[Space, Spaceships] Total ΔV for Interplanetary Travel

[DaltonS]

Originally Posted by Anthony

Realistically, the Oberth Effect means that the total ΔV is less than their sum; how much less depends on your thrust.

The limit case for infinite thrust is ΔV = sqrt( escape velocity ^ 2 + transfer velocity ^ 2 ) - orbital velocity, to either enter or leave orbit. For example, from low earth orbit (escape velocity = 11.2 km/sec, velocity = 7.92 km/s) to a Mars transfer orbit (2.9 km/s) requires sqrt( 11.2^2 + 2.9^2) - 7.92 = 3.65 km/s, which is barely more than the 3.28 km/s required to break orbit.

Okay, let's translate this to "mps" (the speed unit of choice for GURPS Spaceships). The Halfway to Anywhere “Mission to Mars" profile on page 33 of Pyramid #3/79: Space Atlas breaks the 3.4 mps ΔV of the Hohmann transfer orbit into two burns (1.8 mps at Earth orbit and 1.6 mps at Mars) and adds a 0.6 mps burn “to account for a 1.85° difference in orbital planes”. Ve = 6.96 mps (SS1 p.37) making Vo = Ve /sqrt(2) = 4.92 mps. So ΔV = sqrt(6.96^2 + 1.8^2) - 4.92 = 2.27 mps. (I could have directly converted the numbers, but I like using official sources.) Now for the tricky bit; would there be a reverse Oberth effect when making Mars orbit?

Dalton “just when I think I'm getting it, what I get is confused” Spence