Physics, Astronomy & Cosmology / Re: In deriving the Rocket Equation, why does this segment seem to be added twice?« on: Yesterday at 02:11:41 »
Quote from: cmiles0509
why does the equation account for the momentum of both fuel AND rocket, if the fuel's momentum doesn't really matter once it has been used to propel the spacecraft?Physicists really like "conservation laws", because they tell you something about the system which you wouldn't know without the conservation law (or you would have to measure every time).
- Mathematicians and theoretical physicists also like symmetries, as that tells you about the pattern of behavior.
In this case, the calculation you quoted uses the law of "conservation of momentum", which says that the initial and final momentum is the same
- provided you account for all the sources of initial momentum (the rocket containing fuel)
- and also account for all the final momentum (which includes a rocket containing less fuel but travelling faster to the left, and some hot exhaust gas travelling to the right).
If you can get the exhaust traveling faster to the right (without melting the nozzle!), the rocket can go faster to the left.
- Hydrogen and oxygen combustion can get hot enough to melt through anything
- So this normally means finding ways to cool down the nozzle, eg by piping liquid hydrogen through cooling pipes around the nozzle