Antimatter rocket

Antiproton annihilation reactions produce charged and uncharged pions, in addition to neutrinos and gamma rays. The charged pions can be channelled by a magnetic nozzle, producing thrust. This type of antimatter rocket is a pion rocket or beamed core configuration. It is not perfectly efficient; energy is lost as the rest mass of the charged (22.3%) and uncharged pions (14.38%), lost as the kinetic energy of the uncharged pions (which can't be deflected for thrust); and lost as neutrinos and gamma rays (see antimatter as fuel).[3]

Positron annihilation has also been proposed for rocketry. Annihilation of positrons produces only gamma rays. Early proposals for this type of rocket, such as those developed by Eugen Sänger, assumed the use of some material that could reflect gamma rays, used as a light sail or parabolic shield to derive thrust from the annihilation reaction, but no known form of matter (consisting of atoms or ions) interacts with gamma rays in a manner that would enable specular reflection. The momentum of gamma rays can, however, be partially transferred to matter by Compton scattering.[4][5]

One method to reach relativistic velocities uses a matter-antimatter GeV gamma ray laser photon rocket made possible by a relativistic proton-antiproton pinch discharge, where the recoil from the laser beam is transmitted by the Mössbauer effect to the spacecraft.[6]

A new annihilation process has allegedly been developed by researchers from Gothenborg University. Several annihilation reactors have been constructed in the past years where hydrogen or deuterium is converted into relativistic particles by laser annihilation. The technology has been demonstrated by research groups led by Prof. Leif Holmlid and Sindre Zeiner-Gundersen at research facilities in both Sweden and Oslo. A third relativistic particle reactor is currently being built at the University of Iceland. The emitted particles from hydrogen annihilation processes may reach 0.94c and can be used in space propulsion.[7] Note, however, that the veracity of Leif Holmlid's research is under dispute.[8]

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, 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 (${\displaystyle I_{\text{sp}}}$ ~ 10³ sec) due to temperature limitations of the solid. However, the antimatter energy conversion and heating efficiencies are typically high due to the short mean path between collisions with core atoms (efficiency ${\displaystyle \eta _{e}}$ ~ 85%).[2] Several methods for the liquid-propellant thermal antimatter engine using the gamma rays produced by antiproton or positron annihilation have been proposed.[9][10] These methods resemble those proposed for nuclear thermal rockets. 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 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.[11]

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 (${\displaystyle I_{\text{sp}}}$ ~ 2 × 10³ sec). However, the longer mean free path for thermalization and absorption results in much lower energy conversion efficiencies (${\displaystyle \eta _{e}}$ ~ 35%).[2]

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 (${\displaystyle I_{\text{sp}}}$ ~ 10⁴-10⁵ sec), the long mean free path results in very low energy utilization (${\displaystyle \eta _{e}}$ ~ 10%)[2]

The idea of using antimatter to power an electric space drive has also been proposed. These proposed designs are typically similar to those suggested for nuclear electric rockets. Antimatter annihilations are used to directly or indirectly heat a working fluid, as in a nuclear thermal rocket, but the fluid is used to generate electricity, which is then used to power some form of electric space propulsion system. The resulting system shares many of the characteristics of other charged particle/electric propulsion proposals, that typically being high specific impulse and low thrust (An associated article further detailing antimatter power generation).[12][13]

This is a hybrid approach in which antiprotons are used to catalyze a fission/fusion reaction or to "spike" the propulsion of a fusion rocket or any similar applications.

The antiproton-driven Inertial confinement fusion (ICF) Rocket concept uses pellets for the D-T reaction. The pellet consists of a hemisphere of fissionable material such as U²³⁵ with a hole through which a pulse of antiprotons and positrons is injected. It is surrounded by a hemisphere of fusion fuel, for example deuterium-tritium, or lithium deuteride. Antiproton annihilation occurs at the surface of the hemisphere, which ionizes the fuel. These ions heat the core of the pellet to fusion temperatures.[14]

The antiproton-driven Magnetically Insulated Inertial Confinement Fusion Propulsion (MICF) concept relies on self-generated magnetic field which insulates the plasma from the metallic shell that contains it during the burn. The lifetime of the plasma was estimated to be two orders of magnitude greater than implosion inertial fusion, which corresponds to a longer burn time, and hence, greater gain.[14]

