to be, then you do one burn to get to the tranfer orbit from "yours" to "his" orbit. When you meet him you are at same point in space, but still on different (transfer) orbit which would carry you away. You need to perform second correction to get on same orbit as your target. Since you are close to tour target at this point, you already equalized most of your orbital parameters, you just have to equalize you _relative_ speed. Same place + same velocity = same orbit. BUT, if you flip your navball between target and orbital mode, you will see prograde/retrograde markers are different. Decreasing speed relative to target may actually increase orbital speed (relative to Kerbin). So do yourself a service and just forget about "breaking" and "accelerating. You are matching your _relative_ velocities and that's it.

Again, I'm sorry for all confusion. I appreciate the help so much guys . Even though it's confusing, I'm having fun. Solving this problem is entertaining and I find myself laughing when I end up shooting my Rescue ship into the water through too low an orbit and not enough fuel. Plan B will be push into the planet, but I'd really like to nail this orbital maneuver.

Actually my first reaction to your post was "just wait until you get RCS" but there is no fun in that. Improvised rescue missions are way more entertraining then performing mathematicly perfected scenarios :-)