Best orbit for Mun landing (possibly a daft question)

[whoshotdk]

Hello wonderful community,

I'm thoroughly hooked on KSP now and am working my way through career mode. I've managed to get a behemoth of a space station with Science Lab and dock-able lander into a 75km circular more-or-less equatorial orbit around the Mun. However, in true Kerbal style I didn't bother to test it out and so to Jeb's chagrin, the lander has enough fuel to get down to the Mun but not back again to the station at 75Km. I would've thought 200 units of fuel would be enough, but the weight of all Jeb's snacks is just too much. Or, more likely, I'm simply burning fuel really inefficiently.

I'm unable to get back into the game until the weekend and this question has been bugging me, so I'm hoping some kind member can inform me before then.

Basically, I'd like to know - if I lower the orbit of the station to say, 15Km, would that reduce the fuel requirements of my lander? My first instinct is that of course it'll have lower requirements if the destination orbit is lower. However, I then realized that a lower orbit probably equals a lot more speed I think.

Fortunately the station itself is chock full of fuelly goodness, so moving it is most definitely not an issue, even if I have to stick the engines down to 5% and gently fart it into a new position (the center of mass is so far off as to be still back on Kerbin )

Any tips? Thanks!

Edited October 23, 2014 by whoshotdk

[mhoram]

Basically, I'd like to know - if I lower the orbit of the station to say, 15Km, would that reduce the fuel requirements of my lander? My first instinct is that of course it'll have lower requirements if the destination orbit is lower. However, I then realized that a lower orbit probably equals a lot more speed I think.

Yes, the lower your orbit, thel lower your fuel-requirements for the lander.

Getting a ship from a circular 10km orbit to land on the Mun will still take at least 580m/s Delta-V.

[Oafman]

Yes, reaching a lower orbit will save you fuel, but probably not all that much. If you're able to get the lander into a very low orbit with the fuel you have, then lowering the station's orbit might work. But if you can't even get the lander into any sort of orbit, then it doesn't matter where the station is, you won't be able to reach it, so you'll need to get an upgraded lander out there.

[Jimbimbibble]

Fuel savings would likely be a few tens of m/s by lowering the orbit. You also need to consider that at that altitude you would be limited to 10x time warp. However, a different landing/ascent trajectory could make much more of a difference. In the ideal landing, you lower your Pe to just above the surface and get most of your deceleration done just above the ground. You want to time it just right so you finish your burn just above the ground. Ascent follows a similar path where you reach orbital velocity and kick your Ap up to 75km while still just above the ground. Also, you want to be pointed prograde/retrograde at all times.

[radonek]

Basically, I'd like to know - if I lower the orbit of the station to say, 15Km, would that reduce the fuel requirements of my lander?

Sure would. But rasing orbit from 15 to 75km is not that much compared to what getting at orbit takes (about 600m/s) and well within safety margin you should have. Get Flight Engineer mod and ask him what he thinks about your lander. You just need to undock and look at total delta-v in "vessel" tab. If you are under about 1200, you wont make it back no matter what. 1500 should be enough to have a few seconds of "hover" time and a spare bit for small plane change. More then that is a waste since you will be hauling dead weight down and back up.

Also look at TWR of your lander. If you have big engines, you can go close to suicide burn to spare fuel (read: do breaking burn very low) but you need some experience to pull that off. (or lots of loading :-). Generaly speaking, with TWR under 3.0 you stick to shallow descent paths, with 5.0 or more you can fall from higher up. Also, know that ascent from mun work different then from kerbin. You dont need to waste fuel getting off atmosphere, turn to 90 immediately and once you are clear of terrain (should be a second unless you are in really deep crater), you can begin chase the horizon. With apopasis over 5000m you sholud be already leveled and building orbital speed. You can cut off once apoapsis is over 10000m and do circularization burn there. Any orbit over 8000m is good and from there you can either go for your station or bring it down to meet you.

I would advise to bring it to about 32km anyway – its close to surface but high enough for decent timewarp.

[Pecan]

...I would've thought 200 units of fuel would be enough...

The hidden thing in what everyone else has said is "200 units of fuel doesn't say anything about what you can do." A gallon of fuel in your car might let you drive 50 miles. In a Saturn V it probably wouldn't even warm-up the engines.

The critical thing for performance is not just how much fuel but how light the lander (or other vehicle). That performance figure is the "deltaV" and is not shown anywhere in stock KSP. You need a mod like Kerbal Engineer Redux, MechJeb or VOID to show it to you. Otherwise, you can take your vehicle's mass with all the fuel, mass with empty tanks and engine Isp and calculate your deltaV yourself (spreadsheet/calculator recommended) using Tsiolkovsky's rocket equation (link is to Wikipedia).

