[HELP] me get HOME from the Mun! And like ten other questions!

[WestSayid]

Hi fellow Kerbanauts,

Hugely enjoying the game so far. Orbiting is now a breeze (I can routinely get 90x120 parking orbits), and I have succeeded in landing on the Mun twice but no luck yet in getting home. I have the fuel but not the know how. Both times I just got into eccentric Earth orbits and ran out of fuel.

So how do I get home from the Mun? Is there a general guide to follow?

I have some other questions that would greatly improve my space program

GRAVITATIONAL SLINGSHOTS?

One time when I failed to get back to Earth from the Mun, I accidentally inserted myself into a high Kerbin orbit that was only a million miles beyond the Mun (see pic).

So I just hit timewarp and waited for the Mun to catch up to me and slingshot me out of orbit. Only it didn\'t happen. Every time the Mun passed below Jeb just waved at it and totally ignored its gravity. Is this a bug or am I an astrophysics noob or what?

LIFTOFF

I have the throttle at max pretty much throughout the ascent. Is this efficient or is a lower throttle actually better? Is there a way to calculate the ideal?

ORBITAL INSERTION

As I leave the atmosphere and start turning, is it more efficient to aim at the horizon or the prograde marker?

Is it more efficient to burn nonstop into orbit, or to burn to establish an apogee above the atmosphere, cut engines and then burn at apogee? If the latter, how high of an apogee do you aim for?

ORBITING

I know that pointing at 90 (due east) and 270 (due west) gets me into two different equatorial orbits. Is one preferable? Is it true that 90 uses less fuel due to Kerbin\'s rotation or does the game not model this?

HOHMANN TRANS MUNAR INJECTION

I read a tip somewhere that in the 270 configuration you do your TMI burn at Munset over Kerbin, and in the 90 configuration you burn at Munrise. I\'ve tried both versions and it works, the apogee lines up right with where the Mun will be. Why does this work? Why do we need Mun to be at the horizon?

The 270 orbit is 'pro-Mun' (CCW viewed from north, same direction as the Mun\'s revolution around Kerbin) and the 90 orbit is 'retro-Mun.'

Do you experienced Kerbanauts have a preference for proMun or retroMun pre-TMI parking orbit?

MUNAR LANDING

You guys make sure your nav ball is on SURFACE mode, right? That was a big breakthrough for me. Before I was coming down in ORBIT mode and getting a totally wrong reading of my lateral motion relative to the Munar surface. Now I can kill that motion at around 100km with a sideways burn and do a direct vertical descent. During this burn I generally do not aim at the retrograde marker but 'beyond' it with the aim of 'scooting' the marker into the center.

Any other tips for this phase?

LEAVING THE MUN

This is where I\'m currently stuck

ATMOSPHERIC RE ENTRY

Your capsule can\'t burn up in the atmosphere so you aren\'t punished for having terrible re-entry angles and speeds. Do you try to get a realistic re-entry trajectory anyway? Any tips on how to do this? Or do people just generally come down on any old trajectory?

[Guekko]

Hey!! If you speak french you could use the guide i made, you can download it here: http://www.fichier-pdf.fr/2012/04/02/ksp-manuel-d-alunissage/

and here the forum post: http://kerbalspaceprogram.com/forum/index.php?topic=9900.0

If you don\'t, try to download it and look at the pictures, or wait for this week end that i finish translate it.....or learn french

Seriously, look at the pictures I shot, even in french it\'s very explicit.

[Guekko]

To make the Ap (Apogée) point get down to Kerbin, go on the opposite point call Périgé, axe your rocket to the yellow circle with a cross inside on the navigation globe and throttle up....look at your orbit, it goes down, when it cross Kerbin...it\'s done just wait to make the orbit grows throttle up when your axe is with the simple yellow circle.

The two first picts shows what i said.

[WestSayid]

Yes, I know apogee/perigee altering (burn on the opposite side etc). I just don\'t know what to do in general. Do you establish an equatorial moon orbit and then just move your apogee to hit Kerbin? Or do you aim straight for Kerbin upon liftoff? etc

[Kosmo-not]

You want to exit Mun\'s SOI flying opposite the Mun\'s direction of motion. You also want to exit at a speed that will put your Kerbin periapsis within the atmosphere.

