in orbit around mun - how to come back? also parachutes

[draeath]

Updated yesterday (last time I played was 0.15) and stuck MechJeb (and the launchpad obliterator) in. Slapped a rocket together and go!

Click a few buttons, and BAM! Circular orbit at 100km +/- 1km. Click a few more buttons, some waiting, and a few more buttons... and BAM! Orbit around the Mun at 95km +/- 100m. Target at injection burn was 100km, so to be only off by 5km (and to circularize so tightly to it) is quite good! my inclination is 1.2 degrees. I like this. It's what I'd imagine a modernized Apollo Guidance Computer would do.

I believe I have fuel to make it back - how do I go about doing so? If my general understanding of how things works is correct, would this do the trick?

1. Wait for my mun orbit to place the ship on the farthest point away from Kerbin (eg drawing a line from Kerbin through the center of Mun, my ship would be where this intersects my orbit on the side of Mun facing away from Kerbin)

2. Burn prograde - my apoapsis would extend towards kerbin until it hits SoI. I would want to burn a bit longer so I don't smack kerbin but actually fly a bit past it, similar to how it looked transferring into Mun?

3. Should I aim for a circular Kerbin orbit first, or just aerobrake it?

One final question. How are we supposed to attach parachutes now? It doesn't want to snap to the nose of the capsule, and I'm a bit OCD so I don't want to freehand it. Because I couldn't figure this out I went with the mechjeb "unmanned" capsule, so no biggie.

[Tauge]

I've yet to find the best burn time for the return from Mun. Typically what I do is reach Munar escape wait for apoapsis, burn periapsis down to 30km-35km (can actually be as low as you want until/unless reentry heating is added), and reenter. There does seem to be a minor issue with the chute not snapping to the nose cone, but I just move it around a little and it usually snaps in.

[Dusty926]

thrust until kerbin ecounter, then once in kerbin orbit retrograde

[draeath]

OK, so I've got the right idea. Once I'm on an encounter path with kerbin though I can cut thrust until I get closer, to ensure I have fuel for de-orbit retrograde burn?

[AmpsterMan]

I usually like to burn retrograde to the mun's orbit. Reasoning for this is that I am killing the Mun's velocity relative to Kerbin. This means I have to do minimal corrections to my return.

[bitbucket]

First, just get your ship into an orbit that's high enough to not hit the terrain. Thrust when you're 90° away from the line between Kerbin and the Mun, so that your periapsis extends in the opposite direction of the Mun's orbital motion, and get to escape velocity.

When you escape the Mun's sphere of influence you'll be on a very elongated orbit around Kerbin, and it should only take a small amount of retro thrusting (if any) to get home. Get your periapsis below 40km; you'll hit the atmosphere at 3500+ m/s coming in, but should still land on the first pass.

[Tauge]

If your periapsis around Kerbin, after you escape Munar orbit, is <35km you will be returning. No more fuel would be required. Aerobraking will take care of the decrease in speed.

[Dusty926]

OK, so I've got the right idea. Once I'm on an encounter path with kerbin though I can cut thrust until I get closer, to ensure I have fuel for de-orbit retrograde burn?

You can keep burning, that also lowers your perilapsis...uhg...perilapsius-pera-F*CK!

[draeath]

I usually like to burn retrograde to the mun's orbit. Reasoning for this is that I am killing the Mun's velocity relative to Kerbin. This means I have to do minimal corrections to my return.

Not sure what you mean here? While I did not execute the maneuver myself (MechJeb did), I burned "leading" the mun. Mun captured me on the leading side, passing behind Mun's far side and coming back in "front" from the trailing side. (eg if you were to watch from Kerbin, I would appear from behind the Mun, overtake it between Kerbin and Mun, and duck back behind the Mun while it overtakes me).

[draeath]

You can keep burning, that also lowers your perilapsis...uhg...perilapsius-pera-F*CK!

I'm thinking my orbit would start "short of Kerbin (eg I'd faceplant into it) and as my burn continued it would poke out the other side, then grow to be more of a huge ellipse that just happens to pass close of Kerbin?

I don't suppose anyone could show me a diagram or such. I'm thinking it's like doing a Hohmann transfer except instead of getting a nice circular orbit at a greater altitude above Mun, you actually get captured by Kerbin instead? Whaaa where is my line of thinking off?

[Jay_Em]

As for parachutes, I know what you mean. I find that if you adjust the angle loads, and move the parachute slowly towards the nose, it should snap into place as before. It doesn't always work first time, try getting more acute angles and slow movements. Hope this helps.

[RoboRay]

Not sure what you mean here?

He means, burn in the direction opposite the direction the Mun is traveling around Kerbin (Munar retrograde). So, if you are on the near-side of the Mun (you can see Kerbin up in the sky), take off and turn east (heading 090) and hold a course parallel to the Mun's orbital path while you burn. If you are on the far-side, head west. This slows you down relative to the Mun's orbit around Kerbin, so you tend to fall right down toward the planet after leaving the Mun's SOI. You can probably line up an atmosphere-grazing trajectory right here and not require any additional burns.

Edited September 27, 2012 by RoboRay

[draeath]

Ahh... so pretend I'm not in orbit of Mun at all but in Mun's orbit around Kerbin. Then execute the Hohmann Transfer (of course timed so I don't hit Mun) and I should fall away from Mun. Makes sense now!

As for parachutes, I know what you mean. I find that if you adjust the angle loads, and move the parachute slowly towards the nose, it should snap into place as before. It doesn't always work first time, try getting more acute angles and slow movements. Hope this helps.

