How do you get to Moho without using an ion engine or a ridiculous amount of fuel?

[MAFman]

I've been able to get into orbit around Moho using ion engines or by spamming fuel tanks, but never with a reasonable looking spacecraft and especially never crewed. How does one do this?

[jimmymcgoochie]

That's the neat part. You don't.

According to Ye Olde Delta-V Mappe*, you need about 7km/s of delta-V to go from low Kerbin orbit to low Moho orbit, about another 2km/s to land and return to low Moho orbit and a further 5-6km/s to get back to Kerbin (assuming full aerobraking). Call it 13km/s for the ship and 2km/s for the lander.

Spoiler

* Said map:

Chemical engines just aren't going to cut it for Moho as they lack the ISP to get enough delta-V. Your options are a) use the superior ISP of the NERV to build a large and probably quite ugly ship that can carry a small lander to Moho and back- I managed to throw one together that uses two large Mk3 liquid fuel fuselages and four NERVs, with a dinky lander and space for one crew, for less than 140 tons in LKO; or b) make a smaller NERV-powered ship and add mining equipment so it can make enough fuel on Moho to come back. Option A is simpler but would require hours of burns in total to get to Moho and back; option B would probably be lighter to launch to LKO and involve shorter burn times all around, but also comes with the risk of not being able to refuel and getting stranded.

Don't subject yourself to ion engines; they're incredibly efficient, but also incredibly weak compared to the other options- and yet still tens of thousands of times more powerful than real ion thrusters.

Discarding empty fuel tanks along the way with some clever staging is one way to boost your delta-V numbers, since an empty tank is just more dry mass which directly reduces the mass ratio and so reduces delta-V. Extra tanks attached via decouplers with crossfeed enabled or fuel ducts to feed into the main tanks are a good way of doing it, sometimes with extra engines on those tanks to compensate for the really low TWRs you get with those kinds of big, heavy, slow ships.

The other option is c) cheat with mods. Add some stupendously efficient yet reasonably powerful engines (such as those in Near Future Propulsion, for example) and you can get huge delta-V without suffering too much on the hours-long burns front.

[king of nowhere]

by taking gravity assists. lots of them. 

In this mission report, I describe how I started from Ike, orbited moho, and landed back on mun, all within a 6 km/s budget - including landing and takeoff. In this other report, I describe going from eve to Moho on a reasonable budget, and then using several gravity assists to get to duna again on a reasonable budget.

Still, going to moho is rather expensive. You can use gravity assists to get a solar periapsis touching moho and a solar apoapsis touching eve, but you can't do any better than that, and you still have to pay a fairly high price in intercept speed. that cannot be avoided. you can save a lot over more conservative approaches anyway.

The general rule is that the more your orbit is different from the orbit of your target planet, the higher the intercept speed will be. This is why if you go to eeloo from kerbin you end up with 1300 m/s of intercept speed, while if you start from jool you get almost nothing. with moho, your main issue is minimizing intercept deltaV, and you use gravity assists from eve to make your orbit as similar as possible to that of moho. Now, as I discuss in the first mission report, ideally you would want to meet moho at its solar periapsis, but there is also the issue of orbital inclination; it's probably a bit more efficient to use eve to equalize orbital inclination, even if that results in meeting moho far from periapsis.

The trick to multiple gravity assist, again as explained in the first mission report linked, is to enter a resonant orbit. Eve makes an orbit in 261.95 days, so you want to take a gravity assist that will result in a trajectory with an orbital time that will generate a resonance; for example an orbit lasting 175.7 days, so that 3 of your orbits will equal 2 orbits of Eve, and you'll get to eve again for another gravity assist.

8 minutes ago, jimmymcgoochie said:

That's the neat part. You don't.

