Munar Landing - Can't get 200K orbit less than 400m/s

[JoeSchmuckatelli]

I've tried this several times and keep failing.  Trying to land, and I've read that at 200k from the moon, my orbital speed should be 3-400m/s.  Every time I've tried to land - I just don't have enough fuel to get my speed down enough.

 

I inserted at 300 - but bringing the orbit lower increased my speed.

 

What to do?

[Pecan]

The closer you are to a body the faster you need to go in order to maintain an orbit.  At any given altitude there is exactly one orbital velocity - go faster than that and your orbit rises, go slower and it falls.
If you slow down, you fall, which increases your velocity.  Hence the totally obvious orbital mechanics rule "if you want to go faster, slow down".¹

For a landing I usually first establish orbit at 10km or lower - you will accelerate even from that altitude and initial higher speed anyway as you fall towards the surface.  The easiest way to land (but not most efficient) is to burn slowly, keeping your nose facing retrograde (assuming the engines face the other way).  Initially this slows your horizontal/orbital speed but as you start to fall more quickly retrograde rises above the horizon and more thrust is directed towards countering your vertical fall.  Just remember to throttle-up if necessary during the last stages so you land gently enough to survive!

[¹Seriously - it gets even more fun when you start doing orbital rendezvous and docking.  To catch up with something ahead of you, slow down (a bit), so you fall (a bit) and then orbit faster, accelerate to go back up and slow down again once you've closed the angular gap.  To let something behind you catch up, go faster, so you rise and orbit slower.  It's rocket science, it doesn't have to make sense too!  (This is really real, not just a KSP thing, by the way)]

Edited December 15, 2015 by Pecan

[Sampa]

11 minutes ago, Pecan said:

The closer you are to a body the faster you need to go in order to maintain an orbit.  At any given altitude there is exactly one orbital velocity - go faster than that and your orbit rises, go slower and it falls.
If you slow down, you fall, which increases your velocity.  Hence the totally obvious orbital mechanics rule "if you want to go faster, slow down".¹

For a landing I usually first establish orbit at 10km or lower - you will accelerate even from that altitude and initial higher speed anyway as you fall towards the surface.  The easiest way to land (but not most efficient) is to burn slowly, keeping your nose facing retrograde (assuming the engines face the other way).  Initially this slows your horizontal/orbital speed but as you start to fall more quickly retrograde rises above the horizon and more thrust is directed towards countering your vertical fall.  Just remember to throttle-up if necessary during the last stages so you land gently enough to survive!

[¹Seriously - it gets even more fun when you start doing orbital rendezvous and docking.  To catch up with something ahead of you, slow down (a bit), so you fall (a bit) and then orbit faster, accelerate to go back up and slow down again once you've closed the angular gap.  To let something behind you catch up, go faster, so you rise and orbit slower.  It's rocket science, it doesn't have to make sense too!  (This is really real, not just a KSP thing, by the way)]

To add to what he said, with regards to the orbital velocity rule, an imagery to halp you better see what is going on.  I am sure that, when you were younger, you used to spin yourself in a spinning office chair.  If you haven't, I suggest you make a small experiment,  Have someone spin you as fast as they can (start with your arms in closer to your body)  When they get out of the way, put your arms out.  If done right, you will notice that you slowed down when your arms were out and moved faster when your arms were pulled in.  This same principle applies to orbits as well.

[Kerbart]

38 minutes ago, JoeSchmuckatelli said:

I've tried this several times and keep failing.  Trying to land, and I've read that at 200k from the moon, my orbital speed should be 3-400m/s.  Every time I've tried to land - I just don't have enough fuel to get my speed down enough.
(...)
What to do?

Landing is lift-off in reverse. If there's no atmosphere to help you in slowing down, it is going to cost lots of fuel. From low orbit to landing will take about 600 m/s ∆v. You can google "Rocket Equation" if you don't have at hand how much ∆v you have in your craft; MechJeb and Kerbal Engineer can tell you.

If you don't have a tool to give you an estimate of your ∆v budget you can always do it the old-fashioned way: calculate it.

∆v = I_sp×g×ln(m_start / m_finish) with I_sp the specific impulse of your engine (usually somewhere around 300 s; you can look it up in the VAB or in the wiki), g is a standard g of 9.81 ^m/_s.s (a little bookkeeping trick in the delta-v calculations) and you multiply that with the natural logarithm of your wet mass (at the beginning of the burn) and the dry mass. Your max delta-v would be based on total mass with propellant (fuel + oxidizer) divided by total mass without propellant.

If that sounds like abracadabra, consider this: around 20% of the mass of your lander should be propellant (fuel + oxidizer). If it's less, you will not be able to land in one piece.

Edited December 15, 2015 by Kerbart

[JoeSchmuckatelli]

Grumble - it's becoming clear to me that I don't have enough fuel.  I'm pretty sure a 10k orbit will have me going well over 600m/s - and I'm pretty sure I don't have the fuel for that.

