efficiency

[Cannon]

which ship should do the best and why?

[minotard1]

You have to quantify "doing best." Do you want the fastest acceleration, then the top ship. You want the greatest delta-V, then the bottom.

Install Kerbal Engineer or MechJeb and it will calculate these things, and more, for you.

[TheCanadianVendingMachine]

The one with the highest ISP. That is the most efficient

[Dave Kerbin]

The one with the highest ISP. That is the most efficient

Efficiency is a balance between ISP, fuel and mass. Having the highest ISP engine won't help if the engine contributes too much extra mass. The Rockomax 48-7S is a good example of this. Despite having an ISP of only 350 it is still more efficient then the popular LV-909 and LV-N for many small ships simply because it weighs so little. You can see an example of this here, where the 48-7S can take a little 3 ton ship to Duna and back for less fuel then an LV-909 or LV-N.

In fact the question being asked, which of the three designs is most efficient, is answered not based on their ISP but on their weight. The last design, the one with the light LV-909, is the most efficient (>3000 m/s). However it's not because it has the highest ISP. Even if it's ISP was the same as the second one (I'm assuming that's an LV-T30) it would still have about 500 m/s more delta-v just because the engine weighs less then half and that is a significant part of the spacecrafts dry weight. For the same reason the first one, even if it had the same ISP as a nuclear engine (LV-N) would still be last by a huge margin.

[Red Iron Crown]

There's no parameters for the ship's purpose, so it's hard to say which will be best. The first has the best TWR, the last the most delta-V, the second in between the two.

[Cannon]

how do i calculate lsp?

purpose is to get as far as possible

[Kasuha]

You can find the Isp value in part description in VAB/SPH. Note that it depends on atmospheric pressure so all engines have two values, one for Kerbin's atmosphere at sea level, the other for vacuum.

Edited September 6, 2014 by Kasuha

[Jimbimbibble]

Every engine has this parameter listed in the VAB. Right click on any part to show/hide more information. I would strong recommend getting Kerbal Engineer, a very useful mod that will give you a lot of info about your ship. Here's the link.

http://forum.kerbalspaceprogram.com/threads/18230-0-24-2-Kerbal-Engineer-Redux-v0-6-2-10-and-v1-0-7-1

[Dave Kerbin]

To explain this in full lets examine how efficiency is calculated. It is assumed here that efficiency = delta-v. The more delta-v you have the farther you can go (unless you are trying to take off/land, in which case you'll also need a certain minimum thrust otherwise you won't go anywhere). In car terms delta-v is your range, so for example I can drive 350 miles (560 km) on a single tank of gas, or my spaceship can produce 350 m/s of delta-v with the fuel on board.

Delta-v is calculated from three numbers:

• ISP. This is the specific impulse generated for each unit of fuel burned, essentially how much 'push' you get. It is similar in a way to the MPG (L/100km) of your car but ISP does not yet take into account how heavy your ship is.
  
• Fuel mass. This is how much fuel you have to burn, more fuel means more impulse adding up. Like with a car if you take your fuel tank size and multiple it by your MPG you'd have an idea of the range of your car.
  
• Payload mass. If you load up your car with lots of weight then your MPG will go down because there is more to push. The same happens for spacecraft - your actual range depends on how much weight is stacked up against your ISP.
  

The actual formula looks like this:

Delta-v = LN(WetMass / DryMass) * ISP * 9.81

Now the engines you picked actually have very similar ISP (360s, 370s and 390s) and only differ by weight and thrust (how fast they accelerate). To help illustrate how weight and ISP work I've got a little chart here showing how these engines would perform if their ISP values where changed. 300s is very poor, while 800s is very good (same as a nuclear engine). However you can see that even given the poor ISP the very light engine will still get the ship farther. This is because even if the engine is twice as efficient at burning fuel, if you need to carry 3 times the weight it will have less range then the lighter craft.

[table=width: 800, class: grid]

[tr]

[td]Capsule & Fuel Tank[/td]

[td]Fuel[/td]

[td]Engine[/td]

[td]Total Weight[/td]

[td]300s ISP[/td]

[td]390s ISP[/td]

[td]800s ISP[/td]

[/tr]

[tr]

[td]1.1 tons[/td]

[td]2 tons[/td]

[td]9.75 tons[/td]

[td]12.85 tons[/td]

[td]497 m/s[/td]

[td]647 m/s[/td]

[td]1327 m/s[/td]

[/tr]

[tr]

[td]1.1 tons[/td]

[td]2 tons[/td]

[td]1.25 tons[/td]

[td]4.35 tons[/td]

[td]1812 m/s[/td]

[td]2355 m/s[/td]

[td]4832 m/s[/td]

[/tr]

[tr]

[td]1.1 tons[/td]

[td]2 tons[/td]

[td]0.5 tons[/td]

[td]3.6 tons[/td]

[td]2386 m/s[/td]

[td]3102 m/s[/td]

[td]6364 m/s[/td]

[/tr]

[/table]

Now there is a reason these different engines exist in the game, even though the last one is in theory the most efficient in any situation (it has the highest ISP and the lowest weight). That reason is thrust. Thrust is the amount of force being applied by the engine at once (kN). In a way it is related to how quickly the engine is burning fuel and how fast it is applying the delta-v. If you had two engines with the same ISP but one had twice the thrust of the other then it would burn it's fuel twice as fast. In car terms thrust would be speed - both cars might have the same range of 300 miles but one has a top speed of 100mph and the other one can only do 50mph.