According to Ye Olde Delta-V Mappe*, you need about 7km/s of delta-V to go from low Kerbin orbit to low Moho orbit, about another 2km/s to land and return to low Moho orbit and a further 5-6km/s to get back to Kerbin (assuming full aerobraking). Call it 13km/s for the ship and 2km/s for the lander.

the deltaV map does not account for gravity assist. it is also VERY unreliable regarding the navigation around jool, giving completely nonsensical figures for transfers between moons. it's a good tool for beginners, but i long since stopped using it, except to get an estimate of how much deltaV i need to land from low orbit on any given body.

Edited June 29, 2022 by king of nowhere

[king of nowhere]

I can further show this example of going to Gilly (where you can refuel with isru) to moho surface (where again you can refuel with isru) for less than 4000 m/s, which is an easily achievable target for a lander.

1) start from Gilly. Leave it when eve is intersecting the plane of moho's orbit, so you'll be able to avoid a plane change. the spot was eyeballed, because i don't have any tool to do this accurately

you want to lower your eve periapsis, to have lots of oberth effect when you burn for moho. lowering your periapsis from gilly would cost you some 400 m/s. but you have a trick to save fuel there: instead of burning to lower your eve orbit, burn to leave eve, with the minimum amount of thrust. fiddle with the ejection direction and exact thrust, and you'll be ejected in an orbit that's extremely close to eve, and will meet eve again on the next orbit. notice the 250 days eve periapsis, when eve year is 270 days.

the red manuever is a correction burn, mostly a plane change because the inclination with which you arrive at eve will determine the direction you'll be leaving

2) when you pass at eve periapsis make a burn to lower solar periapsis. Of course, your eve periapsis must be angled just right for this manuever to work: that's the purpose of the correction burn (here yellow) and also of the original burn. Make sure, before you leave gilly, that you fiddle with the planned manuever so that now your eve periapsis is facing the right way for this ejection.

I also angled the trajectory to leave eve going downward. so now the inclination with moho will only be 2.2 degrees, a lot less than it normally is.

This manuever wasn't perfect. I return to eve in 250 days, 20 days sooner than desired, and so I am not perfectly in the node and I can't completely cancel inclination. this manuever could have been better. still, that's good enough for most purposes, and it took me 10 minutes to set it up; further refining it would have been a lot harder.

The ejection burn from eve must send your inclination node with moho overlapping moho orbit, so you get an encounter. It must also be close to your solar periapsis, and as close as possible to moho solar periapsis. notice how none of those conditions is respected perfectly, but all are "good enough"

Finally, when passing at solar periapsis, make a small manuever to change the time of your orbit so you can syncronize it to meet moho at a later orbit. In this case, by setting up a manuever forward and clicking repeatedy "next orbit", I was able to discover that in 1 year 120 days I'd be passing very close to moho. at this point, a small 2.5 m/s burn was enough to get an intercept.

the violet manuever is the one I used for this, and I'm also using it to refine the encounter, setting a properly low moho periapsis.

At this point, having followed all those steps to ensure my orbit would be as similar as possible to moho's orbit, I get an intercept deltaV of 1750 m/s - including the cost for circularization. It would be possible to improve on this by up to 400 m/s, in theory - as I said, I've been inaccurate on getting the eve periapsis. However, getting more accurate than that is very hard.

anyway, add 900 m/s to land on moho and you're done.

100 m/s to leave gilly, plus 900 m/s to lower solar periapsis to moho, plus 1750 m/s to capture and circularize on moho, plus 900 m/s to land on moho, plus 20 m/s to orbit gilly, plus 50 m/s of various correction burns: total 3720 m/s.

A ship with isru and 3800 m/s can reach moho and return without staging. Took me half an hour to get that, it's not too difficult if one has the expertise.

It's possible to do it with as little as 3500 m/s if one can also optimize hard the trajectory. but that would require a lot more effort, and i'm not trying it.

[Kryxal]

Make an asteroid miner with lots of fuel, grab an asteroid as extra fuel, and away you go!  I completed a contract to take a class E asteroid to Moho once like this.  I used multiple NERVs, and adjusted thrust output to correct torque.