Moho return

[Rylant]

Hey all. So I am getting closer to the end game. Been to and from Eeloo and most Jool moons. Still struggling a bit with Moho return.

I developed a high delta v lander with refueling capabilities. It does have several nuclear engines as much as I dislike them lol. The problem I realize, was the thrust to weight makes it really hard to land on Moho. I could make a lander which rendezvous with the mother ship, of course then the mother ship can’t refuel. So I need enough delta v to get to Moho, slow down (the tough part), and return. 
 

I am trying to nail down the Moho transfer. I know it’s probably the toughest transfer because of the inclination and the fast travel around  the Sun. You can get to Moho straight from Kerbin with one burn, but slowing down is tight. I know you can literally spend thousands more delta v doing it this way. 
 

How are other people doing it? I am more wasteful with delta v than I would like. Could somebody explain the most efficient burn to Moho? I know it involves a correction burn but I can’t quite figure that out.  I think it involves the correction burn at ascending node or descending node?

 Rylant

[Echo__3]

Perhaps this video tutorial will help.

 

[Starman4308]

While the inclination is a problem, it's not the biggest problem. The biggest problem is that Moho is in a much lower specific-energy orbit: if you transfer straight from Kerbin to Moho, you're traveling several kilometers/sec relative to Moho at the encounter.

What can save you several km/s is using gravity assists off of Eve and Moho to shed some of your orbital energy. It's a little tricky to set up courtesy of the differing inclinations of Kerbin, Eve and Moho, but a careful series of gravity assists can really help you shed energy and enter Moho's SOI with a much lower excess velocity.

The three tips I can give: take careful note of where you encounter a planet when setting up a gravity assist (as you will always wind up returning to roughly the same spot), it can help to set up resonant orbits (e.g. having a 2:1 vessel:Moho orbital period ratio so you re-encounter Moho in two of its orbits), and remember that gravity assists can change inclination as well as orbital energy.

EDIT: And if you're wondering what I mean by "shedding energy", that's basically just going to a lower orbit. While going into a lower orbit means you pick up kinetic energy, you lose more gravitational potential energy than you gain in kinetic.

Edited May 21, 2021 by Starman4308

[Rylant]

40 minutes ago, Starman4308 said:

While the inclination is a problem, it's not the biggest problem. The biggest problem is that Moho is in a much lower specific-energy orbit: if you transfer straight from Kerbin to Moho, you're traveling several kilometers/sec relative to Moho at the encounter.

What can save you several km/s is using gravity assists off of Eve and Moho to shed some of your orbital energy. It's a little tricky to set up courtesy of the differing inclinations of Kerbin, Eve and Moho, but a careful series of gravity assists can really help you shed energy and enter Moho's SOI with a much lower excess velocity.

The three tips I can give: take careful note of where you encounter a planet when setting up a gravity assist (as you will always wind up returning to roughly the same spot), it can help to set up resonant orbits (e.g. having a 2:1 vessel:Moho orbital period ratio so you re-encounter Moho in two of its orbits), and remember that gravity assists can change inclination as well as orbital energy.

EDIT: And if you're wondering what I mean by "shedding energy", that's basically just going to a lower orbit. While going into a lower orbit means you pick up kinetic energy, you lose more gravitational potential energy than you gain in kinetic.

Hey thanks for this suggestion. So I am brutal at gravity assists. So much so that my feeble attempt usually makes it much worse lol. Definitely something I need to work on. I have heard people suggest the Eve gravity assist to Moho as well, but also lots of suggestions to not bother because it can take game years lining them up for a good assist. Of course I have never actually tried it. 
 

Rylant

[AlexinTokyo]

This is an old thread, and unfortunately the pictures have fallen to link rot, but it explains pretty well (I think) a highly efficient and simple way to do a Moho transfer:

 

[Rylant]

thanks guys.  Super helpful.

Wanted to ask, what's the minimum amount of Thrust to Weight for the planet that you would want to land with?  Obviously, it's gotta be over 1.  But 1.4.?  Is that still too low? 

Rylant

[Starman4308]

I've started landing burns on < 1. You don't need to start with TWR > 1*, you just need to end there.

*Throughout, I'll be referring to local TWR, dependent on the body's surface gravity. 

