My first Moho mission

[BRG75]

 

I’m playing on PS4, and I did my first Moho mission this morning!

It was a partial success, I used that Olex calculator tool to work out when I needed to launch, and I got to Moho. The trouble is when I arrived I was going at an insane speed, I put in a manoeuvre node to put me in orbit, and it estimated a 30min burn with my nuclear engine! I didn’t have the fuel for that, so it turned into a flyby with me frantically trying to get the science done as I hurtled through the “space near Moho” zone in seconds.

 

Any idea what I’m doing wrong?

I had a pretty basic ship, a Mk1 Mercury style capsule, some simple science stuff, the big orange fuel tank( Jumbo 64, I think), and the Nerv engine. I thought this would have more than enough fuel to get me into orbit and back home.  Do I need a bigger ship? Different engine? More than one nuclear engine? 

[Superfluous J]

10 minutes ago, BRG75 said:

Any idea what I’m doing wrong?

Possibly nothing, other than using a nuclear engine. What was your burn dV? Insertion burns at Moho can get up over 3km/s. If your nuke only gets you 2m/s² then it's going to take 30 minutes or so to do that.

Ideally you want to do one of two things when going to Moho.

1. Bring more beef. You want the insertion burn to be less than 5 minutes, so you want at least 10m/s² of thrust. I prefer maybe twice that.
2. Do most of your burn in Sun orbit. There's no real time limit here, so instead of using the calculator, go to Moho like you'd rendezvous with a ship: Burn to transfer to Moho's ORBIT, not Moho itself. Then at your Pe (where you meet Moho's orbit, where Moho is somewhere else around the Sun) burn (with a maneuver node so you know you're doing it right before you burn) your orbit down until on the next pass, you enter Moho's SOI. This will do some (and hopefully most) of your burning when time doesn't really matter. If you spend 2000m/s getting your encounter, you'll only need about 1000 at Moho itself, meaning your 2m/s burn will "only" take 10 minutes.

I actually tend to do both of these things.

One thing to keep in mind: Moho is HARD. It's not Eve HARD, and may not even be Tylo HARD, but it's HARD. Harder than any other planet to get into orbit of, for sure. So don't feel bad it's taking a few tries.

[Vanamonde]

The LV-N engine is the most efficient, but has low thrust and takes a LONG time to make a big change to your speed. If you can revert to a save, I'd suggest starting the braking burn 30 minutes from periapsis, even if you're out of the planet's SOI at the time. That's close enough that it shouldn't mess with your intercept much. Though if fuel is a problem, that still won't help.

[Aegolius13]

What was the delta-v figure for the burn?  Moho is really, really sensitive if you have a less-than perfect approach.  You want to make sure you're as close to tangent as possible with Moho's orbit, and you'll likely need to address the plane change issue far enough out that you don't have a huge normal/antinormal burn towards the end.

If you decide to do a bunch of the capture burn prior to hitting Moho's SOI, you can always take that concept one step further and use ion engines.  They're really easy to power with solar that close to the sun, and the very long burn times don't matter which when in deep space. 

There's also an alternative method that involves leaving for Moho when Kerbin is at its ascending or descending node, so that you can match planes as you leave.  Personally I like it a lot better, and it seems to be less prone to unexpectedly-huge capture maneuvers. 

[Kryxal]

I like the plane-matching one too ... specifically where you eject on the same side as Moho's apoapsis.  Arriving at the periapsis and plane intersection is the most fuel-efficient transfer you can manage, just use your first burn to set up a later encounter.

[StrandedonEarth]

Good advice here already regarding getting the capture done, so I'll address the fuel issue: You can get an insane amount of dV from LV-Ns by using radial drop tanks. If nothing else, start slowing down as soon as you hit Moho's SOI if not sooner, dropping tanks in pairs as necessary, and mind your peri-moho. if you've plotted a close pass beforehand (hoping to take advantage of Oberth), a long, early burn will put you on an impact course. I learned that the hard way.

As an example, I had a Mk1 1-seater capsule with 2 LF tanks then the LV-N, with 2 pairs of LF drop tanks on radial decouplers. IIRC correctly this setup was worth around 10km/s dV. Bear in mind this had no science gear (just a probe core), as I was doing a sub-Moho rescue mission.

E: Confirmed specs, OKTO2 probe core and 'chute... 

Spoiler

oops, that was without decoupler or heatshield. With decoupler and full heatshield.... Still no power generation tho....

 

 

Edited May 12, 2019 by StrandedonEarth

[ElJugador]

What I'm going to say is: oh, you poor man. I sent a whole mission to Moho in a modded game, but with the stock engines. Even with 7-8km/s in the tanks, I still wound up completely running out of fuel when I tried to exit Moho and had to send two full tankers the same size as the original ship to get it home. I'm not sure but the LV-N probably doesn't have the muscle to pull off an efficient landing; try using the huge Mk3 LF spaceplane tanks only, using about seven or eight engines and bring a tiny lander to undock from the mothership when you get into orbit, with about 2000m/s in the tanks. That should be easy with only 1 kerbal to deal with. Good luck.

