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Giygus

Landing on the Mun (newb)

Question

First, forgive me for not searching forums for an already created thread.  I looked at FAQ, but it seemed to be more about the forums themselves.

Anyway, I built a lander that's pretty similar to the "stock heavy lander".  Then I built a 2 stage launch rocket to get the lander to orbit and then to the Mun.  The problem is, once I get their and start actually approaching it, I have so much speed (m/s) that I have to burn pretty much all of my fuel to slow craft down enough so I don't explode on impact.

Any general idea what I am missing here?  Like, am I supposed to burn retrograde at a certain altitude or something?  Obviously, parachutes won't work, maybe some sort of air break?

I feel like I am missing some obvious step.

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some light reading post pics people will answer specific questions more often.

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Well, probably better to break the problem down into different bits.

1) The right craft for the job: The stock heavy craft is, quite simply, far too heavy and absolutely not the best thing for the job. You really want to start with the 1-man lander can, add some droppable tanks, and go from there.

However, since you managed to get the heavy lander all the way to the Mun anyway, you must be doing something right. And if you insist on using it, you can do but you really need to have the stage before it get you to the Mun. You could feasibly use the heavy lander to circularise in LMO, land and return to orbit again, but only just.

If you want to use stock craft, however, use the Kerbal X instead. It is vastly overpowered but it has more than enough delta-v to get up to orbit, to the Mun and land. You might not be able to get back to Kerbin though - a really inefficient launch and landing could feasibly leave you a bit short of fuel. Basically, you should be well more than halfway through your burn to the Mun before you switch to the final stage.

2) Transfer from Low Kerbin Orbit (LKO) to Low Munar Orbit (LMO):

You definitely don't want to hit the Mun straight from LKO. The way most people do it is select the Mun as target, then rotate the camera so that the Mun is just peeking over the eastern (prograde) horizon on Kerbin, and drop a maneuvre node at the first point on your orbit on a line between the camera and the Mun. When you draw that node out to Mun orbit, it should give a reasonable encounter. Definition of a "good encounter" is that your Ap (with respect to Kerbin) should be just short of or at the Mun's own orbit.

Since you can't see your Ap once you have an encounter, then that means that you need to zoom out a little and look from above, to check that you're not overshooting. If you don't get an encounter now, don't try and make Ap any higher but rather drag the node around until you hit the Mun. Keep your burn at an absolute minimum.

As soon as you have got an encounter, stay zoomed out so you can see everything and drag that node back and forth along the orbit some more. What you want to see is an intercept where the Munar "Pe" is just behind (i.e to the right, looking from Kerbin) the shadowy Mun image (which is the Mun's position when you reach that periapsis). Your path afterwards should shoot off to the left. Ideally, you'd want that Pe to be somewhere around 20-40km, but you'll probably need to add a bit more prograde to your burn to get that. Your final burn should be in the 850 m/s range.

Now burn. Warp to the encounter (but warp slowly, about 100x max, when actually crossing the boundary to avoid errors), and as soon as you're in the Mun's SOI plot a circularising maneuvre. That should cost you about 250 m/s.

3) Landing on the Mun:

However you want to do it, the most efficient landing profile is always the shallowest, but it is also hard to be safe while coming in shallow. You basically want to burn on the dark side of the Mun to bring your Pe down so that it just grazes the surface in the sunlight. At this point you have two options, neither of which is absolutely optimal for fuel but each of which gives you some room for error (but not the same kind of error):

Option 1: constant altitude burn (reduces the risk of running vertically into the ground, generally due to insufficient thrust): plot a node that brings Pe down until the line is broken by something on the surface, then back up a bit to give yourself about 2000m extra room. Burn that node. Just before you get to Pe, start burning retrograde and watch your climb/dive rate at the top of the screen. As you reach Pe it should be level and may show a slight rate of climb until you slow down, then it will start to fall significantly. You basically want to keep it at a gentle dive as you kill your speed. On your main view, check that you don't seem to be making a controlled flight into terrain.

