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farmerben

Moho blues

Question

I'm trying to land on Moho and return a sample to Kerbin.  The landing part is easy, getting back is the hard part.  I find myself using ridiculous amounts of dV.  It takes me almost 2000 dV just to match inclination.  I'm OK at gravity assists if they're fairly easy to eyeball, but I can;t seem to set up anything especially complex.  Perhaps I need some mods to help find exact transfer windows...The inclination changes are the real killer.  I have learned to launch from Kerbin when I'm near the ascending node of Moho (roughly eyeballing it) but didn't manage to get much of the inclination change I needed while in Kerbin's SOI.

 

Tips of navigation mods?

Tips of better inclination changes?

  

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Inclination burns in low Solar orbit are a huge pain, as you see. There are two ways to mitigate them:

  1. Do it as far from the Sun as is reasonable.  When you leave Moho burn all the way up so your orbit crosses Kerbin, and then see where the An and Dn are. Do your inclination burn at the further out one.
  2. (better) Don't do one! Leave Moho while it is at the An/Dn for Kerbin (you can tell this in map mode by looking at the solar system from the side, and lining up the orbits of Moho and Kerbin so they're both just lines. Where those two lines cross is where Moho should be when you leave. Then, you will encounter Kerbin's orbit at the other (An or Dn) node on the other side, and can combine your slowing down and inclination change in one big burn + aerobraking. Or if all else fails send up a rescue rendezvous ship that's designed specifically to meet the doomed Moho return ship as it flies past, pick up the crew and science - or simply claw the whole ship - and return home.

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For a return from Moho, I tend to use a slingshot at Eve, it lets you do a fair bit of plane correction while you're at it.  I don't worry too much about getting to Kerbin itself, just to a point on the orbit, then hang around for a while on that orbit.

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3 hours ago, farmerben said:

Perhaps I need some mods to help find exact transfer windows...


MechJeb will do that, and calculate a burn that will place you on a direct trajectory from where you are to where you want to go.  (That is, it combines the ejection burn and the plane change burn into a single burn.)  It's much cheaper than doing plane change burns out in solar orbit.  Then you can either manually fly the node, or let MJ do it for you.

"Cheaper" is of course a relative term, it's expensive to transfer Moho <-> Kerbin no matter how you cut it.

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8 hours ago, farmerben said:

Perhaps I need some mods to help find exact transfer windows...

Yes. Yes, you do. Or just the online calculator. Trying to do a Moho return mission without one would be like throwing darts at an archery range... blindfolded.

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8 hours ago, DerekL1963 said:


MechJeb will do that, and calculate a burn that will place you on a direct trajectory from where you are to where you want to go.  (That is, it combines the ejection burn and the plane change burn into a single burn.)  It's much cheaper than doing plane change burns out in solar orbit.  Then you can either manually fly the node, or let MJ do it for you.

"Cheaper" is of course a relative term, it's expensive to transfer Moho <-> Kerbin no matter how you cut it.

I am using MechJeb maneuver planner tool.  Many of the functions only work for the SOI I'm in.  There is some information about the next SOI (brown line), but I'm sorta eyeballing the brown line.  

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31 minutes ago, farmerben said:

I am using MechJeb maneuver planner tool.  Many of the functions only work for the SOI I'm in. 

The functions that only work for the SOI you're in aren't the one's you're looking for...  :) :)   Try the Advanced Transfer function in the Maneuver Planner window.

I'm not sure what you mean by "eyeballing the brown line".

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Dont bother correcting plane inclination. Instead go for a transfer to moho/out of moho at the ascending/descending node for an intercept. Saves you a lot of DV that way. Trade off is that there are fewer transfer windows. Correct me if I'm wrong, but moho orbits fast enough that that shouldn't be a real problem tho.

That's how I do all my transfers if I dont care about inclination when I get there. 

Edited by ZL647

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Tip 1: Launch from Moho to Kerbin or Eve, but do it only when Moho is at it's Pe around the Sun. The reason being is that you use the Oberth effect around the sun when doing your ejection burn at LMO. This shaves off some Dv requirement when doing the final ejection burn.

Tip 2: It's recommended before by ZL647. To further explain this. Try to leave Moho when it's plane intersects the ascending/descending node of Eve's or Kerbin's. When targeting your destination (Kerbin) it will visualize the An or Dn in map view. (you probably already knew this, just saying)
The plane change is the biggest Dv waste when coming from Moho. Even if Eve is not at the proper location I can park in a 1:3 or 1:2 Eve-Moho resonant orbit until Kerbin or Eve catches up to the desired spot and then do a burn at Pe around the Sun to get to Eve/Kerbin.

Tip 3: I take it you only want to bring the Kerbals back to Kerbin? So a small Mk 1 or Mk 1 lander can per each pilot should suffice? Have you thought about electric propulsion yet? (Dawn electric motor)
I know this puts people off due to their long low TWR and consequently longer burn times, and then a Moho ejection burn, am I nuts? ...