Don't know what you mean?

[AmpsterMan]

Not sure what you mean here? While I did not execute the maneuver myself (MechJeb did), I burned "leading" the mun. Mun captured me on the leading side, passing behind Mun's far side and coming back in "front" from the trailing side. (eg if you were to watch from Kerbin, I would appear from behind the Mun, overtake it between Kerbin and Mun, and duck back behind the Mun while it overtakes

me).

If you don't mind waiting four to five hours for me to get home from work, I'll show you in game what I mean and a few diagrams of why I believe it to be the most efficient. Don't worry, no maths involved; it's just that these ideas habe been floating around in my head and I want to know what the community thinks about it.

[draeath]

Thanks Ampster! I need to wait another 4-5 hours myself (stupid work... lol)

Math is OK if you need to use it to show something. I'll dig up TI-84 and try to prod my brain into remembering how to use it

I've got MechJeb, so I can get to all those fancy numbers.

[AmpsterMan]

Thanks Ampster! I need to wait another 4-5 hours myself (stupid work... lol)

Stupid work indeed...

So what RoboBay said is pretty much it. If you understood him than you can do it. Still here is a linky to a PDF of what I meant with pretty hand drawn pictures and some pseudo algebra. Sorry if you don't have Adobe, I forgot to scan my notes as JPEGs. I know diddly squat about rockets, math, or rockets and math in general so please excuse me if it all looks like a garbled mess .

Also, welcome back to KSP

[Frederf]

I've started thinking in terms of "leaving out the back door." Imagine the Mun is a car and your exit burn is jumping from the car. For return to Kerbin you want to reduce your forward speed as much as possible by jumping against the direction the Mun/car. If the Mun is orbiting counter-clockwise (eastward) with respect to Kerbin then you want to exit its SOI going westward.

I'm still not sure what's the best way to exit a SOI. Should I make a long elliptic lobe facing Kerbin and start my escape burn near the edge or should I try to point the lobe opposite to the Mun's travel and burn to escape with excess speed equal to the Mun's orbital speed.

[Peppe]

For the parachute you can view the attachment point level/little above and aim way high then you think it needs to be and it will snapdown to the center point. Usually just have to make sure your mouse doesn't hit any other surfaces and mouse near the green point.

Still working on orbits. The transfer calculator works planet to planet or moon to moon. Am I missing something or shouldn't they be transfer calcs for moon to planet? Or is the most efficient thing to come off the retrograde side of a moons orbit... get picked up by the planets SOI and just kill velocity to be pulled in?

[maltesh]

I've started thinking in terms of "leaving out the back door." Imagine the Mun is a car and your exit burn is jumping from the car. For return to Kerbin you want to reduce your forward speed as much as possible by jumping against the direction the Mun/car. If the Mun is orbiting counter-clockwise (eastward) with respect to Kerbin then you want to exit its SOI going westward.

I'm still not sure what's the best way to exit a SOI. Should I make a long elliptic lobe facing Kerbin and start my escape burn near the edge or should I try to point the lobe opposite to the Mun's travel and burn to escape with excess speed equal to the Mun's orbital speed.

For Munarl returns, getting into a hypoerbolic trajectory from very low munar orbit that points straight out the trailing pole of the Munar SOI (the point on the Munar SOI directly behind the Mun in its orbit), with a velocity that puts your spacecraft near the of the Mun's orbital velocity as you cross that SOI is going to be the most efficient way to do it, as it takes the best advantage of the Oberth Effect.

If your low (10kmish) orbit around the Mun takes you eastward across its surface, you want to burn a little before you get the middle of the near face. If it takes you westward across its surface, you want to burn a little before you get to the middle of the far face.

This video from a bygone era before Patched Conics illustrates the basic technique, except I didn't get into orbit first, and instead burned from the surface into an approximation of the desired hyperbolic path.

Ideally, I would have wanted my departure course to be aligned parallel to the Mun's orbit for minimal fuel costs.

[bitbucket]

Seems that it works a bit better if you start your munar escape burn 40-60° clockwise from the direct line between Kerbin and the Mun. Doesn't matter which way you're going, the end result shound be similar.

[Felsmak]

Are you in a clock-wise orbit? If yes, just wait until your ship is at its farthest away from Kerbin, so that you can draw a straight line between it, the Mun and Kerbin. Then, you just burn prograde until you leave the Mun's SOI. This way, you also kill the orbital velocity you gained from the Mun, so you'll get a future Kerbin-orbit that will make you fall straight back home.

[Nibb31]

In orbit around the Mun, set yourself above the prograde line of the Mun's orbit, and burn to 850 or 950m/s. This should put you on a return trajectory to Kerbin.

[Nibb31]

In orbit around the Mun, set yourself above the prograde line of the Mun's orbit (so that the Apoapsis of your burn will put itself on the trailing side of the Mun), and burn to 850 or 950m/s. This should put you on a return trajectory to Kerbin and takes advantage of the Mun's orbital velocity to give you a bonus.

[Nibb31]

In orbit around the Mun, set yourself above the prograde line of the Mun's orbit (so that the Apoapsis of your burn will put itself on the trailing side of the Mun), and burn to 850 or 950m/s. This should put you on a return trajectory to Kerbin and takes advantage of the Mun's orbital velocity to give you a bonus (you will be moving backwards while the Mun will be moving forwards).

[RoboRay]

Are you in a clock-wise orbit?

That would depend on if you were looking at it from above the plane of the ecliptic or from below it.

Prograde/retrograde (in regards to the rotation of the object being orbited) and even easterly/westerly eliminate the ambiguity.