best way to get to Moho

[BFGfreak]

I recently attempted to send a probe over to Moho in preperation for a manned mission, and while everything I did went better than planned (I had enough fuel in my assent stage to fling me into interplanetary space for starters) when I finally got to Moho's SOI I was hauling at 4k dV, which unfortunately was too much for both my nuke engine and ion engine to cancel out resulting in my probe now xenonless in a solar orbit. Anyway I was wondering if anyone had any tips for reducing that Delta V I have to burn off for my next mission?

[Eric S]

On that tranfer orbit, I usually start burning before I hit Moho's SoI. I think they got rid of Moho's atmosphere, so you can't aerobrake there anymore, if you ever could, so if you're in a low TWR craft, starting the breaking burn early is your only efficient and practical option. If you hit Moho's SoI precisely enough, you've probably got plenty of time to break while in it, so you might want to fine tune your intercept (which does sound faster than it's supposed to be).

I imagine that a combination gravity slingshot/aerobrake maneuver at Eve would be possible, but waiting for the planets to align just right... Oh, wow, I have to try it now. A transfer orbit to Eve, swing around Eve so that I go from moving faster than eve to slower than eve, maybe even aerobraking at Eve to adjust my exit velocity to what I'd need for a Moho transfer orbit. Oops, never mind the aerobraking, my exit velocity for an Eve-> Moho transfer orbit would have to be higher than my entrance velocity after a Kerbin->Eve transfer. So the question becomes, can I use Eve's gravity to flip a U-turn without getting so close to Eve that I hit its atmosphere?

Hmmm... it looks like that would only reduce the breaking delta-V required by about 800 delta-V and require some acceleration in Eve's SoI, so probably not enough to be worth it from a technical standpoint. On the other hand, I think it would still be the kind of "I can't believe I pulled that off!" moment that really makes this game worth playing.

[Amaroq]

Uhh .. keeping it a bit simpler:

Make sure your intersection with Moho's orbit is a perfect tangent.

If, for example, you still have a difference in inclination, or if your periapsis is inside Moho's orbit but you have an intersect on the way "in" or the way "out", you're going to need too much delta-V.

If, instead, you've got an inclination difference of no more than 0.1 degrees, and the projected intercept is at your projected solar periapsis, and you make a little adjustment or two as you get close to ensure that your Moho periapsis is low to the surface (< 20km?), you should do just fine .. as Eric S. pointed out you could brake before entering Moho SOI if you have a really low TWR.

Those changes have literally made the difference between a braking burn requiring 5000 d-V and requiring about 2500 d-V for me -- they feel really "slight" in terms of what your solar orbit looks like, but they make a big difference.

[Eric S]

Right, I got derailed. I'm good at that. For the normal method, you want to match orbits with Moho as close to parallel to Moho's orbit as possible to minimize radial velocity relative to the sun. You'll want to match Moho's inclination to minimize vertical distance and velocity, but this isn't as critical since changing your inclination in solar orbit, especially if your ascending/descending node is lower in your orbit, is very expensive. You should be fine-tuning your Moho intercept using small burns early on in the trip to achieve a minimal periapsis so that the oberth effect maximizes your breaking. Ideally, you'll center your breaking at your periapsis to maximize the oberth effect, but you may need to start the burn early. Don't burn too early though, as that will cost fuel, you'll want to have the burn as close to centered on the periapsis as you can but still achieve a capture.

[Merinsan]

One thing I worked out last night is Moho has an elliptical orbit, so you want to get your solar orbit apoapsis to be opposite Mohos' apoapsis, which means you need to escape Kerbins SOI prograde at that point. Then make sure your transfer burn happens at the apoapsis.

Last night I did just what you did, and ran out of fuel with about 400 delta V left to get a capture. I'll probably redesign the ship to get more fuel there so I can get a capture, but before I do that, I'm going to have to try the tips here. One question I have about the oberth effect is how much it helps?

I'm already adjusting my encounter as far out as I can (which is hard without being able to zoom on it!), but aim for about 100km periapsis, since I have my station orbiting there. How much delta V would I save by dropping it to say 10km?

