Interplanetary orbital mechanics calculator

[ocf81]

Ok, I\'ve managed to land this huge rocket on the Mün which is capable to travel a lot further still (using only stock parts 8) (save for the huge lander legs from NovaPunch pack to land the bigger stages))

Now I want to go to Minmus but I can\'t figure out the following:

1: the optimal launch window to reach Minmus.

2: The right escape velocity to get to minmus in one shot.

The first is caused primarily because I have no Idea what the travel time is when launching in the optimal window. I know that I need to launch a little ahead of minmus, but I often miss and need extensive corrections when I miss to save the orbit to prevent going around Kerbin a few times before reaching Minmus.

I believe the second is more or less because KSP can\'t look ahead more than one SOI, but I reckon there should be a way to calculate the proper speed. Sometimes I am to slow and need an extra burn, sometimes I shoot straight into the Sol SOI. The margin seems to be small if you want to do it in one jump.

Does anyone know of a calculator for KSP which can help me with this?

[scotty110]

i\'ve never made a minmus attempt yet but i would imagine it would be similar to getting to the mun. make an orbit around kerbin and burn as soon as minmus appears on the horizon.

[wildkittyv1]

I am by no means an expert and I leave my suggestion as only that.

If I was sitting on the Mun and wanted to get to Minmus, I\'d do one of a few things, which all boil down to the same basic concept. You want to only go in one direction with this shot, in my mind. You don\'t want to be using orbits (unless you can time the 'single shot' using one to reduce fuel consumption) for this. Find a route across the system that is as close to a straight line as possible. It doesn\'t necessarily mean sitting on the Mun for a week so the closest path comes to you (if you\'re trying to be at least semi-realistic about it), but if it\'s only 30 degrees off from you and there\'s no orbital interference going on, burn at it and adjust when you\'re halfway there and don\'t have any interference. When you start to toy with orbital paths, things can get a bit complicated because you have to mentally adjust for the curvature given to you at the time of 'zero acceleration', you know, your escape is pinpointed in one place, and the direction you assume you\'re coming from is completely different because the orbital map doesn\'t show what happens while you will be caught in orbit of another planet while in the orbit of a different one - you onl see the exit point and have to imagine what happens in between.

Regardless, you want to see how fast Minmus is moving in correlation with the Mun at your current location on its surface. From there you want to try and plan a shot at a tangent point between the Mun and Minmus. You say fuel\'s not a problem, but depending on when you make the initial burn, you could push yourself way off course and may need to spend more fuel than you initially intended. This is going to be all about making sure the first burn is correct. Better to be safe than sorry... I hope with all that extra fuel you have you filled some cargo space with food stuffs, as you might want to wait a day or two to get the optimal and easiest exit, and hungry Kerbals are stupid(er) Kerbals.

[ocf81]

i\'ve never made a minmus attempt yet but i would imagine it would be similar to getting to the mun. make an orbit around kerbin and burn as soon as minmus appears on the horizon.

That would be true if launching from Kerbin, but I\'m launching from the Mün. Since the Mün is orbiting the same body as my target, it isn\'t as simple as a Kerbal based shot where you start at the center of the (locally) dominant gravity well and everything is relative to that well. With a Kerbin based shot you do an injection shot when the target is on the horizon and hope then for the best. The relative speeds of the launch body and the target body make it a tad more complex. (although I suspect that it should be possible to do it unassisted once I\'ve found a few good rules of thumb)

In the case of Minmus, its inclination adds to the trouble as well. You have to make an out of plane orbit which shoots in the right direction taking in account the curve and the travel time.

BTW, I did end up on Minmus several times, but most often I need to so some orbiting and quite a bit of orbital adjustments around Kerbin to be able to intercept Minmus. I want to eliminate that time and manoeuvring as much as possible so I can try to do it with a smaller rocket.

[PakledHostage]

You could look up WxEcho\'s orbit mechanic. I\'ve never used it so I don\'t know its limitations but it might help you. I\'m not sure that there\'s very much we can offer other than that. We\'d need a lot more information about what you\'re trying to do if you want a better answer. The problem is you not only need to know how fast you should be going but in what direction. It is possible to work out the required burn for an efficient transfer (even without resorting to MechJeb), but it will take careful planning.

