Why landing on Moho is damn difficult

[Xurkitree]

Moho is very irritating to get to and back. I've only done it successfully with ions, and luck.

[Nich]

I have not done Moho but I recently speced out a direct Mercury transfer and I can tell you Oberth does matter.  It was something like 10km/s vs 13km/s to capture if you did everything right.

0.1 create a manouvor just barely exiting earth/kerbin SOI

0.2 in the suns/kerbols SOI create a manouvor at Mercury/Moho AN or DN.  This will give you the time to the transfer

1. Have to fix inclination with Kerbin/Earth ejection burn while bringing PE down to Mercury/Moho level.  Mine was 6330 if in the correct plain or 8500 with a 43ish degree plane change combined with the required ejection burn

2. Burn retrograde at PE to set up an encounter in the next 2/3 orbits (lots of different tricks to do this).  I spent about 800 dv on the second orbit but if I had spent 250ish on the first orbit I could have gotten the same intercept

     -time/angle math

    -Place a node a PE.  Place another node shortly after it.  Click next orbit until you have a decent approach.  Make adjustments on the first node.  It should have the multiplier of the multiple orbits so a smaller burn will have a larger impact.

3. finally the final burn will be a long burn so you will not see as much oberth benefit as instantaneous burn but it will be significant.  I would plan for no oberith and use oberth as much as possible.  your savings will probably get used up in correction burns and cosine losses

[Superfluous J]

Here's a video on how to get to Moho (as was said, landing there isn't really the issue) for far less than the 5-digit budgets many bring. Bonus: It's older than this thread!

Watching it again, I'd not bother fixing my inclination though I don't know offhand how much it hurt at Kerbin, or saved at Moho.

[ExtremeSquared]

Definitely something to be said for stopping at Eve on the way, especially for complex mothership/lander/return missions.
1: If you have a Gilly ISRU mine, a refuelable transfer/insertion tug can be topped off there.
2: Gilly --> Eve gravity assists can be a bit simpler than interplanetary assists.
3: It takes some of the edge off of every single component of the deltaV requirements.

[Red Iron Crown]

21 minutes ago, ExtremeSquared said:

Gilly --> Eve gravity assists can be a bit simpler than interplanetary assists.

What do you mean by this? Gilly doesn't have the mass to do a meaningful gravity assist, and Eve is only useful for interplanetary assists. 

[Guest]

On 1/1/2014 at 12:10 PM, Pawelk198604 said:

I made several mission to Moho, all crashed. I have problem with orbital insertions?

moho takes a great amount of fuel. I would say for your braking stage and your lander I would do this for a beginner

Cruise: 6200 

Lander: 1800

[MechBFP]

Mining is generally the way to go on Moho, otherwise you need a MASSIVE rocket with lots of asparagus staging.

Here is my lander. I have cross feed turned on for all the decouples so that I can land with all my engines using all my fuel. I then refuel and have way more than enough dV to get home  

[Muetdhiver]

Note that one can save about 1500 m/s in dV with a good Eve assist.

My dream is to replicate something like bepiColombo or Messenger with multiple moho slowdown passes before insertion.

Using pykep I can see solutions in the 3.5 km/s dV range for a Kerbin-Eve-Moho sequence.

But yeah, moho is a tough one.

Edited May 17, 2019 by Muetdhiver

[ExtremeSquared]

2 hours ago, Red Iron Crown said:

What do you mean by this? Gilly doesn't have the mass to do a meaningful gravity assist, and Eve is only useful for interplanetary assists. 

I mean dropping from Gilly's orbit to slingshot by Eve. Eve will still provide some assist from its own high orbit.

Just managed this low equatorial Moho orbit from Eve with around 4km/s total deltaV cost. Definitely competitive for a brute-force direct transfer without a high-quality transfer window calculator or waiting for optimum ascending/descending nodes. Capture into circular orbit was around 2km/s, which by my notes is ~20% easier than when done direct from Kerbin.

And Gilly allows surface refueling of single-stage 0.05 TWR monstrosities like this.

[Red Iron Crown]

9 hours ago, ExtremeSquared said:

I mean dropping from Gilly's orbit to slingshot by Eve. Eve will still provide some assist from its own high orbit.

Ah, so not a gravity assist at all, just dropping to low Eve orbit for better Oberth effect. 

[ExtremeSquared]

Isn't a gravity assist just using the gravity well of any intermediary body to aid transfer to a third? The SOI hierarchies in KSP don't really matter, I think, but I may have the terminology wrong.

[Xurkitree]

Is it possible to split up the capture burn by burning enough to create a resonant orbit, thereby saving DeltaV by the Oberth Effect?

[Superfluous J]

1 hour ago, ExtremeSquared said:

Isn't a gravity assist just using the gravity well of any intermediary body to aid transfer to a third? The SOI hierarchies in KSP don't really matter, I think, but I may have the terminology wrong.

A gravity assist is (in a universe with SOIs) entering an SOI from outside, altering your trajectory within that SOI with the gravity of the body, and then exiting the SOI in a different direction than you entered.

Leaving Gilly's SOI doesn't count in this case as Gilly is inside Eve's SOI. You can only use Eve to Gravity Assist by entering Eve's SOI from Sun's SOI, and then exiting Eve's SOI again into Sun's SOI.

This is also why the maneuver they did in Interstellar was not a gravity assist. Or anything useful in that case but hey.

NOTE: Leaving Gilly to go deep in Eve's gravity well so that when you burn at Pe you get an Oberth boost is a great idea in many circumstances. It's just not a gravity assist.

Edited May 18, 2019 by 5thHorseman

[ExtremeSquared]

Just looked it up, and I've been using the term wrong for a while. Gravity assists are unpowered maneuvers. Oberth maneuver is what was done. Regardless of whether SOI or N-body. Thanks for the correction.

[Nich]

On 5/18/2019 at 12:18 PM, Xurkitree said:

Is it possible to split up the capture burn by burning enough to create a resonant orbit, thereby saving DeltaV by the Oberth Effect?

Actually this is very true. I totally forgot about this.  You would also be getting a small gravity assist by doing this as well.  Problem I have had is this is very difficult to do with the given tools most of the time I spend more dv making corrections then I actually save.  However with IONs it could be totally worth it

[schwank]

On 1/1/2014 at 2:37 PM, sporkafife said:

This may sound a little crazy, and I haven't tried it so it might be a complete bust, but maybe an ion engine would be good to put a probe into Moho orbit. With the sun so close, you could probably get enough power to run it with the smallest solar panel, so the t/w ratio wouldn't be too terrible. In fact, I wanna go try it now

That only works if you are positive you will be inserting on the sun facing side. Usually you are on the dark side when inserting but it can be done with planning. 

[Nich]

I saw someone make a Moho transfer vehicle for 4-8 kerbals with only 8 or 16 ions and something like 27 km/s dv

[Muetdhiver]

15 hours ago, Nich said:

Actually this is very true. I totally forgot about this.  You would also be getting a small gravity assist by doing this as well.  Problem I have had is this is very difficult to do with the given tools most of the time I spend more dv making corrections then I actually save.  However with IONs it could be totally worth it

Indeed. With ions the best would be a Kerbin-Eve-Moho-Moho sequence. (Much slower insertion)

I don't think getting the resonant moho orbit would be too hard.

The issue with ions is that low TWR is not great for a direct moho capture. A double braking burn seems much easier.

Edited May 21, 2019 by Muetdhiver