Minimus to Jool

[Tokay Gris]

Problem is this:

I do know that a trajectory from Minimus (refueling there), passing Kerbin, burning at that periapsis will get me to Jool (and the joolian moons) a lot cheaper than starting from Kerbin LKO.

But how on Kerbin am I going to plan that?

First, I need a window for a Kerbin transfer, I figured. Then before that take the plunge towards Kerbin from Minimus at just the right moment that the vector leaving Kerbin is parallel to Kerbins motion within the kerbal system.

There the problem starts: What position needs Minimus in relation to Kerbol be in? The timing is not that important, since Minimus' sidereal period is 12 Kerbin days, so I should still get a decent window for Jool within those 12 days.

And the second problem: How do I compensate for Minimus' inclination? I come in at 6° relativ to the plane of ecliptic (IF the point of departure does not happen to be at the point where Minimus' plane intersects with the plane of ecliptic. So say: between 0° and 6°).

Do I need to change that during the plunge towards Kerbin? That seems to be the best idea. I won't be able to get a 0° inclination, but that should not be necessary. Important is the vector I come out of. If I leave Kerbins SOI roughly within the plane, I should then get a fairly level inclination relative to Kerbol, am I?

Any tips on that mission profile? What should I look out for?

I do have a kethane rig stationed on Minimus, including two huge refueling tankers that can land there.

I even suspect I could land the whole vessel as is on Minimus (except the remaining lifting stage, of course. The center of the asparagus.

I was planning to refuel that and use the 800 m/s it gives (since I have to refuel anyway, so that is just one more tanker) for the first leg out of Kerbins SOI.

So the plan is to get that vessel (if refueled just short of 300 tons) out to Minimus, refuel it there using the kethane rig, then do the ascribed trajectory.

[Claw]

Problem is this:

I do know that a trajectory from Minimus (refueling there), passing Kerbin, burning at that periapsis will get me to Jool (and the joolian moons) a lot cheaper than starting from Kerbin LKO.

Yes it's cheaper, but oh so much more painful...

But how on Kerbin am I going to plan that?

First, I need a window for a Kerbin transfer, I figured. Then before that take the plunge towards Kerbin from Minimus at just the right moment that the vector leaving Kerbin is parallel to Kerbins motion within the kerbal system.

There the problem starts: What position needs Minimus in relation to Kerbol be in? The timing is not that important, since Minimus' sidereal period is 12 Kerbin days, so I should still get a decent window for Jool within those 12 days.

And the second problem: How do I compensate for Minimus' inclination? I come in at 6° relativ to the plane of ecliptic (IF the point of departure does not happen to be at the point where Minimus' plane intersects with the plane of ecliptic. So say: between 0° and 6°).

Do I need to change that during the plunge towards Kerbin? That seems to be the best idea. I won't be able to get a 0° inclination, but that should not be necessary. Important is the vector I come out of. If I leave Kerbins SOI roughly within the plane, I should then get a fairly level inclination relative to Kerbol, am I?

You'll need Minmus nearly "in front" of Kerbin in relations to Kerbin's orbit around the sun. You'll actually want to drop a bit before that, but you may need to play with the exact placement to give you the best angle toward Jool.

To compensate for the inclination, you can try to adjust your aim at Kerbin (on the inward leg) and let gravity sling you around a bit straighter. You should also be presented with a chance along the way to adjust the inclination, where your craft is passing through Kerbin's equatorial plane. Assuming the AN/DN nodes happen away from LKO, the dV to adjust the plane should be pretty small.

Any tips on that mission profile? What should I look out for?

I think you already covered the major headaches. Also realize that it takes several days dropping in from Minmus to get to Kerbin. You'll need to factor that in to your decision on "when to go" to meet your Jool window.

I do have a kethane rig stationed on Minimus, including two huge refueling tankers that can land there.

I even suspect I could land the whole vessel as is on Minimus (except the remaining lifting stage, of course. The center of the asparagus.

I was planning to refuel that and use the 800 m/s it gives (since I have to refuel anyway, so that is just one more tanker) for the first leg out of Kerbins SOI.

So the plan is to get that vessel (if refueled just short of 300 tons) out to Minimus, refuel it there using the kethane rig, then do the ascribed trajectory.

Personally, I think you'd be better off periodically hauling that fuel to Munar orbit. Dealing with starting your inward journey from the Mun is a lot easier than from Minmus, for all the reasons you're asking, and the difference in dV between the two is only around 100 m/s. The windows occur more often, and the inward journey is only a few hours long, instead of days.

