Whats the best way to get to moho

[Unknown Venom]

So I've been trying to find a way to get to moho. Do I use Jool or just drop down near Kerbol?

[FullMetalMachinist]

Welcome to the forums @Unknown Venom! This reply to a fairly old forum thread is a good tutorial on how to get to Moho:

Some of the dV numbers might be a bit off because it was from an earlier version of the game, but otherwise it's a great method. 

As for going to Jool first, that's a terribly bad idea. You can certainly use it for a gravity assist to bring your periapsis down close to the sun, but the problem with that is that you'll be left with a very eccentric orbit, which means your breaking/capture burn at Moho will be absolutely huge. So huge, in fact, that it'll likely end up being more expensive (dV wise), and it'll also take much longer to go that far out just to come back in. 

[Streetwind]

9 hours ago, Unknown Venom said:

So I've been trying to find a way to get to moho. Do I use Jool or just drop down near Kerbol?

Moho is the highest dV cost destination in the game. It's also the one that varies the most in difficulty and cost depending on the timing of your chosen transfer window, due to Moho's significant inclination and eccentricity. And in addition to that, it's the one most sensitive to errors and imprecisions, due to the high speed with which Moho screams around the sun. Getting to Eeloo is a walk in the park compared to getting to Moho, so don't feel bad about being stumped here.

There are basically two ways:
- The first is your typical Hohmann transfer. You make a single departure burn in low Kerbin orbit that gets you an encounter, and you make a braking burn once inside Moho's SoI. It works just the same as any other interplanetary transfer, and is the fastest and most direct option. The problem with it is that because Moho is in such a tight, fast orbit, you can't really eyeball the transfer window like you can for other planets. The ideal time for departure is an intervall of maybe an hour or two at best, and deviating from it at all increases your dV cost significantly. So you probably want a tool to assist you, unless you like being really persistent with trying out maneuver nodes orbit after orbit until you find one that's acceptable to you. A good tool is a mod called Transfer Window Planner by TriggerAU, which is an ingame port of this web calculator.
- The second one is the way macollo describes in the post linked above. Because the Hohmann transfer window is so finnicky and mistakes cost so much extra dV, that appraoch says "screw transfer windows" and instead departs when lined up with Moho's ascending node/apoapsis (they conveniently fall in the same spot). When you do this, you will not get an encounter. The trick is to stay in that orbit and loop around the sun until you do - or, more realistically, until you get a near miss. At that point you perform a course correction to force that encounter. Depending on your choice of transfer window and the magnitude of your course correction, this approach can give you a cost similar to a perfect Hohmann transfer, and definitely much less than a botched one. The downside is that you may potentially be waiting for a long, long time to get an encounter. For most people playing stock KSP, this is an acceptable tradeoff; but if you like to play with life support mods, and your vessel is crewed, this might not be the best option for you.

 

Edited March 25, 2016 by Streetwind

[Plusck]

I've been going to and from Moho quite a lot recently. I've tried a variety of methods, including several attempts at using a gravity assist from Eve.

- gravity assist from Eve: this can save you some dv, but the most important factor is getting close to AN/DN for Eve's orbit relative to Moho's which is very difficult to arrange. I never got a good alignment, so dv saved was not a lot in return for a lot of time spent (both game time and playing time) adjusting the Eve fly-by.

- direct Hohmann transfer at Moho's AN/DN: by far the simplest and quickest method for playing time, and also only slightly more expensive than using Eve for a gravity assist. I've also taken to eyeballing the angle from launch: you want to launch at shortly before midnight on Kerbin at exactly the same angle as Moho's orbit compared to Kerbin's, or shortly before midday at the opposite angle (compared to the equator). When you reach Moho's orbit at Pe, simply do the same as you would for an orbital rendezvous: drop your first node at the intersect chevron, drop a second node a quarter-orbit later so that you can see your encounter the next orbit around, then adjust the first node until you get an encounter. Add normal/anti-normal as required to improve the encounter, this should only add a few m/s to the burn. In any event, the Oberth effect for the capture burn at Moho is not huge so you don't lose much by slowing down in sun orbit at Pe to bring Ap down.

- transfer window: fastest for in-game time, but can be very expensive for your mid-course correction at AN/DN (depending how far down Kerbol's gravity well it is). Eyeballing from launch to get the right launch angle can help significantly, but total cost will always be expensive.