If you start from low Mun orbit, your burn point will be something like 15° (I don\'t know exactly) after you pass the Mun\'s orbit line on its prograde side. A velocity of 850m/s will definitely put you within the atmosphere.

[Guekko]

Make your Mun Apogée orbit goes about 1/5 of the distance Mun/Kerbin, go to the Apogée, you will be catch by Kerbin gravity. If you want to be realistic as possible, to go back kerbin, you must go back in atmosphere most horizontal as possible, if you go back in a vertical way, you will have to use all your engine power, and make your crew get a lot a G, about 15 to 25......a human body can only survive to 13 max, and over 11G even train astronaut fall....asleep (don\'t know how to say 'coma' in english). If you go into atmosphere horizontal way you will get about 5 to 9G.

I hope i\'m understandable ....

[Kosmo-not]

Don\'t listen to Guekko. The optimal trajectory for getting back to Kerbin from the Mun will always be hyperbolic (there is no apoapsis).

Here\'s roughly how you calculate the velocity needed to reenter Kerbin\'s atmosphere from the Mun:

Calculate velocity needed at Kerbin apoapsis to have a periapsis of 30km: 157m/s

Subtract that from Mun\'s orbital speed to get the velocity you need to exit Mun\'s SOI with: 542 - 157 = 385m/s

Use the conservation of specific orbital energy equation to calculate the velocity you need to reach at your orbital altitude.

You end up with: 310*sqrt(a+1555)/sqrt(a+200), where \'a\' is your altitude above Mun\'s surface.

All you have to do then is make sure you end up with a trajectory that is retrograde from the Mun\'s orbital direction.

To answer your other questions:

'GRAVITATIONAL SLINGSHOTS?'

You just didn\'t return to Mun\'s SOI (2,500,000m)

'LIFTOFF'

A thrust to weight ratio of a little over 2 is ideal. More or less will decrease your fuel efficiency while you are in the lower atmosphere.

'ORBITAL INSERTION'

Burning towards the horizon will eliminate gravity drag. However, burning at a higher angle will get you out of the atmosphere faster. You have to make a judgement call.

'ORBITING'

Kerbin\'s rotation gives you about a 200m/s boost towards 090° and a 200m/s penalty going 270°.

'HOHMANN TRANS MUNAR INJECTION'

The periapsis will always be in the same spot for this transfer orbit. The math for the TMI works out to having the mun at about a 23° angle below the local horizon at periapsis. This is roughly when the Mun is on the eastern horizon. 090° is the pro-Mun orbit (and I prefer it for the reason stated under 'ORBITING')

'MUNAR LANDING'

It sounds like you\'re doing your braking burn too early. I don\'t descend this way. I always insert into an orbit with a periapsis 5-10km above my landing point. I don\'t have to worry about having a late braking burn. The method I use does use a little more fuel than a well executed direct descent.

'ATMOSPHERIC RE ENTRY'

A periapsis of 30km will bring you down with 2G\'s max.

[UmbralRaptor]

To answer what I can:

So I just hit timewarp and waited for the Mun to catch up to me and slingshot me out of orbit. Only it didn\'t happen. Every time the Mun passed below Jeb just waved at it and totally ignored its gravity. Is this a bug or am I an astrophysics noob or what?

It\'s because KSP only calculates gravity from one body at a time. If you\'re not within the Mün\'s Sphere of Influence, it cannot effect you.

LIFTOFF

I have the throttle at max pretty much throughout the ascent. Is this efficient or is a lower throttle actually better? Is there a way to calculate the ideal?

The rule of thumb is that you want the drag on your rocket to be about the same as gravity. For low TWR rockets, just leave it on full throttle. For high TWR (stock) rockets, Closette made a table:

Altitude (m) Target speed (m/s) - rounded to nearest 5 m/s

0 97.3- you\'d better get a move on!

500 105 - you should be catching up by now!

1000 110

2000 120

3000 130 - usually the first benchmark I have time to look at

5000 160

8000 215

10000 260

13000 350

15000 425

16000 470 - most of us are at least thinking about staging and pitchover maneuvers by now!

..

