How to match satellite orbit

[daterxies]

Hey.. using mechjab and just cant figure out how to get this...

http://puu.sh/muZX7/551cbdcbdb.jpg <- Mission specifics... i was able to get the right AP and Pe and Inclination, but the longitude of ascending node was enough aparently to make it all messed up? I should took a screenshot at the time, i had the right orbit, just it w.as off of the purple path.. spend 45 mins trying to figure it out and [snip] eventually ran out of fuel


http://puu.sh/mv06F/d78574731c.jpg

So with Mechjab WHAT settings do i have .to set to get into that correct orbit., with the correct tilt and all?? Games so fun, but this is driving me nuts..

What i basically did was set. ascent to 2938km... once there .changed me AP and Pe to correc.t, same with the inclinations .32.3.. and it was way off still.. Couldnt figure out about the Ascending node because i would .put 97.9 deg in there and it .wouldnt match up to the purple line.

Any ideas .or tips?

Edit

http://puu.sh/mv1Em/1599207bc2.jpg This is what i always come up with.. dont understand what im doing wrong. I set all the settings i believe correctly.. but its not working out

[Aethon]

Ditch Mechjeb and learn to play the game for yourself.  Download Kerbal Engineer Redux instead.

Go to the map view from the tracking station and double click Kerbin so the camera is focused on the planet.  Spin around the planet until you're looking at the orbit exactly edge on.  Time warp until the launchpad is under the specified contract orbit or very slightly before, and launch then.  If the part of the orbit north of the equator is on the right hand side you will do your gravity turn to the north and east, if the part of the orbit south of the equator is on the right hand side you will do your gravity turn to the south and east.  Watch the AN/ Dn in the map view to make sure it is decreasing toward 0.

Hope this helps, and welcome to the forums.

[Plusck]

While Aethon's first sentence sounds harsh, I wholeheartedly agree. I accept that MchJeb's features are great for doing routine stuff once you have got to the stage that they have become routine, but if you start off with it you'll never get a feel for all of the mechanics you need to play the game.

As for getting to that orbit: first thing you need to know (not possible to see on a screenshot) is which direction the orbit is in. If it's anti-clockwise when you look at it from above, then that's fine and easy. If it's clockwise, then you need to start off with a westwards launch (against the spin of the planet).

I'm writing this on the assumption that it's the right way around (so you take off relatively normally to get to it). However, you wouldn't be the first person to make the mistake of going the wrong way to satisfy a satellite contract. If it's clockwise, just go the other way...

So, in any event, Aethon's advice is the most efficient, without a doubt. Take off from KSC as the orbit passes overhead and match the orbital plane as you do your gravity turn.

However, you can simplify things for yourself if you accept some dv losses, because the plane-change angle from equatorial LKO is significant but not too huge. It looks to be about 30°, which means that it'll cost you half of your orbital velocity to change plane, and you can set things up so that (a) your orbital velocity is relatively low when you do it and (b) you integrate a slight angle to every burn to reduce the full cost of changing plane.

First step (whether following Aethon's advice or not): launch to LKO very shortly before the target orbit crosses the equator at KSC.

To be efficient, you'll try to do as Aethon said and get to LKO with more or less the right angle, with AN/DN reducing to zero. If you don't and you end up with a more-or-less equatorial orbit, you'll spend more fuel but the next bit is still valid:

At the end of your ascent, you'll generally have about 1 minute of doing nothing while you wait to reach Apoapsis to circularise. Use that time to see how far off the target orbit's angle you are. The reason why is because you want to make any plane change at as slow a speed as possible, and this is one of those times that you're going slower than full orbital speed. Circularise while adding a slight (max 15° or so) normal / antinormal component to reduce the plane-change angle. Watch what is happening in map view and make sure you don't overdo it, i.e. don't make your LKO too high and don't overdo the adjustment towards the angle of your target orbit (if there is any significant change in angle needed, that should be mostly left to step 3 below).

Next step: burn up to near the target orbit's Ap while adding a slight plane-change angle

Assuming that your angle is still quite far off, place a manouvre node at AN or DN for the burn up to the target orbit, as close as possible to the target Ap. And really Ap, not Pe.

This is because when you get there, you will be going at the slowest speed ever since you lifted off from KSC, and that is the time you want to do the biggest change to the angle of your orbit.

However, again, adding a slight normal/antinormal vector is not going to cost you too much. So when you are placing your manouvre node, start with prograde to get your orbit up, note the m/s cost of the manouvre, and then add the normal/antinormal vector up to a relatively low limit (like 10% of the cost of the purely prograde burn). Be aware that adding a normal or antinormal vector will change the final Ap, so adjust prograde/retrograde to compensate.

