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Moho mission troubles


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I've spent a lot of time constructing and testing a bunch of vessels to my upcoming Moho mission, since Kerbal Alarm Clock notified me that the launch window will open in a couple of weeks. When every single craft was tested and I was extremely satisfied with the result, I finally plot a course to Moho, ~2100m/s dV from 241km circular Kerbin orbit (which is a bit higher then Wiki told me, but who am I to argue with MechJeb :sealed:).

The problem is that I was pretty sure that 6500 dV is more then enough for a Tow to deliver a mission to whatever place in Kerbol system (maybe except Eeloo, I've never been there). Imagine my disappointment when I've realized that with ~4500m/s dV left upon arrival to Moho SOI i can barely inject into orbit, not even thinking about continuing my mission. My super fancy spaceship was stranded in Moho hight orbit waiting for a refuel.

So, what I want to know - was that my mistake or 6500 dV for a Tow is not enough to get from Kerbin orbit to Moho low orbit and then get back?

Was my transfer maneuver that bad (tried both MechJeb plotted and manual), or 4200-4700m/s braking is just Okay for Moho orbit injection?

(I'm a little surprised that most of tutorials and stuff like that doesn't even mention Moho as a place to go, or just saying that 'It's just another Mun')

Edited by TC One
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I once got Moho encounter at 6 km/s, but it was a sloppily plotted intercept. 4 km/s for Moho encounter is a rather usual value for usual plotting (plus 2 km/s for Kerbin ejection plus up to 1 km/s plane chnge, if done midcourse). So you getting to Moho with 6,5 km/s total delta v is rather normal.

The optimal transfer profile (if not using slingshots) is to eject from Kerbin into the plane of Moho orbit (it's much cheaper than ejection into ecliptics + plane change) and catch Moho near its periapsis (fortunately, major axis of Moho's orbit is close to the line of orbital plane intersection), which makes Moho insertion at slightly over 2 km/s and total LKO-Moho orbit transfer below 5 km/s. And if the same kind of transfer trajectory is used for the flight back, 7 km/s might be just enough for transfers there and back. But you'd better have at least 9-10 km/s if you aren't sure about your navigation skills.

Technically, Moho is nothing too special, except for huge delta v requirements. And huge delta v cost of some mistakes in navigation.

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I'm aroud Jool for the moment, doing multiple missions to each moons. Tranfert to other bodies can result in very different costs. I fell short of fuel twice (finishing plan change with RCS... then waited for the station to send a tug). I've noticed that AlarmClock or MechJeb can make poor calculations but maybe I don't use them correctly.

The ejection inclinaison is very hard to get right because you have to be in the correct alignment at the proper time. For my Eeloo mission (which isn't arrived yet) I had to redo it several times. Finally, the DV cost is way higher than the number I can find on various tools or sites. I even had to make mid course correction.

Even if it's more costly, a basic Hoffman transfert maybe the way to go to avoid bad news on arrival. It's much easier to aim for a shallow angle. But I'm very far to be an expert.

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Moho is surprisingly hard; it's close to the sun, so your velocity is always pretty high, and it doesn't have much gravity or any atmosphere, so it doesn't do anything to help you out :/

I tend to ignore it, at least as far as manned missions go, since it's not very exciting for how hard it is to get to. I mean, maybe if it was a frikkin' lava world with a hot orange atmosphere or something... ;)

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@ Alchemist

Thanks for the tip about target's periapsis. It's so obvious, when someone else say that out loud :D

@ Warzouz

Regarding MJ and KAC - they are pretty good, really. But looks like sometimes MJ tends to recommend the most recent transfers instead of the maximum efficiency, however you can always wait a bit and find a new variants from it's porkchop selection.

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I had most difficulties with porkshop. (My station orbits at 500km around Laythe) I asked for an orbit from Laythe to Bop and it offers 1 node which targeted Laythe (!), updated it's calculation and added another very costly node which still been in orbit of Laythe, then again a third node which had a very steep encounter with Bop (which braking should hav been much higher than calculated). I managed to do a classic hoffman to Bop for a third of the DV and a very low capture cost. But I surely not using this tool correctly.

