When planning a rocket, thrust tells you how much force an engine produces, but not how quickly it uses propellant. Specific impulse, written Isp, supplies that missing measure of propellant efficiency. A higher-Isp engine consumes less propellant to produce the same thrust. section 8.5
Think of Isp as a rocket’s counterpart to gas mileage, with an important difference: a rocket can keep moving without firing its engine. Rather than distance per unit of propellant, Isp is expressed in seconds—the time a unit of propellant could sustain a force of 1 G. section 8.5
g = 9.80665 #1
function mass_flow(thrust::Number, Isp::Number) #2
thrust / (Isp * g)
end
The Julia function converts thrust and Isp into propellant mass flow: `mass_flow = thrust / (Isp * g)`, where `g` is 9.80665. At fixed thrust, increasing Isp reduces consumption per second. section 8.5
Nine engines firing together
Nine engines producing 845,000 units of thrust each provide 7,605,000 in total. With Isp of 282 seconds, `mass_flow(7.605e6, 282)` returns about 2,750 units of propellant mass per second. section 8.5
In a time-step simulation, the rocket uses thrust and its engine’s Isp to calculate mass flow, then consumes `mass_flow × Δt` from the tank. A cluster of identical engines multiplies total thrust and engine mass by the engine count, but keeps the same Isp. A cluster of different engines raises a separate question: how to determine its combined Isp. section 8.7 section 8.7