If one or two dawn engines have to push one or two lander cans the vessels peak TWR can be quite reasonable. For reference a single Dawn can orbit a small lander can from the Munar surface so it's doable for a Moho ejection burn at 4x timewarp. You may not like the slow electric dawn motor because many of you don't but it will seriously take the complexity from a entire vessel when used for transferring Kerbals from Moho back to Kerbin.
 

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A couple of lateral thoughts to reduce dV requirements...

How lean is your return craft? You could make it just a mk1 capsule and dump the science experiments, landing gear, etc before leaving Moho. A single nuke engine and drop tanks is a pretty good way to keep it light, or you could use ions and possibly a disposable lander. Even lighter is to abandon re-home your scientist on Moho and return just a Experiment Storage Unit. 

Don't aim for an orbit at Kerbin on the return. A mk1 capsule and a heatshield with 20% ablator can re-enter safely at any speed from anywhere. 

 

Edited by Foxster

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I'm surprised nobody has mentioned the mods that I use for interplanetary transfers:

Transfer Window Planner will help find the most efficient burns. It may not be perfect, but it will at least get you into the ballpark. And the maneuver info it gives you can be copy and pasted directly into.... Precise Node!

The way to not have to worry about inclination is to get the An or Dn to be where your craft will intercept Kerbin

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22 hours ago, DerekL1963 said:

The functions that only work for the SOI you're in aren't the one's you're looking for...  :) :)   Try the Advanced Transfer function in the Maneuver Planner window.

I'm not sure what you mean by "eyeballing the brown line".

Well that't part of my problem.  I guess I cut my teeth on MJ in career mode and was only using the basic options.   

The difficulty I'm having now is returning to Kerbin using Advanced Transfer and requiring a >4000 DV capture burn. 

My return stage uses a command seat and ion engine.  With lower velocity I can aerobrake this type of vessel, keeping the kerbal in the shadow of the other parts.  This time he blows up at 69000m.  Also my periapsis is in Kerbin's shadow.  

I can probably do better with transfer, and try heat shields.

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1 hour ago, StrandedonEarth said:

Transfer Window Planner will help find the most efficient burns. It may not be perfect, but it will at least get you into the ballpark. And the maneuver info it gives you can be copy and pasted directly into.... Precise Node

For mods, you can't beat the simplicity of Astrogator. It basically does automatically what you did manually. It's so easy, I feel bad using it in my videos so only use it when I'm playing "by myself."

 

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5 hours ago, farmerben said:

... a >4000 DV capture burn. 

My return stage uses a command seat and ion engine.  With lower velocity I can aerobrake this type of vessel, keeping the kerbal in the shadow of the other parts.  This time he blows up at 69000m.  Also my periapsis is in Kerbin's shadow.  

I can probably do better with transfer, and try heat shields.

If you can't aerobrake, the capture on return from Moho should cost you exactly as much as it cost to make the burn on the way out. From memory, that's about 2400-2600 m/s. If it's much more than that, your approach is wrong. Also don't forget you can reduce some of that capture burn by using the Mun to give a gravity assist.

The very easy (and second most efficient) way of getting to and from Moho is on AN/DN with respect to Kerbin. You want to arrive at (and leave from) Moho when it is at its AN/DN that is closest to the sun.* Obviously, also, you want to start the burn as close to the planet as possible to maximise the Oberth effect. You also want to include the plane change with that initial burn (unless you're aerobraking on arrival, in which case you don't really care about the plane).

(* this is because Moho is going faster, therefore the difference in speed between your transfer orbit and Moho's orbit is at its lowest. When Moho is nearer its Ap, it's going a lot slower and you need a whole lot more dV to make the transition between Moho's orbit and your transfer orbit)

It doesn't much matter where your destination planet is when you start, because it is pretty much guaranteed not to be where you want it. You simply can't make a direct transfer, so instead you need to go round the sun for a whole extra orbit and use that second pass at Ap or Pe to time your arrival perfectly.
Going from Kerbin to Moho is easy: match the plane almost perfectly, with your Pe just touching Moho's orbit, then when you get to Pe plot a retrograde burn that perfects the plane and makes you hit Moho at Pe the next time around.
Going from Moho to Kerbin is trickier. Either you plot the exact reverse (which is finicky, since you need to plot that second burn in sun orbit at Pe while you're still in Moho's SOI, at virtually the same location) or you have two other options: (1) plot the burn to touch Kerbin's orbit but only do about 1/2 to 3/4 of the resulting burn, then wait until you're in sun orbit to plot the two burns that will get you up to Kerbin's orbit at exactly the right time one orbit later, or (2) plot the same burn and do it all, then at Ap add a prograde burn to catch Kerbin the next orbit around. The second option adds a lot of time to the return trip.

The harder (and most efficient) way of getting to and from Moho is via Eve. It's tricky, because AN/DN between Moho and Eve is not anywhere near AN/DN between Eve and Kerbin, so the plane change that you can get from passing by Eve is only part of the plane change that you'll need to make to get an intercept trajectory. Still, if you get it right it does give significant savings and a much lower capture burn at Kerbin if you can't aerobrake.

 

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The thrust from that second ion engine is occluded, isn't it?

 

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41 minutes ago, bewing said:

The thrust from that second ion engine is occluded, isn't it?

 

Nope. You can't occlude an ion engine. 

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