 

(I do really appreciate the responses - but my options at this point is to either kill a good kerbil or leave her stranded in lunar orbit...  Probably do that.

[Snark]

By the way:  If you're spending fuel to "lower your orbit" (i.e. move your periapsis downward), you're doing it wrong.  You're wasting fuel and Hermann Oberth is spinning in his grave. 

The "right way" to approach the Mun is to set up your initial ejection burn from LKO so that when you approach the Mun, your periapsis is already as low as possible.  Like, toe-scrapingly low, just barely high enough to avoid smacking into any inconvenient mountains.  A munar periapsis of 10-15km is good.  Higher, like anything up to 30 km, is okay too, but the lower the better.

Right now there are a few questions that are likely going through your head:

1. Why?
2. How do I set up that low periapsis?
3. Okay, I've set up the low periapsis, now what do I do?

As for the why, the answer is "because physics."  Specifically, Oberth effect.  Basically, the faster you're going when you fire your engine, the more bang for your buck you get.  Therefore, you really want to do all your burns at low altitudes, since that's when you're going fastest.

As for the how:  Use a maneuver node, like this:

1. Get to LKO.  Aim for a circular parking orbit that's as low as possible (under 90 km).  The lower the better for conserving dV.
2. Then set up the map view so you're looking straight down at Kerbin's north pole.
3. Rotate the map so that the Mun is just slightly above the 3 o'clock position
4. Place a maneuver node at the 6 o'clock position and drag its prograde marker until your Ap rises up to the Mun's orbit.  You should see a Mun intercept.  (This will take around 850 m/s of dV for the node).
5. Now grab the maneuver node itself by the big white circle in the middle and gently drag it forward and back.  Watch what happens to your munar periapsis when you do this-- it will go up and down.  Slide the node around until you find the spot where your munar periapsis is the lowest possible.  (It may be underground, i.e. the periapsis marker disappears, meaning "impact the Mun").
6. Drag the prograde/retrograde handles of the node until you've adjusted the periapsis to something low-but-safe, like 10-15 km.
7. Repeat steps 5-6 as needed.  You now have an optimal, cheapest burn to give you exactly the munar approach you want.  It will likely be around 850 m/s of dV, plus or minus depending on how high your parking orbit is.

Okay, now you just coast to the Mun.  You will approach and get really close to the surface.  Right when you're at munar periapsis, burn retrograde.  You'll be going really fast when this happens, don't worry about it, it's a good thing.  The amount you should burn retrograde is enough to circularize your orbit.

Congratulations, you're now in a very low circular orbit around the Mun.  From here, landing is the best deal you're gonna get.  Pick your spot, do a retrograde burn a ways westward from there to drop your orbit into the ground, then do a suicide burn to land.  Ta dah!

Edited December 16, 2015 by Snark

[JoeSchmuckatelli]

Snark - I have to hand it to you; EVERY time you respond to any of my posts - you end up answering 11 questions already percolating in the back of my head (as well as giving me a trick I did not even know about!)  Thanks very much - more stuff to try; i.e. I may actually have a good enough rocket - I just need to fly it better!

 

To the other guys: thanks very much for your responses as well!  Snark's been showing up on a lot of my 'ask a random question about something I just don't ken' posts - so he gets the 'special' thanks.

Edited December 15, 2015 by JoeSchmuckatelli

[JoeSchmuckatelli]

UPDATE: I tried what you guys suggested on a new lander mission: except I chose Minimus due to its lower gravity and the fact I think I can land there without killing my Kerbil (Getting her back (Kerbil women in STEM!) will be another story).

 

Getting the hang of maneuver nodes is an ENORMOUS help - hit Minimus orbit on the first try - still have fuel (not much) in my second to last stage.  Thinking I can use it, plus the last stage fuel to set down gently enough for my scientist to get a soil sample or two.

 

Up until today - all my orbits and other missions have pretty much relied on brute force and guesswork - really nice to see how you're supposed to use maneuver nodes - and how simple they are to use!

[Snark]

10 hours ago, JoeSchmuckatelli said:

Up until today - all my orbits and other missions have pretty much relied on brute force and guesswork - really nice to see how you're supposed to use maneuver nodes - and how simple they are to use!

Yeah, KSP is one of those games where button-mashing just won't cut it.  (I still remember the sense of glee when I "got" maneuver nodes.)  You really have to understand what you're doing. 

 

[Pecan]

14 hours ago, JoeSchmuckatelli said:

UPDATE: I tried what you guys suggested on a new lander mission: except I chose Minimus due to its lower gravity and the fact I think I can land there without killing my Kerbil

Minmus is MUCH easier for first landing missions because of its gravity and wide flat areas.  In order to get there, however, you've almost certainly had to use the manoeuvre nodes to change your orbital inclincation to match planes with Minmus as well as just increase your Ap to get there.  Give yourself another pat on the back for that.