32000 2250 - this is equal to the orbital speed, so by now you should be pushing over hard for orbit, and air drag is not as important above this altitude.

ORBITING

I know that pointing at 90 (due east) and 270 (due west) gets me into two different equatorial orbits. Is one preferable? Is it true that 90 uses less fuel due to Kerbin\'s rotation or does the game not model this?

Which one is preferable depends on if the orbit has some specific property you want. A 90° orbit requires ~350 m/s less ?V than a 270° orbit.

HOHMANN TRANS MUNAR INJECTION

I read a tip somewhere that in the 270 configuration you do your TMI burn at Munset over Kerbin, and in the 90 configuration you burn at Munrise. I\'ve tried both versions and it works, the apogee lines up right with where the Mun will be. Why does this work? Why do we need Mun to be at the horizon?

I *think* it\'s just a coincidence of geometry, but am unsure.

MUNAR LANDING

You guys make sure your nav ball is on SURFACE mode, right?

Yep.

ATMOSPHERIC RE ENTRY

Your capsule can\'t burn up in the atmosphere so you aren\'t punished for having terrible re-entry angles and speeds. Do you try to get a realistic re-entry trajectory anyway? Any tips on how to do this? Or do people just generally come down on any old trajectory?

I tend to do whatever. If you want a slow, low G reentry when coming in from the Mün, aim for a periapsis of ~30 km. (Ship dependent)

[Kosmo-not]

Just incase anyone missed it, I edited my post to include answers to questions.

@UmbralRaptor

How is the re-entry periapsis of 30km ship dependent?

[UmbralRaptor]

Just incase anyone missed it, I edited my post to include answers to questions.

@UmbralRaptor

How is the re-entry periapsis of 30km ship dependent?

Varying drag coefficients. IIRC, something with especially draggy parts will slow down at higher altitudes, while one with especially low drag parts will need to go lower into the atmosphere to avoid multiple passes.

I suspect you\'ve worked it out in more detail?

[Kosmo-not]

Varying drag coefficients. IIRC, something with especially draggy parts will slow down at higher altitudes, while one with especially low drag parts will need to go lower into the atmosphere to avoid multiple passes.

True. With stock parts, the overall drag coefficient is pretty much 0.2

I haven\'t worked with any mods yet.

[Zephram Kerman]

All you have to do then is make sure you end up with a trajectory that is retrograde from the Mun\'s orbital direction.

For this part, it is simple. While sitting on the surface of Mun, think about where Kerbin is located. If you are on the 'face' of Mun, Kerbin should be somewhere in the sky. In this case, you should go east-ish. Conversely, if you are on the 'back' and Kerbin is not visible, go west-ish. Both of these cases have the effect of cancelling Mun\'s orbital velocity, so that you fall back to Kerbin once clear of Mun\'s SOI. It helps to adjust your trajectory to compensate for latitude.

(By the way, SOI is Sphere Of Influence. Have you already learned about this? It\'s not normal astrophysics, but more like Aristotlean astronomy. Your flight path is affected by gravitational attraction from only one body.)

To answer your question about throttle optimization, it is a dilemma between fuel wasted hovering and exponentially increasing air friction. Historically, this is named the 'Goddard Problem'. Basically, build your rocket to be powerful enough for a Thrust-To-Weight ratio of 2:1. Use Closettes table of terminal velocity as your ascent profile. (Thanks UmbralRaptor! Copy this table and staple it to the inside of Jeb\'s visor!)

Which brings me to the gravity turn. It\'s another dilemma like the Goddard Problem, but this time in 2 dimensions. You need horizontal velocity to reduce the effect of gravity. But there is also that pesky atmospheric drag, so you need altitude to escape drag. I just realized the other day that a rocket with TWR of 2:1 cannot keep up with terminal velocity past 10 Km or so. When following that ascent profile table, simply start your gravity turn when you can\'t keep up. In this way, air friction is minimized, and you get that gravity turn bonus as soon as possible.

[WestSayid]

Thanks for the help guys. I just came back from the Mun. And how\'s this for a picturesque re-entry ;D

My method was to enter a Munar equatorial orbit and then burn when I was going counter to the Mun\'s direction around Kerbin (which happened to be on the dark side).