 

[Unknown Venom]

 

 

On 3/25/2016 at 4:14 AM, Plusck said:

I've been going to and from Moho quite a lot recently. I've tried a variety of methods, including several attempts at using a gravity assist from Eve.

- gravity assist from Eve: this can save you some dv, but the most important factor is getting close to AN/DN for Eve's orbit relative to Moho's which is very difficult to arrange. I never got a good alignment, so dv saved was not a lot in return for a lot of time spent (both game time and playing time) adjusting the Eve fly-by.

- direct Hohmann transfer at Moho's AN/DN: by far the simplest and quickest method for playing time, and also only slightly more expensive than using Eve for a gravity assist. I've also taken to eyeballing the angle from launch: you want to launch at shortly before midnight on Kerbin at exactly the same angle as Moho's orbit compared to Kerbin's, or shortly before midday at the opposite angle (compared to the equator). When you reach Moho's orbit at Pe, simply do the same as you would for an orbital rendezvous: drop your first node at the intersect chevron, drop a second node a quarter-orbit later so that you can see your encounter the next orbit around, then adjust the first node until you get an encounter. Add normal/anti-normal as required to improve the encounter, this should only add a few m/s to the burn. In any event, the Oberth effect for the capture burn at Moho is not huge so you don't lose much by slowing down in sun orbit at Pe to bring Ap down.

- transfer window: fastest for in-game time, but can be very expensive for your mid-course correction at AN/DN (depending how far down Kerbol's gravity well it is). Eyeballing from launch to get the right launch angle can help significantly, but total cost will always be expensive.

 

I see, how about if you use the Principia mod

[GoSlash27]

Another option:

 A Hohmann transfer from low Munar orbit to Moho should only cost 1,535 m/sec instead of the usual 1,710. It's not much in the way of savings, but it's something.

 You could probably save even more by escaping Mun to swing low by Kerbin before departing, but I haven't calculated that.

Best,
-Slashy

[GoSlash27]

Oh, yeah (derp)

I was going to calculate out the DV for the high eccentricity departure, and realized I didn't actually have to!

The first burn is 270 m/sec from low Munar orbit to get a 70 km periapsis over Kerbin. When you arrive at periapsis, you have 860 m/sec of "free" DV (the amount it would take to get a Munar intercept), so this can be subtracted from the total DV to Moho from LKO of 1,710 m/sec.

Therefore total DV for this maneuver would be 1,710-860+270= 1,120 m/sec.

[Geschosskopf]

On 3/24/2016 at 6:01 PM, Unknown Venom said:

So I've been trying to find a way to get to moho. Do I use Jool or just drop down near Kerbol?

Welcome aboard.

The hard part about Moho isn't so much in getting there from Kerbin, it's STOPPING there once you arrive.  If you're not extremely careful with the transfer, you will often find that capturing into a Moho orbit takes ungodly amounts of dV, like 4000-6000m/s.  This has been the ruin of many 1st-time Moho expeditions and is what has given Moho its evil reputation   Because the cost of the transfer burn is essentially fixed due to orbital altitudes of Kerbin and Moho, the objective is to reduce the capture burn down to something reasonable, say 1000-1500m/s, without simply shifting this cost to the transfer burn.

At any planet, the cost of the capture burn is proportional to the angle between the ship's trajectory and the planet's orbital path.  The closer these lines are to being parallel, the lower the relative velocity between the ship and the planet, so the lower the capture burn cost.  The closer these lines are to being perpendicular, the greater the capture burn cost.  Therefore, the objective is to approach Moho at as small an angle as possible.  This is achieved by making the ship's orbit as similar to Moho's as possible.  Because a direct transfer from Kerbin to Moho makes a rather elliptical orbit not at all similar to Moho's reasonably circular orbit, it's apparent that you must somehow lower the ship's solar Ap from the  altitude of Kerbin's orbit down closer to Moho's altitude.  The various tricks experienced players use to get to Moho on the cheap perform this lowering of the solar Ap in different ways.

There is also the matter of Moho's inclination.  The further Moho is above or below Kerbin's orbital plane by the time you get there, the more dV the transfer burn itself costs, either up front on a ballistic transfer or en route with a plane change.  This can be a significant amount so experienced players avoid it as much as possible by arriving at Moho only when it's at or near its AN or DN with Kerbin.  This has NOTHING to do with conventional transfer windows based on the phase angle between Moho and Kerbin, which don't take this into account.  Therefore, experienced Moho explorers totally ignore transfer windows between Kerbin and Moho and leave only when they need little or no inclination change (more on this in a while).