Option 2: suicide burn (reduces the risk of running sideways into something): plot the same node as before, but as soon as the line is broken by something on the surface, drop Pe a bit more (edit: actually quite a lot more, to be sure of avoiding terrain) so that you have a clear line running into the ground. Then at that exact point, plot a second node and drag the retrograde marker until it "reverses" itself. Basically that means a dead stop. Burn the first node (the one that brings the Pe down to the surface) and delete it.

Your second node should now give you the burn time you need to come to a dead stop, and the time until you get there. Drag that node back up your path for either (a) if living dangerously, 3/4 of that burn time, or (b) if wanting to be safe, slightly more than that burn time. Now, press F5. Rather than actually following the burn marker, simply point retrograde (using SAS retrograde if you have it) and burn at the instant you reach the node. If you chose option (a) you should come to a dead stop a handful of metres above the surface. If you chose option (b), you should be several hundred metres higher. If you hit the ground anyway, press and hold F9 and start burning a little earlier next time...

Whatever option you choose, keep in mind that the surface is NOT perfectly synched with the orbit lines. Your orbit may look to lie above the surface, but it might not be the case in reality. If you have a very high TWR (like the Kerbal X has) you can limit the danger of hitting the ground unexpectedly by giving a significantly steeper incoming angle.

Final phase of landing: The final stage is hard for beginners. You basically want to follow the retrograde marker up until it is at the zenith. Remember that burning towards retrograde will always "push" the marker around on the NavBall, so if it drifts off the zenith, aim to the other side to bring it back. "Retrograde hold" on SAS is a great help here, meaning you can concentrate on your speed instead.
You want to land at a fairly low speed. 3-4 m/s is ideal. 7 m/s is a hard-ish landing. 10 m/s can break things.
However, you don't want to fall from 1000m at 5 m/s - it will guzzle fuel.

In daylight, you can see how far you are off the ground by your ship's shadow. As long as the shadow is tiny and far off, let yourself fall. Watch your velocity, and as the speed rises do a couple of short burns to check how quickly you can slow down. As the shadow gets bigger and closer, prepare to slow more. Try to leave off the throttle until as late as you dare. Again, F5/F9 may be your friend.

If you need to pick your landing site visually (to avoid slopes) you need to be able to control the ship as it appears on the screen. For this purpose, I always roll the ship so that the brown area forms a crescent at the top of the navball. In this position, with the camera (normal auto setting) following above and behind the ship, left and right work as expected, and climb/dive are reversed.

Edited by Plusck

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Two main areas to work on:

  • Ship design
  • Navigation

With regard to navigation, there are many possible ways to go wrong... hard to give specific advice without having a feel for where you're running into difficulty.  Some questions for you:

  • How much dV are you using to get from LKO to the Mun?
  • When you arrive at the Mun, how much dV are you spending to capture?
  • What is your periapsis upon arrival at the Mun?  (i.e. what altitude are you braking at)?

As for the lander design, a few suggestions:

  • Do you have a reason for those angled engines?  If you just like the way it looks, fine... just pointing out that having them pointed at angles like that is wasting fuel due to cosine losses.  For that matter, why the extra engines?  that single Poodle should be plenty of engine to land that ship on the Mun.
  • Why all the Vernors and RCS thrusters?  If you're not docking anywhere, you can get rid of them and save a fair amount of weight.
  • It's hard to tell exactly, but do you have three landing legs?  Suggest upping that to 4... it's only a little extra mass, and it'll make your ship considerably more stable when landing.

Regarding your mission profile:

  • Is this your first Mun landing, or do you already have experience with smaller/lighter craft?  Larger ships can be more unwieldy and tricky to fly-- if you've never been to the Mun before, it may be worth considering going with a smaller/lighter craft to help you learn the ropes.
  • Have you been to Minmus yet?  If you're finding the Mun difficult, you might try Minmus-- it's actually considerably easier to land there (and requires less dV); the speeds are a lot slower, it's a much more forgiving environment, and there are lots of nice flat places to land.  If you're new to the game, Minmus can be a great place to practice before you tackle the Mun.

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Minmus is much easier to land on than the Mun.  Pictures would help us help you.

 

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OK, wow, and I thought getting off the ground would be the hard part of the game.  Is this even a game any more? :P

24 minutes ago, Aethon said:

Pictures would help us help you.

