Editing Relativistic rocket (section)

==Matter-antimatter annihilation rockets==
It is clear from the above calculations that a relativistic rocket would likely need to be antimatter-fired.{{or|date=April 2024}} Other antimatter rockets in addition to the photon rocket that can provide a 0.6''c'' specific impulse (studied for basic [[hydrogen]]-[[antimatter|antihydrogen]] annihilation, no [[ionization]], no recycling of the radiation<ref name="Analysis of relativistic rocketry; S. Westmoreland; Kansas State University">{{Cite journal|arxiv=0910.1965|last1=Westmoreland|first1=Shawn|title=A note on relativistic rocketry|journal=Acta Astronautica|volume=67|issue=9–10|pages=1248–1251|year=2009|doi=10.1016/j.actaastro.2010.06.050|bibcode=2010AcAau..67.1248W|s2cid=54735356 }}</ref>) needed for interstellar flight include the "beam core" [[pion]] rocket. In a pion rocket, frozen antihydrogen is stored inside electromagnetic bottles. Antihydrogen, like regular hydrogen, is [[diamagnetic]] which allows it to be [[Electromagnetic force|electromagnetically]] [[Magnetic levitation#Diamagnetic levitation|levitated]] when refrigerated. Temperature control of the storage volume is used to determine the rate of [[vaporization]] of the frozen antihydrogen, up to a few grams per second (hence several peta[[watt]]s when annihilated with equal amounts of matter). It is then ionized into [[antiproton]]s which can be electromagnetically accelerated into the reaction chamber. The [[positron]]s are usually discarded since their [[Electron–positron annihilation|annihilation]] only produces harmful [[gamma ray]]s with negligible effect on thrust. However, non-relativistic rockets may exclusively rely on these gamma rays for propulsion.<ref name="futureofthings; positron engines">{{cite web |url=http://thefutureofthings.com/3031-new-antimatter-engine-design/ |title=New Antimatter Engine Design|date=29 October 2006 }}</ref> This process is necessary because un-neutralized antiprotons repel one another, limiting the number that may be stored with current technology to less than a trillion.<ref name="science.nasa.gov examining antimatter as a propellent">{{cite web|url=https://science.nasa.gov/science-news/science-at-nasa/1999/prop12apr99_1/ |title=Reaching for the Stars - NASA Science |publisher=Science.nasa.gov |date= |accessdate=2015-06-21}}</ref>

===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]].