I thought this would be enough to reach Moho (update)

[jfull]

I was wrong.

I got it to Moho's SOI, but it would have taken an 18 Minute burn to brake and circularize. I probably could have done it, but it would have meant stranding the Kerbals there.

Luckily I could go back to a quick save before I left Kerbin.

Anyone have any tips for Moho?

Update: actually made it there and back!

Javascript is disabled. View full album

Edited December 1, 2013 by jfull

[Jim DiGriz]

you need to meet moho at its aphelion. since you leave kerbal on a hohmann transfer at a 180 degree angle away from that, you need to be burning from kerbin when you're roughly in line with moho's perihelion. not every transfer orbit from kerbin to moho will work... basically stay away from getting deep in the sun's gravity, and use moho's eccentricity in your favor...

[koshelenkovv]

Travelling off chance can be dangerous.

Please, use this.

Anyway, Moho rockets must be exceptionally large.

Even one man rocket with ultralight lander will look like this:

Edited December 1, 2013 by koshelenkovv

[maltesh]

you need to meet moho at its aphelion. since you leave kerbal on a hohmann transfer at a 180 degree angle away from that, you need to be burning from kerbin when you're roughly in line with moho's perihelion. not every transfer orbit from kerbin to moho will work... basically stay away from getting deep in the sun's gravity, and use moho's eccentricity in your favor...

Actually, arriving at apoapsis is using Moho's eccentricity against yourself. When your spacecraft is doing a Hohmann transfer deeper into the well, when your spacecraft reaches its inteded final altitude, it's moving faster than the circular orbit velocity for the distance it descends to. When Moho is near periapse, it's also moving faster than the circular orbit velocity for its distance, whereas when Moho's near apoapse, it's moving slower than the circular orbit velocity for its distance.

When you actually run the numbers the result is that relative velocity on arrival to Moho is /higher/ when meeting Moho at apoapsis than it is when meeting Moho at periapsis. resulting in more delta-V being required for the capture burn.

Desmos graph that produces the above image is here. https://www.desmos.com/calculator/l8r5pwjelg

In the image above, the horizontal axis is distance from the sun in millions of kilometers, and the vertical axis is velocity in km/s. It also makes the simplifying assumption that Kerbin's orbit is coplanar with Moho's.

The orange curve is Moho's orbital speed as a function of its distance from the sun.

The blue curve is the periapse arrival speed on a Hohmann transfer from Kerbin as a function of the distance from the sun.

The green curve is a straight subtractive difference between the orange and blue curves.

The red curve is relative velocity on Moho SOI entrance, taking into account arrival geometry resulting from Moho's eccentricity, and the vertical black line is the distance from the sun equivalent to Moho's semimajor axis. As shown, relative velocity on arrival is higher, by more than 1.5 km/s, when moho is near apoapse than when moho is near periapse.

Alas, accounting for inclination wound up pushign things a bit further mathematically than I felt comfortable handling, so I didn't make a graph of that (and I'm not sure Desmos could produce a graph of that, anyway.)

[jfull]

Travelling off chance can be dangerous.

Please, use this.

Oh don't worry, I always use an interplanetary calcuator... though this one might be better than the one I've been using

So, what I'm getting from this is that I should try to reach Moho when its orbit brings it closer to Kerbol, that way there'll be less of a difference in relative velocity? Is that correct?

no matter what, I'll be packing a bit more fuel when I try again

[maltesh]

Yes, the difference in relative velocity on arrival is lowest when Moho is near its periapse, given Moho's eccentricity, (though I assumed an equatorial solar orbit.)

In general, Moho is the hardest planetary target in KSP and is pretty much guaranteed to cost more delta-V than you think it will, so bring plenty extra. Also, most of the delta-V maps assume circular, equatorial orbits for the planets. As such, the numbers that tend to be produced for Moho tend to be off by quite a bit. The aforementioned Alexmoon site is one notable exception.

[DMagic]

Yes, meeting Moho when it is closest to the sun is the ideal time. The initial burn from Kerbin to Moho will take slightly more delta-v, but the final burn, to orbit Moho, should take much less delte-v. That said, the reason you're getting a really huge orbital injection burn is that you aren't getting an ideal intercept with Moho. Your intercept should occur at your closest approach to the sun, anywhere else means a much bigger orbital injection burn.

Don't use delta-v maps for Moho, they assume all kinds of ideal situations which make them basically useless. There are some interplanetary trip calculators that take into account Moho's orbit and will tell you when and in what direction to burn.

Another way to reach Moho is to treat it like a docking encounter. Don't bother with waiting for transfer windows or figuring out the ideal burn times. Just burn until your orbit just touches Moho's then make adjustments to set up an encounter, the same way you do with any other orbital rendezvous. It's not quite as efficient as an ideal transfer, but it's much easier and usually ensures a much smaller orbital injection burn.

[Jim DiGriz]

Weird, meeting it at perihelion didn't work for me at all the one time I tried it... I can't argue with that graph though...

[koshelenkovv]

Another way to reach Moho is to treat it like a docking encounter. Don't bother with waiting for transfer windows or figuring out the ideal burn times. Just burn until your orbit just touches Moho's then make adjustments to set up an encounter, the same way you do with any other orbital rendezvous. It's not quite as efficient as an ideal transfer, but it's much easier and usually ensures a much smaller orbital injection burn.

Actually, for Moho and Eeloo it's more effective than classic Hohmann transfer with plane alignment. And this way does not require precise timing or precise execution of maneuvers, so it's much easier to just eyeball, fiddle with node until markers appear, and correct orbit later in flight.

[Superfluous J]

I just posted this on YouTube. I wasn't even super efficient and got there in 5678 dV. I used a special ship with exactly 10k dV so I could more easily determine the total dV used.

[jfull]

Thanks for the help everyone!

Updated with the completed mission.