Engine efficiency

[Talonsin]

Is there any difference between engines and their efficiency, besides the nuclear ones? I see a mainsail and then another one with half the thrust, does that mean it uses half the fuel as the mainsail?

Thanks

[allmhuran]

Fuel efficiency is measured by ISP, which means specific impulse. This is essentially a measure of delta-V per unit of fuel. The nuclear engines have high ISP in vacuum, but low in atmosphere.

What gives an engine high ISP is a high exhaust velocity.

But there's more to efficiency than just ISP. For example, one of the little 909's has very high ISP, but if you stick it at the bottom of a heavy rocket it will use up almost all the fuel without even being able to get you off the pad! In other words, you need enough thrust to have an efficient launch profile. This is why mainsails are not a bad choice for the first stage of a large rocekt despite their low ISP. They give you oodles of thrust, which they provide by throwing huge quantities of exhaust mass out the back of your rocket at an alarming rate, but at relatively low exhaust velocity (compared with a nuclear engine in space)

Overall, LVT30's are fantastic little engines. They weigh less than LVT45s but produce more thrust, and have at least decent ISP even in the atmosphere. The only drawback is that they do not have any vectoring, but that's OK.. as long as you have at least one engine with vectoring as well, you should be fine. Indeed, having engines on radial stacks that *don't* vector their thrust can be advantaeous for stability and efficiency. Long story short: LVT30's are really great engines to stick on booster columns.

Edited August 8, 2013 by allmhuran

[capi3101]

The wiki's got a pretty good chart that compares liquid fuel engines to one another.

I've gotten to where I like Skippers a great deal, especially as a middle-stage engine. Used to have a thing for Aerospikes (on account of their relatively high Isp at all altitudes), but the fact that they don't vector has given me a few too many spinning rockets...

Really, which engine you need all depends on the payload. Bigger loads need the engines with higher thrust; no way around that, really.

[Tiron]

Less complicated Version: The 'Isp' statistic indicates the fuel efficiency, which is independent of thrust. The efficiency varies with altitude, and smoothly slides between the 'Atmosphere' and 'Vacuum' numbers as the pressure decreases. Interestingly, the lack of a third point at a higher-than-Kerbin's-Sealevel pressure means that if you go someplace with a thicker atmosphere, like Jool or Eve, the efficiency flatlines at its 'Atmosphere' level even if you're at several atm of pressure.

The key thing is that burn rate is determined by a combination of the thrust and the Isp. Two engines with the same Isp but different thrust will burn fuel at different rates. The LV-T30 and LV-T45 for example. The LV-T45 burns fuel at a slower rate because it produces less thrust. Crucially, however, because their Isp stats are the same, you'll get the same effect out of both of them: The T30 just does it a bit faster.

The exception is during ascent, because if you don't have enough thrust to efficiently overcome atmospheric drag and gravity, the extra efficiency gets wasted while you're 'stuck in the mud', so to speak. Using larger numbers of the more fuel efficient engines doesn't always help either, because the extra weight reduces the overall efficiency of the rocket, which can easily end up more than compensating for the more efficient fuel burn of the engines (LV-Ns are especially guilty of this, being so absurdly heavy).

In space, the thrust level almost doesn't matter: if your thrust is low you just do a longer burn. In this case higher Isp is almost always better. The sole exception is if your overall thrust to weight ratio ends up being so low that you can't complete the maneuver during the window you have to do so. This doesn't happen as often as you'd think, except with Ion powered probes that are way too heavy.

Edited August 8, 2013 by Tiron

[rkman]

I see a mainsail and then another one with half the thrust, does that mean it uses half the fuel as the mainsail?

Yes it uses half the fuel but because it also produces half the thrust, the efficiency of those engines is the same.

As others have pointed out, the measure of engine efficiency is "ISP", higher is better.

[numerobis]

There's five main variables:

- the mass of the engine

- the thrust produced by the engine

- the Isp of the engine

- drag of the engine part

- whether the engine has thrust vectoring

Obviously drag only matters in the atmosphere.

If you keep the other variables fixed, higher Isp, lower mass, higher thrust, lower drag are the right directions to go. And you probably want some thrust vectoring, but not all engines need it -- just a few of them do, near the center of the rocket.