So, what do you do?  Well, as a general rule in KSP (and probably real life as well), you can trade fuel for travel time.  If you don't care how long the trip takes, you can usually get there for very little fuel by taking an indirect route.  Conversely, the shorter the travel time, the more fuel it requires.  Elapsed mission time in KSP doesn't matter in stock games.  If you're using a life support mod, however, you need to pack enough supplies for the trip or use DeepFreeze.  Indirect transfers to Moho typically take about 1-2 years of gametime because they usually require you to do 1 or more solar orbits.

Anyway, probably the most efficient way to get from Moho to Kerbin is to leave Kerbin when it's at an AN or DN with Moho's orbit.  Pay no attention to where Moho itself is at the time, only where Kerbin is.  Do a transfer burn from Kerbin so that your trajectory hits Moho's orbit at the AN or DN with Kerbin that's on the other side of the sun.  Thus, once you leave Kerbin's SOI, your solar Pe is at Moho's orbital altitude and your solar Ap is at Kerbin's altitude.  Once you get to Pe (which is on or near Moho's AN/DN with Kerbin's orbital plane), burn retrograde to lower your solar Ap down to somewhere between Eve and Moho so your orbit is fairly circular, minimizing the angle at which you'll meet Moho.  Then you wait, tweaking your Ap as needed, until you meet Moho at your ship's Pe, which might require making several solar orbits.  This pretty much eliminates all the inclination change you'd otherwise have to do and also minimizes the capture burn, although you effectively need more on the transfer due to lowering your solar Ap with thrust.  The name for this method is the bi-elliptic transfer.

Not quite as efficient but still pretty good is to use a gravity brake at Eve.  In this scenario, again you totally ignore Moho's position when you do the transfer burn.  Instead, you leave Kerbin on a regular transfer window to Eve.  You tweak your departure burn from Kerbin (and maybe tweak your trajectory en route or once in Eve's SOI) so that your ship will pass Eve on the sunny side and somewhat above or below its orbital plane.  Adjust your Eve Pe altitude so that once you leave Eve's SOI, your solar Pe will be at or near Moho's orbit.  Also adjust your Eve Pe vertically to get as much inclination change as possible towards matching planes with Moho.  Once you leave Eve's SOI, your solar Pe will be at Moho's altitude and your solar Ap will be at Eve's altitude, and you'll have done some (but never all) of the plane change.  From here, it's a matter of burning to match planes with Moho and then tweaking your solar Ap and/or Pe as needed to intercept Moho.  Because you're matching planes with Moho, you can intercept it at any point around its orbit instead of having to hit it at an AN or DN with Kerbin.  This means  you can usually hit Moho within 1 solar orbit instead of maybe having to wait several solar orbits, so travel time this way is usually rather shorter than with a bi-elliptic transfer.  And the actual transfer burn is minimized because you only burn to go to Eve and its gravity does the work of lowering your solar Ap.  However, you make up for that in usually having to do a fairly significant plane change burn to match planes with Moho, and this usually makes this method require somewhat more fuel than a bi-elliptic transfer.

With both methods, the capture burn can be further reduced by gravity braking off Moho itself.  The 1st time you meet Moho, pass it on the sunny side to further lower your solar Ap so that now your orbit is very similar to Moho's.  Then do another solar orbit and finish capturing the next time around.  However, this will add even more travel time.

So there you are, 2 indirect methods for getting to Moho that cost significantly less in total (including the capture burn) than trying to do a conventional direct Hohmann or ballistic transfer based on phase angles.  Both take considerably longer than direct transfers however.  The opportunities to do bi-elliptic transfers happen regularly twice per year as Kerbin passes the AN and DN with Moho's orbit.  The opportunities for Eve gravity assists happen with the regular Eve transfer windows rather less frequently.  So take your pick.

Have fun

 

 

 

[Streetwind]

20 hours ago, Unknown Venom said:

I see, how about if you use the Principia mod

Then all bets are off and you won't find anyone to help you here... the trajectories are so sensitive to initial conditions that there simply is no such thing as a "do this and you are guaranteed to arrive" guide. You need to custom-tailor a flight plan to your exact situation and vessel, using the tools Principia offers you - though IIRC, it cannot plan ahead far enough for an inteprlanetary trip in its current state of development.

So you best bet would be to bring twice the dV you expect to be needing, and eyeball everything.

Edited March 27, 2016 by Streetwind