Redezvous in Solar Orbit

[davidpsummers]

I'm about to rescue an astronaut stranded in solar orbit. He is orbit crosses the Keribin orbit but has a higher periapsis. Now I can do a Kerbin escape when the planet is at the point where the orbit crosses and do an ordinary rendezvous. (set the target, look for the closest approach, burn to improve it, etc.) But I was reading stuff about interplanetary transfers and it sounds like its is possible to have a faster and more efficient path. However, everything I read is about going from one circular orbit to another circular orbit, not going from a circular orbit to a similar elliptical orbit. Anyone know where I can find info on that?

Edited June 7, 2014 by davidpsummers

[Sirine]

Screenshoot help explain the situation.

[davidpsummers]

OK, here is a link to a screen shot of the situation...

https://www.flickr.com/photos/doctordavid/14349264354/

The stranded ship has the lime-green orbit and Kerbin is the light blue.

My ship is in orbit around Kerbin.

[meyst]

That's a significant challenge and you have some options.

Maths, I think this is a Lambert's problem. If you understand the terminology to get the orbits detailed information you can use Alex moon's excellent launch window planner to add your stranded ship as 'a body' in the solar system and it will chart possible intercept courses as a delta Vee pork chop plot, recommend the most efficient option and give you the information needed to execute it.

You can wait, after a few years the stranded ship should end up with a relatively close intercept to Kerbin and when it does send your rescue ship out such that the ship you are sending out has the exact same Periapsis and patiently narrow the gap every subsequent orbit by the usually method of using a slightly higher Apoapsis if you are ahead of it and slightly lower Apoapsis if you are behind it and need to catch up. I recommend you practice matching and closing with an eccentric orbit first. Launching two probes into orbit together, sending one out on an orbit just short of a Mun transfer and intercepting it when the probe on the eccentric orbit returns.

Create opportunities. Build a big multi-stage rescue rocket with a ridiculous amount of delta V, around 15,000 - 20,000 M/S available once in Kerbin's orbit sounds about right to me (the Moar approach)

option A) (The even Moar approach) Use the brute force to make an ugly high energy intercept, match the targets orbit when you get close.

option Put the rescue ship in a circular orbit around the sun (the lower your solar orbit the easier and faster it is to make an intercept but the more fuel you will burn doing it) use the in game maneuver node tool to plot and execute an orbit from this circular orbit that's Apoapsis intercepts a part of the targets orbit (higher intercepts above the sun with the target will be easier to work with due to lower relative velocities at time of intercept)

I think unmanned rescue ships are best, if it doesn't work out right you aren't inadvertently creating more new lost in space problems.

[Kasuha]

Not very effective but fast:

- time warp until kerbin is between the ship and sun (better leave the ship slightly ahead)

- launch directly towards the ship

- perform rendezvous as if you're in Kerbin orbit (just distances are bigger and encounter markers less reliable)

- after rescuing the ship, burn (more or less) directly towards Kerbin to get intercept

Edited June 5, 2014 by Kasuha

[davidpsummers]

I have a ship capable of 3700 m/s (or a bit more, I added a smaller stage after the calculation). I did a rough estimate on the Launch Window Planner and there were plenty of trajectories below this. (Some were only just over 1000 m/s, which is good since I need to get back). I did have questions about how to apply it....

Regarding my targets orbit...

-My Acending/Descending Node wrt Kerbin is only 0.1 degrees. So maybe I can just assume an inclination of 0 (which makes the longitude of ascending node 0). Think I can get away with this?

-Argument of pariapsis is the angle of the peripasis wrt a reference direction, I assume that is Kerbin? I'll have to estimate that since the game won't give it to me?

-It wants the time of the last passage by the periapsis, or the next one?

-So the info it gets me is an ejection angle and the ejection delta-V. If they are 166 degree to prograde and 871 m/s, I look at the prograde marker, add 166 degree, point in that direction and burn for 871 m/s. I guess I will have to use a burn set up for the right amount of time, and move it until it gives the right angle wrt prograde on the ball?

[davidpsummers]

Not very effective but fast:

- time warp until kerbin is between the ship and sun (better leave the ship slightly ahead)

- launch directly towards the ship

- perform rendezvous as if you're in Kerbin orbit (just distances are bigger and encounter markers less reliable)

- after rescuing the ship, burn (more or less) directly towards Kerbin to get intercept

Well, I was going to wait until Kerbin crosses the target ships orbit. But away from Kerbin until I (with as little left over dV as possible) escape Kerbin and do a rendevous as normal. I think you are saying do this when the sun, Kerbin, and the ship are lined up instead? Is that better?

[Kasuha]

Well, I was going to wait until Kerbin crosses the target ships orbit. But away from Kerbin until I (with as little left over dV as possible) escape Kerbin and do a rendevous as normal. I think you are saying do this when the sun, Kerbin, and the ship are lined up instead? Is that better?

