Building an efficient rocket

[Scarredclown]

I'm triyng to build a bigger and more efficient rocket, but the bigger i built, the more fuel it need to get in orbit resulting in getting at the same point in orbit with the same amount of fuel left... How to know if your rocket is balanced in weight and speed and fuel ?

[Claw]

Well, unfortunately in pure stock you have to do some work to figure out how much delta V and TWR your rocket has. If you are unfamiliar with those terms, deltaV is a measure of how much maneuvering your rocket can do and TWR is the rocket's Thrust to Weight ratio. So those are the numbers you need to balance in weight, speed, and fuel.

If you're not using any mods, get a pencil and paper out (or build a spreadsheet) because it's a lot of numbers to keep track of:

To figure out how much deltaV your craft has, you need to know the fueled mass, empty mass, and ISP of the engines. Then use the rocket equation: http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

For TWR, you need the mass of your rocket and the cumulative engine thrust for each stage. Then it's simply Thrust/Mass.

If you are not opposed to mods, MechJeb (MJ) or Kerbal Engineer Redux (KER) will provide you deltaV, TWR, and a whole ton of other information. I have used both and prefer MechJeb (mostly for layout reasons), but for some reason some people have a huge personal problem with MechJeb. MJ includes an autopilot (if you want to use it), but both will provide the info you're asking about.

[Worst at Video Games NA]

When trying to figure out rocket efficiency, I always remind myself that each stage negatively impacts the stages below it. i.e. if you want your final stage to have several fuel cells and lots of mass, that means every subsequent stage that's responsible for lifting that monstrosity has to be that much more powerful, and by the time you reach the very first stage it'll take a tremendous amount of impulse to move your rocket even a little.

If you keep that in mind, and make sure the stages on top are very light, allowing your stages below it to do most of the work, then your rocket should be much more efficient. Think pyramids when deciding how to shape your rocket.

[mhoram]

A while back I held a challenge about efficient rockets with respect to payload fraction. You might find a few ideas there.

I usually design my rockets from top to bottom and try to keep TWR as low as possible while still being able to fulfill the goal I have in mind for the respective stage.

A low TWR means less mass for engines.

If you don't use addons, try to reduce the number of engines and see if you can still get to orbit. If not, then you probably need the engine.

Another thing to keep in mind is the ISP of the engines. A engine is more efficient, if it's ISP is higher - this value is visible in the VAB.

[Kasuha]

There's a ratio called payload fraction. It is ratio of rocket's initial mass to the mass of payload it is able to carry to its destination. For travel from Kerbin surface to Kerbin orbit about the best reachable ratio is about 20%, meaning if you try very hard, you may deploy 1/5 of your initial rocket's mass in orbit. In most cases rockets are nowhere near that efficient and payload fraction 5-10% is common.

That means if you have 100 tons of payload, you'll need to launch about 1000 tons of rocket.

With given set of engines, there's no way around that - you may get above 20% with the recent engines but I strongly doubt you can beat 25% limit. The only thing you can improve with bigger rockets is granularity of engine thrusts and fuel tank sizes.

If you want to be more efficient, you need to build spaceplanes or at least use jets for initial lifting as they have ridiculously high efficiency. They may increase your part count a lot, too, and spaceplanes have many of their own limits as well - they're not really suited to be scaled way up in current KSP.

Edited June 1, 2014 by Kasuha

[Raptor831]

Also, you should consider how you are flying your rocket into orbit. If you fly to 10km pointing straight up, pitch over 45 degrees, then burn until your Ap is at the right spot, then you're wasting some fuel. There was an "Efficient Orbit" challenge here somewhere (found it: link), which you could mine for details. There are some MechJeb auto-ascent settings you could use in that thread, so you should be able to mimic some attempts.

For stock KSP, you want an initial TWR (thrust-to-weight ratio) of around 1.5 - 2. Any more, and the air starts pushing back too much in the lower atmosphere and you waste fuel. Once you're above around 30km it doesn't matter. It's also more efficient to throtle back in the upper atmosphere as you travel to apoapsis. If you have a huge burn to make at Ap, then you'll waste some fuel during that burn as well. Ideally you want one long burn from launch to Ap that gets you in a circular orbit.

Hope that helps, and good luck!

[cantab]

I've found the advice here useful

http://wiki.kerbalspaceprogram.com/wiki/Basic_maneuvers

During the vertical phase of your ascent, watch your speed against those "checkpoint" values. If you're going much faster, throttle back your engines. For solid boosters, you can right-click them in the VAB and reduce their thrust. The lower thrust will make them burn for longer so you don't lose out. If you're having to throttle way back, use smaller or fewer engines. If you're going much slower, you could add some boosters or use more or bigger engines, though a slow launch isn't all that bad IMHO.

The overall method there isn't the most efficient, but it's not bad.