[michaelhester07]

Delta V is the big number. I really wish Squad would provide fuel measurements in terms of that in stock because this game really is rocket science. They already show the navball maneuvers in terms of deltaV.

Landing on the mun and getting back to orbit I've found requires in the neighborhood of 1500 d/V. 2500 if you want to get back to kerbin with your lander. Without a mod like Kerbal Engineer or calculating the deltaV yourself you won't know if your craft is capable of that.

A simple recipe for a mun lander. Left to right is top to bottom

Mk1 parachute, mk1 capsule, TR-18 separator , FL-400 tank, 3x Landing gear (medium size), LV 909

A last note: any part with a mass less than .01 doesn't actually count towards your ship's weight (it's regarded as weightless). You can go nuts with OX-STAT panels, Batman 100 size batteries, and Communotron 16 antennas, as well as all the small science experiments and your ship still has the same delta V as if you didn't have any of that.

[bakanando]

If you can get to a low orbit around the mun with the fuel, try using RCS thrusters to gain the last few kilometres between the lander and station. Lowering the station closer will also save you fuel in the long run. For example, a hohmann transfer from 60k orbit to 200k will need about 100 delta-v. This is extra delta-v you will need to use every time you want to go down to or up from the mun surface. It will also help with rendez-vous maneuver window calculations. Also, what Pecan said. The engine thrust doesn't change your velocity but your momentum, which takes the ship's mass into account. The more massive/heavier your ship is, the more burn time (and fuel) you will need to change your ship's velocity.

[cantab]

There's always the option of bailing out. A Kerbal's jetpack packs around 600 m/s of delta-V. That's almost enough to make Munar orbit by itself and will certainly get you into a stable orbit after the lander's fuel runs out. Don't forget to grab the science from the command pod before flying off!

Unless you use mods you have fewer instruments on EVA so flying can be tricky. You can still hit M to view the orbital map, that's crucial. Once you're close to your target, you want to make the marker appear still, that means you're heading right at it. If it appears to drift cancel that drift with little dabs of thrust. Use the changing distance figure to judge your approach speed.

Edited October 22, 2014 by cantab

[Kerbart]

The big question is: with how much DV do you start your descent? Engineer and Mechjeb can tell you that.

If you start with 4000m/s then it's your landing technique. If it's 1300 m/s you might want to consider bigger fuel tanks.

For landing it's easy. After your initial descent you have to kill your vertical velocity at one point. That's often done way too early. You know the thrust of your engines. You know the mass. Using Newton's equation f=m×a you can now derive your deceleration (worst case, it'll get better as you burn off fuel). You can then build a table of height vs speed and use that as a guideline as to when to start your burn. Nothing is more exciting as plunging to Mun at 140 m/s at 1000m above the surface and still coming to a soft landing!

[Padishar]

A last note: any part with a mass less than .01 doesn't actually count towards your ship's weight (it's regarded as weightless). You can go nuts with OX-STAT panels, Batman 100 size batteries, and Communotron 16 antennas, as well as all the small science experiments and your ship still has the same delta V as if you didn't have any of that.

This isn't because the parts have a mass less than 0.01, rather it is because certain parts are explicitly marked in their .cfg file as having no physical significance. This flag can be set on any part but most of the ones with it set in stock KSP are small parts that can only be surface attached. This includes the 100 & 400 batteries, the OX-STAT panel, the small experiments, lights, ladders and the plane landing gear (and possibly a few others).

[Streetwind]

Landing on the mun and getting back to orbit I've found requires in the neighborhood of 1500 d/V. 2500 if you want to get back to kerbin with your lander.

That actually depends somewhat on where you land, what kind of orbit you land from and take off into, how high an elevation you land on, how much TWR you have, and how much reckless luck aptitude you have for eyeballing that suicide burn correctly. I've had a Mun lander that, with 1850 dV, was able to brake itself out of a 10 km orbit, land, launch again into a 10 km orbit and then drop its periapsis into Kerbin's atmosphere. Well, most of the time. On the really far northern and southern biomes, I sometimes ran out of fuel on the transfer back to Kerbin and had to get out and push a bit. I'd probably have needed 1900 for those.

So my personal target number for return capable Mun landers is 2000 dV, just to have a buffer for when I accidentally come down on a steep slope and need to deflect. But the number people are comfortable with will vary from person to person, simply because their approaches to landing are different.