Similarly to how you need to have a target planet at appropriate phase angle for optimal approach, you need that ship at appropriate phase angle as well. It's just that the angle is not that big since the ship's orbital period is not that different from Kerbin's as it is e.g. for Duna. So I'd give that ship about 10-15 degrees of phase angle at the most distant point, the closer to Kerbin it gets the less. You don't need to be extremely precise since then you'll be correcting your approach anyway (I did write it is not the most fuel efficient approach).

[allmhuran]

You may not need to send anything out into solar orbit...

Let's assume you don't send another ship. Your Kerbal has about 500m/s of delta-V in their EVA pack. That's going to be more than enough.

Stay in your ship for now, set Kerbin as your target, and put down a maneuver node at the point where your orbit crosses Kerbin's orbit. This should be at or near your current closest approach.

Put a second maneuver node 90 degrees behind that (actually 270 degrees ahead). When you place the second node, you will see the "target position at closest approach" shift. That's because we are now looking at the closest approach one orbit ahead.

Is the target position now very close to the intersection point? If not, right click on the second node you placed, and click the bottom right button to set it "one orbit ahead". The target position will shift again, indicating what is going to happen at the crossing point two orbits from now.

Repeat this procedure until the target position is "pretty close". A few cm away on screen. It shouldn't take more than two or three orbits.

Now go back to that first node. If the target position marker is currently ahead of the intersection point, it means you're getting there a little bit too late after three orbits. So add a little retrograde to that first maneuver. Keep adding retrograde until you get an intercept. It won't take much delta V... almost certainly less than 50 m/s, maybe just a few m/s. If the target position marker is behind, you need to add a bit of prograde instead.

Let's assume you needed to add a bit of retrograde for the rest of this example, and that the intercept will happen three orbits from now.

Now warp around to the that first node you placed at the intersection of the orbits. Orient your ship so that it is facing prograde.

It's time to get out. Delete the maneuver nodes, you don't really need them anymore and they just make things confusing.

EVA, and orient your kerbal so that he is facing directly down the line of the ship, and towards the back. He will now be facing retrograde.

Use your EVA pack to thrust forwards, ie, retrograde. Your EVA pack has about 500m/s, so use this as a benchmark (.5 EVA fuel will be about 1/10th of the total, or about 50m/s). Keep switching in and out of map view with kerbin as the focus in order to see your kerbin periapsis shift. Get it as low as you can. Towards the end you will just be tapping the EVA jets for a split second to make changes. Keep deleting and adding the maneuver node back so you can click on the "next orbit" button as necessary to see what is going to happen.

Now, you're probably going to have to make a correction or two. You will not have delta-v indicators, or current velocity indicators, while on EVA. But you're not going to need them.

Warp around until you are in the Kerbin intercept orbit (in our example, 3 orbits later). Plop down a maneuver node. It doesn't really matter where. Now pull the radial in or radial out handles as necessary to bring your periapsis lower. We are using radial in and radial out *only*, because it's easy to use the sun as a reference for which way you are facing. Facing the sun? Radial in. If you need to burn radial out, face the sun, then swing the camera around so you are looking directly at your Kerbal's face, and tap the forward EVA key to turn him 180 degrees.

If you need to make normal corrections it's a bit harder. Hopefully you won't have to, because we don't care about whether we are coming in equatorially. We just need a periapsis low enough for capture via aerobrake. That's going to be at around 35-40km, by the way.

Again, you can't see what your required delta-v is, nor how much delta-v you have spent, so keep switching in and out of map view when you use your EVA pack to see the effect of your "burns". Remember, you don't weigh much, so small taps result in significant changes.

Now, all of that is probably pretty hard to follow... so I made a video for you!

http://youtu.be/LNxk4w99gbc

[cantab]

-My Acending/Descending Node wrt Kerbin is only 0.1 degrees. So maybe I can just assume an inclination of 0 (which makes the longitude of ascending node 0). Think I can get away with this?

-Argument of pariapsis is the angle of the peripasis wrt a reference direction, I assume that is Kerbin? I'll have to estimate that since the game won't give it to me?

-It wants the time of the last passage by the periapsis, or the next one?

You can read most of these out from the persistence file. Alternatively VOID's extended orbital information gives you most of it. And the time of any periapsis passage will do.

I'm not sure exactly how I'd do this rendezvous, but I can think of a couple of methods:

If you aren't in a hurry, wait until Kerbin's around the target's periapsis, then leave Kerbin's SOI. Put yourself in a phasing orbit, probably between Kerbin's and the target's but maybe wider than the target's, such that in a few orbits you'll have an intersect near your shared periapsis.

If you are in a hurry, wait until Kerbin's "catching up" on the target, then leave, taking a short(ish) diagonal trajectory between the two orbits to an intersect.