Once you are in orbit speed (thrust) doesn't matter as much. You aren't going to fall back to the ground so you have lots of time to get where you want to go which means if the slower car has a longer range you can take it. However when you are trying to get into orbit (or land without crashing) you need a certain amount of thrust because you are fighting gravity. Think of it like trying to drive up a hill - that slow efficient engine might not have enough power and it just falls back down.

The 3rd engine you picked, the LV-909, has a thrust of only 50 kN. That is very weak and it means that it can only lift a small amount of weight in Kerbin's gravity (9.81 m/s). The actual amount can be easily found by dividing: 50 / 9.81 = 5.81 tons. So if your total mass is more then about 5.8 tons your rocket will just sit on the launchpad because the engine can't get it off the ground. By comparison the largest engine you picked has a thrust of 3200 kN. That means it can lift about 326 tons off Kerbin which is much more useful if you want to build something bigger.

[Red Iron Crown]

purpose is to get as far as possible

The third design should carry you furthest. Enough TWR to get off the pad, most delta-V.

[LordFerret]

I'm running vanilla, no MecJeb, no nada ... so for me a lot of design is trial and error - a lot of manual calculation and pre-flight testing ..... SWAG!

[KerikBalm]

how do i calculate lsp?

purpose is to get as far as possible

Isp is listed for each engine, you dont calculate it.

What you want is the most delta-V (to go the farthest...)

\Delta v = v_\text{e} \ln \frac {m_0} {m_1}

http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

Add up the weight of the parts with and without fuel, divide the weight with fuel by the weight without fuel. Take the natural logarithm of this, and multiply it by the ISP.

The one with the highest value is most efficient for most purposes (note that launching from kerbin requires a thrust to weight ratio greater than 1, if you don't have that, you won't take off, and you don't get anywhere, a TWR just barely above 1 will waste too much fuel fighting Kerbin's gravity.)

[Red Iron Crown]

\Delta v = v_\text{e} \ln \frac {m_0} {m_1}

We need LaTeX functionality on the forums, stat!

Seriously, if ever a community forum needed robust equation formatting it's this one.

[cantab]

The first one is clearly silly, but to elaborate, too much mass of engine compared to the fuel and payload. Using the KR-2L would be equally silly and inefficient, despite that engine's good vacuum Isp.

The other two are both sensible. In deep space the design with the LV-909 will be better, as much because the engine's lighter as because it's more efficient. Not entirely sure about launching from Kerbin, there's probably not an awful lot of difference.

[maccollo]

The third one is the best by any reasonable figure of merit .

It has the best mass fraction and ISP.

It has 50% more delta V than the second one.

On top of that it's also the lightest and the cheapest.

It's TWR, 1,42 on Kerbin, is good enough for the vast majority of situations.

[Rusty6899]

Isp is the specific impulse and is calculated by the following equation...

Isp = thrust / (mass flow rate * standard gravity)

Standard gravity is 9.81m/s2 (This value is used anywhere in the universe it isn't just to local gravity)

Mass flow rate is the mass of fuel that is burnt each second at full thrust.

KSP gives you the Isp, so you don't really need to worry about calculating it. The game fairly recently started to include mass flow rate in it's part description, making it easier to calculate the length of each burn (previously you had to calculate mass flow rate from the Isp and thrust).

Basically, the Isp is a measure of fuel efficiency. It is easier to understand if you consider that it is just asking the questions; How much thrust will I get? And for how long? Higher thrust engines will burn through fuel much quicker than low thrust engines, in general, and tend to have lower Isp.

The Isp will not always be the most important factor in selecting an engine for a stage. Often a lighter engine, with a lower Isp will allow you to take a smaller amount of fuel to fulfill your delta-V requirements. Keeping your mass low will often be as great a consideration. Remember that for every tonne of payload, you will generally require roughly 10 tonnes to get into orbit.

I would strongly recommend that you learn how to calculate delta-V and make an excel spreadsheet to allow you to (fairly) quickly design ships. Failing that, you would be best getting a mod that gives you a delta-V readout. I originally played the game by trial and error and managed to return from Duna, refueling multiple times. As soon as I learnt how to calculate delta-V, my ships became much more efficient, much more reliable for getting to where I wanted to go, and crucially, the part counts were reduced by a factor of 5.