GURPS Spaceships Design Spreadsheet

[ericbsmith]

Originally Posted by DaltonS

Some fuel usage issues:[*]The fuel usage duration calculation for reaction engines seems to assume constant thrust rather than constant acceleration. This means at engine burnout the actual acceleration = a × 20/(20 - number of tanks used up) where a = nominal acceleration. Fuel duration for constant acceleration = ΔV × K/a hours where K=0.045 for liftoff and 0.0455 for space. There should be an engine (or ship) option as to which is constant.

This is a direct result of the way that the rules calculate the amount of fuel for calculating delta-V; the rules essentially multiply the amount of fuel available based on the number of tanks installed; by extension, the duration is multiplied by the same factor. This results in the duration being calculated by the first method. The second method may be possible, but would take some work to do.

Originally Posted by DaltonS

[*]If the ship has more than one reaction engine that use the same fuel tanks, ΔV and duration for each engine is calculated using the entire fuel supply. Could you set up a "fuel profile" entry like the "power profile" entry that could assign a fixed amount of fuel to each engine's use?

It is true that duration and delta-V is calculated as if all appropriate fuel tanks are assigned to each engine (or the set of identical engines). Again, while it may be possible to set up a fuel profile, this would take a bit of work. The biggest issue here is that the number of tanks, and thus the delta-V tank multiplier, ends up needing to get calculated for each profile based on which tanks are assigned to which engines. That gets messy and complicated; not impossible, but messy and complicated.

Originally Posted by DaltonS

“who'd still like an answer to his question about collapsible tanks

I don't see why you couldn't pump fuel the from the collapsible tanks into main tanks while using the fuel from the main tanks. The main reason for the limitation is to prevent a ship from using only collapsible tanks, as the standard fuel tanks also include the pump machinery and fuel lines to the engines while the collapsible tanks are meant to be just the tank itself, set up in a cargo bay.