Is Starship Rocket Physics Any Different? Not at All!
Yesterday we discussed what SpaceX super heavy rocket will be used for, and who and why backs this project:
Now, let’s look at this project from a physics perspective: is it any different to launch a super heavy rocket than launching light rockets, bundled together?
Finding no physical/mechanical difference makes yesterday’s story even more reasonable. Now, since there is no physical difference, is there a significant chemical difference, which might boost propulsion? Often rockets and missiles use nasty chemicals (as fuel and oxidant). The least aggressive for the environment is flying on barrels of kerosene (C₁₂H₂₆ − C₁₅H₃₂) and oxygen (O₂). SpaceX currently tests methane (CH₄) fuel, with eventual goal of switching to hydrogen (H₂). Does that change Starship propulsion a lot?
Propulsion efficiency is estimated as a ratio of the output volume (which is proportional to the number of output molecules) to the mass of these molecules: output gas volume per unit of mass.
- Burning hydrogen produces 2 units of gas volume from 36 units of mass. Efficiency of propulsion based on hydrogen burning is ~0.056.
- Burning methane produces 3 units of gas volume from 80 units of mass. Efficiency of propulsion based on methane burning is ~0.038.
- Burning kerosene produces 25 units of gas volume from 762 units of mass. Efficiency of propulsion based on kerosene burning is ~0.033.
So far, there is almost no propulsion gain in replacing kerosene with methane.
P.S. Here we have not discussed differences in burning temperature (raising gas temperature increases its volume). But burning temperatures are close (of the same order of magnitude) for these fuels.
