Good way to visit Moho

[MagicFireCaster]

Hey all, so a week ago i tried my first trip to Moho but i wasn't able to come back. I've realized is not as easy as visiting other planets so i came for some advice.

So here are my main objetives an issues:

How much delta-V i'm going to need for a round trip (with landing off course) ?

What is the best way to transfer to moho timewise?

Besides an apollo like lander what other strategies can i use? (I can consider an Eve assist)

Ablator stills runs out from heat shields when you visit moho's orbit? 

I have no craft to show since the one i was talking about failed, i'm going to build a new one so do you have any ideas for it? 

Thanks for your help! 

[Foxster]

Without flybys or slingshots elsewhere, from Kerbin orbit you want about 6000dV to the surface of Moho. About another 5000 back to Kerbin orbit. That's a kinda worse case, there are opportunities for savings on that. 

So, nukes and/or ions are your friend. 

[Superfluous J]

Foxter's got the numbers about right. You can get to Moho orbit from Kerbin orbit for under 5000 m/s direct, but it's not the easiest run. 6000's far safer. Moho's about 900m/s down and another 900 back up, too. And you need at LEAST 2500 or so to get back to Kerbin, and that's if you aerobrake all the way down. It's not undoable, but it will be hard. Better to bring along an extra 1000-2000m/s for the return trip to slow down at Kerbin.

As far as getting there, ignore everything anybody's ever said about transfer windows. Leave when you're at an An/Dn for Moho from Kerbin (You can see this by plotting an ejection from Kerbin and setting Moho as your target. Wherever those lines intersect Kerbin's orbit, time warp until Kerbin's there). Leave there, and in one big burn from LKO get your Sun Pe down touching Moho's orbit, until you get an encounter. Then treat it like rendezvousing with a ship in Kerbin orbit. i.e., go around Sun a time or two, burning at your Pe to make it so the next time you pass by, Moho's there as well.

[Foxster]

Here's a stock lander and Kerbin-return craft....

That's 6 ions, 16 solar panels. One large tank of xenon. Weighs about 3.5t. Has about 6700dV and a TWR of 1.3 at Moho. 

You can use one gigantor panel stuck on top instead but it will increase the chances of losing power due to occlusion at some point. 

It will get a Kerbal down to Moho from orbit and back to Kerbin surface OK, though you will lose solar panels and engines on re-entry and landing. 

 

[Plusck]

Most important for Moho is when you go.

The best time to leave is on relative AN/DN. That happens twice a year, but of the two options the cheaper one is when you reach Moho's orbit when it's closest to the sun.
This is because you benefit from Oberth when leaving Kerbin, but much less so when arriving at Moho. It's not very expensive to eject from Kerbin at high velocity, but it is expensive to capture at high velocity, and there is less of a difference between your speed at solar Pe and Moho's orbital speed when Moho is closest to its solar Pe.

I've tried using Eve for an assist, but it didn't help me much because I didn't hit Eve at the right time. It's difficult because the two orbits are inclined in quite different directions, so again, precise timing is key. I might try again shortly but I'm really not convinced it'll help all that much.

The cheapest way to do it in my experience, is to get into LKO, and then eject at relative AN/DN, with an inclination of about 15° opposite to the inclination of Moho that you see in map view. Since you are ejecting Kerbin retrograde, this inclination is reversed when you leave Kerbin's SOI. No need to get an encounter, just touch Moho's orbit at Pe with minimal difference in inclination.
At Moho Pe, set a maneuvre node, then a second node about a quarter orbit later. Pull retrograde on the first node until you get an encounter, then add some normal/anti-normal to that burn to reduce inclination further still. With little extra cost it might even be possible to eliminate all relative inclination at this stage. You can forget about the second node, or use it to fine-tune your encounter.

With this method, I typically get a capture burn of about 1800 m/s or so. The pre-encounter burn at Pe is typically in the 600-700 m/s zone, and the LKO ejection burn is around 2400 m/s. From there to the surface requires about 1000m/s.
Time-wise, you are going to lose time doing that extra orbit around the sun, but it's a short year (IIRC about 200 days).
Going back you just reverse the process, so as long as you're planning to re-enter Kerbin's atmosphere, you only need about 2600 m/s or so.
With a reasonable safety margin, that gives total dv in LKO of about 8km/s for your main ship, plus 2km/s for your lander.

I've never suffered from excess heat/ablator loss at Moho. Drills/ISRU need decent cooling but that's the only real issue I've had.

[Snark]

+1 to the advice of previous posters in this thread, regarding "launch when you're at the AN/DN of Moho's orbit with respect to Kerbin's."

A few other points to consider when designing a Moho mission:

Getting a Moho encounter after leaving Kerbin.

Spoiler

5thHorseman already covered how you leave Kerbin pretty well:

10 hours ago, 5thHorseman said:

As far as getting there, ignore everything anybody's ever said about transfer windows. Leave when you're at an An/Dn for Moho from Kerbin (You can see this by plotting an ejection from Kerbin and setting Moho as your target. Wherever those lines intersect Kerbin's orbit, time warp until Kerbin's there). Leave there, and in one big burn from LKO get your Sun Pe down touching Moho's orbit, until you get an encounter.

...but once you arrive at solar Pe (i.e. where you cross Moho's orbit), there's the question of "how to get a Moho encounter".  5thHorseman touches on this:

10 hours ago, 5thHorseman said:

Then treat it like rendezvousing with a ship in Kerbin orbit. i.e., go around Sun a time or two, burning at your Pe to make it so the next time you pass by, Moho's there as well.