From your picture, your target orbit's Ap is unlikely to be at the AN/DN. That makes it a little more complicated because you'll need to add a radial component later, but it's still not a significant problem. The plane change is the most expensive part of all this anyway, so concentrate most on reducing the AN/DN angle a bit. But don't try to reduce the angle too much at this stage either: it's better to wait until the next stage for that.

Final step: circularise at Ap with the remaining plane-change vector

If you've done all this right, you'll reach Ap exactly as it touches your target orbit. And you'll be going relatively slowly. Much more slowly than in LKO. So this is the time to make the biggest plane-change burn if you have to. Simply place a manouvre node and match orbit to the target orbit. That means adding a significant prograde vector and a normal/antinormal vector.

Since you are unlikely to be at the target orbit's Ap, you will also need to add a radial in/out vector. Do this all at once, it's cheaper than doing it all in separate stages.

And voila, burn and you're done.

 

And finally, if the target orbit is clockwise, not anticockwise, then you simply need to follow the same simple steps but start off going westwards (270° not 90°) from KSC instead.

Edited January 14, 2016 by Plusck

[tutike2000]

I'd suggest just using SmartASS from MechJeb, instead of going cold turkey.

MJ doesn't have the option to match a contract's orbit, although it would not be impossible to implement, I think.

If you absolutely have to use MJ, circularize at the target Pe or Ap, then change inclination to match the target orbit when you intersect it. Inefficient, but it will work.

[wumpus]

Note: if your inclination change is expensive (usually after you botch the angle, but sometimes you want to launch two or more satellites on one booster with different required orbits), the most efficient time to fix it is during your circularization (raise PE to the correct height) burn.  My understanding is that real rockets launched from Canaveral to an equatorial GSO often will go excessively high (beyond GSO AP), change the inclination from Florida's to the Equator, then reduce the orbit and circularize (not sure the exact order).  Since Kerbal Space Center is on the equator, you never had to do this for single satellites (assuming you matched the orbit on liftoff).

[Aethon]

9 hours ago, Plusck said:

first thing you need to know (not possible to see on a screenshot) is which direction the orbit is in.

The OPs' first link shows the contract which lists a prograde inclination.

[numerobis]

It would be easy to implement it in MechJeb -- it already does rendezvous.

One note to what Plusck said: wherever you see "radial" in that writeup, replace it with "normal" -- you want to burn towards the purple triangle a bit (or away from it). The MechJeb maneuver node editor is actually very helpful for this.

The main point is that if want to burn 1 km/s prograde and 500 m/s normal, doing it in two burns is obviously 1.5 km/s. But doing it in one is only 1.12 km/s. As you can see, merging the burns is really quite important.

 

(Edit: I have no idea what brainfart caused me to write the struck-out parts.)

Edited January 16, 2016 by numerobis

[Plusck]

14 minutes ago, numerobis said:

It would be easy to implement it in MechJeb -- it already does rendezvous.

One note to what Plusck said: wherever you see "radial" in that writeup, replace it with "normal" -- you want to burn towards the purple triangle a bit (or away from it). The MechJeb maneuver node editor is actually very helpful for this.

The main point is that if want to burn 1 km/s prograde and 500 m/s normal, doing it in two burns is obviously 1.5 km/s. But doing it in one is only 1.12 km/s. As you can see, merging the burns is really quite important.

Um, I only mentioned radial once twice and I really meant radial. Because there is every chance the satellite will not have its Ap at the target Ap.

5 hours ago, Aethon said:

The OPs' first link shows the contract which lists a prograde inclination.

Yes, I didn't see the first pic, only the second one.

Of course, that means that the radial component will be tiny...

Edited January 15, 2016 by Plusck

[numerobis]

Weird. You did indeed mention it in exactly the right context. What was I thinking? Apologies!

[MarvinKitFox]

An orbit consists of 4 things:

 

1)Longitude of ascending node

2)Inclination

3)Apogee

4)Perigee

 

You are matching 2+3+4 perfectly, and utterly ignoring (1)

 

When launching from the surface, you want to consider these four parameters IN THAT ORDER.

Trying to get them right in another order is either inefficient, or downright difficult.

 

1) First make sure you launch at, or near, the correct time. The easy way to do this is to view the intended orbit from the side, and only launch when the launchpad is directly under the path, or will be in less than 5 minutes.

2) Only once (1) is aligned, launch. Aim so as to match the inclination. (optimal is a bit further north/south than the desired inclination, due to Kerbin's rotation, but matching inclination angle is a good first approximation)

3) Once in low orbit, stretch your Apogee to the intended height. Do this maneuver when on the exact opposite side of the planet from where you want your Apogee point to end up.

4) At Apogee, change your Perigee to match the intended orbit. This is also the best time to fix any remaining inclination error.

 

In KSP, it is *much* easier to do all these things by eyeballing the map display, that by trying to match the exact numerical parameters of the orbit.