Solid vs liquid fuel

[Capt. Hunt]

I've got a booster from a certain mod, that uses an LF/O mixture.  I tried replacing the LF&O one for one with Solid fuel, and it came out with significantly higher delta-v.  However, I want to maintain the same performance as the original booster.  So, assuming the same engine stats, how much less Solid fuel do I need?

Edited October 20, 2016 by Capt. Hunt

[Avonix]

Depends on the stats for the engine. Could you post the specific impulse/weight/thrust etc?

[Capt. Hunt]

maybe I'm missing something here, but, if the only thing I'm changing is the type of propellant, shouldn't the effect be the same for any engine?

[Zhetaan]

Correct.  The effect results from the properties of the fuel, not the engine, and thus would be the same for every engine you altered in this way.  Delta-v derives from three things only:  the exhaust velocity of the engine--unchanged if you didn't touch the I_sp, the dry mass--also unchanged, and the wet mass, which, assuming you did not change the number of units in the fuel load, must mean the fuels have differing intrinsic properties.  SolidFuel has a density of .0075, whereas LF and Ox each have a density of .005.  This alters the mass ratio and is the source of your extra delta-v.

Edited to add:  Actually, there's more going on, given the bipropellant versus monopropellant nature of the fuels, but it's always more complicated than it says on the package.  The good thing of it is that LF and O are given the same density (totally unlike real life), so you can simply treat them as one resource and add them together.  Given that you are seeing a delta-v increase, I assume that you are adding the previous LF and O values together and giving the engine the same number of units of SolidFuel as that sum.  Reduce that SolidFuel load by one third and you should see the correct performance.

Edited October 20, 2016 by Zhetaan

[Capt. Hunt]

cool, thanks

Yeah, I was subbing the solid for the combined volume of both LF and O.  I'll try cutting it by a third