Doing a Kerbin Slingshot from Mun orbit to Duna (or anywhere else)

[BlackHat]

I have a Sat in Mun Orbit which no longer is needed, Since it is fully fueled, I wanted to use it to send to Duna (or eve or anywhere else) to get Science!!

As long as I get the right ejection angle (pro-grade for Duna, retro-grade for Eve) will I be able to save enough fuel by sling-shotting around Kerbin to make it worthwhile to set up the maneuver, or am I better off just going directly from Mun orbit? (assuming the Mun is on the correct side of the orbit of Kerbin to do the pro-grade or retro-grade solar orbit)

If I will only save 50-100ms or less, then its not worth it to me. But if I can save 500-600ms or more, that would be.

The only problem I can think of is if the Mun is poorly positioned for a slingshot to the destination.

I do vaguely recall seeing a Scott Manely video (I think) where he tried something like this (from Minmus) but if I recall correctly messed up the timing I think, missed the window, and spent more fuel trying to get the final encounter)

Any thoughts?

Edited October 18, 2015 by BlackHat

[kStrout]

Sounds like you could probably save a lot of fuel getting a slingshot from Kerbin, It only takes about 100-200m/s to escape from the moon and get back to a low periapsis around Kerbin(it's been a while since I've done this so don't count me on this figure), and from there it should only take about 200m/s at periapsis to get to Duna. But I have no Idea how much fuel you will need for a direct burn so set up a maneuver node and if it is more that 500m/s or so, go for the Kerbin slingshot.

[Tatonf]

After some testing, from a 10 000m equatorial orbit around the Mun :

- Exiting Kerbin's SOI cost 200 m/s

- Touching Duna's orbit cost 500 m/s

So, if you can set some perfect gravity assist, you can save 300 m/s.

[Superfluous J]

You will save fuel. If you have a ship in LKO you can use it to set up a maneuver burn to Duna (Hint: precise node and future orbits help a lot). Don't perform that burn, but while controlling your Mun orbiter you can target that ship and see its maneuver node.

Then, you can time leaving Mun such that your periapsis comes down to touch that maneuver node (or get close), and then you can be confident that burning at periapsis will send you to Duna.

There's a lot of timing and logistics, and if you don't appreciate that kind of thing it's not worth the mental gymnastics. However seeing as you asked I suspect you DO appreciate that kind of thing

[BlackHat]

Cool, Thanks for the replies, I have another 20-30 game days before the window, so I will definitely play around with doing the nodes and check it out.

[Yasmy]

Going from the Mun to Duna or Eve, you are better off just going directly from the Mun, and not sling-shotting around Kerbin. For other destinations, a Kerbin slingshot is more fuel efficient. Here's an approximate table comparing the cost of slingshots versus direct transfers, assuming no plane change maneuvers and circular orbits. While it is somewhat inaccurate in absolute terms, in comparison terms it should still be useful.

[Tatonf]

Here's an approximate table comparing the cost of slingshots versus direct transfers

If I understand the table correctly, it's not a slingshot versus direct transfer. It compares the costs between

1) Getting out of the moon with a periapsis just above the moon's parent and then using the parent's Oberth effect to reach destination

2) Transfering directly from the moon to the destination

3) Transfering directly from the moon's parent to the destination

The table is here to know if it's better to use the moon's parent Oberth effect rather than the moon's Oberth effect, or not.

[BlackHat]

Going from the Mun to Duna or Eve, you are better off just going directly from the Mun, and not sling-shotting around Kerbin. For other destinations, a Kerbin slingshot is more fuel efficient. Here's an approximate table comparing the cost of slingshots versus direct transfers, assuming no plane change maneuvers and circular orbits. While it is somewhat inaccurate in absolute terms, in comparison terms it should still be useful.

Nice thread and chart... Looks like a slingshot from Minmus would be cost effective, but not from the mun. I will still play around with the numbers and nodes on my game but suspect the chart is accurate.

Thanks

[Yasmy]

If I understand the table correctly, it's not a slingshot versus direct transfer. It compares the costs between

1) Getting out of the moon with a periapsis just above the moon's parent and then using the parent's Oberth effect to reach destination

2) Transfering directly from the moon to the destination

3) Transfering directly from the moon's parent to the destination

Yes. Number 1 is what I'm calling a powered slingshot. Your terminology may differ. Heck, everyone's terminology may differ. I'm OK with that. Escape the moon, dropping your periapsis to the parent, and complete the burn at the parent. The delta-v cost is the combined cost of dropping down to the parent and executing the transfer burn.

The table is here to know if it's better to use the moon's parent Oberth effect rather than the moon's Oberth effect, or not.

Yup. I created the table for two reasons:

1) To help understand the tradeoffs involved in different mining operation locations vs. fuel depots around the parent, and

2) To know when to transfer directly back to Kerbin from a moon, vs. dropping down to the parent to complete the burn.

Edited October 18, 2015 by Yasmy

[Tatonf]

Ok, I think I undestand. I'm not really ok with using differents terminology though...

A slingshot is when you have a spacecraft on an hyperbolic encounter trajectory with a body. This is not the case here.

I don't know if "slingshot" is an official term but I think it's the most logical way of using it.

[Yasmy]

I don't disagree with you. I can be overly pedantic myself, and it is always best to try to be clear here. I don't know if there is a standard term for this particular maneuver, though on these forums, some people call it an Oberth maneuver. I don't know if people call it that elsewhere, or if there is just no accepted term for it. I don't like that term because the Oberth effect is always in effect. All fuel-ejecting maneuvers are Oberth maneuvers. Some are just better than others. Anyway, I thought it was somewhat appropriate to call it a powered slingshot since you come in from a different SOI and end up on a hyperbolic orbit. I should be clear when I'm playing fast and loose with terminology.