Oh bugger... Injection burn at Moho

[Tokay Gris]

So I am currently in the process of an all out attack on all planets and some moons.

The first part of the mission to Moho wasn't such a big success. An orbiter (Bill and Bob Kerman) didn't actually orbit and just did a low (and fast!) flyby and is now on route back to Kerbin.

The lander actually was a success in part. It did land, it did get to orbit again. But because of the unbelievable injection burn to even reach Moho orbit from interplanetar space hasn't got enough delta-V to get the Kerbals and the science equipment back to Kerbin. So a fuel mission was launched.

And AGAIN! 4300 m/s injection burn to get into orbit.

What am I doing wrong?

Basic profile is a Hohman transfer to the orbit of Moho, a rather big correction burn mid-flight... and then this HUGE burn to get caught by Moho.

In this case a possible burn. It is a tanker after all and had around 10.000 m/s left just before the injection burn was started. That should be enough to get fuel to those two Kerbals and allow them to get back to Kerbin with the equipment.

According to delta-V charts, Moho is listed as needing 2200 m/s injection burn.

What am I doing wrong to get almost double that?

Edited December 13, 2013 by Tokay Gris

[Scarecrow88]

I found Moho was one of the most difficult planets to visit with a manned lander. If I remember rightly I too had to send a refuelling mission due to the insane burn required to get captured.

[Rhomphaia]

Not doing anything wrong, just that the charts are particularly optimistic about Moho

Try this instead

http://alexmoon.github.io/ksp/

[Specialist290]

Short answer: Moho is hard.

Slightly longer answer: The problem here is probably not with your procedure, but indeed with the chart itself -- or rather, with some of the assumptions the chart makers go with when putting together these delta-v charts. Moho's orbit has the three disadvantages of being deep in the system's gravity well, having a relatively high eccentricity, and also having a rather significant difference in inclination from Kerbin. Many delta-v charts, however, are constructed on the assumptions of idealized circular orbits with minimal inclination differences, which works well enough for most planets but not all that well for Moho.

I'd personally recommend using alexmoon's Launch Window Planner to help out with this one, as it will actually print out a "porkchop plot" to help you determine the best time to launch within a margin of windows. Arrowstar's Trajectory Optimization Tool is quite handy for this as well, and can also be used to calculated gravity-assisted trajectories to Moho that rebound off Eve, to save delta-v (though those take a while to calculate properly).

[Tokay Gris]

The problem is not getting to Moho. It is clear that the transfer itself is either done at the right point (reaching Moho when its orbit clips the plane of Kerbins orbit) or requires a big correction burn.

What I don't get is that the injection burn (the burn needed to get caught by Moho and stay in orbit) is so insanely huge.

I just found out that my fuel tanker got there al right but did not bring that much fuel to Moho. It might be enough, but I am not yet sure about the science equipment. Oh bugger. Maybe another fuel mission. Maybe with my new monster... THAT should be able to get enough fuel there.

[Sirine]

Agree with Rhomphaia. Any other planet you can skip the launch windows...except Moho...

Delta-V differences (gaps) too much...

1) You need to adjust the inclination angle...as far away from Sun as possible.

2) You need to get closes as possible (Following, it Moho line pattern) to Moho line of orbit (The line Moho Orbiting Sun), before you go in Moho SOI to get capture.

Edited December 13, 2013 by Sirine

[Tokay Gris]

Ok... so I will mark this answered...

And probably send another tanker to Moho. I will see after I transferred the meager leftover fuel from the first one to the actual vessel.

[maccollo]

If you do things properly, the Moho injection burn should be a little more than 1500 m/s.

Due to Moho's extreme orbit correct procedure a bit different than other planets.

The most efficient way to get to Moho is as follows:

1: Forget about phase angles

Moho's eccentricity, tight orbit, and inclination means the difference between just any encounter and a good encounter is massive.

The best way to encounter Moho is to line up your periapsis with it's periapsis. Therefore if you want a good encounter you must always depart when Kirbin is lined up with the apoapsis of Moho.

2: Combine your escape burn with your inclination change

Moho actually has one thing going for it. While it has a high inclination, the orbital plane nodes are actually aligned with the periapsis and apoapsis.

This means you should do the inclination change at the same time as you do your escape burn from Kirbin. This will only cost 300 m/s or so.

If you perform the inclination change after you have escaped Kirbin it will cost 800 m/s

3: Orbit the sun until you get a near encounter

Now you just orbit around the sun until you have a somewhat decent near encounter. You can use one maneuver node right after you pass the periapsis to check 2 orbits ahead.