Thanks for any help you can give, but I am unsure of what you would like a picture of?  I would assume the lander and the orbit line down to the Mun, which I guess I am plotting all wrong anyway, reading around.

 

39 minutes ago, Nich said:

...some light reading...

Light, yes, "light" :wink:

I skimmed through them, will read them later when I have more time.

And thank you Plusck for the info, will take a bit to digest.  I will try and post a screenshot of the lander/return vessel itself at least.

7lpm6Kc.jpg

http://i.imgur.com/7lpm6Kc.jpg

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55 minutes ago, Giygus said:

Light, yes, "light" :wink:

I skimmed through them, will read them later when I have more time.

And thank you Plusck for the info, will take a bit to digest.  I will try and post a screenshot of the lander/return vessel itself at least.

http://i.imgur.com/7lpm6Kc.jpg

Hmm, well that lander should definitely have enough delta-v to get to the Mun and down to the surface from LKO. However, the legs don't appear to be able to touch the ground...

Also, all that Monoprop is really quite unnecessary. In the original Super-Heavy Lander, it was needed for the huge number of very effective RCS boosters. In yours, you've got Vernors doing a lot of the work. You could probably get away with about a sixth of that Monoprop.

 

So I suspect the problem is simply your Mun encounter. If you get it very wrong you could end up approaching the surface at a very high speed, but if you go straight into low orbit you'll only need 250 m/s circularising, 30 m/s deorbit and 600 m/s landing burn + time fighting gravity (say 100 m/s dv spent per minute off the ground) as you drift down to your actual landing site. And that lander theoretically has around 3000 m/s in it.

 

(edit: also, the Poodle will burn for nearly twice the time of the three Terriers, so unless you rebalance fuel or use only the Poodle until neary half of its fuel is gone, you are going to have engines cut out while landing.

Also, fun fact: because they only burn for part of the time that the Poodle is burning, and because of those heavy Monoprop tanks, the Terrier side assemblies (tank, engine, monoprop + vernors) actually subtract from the total delta-v of the lander. And if you only stage them once the Poodle's tank is half-full, they only add less 10% than to the starting TWR.

And finally, I don't see a heatshield there. And that is a drogue chute, it will not slow you enough to land back on Kerbin.)

Edited by Plusck

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17 minutes ago, Snark said:

Two main areas to work on:

  • Ship design
  • Navigation

With regard to navigation, there are many possible ways to go wrong... hard to give specific advice without having a feel for where you're running into difficulty.  Some questions for you:

  • How much dV are you using to get from LKO to the Mun?
  • When you arrive at the Mun, how much dV are you spending to capture?
  • What is your periapsis upon arrival at the Mun?  (i.e. what altitude are you braking at)?

As for the lander design, a few suggestions:

  • Do you have a reason for those angled engines?  If you just like the way it looks, fine... just pointing out that having them pointed at angles like that is wasting fuel due to cosine losses.  For that matter, why the extra engines?  that single Poodle should be plenty of engine to land that ship on the Mun.
  • Why all the Vernors and RCS thrusters?  If you're not docking anywhere, you can get rid of them and save a fair amount of weight.
  • It's hard to tell exactly, but do you have three landing legs?  Suggest upping that to 4... it's only a little extra mass, and it'll make your ship considerably more stable when landing.

Regarding your mission profile:

  • Is this your first Mun landing, or do you already have experience with smaller/lighter craft?  Larger ships can be more unwieldy and tricky to fly-- if you've never been to the Mun before, it may be worth considering going with a smaller/lighter craft to help you learn the ropes.
  • Have you been to Minmus yet?  If you're finding the Mun difficult, you might try Minmus-- it's actually considerably easier to land there (and requires less dV); the speeds are a lot slower, it's a much more forgiving environment, and there are lots of nice flat places to land.  If you're new to the game, Minmus can be a great place to practice before you tackle the Mun.

Snark:  First, thanks for your time.  2nd, I will have to give it another try to answer your very smart questions about dV, as I can't say I know, but I am sure I am being inefficent.  I can answer the simpler ones though. 

The 3 engines were put there to stop me from falling over when I landed.  I only got to the moon once before (without exploding) in a very differently designed ship (It was pretty tall).  All the RCA stuff was to either help me from tipping over, or righting me when I did tip.  And there are, in fact, 4 legs.  Will work more on picture taking skills.

