I want to get orbit the Mun and get back to Kerbin more efficiently

[JackBush]

Nine times out of ten, once I've gotten into orbit around Mun (if I get that far!), I don't have sufficient fuel left to get back to Kerbin. I've tried adding fuel to the bottom stage, but that hasn't helped. I've tried adding to the top (last stage) but with same results. I have done it, but only about twice, I think. I'm doing something wrong but I don't know what.

My purpose is to do more science around the Mun. I'm not ready to try a landing yet. I'd like to get this whole business down pat first.

Edited November 12, 2015 by JackBush

[Tex_NL]

Plenty of good tutorials out there if you know where to look.

[Oafman]

Please post a picture of the whole craft at launch, so people can see where extra efficiencies might be gained.

[Snark]

+1 to posting pics of the rocket.

A Mun mission requires the following dV:

1. Get into LKO, around 3000-3400 m/s depending on ship design
   
2. Transfer from LKO to Mun, around 800 m/s
   
3. Enter munar orbit, around 300 m/s
   
4. Land on the munar surface, around 800 m/s with an efficient suicide burn
   
5. Take off from the mun, 800 m/s with an efficient ascent profile
   
6. Head home to Kerbin, around 300 m/s
   
7. Reentry and landing, 0 m/s.
   

Possible problems you may be having:

• You just don't have enough dV. If you know accurately how much dV you have in each stage of the rocket, that takes a lot of trial-and-error guesswork out of it. The Tsiolkovsky rocket equation helps with this. In KSP terms, to find the dV of a stage: divide the starting mass by the ending mass, take the natural logarithm, multiply by engine Isp, multiply by 9.81, and that's your dV in m/s for the stage.
  
• Inefficient munar capture. Make sure you're approaching the Mun with a low Pe when you depart from Kerbin, so that you can make your Mun capture burn at a low altitude to take maximum advantage of Oberth effect.
  
• Inefficient landing. This is step 4 above. Search for "suicide burn" in the tutorials and learn how to do a good one.
  
• Inefficient munar takeoff. Make sure your ascent profile is reasonable. Basically, crank your ship over to practically horizontal immediately upon liftoff, then punch the throttle to 100% and get yourself into as low a circular orbit as you can.
  

Those are the biggest pitfalls to a munar mission. It's also possible to get fancy with an Apollo-style mission, where you leave an orbiter around the Mun and descend to the surface with just a lander, which comes up and docks before heading home. This has the advantage that you don't have to transport the fuel for step #6 down to the surface and back again, but it's a more challenging design and if you're new to munar landings, may be better to stick with a simple land-and-return.

Also, one last "emergency cord" that you can pull: If it turns out that you have almost enough fuel but not quite to get home from Kerbin-- i.e. if you can get into Munar orbit again after landing, but then can't get home because you don't have that last 300 m/s or so-- then there's one measure you can do in extremis: send your pilot home without a ship. Kerbal EVA packs are ridiculously overpowered and have around 300 m/s of dV in them, which is enough to send you home from orbit around the Mun. And kerbals are surprisingly tough: they can reenter Kerbin's atmosphere, fall out the sky, and land on the surface (sea or land, doesn't matter) unharmed.

So if it comes to that, then once you get into munar orbit, you just go EVA, take the science out of your instruments and/or command pod, and go home. For a "naked" kerbal, you want to set your re-entry periapsis much higher than you would for a ship, since kerbals are both draggier and less heat-resistant than ships are. A good Pe for an EVA kerbal is up around 45 km (or a smidgeon higher), as opposed to around 30km for a ship.

Edited November 5, 2015 by Snark

[FlyingPete]

There's also the somewhat amusing method of 'get out and push' if you're just a few m/s short of getting a re-entry trajectory. Point the ship prograde, then EVA a Kerbal, get behind and use the EVA pack to thrust forward. Theoretically this gives you infinite dV (since the EVA pack is refuelled when you return to the ship) but it's not really practical for anything more than those last few m/s. Once the Pe is inside the atmosphere, you'll go back to Kerbin eventually even if it takes a few passes.

[Tatonf]

For beginners I would recommend a 6500-7000 m/s delta-V budget.

[Highguard]

Section 3 of this post by Pecan is a really good explanation of how to design a vehicle for Kerbin lunar missions. Those are for landings, but the principles will help your missions too.

[GeneralVeers]

Nine times out of ten, once I've gotten into orbit around Mun (if I get that far!), I don't have sufficient fuel left to get back to Kerbin. I've tried adding fuel to the bottom stage, but that hasn't helped. I've tried adding to the top (last stage) but with same results. I have done it, but only about twice, I think. I'm doing something wrong but I don't know what.

My purpose is to do more science around the Mun. I'm not ready to try a landing yet. I'd like to get this whole business down pat first.

