I showed this when I made a stage LIGHTER than the LV909 version (by removing the oxidizer) and still had a lower dV. When most of the mass is engines, it doesn't go up as much.
If most of it is fuel, then near the end of the tank, the dV per unit burned goes WAY up. The real issue here is how much of the mass of the rocket is engines vs fuel. (IE I need 5 units of fuel to produce X thrust vs 10 units to produce X thrust, but I have Y units of fuel whichever one I use). Hat, do I misunderstand this completely? If two rockets had equal mass, but one had an Isp of 345 and one had an Isp of 800, wouldn't the latter have over 2x the dV of the former? Your initial equation doesn't seem to account for having the same fuel available, but needing half of it. As the craft gets bigger, this differential will drop, but it would need to be a VERY big craft for the LV-N to come out ahead.įuel mass would alter this, but it's very low compared to everything else - fuel mass is very low in KSP. This means that for a given unit of fuel, and in isolation, the LV909 is five times better at putting momentum into itself. Momentum has a unit of mass, so we need to divide by the engine mass (technically the mass of the entire craft, but engines are big part of it) and that's when it starts looking ugly. Without correcting for thrust, as we don't care about that for these purposes, we get 345x10 and 800x5.
You can crudely multiply that by the Isp to get (something proportional to) the amount of momentum imparted. LV-Ns create their thrust using 5 mass-units of fuel. LV909s create their thrust using 10 mass-units of fuel. LV-Ns are still great, you just have to use the appropriate liquid-fuel-only tanks, the 1.25m tanks used here have the same mass ratio as liquid fuel/oxidizer tanks.
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