Going Moho via Eve

[Musil]

I was playing around with different ways of getting a gravity assist from Eve to get to Moho. I discovered... quite disappointed, that it doesn't seem possible to get the inclination change and the retro push at the same time. I tried it getting two separate encounters with Eve (and once you have done the first change to your orbit, getting the second encounter takes about 7 in-game years, unless you want to burn as much fuel as you save), but this happened:

-If I did the inclination change first, when I tried to get the retro assist the encounter changed my inclination anyways, so there was no point to the first encounter.

-If I did the inclination change first, not only was it a lot harder to get the second encounter, I was just unable to get Eve where I needed it to make a decent deceleration.

So I think it is probably best just to settle for one of the assists. But I am wondering witch one. These are the pros and cons:

-Getting the inclination change saves about 1500-2000 d-v. That's nice, but then my relative velocity to Moho is still very high when I get there. Also, I wonder if doing the inclination change at, say, Kerbins altitude, would not be a lot cheaper.

-Getting the retro assist can make my relative velocity much less, but I still have to do the plane change, and much lower than otherwise.

I did get to Mohos SOI on one occasion, combining the two assists... Almost managed to land, I was about 500 m/s short of d-v and the ship crashed horribly. The ship had only 12.000 m/s at launch, so that was not bad at all... If I would put a nuke on the deceleration stage I would get there, but I was too impatient and wanted a poodle, just so that the burns wouldn't be so long every time.

So, if you have gone to Moho without just putting a dozen jumbo oranges in you ship to do it, what's your way of getting there?

Edited March 17, 2015 by Musil

[tsotha]

You might want to have a look at metaphor's post in this thread. From KSC to the surface of Moho he used 7217 dV.

http://forum.kerbalspaceprogram.com/threads/74375-Lowest-Delta-v-to-Moho/page3

[Musil]

Thanks! That's really good. It hadn't occurred to me to use Moho for the assists, because it's so small... But that is a good way to do it. Going to try it.

[Alchemist]

I once did Dres-Eve-Eve-Moho maneuver

https://youtu.be/eRw1ZojMhzo?t=36m54s

Maybe that will be helpful. It ended with the perfect Eve-Moho transfer orbit.

Some tips about such slingshots:

*If you don't use additional burns at periapsis, you get the same relative velocity to the planet you used, but in different direction. And that can change your solar orbit velocity quite significantly.

*The only limitation to the angle between approach and escape vectors is defined by your speed ant the planet's terrain/atmosphere. But you will never get the 180 degree turn in one go.

*That means, you can't turn trajectory with periapsis at given planet's orbit into trajectory with apoapsis at given planet's orbit.

*You may want to intercept the planet's orbit at an angle. For Kerbin-Eve-Moho maneuver you can have Kerbin-Eve transfer trajectory with periapsis below Eve orbit. This way you'll get more manageable velocity change angle (you'd probably still aim for optimal Eve-Moho transfer) and some extra speed you'll need to get the good transfer (note that in my video it was Dres-Eve trajectory with close to optimal parameters, optimal Kerbin-Eve transfer won't probably give you enough velocity at Eve to get the optimal Eve-Moho transfer)

*If you use double slingshot (because you have to change your velocity vector too much), aim for intermediate orbit with the same period (try escape vector perpendicular to the planet's velocity). This way, you can encounter the planet again after one orbit (or even half the orbit, ifthe intermediate obit differs from the planet's orbit just in inclination). Otherwise, it'll just take longer.

*The biggest trick with Moho is encounter velocity. The perfect transfer (the same plane, meeting Moho at its and your periapsis) may result in less than 2 km/s orbital insertion Delta v (especially, if done with Eve slingshot; perfect transfer directly from Kerbin requires slightly more), usual Kerbin-Moho transfer often results in about 4 km/s and a sloppy encounter can yield even 6 km/s of insertion burn.

Edited March 15, 2015 by Alchemist

[Armchair Rocket Scientist]

Thanks! That's really good. It hadn't occurred to me to use Moho for the assists, because it's so small... But that is a good way to do it. Going to try it.

That's how MESSENGER did it. One flyby of Earth, two of Venus, and three of Mercury before the final orbital insertion.

[Musil]

Thanks, @Alchemist.

Yes, you can't get a 180º change, but at Eve it is possible to get 90º. If you get into it's SOI perfectly at the dark side, you can get a very good turn in the retrograde direction. I managed to reduce my ships orbit from one with apoapsis at Kerbin and periapsis at Moho to an orbit with apoapsis at Eve and periapsis halfway between Eve and Moho, but I think I had some deceleration from the atmosphere too, lowering my speed significantly without getting capture.

I am not sure I get the point of going to Dres first... Or was that part of another mission?

@Armchair Rocket Scientist:

Yes, I was aware that the Messenger probe used several gravity assists, but I thought they had all been at Venus. Should have done my homework better.

I find your name funny, btw. I majored in philosophy, and philosophy it is usually described as "armchair science", so being an armchair rocket scientist seems the perfect way to describe a KSP player

[Alchemist]

I am not sure I get the point of going to Dres first... Or was that part of another mission?

The challenge was to do both Eeloo and Moho with one ship, but one of the bonus points was rendezvous with another craft and crew transfer. Somehow, I decided that sending the second craft to either destination is not the best idea and chose Dres as the meeting point. Only then I realized that there's way less fuel than would be required for direct flight to Moho (in this configuration 1 full ship is good for usual LKO-Moho-Kerbin flight; however if you enlarge the tank a bit more you an squeeze multiple landings at different locations on Moho), therefore I tried using Eve. Actually, I barely had any idea what I was supposed to do there, starting with the attempt to to get that 180 degree turn... Well, I was aiming for tht optimal trajectory, but had no idea I've have just the right velocity (well, there was some extra, but minimally)

Also, now looking at the intermediate orbit (Ap between Kerbin and Duna, Pe between Eve and Moho and quite some inclination) it seems that in Kerbin-Eve-Moho flight the ultra-cheap Moho encounter would require initial Kerbin-Eve transfer to already have periapsis close to Moho and the shingshot to work more like the plane change. But then will there be really much Delta v difference from directly using the cheapest Kerbin-Moho transfer? (For that you'll need to directly launch from Kerbin into inclined transfer orbit in the same plane as Moho)

By the way, slingshots with your starting point (as the first slingshot) or your destination (as the last one) really do much only in case if the planet has rather eccentric orbit - then you can get second encounter at different distance from the Sun with notably different relative velocity (also applicable to double slingshots at eccentric planets). Another interesting trick is bouncing between 2 planets (like Kerbin-Eve-Kerbin-...) taking advantage of their relative inclination to gain free velocity