# Rocket Question

1. Apr 16, 2008

### joker_900

1. The problem statement, all variables and given/known data

I've done the first part, I'm just posting it for completeness

A rocket at rest in deep space has a body of mass m and carries an initial mass
M of fuel, which is ejected at non-relativistic speed v0 relative to the rocket. Show that
the speed of the rocket vf after all the fuel is ejected is given by

vf = v0 ln(1 + M/m)

Now consider the case of a relativistic rocket, where matter/antimatter fuel is
annihilated and expelled from the rocket as photons. Show that the final speed of this
rocket is given by

1 + M/m = [(c + vf)/(c - vf)]^0.5

2. Relevant equations

3. The attempt at a solution

So for the second bit: I called the mass at any instant n, and attempted to conserve momentum:

y1(n + &n)v = y2(v + &v)n - p

Where y1 is the gamme for the velocity before the emission of a small fuel element &n (the & is supposed to be a delta), and y2 is the gamme for the velocity of the rocket after emission.

y1(n - dn)v = y2(v + dv)n - E

Where E is the energy of the emitted photon(s). Here's where I get stuck. Is E = &nc^2 or E = y1 &nc^2 or something else?

2. Apr 17, 2008

### Shooting Star

Consider the initial and final states only.

Let E and p be the magnitudes of the total energy and total momentum of the photons respectively. (It does not matter that the photons have been emitted at different times, since all of the travel at the same speed c anyway.) Let g denote gamma(v).

E = pc (for photons).

P = mgv, since the magnitude of the final momentum of the rocket must be equal to that of the photons.

Now use the fact that the initial energy must be equal to the final energy of the rocket and the photons.