Getting to Moho - issues calculating the right moment

[Dieselpower]

So I've been trying to get to Moho for the last days (!), and I haven't been a bit effective.

Getting to other planets with the same design (Jool's moons are no problem) seems to be a walk in the park compared to this mission.

Hopefully, you guys can help me out?

First, let me explain what I have been trying.

1. I enter into an orbit at around 620km with around 3600 Units of fuel left. 4 Nerva engines are ready to propel a landing stage into trans-planetary orbit.

- in this example, I've parked at a ~614 circular orbit.

2. I have Mechjeb & Protractor Mod strapped on my rocket, so I use them to calculate when I have to burn. But four problems arise, listed them as a to d.

a. The suggestions that Protractor does are absolutely not in line with those from http://ksp.olex.biz/.

http://ksp.olex.biz/:

Phase Angle: -251.79°*

Ejection Angle: 110.38°

Ejection velocity: 3357.56 m/s

Protractor Mod:

*) Note that I add 360 to arrive at positive values: 117.33 for Protractor and 108.21 for ksp.olex

b. Ejection Angle NEVER occurs at indicated Phase Angle. In both calculations. This makes sense since you can be at any point in the orbit when the right Phase Angle to Moho occurs. But how should I round it off? Burn before Phase Angle occurs at the right ejection angle? Or after?

c. When should I start burning? When I burn according to the Protractor mod, at a vessel eject angle of 69, my vessel eject angle continues increasing.

At the end of my burn, when I've compensated the delta speed, I have a ejection angle of 108!

d. My ejection velocity is not accurate enough to bring at Moho's orbit. Adding /decreasing speed does not lead to being captured in orbit of Moho.

* see that I officially still have to burn 255 m/s, but doing so gets me way inside orbit:

3. In the end, I stop burning until my PE intersects Moho's orbit, and I hope (in vain) that I might get caught in the next 20 orbits or so.

When that does not work out, I try to decrease AP and/ or change inclination to Moho similar levels until fuel runs out.

Never have I had a predicted gravity influence of Moho.

Also, since Moho's orbit is eccentric and inclined, I have trouble determining what is more important: matching inclination or eccentricity?

Thanks for your help!

[Tubal]

I feel your pain. I have been trying on and off for a few days too, but I gave up and started mapping Jool's moons.

AFAIK it's very important to match inclination, expecially when you're relying on luck and time to get you into the SOI. If your inclination is off it could be thousands of years before all the numbers line up.

I've been trying to use both MechJeb and the protractor. One strategy I've used is warping to periapsis or apoapsis and burn to get the intersection of my and Moho's orbits to move around, hoping I'll catch the SOI at some point. No luck so far.

Rocket scientist I ain't.

[Excalibur]

Perhaps this post I made in response to someone else's Moho troubles may help. The OP claimed it got him there on the second try, no MechJeb or protractors required.

http://kerbalspaceprogram.com/forum/showthread.php/21526-A-easier-way-to-get-into-Moho-s-orbit

[Xcizer]

I don't recommend using mods to help do to them being unreliable. If anything they have usually failed me in the past with everything except orbiting kerban.

[numerobis]

Ideally, launch when the phase angle matches. Don't launch and then sit up in orbit.

The phase angle doesn't change that fast, so you can wait for a Kerbin day and get the right ejection angle. Or you can launch into a low Kerbin orbit and time warp to the right spot in the orbit.

[AndreyATGB]

Ideally, launch when the phase angle matches. Don't launch and then sit up in orbit.

The phase angle doesn't change that fast, so you can wait for a Kerbin day and get the right ejection angle. Or you can launch into a low Kerbin orbit and time warp to the right spot in the orbit.

Yes it does for Moho. One orbit around Kerbin at 100km will change the angle by at least 0.5 degrees. I'd recommend OP to use MJ's angles. You use KSP Olex to see the angle and stop time warp about 1-2 degrees before. I see Olex says -251 degrees, so that's about 109 for MechJeb. Also use a lower parking orbit, you'll take better use of the Oberth effect as well as be more accurate with the angles. If you didn't increase the comic draw limit, do so and plan your whole trip with nodes. First thing you do is put a node at the indicated ejection angle and use roughly as much dV as indicated on the website. You then place a second node on or close to the first inclination node you see, this is likely half way to Moho's orbit. You then fix your inclination with a second node. The closest approach things should show up on screen now. If they are very far from each other, I recommend moving your first node around your Kerbin orbit, this can have a huge impact. You can adjust then with a 3rd node when they're close enough.