A last note: any part with a mass less than .01 doesn't actually count towards your ship's weight (it's regarded as weightless). You can go nuts with OX-STAT panels, Batman 100 size batteries, and Communotron 16 antennas, as well as all the small science experiments and your ship still has the same delta V as if you didn't have any of that.

It doesn't depend on the mass of the part at all, it depends on whether the part is defined as physicsless in its part.cfg file

[whoshotdk]

Thanks very much for all your kind replies;

I installed Mechjeb and am now fighting an internal battle not to use it to do everything for me

However, its custom panel feature is awesome and I can now see all the important stuff like weight, delta-v and planetary angles all in a single window. Smart A.S.S and auto-executing manoeuvres is so helpful. The best - absolute best - indicator I’ve found though, is the ‘suicide burn’ countdown. MechJeb should totally be built-in by default in my opinion!

So, my lander had a delta-v of nearly 1300 (vac) - maybe doable by some but certainly wasn’t enough for me. I was able to get some time last night to take the lander back to Kerbin and re-build it.

Not launched yet as I can’t afford to launch the bloody thing, but now at least it itself has 3018 dV. I’m quite confident now that even I’ll be able to get down to the Mun and back from 75Km

oafman: thanks for the suggestion, I have indeed flown the lander back to Kerbin to do an upgrade.

mhoram, radonek, kerbart: thanks for the minimum delta-v requirements; its great to have a figure to aim for without having to trial-and-error it.

Jimbibble: thanks for the heads-up on the time warp issue; I didn’t think of the pain-in-the-ass that would be.

pecan, michaelhester07; great info on what delta-v actually is - ill be honest I was basing everything on the isp of my engines; though that is another number I’m not totally au fait with yet.

bakanando: so many times I've had to resort to RCS thrust to complete a burn properly and in some cases I’ve even had to get out and push Unfortunately I’d decided to go down with only 40 units of mono-propellant so even that wasn’t enough this time!

cantab; i was so ready to consider a bail out upon your suggestion but luckily i was able to retrieve a not-too-old save from my backup machine, hence I could start the game from where the lander hadn’t gone down to the Mun yet. If that hadn’t been possible I would definitely have given it a go.

padishar, streetwind: I had no idea i could stuff my vessels chock-full of batteries, ox-stats without fear of increasing the overall mass or de-centering the center of mass. Now I can keep the lights on

Thanks again for all your help; can’t wait to get back to the game!

[MarvinKitFox]

So, my lander had a delta-v of nearly 1300 (vac) - maybe doable by some but certainly wasn’t enough for me.

1300, from a 75km Mun orbit.

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ummmm... erm.. Yes i think that is enough.

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hmm.. you *are* planning on landing on some of the higher bits, right?

From 10km orbit, 1300 is plenty.

If your ship has decent TWR, and you have a Black Belt in suicide-burn-landings.

Personally, I would want 1500.

Query:

WHY did you plonk your refueling station in an equatorial orbit. Don't you plan to go to the Polar regions at all?

On a polar orbit, you get to fly over every single square foot of the Mun, eventually. No fuel penalty to land in a funny spot.

From equatorial, you would have to pay a penalty of almost 1400m/s to fly to a pole, land, and return. In excess of the about 1250 needed for a landing & return directly under your station path.

[Pecan]

Congratulations on all that. Squad won't put the numbers in-game because they say it's "maths, not fun" but most of us who actually play it (~80% in a fairly recent forum poll) think, as you say "its great to have a figure to aim for without having to trial-and-error it."

Now you can check your vehicles' deltaV you will almost certainly want a deltaV map (others are available), which tells you how much deltaV you need to move between different bodies in KSP. There is a discussion of all this in the tutorial in my signature (especially Chapter 4, section 8) and right at the end, Appendix 2, is a summary of deltaV requirements in case you prefer thatto adding them all up on the deltaV maps.

[radonek]

I installed Mechjeb and am now fighting an internal battle not to use it to do everything for me

You are stronger then me then. Long time ago I decided that only way to stay firm is to not install mechjeb at all. Which means that my first attempt at docking was two hours of nerve-stretching odyssey :-) but definitely worth every second.

Unfortunately I’d decided to go down with only 40 units of mono-propellant so even that wasn’t enough this time!

