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Wijbrandus

Achieving orbit around the Mun w/MechJeb - help requested

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I apologize if this is in the wrong folder. It appeared to me the most appropriate. Yes, I'm a newbie, and I'm having a lot of fun and frustration with the learning curve.

Since I am new to the game, I am using MechJeb to get over the learning curve. I am trying to send a probe to the Mun, but I am failing once I get there. The first probe crashed into the planet, and the second ended up in a slingshot to deep space. Part of my problem is fuel supplies, and part of my problem is working the MechJeb interface when I get to the Mun and not knowing how to establish an orbit there.

Here is what I'm doing. Tell me where I'm going wrong, if you can. There is something I am missing, or perhaps I'm over-correcting things.

1) Launch and establish a stable orbit around Kerbin. I aim for 200km, and then I use MechJeb to circularize and bring my eccentricity to essentially zero. I hope to have enough fuel left at this point to make the trip successfully.

2) I use the Maneuver Planner to set up a Hohmann transfer to the Mun. This appears to work perfectly, and I'm off in flight. I see the path intersecting with the Mun and a fishhook path establishing the flyby. There is also a little circle that says "Encounter". I don't know what that means.

3) When I get close to the encounter point, I set the "Fine tune closest approach to target". However, if I am already in the SoI of the Mun, I cannot select it as a target, and the Maneuver Planner requires it.

Now comes the problem. I'm within the Mun's SoI. I'm flying close by, or sometimes not that close. I want to establish an orbit. What do I do? I can't set MechJeb to circularize because it goes all wacky. It's like it is trying to orbit Kerbin again. Maybe I'm not close enough. But it's frustrating the hell out of me when it says "This maneuver requires a target" and I can't select the Mun as a target to conduct the maneuver.

Help!

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Since I am new to the game, I am using MechJeb to get over the learning curve.

It may be having the opposite effect. It's worth experimenting a bit so you get a basic idea of how your path changes in response to burns.

What you need to do in this case is quite simple. You're flying past the mun because you have too much speed, so you need to slow down. How do you slow down? Turn around and burn the opposite way.

The way I'd do it using mechjeb is to open Smart A.S.S. and hit the retrograde button. Assuming your your thrust is in the same direction as your capsule then that'll let you burn retrograde and lose speed. The curve of your path will increase and once you'll lost enough speed you'll be unable to escape from the mun and will be on a circular orbit. Keep burning and it'll curve more and you'll drop out of orbit as your path intersects the surface.

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I generally recommend MechJeb as well as ORDA, for newbies and experienced players alike, because they help you understand aspects of the game more passively (seeing what is done rather than figuring how to do it yourself).

General must-knows on KSP Orbital Map:

"... Encounter" means you are entering (encountering) its Sphere of Influence, basically its gravitational well.

"... Escape" means the point where you leave the body's SoI.

Now, you stated a fish hook, and that it gives no periapsis, this means that your orbital trajectory (flight path) is such that you are slowed to 0 by the Mün, accelerated towards its surface, where your logical flight ends. However the conical mathematics continue on with the calculations, which results in the Kerbin SoI escape trajectory that is physically impossible to achieve, due to the fact that you smash into the surface.

I recommend the following:

Get up into a 200km parking orbit. Now this must be done at a 90° heading (prograde orbit). Now that you have a prograde orbit just like the Mün's, you must wait until a transfer window (usually every 40 minutes) opens for you.

The Mün would be about 2/9ths behind the intercept your burning for when your planning. This means if your a rocket expert you could just launch on a KSC-to-Mün path, but that has many more variables associated (like the slowness of the ascent). Anyways, because you are going for a mun intercept in a prograde orbit, you can easily insert yourself in either a prograde or retrograde orbit around the Mün, there isn't too much of a difference, it just denotes the direction of the orbit (like going from west to east or east to west).

The best way to do this is to put yourself on a slingshot: make sure that you get the Kerbin Escape circle as well as a Mün Periapsis. You can fine tune your periapsis on your way there, but for now aim for your wanted periapsis allowing 50km error.

The trick to this injection is that with just 1000 m/s dV you can insert yourself into Munar orbit, because the injection burn at Kerbin costs roughly 800 m/s and the braking burn at the Mün costs a lot less, usually around 200 m/s. If you wish to have a polar or inclined orbit, perform the same steps, but your parking orbit has to be inclined from the start to be the most fuel efficient, and think ahead: take into account your ground position and the Mün's position, in order to get a transfer window sooner (as your on different orbital planes). Inclination can also be performed in transit from a regular equatorial injection.

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Ok thanks for the tips. I've finally found a launch design that will get my probe into orbit with enough fuel to establish orbit on the Mun.

Kethane scanning has begun. :)

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