I burned until my periapsis was inside Kerbin and then waited. Unfortunately something weird happened. My whole orbit moved outside of Kerbin as the Mun revolved around it. So instead of hitting Kerbin head on it got me into a highly eccentric Kerbin orbit. No worries though, I still had half a tank of fuel, so I burned at the apoapsis of that ellipse and made it home. Total flight time 09:52.

Next step is to try to get an Apollo-esque Munar orbit before I land. I have been doing direct descent from Hohmann transfer, because my rocket has lots of fuel and I can easily aim myself for a crater.

And, I\'ll see if I can manage a more fuel-efficient return from the Mun.

[Kosmo-not]

Seems like you didn\'t think about the change of sphere of influence. The orbit you see on the map is relative to only the celestial body it it around and does not take into account what happens after a change in SOI.

[WestSayid]

Yes, this one-body physics is confusing....

I managed to get a very nice lunar orbit. Pics in this thread. http://kerbalspaceprogram.com/forum/index.php?topic=9963.0

[Ivan Ivanovich]

I\'ll try to answer what I can answer.

SLINGSHOTS

Your orbit looks a bit like it is not quite 'in sync' with the Munar orbit. I.e. tilted towards it. Of course the Mun will only 'catch' you if you are close enough, just being as far away as the Mun, even if your viewing angle looks like you\'re right 'on' it won\'t suffice. Are you perfectly sure that you were REALLY close to the Mun, not just on the same side of Kerbin?

LIFTOFF

There is actually a chart (it was posted here), where atmospheric drag outweighs impulse gain from max thrust. As a rule of thumb, this seems to be the case at a TWR (thrust-weight ratio) of roughly 2:1. So far, I didn\'t manage to build a Mun-capable rocket with a TWR coming even CLOSE to 2:1 (mine usually have a TWR of about 1.2:1 to 1.4:1). So I guess as long as your rockets are 'too heavy' to be near 2:1, full throttle it is.

ORBITAL INSERTION

There is no absolute answer to this, it depends on too many variables. It IS in general more efficient to keep the engines on and on full throttle (at least as soon as atmospheric drag is no issue anymore) due to the law of conservation of momentum, but that may not be in your interest since it might burn fuel to get where you don\'t want to go (e.g. it would lead to a high Apogee opposite of your intended flight direction).

My usual approach is to cut the engines when my apogee is high enough to stay out of the atmosphere even after I leave it (as long as you\'re in the atmosphere, your apogee will fall due to drag loss) AND I\'m certain I will have enough thrust to keep the AP ahead of me when I turn them back on. Hence I tend to bank rather early in the flight and get to a mostly horizontal thrust well before leaving the atmosphere (usually I have about 2200 m/s when my engines get turned off, most of it horizontal movement).

ORBITING

It is modeled that way and yes, you need less fuel orbiting towards 90°.

HOHMANN TRANS MUNAR INJECTION

The 'fact' that you burn at rising/setting Mun is due to how fast the Mun revolves around Kerbin and how far away it is from Kerbin. I\'d guess the game was built that way to make the trip there easier. Personally, I burn based on the overview map, because then it is (mostly) independent of my actual altitude when initiating the TMI. I burn when I fit the Mun in the fully tilted orbital map right into the upper right corner of my screen (at the correct zoom), with Kerbin as the target and my craft right 'below' Kerbin on the overview map. That puts me a bit ahead of Kerbin, despite aiming for an Apogee that\'s a bit short of the Mun orbit. I guess it\'s about the least fuel consuming option I have.

The reason for this is simple: Your Apogee will be opposite to where you initiate the TMI. Since your previous Ap/Pe pair was insignificant compared to the target Apogee (about 70km vs. 11.000), you WILL be at Perigee when you shut the engines off. In other words, you will fly towards wherever your opposite end of the orbit is pointing. Add that the trip to the Mun takes approximately 1/6 to 1/4 of a Munar orbit around Kerbin and you know why you burn at Munrise/Munset.

