[Spaceships] Engine size and performance

the-red-scare · 04-13-2021, 07:07 AM

For the acceleration of a reaction engine to remain constant, the propellant mass flow rate has to scale with the mass of the ship. So a 3x increase in ship size means a 3x increase in mass flow, a 3x increase in thrust, and therefore equal acceleration.

Does that then mean the cross-sectional area of the nozzle throat (or equivalent region in an electric or pulse-type engine) also scales with mass? And therefore the surface area required for a given type of engine scales with mass?

What I’m getting at is, a SM +7 ship has twice the surface area of an SM +6 ship. Do its engines take up twice the surface area on the rear hull (scaling with area as most parts will) or three times the surface area (scaling with mass to accommodate three times the mass flow)?

If the latter (causing the engines to be ridiculously huge at large sizes) should performance decrease, perhaps in a Speed/Range kind of way, as ships get bigger? Could be a justification for “fast” fighters and “slow” capital ships…

04-13-2021, 07:07 AM	#1
the-red-scare Join Date: Sep 2014	[Spaceships] Engine size and performance For the acceleration of a reaction engine to remain constant, the propellant mass flow rate has to scale with the mass of the ship. So a 3x increase in ship size means a 3x increase in mass flow, a 3x increase in thrust, and therefore equal acceleration. Does that then mean the cross-sectional area of the nozzle throat (or equivalent region in an electric or pulse-type engine) also scales with mass? And therefore the surface area required for a given type of engine scales with mass? What I’m getting at is, a SM +7 ship has twice the surface area of an SM +6 ship. Do its engines take up twice the surface area on the rear hull (scaling with area as most parts will) or three times the surface area (scaling with mass to accommodate three times the mass flow)? If the latter (causing the engines to be ridiculously huge at large sizes) should performance decrease, perhaps in a Speed/Range kind of way, as ships get bigger? Could be a justification for “fast” fighters and “slow” capital ships… Last edited by the-red-scare; 04-13-2021 at 04:14 PM.