Rendezvous timing

[bradbamstl]

I\'m trying to rendezvous with a ship I already have in orbit, and I\'ve worked the math to know when I have to launch the second rocket to meet the first. All I need is to know the longitude of the orbiting craft to know when to hit space. I saw a plugin which sends data to another IP address, as a mission control almost. Any ideas on how to have that info in front of me so I can launch at the right time?

[Kosmo-not]

Is this math on launch timing going to be released to the community? I am very interested in your work.

[bradbamstl]

Just algebra and trig. Time the MET of the second spaceship from launch to MECO, and the longitude at MECO. Then using the period of the orbiting spaceship, find the angle traveled by spaceship 1 and subtract that angle from Spaceship 2 MECO Longitude. Using the MechJeb plugin I can usually maintain a relatively consistent launch, so if I know when the orbiting spaceship 1 is at that point, I can launch spaceship 2, so that after a circularizing burn of spaceship 2, then first is arriving to that point. I am a very mediocre mathematician, so if anyone can help me explain this as well, I would appreciate that as well. HA.

[Zephram Kerman]

Simplest way would be to use MechJeb on the orbiting ship to monitor its longitude. Fly that one until almost in position, then switch to the launch pad.

If you have very consistent orbit altitudes, you could also put something on the ground to mark that spot.

But wouldn\'t it be nice to have a position readout of other ships? I think that\'s what you\'re really asking for. Unless something like that is available, I can only think of 'close enough' ideas like those above.

On the other hand, aligning the altitude, plane, and phase of an orbit is all part of the challenge! So just launch at any old time, and enjoy the extra challenge.

[bradbamstl]

I swear I saw something like that, but that is exactly what I\'m looking for. A sort of mission control for all crafts while flying your own. It would be a great asset when docking becomes available, and Gemini 6/7 and the space shuttle have mission control backup anyway. And the Apollo 11 lunar landing reference is hilarious.

[Ydoow]

Load up the craft in orbit.

Set it with a heading towards east or west. Then observe the NavBall and record the time it takes to travel so many degrees, say 15 degrees.

You could alternatively calculate that out using it\'s orbital velocity and altitude. Or just use the Orbital period provided by MechJeb and divide by whatever degrees you want to know. I would guess knowing degrees/sec would be most helpful

Now wait until KSC is directly below your craft (on either side of the planet).

You can tell when it is there because the purple/pink marker on the NavBall should line up right over the orange thingy in the NavBall. Mark down the MET.

Now you know at what time the craft is exactly over KSC, and you know how many degrees it travels per second (or minute).

You can then do simple math to figure out at what degree your orbiting craft is at in relation to KSC by adding to the MET you marked down.

I don\'t think I need to explain the math since you figured out the timing of making them meet, which is more than I\'ve ever done

Also, what do MET and MECO stand for?

I know MET is the current Mission Time, I just don\'t know what the E stands for.

MECO I have no idea beyond the M

[Kosmo-not]

I usually launch so that I will be trailing my rendezvous target by a little and at a lower altitude. This way, I catch up to it and do a precision rendezvous using my method. I usually obtain rendezvous in less than one full orbit.

[closette]

MET = mission elapsed time

MECO = main engine cut off

Can\'t add any more here - what you\'re doing is great, and worth writing up as a tutorial.

[Ydoow]

I agree with closette, a tutorial is in order. Or at least just a quick run through of the math in a bit more detail/organized fashion.

I might try this out when I get home, then again I always say that and school takes over

[Kosmo-not]

Basically, what he\'s doing is determining a launch phase angle (the angle formed by one craft, the center of the celestial body, and the other craft) between the two craft by starting them out at zero phase angle and seeing where they end up after orbit is achieved. Comparing the longitudes of the craft after orbit is complete will give you the same magnitude of the phase angle that is needed at the time of launch.

I hope this explains it well enough.

[Ydoow]

Basically, what he\'s doing is determining a launch phase angle (the angle formed by one craft, the center of the celestial body, and the other craft) between the two craft by starting them out at zero phase angle and seeing where they end up after orbit is achieved. Comparing the longitudes of the craft after orbit is complete will give you the same magnitude of the phase angle that is needed at the time of launch.

I hope this explains it well enough.

At first, you made my understanding less than what it was before lol.

But a third read through and I think I understand what you\'re saying.

It\'s just simply launching both craft into orbit when they\'re both exactly at the same place (Both at KSC, ship 1 in orbit).

Once Ship 2 gets a circular orbit, measure the distance between Ship 1 and Ship 2 by determining the angle between them all

Ship 1>Kerbin>Ship 2

Then just use that angle/distance to know early in advance you launch Ship 2 to meet up with ship 1.

So then what I suggested would still work, no?

