Editing Relativistic rocket (section)

===Design notes on a pion rocket===
The pion rocket has been studied independently by Robert Frisbee<ref name="R. Frisbee's comprehensive analysis of pion propulsion">{{cite web|url=http://www.relativitycalculator.com/images/relativistic_photon_rocket/ANTIMATTER_ROCKET_FOR_INTERSTELLAR_MISSIONS.pdf |title=How to Build an Anitmatter Rocket for Interstellar Missions |publisher=Relativitycalculator.com |accessdate=2015-06-21}}</ref> and Ulrich Walter, with similar results. Pions, short for pi-mesons, are produced by proton-antiproton annihilation. The antihydrogen or the antiprotons extracted from it will be mixed with a mass of regular protons pumped into the magnetic confinement nozzle of a pion rocket engine, usually as part of hydrogen atoms. The resulting charged pions have a speed of 0.94''c'' (i.e. <math>\beta</math> = 0.94), and a [[Lorentz factor]] <math>\gamma</math> of 2.93 which extends their lifespan enough to travel 21 meters through the nozzle before decaying into [[muon]]s. 60% of the pions will have either a negative, or a positive electric charge. 40% of the pions will be neutral. The neutral pions decay immediately into gamma rays. These can't be reflected by any known material at the energies involved, though they can undergo [[Compton scattering]]. They can be absorbed efficiently by a shield of [[tungsten]] placed between the pion rocket engine reaction volume and the crew modules and various electromagnets to protect them from the gamma rays. The consequent heating of the shield will make it radiate visible light, which could then be collimated to increase the rocket's specific impulse.<ref name="Analysis of relativistic rocketry; S. Westmoreland; Kansas State University"/> The remaining heat will also require the shield to be refrigerated.<ref name="R. Frisbee's comprehensive analysis of pion propulsion"/> The charged pions would travel in helical spirals around the axial electromagnetic field lines inside the nozzle and in this way the charged pions could be collimated into an exhaust jet moving at 0.94''c''. In realistic matter/antimatter reactions, this jet only represents a fraction of the reaction's mass-energy: over 60% of it is lost as [[gamma-rays]], collimation is not perfect, and some pions are not reflected backward by the nozzle. Thus, the effective exhaust speed for the entire reaction drops to just 0.58c.<ref name="Analysis of relativistic rocketry; S. Westmoreland; Kansas State University"/> Alternate propulsion schemes include physical confinement of hydrogen atoms in an antiproton and pion-transparent [[beryllium]] reaction chamber with collimation of the reaction products achieved with a single external electromagnet; see [[Project Valkyrie]].