...but it's worth mentioning that you can get an encounter for less dV if you plan an encounter multiple orbits ahead.  In other words, "I do a small burn at solar Pe now so that I get a Moho encounter N orbits from now."  There are some tricks you can play with maneuver nodes in order to plan multiple-orbits-in-the-future encounters.  I won't go into it unless you ask, since I don't know if you're already familiar with the technique.

 

Deciding "to mine, or not to mine"

Getting to Moho takes a whole bunch of dV.  Getting back from Moho also takes a whole bunch of dV.  One option is to send a mining ship so you can fuel up your return ship once you get there.  Advantage:  don't have to ship all the fuel for a round trip.  Disadvantage:  drills and ISRU refineries are heavy.  Neither one's a clear win, it depends on what you plan to do.  If you plan on going biome hopping to hit lots of spots all over Moho, then you should really consider the mining option.  If it's just touch-down-once-and-then-go-home, maybe not so much.  (And of course, if your return ship is ion-powered, you can't mine for xenon.)

 

Consider leaving your return ship in Moho orbit.

Going from Moho orbit down to the surface and back again takes quite a lot of dV, a lot more than on the Mun.  And once you're back in Moho orbit, you need a lot of dV to go home.  So if you can avoid schlepping your return-to-Kerbin fuel down to the surface and back, it's seriously worth considering.  (Especially since you need a reasonably high TWR for your lander, but for the return ship you may be optimized for high-Isp, low-TWR, which means it may not make a great lander.)

You don't have to do this... but it can be worthwhile, depending on your mission design.

 

Consider a stop at Eve on the way home.

It takes quite a bit less dV to go from Moho to Eve than from Moho to Kerbin.  You can set up a refueling station at Gilly (it's really easy to land on and mine).  So, one way to shave some dV off the Moho round-trip is to head to Gilly first to top off your tanks before continuing on to Kerbin.  Again, you don't have to do this (I expect most people don't), just want to mention it as an option.

 

Don't bother with equatorial orbit at Moho arrival.

A fairly common scenario when arriving at a planet or moon:  try to get into an equatorial orbit, which is where you park your transfer stage, as you send down your lander, and your lander lands on the equator.  Reason:  It means that the parked ship's orbit stays over the lander, which makes it easier for the lander to get back to the orbiter for rendezvous.

Except Moho.

Moho rotates really slowly-- it takes weeks to make one rotation.  For all practical purposes, it might as well not be rotating.  So when you arrive, you can be orbiting any which way, and your lander can rendezvous easily with an orbiter.

 

Don't worry about orbiting Moho east-to-west or west-to-east.

Similarly, it doesn't matter whether you're orbiting Moho east-to-west or west-to-east; there's basically no rotational velocity to worry about.

This is relevant because a lot of folks go to Moho using ion drives.  Which means you want sunshine while you're braking into orbit, or accelerating away to go home.  This can be awkward if you're in the planet's shadow.  So... if this is an issue for you, don't worry about trying to hit a standard west-to-east orbit upon arrival.  Pick your approach so that you're passing over Moho's sunlit side, and have plenty of electricity for those ions.

[FullMetalMachinist]

15 hours ago, MagicFireCaster said:

Ablator stills runs out from heat shields when you visit moho's orbit? 

Since no one has answered this yet, I'll take a stab. Heat shield ablator evaporates away when it hits a certain minimum skin temperature. Where that heat comes from doesn't matter. It could be from reentry heat, or heat generated by ISRU or a Nerva engine; doesn't matter. So being at Moho's orbit you're closer to the sun, and therefore get pretty roasty, thus you're likely to lose ablator. Unfortunately I can't recall what the magic minimum temp is, but it's probably in the part config somewhere. 

[Snark]

2 hours ago, FullMetalMachinist said:

Since no one has answered this yet, I'll take a stab. Heat shield ablator evaporates away when it hits a certain minimum skin temperature. Where that heat comes from doesn't matter. It could be from reentry heat, or heat generated by ISRU or a Nerva engine; doesn't matter. So being at Moho's orbit you're closer to the sun, and therefore get pretty roasty, thus you're likely to lose ablator. Unfortunately I can't recall what the magic minimum temp is, but it's probably in the part config somewhere. 

The threshold temperature is pretty darn low-- 500 K.

http://wiki.kerbalspaceprogram.com/wiki/Parts/Aero/HeatShield/HeatShield1.cfg

...note the "ablationTempThresh" value.  I think it's a bit silly that it's so low-- seems like it ought to be higher.  A snippet of ModuleManager config could do the trick, if you want to tweak it.

One way to keep it from evaporating would be to have some of the foldable radiators on your ship, actively running.  They'll suck heat out of whatever's hottest on the ship (anywhere, placement doesn't matter).  So if you've got enough radiators going (again, make sure it's the foldable ones), I suppose those ought to keep the shields cooled down enough so they stop leaking.

Why foldable?  Two reasons.  First, they take heat from anywhere on the ship, so you can have them mounted anywhere.  Second (and this is really important for solar-heating problems), they auto-rotate to stay edge-on to the sun.  The static panels could actually hurt more than they help, by absorbing sunlight heat, unless you manage to keep them manually turned edge-on to the sun all the time, which isn't super practical.

[MagicFireCaster]

Thanks a lot to all of you for the answers, Considering my options i will try to transfer on mohos AN/DN,  i will probably be better is i make a Gilly refueling station, pack some radiators and extra delta-V in case of something 

[DerekL1963]

11 hours ago, Snark said:

This is relevant because a lot of folks go to Moho using ion drives.  Which means you want sunshine while you're braking into orbit, or accelerating away to go home.  This can be awkward if you're in the planet's shadow.  So... if this is an issue for you, don't worry about trying to hit a standard west-to-east orbit upon arrival.  Pick your approach so that you're passing over Moho's sunlit side, and have plenty of electricity for those ions.

Another way to do this is to come in over the poles.