As for going to Minmus, it never occurred to me.  I am kind of approaching this game a bit linearly.  Orbit around Kerbin lvl 1, going to Mun lvl 2, etc.  If I can't get a redesigned ship to work ill try there next.

19 minutes ago, Plusck said:

Hmm, well that lander should definitely have enough delta-v to get to the Mun and down to the surface from LKO. However, the legs don't appear to be able to touch the ground...

Also, all that Monoprop is really quite unnecessary. In the original Super-Heavy Lander, it was needed for the huge number of very effective RCS boosters. In yours, you've got Vernors doing a lot of the work. You could probably get away with about a sixth of that Monoprop.

 

So I suspect the problem is simply your Mun encounter. If you get it very wrong you could end up approaching the surface at a very high speed, but if you go straight into low orbit you'll only need 250 m/s circularising, 30 m/s deorbit and 600 m/s landing burn + time fighting gravity (say 100 m/s dv spent per minute off the ground) as you drift down to your actual landing site. And that lander theoretically has around 3000 m/s in it.

Plusck:  Again, thanks for the time.  It did not dawn on me about the monoprop. I wanted less then the big orange tank that goes in line on rockets, so I went with the propane looking ones.  I did not realize that 6 of them = 2 Volkswagens in weight.  I do have a question here:  You are saying the RCS boosters (I assume you mean the ones with 4 nozzles) are very effective, and are implying that the "Vernors" are less so.  In the design screen, it says the Vernors have thrust power of "12", and the quad has "1", is there something I am missing?

As for orbiting the Mun, what I was doing was coming in from Kerbin, waiting till I was on the light side of the moon (without going into orbit around moon) hitting the Retrograde button, and turning on the engines until I fell straight down.  Then I pointed straight up (blue side of the ball) and played with engines until my m/s was under 50 as I fell (I would have to toggle on and off).  When I got close, I would turn on the 3 engines on the side so I would stay straight, then land.  Not very efficient, I expect you will find.

21 minutes ago, Vanamonde said:

That will be very helpful yes. :cool:

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7 minutes ago, Giygus said:

As for going to Minmus, it never occurred to me.  I am kind of approaching this game a bit linearly.  Orbit around Kerbin lvl 1, going to Mun lvl 2, etc.  If I can't get a redesigned ship to work ill try there next.

It's worth considering.  Minmus is a bit trickier than the Mun in one and only one regard, which is that its orbit is slightly inclined from Kerbin's equator by a few degrees, so navigation matters somewhat more.  But other than that, it's much easier than the Mun, in every way:

  • Takes less dV to get to Minmus orbit than Mun orbit, = smaller lighter cheaper easier-to-control ship.
  • Takes much less dV to get from Minmus orbit to the surface, = much smaller lighter lander, = much much smaller lighter ship.
  • Low gravity = landing's happening at 1/3 the speed as the Mun = you have 3x the time to correct your goofs = very forgiving.
  • Low gravity also means much more forgiving in terms of fuel efficiency if you're not nailing your landing technique quite right.
  • Lots of big, perfectly flat, perfectly level areas to land on, = don't have to worry so much about tipping over
  • The gravity is so low that even if you do tip over, you can probably turn the ship right-side-up just using your reaction wheels
  • The dV requirements to get home are so forgiving that it's practically impossible to get stranded on Minmus.  Even if you land and are completely out of fuel, a kerbal on EVA can go all the way home to Kerbin from the surface of Minmus using just his jetpack!  (Of course, he'd burn up on reentry, so you'd have to do an interesting rescue mission...)
16 minutes ago, Giygus said:

You are saying the RCS boosters (I assume you mean the ones with 4 nozzles) are very effective, and are implying that the "Vernors" are less so.

Actually, that's exactly the opposite of what he's saying.  :) The Vernors are quite powerful, and they use the same fuel as the rest of your ship, so you can ditch lots of monoprop.  You really don't need so much.