Although there are some people who don't mind dropping in from Minmus.

Edited May 11, 2014 by Claw

[Tokay Gris]

Personally, I think you'd be better off periodically hauling that fuel to Munar orbit.

That would not be a problem. I have a rig on Mun as well. The tankers are a bit bigger (more engines), but otherwise it is the same.

Dealing with starting your inward journey from the Mun is a lot easier than from Minmus, for all the reasons you're asking, and the difference in dV between the two is only around 100 m/s.

Thats all? Really? Wow.... Since this vessel is supposed to have enough delta-V even if started from LKO, these 100 m/s "won't fatten the cabbage". (Deutsch: Davon wird der Kohl auch nicht fett....)

And if the reason for mission failure is a mere 100 m/s, I would be surprised immensely.

(I DO know that Apollo 11 was a lot closer to failure than 100 m/s... but this is not NASA. This is KSP... And "Delos Interplanetary" at that. I usually overdo delta-V)

Thanks!

[Claw]

That would not be a problem. I have a rig on Mun as well. The tankers are a bit bigger (more engines), but otherwise it is the same.

Thats all? Really? Wow.... Since this vessel is supposed to have enough delta-V even if started from LKO, these 100 m/s "won't fatten the cabbage". (Deutsch: Davon wird der Kohl auch nicht fett....)

And if the reason for mission failure is a mere 100 m/s, I would be surprised immensely.

(I DO know that Apollo 11 was a lot closer to failure than 100 m/s... but this is not NASA. This is KSP... And "Delos Interplanetary" at that. I usually overdo delta-V)

Thanks!

If you're launching fuel from the Mun, it will obviously cost you more dV to get it up there than launching it from Minmus. However, that wouldn't effect your interplanetary craft directly.

100 m/s is a rough guide and it'll probably be slightly more of a difference than that. The exact amount will depend on how low in orbit your refuling station is for Minmus/Mun, etc. But for the most part, you get most of the dV benefits with much fewer drawbacks when using the Mun compared to Minmus.

Consider that it's only about 70 m/s more to get to Minmus than Mun (from LKO). You'll also have to burn a bit more dV to escape Mun and do your Kerbin pass than you would to escape Minmus for your Kerbin pass, but the Mun also doesn't require an inclination change. In my opinion, the real savings is that it's a lot easier to align your departure window from Mun, which will save you dV on the interplanetary burn.

I'm sure someone could do all the exact math, but to me the ease of coming from Mun outweighs the small dV benefit from Minmus.

[cantab]

There the problem starts: What position needs Minimus in relation to Kerbol be in? The timing is not that important, since Minimus' sidereal period is 12 Kerbin days, so I should still get a decent window for Jool within those 12 days.

Opposite where you would make the burn if you were leaving from LKO, so that when you leave Minmus and drop your periapsis, that periapsis is where you need to make your final ejection burn. You might find it helpful to set a node on a spare craft in LKO for reference. As mentioned, it's worth factoring in the time taken to reach periapsis.

And the second problem: How do I compensate for Minimus' inclination? I come in at 6° relativ to the plane of ecliptic (IF the point of departure does not happen to be at the point where Minimus' plane intersects with the plane of ecliptic. So say: between 0° and 6°).

Do I need to change that during the plunge towards Kerbin? That seems to be the best idea. I won't be able to get a 0° inclination, but that should not be necessary. Important is the vector I come out of. If I leave Kerbins SOI roughly within the plane, I should then get a fairly level inclination relative to Kerbol, am I?

I think I'd sort things out at the Kerbin ejection burn. As you say, what matters is the ejection vector, and you're not starting massively out of plane. You could make a correction at the ascending/descending node but I don't think it'll save that much.

Any tips on that mission profile? What should I look out for?

TWR during the Kerbin ejection burn, since you can't split it. Darkness if you're using solar-electric propulsion (but you probably aren't). And the Mun messing things up.

[Claw]

...

I think I'd sort things out at the Kerbin ejection burn. As you say, what matters is the ejection vector, and you're not starting massively out of plane. You could make a correction at the ascending/descending node but I don't think it'll save that much.

...

I would disagree with this on a minor point in that it can be more expensive to adjust the inclination if done at the wrong time. The worst case inclination change from Minmus can cost over 300 dV, thereby reducing a good deal of the dV you were trying to save. Inclination change done at the nodes is typically less than 20 dV.