Also, I've noticed the calculation of alarmClock don't give me an encounter at the specified date. I'm usually several orbits from an encouter. For now I reverting to simplier trajectories, or guessing it with MechJeb Node Editor.

Usually, I try with MJ to see what I get (at the date offered by Alarm Clock or Window Planner). I'm usually not convinced by what I found so I guessing by ploting an approximative prograde escape burn then tweeking with the time setting until I find something nice. Then I fine tune the burn (prograde, radial, normal).

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Oh goodness, happened to me too. I just now got Jeb in an orbit around Moho; I had launched him thinking he would be back in time to lead a mission to Duna. That mission has come and gone, and he also missed being on the Eve crew now in LEO.

I think I made the same mistakes as others. First, I didn't pay attention to inclination at launch. I should have gotten into LKO in Moho's plane, not in Kerbin's equatorial plane. Correcting later was very expensive. Second, I should have paid attention to the velocity at intercept rather than just thrust until I got an encounter. That first encounter was at ludicrous speed, and while plotting the maneuver node I realized I would never have time to decelerate for capture on 60kN of thrust. So I let that one go, and a Kerbal year or two later managed to get an orbit using the reserve fuel in the lander. I think i have enough to get home, but a landing is off unless I can send a refueling ship.

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A tip on getting into transfer trajectories with high inclination:

1) you might start in equatorial Kerbin orbit. It can be fixed. Ensure that Kerbin is near the target planet's orbital plane.

2) plot the ejection so that your interplanetary transfer gets right into the correct plane. Yes, use normal/antinormal vector in addition to pograde. Yes, it's a ridiculous number. No, you won't be spending this much.

3) Burn prograde at the maneuver node (or slightly before it, if you have low TWR) until your apoapsis is like 2500 km (or higher, but you don't win much by getting much higher). (if your TWR is teribly low, you might need several burns to do it)

4) Now at the apoapsis perform plane change so that you are in the plane of the previousy plotted ejection trajectory

5) recalculate the interplanetary transfer

6) PROFIT!

Alternatively, you can calculate ejection on a small probe and then change the probe's orbit so that it's in the plane of your ejection trajectory. Then use the probe as the reference to launch your actual craft in the properly inclined Kerbin orbit.

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Does getting into LKO in Moho's plane actually make much difference? Soon as you leave Kerbin's SOI, you find you've got 8km/s (or more) of momentum travelling in the direction of Kerbin's orbital plane. The few hundred m/s that you might have had vertically quickly disappears in the minutiae...

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I love linking this, because it shows how cheaply you can get to Moho, IF you're really, really proficient at gravity assists. Gravity assists are the key to advanced gameplay-- they save loads of delta-v, are fun to plot and fly, and makes ridiculous multi-planet missions feasible.

In short, by using 8+ gravity assists off of Eve and Moho, metaphor was able to land on Moho with only 2900 dV from LKO. So even with this uber-piloting (well beyond the level of me or you), getting from LKO to Moho took half of your 6500 dV.

- - - Updated - - -

Does getting into LKO in Moho's plane actually make much difference? Soon as you leave Kerbin's SOI, you find you've got 8km/s (or more) of momentum travelling in the direction of Kerbin's orbital plane. The few hundred m/s that you might have had vertically quickly disappears in the minutiae...

And eddiew, your question is one I've been wondering about, too. It seems to me that where KERBIN is relative to Moho's plane is way more important than what plane you orbit in LKO.