Just know it\'s orbital Velocity, and the MET when the craft is right over KSC. You could then use the current MET to figure out at what angle that craft is at by subtracting it from MET over KSC

[Kosmo-not]

At first, you made my understanding less than what it was before lol.

But a third read through and I think I understand what you\'re saying.

It\'s just simply launching both craft into orbit when they\'re both exactly at the same place (Both at KSC, ship 1 in orbit).

Once Ship 2 gets a circular orbit, measure the distance between Ship 1 and Ship 2 by determining the angle between them all

Ship 1>Kerbin>Ship 2

Then just use that angle/distance to know early in advance you launch Ship 2 to meet up with ship 1.

So then what I suggested would still work, no?

Just know it\'s orbital Velocity, and the MET when the craft is right over KSC. You could then use the current MET to figure out at what angle that craft is at by subtracting it from MET over KSC

You understand it perfectly. We don\'t need the distance between them, only the angle.

Yes, your suggestion will work. You\'re basically converting that angle into time be dividing the phase angle by the angular velocity of the orbiting craft and subtracting it from the future time that the orbiting craft will pass over KSC (don\'t forget about Kerbin\'s rotation for the future MET).

[Ydoow]

So...why hasn\'t this been done before!?

[Kosmo-not]

So...why hasn\'t this been done before!?

In my case, since I don\'t use mechjeb, human reaction time. Just one second of error means you\'ll be off by 2km.

[bradbamstl]

Ydoow, that helps a lot. Didn\'t think I\'d get any feedback. 2 questions

1. What is the rate of rotation of Kerbin?

2. When I hit esc to get back to the tracking station, it freezes time. does it resume when I get back to the tracking station exactly where it left off? So if I know where it is at one point, will I be able to calculate where it is 2 minutes later while switching ships? Is there a way to switch directly without the tracking station? Can anybody give me, in 140 characters or less, a complete education and set of practical skills in aerospace engineering and computer programing?

#thanks

[Ydoow]

Ydoow, that helps a lot. Didn\'t think I\'d get any feedback. 2 questions

1. What is the rate of rotation of Kerbin?

2. When I hit esc to get back to the tracking station, it freezes time. does it resume when I get back to the tracking station exactly where it left off? So if I know where it is at one point, will I be able to calculate where it is 2 minutes later while switching ships? Is there a way to switch directly without the tracking station? Can anybody give me, in 140 characters or less, a complete education and set of practical skills in aerospace engineering and computer programing?

#thanks

1.) I can\'t remember the rotation, but just put a craft on the launchpad, then click the Speedometer and it will switch to show you your speed in an Orbital Rotation. Since you\'re rotating at the same exact speed as Kerbin, it will tell you how fast you are 'Orbiting' Kerbin i.e. the rotation of Kerbin.

I\'m not sure if this will matter though. I considered it when I was posting my first reply, but once you leave the ground Kerbin\'s rotation shouldn\'t matter.

I might be wrong though.

2.) Time does freeze when you press escape.

If you select 'End Flight' the game will act as if you never launched that craft; time will be reset back to pre-launch conditions.

If you go to the Tracking station, it should pick up time where you left off.

I do not believe waiting at the Space Station screen, or the VAB screen allows time to pass. Only if you are in the Tracking Station, or in the middle of a Flight.

I have no evidence to back this up. It\'s only what seems logical.

Except for the 'End Flight' part. I know that is true.

[semininja]

1.) I can\'t remember the rotation, but just put a craft on the launchpad, then click the Speedometer and it will switch to show you your speed in an Orbital Rotation. Since you\'re rotating at the same exact speed as Kerbin, it will tell you how fast you are 'Orbiting' Kerbin i.e. the rotation of Kerbin.

I\'m not sure if this will matter though. I considered it when I was posting my first reply, but once you leave the ground Kerbin\'s rotation shouldn\'t matter.

I might be wrong though.

2.) Time does freeze when you press escape.

If you select 'End Flight' the game will act as if you never launched that craft; time will be reset back to pre-launch conditions.

If you go to the Tracking station, it should pick up time where you left off.

I do not believe waiting at the Space Station screen, or the VAB screen allows time to pass. Only if you are in the Tracking Station, or in the middle of a Flight.

I have no evidence to back this up. It\'s only what seems logical.

Except for the 'End Flight' part. I know that is true.

Actually, ending the flight doesn\'t reset time to pre-launch; only restarting the flight will do that.

I believe the tracking station does run time from where you last left flight mode, as opposed to continuing while you are (for example) at the VAB. I\'m not certain about that, though it would be easy to check.

[hubbazoot]

I would LOVE LOVE LOVE LOVE an MET to a position on your current course on mouseover, that\'d be mucho handy for doing this sort of timing stuff.

And semininja, you\'re right, time stays 'current' when you switch back to VAB.