Or you could take a third option: don't send a rescue mission at all. I can't imagine it would take more than a small correction from it to get onto a Kerbin aerobrake trajectory. Monopropellant, residual fuel, or even Get Out And Push might be able to it, and EVAing the Kerbal back to low Kerbin orbit on his own would certainly be possible. (Challenging, but possible).

[davidpsummers]

You can read most of these out from the persistence file. Alternatively VOID's extended orbital information gives you most of it. And the time of any periapsis passage will do.

I'm not sure exactly how I'd do this rendezvous, but I can think of a couple of methods:

If you aren't in a hurry, wait until Kerbin's around the target's periapsis, then leave Kerbin's SOI. Put yourself in a phasing orbit, probably between Kerbin's and the target's but maybe wider than the target's, such that in a few orbits you'll have an intersect near your shared periapsis.

If you are in a hurry, wait until Kerbin's "catching up" on the target, then leave, taking a short(ish) diagonal trajectory between the two orbits to an intersect.

Or you could take a third option: don't send a rescue mission at all. I can't imagine it would take more than a small correction from it to get onto a Kerbin aerobrake trajectory. Monopropellant, residual fuel, or even Get Out And Push might be able to it, and EVAing the Kerbal back to low Kerbin orbit on his own would certainly be possible. (Challenging, but possible).

The persistence file is a link on the site?

I found some places that would give me detailed info, but they all involved having already installed a part on the ship.

Do you know why the entry for when you want to leave doesn't go below a year?

[juanml82]

The persistence file is a link on the site?

I found some places that would give me detailed info, but they all involved having already installed a part on the ship.

Do you know why the entry for when you want to leave doesn't go below a year?

The persistent file is the savegame. Go into the savegames folder, get into proper subfolder and you'll see persistent.ess and quicksave.ess

(well, I think ess is the extension. The savegame, in any case)

[Claw]

The persistent file is the savegame. Go into the savegames folder, get into proper subfolder and you'll see persistent.ess and quicksave.ess

(well, I think ess is the extension. The savegame, in any case)

Close, persistence.sfs and quicksave.sfs in the saves/Save_Name folder. But the rest is right.

[davidpsummers]

You can read most of these out from the persistence file. Alternatively VOID's extended orbital information gives you most of it. And the time of any periapsis passage will do.

OK, thanks. I found it. It gives...

SMA = 16195287240.3846

ECC = 0.161534375525393

INC = 0.134881427300168

LPE = 8.50860896655636

LAN = 195.382865403951

MNA = 2.03855206470513

EPH = 88196591.6516588

The only number I can't pull out is which one is the "Arguement of Periapsis"? Thought I guess "LPE is the "longitude of the periapsis" and wikpedia says "Especially in discussions of binary stars and exoplanets,the terms "longitude of periapsis" or "longitude of periastron" are often used synonymously with "argument of periapsis." so I can use LPE for that?

(Oh, and when I add the time to the next periapsis to the calander time, it is 365 days in a year?)

Using the dV of the suit seems a bit like cheating, and I have the rocket ready anyways and it will teach me stuff I can use later.

Just to confirm, one I have Alex's launch window, I burn at the ejection angles listed for the ejection delta V?

[davidpsummers]

Well, my rescue ship is on its way. Since Kerbin crossed the orbit in question, I just ejected from the Kerbin SOI with as small a burn as possible (to dump me into Kerbin's orbit as closely as possible) and then treated it as a standard rendezvous. It seemed safer than the transfer calculation (and the odds that the map would show the intercept as I set up the maneuver mode seemed non-existent).

In the end I had to actually raise my apoapsis past that of the target (to no have to go around the sun for a few years) so it probably wasn't as efficient.

Here is a picture of the ship just before it left my space station (from the twin orange tanks on down, boy was it wobbly !!!!)

- - - Updated - - -

Well, my rescue ship is on its way. Since Kerbin crossed the orbit in question, I just ejected from the Kerbin SOI with as small a burn as possible (to dump me into Kerbin's orbit as closely as possible) and then treated it as a standard rendezvous. It seemed safer than the transfer calculation (and the odds that the map would show the intercept as I set up the maneuver mode seemed non-existent).

In the end I had to actually raise my apoapsis past that of the target (to no have to go around the sun for a few years) so it probably wasn't as efficient.

Here is a picture of the ship just before it left my space station (from the twin orange tanks on down, boy was it wobbly !!!!)

[Kryxal]

It's how I would have worked it, let the other ship catch up with you...

[davidpsummers]

Well, I have picked Bill up. He was there for about 10 years (I stranded him when I couldn't even do orbital flights and it took my a long time to build up). He has another year getting back, but he is in a Kerbin return trajectory. I only needed a short burn to get back because it turned out in a year he was going to make a close approach to Kerbin. (He still needed rescuing because without fuel, a close approach was still a "near miss".)

[cantab]

Congrats on the success!

Oh, and as for "why the entry for when you want to leave doesn't go below a year", that's because the game calendar starts at year 1, day 1.