MUNAR LANDING

Personally, I get into a Munar orbit first. It\'s not necessarily a stable orbit, more a result of the way I get there. First I install a Perigee of about 5-10km, then at that perigee I bring down the Apogee (which then turns into the Perigee, of course) to the ground a bit behind my intended target area. That usually gets me in position for landing at about 2000 meters. There I do a burn to kill lateral speed, put my rocket upright, zero the lateral speed with RCS and then, well, land. Personally I think there is no easier way.

LEAVING THE MUN

Easy as pie. Start, bank towards 90° and get into an orbit. Altitude doesn\'t really matter, just keep it low to conserve fuel. Keep an eye on the overview map, it\'s VERY easy on the Mun to suddenly end up with an Apogee of 20+ km and you don\'t know where it came from. Just put enough thrust into it to get an Apogee that\'s a bit ahead of you, switch to the overview map and keep that AP ahead of you a few meters until you have a more or less stable orbit. Then wait until you reach the 'orbit line' of the Mun around Kerbin on the 'front' of the Mun (i.e. the side the Mun is going to. It\'s where your flight indicator points towards Kerbin. There, throttle up to about 900m/s. Yes, that results in an escape trajectory from the Mun that points to the 'left' of Kerbin. No problem there. You see, as we move, and Kerbin does, it will catch us. You will end up in a very elliptical orbit, but that\'s no biggie. Now, depending on how much fuel you have left, either lower your Apogee a little to enable a smooth landing or, if you\'re low on fuel, wait until you\'re at Apogee and lower the Perigee down to Kerbin\'s ground (or at least about 30km above, give or take depending on your Apogee, the higher the Apogee, the lower you\'d have to get the Perigee unless you don\'t mind taking another round around the marble).

Note that in 'reality' you would want to get a low Apogee to ensure a smooth reentry. The higher your Apogee, the faster your reentry speed, resulting in very high g-force and (as soon as it is implemented) the risk of burning up in the atmosphere. But for now that\'s not really an issue.

And yes, it\'s very possible to do a reentry with 2.5g and less.

[MrMoog]

The easiest way to get back to Kerbin from the Mun I found is to get in orbit eastward around the Mun, when you see Kerbin rise on the horizon, you boost prograde until an apoapsis of 5-10 millions meters. 13 millions is the real number, but it work below that if you\'re low on fuel.

Your orbit will look weird and counter intuitive, but as the Mun rotate around Kerbin, the orbit will gently turn to cross the SOI of Kerbin, and put you in a nice orbit around it.

[WestSayid]

Thanks for the help everyone. Munar missions are starting to become easier with practice. Yes, orbiting first turned out to be much easier than direct descent!

[Kosmo-not]

The easiest way to get back to Kerbin from the Mun I found is to get in orbit eastward around the Mun, when you see Kerbin rise on the horizon, you boost prograde until an apoapsis of 5-10 millions meters. 13 millions is the real number, but it work below that if you\'re low on fuel.

Your orbit will look weird and counter intuitive, but as the Mun rotate around Kerbin, the orbit will gently turn to cross the SOI of Kerbin, and put you in a nice orbit around it.

If you\'re low on fuel, you want to take as much advantage of the Oberth effect as you can. For the mun\'s case, you will always end up with an munar escape trajectory for a Kerbin orbit that intersects the atmosphere or lower without doing any additional burns. You would probably be surprised at how a couple extra m/s on your TKI burn can translate into many more extra m/s at the SOI boundary.

For example:

Case 1:

From a 20km mun altitude, it takes 734 m/s to end up with 0m/s at the SOI boundary (you will have to have a vertical trajectory to accomplish this). It obviously takes a little more velocity to reach the SOI boundary from an orbit. In the vertical case, having 734m/s, you will end up with 542m/s tangent to Kerbin. If you plan to reenter Kerbin\'s atmosphere, you have to expend around 385m/s more. That\'s 734+385=1119m/s total.

Case 2:

If you instead burn to 829m/s from 20km with a trajectory that will take you retrograde from the mun\'s orbit, you will end up with 157m/s tangent to Kerbin if you do it right. If you do it perfectly, it will intersect Kerbin\'s atmosphere at 30km for a comfortable reentry. This also means there is no additional burn, so your total delta-V expense in this case is 829m/s

There is a 290m/s difference between the two, and that equates to fuel. All you had to do to get from case 1 to case 2 was burn for 95m/s more.