 

17 minutes ago, Giygus said:

As for orbiting the Mun, what I was doing was coming in from Kerbin, waiting till I was on the light side of the moon (without going into orbit around moon) hitting the Retrograde button, and turning on the engines until I fell straight down.  Then I pointed straight up (blue side of the ball) and played with engines until my m/s was under 50 as I fell (I would have to toggle on and off).  When I got close, I would turn on the 3 engines on the side so I would stay straight, then land.  Not very efficient, I expect you will find.

Yes, that's inefficient, but just how inefficient it is depends on your altitude when you brake to a stop like that (high = bad).  The optimal Mun landing looks like this:  Set up your approach trajectory from Kerbin so that you arrive at the Mun with a very low Pe-- as low as you can go without having to worry about smacking into any inconvenient mountains.  10-15 km is low enough.  Wait until you're right at Pe, then thrust retrograde until you capture (form a closed orbit), and then keep on thrusting until you lower your Ap to about the same as your Pe.  Now you're in a nice, low orbit around the Mun, having arrived there efficiently.

Being in orbit first isn't strictly necessary, but it doesn't cost you much over a direct landing, and it gives you a big benefit in terms of giving you the leisure and flexibility to pick where you want to land (e.g. somewhere relatively flat).

To land, first you do a retro-burn that lowers your Pe into the terrain, then when you get fairly close to the ground, you retro-thrust to brake to a halt right at ground level.  Judging when to start the retro-thrust is a bit dicey, because if you wait too long to start, you go splat.  That's why this technique is called a "suicide burn", and takes some practice and/or math.  :)

If you're new to all this, don't worry too much about trying to get an optimal, perfect landing-- plenty of time later to perfect your technique.  Just focus on trying not to do the egregiously bad things that waste lots of fuel.  The main thing is:  do your braking horizontally at low altitude where possible, and don't spend lots of time "hovering" (or descending super slowly, which amounts to the same thing)-- that eats up lots of fuel.

 

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40 minutes ago, Giygus said:

...

I do have a question here:  You are saying the RCS boosters (I assume you mean the ones with 4 nozzles) are very effective, and are implying that the "Vernors" are less so.  In the design screen, it says the Vernors have thrust power of "12", and the quad has "1", is there something I am missing?

Yes, sorry I wasn't as clear as I meant to be. And you're not missing anything.

What I meant about the Vernors doing most of the "work" is that they all fire together, but the Vernors will provide most of the thrust while the RCS boosters will help with rotation by being higher up. Since the Vernors are providing a lot of control thrust, you shouldn't need to fire RCS much. The monoprop-fuelled RCS boosters will therefore help for stability, but won't actually be on for long. Therefore they won't need much monoprop.

The original super-heavy lander, on the other hand, is peppered with those single RCS nozzles (each with a power of 2, compared to 1 for each direction for the 4-way boosters). This was what I meant by the original's "very effective" RCS: it has a ton of nozzles (at least 4 for each direction, I think), spaced out around the centre of mass of the ship whichever way you look at it. Its RCS system is therefore very effective despite the substantial mass of the lander. I certainly never put that much RCS control on my builds.

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OK, so after failing at even getting my redesigned setup into orbit in my free time on Saturday, I decided to give Vanamonde's guide a shot.  Worked pretty well, so kudos on a well thought out guide.  I almost ran out of fuel on the return trip, but still made it back safe with a bit to spare.

Will try and take what I can learn from the launch vessel and return ship and see what I can do with it.

At least I am learning from my mistakes.

On 2/27/2016 at 10:11 PM, Vanamonde said:

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3 hours ago, Giygus said:

OK, so after failing at even getting my redesigned setup into orbit in my free time on Saturday, I decided to give Vanamonde's guide a shot.  Worked pretty well, so kudos on a well thought out guide.  I almost ran out of fuel on the return trip, but still made it back safe with a bit to spare.

Will try and take what I can learn from the launch vessel and return ship and see what I can do with it.

At least I am learning from my mistakes.

That's the spirit.

As for getting back: I'm not sure if you know or if you were just very low on fuel when trying, but it's actually extremely cheap to return from the Mun (and Minmus). This video explains it:

 Starts at 19:20

 

Beware re-entry heat, but you should be fine (just point in retrograde direction)

Edited by Sir_Robert

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