Minmus refueling?

[Colonel_Panic]

Hi, so I know it takes very little delta-v to achieve kerbin escape from minmus orbit, however, is it worth it given the fact that youhave such a high kerbin orbit and a second body to line up for ejection angles? I'm thinking about building a fuel depot there, but I'm concerned I'll lose too much deta-v by ejecting outside the ideal transfer window, due to waiting for minmis to reach the right point in kerbin orbit.

[Pontiac]

What I've done is plopped a gas station at 750km above Kerbin for interplanetary travel. I don't know if its any better than keeping it at 100, 200, 250 or 300k, but I do know that above 750k, I can fast forward for those months long waits for a proper orbit to appear.

[Colonel_Panic]

What I've done is plopped a gas station at 750km above Kerbin for interplanetary travel. I don't know if its any better than keeping it at 100, 200, 250 or 300k, but I do know that above 750k, I can fast forward for those months long waits for a proper orbit to appear.

For this I generally try to warp from the launchpad, but if I forget a higher orbit can be useful... of course you can always just select another ship from the tracking station to warp faster.

At higher orbits there's always a tradeoff, higher energy for little to no oberth effect, and I find that I waste far more fuel reaching and circularizing at a high orbit before ejecting, than I would just ejecting to begin with. Using minmus as a waypoint though I could establish an ultra-low, (relatively) high-speed orbit around minmus, and use minmus ejection to boost my kerbin ejection almost for free.

[Sephta]

What I do for interplanetary travel: Get to the Kerbol SoI, then set up the Homann-transfer (with the nav-system it's fairly easy then...) In that case I have to wait in a Kerbol orbit and don't need any angles in the kerbin system.

If you go interplanetary like this, a refueling depot at minimus makes a lot of sense, mine's mobile and at Mün atm - untill it's mined empty.

[Colonel_Panic]

What I do for interplanetary travel: Get to the Kerbol SoI, then set up the Homann-transfer (with the nav-system it's fairly easy then...) In that case I have to wait in a Kerbol orbit and don't need any angles in the kerbin system.

If you go interplanetary like this, a refueling depot at minimus makes a lot of sense, mine's mobile and at Mün atm - untill it's mined empty.

hmm, this brings up another point, since again you can't use the oberth effect to boost your apoapsis when doing a hohmann transfer from kerbol orbit, what's the hidden delta-v cost of doing it that way vs timing your transfer to coincide with your ejection burn?

[Sephta]

hmm, this brings up another point, since again you can't use the oberth effect to boost your apoapsis when doing a hohmann transfer from kerbol orbit, what's the hidden delta-v cost of doing it that way vs timing your transfer to coincide with your ejection burn?

Honestly, I didn't calculate. Kerbin orbit to Duna orbit is about 800m/s, it takes (all in all) about 1000m/s to leave Kerbin SoI from LKO (100km). So for a Duna return mission I have to have an interplanetary stage capable of around 3600m/s. But you can refuel easily on Ike there. My interplanetary ship has a dV of 8000m/s fully loaded and fueled, basically it can go anywhere in the system, as long as I can refuel on a something with less than 3.0m/s² and no atmo.

[Brofessional]

It's worth it. After refuelling at Minmus I can usually escape Kerbin with 90%+ full fuel tanks.

It doesn't really matter when you leave since Kerbals don't need life support, you can just leave Kerbin and orbit the sun until your angle is right.

[Grizzlol]

It has been proven in practice in a challenge and in theory using maths and plots that the most efficient way to transfer to Duna (and therefore any other planet, from Kerbin) is by dropping from a Minmus orbit to an elliptical Kerbin orbit at about 100 km periapsis, and timing it so that the transfer burn to the destination is executed at the Kerbin periapsis. This does not, however, take into account any possible gravity assists that may or may not further reduce the delta-V requirements, nor does it assume that you take into account the efforts of placing a station around Minmus to begin with.

