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Post by jfmoyen on Jan 3, 2019 21:25:35 GMT
Dear Board, Maybe I'm overlooking something simple, but I'm struggling to plan a Hohmann transfer between two bodies. Take, for instance, my current scenario (Uranian cargo run). I'm supposed to transfer from one moon (Miranda) to another (Titania), using a Hohmann ellipse. This seems simple enough (burn prograde, ideally at apoapsis, until the new apoapsis is tangent to the target orbit). The problem, of course, is orbital phasing. How can I find out -- other than by endless trial and error -- the right launch window, and thus decide when to burn ? Something like a pork chop plot (https://en.wikipedia.org/wiki/Porkchop_plot), or an equivalent tool (MechJeb, anybody ? ) would help. Of course, the problem is more general than this simple scenario, as pretty much any scenario except the "sitting duck" cases requires orbital transfer, or at least phasing... so a way to determine the best burn date would be very useful ! Thanks for help !
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Post by apophys on Jan 4, 2019 18:48:08 GMT
Welcome, jfmoyen ! Trial & error is basically it. Just do a rough burn plan at some point of the orbit, and see how far out of phase you are. Try at a different nearby point; see if you're closer or farther, and you should get a good idea of where you actually need to burn from. When you get a decent enough trajectory, you can fine-tune the phasing with the radial (green) knob. For Uranian Cargo Run, the ideal transfer timing is around 1 day 13 hours from the start (you'll need to escape Miranda first though, or you'll crash into it). My ending maneuver illustrates another case where you need trial & error for phasing. I'm doing an Oberth capture at Titania (disturbingly close to the surface... ), and want to use just enough dV that one orbit will put the craft in phase with the target when I burn again for it. There's no tool I know of that can make this easier, but it's not really that bad. Btw, this run is my lowest dV use on the stage, at 2.95 km/s.
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Post by jfmoyen on Jan 5, 2019 14:32:24 GMT
Thanks for the answer - a bit disappointed that there are no better tools, but at least I know !
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Post by AtomHeartDragon on Jan 6, 2019 12:22:30 GMT
I agree that COADE interface is more streamlined than it can afford to be and it hurts usability. Still, if it's any consolation, you can get really good at eyeballing your orbital manoeuvres with a bit of practice.
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Post by gyratron on Jan 7, 2019 18:59:15 GMT
Firstly, you should burn at periapsis to take advantage of the oberth effect, not apoapsis, unless you're using a very low thrust engine in which case I guess it doesn't matter much. Generally all orbit changes should be performed as close to a gravity well as possible except when changing plane or orbit direction, in which case you should be as far away as possible. To find a good launch window the easiest way is just to plot a burn which will intersect the desired orbit nicely and then play with the time slider handle (the grey one) until the destination appears next to the match orbit icon when you hover over it. If the destination is at a planet or moon you will probably find that the trajectory suddenly widens when you get close to lining it up as the destination body starts pulling you in, in this case you just have to reduce the tangential a bit and then go back to the time slider to line it up again.
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