If redesigning the lander is something you're willing to do, start small. I have experimentally verified the following myself: a lander with nothing but the basic 90-unit fuel tank, a single Spark engine, the black one-seat command capsule, and the basic nosecone parachute can take off from the Mun, reorbit, make it back to Kerbin, and land safely with fuel to spare.

I've tried adding fuel to the bottom stage, but that hasn't helped.

You added fuel......did you also add engines? Just adding fuel will allow your ship to fly longer, but it will also fly slower, and as you pile on more fuel tanks, each additional unit of fuel will do less and less for you. The basic rule is that you want to get out of Kerbin's atmosphere and into orbit as quickly as possible. So, when adding oomph to the liftoff stage, put on an addtional engine or two.

I've tried adding to the top (last stage) but with same results.

Adding weight on the top is usually counterproductive. As a general rule with the rockets I build, the launchpad weight is five to ten times the weight of the final payload that makes it into Kerbin orbit. Add one pound at the top, and you need ten pounds at the bottom to lift it. So focus on taking weight off your lander. Get a lander design you like, then work on putting enough muscle on the liftoff and transfer stack. If your liftoff comes up just a tad short, grab a few solid rocket boosters and radial-mount them to the first stage.

[Streetwind]

Plenty of good tutorials out there if you know where to look.

( embedded video )

Attention: that video is from summer 2013 and will not necessarily represent a working solution for the current state of the game. Should have googled a little harder perhaps?

The real treasure trove of up-to-date tutorials can be found here: http://forum.kerbalspaceprogram.com/threads/28352

[JackBush]

Please post a picture of the whole craft at launch, so people can see where extra efficiencies might be gained.

This is the craft I've been using, with and without additions to try to make life easier once I have the orbit on the Mun. As I said, at this point I'm not ready to try a landing, just collect science and get home.

http://imgur.com/NiI2MhB

[Tex_NL]

Attention: that video is from summer 2013 and will not necessarily represent a working solution for the current state of the game. Should have googled a little harder perhaps?

The real treasure trove of up-to-date tutorials can be found here: http://forum.kerbalspaceprogram.com/threads/28352

True, some things have changed. Most notably the aerodynamics and a some engine tweaking here and there. But the basic theory is still the same.

The most important thing that has not been changed is the sheer amount of very useful tutorials that can be very easily found if you actually bother to look for them.

[Alshain]

This is the craft I've been using, with and without additions to try to make life easier once I have the orbit on the Mun. As I said, at this point I'm not ready to try a landing, just collect science and get home.

http://imgur.com/NiI2MhB http://imgur.com/NiI2MhB

Your swivel is a bit under-utilized. It can handle a much higher load than that without the SRBs. You can make it cheaper. Here is a full lander at only 20k funds.

OP, I don't have a landing tutorial, but to get the most out of your launches, watch this. The more you can save on launch, the more you get to use going to the Mun.

Edited November 5, 2015 by Alshain

[manaiaK]

Jack,

congrats on learning how to post a picture!

As Alshain said, you want most of the fuel in the lower stage. Move one of the 400s down below the decoupler, and maybe add another 200 down there too. Note that in Alshain's craft the liquid-fuel engine fires at the same time as the boosters (see "more minor" tip below).

A few small things:

The Mk1 Cockpit is heavy compared to the Mk1 Command Pod, so use that instead. With the pod, you'll only need one round Mk16 parachute on top, for an additional mass saving of one less parachute.

More minor: if you put the next-stage engine into the same stage as the decoupler below it, you get an instant start-up so you don't lose any speed from atmospheric drag. This applies to the engine in stage 4 and the decouplers in stage 5. Click on the engine symbol and drag it down into stage 5. Or use Alshain's method -- drag the engine down to stage 6, in which case add even more fuel for the engine.

The Science Junior can be brought back with the service bay and pod if you put the heat shield and decoupler underneath it. You should still only need the one parachute, I think. If you discard it you will need your Kerbal to EVA to it and collect the science from it before returning to Kerbin.

If you can get to the Mun OK and get into orbit, then the problem might be how you're trying to get back. This is a two-stage process. First burn so you *just* escape the Mun (as shown in Map view). This should only take 200 m/s of delta-vee (not very much fuel.)

Once you are back in orbit around Kerbin, burn retrograde until your Pe drops into the atmosphere of Kerbin. (In map view hover your mouse over the Pe while you're burning; do it slowly using the Shift key rather than Z.)

(This requires a variable amount up to say 500 m/s depending on the previous maneuver and where you do it.) This second burn is most cheaply done at Ap, but anywhere within 30 degrees of it will be fine.

[Snark]

+1 to manaiaK's excellent advice on saving mass by using the Mk1 pod with a single chute.