You should only have to orbit a few times to get close enough.

Once you get a near encounter you can do a small correction burn to get a very close encounter. This shouldn't take more than 50 m/s

4: And the result is

A final injection burn of around 1550 m/s, and a combined delta V from LKO to Moho of just under 4000 m/s

Edited December 13, 2013 by maccollo

[FlamedSteak]

I don't think you're going to get a better answer than that.

[Kasuha]

3: Orbit the sun until you get a near encounter

Now you just orbit around the sun until you have a somewhat decent near encounter. You can use one maneuver node right after you pass the periapsis to check 2 orbits ahead.

You should only have to orbit a few times to get close enough.

Once you get a near encounter you can do a small correction burn to get a very close encounter. This shouldn't take more than 50 m/s

I would add to that point: Moho has terrible Oberth effect, you don't save much by burning near its surface.

Therefore you shouldn't feel yourself constrained to that mentioned 50 m/s - you can save a lot of time orbiting the Sun by performing retrograde burn at the intersect point which will bring your apoapsis down towards Moho's and at the same time ensure you get an encounter the next orbit or the orbit after that.

The more you brake there, the less will you need to brake on the Moho intercept. The difference is just in Moho's Oberth effect which I already wrote above is terrible. And you reduce the number of orbits you'll need to wait for close encounter to just one most of the time .

[Geschosskopf]

Thanks, Maccollo. I'll try that next time .

BTW, just a general note for going anywhere, to avoid being surprised by an unexpectedly huge capture burn. To minimize the size of the capture burn, you want your ship's path to cross the target planet's orbit at as small an angle as possible. IOW, your ship and the planet are almost on parallel paths when they meet. The greater the angle between your path and the planet's orbit, the less of your existing velocity counts toward your desired final orbit, and the more you have to work to bend your vector in the desired direction.

Now, I'm not claiming this make the whole flight more efficient. I'm just saying this minimizes the capture burn. But if you're tight on fuel or have a very low TWR (so long burn time in a short SOI window), anything that makes the capture burn smaller is a big help. With Moho, capture burns range from a low-angle optimum of about 1500 as Maccollo says, to an obscene maximum of about 7500m/s when meeting Moho at nearly right angles.

[Moar Boosters]

Well..... there's the Scott Manley approach where you do everything extremely efficiently......... but there's also the Moar Boosters approach.

After my first failed mission there, for both my subsequent visits I just engineered my spacecraft to have 5k delta V reserved for slowing down.

[capi3101]

The best way to encounter Moho is to line up your periapsis with it's periapsis.

Anybody got a date as to when that occurs? I know Kerbin's got an orbital period of 106.5 days or so, so it'll reach that same spot that often, but a reference date from which to extrapolate other "launch windows" would be very helpful.

Tried this the other day; pretty sure I screwed it up again.

[Tank Buddy]

Apparently if you make a flyby of Eve you can use the Oberth Effect to slow down, then your Moho capture burn would be less. Never tried it myself.

[capi3101]

Made it! Took ~8,000 but again I think that's because I had the wrong departure date.

[JiWint]

I probably had too much delta V,because I had absolutely no problems with moho,it was pretty similiar to landing at dres or so

[MartGonzo]

Some great tips in this thread, thanks to everyone

I'll give it another try, maybe even go for a landing

[KerikBalm]

I also ended up using more dV than I intended getting to Moho.... almost aborted the landing or even capture burn... but Danbro Kerman had a high courage stat (and high stupidity stat), and went for it. I landed Apollo style (of course), and am now in a 45 degree orbit... not equatorial, not polar... I'm not sure I can get back now (been doing other stuff in LKO the moho->kerbin window hasn't come up)

I do know that the interplanetary (IP) stage does not have enough dV in rocket fuel to get back to kerbin.... its packing 2 nukes and a lot of xenon. Due to the mass fraction, I'm going to bring the lander along, refuel it, and when the IP stage runs out of liquid fuel, I'll use the lander as a 2nd stae, and I hope to get back with the lv-909 (it also has 800 units of xenon gas and ions...)

I might be able to take the IP stage back to LKO for aerocapture and refueling/use with ions...

[Kasuha]

I also ended up using more dV than I intended getting to Moho.... almost aborted the landing or even capture burn... but Danbro Kerman had a high courage stat (and high stupidity stat), and went for it. I landed Apollo style (of course), and am now in a 45 degree orbit... not equatorial, not polar... I'm not sure I can get back now (been doing other stuff in LKO the moho->kerbin window hasn't come up)

I do know that the interplanetary (IP) stage does not have enough dV in rocket fuel to get back to kerbin.... its packing 2 nukes and a lot of xenon. Due to the mass fraction, I'm going to bring the lander along, refuel it, and when the IP stage runs out of liquid fuel, I'll use the lander as a 2nd stae, and I hope to get back with the lv-909 (it also has 800 units of xenon gas and ions...)