The question is if you really want to take the effort of doing all of this. To Duna, specifically, the gain is not much, but to any distant targets it could be quite significant. Unfortunately, all of the posts I know about regarding this was lost in the attack a while ago.

Edited June 3, 2013 by Grizzlol

[Colonel_Panic]

It has been proven in practice in a challenge and in theory using maths and plots that the most efficient way to transfer to Duna (and therefore any other planet, from Kerbin) is by dropping from a Minmus orbit to an elliptical Kerbin orbit at about 100 km periapsis, and timing it so that the transfer burn to the destination is executed at the Kerbin periapsis. This does not, however, take into account any possible gravity assists that may or may not further reduce the delta-V requirements, nor does it assume that you take into account the efforts of placing a station around Minmus to begin with.

The question is if you really want to take the effort of doing all of this. To Duna, specifically, the gain is not much, but to any distant targets it could be quite significant. Unfortunately, all of the posts regarding this was lost in the attack a while ago.

That sounds highly effective yet extremely difficult and complicated to pull off. Has anyone actually worked out any sort of phase-angle calculator that takes this method into account? It seems to me you'd need to figure out how long it takes to return to kerbin periapsis from minmus, the ideal return vector, the ideal ejection vector, the timing of the transfer window, and the timing of the minmus orbit to the opposite of the ejection vector, and make it all line up with your destination... perfectly doable in theory, but I'd much rather have some sort of calculator to plug the numbers into and do the math for me, than try and eyeball it.

[Grizzlol]

The Protractor add-on has functionality specifically for this, in "advanced" mode. Not sure if Mechjeb does this. If you do not like add-ons then you just need to play with the nodes a little. It is really not that difficult, you just have to make sure that the ejection angle ends up about the same as it would have been doing a normal transfer from LKO. It does not need to be perfect. You get many opportunities to make corrections, and even if it is not perfect, you will still reduce the overall delta-V requirements compared to a standard LKO transfer.

If you do attempt this, you will find that the numbers do not line up perfectly because Minmus will (almost) never be in the perfect spot. It does not need to be. I would highly recommend using nodes to see where you will end up regardless of using an add-on or not, though.

Edited June 3, 2013 by Grizzlol

[mk78123]

This mod allows you to calculate a path from a moon to a different planet, eg minmus-Kerbin-Duna. http://kerbalspaceprogram.com/protractor-for-ksp-0-18/

[Colonel_Panic]

The Protractor add-on has functionality specifically for this, in "advanced" mode. Not sure if Mechjeb does this. If you do not like add-ons then you just need to play with the nodes a little. It is really not that difficult, you just have to make sure that the ejection angle ends up about the same as it would have been doing a normal transfer from LKO. It does not need to be perfect. You get many opportunities to make corrections, and even if it is not perfect, you will still reduce the overall delta-V requirements compared to a standard LKO transfer.

If you do attempt this, you will find that the numbers do not line up perfectly because Minmus will (almost) never be in the perfect spot. But as I already mentioned, it does not need to be perfect. I would highly recommend using nodes to see where you will end up regardless of using an add-on or not, though.

This mod allows you to calculate a path from a moon to a different planet, eg minmus-Kerbin-Duna. http://kerbalspaceprogram.com/protractor-for-ksp-0-18/

huh... I actually have protractor already, I was just unaware it had that functionality.

[Grizzlol]

The advanced button will add the position of the moon. The alignment of the departure and destination bodies (not the moon) is the most important factor, the rest do not need to, and will not, be perfect. You can always adjust the transfer orbit in several ways, the bodies' position you can not. The position of Minmus can be off by quite a bit and still be effective. There are too many factors to rely entirely on the add-on, you will need nodes to make up for that. By burning before or after the Kerbin periapsis, you can adjust for Minmus being at the wrong position, which it most likely will be.

Edited June 3, 2013 by Grizzlol