Previous posters give good advice about moving fuel downward from the 3rd stage to the 2nd. A word as to why that's a good idea:

An efficient multi-stage rocket should exponentially shrink with each successive stage. That is, you want each stage to be a fraction of the last one. (Exactly what the fraction should be depends on your design, but in general it should be no more than half). You're packing 9 tons of LFO, but you've got 4 tons in the 2nd stage and 5 tons in the 3rd, which is backwards. If you rearrange that to a better exponential pattern (i.e. the 3rd stage has half the fuel of the 2nd), you'll get more bang for your buck. So move some fuel downward, so that you have 6 tons in your 2nd stage and 3 tons in your 3rd stage.

Also: How do you have the thrust on your SRBs configured?

One feature that a lot of folks who are new to KSP don't realize: even though you can't throttle SRBs while they're burning, you can decide their thrust level at design time in the VAB (right click on them and choose a thrust percentage; they're 100% by default).

Here's why it's important: Just eyeballing your photo, I'd estimate your launchpad mass at around 37 tons. Thumpers get 250 kN thrust at sea level, so that would put your TWR at about (250 * 3) / (37 * 9.8) = 2.07. That's pretty high-- you'll end up going too fast too low, and wasting a lot of your fuel fighting aerodynamic drag. (You're getting a lot of extreme Mach effects when you launch, amirite?)

To avoid that, it's best to have a launchpad TWR of no more than 1.5. To get that, you'd need to set your SRB thrust level at around (37 * 9.8 * 1.5) / (250 * 3) = 72.5% (adjust as needed if I've guessed your mass wrong at 37 tons). Doing this will give you a longer, gentler ride: you'll be going slower when you're lower down (so not so much wasted fuel fighting drag), and by running at lower thrust level, your SRBs will burn longer.

In fact, you could even pack some more SRBs on there. Even if you're bumping up against part-count limits, dropping down to 1 parachute from 4 will save you three parts, so you can spare 1 more SRB with nosecone and decoupler, right?

So you could go from 3 Thumpers to 4. If you do that, you'd need to lower the thrust even further-- your mass would go up to around 45 tons, so to maintain a 1.5 TWR, you'd want the SRBs to be at (45 * 9.8 * 1.5) / (250 * 4) = 66%.

Even better, you can stretch your SRB burn time even farther if you take those four SRBs and give them different thrust levels. Instead of adding one group of 4, you can add two groups of 2. Give one of them a thrust higher than 66%, and the other a thrust lower than 66% by the same amount-- for example, you could run pair A at 80% and pair B at 52%. All four of of them would be activated together at launch, but pair A will burn out (and be jettisoned) first, giving you additional burn time on pair B until it burns out and you activate the Swivel.

[KerikBalm]

Well, nobody is going to mention a free return trajectory? if he just wants science from around the Mun, then a Munar flyby should be fine, no?

Or does he want to orbit and take EVA reports from above all the biomes possible?

[the_pazter]

Try using Asparagus staging and using three stages,

1- Launch

2- Orbit insertion and transfer

3- Capture and Free Return

Let me know how it goes

- the_pazter

[Alshain]

No Asparagus. Asparagus + KSP 1.0 = BAD. LFO engines cost way too much to be using them for boosters. Use a 2 stage center stack with SRB boosters.

[JackBush]

Excellent stuff. Thanks.

- - - Updated - - -

This is great. Thanks. Where do I find out what my TWR is? I've just added KER, by the way, but I'm not sure what all it's telling me.

- - - Updated - - -

How do you do a Munar flyby?

[Snark]

Where do I find out what my TWR is? I've just added KER, by the way, but I'm not sure what all it's telling me.

I don't use KER myself, so someone else could help you with that. But TWR is straightforward enough that you can get it without resort to KER.

It's just your total engine thrust divided by your weight. So if you want to know your launchpad TWR, just add up the kN thrust of all the engines you fire at takeoff (you can get that number off the parts list in the VAB, be sure to use the ASL value and not the vacuum value), then divide by the total mass of your ship (it's available in the engineer's report at bottom right), then divide by 9.81 m/s2 (Kerbin gravity), and there you are.

I hear KER is very informative, I expect it probably just tells this number somewhere.

How do you do a Munar flyby?

This just refers to flying past the Mun (i.e. entering its SoI) without necessarily entering orbit or landing. It's cheaper in terms of dV than orbit, since you don't have to spend any fuel once you get there-- just go flying past. If you arrange it right, you can get a "free return" and come back to Kerbin afterwards without spending fuel, so the only fuel it takes beyond getting to low Kerbin orbit is the roughly 800 m/s burn to boost out to the Mun.

There are lots of good tutorials out there about how to go to the Mun, since that's generally the first thing players want to do after getting to Kerbin orbit. The TL;DR here is: get into low Kerbin orbit, wait until you see the Mun peeking over the eastern horizon, then burn prograde until your raise your apoapsis up enough to get a Mun encounter (it will take around 800 m/s). There's a lot more detail around that if you care about more details (such as whether you want to come back home again, whether or not you have maneuver nodes unlocked in the tracking station, etc.) but that's the gist of it.

[JackBush]

Thanks for your help, Snark.