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What's the Best Way to Establish a Retrograde Orbit Around Kerbin


Brigadier
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KSP 1.0 32-bit, Win 7 SP1 64-bit

Tons of mods but I don't think they're relevant to the question.

I've started my first career game and been having quite a bit of fun with it. I've received two contracts, one to rescue a Kerbal from an 80 x 80 km retrograde orbit and another to place a satellite in a polar orbit around Kerbin.

I've attempted to launch a couple of flights to meet the parameters of the orbits and I'm either completely underestimating the dV requirements or I'm missing some orbital mechanics detail since I cannot reach these orbits without huge dV changes. In the first case, I've tried launching directly into a retrograde orbit since I believe that launching into a prograde orbit and then trying to reverse it would be silly.

In the second case, the 9000 x 9000-ish km, 90 degree target orbit is currently offset (rotated) from a direct launch to polar orbit by about 60 degrees but I can't seem to rotate my orbit sufficient without huge dV requirements. I've resigned myself to wait until Kerbin moves around the sun until the target orbit can be reached with a polar launch from KSC.

I believe these to be sort of the same problem. Am I correct and is there a better way that I'm not considering? Do I just need to design more dV into my boosters?

I appreciate any help, but I don't need a detailed solution preferring to do as much as I can myself, i.e. please don't give me too much info. I just want to know if I'm going about this completely the wrong way.

Cheers,

Edited by Brigadier
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The rescue mission is easy. I take it the kerbal is on the equator? They usually are. So yes launching into a retrograde orbit is just a matter of a bigger booster. It's quite expensive to go against the rotation of the planet.

Your may have to explain the second issue a little better. A 9000x9000km polar orbit? As in a 90 degree inclination? That should be relatively easy as well. Again just needs a bigger rocket.

All you should have to do is eye up when the KSC rotates under the target orbit. Should be twice per day. However. If you look at your target orbit it will have arrows traveling in the direction you need to be going. You need to launch on the correct side of the orbit so your going in the designated direction.

Make your inclination adjustments at an ascending node before circularization if you can manage. But as long as you get the launch pretty good that shouldn't cost much either way.

Also look into getting a data read out mod such as mechjeb or kerbal engineer so you don't have to fly in the map view and depend on your eye. You need to go by the numbers.

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I strongly recommend exploiting a Mun-assisted plane change in either case. The 9000 km orbit is only slightly inside Mun's SOI, so you're just redirecting dV already expended by exploiting Mun's gravity. Making the targetted plane change is a case of timing your Mun intercept so that you escape Mun on a polar inclination rather than an equatorial one. In other words, intercept Mun when Mun is on or close to the targeted orbital plane, and make a polar slingshot rather than an equatorial one to return to Kerbin with a new (and mostly free) inclination.

This will probably cost peanuts compared with attempting a brute-force orbital reversal.

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The retrograde launch is simply a matter of launching west instead of east. It'll cost you an additional 350 m/sec DV because you're overcoming Kerbin's rotational speed instead of using it to your advantage.

For inclined orbits, you launch into the plane of the orbit. KSC is directly under the desired orbit plane twice daily. It will cost you somewhere between 0-350 m/sec depending on the inclination. Keep in mind that you are carrying 175 m/sec velocity to the east when you launch due to Kerbin's rotation and you'll have to steer a few degrees to compensate for that.

Best,

-Slashy

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For a polar orbit, by far the most efficient way is to wait until KSC is under the target orbit. Then you launch north/south (depending on the direction of the orbit) with a slight westward component to cancel out the rotation of Kerbin. Other highly-inclined orbits can be reached by a similar method. For a retrograde equatorial orbit, simply turn west rather than east, and make sure you've got some extra dV to account for Kerbin's rotation.

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For plane changes, you need to be going as slow as possible, the best way to do it is to get your apoapsis in the plane of the target orbit and do the maneuver there, the slower the better.

Also launch while KSC is directley under the target orbit, that way you will be roughly in the correct plane after launching.

Edited by peachoftree
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For a retrograde orbit, just launch directly into a retrograde orbit. You'll need about 400 m/s more to launch into a retrograde orbit, as you normally get ~200 m/s for free from the rotation of kerbin.

By launching retrograde, you need to cancel those 200m/s and then get to the same orbital velocity without the "free" 200 m/s.. for 400 m/s more total.

As to the second case, I'm not sure what you mean, there should be launch windows into any polar orbit twice per day... on where you head north, one where you head south.

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By the way: in the default map view, it can be tricky to eyeball conveniently just when your ship is directly under the target polar orbit, since the viewpoint keeps changing as Kerbin rotates and you have to keep fiddling with the camera to keep the target orbit lined up.

It's a bit easier if you double-click on Kerbin so that the map view is centered on Kerbin itself rather than your ship sitting on the launch pad.

If you do that, then all you need to do is rotate the map camera view until you're looking at the target orbit edge-on (i.e. it appears as a line, rather than an ellipse). Then just warp until your ship rotates to that line and it's time to launch.

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Wow, you guys have been great and, see, I knew it was something I was forgetting (forest...trees). I should have realized that the target polar orbit would have to pass over KSC twice per day - I didn't think it through and thought that I'd have to wait for Kerbin to move in its orbit (doh).

Beefier lifters, aye.

I strongly recommend exploiting a Mun-assisted plane change in either case. The 9000 km orbit is only slightly inside Mun's SOI, so you're just redirecting dV already expended by exploiting Mun's gravity. Making the targetted plane change is a case of timing your Mun intercept so that you escape Mun on a polar inclination rather than an equatorial one. In other words, intercept Mun when Mun is on or close to the targeted orbital plane, and make a polar slingshot rather than an equatorial one to return to Kerbin with a new (and mostly free) inclination.

This will probably cost peanuts compared with attempting a brute-force orbital reversal.

I had thought of this but discounted it for dV reasons. I'll have to try to engineer this solution just to learn from the experience, and compare to the brute force approach.

Thanks to all.

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