[ocf81]

You could look up WxEcho\'s orbit mechanic. I\'ve never used it so I don\'t know its limitations but it might help you. I\'m not sure that there\'s very much we can offer other than that.

Unfortunately Wx_Echo\'s orbit mechanic hasn\'t been updated to include Minmus. Thank you for your suggestion though, it\'s much appreciated

We\'d need a lot more information about what you\'re trying to do if you want a better answer. The problem is you not only need to know how fast you should be going but in what direction. It is possible to work out the required burn for an efficient transfer (even without resorting to MechJeb), but it will take careful planning.

What I\'m trying to do is go from the Mün to Minmus. Preferably when they\'re near either their ascending or descending node, so I don\'t have to aim out of the Kerbin\'s ecliptic plane, but I\'ll also try it when they are very close together somewhere else in their orbits.

Come to think of it, I think that, in addition to the mission timer, KSP needs a date/time system so you can determine launch windows or calulate launch windows.

[PakledHostage]

Well, here\'s what I would do. (I haven\'t tried it yet but there\'s no reason it shouldn\'t work):

An elliptical transfer from 50 km above the Mun to 10 km above Minmus (Pe = 11650 km altitude above Kerbin\'s surface, Ap = 46470 km altitude) takes 75.023 hours. Minmus moves 90.246 degrees in its own orbital plane in that time. As Minmus\' orbit is only inclined at 6 degrees from the Mun\'s, and given the accuracy to which we know the Mun and Minmus\' position at any time, it is reasonable to neglect Minmus\' orbital inclination when calculating its angular displacement from the Mun¹. For this transfer orbit, a window occurs every time the Mun is located 89.8 degrees behind Minmus. This first occurs at 1d, 17hr, 51 minutes UT. It occurs again every 44.324 hours after that.

In reality, you\'d have a hard time flying that transfer because of the nature of the required Munar escape trajectory (it needs to be elliptical rather than parabolic or hyperbolic). A more realistic transfer is a 11850 km x 46550 km orbit, which would take 75.559 hours from Pe to Ap. Actual transfer time from transfer orbit insertion burn to Minmus intercept is closer to 79 hours due to the time it takes to escape the Mun\'s SOI. The extra transfer time has a significant effect on the timing of the window.

To insert into that transfer orbit from a 50 km circular, prograde, equatorial Munar orbit, our Kerbals would have to accelerate to 695.5 m/s starting the instant that the last bit of Kerbin\'s upper limb sets below the Mun\'s horizon, as seen from the 50 km high circular orbit (all described manoeuvers should occur within the Mun\'s equatorial plane). That\'s a delta-V of only 185.1 m/s and results in an elliptical orbit with Ap outside the Mun\'s SOI (Ap = 3050 km). If they flew the transfer orbit insertion burn accurately, then our Kerbals should find themselves in a 11850 km x 46550 km orbit about Kerbin after leaving the Mun’s SOI. That transfer orbit should intercept Minmus with only minor orbital trim manoeuvres.

As for intercepting Minmus near the ascending or descending node, you\'d have to wait a long time before a launch window results in a Minmus intercept near one of the nodes. The earliest such option that results in Minmus intercept within 1000 km of the Mun\'s orbital plane occurs at 12d, 19h, 53 min UT (intercept ~850 km below Mun\'s orbital plane). The next one after that occurs at 31d, 7hr, 7min UT (intercept ~275 km above Mun\'s orbital plane). Fortunately though, it doesn\'t take much Delta-V to adjust your orbital plane enroute to Minmus if you start your transfer at one of the other windows, so you should be able to use any of the windows. Just be sure to start your insertion burn at exactly the instant that Kerbin sets, and try to have that burn start coincide as close as possible with the windows that I\'ve given above.

I hope this helps.

PH.

¹I made up a MathCAD spreadsheet that takes the orbital inclination into account, but the difference is insignificant for the purposes of planning this type of transfer. The maximum error that results from assuming that the Mun and Minmus are coplanar when calculating their angular displacement is on the order of 0.16 degrees. This can safely be ignored because it is silly, after all, to measure with a micrometer when you\'re marking with chalk and cutting with a chainsaw... The Mun moves through 0.16 degrees every minute. Minmus moves through 0.16 degrees every 8 minutes.

[ocf81]

I haven\'t been able to try yet, but your post has certainly given me a rough idea how long it takes/how much delta-v it will take and when to launch from the Mün. THNX!