The theoretical optimum depends on the moon and engines in question, as it's a tradeoff between minimizing engine mass and minimizing gravity losses. High TWR reduces gravity losses, as you spend less effort hovering, but involves a larger engine mass.

Smaller bodies will tend to favor higher TWR, as there's less time to burn off propellant, and the mass cost of increased TWR is lesser. With bodies like Tylo, you can burn off so much propellant in the early stages of your descent that your TWR at landing is noticeably higher, and trying to increase TWR means a lot of engine mass due to Tylo's high surface gravity. With a 30 ton craft at Gilly, going from a TWR of 1.5 to 3.0 means adding a second ion engine.

In practice, higher TWR landings are easier, especially if you want to land on a precise spot. If you descend too fast and are about to smack into terrain, high TWR lets you pull up much faster.

EDIT: It also depends on whether you expect to utilize ISRU refueling for the ascent. If your vessel has a TWR of less than 1.0 when fully loaded, you are not going to be able to take off with a full tank. When landing, minimum fuel mass coincides with highest gravity losses (just before touchdown), whereas for ascent, highest gravity losses occur at maximum fuel mass.

Edited May 21, 2021 by Starman4308

[Hoozemans]

On 5/20/2021 at 10:51 PM, Rylant said:

So I am getting closer to the end game.

What is the 'end game'? Is there such a thing?

[Hotel26]

What's worked best for me is to depart at Moho's Ascending Node and reduce PE to Moho's altitude at its opposite DN -- and it additionally works as you drag out the injection maneuver node, to dial in the inclination adjustment.  This latter doesn't have to be fussy: if you can get the inclination down from 7 deg to < 1, you're getting it with Oberth and you can finish it at the other end with Oberth.  The big benefit is it makes getting an encounter a lot easier because your arrival orbit is so much better aligned with Moho's, meaning the encounter is not a pinhole intersection.

This is, I think, a better approach than using a transfer window (although you still have to go at a particular time).  The cost will be that, when you arrive at Moho's DN (and your ultimate rendez-vous point), you will need one more Moho orbit (approx.) to align that encounter.  The benefit is its about the simplest technique I know for Moho.

Note that Moho's DN is not too far from its own PE, which would be the optimum rendez-vous point in terms of matching orbital speeds (ignoring inclination).  This is why DN arrival is preferred to AN.

Here's the link, read through this: https://forum.kerbalspaceprogram.com/index.php?/topic/196048-when-is-the-annual-moho-transfer-again/

 

Edited May 22, 2021 by Hotel26

[Hotel26]

On 5/21/2021 at 4:51 AM, Rylant said:

think it involves the correction burn at ascending node or descending node?

You were on the right track!

[king of nowhere]

On 5/20/2021 at 10:51 PM, Rylant said:

I developed a high delta v lander with refueling capabilities. It does have several nuclear engines as much as I dislike them lol. The problem I realize, was the thrust to weight makes it really hard to land on Moho. I could make a lander which rendezvous with the mother ship, of course then the mother ship can’t refuel. So I need enough delta v to get to Moho, slow down (the tough part), and return. 
 

you have a few ways around that

1) make a nuclear tug with low thrust to bring you in Moho's orbit. Once in Moho's orbit, detach the lander. make sure that the lander has large tanks, so it can go up and down with much spare fuel. refuel the lander, bring it back to the tug, and transfer fuel to the tug. then land the lander again, and dig more fuel. repeat until the tug is refueled.

2) make a slightly bigger tug. Do not bother with refueling it. in fact, if you drop the mining equipment in favor of fuel, you're probably good already. reach Moho. drop the lander. recover the lander. Now, going to Moho requires at least 3000 m/s, unless you are willing to spend many decades making slow gravity assists. but going away FROM Moho is much cheaper: with 1000 m/s you can get to Eve, use an Eve gravity assist to go everywhere. You can get an intercept to kerbin, where you'll aerobrake (you'll need a heat shield). So, refueling at Moho is not all that necessary, you only need a fraction of what you spent going in

3) instead of going to Moho, go to Gilly. Refuel at Gilly. Now go to Moho from Gilly - falling down on Eve first, to get Oberth effect. It's much cheaper to go to Moho this way, so you won't need refueling at Moho. In fact, you can drop your mining equipment at Gilly for reduced weight