Could someone explain a simple way to determine engine efficiency?

[manollette]

The wiki's got WAY too much math, and while I understand that's a vital underlying component of the game, it's beyond me at this point. I'm mostly looking at rockets for in a vacuum, though a bit of understanding may help me with my launches as well.

I THOUGHT that an engine's efficiency was in the ratio of max thrust to liquid fuel comsumption at max, but now looking at stats I'm seeing wildly varying ISP as well, and am not sure what to think.

If a specific example would help, I'm comparing the atomic rocket motor to the terrier for deep space use, and while the atomic rocket clearly allows more efficiency by not requiring oxidizer in your fuel, the fact that it's 6 times the weight of the terrier is telling me that unless I'm loading up on a ton of fuel for one engine, it's really not that much better. Help?

[Ogre420]

https://www.youtube.com/watch?v=s90tSM0oowo

[AlexMBrennan]

I would just get the Kerbal Engineer Redux mod and compare the calculated delta V values for various engines when in doubt.
For any larger or long term missions you are almost certainly better off with the LV-N than anything else.

[Empiro]

There is no simple way, but you're getting the intuition behind it. There is a web tool here that calculates the most mass-efficient engine given payload mass, desired delta-V, and desired TWR:
http://meithan.net/KSP/engines/

Note that the LV-N is efficient not because it doesn't use oxidizer, but rather because it has an ISP of 800, which is much higher than the Terrier's 345. Make sure that you're also using the Liquid Fuel only tanks for the LV-N, otherwise, you're losing quite a bit of delta-V due to excess wasted mass in the tanks themselves.

General rules of thumb:
-If you need lots of delta-V, then higher ISP engines are usually better, even if heavier
-If your ship is very light, then lighter engines tend to be better -- you can't have half an engine, so the mass matters a lot. For larger ships, you can always use multiple small engines for higher TWR
-TWR usually doesn't matter, but < .1 usually leads to impractically long burns. This is why I never use ion engines. It has amazing ISP, but terrible TWR.

[Patriot03]

Kerbal engineer makes it child's play. Put one engine on see the DeltaV, put the other on and see if it increases or decreases.

[manollette]

Thanks for the explanations and video links guys. I'll test around a bit and see what I can figure out. Trying to do a mod-free game, but if that helps give me ideas for playthroughs later then I might have to try it out too.

[ghpstage]

Originally posted by manollette:

Thanks for the explanations and video links guys. I'll test around a bit and see what I can figure out. Trying to do a mod-free game, but if that helps give me ideas for playthroughs later then I might have to try it out too.

Are you avoiding mods as a principle in this game, or do you just want to avoid adding new cheaty parts?
Kerbal Engineer only adds two parts, both of which are almost entirely inconsequential (tiny computer parts that only allow Kerbal Engineer info in flight). The purpose of the mod is to give easy access to information that is useful in designing craft and in orbital manoeuvres, things like automatic calculation of dV and TWR from the perspective of any planets/moon and in cases where atmosphere is present. Stuff that really ought to be in stock as having to redo the calculations with every change in rocket parts, or during missions gets old fast.