Comparing efficiency of engines (fuel densities)

[Zedny]

Everyone is saying, that you should burn the least efficient fuel first, but it got me thinking. Is this thing really just as simple as that?

How can you compare the Specific impulse of an LFO, a SRB, an Ion Engine and the RCS thrusters with their different fuel densities?

Basically, which would give you more ÃŽâ€v? Slowly using up some denser (heavier) fuel therefore wasting energy on bringing it with you longer, or taking lighter fuel but burning through it faster?

I know it is almost irrelevant in KSP, since each type has it's own specialised use, but I would still like to read your thoughts about the subject. Should this be investigated and if so, how could you put math behind it? It's not rocket science or anything and I don't want any complicated equations just some simple stuff to compare the values. My first idea was to divide ISP by Mass, but it just didn't seem scientifically accurate enough.

Edited November 6, 2014 by Zedny

[Taki117]

This really only changes if you are using the special engines in the KSP Interstellar mod. Liquid engine efficiency is determined by the specific impulse of the engine you are using.

[Starman4308]

Everyone is saying, that you should burn the least efficient fuel first, but it got me thinking. Is this thing really just as simple as that?

Yes.

How can you compare the Specific impulse of an LFO, a SRB, an Ion Engine and the RCS thrusters with their different fuel densities?

Higher specific impulse is better. Density is mostly irrelevant, although low-density fuels like liquid hydrogen do take a lot of volume, requiring larger (and heavier) tanks for the same mass of fuel.

Basically, which would give you more ÃŽâ€v? Slowly using up some denser (heavier) fuel therefore wasting energy on bringing it with you longer, or taking lighter fuel but burning through it faster?

Disregarding the mass of fuel tanks, and assuming identical specific impulse, X mass of low-density fuel gives you the exact same delta-V as X mass of high-density fuel. The sole difference is that you will use a larger volume of low-density fuel, but the rocket equation doesn't care about volume, it cares about mass.

I know it is almost irrelevant in KSP, since each type has it's own specialised use, but I would still like to read your thoughts about the subject. Should this be investigated and if so, how could you put math behind it? It's not rocket science or anything and I don't want any complicated equations just some simple stuff to compare the values. My first idea was to divide ISP by Mass, but it just didn't seem scientifically accurate enough.

Isp/mass is nonsensical. The only thing that matters is the rocket equation: delta-V = G * Isp * ln(full mass/dry mass)

[NathanKell]

Isp relates fuel flow in mass units to thrust. You don't have to worry about fuel density, always burn your least efficient fuel first.

[Zedny]

Thanks for the quick and informative replies.

Isp relates fuel flow in mass units to thrust.

I didn't think of that...

[Streetwind]

To better understand what Isp actually is, I always find it helpful to visualize how it is defined:

"This engine, when set to produce 1 kN of thrust, will run for this long on 1 ton* of fuel before running out."

And that is the reason why we are describing the fuel efficiency of a rocket engine with a value given in seconds, of all things.

As you can see, the specific impulse is completely unrelated to engine type or fuel choice. It works for solid boosters just as well as for liquid monopropellants, liquid bipropellants and electric plasma thrusters. An LV-909 in a vacuum will run for 390 seconds on 0.45t liquid fuel + 0.55t oxidizer if you manage to throttle it to exactly 1 kN of thrust. Meanwhile, a PB-ION engine producing that exact same 1 KN of thrust would run constantly for an entire 4200 seconds before its one ton of xenon gas is depleted.

You can time this with a stopwatch ingame if you wish

* Strictly speaking the units are 1 N and 1 kg, but the above units are more fitting for what we see in KSP. Since both are just multiplied by 1000, it still works.

Edited November 6, 2014 by Streetwind

[pellinor]

Shouldn't it be 9.81N and 1kg? Coming from this 'g' in the formula that was introduced so that working with imperial or metric units did not produce different numbers.

As I understand, ISP is the time that an arbitrary amount of fuel would provide 1g of acceleration on its own weight.

[Starman4308]

To better understand what Isp actually is, I always find it helpful to visualize how it is defined:

"This engine, when set to produce 1 kN of thrust, will run for this long on 1 ton* of fuel before running out."

There is another way to visualize it I like: "It would take Kerbin surface gravity this long to accelerate something to the same speed as the exhaust gases", which lets you tie it back to the exhaust velocity in the other form of the rocket equation.

[NathanKell]

Yep, missing the factor of 9.80665 (or, rounded to 9.81...or, if you're KSP, it's rounded to 9.82 because...who knows.)

I goofed earlier too: it relates *weight* of fuel to thrust, so lb vs lb, or N vs N. If you're comparing mass of fuel and thrust, you need g0 in the mix.

[KerikBalm]

Everyone is saying, that you should burn the least efficient fuel first, but it got me thinking. Is this thing really just as simple as that?

For a single stage craft, yes.

If you are staging, it may be worth it to discard a stage with a LV-N at 800 ISP, to proceed onward with a smaller and lighter stage with a 48-7s getting 350 ISP, and on top of that, you may not want to use up all your monoprop-> discard the rest of the stage so its just the landercan/mk1pod and the 15/7.51monoprop it carries, and then use up the monoprop flying just your pod/lander can.

But without decoupling anything, yes, it is that simple.

[numerobis]

It's as simple as that in interplanetary travel. When you're lifting off or landing, there are other considerations like making sure you have enough thrust.

For liftoff, high TWR engines tend to be low Isp, so there's alignment. For landing, it's misaligned: you'll do the transfer with the high Isp stage and land on a high TWR, low Isp stage.

[OhioBob]

The general rules of thumb for maximizing your payload fraction are,

1. Stages with higher specific impulse should be above stages with lower specific impulse.

2. More delta-V should be provided by the stages with the higher specific impulse.

3. Each succeeding stage should be smaller than its predecessor.

4. Similar stages should provide the same delta-V.

[Dispatcher]

My chart & graphs compare single (and for radials two [think plane] and three [think vertical rocket]) engine performance for all engine types. Testing has actually continued into v. 0.25 though the chart still indicates an earlier version. The chart shows results for three different kinds of tests: in one, the craft mass is maxed by adding "fuel" and in practical terms it can't lift any more mass without crashing after being drop fired from the launch stability clamps at full extension. In another, the test is similar but half of the maximized mass is non-fuel payload. The final test is done using the smallest available stackable "fuel" tank. The graphs show results of the "50% non-fuel mass" test. One graph includes the rapier engine; the other excludes that data in order to allow for a "zoomed in" view of the other engine results. The results are simply the achieved altitudes. To paraphrase the chart, your results may differ.

You may find some correlations between my results and your flight experiences. Happy Kerbal-ing!