[Eric S]

To be honest, I'm not sure. It might not save you anything in the long run, since while it might save you braking delta-V, you'd then have to use delta-V to raise your periapsis. I was mostly thinking of the intercepts I'd get when I wasn't trying to be precise, and my capture periapsis would be well outside the orbit of any moons the planet had. THAT you want to avoid.

On the other hand, if you brake at a really tight periapsis until you've got a high apoapsis, where you burn to raise your periapsis to where you want, then when back at periapsis you burn to put your apoapsis where you really want it, that could take delta-V than taking your apoapsis where you want then raising the periapsis. I think this would be the basis of a bi-elliptic transfer.

[Jason Patterson]

You definitely want to encounter Moho at its periapsis, because the velocities will differ by less at that point. Other than that, it's just a matter of bringing along enough delta-v. It's a high delta-v mission; there's no avoiding it.

[Panichio]

What would happen if you got an intercept at your ships solar apoapsis? I was thinking that it would require less dV, because you would already be at a smaller speed. I'm not sure how much of a difference it would make though, and if it would at all.

[Merinsan]

You definitely want to encounter Moho at its periapsis, because the velocities will differ by less at that point. Other than that, it's just a matter of bringing along enough delta-v. It's a high delta-v mission; there's no avoiding it.

I didn't think of that. I'll have to try that too. I noticed the transfer burn differed by about 700 when I aimed to intercept at moho periapsis or apoapsis. My own apoapsis was in a nowhere type of position because I didn't think about where I wanted to eject from Kerbin. So based on what you are saying, I would want to leave Kerbin when it's at the same side as Mohos apoapsis, for the most efficient burn transfer to meet Moho at it's periapsis.

What would happen if you got an intercept at your ships solar apoapsis? I was thinking that it would require less dV, because you would already be at a smaller speed. I'm not sure how much of a difference it would make though, and if it would at all.

That would require you burning to get your periapsis lower than the orbit of Moho, which would use even more fuel.

[Jason Patterson]

I didn't think of that. I'll have to try that too. I noticed the transfer burn differed by about 700 when I aimed to intercept at moho periapsis or apoapsis. My own apoapsis was in a nowhere type of position because I didn't think about where I wanted to eject from Kerbin. So based on what you are saying, I would want to leave Kerbin when it's at the same side as Mohos apoapsis, for the most efficient burn transfer to meet Moho at it's periapsis.

Yep. You do use a bit more fuel on the ejection burn, but when you meet up with Moho you save more than the excess that it took to get there. The other thing that can help is trying to get your encounter at or near one of the orbital nodes, since that can keep you from needing to make any sort of normal burn at all. That's not going to line up with Moho's periapsis though, of course.

[Merinsan]

Yep. You do use a bit more fuel on the ejection burn, but when you meet up with Moho you save more than the excess that it took to get there. The other thing that can help is trying to get your encounter at or near one of the orbital nodes, since that can keep you from needing to make any sort of normal burn at all. That's not going to line up with Moho's periapsis though, of course.

I'm going to have to try that too. I used about a quarter of my fuel just doing the plane correction to match the plane of Moho (forget what the delta v was). I have done an intersect close to an orbital node before, but I've completely ignored it getting my base set up. I know it doesn't take much to change plane while in orbit of Moho. Just the other day I actually reversed the direction of orbit on a ship that was going the wrong way (got there before I got the station set up). I was convinced I wouldn't be able to, but the delta v was only about 600 I think. Now I have fuel for the landing it originally went there for.

[Panichio]

That would require you burning to get your periapsis lower than the orbit of Moho, which would use even more fuel.

Oh right...

I think in the end though, the best way to get to Moho is just cram-pack as much dV as you possibly can, and design a ship with a high T/W ratio.

[RoboRay]

You don't need a high TWR, if you plan ahead. I had to start my mapping probe's ion engine for the orbital insertion an hour before even entering Moho's SOI, but I managed the capture.