Combustion Jet

The equation of motion for rockets is mrv˙r=uem˙r,m_\textrm{r}\,\dot{v}_\textrm{r}= u_\textrm{e}\, \dot{m}_\textrm{r}, where ueu_\textrm{e} is the speed of the exhaust shooting out the back. The final velocity of the rocket is a few multiples of the exhaust speed, so it's quite important, but in physics textbooks, ueu_\textrm{e} is usually treated like a freely adjustable parameter whose origin and precise value is a detail left for engineers to worry about. With a little bit of approximation, we can do better than that.

Estimate ueu_\textrm{e} for the combustion of methane (CHX4),(\ce{CH4}), the propellant used by SpaceX for its Raptor rocket engine program.

Details and Assumptions:

  • The specific enthalpy of combustion for methane is ΔcH890 kJ/mol.\Delta_c H \approx \SI[per-mode=symbol]{890}{\kilo\joule\per\mole}.
  • Assume that all the energy released in combustion goes into the kinetic energy of the gas.
  • Beyond its performance, methane is nice because it could potentially be produced on other planets via the Sabatier reaction COX2+4HX2CHX4+2HX2O.\ce{CO2 + 4H2 -> CH4 + 2H2O}.
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