= Photon rocket
{title2=$M_f/M_i=e^{-|\Delta\eta|}$}
A <photon rocket> converts stored mass-energy into a collimated beam of <photons>. In its instantaneous <rest frame>, <energy> conservation gives $c^2dM=-dE_\gamma$, and <momentum conservation> gives $Mc\,d\eta=dE_\gamma/c$. Therefore a monotone change in <rapidity> costs $M_f/M_i=e^{-|\Delta\eta|}$. The rocket's <rest mass> includes its fuel and stored <internal energy>; inefficiency or an angular spread of the exhaust increases the fuel requirement.
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