Generally, the non-maths method I\'ve been using is to establish a similar orbit about 1km larger, but with the Ap and Pe about as close as I can get them (so they follow the same radial line), and then just fast forward until they get close and shoot in. Seems to work pretty well, but makes for some long mission times.

Info about different planets\' orbitals and such can be found on the wiki:

http://kerbalspaceprogram.com/~kerbalsp/wiki/index.php?title=Kerbin

Also, to make the time lapse faster, I usually leave a vehicle on/near the pad to switch to for the time lapse, and then switch back.

[Ydoow]

I\'m currently attempting this rendezvous timing myself and have found some issues with my suggestion, specifically that the Purple Markers don\'t actually line up with my orbits.

I\'m not sure why. I guess it\'s not exactly in line with the launch pad.

Anyway, I\'m using MechJeb instead. If you click the 'Landing Pilot' it provides you with the exact coordinates of KSC.

I\'m going to use this in combination with MechJeb\'s surface Information to calculate a pretty precise degrees/second unit and also when it\'s right over KSC to find the MET

Edit: Largest issue I\'m seeing now is that the altitude is being calculated from the command pod, and not the center of the craft.

This means your apoapsis/periapsis will always be changing thus affecting your orbit\'s eccentricity. It causes micro-variations in your orbital period and degrees/second.

I recorded mine for 90 degrees at 10 degree increments at 200km orbit. Each 10 degree segment varied between 63 seconds and 68 seconds, so +- 5 seconds roughly. That\'s kinda big if you ask me, for only traveling 10 degrees.

I saw my eccentricity climb from .000139 -> .000152

[hubbazoot]

What\'s your Ap and Pe? round orbits are very difficult to actually rendezvous with, due to the gratuitous rounding error at 2x and 1x time acceleration.

What I\'d recommend doing if you\'re trying to match up with a round orbit is to make one that\'s just eccentric enough to be stable but has a matching Ap and a lower Pe.

As soon as you either 'switch away' from the craft with the round orbit or accelerate up to 5x, it\'ll run on the rails. Here\'s the short version of how I do an orbital rendezvous:

1. Get into orbit, ~70km range (for me)

2. Match planes with the target orbit. Looking at your target orbit edge-on can make this much easier to find.

3. Match up Ap points, within about a km or two.

4. When you AND target craft are near Ap at the same time, accelerate to match up Pe\'s.

5. When target craft is within about 4km of you, you can start 'shooting from the hip' and accelerate straight in. Depending on how far away you are, this can take quite a bit of fuel. Don\'t worry about your map at this point in time, just keep trying to match position and velocity.

[Kosmo-not]

What\'s your Ap and Pe? round orbits are very difficult to actually rendezvous with, due to the gratuitous rounding error at 2x and 1x time acceleration.

What I\'d recommend doing if you\'re trying to match up with a round orbit is to make one that\'s just eccentric enough to be stable but has a matching Ap and a lower Pe.

As soon as you either 'switch away' from the craft with the round orbit or accelerate up to 5x, it\'ll run on the rails. Here\'s the short version of how I do an orbital rendezvous:

1. Get into orbit, ~70km range (for me)

2. Match planes with the target orbit. Looking at your target orbit edge-on can make this much easier to find.

3. Match up Ap points, within about a km or two.

4. When you AND target craft are near Ap at the same time, accelerate to match up Pe\'s.

5. When target craft is within about 4km of you, you can start 'shooting from the hip' and accelerate straight in. Depending on how far away you are, this can take quite a bit of fuel. Don\'t worry about your map at this point in time, just keep trying to match position and velocity.

Circular orbits are the easiest to rendezvous with. Eccentric orbits are more complicated to deal with.

[hubbazoot]

Kosmo, that\'s my general algorithm for ANY orbit.

[Zephram Kerman]

What\'s your Ap and Pe? round orbits are very difficult to actually rendezvous with, due to the gratuitous rounding error at 2x and 1x time acceleration.

Are you saying perfectly round orbits drift more than eccentric ones due to rounding errors or something? That would explain a lot.

[SubOrbitalGuy]

Hi,

I\'m trying to rendezvous with a ship I already have in orbit

-snip-

maybe I\'m missing something obvious, but isn\'t the easiest method to meet in orbit using the one described by Kosmo-not: using a smaller orbit to slowly get at the exact distance (the km according to his table) and just adjust your AP from there?

http://kerbalspaceprogram.com/forum/index.php?topic=9726.0

The thing is, even if you time it nearly perfectly, at speeds above 2 km/s you will always miss your target anyway.

So using the timing-method you are trying would be good for improving optimizing the liftoff for the first (lower) orbit.

[hubbazoot]

Are you saying perfectly round orbits drift more than eccentric ones due to rounding errors or something? That would explain a lot.

I\'d use the word sporadic over eccentric, because eccentricty is a matter of roundness. So, less eccentric orbits are more sporadic.