That is more then enough. RCS is very inefficient – you will be better off if you take same weight in ordinary fuel. You don't really need rcs outside terminal phase of docking. And if you do things right, you can dock decent lander with about 2 units of monoprop. (by hand, I hear mechjeb does not exactly shine in this)

Anyway, for first attempt its better to skip docking and go for direct ascent. Its inefficient since you carry your departure fuel and chutes down and back up The Well, but Mun's well is not that deep and its a WAY simpler mission profile.

[whoshotdk]

1300, from a 75km Mun orbit.

Query:

WHY did you plonk your refueling station in an equatorial orbit. Don't you plan to go to the Polar regions at all?

On a polar orbit, you get to fly over every single square foot of the Mun, eventually. No fuel penalty to land in a funny spot.

From equatorial, you would have to pay a penalty of almost 1400m/s to fly to a pole, land, and return. In excess of the about 1250 needed for a landing & return directly under your station path.

Holy crap, I didn't know that! Now that I think about it though it does seem obvious. I kinda ended up in a more-or-less 0deg inclination when I arrived (having launched into a similar orbit round Kerbin first) so I just left it. Funnily enough, I usually DO end up in a high inclination orbit (not on purpose however) whenever I aim for another body but this time I was happy to keep my planes (Kerbin, Mun, Station) in approximate alignment. Now I know better; perhaps I'll try to push the station round to a more polar-esque one.

I'm more of a no-belt-brown-trousers kinda guy when it comes to suicide burns, but Mechjeb's suicide burn countdown timer is very helpful.

Thanks for the info!

[whoshotdk]

Anyway, for first attempt its better to skip docking and go for direct ascent. Its inefficient since you carry your departure fuel and chutes down and back up The Well, but Mun's well is not that deep and its a WAY simpler mission profile.

Direct ascent; you mean straight from Kerbin and back? Yeah I am wondering if I should've stuck with that; I blew my 700k budget on the station and have yet to reap a reward. My lander actually has 4x goo, 4x thermometer and 1x science junior (all the science tech I have right now) so I aim to collect science with those from every biome on the Mun. Once thats over I guess I'll use the station as a re-fuelling stop-off on the way to other planets so hopefully it'll repay me eventually.

[cantab]

On the downside if you're meeting a station in polar orbit you need to wait for a suitable departure window from LKO, or expend delta-V matching inclinations. The return trip's not so bad but you'll still be waiting for things to line up.

[radonek]

Now I know better; perhaps I'll try to push the station round to a more polar-esque one.

Baaad idea that one. Plane changes are prohibitively expensive on low orbits and you are better off on equatorial orbit anyway. Polar orbits are great for sightseeing (albeit kinda boring since you spend a lot of time waiting ) but equatorial is more efficient (if you are orbiting in direction of planet's rotation). If you really want polar orbit, do it on arrival, imediately after SOI change. Plane changes are cheap when you are slow. Once you fall down the well you gather a lot of speed and any planar/radial maneuvers becomes a huge pain.

Direct ascent; you mean straight from Kerbin and back? Yeah I am wondering if I should've stuck with that; I blew my 700k budget on the station and have yet to reap a reward. My lander actually has 4x goo, 4x thermometer and 1x science junior (all the science tech I have right now) so I aim to collect science with those from every biome on the Mun. Once thats over I guess I'll use the station as a re-fuelling stop-off on the way to other planets so hopefully it'll repay me eventually.

That is a good plan, but you will be doing it all over again once you aquire seismo and gravioli detectors. Its hard to get them soon, so I usualy make one or two "direct" visits (and flyby to collect hi-over and low-over science) to get a tech boost. Polar biomes are good candidates - they are too far from equatorial orbit anyway.

[merendel]

I installed Mechjeb and am now fighting an internal battle not to use it to do everything for me

I personaly prefer Kerbal Engineer Redux. Can get most of the relivent information but no lazy mode tempting me to use it all the time. One notable lack is no suicide burn CD but its possible to get a prety good guess. Set a manuver node right at the impact point and pull the retrograde till it starts fliping. Thats how much you need to burn to stop and you can get your burntime for that. KER will tell you time to impact so as long as you start burning before burntime is less than that your normaly OK.

SmartASS is nice but its goten me into trouble plenty of times, I just prefer manual. I'm also runing remote tech which has something similar to a striped down smartASS and auto node execution. Normaly its for probs so you can preplan a maneuver and have it execute even when they are uncontrollable due to signal constraints but I did tweek it so command pods have access to that bit. Use it for those long burns with ions or motherships with only a couple LVN's

[Padishar]

One notable lack is no suicide burn CD but its possible to get a prety good guess.

KER 1.0 recently added a variety of suicide-burn related readouts. See this post for details...