Editing Antimatter rocket (section)

===Thermal antimatter rocket: heating of a propellant===

This type of antimatter rocket is termed a '''thermal antimatter rocket''' as the energy or heat from the annihilation is harnessed to create an exhaust from non-exotic material or propellant.

The '''solid core''' concept uses antiprotons to heat a solid, [[Atomic number|high-atomic weight]] ('''''Z'''''), refractory metal core. Propellant is pumped into the hot core and expanded through a nozzle to generate thrust. The performance of this concept is roughly equivalent to that of the [[nuclear thermal rocket]] (<math>I_{\text{sp}}</math> ~ 10<sup>3</sup> sec) due to temperature limitations of the solid. However, the antimatter energy conversion and heating efficiencies are typically high due to the short [[Mean free path|mean path]] between collisions with core atoms ([[efficiency]] <math>\eta_e</math> ~ 85%).<ref name=ADA446638/>
Several methods for the '''liquid-propellant thermal antimatter engine''' using the gamma rays produced by antiproton or positron annihilation have been proposed.<ref name=Vulpetti1987>{{cite journal |last1=Vulpetti |first1=G. |title=A further analysis about the liquid-propellant thermal antimatter engine design concept |journal=Acta Astronautica |date=August 1987 |volume=15 |issue=8 |pages=551–555 |doi=10.1016/0094-5765(87)90155-X |bibcode=1987AcAau..15..551V }}</ref><ref>{{cite web |title= Positron Propelled and Powered Space Transport Vehicle for Planetary Missions |url= http://www.niac.usra.edu/files/library/meetings/fellows/mar06/1147Smith.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.niac.usra.edu/files/library/meetings/fellows/mar06/1147Smith.pdf |archive-date=2022-10-09 |url-status=live |last= Smith |first= Gerald |author2= Metzger, John|author3= Meyer, Kirby|author4= Thode, Les |date=2006-03-07 |access-date=2010-04-21}}</ref> These methods resemble those proposed for [[nuclear thermal rocket]]s. One proposed method is to use positron annihilation gamma rays to heat a solid engine core. [[Hydrogen]] gas is ducted through this core, heated, and expelled from a [[nozzle|rocket nozzle]]. A second proposed engine type uses positron annihilation within a solid [[lead]] pellet or within compressed [[xenon]] gas to produce a cloud of hot gas, which heats a surrounding layer of gaseous hydrogen. Direct heating of the hydrogen by gamma rays was considered impractical, due to the difficulty of compressing enough of it within an engine of reasonable size to absorb the gamma rays. A third proposed engine type uses annihilation gamma rays to heat an ablative sail, with the ablated material providing thrust. As with nuclear thermal rockets, the [[specific impulse]] achievable by these methods is limited by materials considerations, typically being in the range of 1000–2000 seconds.<ref name=Vulpetti1989>{{cite journal |last1=Vulpetti |first1=Giovanni |last2=Pecchioli |first2=Mauro |title=Considerations about the specific impulse of an antimatter-based thermal engine |journal=Journal of Propulsion and Power |date=September 1989 |volume=5 |issue=5 |pages=591–595 |doi=10.2514/3.23194 }}</ref>

The '''gaseous core''' system substitutes the low-melting point solid with a high temperature gas (i.e. tungsten gas/plasma), thus permitting higher operational temperatures and performance (<math>I_{\text{sp}}</math> ~ 2 × 10<sup>3</sup> sec). However, the longer mean free path for thermalization and absorption results in much lower energy conversion efficiencies (<math>\eta_e</math> ~ 35%).<ref name=ADA446638/>

The '''plasma core''' allows the gas to ionize and operate at even higher effective temperatures. Heat loss is suppressed by magnetic confinement in the reaction chamber and nozzle. Although performance is extremely high (<math>I_{\text{sp}}</math> ~ 10<sup>4</sup>-10<sup>5</sup> sec), the long mean free path results in very low energy utilization (<math>\eta_e</math> ~ 10%)<ref name=ADA446638/>