But here's an explanation from the link I shared:

"Yeah, so first I sent a test probe into equatorial low Kerbin orbit, then made a maneuver node from that probe to get to Eve. I had to cross Eve's orbit at an ascending/descending node (so that I would actually intersect Eve's orbit instead of going below/above it). So I added a liberal amount of inclination change to the maneuver node to make sure that happened. As you can see from the picture, it's going about 30 degrees towards the south (that node had about 1200 m/s prograde component and 1600 m/s southward component, for a total of 2000 m/s (Pythagorean theorem)). But it's inefficient to do such a huge inclination change when you're already in orbit. Much more efficient to do it during launch. So I waited until the KSC was right under the maneuver node and I launched the ship into an orbit inclined 30 degrees to the south of an equatorial orbit. It doesn't cost much delta-v to launch into an inclined orbit rather than an equatorial one, you lose maybe 50 m/s from not taking full advantage of Kerbin's rotation. But now, if you remake that same maneuver node in the new orbit, you don't have to add any inclination change, you can burn straight prograde to get the same effect, for only ~1200 m/s. Ships do this all the time in reality, when they need to go to an interplanetary destination where they need an inclination change, they first launch into an inclined orbit around Earth to save delta-v."

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For Eeloo, I did with an inclination at launch.

- I waited for the correct time in the KAC window

- I launched half an orbit before the correct ejection angle is set (to Eeloo, it's on the dark side)

- I set MJ for a 100km orbit with 15/20° inclination south.

- You have less than half an orbit to plot for an encounter

- The ship goes down and arrive at the equatorial node with the upward inclinason and ejection angle.

- At that point burn prograde to escape and more.

- After escaping Kerbin SOI, check your inclination angle. If it's off course, retry launch.

- The plane correction should be quite low.

Obviously, the escape burn can't be done witha low TWR or you'll be way off course (or too complexe for me to plan). I've designed a mixed first and second stage with a Kerbodyne engine, 2 LV30 and 4 LVN burning togethers. The TWR is high on the begining of the burn and decrease as I decouple the low efficiency engines.

My Payload is 1 SSTO and some refueling mini station with a Lab and a return ship. My goal is to do 3 missions to Eeloo with 2 hops each (so 6 biomes visited).

Arriving in 300 days after a 2 year trip. They even arrive after the Jool mission is finished (which is a 400T space station around laythe with 5 SSTO).

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Does getting into LKO in Moho's plane actually make much difference? Soon as you leave Kerbin's SOI, you find you've got 8km/s (or more) of momentum travelling in the direction of Kerbin's orbital plane. The few hundred m/s that you might have had vertically quickly disappears in the minutiae...

It's not exactly "LKO in Moho's plane". The actual LKO plane will even be inclined the other way around. It's the difference between ejection that puts you already in the plane of Moho's orbit (for minimal additional cost) and the plane change maneuver (approaching 1 km/s) after the equatorial ejection (because of Oberth effect and just summation of perpendicular vectors)... or having like an extra km/s of relative velocity if you run into Moho without matching the plane

- - - Updated - - -

For Eeloo, I did with an inclination at launch.

- I waited for the correct time in the KAC window

- I launched half an orbit before the correct ejection angle is set (to Eeloo, it's on the dark side)

- I set MJ for a 100km orbit with 15/20° inclination south.

- You have less than half an orbit to plot for an encounter

- The ship goes down and arrive at the equatorial node with the upward inclinason and ejection angle.

- At that point burn prograde to escape and more.

- After escaping Kerbin SOI, check your inclination angle. If it's off course, retry launch.

- The plane correction should be quite low.

For Eeloo (if you launch along the Kerbin/Eeloo orbital plane intersect) it would end up cheaper if you have the transfer orbit in the plane of Kerbin's orbit and arrive at Eeloo with high relative inclination. This maneuver is always cheaper if the plane change is done at the planet furthest from the Sun (during Eeloo insertion not during Kerbin escape)

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...(I'm a little surprised that most of tutorials and stuff like that doesn't even mention Moho as a place to go, or just saying that 'It's just another Mun')

"Exploring The System" will tell you it takes more than 6,600m/s each way (unless you are getting gravity assists.

The really big burn, as you've discovered, is for slowing down into a Moho orbit as you have 'fallen' all the way from a Kerbin altitude around the sun and are going much faster than the planet.

For the same reason, it helps to start your injection burn early and hard unless you have a decent TWR on arrival.

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