[Zhetaan]

On 5/12/2019 at 9:30 AM, BRG75 said:

Any idea what I’m doing wrong?

[...]

Do I need a bigger ship? Different engine? More than one nuclear engine? 

You're doing nothing wrong.  Moho is one of the challenge planets of the game, along with Tylo and Eve.  Tylo is difficult to land on, Eve is difficult to return from, and Moho is difficult to reach.  To put it in perspective, there's a reason why, in real life, Venus has been visited by 27 at least partially successful probe missions, but Mercury has been visited by only two. Read this (the second paragraph under the heading 'Interest in Mercury' is the most relevant to the discussion) and know that you're not alone in appreciating the difficulty here.

Successful, efficient missions to Moho require a bit of unusual thinking and a willingness to break the usual rules of transfers.  You certainly can choose to load a lot of fuel and essentially 'brute-force' the transfer, but as others have said, Moho trips benefit from two-part transfers not unlike orbital rendezvous.  Remember that rendezvousing two ships in orbit of Kerbin and rendezvousing a ship and a planet in orbit of the Sun are the exact same manoeuvre; the difference is only the scale.  The reason why we normally bother with a complicated transfer to a planetary encounter instead of just to a planetary orbit is because it normally costs less in fuel to do it that way (which is true of ship-to-ship rendezvous, too, but the savings scale with the rest of it) and because the time spent waiting if you do it in two steps is impractical.  Phasing orbits to rendezvous a rocket with a space station in low Kerbin orbit, for example, takes a few hours at most.  Phasing orbits to rendezvous a rocket with Duna if you don't get the encounter on the first try can take years.

One thing to remember about encounters with Moho is that it is not strictly necessary to match orbital planes.  If you launch from Kerbin to intercept Moho's orbit at the ascending or descending node, then you can encounter Moho without needing a plane change.  The trick is to get to the node and then burn for an encounter on the next pass.  It will make the capture cost a bit more (because Moho's prograde and your prograde are not strictly parallel in that case), but the savings on the plane change is usually worth it.

Another trick to try is an Eve gravity assist--or technically, two Eve gravity assists.  The K-E-E-M route is not terribly difficult as gravity assists go; it simply requires a bit of planning.  The thing to do is to first find an Eve-Moho transfer window (there are tools for that, as you already know).  Then you need to get a trajectory from Kerbin to Eve that arrives exactly one Eve-year (just under 262 days) before that Eve-Moho transfer window.  As always with interplanetary transfers, some windows are better than others, but don't be too choosy or you'll spend all of your time waiting and none of it launching.  Adjust your Eve encounter so that you approach Eve from the retrograde direction and get a slowing assist to an elliptical orbit with a period of exactly 261.9 days.  You can afford to be off a bit on the other legs, but this one is critical: 261.9 days for the middle (Eve-to-Eve) leg.  On the next encounter, you'll need to burn a bit to get the Moho encounter (even two assists from Eve are not quite sufficient for that), and that will have you arriving at Moho with a much-reduced relative velocity that ought to make capture more straightforward.

Good luck!

Edited July 2, 2019 by Zhetaan

[radonek]

IMO easiest way to get to Moho is just to pack more fuel, get to some lower solar (kerbolar?) orbit (possibly with Eve assist) and do a transfer from there instead of falling all the way from Kerbin.  Yes, it's a waste of fuel, but so is hauling big TWR down there, especially if one have big craft that needs something like mainsail to slow down. Eve assist may be more efficient, but tricky to set up and you still arrive with a lot of speed to shed. If your intermediate orbit is low enough, Moho capture burn is not much bigger then, say, Mun one. Again, this is not best way to do it, but easiest to start with.

[Guest]

I did try two attempts to land on Moho and it took a pretty chunky amount of fuel. I made a rocket with 13000 M/S. It got me there fine, for a more budget of fuel, I would do a lot of Gravity assists. It helps bring down your speed and is used in the real world with Belicombo.

Above: My second attempt worked. Happened two weeks ago

[Laie]

Been there, done that -- I've been running on ions and had more than enough dV, but with the flyby only lasting 20 minutes or so I could only wave a brief hello.

On 5/12/2019 at 3:30 PM, BRG75 said:

Any idea what I’m doing wrong? 

Using an ordinary launch window calculator to get to Moho.

Due to the large difference in inclination, and being close to the sun, ordinary transfer windows will send you on a trajectory that encounters Moho at ridiculous speed, while Moho is really small (little more than the Mun, really) and thus cannot provide much assistance with capture.

The smarter (and often more efficient) way of getting to Moho is to treat it like a rendezvous:

1. leave Kerbin at a time when it crosses Moho's inclination, and try to get as much of the inclination change fixed at departure.
2. make it so that your PE around the Sun about touches Moho's orbit, nevermind if Moho will be there or not.
3. at (or near) solar PE, burn retrograde so that you will encounter Moho the next time you come around.

You can save a lot of dV at step 1 if you start into an inclined orbit to begin with. Being a console player, you have limited means to figure out which inclination and LAN you need, however. Perhaps your best bet is a polar orbit: at the day of departure, you want your orbit to be directly over the terminator. Take off either at dawn just as the sun first peeks over the horizon, for example.