Comparing equipment

[robotroadkill]

I am a bit of an add-on monger, and now I have more parts than I know what to do with. I've been trying to come up with some sort of a standardized metric for comparing like items (namely engines, tanks, and boosters)

For boosters, I use:

BI=(T*BT)/M

Booster index = (Thrust * Burn Time)/Mass

I am ignoring dry mass because, 1. I usually jettison them after they burn off and 2. I am assuming the mass doesn't change as the booster burns (whether it does or not in the game, I have no idea)

The tanks are trivial: TI=C/M (Tank Index = Capacity/Volume) Although with tanks, capacity alone is probably the most important part of what goes into which stage (most of my tanks have a similar TI score close to 200 anyway). Again, dry mass is ignored because empty tanks don't stay attached to my rockets for very long.

The trickier one for me is the engines, mainly because some of them have a mass <1.

EI=MT/(BR+M)

Engine index = MaxThrust/(BurnRate+totalMass)

Adding BurnRate and totalMass is arbitrary, except that I want mass to contribute to the denominator. I don't know if engine performance is actually affected by these parameters in this way. Simply dividing by mass has the adverse effect of increasing the score for those engines with 0<mass<1

Another option is to just subtract mass from the total score (lessening its effect on the score greatly)

EI=(MT/BR)-M

Here's a comparison of the two engine indices (6th and 7th column)

Of course these values (especially EI and BI) can't be used by themselves to make decisions, since an engine with very low thrust can have a high score (MM1 ION, for example). These values should be used as a guide to choose among candidates for a given job.

Anyway, enough of the blabbering, and on to the questions...

I would like to incorporate drag in these indices, but I have no idea how it is implemented in the game. Is it set value for a part, or a coefficient that interacts with velocity?

Does anyone with better math skills and/or understanding of game mechanics have any suggestions for improvements on these somewhat naive equations?

Have you come up with an entirely different system? Let's hear it!

[Corona688]

Specific impulse would be a better way to categorize them I think. Or total energy.

Lots of modded engines throw math out the window, you've got enough fuel and thrust to do anything you want, up to and including infinite fuel. But categorizing them like this would also show just how balanced or unbalanced some are.

[DoctorEvo]

Yes, specific impulse is always a good reality-check. Solid-fuel chemical engines shouldn't exceed about 290 s (230ish is more appropriate for a low-altitude booster); while liquids should be no more than 340 s in-vaccuum and 300 at sea level for dense fuels, or 370 SL/450 vac for hydrogen.

As for liquid engines, thrust:weight ratio is another factor that should be checked. 70:1 is pretty typical for high-power engines; 100:1 is not unheard of. Low-thrust engines for orbital maneuvering may be much lower.

Also, the mass ratio/empty weight of fuel tanks/solid motors should be checked, as in some cases it can have a profound impact on rocket performance.

[robotroadkill]

Thanks for the info, you two!