[Superfluous J]

Here's how I get to Moho. I'm not saying it's the best or cheapest way but it works for me.

Wait until Kerbin and Moho's orbits intersect. I don't think there's a way to see ascending and descending nodes (sadly!) but if you line it up in map mode so that both orbits look like a line, you want to do everything when Kerbin is on the intersection between those two lines.

Launch your ship, get it into orbit, and do your Moho transit burn. Burn like you learned to burn for a "real" Moho transit, but unless you're EXTREMELY lucky you won't actually meet with it at Kerbol periapsis. No bigs.

Once you've left Kerbin SOI, you'll be very close to apoapsis in an orbit around Kerbol, near the ascending or descending node of Moho. Do your burn to align the orbits so your AN and DN are as close to 0 as you can get. After you've got this, your periapsis will likely have moved, burn again to get it touching Moho's orbit again.

Now you've got an orbit that will touch Moho's orbit on periapsis. Set Moho as a target (unless you did that already) and create a maneuver node at your periapsis (where it touches Moho's orbit). Fiddle with bringing your apoapsis down until you get the "near encounter" white markers. Then tweak your maneuver node until those turn into an actual encounter. Tweak further if you'd like, but this is good enough.

You should be able to handle it from here. This has worked for me every single time. It takes an extra orbit but it's fairly Delta-V friendly and very easy once you've done it a couple times.

Edited October 20, 2013 by 5thHorseman
added clarifications

[Kasuha]

Yes it does for Moho. One orbit around Kerbin at 100km will change the angle by at least 0.5 degrees.

Or about 5 m/s dv lost. If that much. Come on.

Also use a lower parking orbit, you'll take better use of the Oberth effect as well as be more accurate with the angles.

Higher parking orbit is good for time warps. Yes, it's not good for Oberth effect but the difference is not that bad.

If you didn't increase the comic draw limit, do so and plan your whole trip with nodes. First thing you do is put a node at the indicated ejection angle and use roughly as much dV as indicated on the website. You then place a second node on or close to the first inclination node you see, this is likely half way to Moho's orbit. You then fix your inclination with a second node. The closest approach things should show up on screen now. If they are very far from each other, I recommend moving your first node around your Kerbin orbit, this can have a huge impact. You can adjust then with a 3rd node when they're close enough.

It's conic draw limit, not comic.

I find this approach ineffective. Interplanetary inclination change is very costly. If you need inclination change, you should do it in the initial burn, using Oberth effect too. Yes it may mean you'll be leaving Kerbin at weird angle. But the dv you need for it is still okay. Inclination change in Sun orbit in high speed you're gonna get till you reach the AN/DN will cost you several hundred m/s dv.

The only thing you should need to do in interplanetary space are small corrections to overcome game's precision limits.

[Timo M]

On Sunday, October 20, 2013 at 0:01 PM, Kasuha said:

Or about 5 m/s dv lost. If that much. Come on.

Higher parking orbit is good for time warps. Yes, it's not good for Oberth effect but the difference is not that bad.

It's conic draw limit, not comic.

I find this approach ineffective. Interplanetary inclination change is very costly. If you need inclination change, you should do it in the initial burn, using Oberth effect too. Yes it may mean you'll be leaving Kerbin at weird angle. But the dv you need for it is still okay. Inclination change in Sun orbit in high speed you're gonna get till you reach the AN/DN will cost you several hundred m/s dv.

The only thing you should need to do in interplanetary space are small corrections to overcome game's precision limits.

Yes，matching inclination at initial burn is much more efficient. Idk why people do it in kerbol orbit

 

[Nefrums]

I use KSP Launch Window Planner by alexmoon. 

[Warzouz]

10 hours ago, Timo M said:

Yes，matching inclination at initial burn is much more efficient. Idk why people do it in kerbol orbit

You answered a 2 years old question !