The antimatter-driven P-B¹¹ concept uses antiprotons to ignite the P-B¹¹ reactions in an MICF scheme. Excessive radiation losses are a major obstacle to ignition and require modifying the particle density, and plasma temperature to increase the gain. It was concluded that it is entirely feasible that this system could achieve I_sp~10⁵s.[15]

A different approach was envisioned for AIMStar in which small fusion fuel droplets would be injected into a cloud of antiprotons confined in a very small volume within a reaction Penning trap. Annihilation takes place on the surface of the antiproton cloud, peeling back 0.5% of the cloud. The power density released is roughly comparable to a 1 kJ, 1 ns laser depositing its energy over a 200 μm ICF target.[16]

The ICAN-II project employs the antiproton catalyzed microfission (ACMF) concept which uses pellets with a molar ratio of 9:1 of D-T:U²³⁵ for Nuclear pulse propulsion.[17]

The loss of mass specific to antimatter annihilation requires a modification of the relativistic rocket equation given as[23]

${\displaystyle {\frac {M_{0}}{M_{1}}}=\left({\frac {1+{\frac {\Delta v}{c}}}{1-{\frac {\Delta v}{c}}}}\right)^{\frac {c}{2I_{\text{sp}}}}}$

(I)

where ${\displaystyle c}$ is the speed of light, and ${\displaystyle I_{\text{sp}}}$ is the specific impulse (i.e. ${\displaystyle I_{\text{sp}}}$=0.69${\displaystyle c}$).

The derivative form of the equation is[3]

${\displaystyle {\frac {dM_{\text{ship}}}{M_{\text{ship}}}}={\frac {-dv(1-I_{\text{sp}}{\frac {v}{c^{2}}})}{(1-{\frac {v^{2}}{c^{2}}})(-{\frac {I_{\text{sp}}v^{2}}{c^{2}}}+(1-a)v+aI_{\text{sp}})}}}$

(II)

where ${\displaystyle M_{\text{ship}}}$ is the non-relativistic (rest) mass of the rocket ship, and ${\displaystyle a}$ is the fraction of the original (on board) propellant mass (non-relativistic) remaining after annihilation (i.e., ${\displaystyle a}$=0.22 for the charged pions).

Eq.II is difficult to integrate analytically. If it is assumed that ${\displaystyle v\sim I_{\text{sp}}}$, such that ${\displaystyle (1-{\frac {I_{\text{sp}}v}{c^{2}}})\sim (1-{\frac {v^{2}}{c^{2}}})}$ then the resulting equation is

${\displaystyle {\frac {dM_{\text{ship}}}{M_{\text{ship}}}}={\frac {-dv}{(-{\frac {I_{\text{sp}}v^{2}}{c^{2}}}+(1-a)v+aI_{\text{sp}})}}}$

(III)

Eq.III can be integrated and the integral evaluated for ${\displaystyle M_{0}}$ and ${\displaystyle M_{1}}$, and initial and final velocities (${\displaystyle v_{i}=0}$ and ${\displaystyle v_{f}=\Delta v}$). The resulting relativistic rocket equation with loss of propellant is[3][23]

${\displaystyle {\frac {M_{0}}{M_{1}}}=\left({\frac {(-2I_{\text{sp}}\Delta v/c^{2}+1-a-{\sqrt {(1-a)^{2}+4aI_{\text{sp}}^{2}/c^{2}}})(1-a+{\sqrt {(1-a)^{2}+4aI_{\text{sp}}^{2}/c^{2}}})}{(-2I_{\text{sp}}\Delta v/c^{2}+1-a+{\sqrt {(1-a)^{2}+4aI_{\text{sp}}^{2}/c^{2}}})(1-a-{\sqrt {(1-a)^{2}+4aI_{\text{sp}}^{2}/c^{2}}})}}\right)^{\frac {1}{\sqrt {(1-a)^{2}+4aI_{\text{sp}}^{2}/c^{2}}}}}$

(IV)

The cosmic background hard radiation will ionize the rocket's hull over time and poses a health threat. Also, gas plasma interactions may cause space charge. The major interaction of concern is differential charging of various parts of a spacecraft, leading to high electric fields and arcing between spacecraft components. This can be resolved with well placed plasma contactor. However, there is no solution yet for when plasma contactors are turned off to allow maintenance work on the hull. Long term space flight at interstellar velocities causes erosion of the rocket's hull due to collision with particles, gas, dust and micrometeorites. At 0.2${\displaystyle c}$ for a 6 light year distance, erosion is estimated to be in the order of about 30 kg/m² or about 1 cm of aluminum shielding.[24][25]