I might be able to take the IP stage back to LKO for aerocapture and refueling/use with ions...

If you can push your apoapsis to Eve level, you can gravity slingshot off it and get to Kerbin. It may take a while and need some orbit fine-tuning, though.

[lagrangewei]

using the suggestion, i did a variation. spend 2300m/s to leave kerbin while matching the inclination. when i reach moho's orbit, i trim my orbit toward moho while timing an intercept, which cost me 800m/s. i spend 1000m/s on full circulation around moho for a total of 4100m/s. this indeed helps cut delta-v by 1/3, but cost me an extra 200 days...

spend 3000m/s on return trip.

within the budget of 8000m/s of my interplanetary carrier. i use the lander to discard one of my tanks during descend and undock just before retrofire for landing, losing the tank probably gave me an extra 600m/s to play with. overall it was a great mission, much longer and complex than the duna mission, and moho was a rough hilly little planet to land on. :\

[flatbear]

D/v doesnt seem to be as much an issue as just getting an encounter, I can get as close as 33000km but no gravitational encounter, if I change anything, the encounter node either gets further apart or disappears.  How do I do a second correction burn?

[Plusck]

Well, this is a bit of a necro... but the procedure suggested by maccollo is spot on and is still the best way to go. Though I only orbit the sun once (a retrograde burn at Moho Pe will always get an encounter the next orbit around).

 

3 hours ago, flatbear said:

D/v doesnt seem to be as much an issue as just getting an encounter, I can get as close as 33000km but no gravitational encounter, if I change anything, the encounter node either gets further apart or disappears.  How do I do a second correction burn?

You seem to have a lucky encounter there - close to AN/DN and (if I'm seeing this right) quite close to Moho's Pe.
The trouble is that you will be going so fast when you get down there that a slight change in orbit makes a big difference in where you'll be relative to Moho. I suspect that you're missing the encounter because you aren't quite reaching Moho's orbit: if you do lower Pe any more, though, you're going to be speeding ahead of Moho.

However, you don't have to get much closer than that, really. You're probably going to have to make a midcourse correction anyway.

If you want to be sure to catch Moho on that encounter (rather than burning retrograde there to catch it the next time around), just make sure that if anything, Moho is slightly behind you at that encounter point when you finish your burn from Kerbin. Certainly not in front if you can help it.
Then, when you make the correction burn (a bit less than halfway, for efficiency's sake), you add a slight retrograde+radial out component. This will slow you down slightly, so Moho will catch up at that encounter point. Added bargain: slowing down a touch will reduce your injection burn a touch.

And if you miss Moho this time, just set a node to burn retrograde as you touch its orbit, then set a dummy node a quarter orbit later. Go back and adjust the retrograde burn until you meet Moho one orbit later. Then use the dummy quarter-orbit node to fine-tune the encounter.

[MitchS]

Does anybody have any follow-up on @capi3101's question about when that ideal transfer window that @maccollo described occurs? (Can't quote the post on my work computer--it's up there in the middle of this page, on 12 December 2013.) I'm sure someone has a quick way to gather that information, but I certainly don't. 

Sorry for the necromancy, but I think that's a major punchline of this thread that somehow didn't get delivered.

[Superfluous J]

30 minutes ago, MitchS said:

Does anybody have any follow-up on @capi3101's question about when that ideal transfer window that @maccollo described occurs? (Can't quote the post on my work computer--it's up there in the middle of this page, on 12 December 2013.) I'm sure someone has a quick way to gather that information, but I certainly don't. 

Sorry for the necromancy, but I think that's a major punchline of this thread that somehow didn't get delivered.

I don't know what to say beyond what he did. That first picture says it all. Launch when Kerbin is nearest to Moho's apoapsis around the Sun. You determine this by looking at map mode. Moho's eccentric orbit is pretty easy to gauge just by eye.

I'm not sure I agree with that but may do some test transfers tonight. I prefer to launch when Kerbin is at Moho's An or Dn, knowing that one of those (the one nearest the Ap, for a similar reason to why @maccollo suggests going exactly at Ap) is more efficient but not bothering anyway because I'd rather be able to launch sooner, generally. Maybe I'll test that too and see how much more efficient it is.

Edited January 4, 2017 by 5thHorseman

[MitchS]

@5thHorseman Thanks!