An antimatter propulsion spacecraft is a proposed hypothetical class of space technology that utilize antimatter for power within spacecraft propulsion. There are several designs that attempt to accomplish this goal.
The theoretical advantage of yield by conversion of fuel using the antimatter-matter annihilative reaction is energy production at annihilation is the maximal currently known, as the specific energy of antimatter is the most energy dense.
Certain theoretical integral antimatter thruster performances have the necessary parameters for interstellar missions.
Annihilation Protonic annihilation produces charged particles which can be confined and directed magnetically which is not the case in electronic. Antiproton annihilation reactions produce charged pions, in addition to neutrinos and gamma rays.
Problems in design The chief practical problems are creating antimatter and storing it. Storage Most storage schemes proposed for interstellar craft require the production of frozen pellets of antihydrogen. This requires cooling of antiprotons, binding to positrons, and capture of the resulting antihydrogen atoms - tasks which have, , been performed only for small numbers of individual atoms. Storage of antimatter is typically done by trapping electrically charged frozen antihydrogen pellets in Penning or Paul traps. There is no theoretical barrier to these tasks being performed on the scale required to fuel an antimatter rocket. However, they are expected to be extremely (and perhaps prohibitively) expensive due to current production abilities being only able to produce small numbers of atoms.
Energy output effect Generally, the energy from antiproton annihilation is deposited over such a large region that it cannot efficiently drive nuclear capsules. Antiproton-induced fission and self-generated magnetic fields may greatly enhance energy localization and efficient use of annihilation energy. Extraction A secondary problem is the extraction of useful energy or momentum from the products of antimatter annihilation, which are primarily in the form of extremely energetic ionizing radiation. The antimatter mechanisms proposed to date have for the most part provided plausible mechanisms for harnessing energy from these annihilation products. The classic rocket equation with its "wet" mass ()(with propellant mass fraction) to "dry" mass ()(with payload) fraction (), the velocity change () and specific impulse () no longer holds due to the mass losses occurring in antimatter annihilation. Modified relativistic rocket equation
The loss of mass specific to antimatter annihilation requires a modification of the relativistic rocket equation given as
where is the speed of light, and is the specific impulse (i.e. =0.69).
The derivative form of the equation is
where is the non-relativistic (rest) mass of the rocket ship, and is the fraction of the original (on board) propellant mass (non-relativistic) remaining after annihilation (i.e., =0.22 for the charged pions).
Eq.II is difficult to integrate analytically. If it is assumed that , such that then the resulting equation is
Eq.III can be integrated and the integral evaluated for and , and initial and final velocities ( and ). The resulting relativistic rocket equation with loss of propellant is
Radiation damage Another general problem with high powered propulsion is excess heat or waste heat, and as with antimatter-matter annihilation also includes extreme radiation. A proton-antiproton annihilation propulsion system transforms 39% of the propellant mass into an intense high-energy flux of gamma radiation. The gamma rays and the high-energy charged pions will cause heating and radiation damage if they are not shielded against. Unlike neutrons, they will not cause the exposed material to become radioactive by transmutation of the nuclei. The components needing shielding are the crew, the electronics, the cryogenic tankage, and the magnetic coils for magnetically assisted rockets. Two types of shielding are needed: radiation protection and thermal protection (different from Heat shield or thermal insulation). Relativity Finally, relativistic considerations have to be taken into account. As the by products of annihilation move at relativistic velocities the rest mass changes according to relativistic mass–energy. For example, the total mass–energy content of the neutral pion is converted into gammas, not just its rest mass. It is necessary to use a relativistic rocket equation that takes into account the relativistic effects of both the vehicle and propellant exhaust (charged pions) moving near the speed of light. These two modifications to the two rocket equations result in a mass ratio () for a given () and () that is much higher for a relativistic antimatter rocket than for either a classical or relativistic "conventional" rocket.
Other general issues
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 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.Space Charge NASA science news, April 6, 2011There and Back Again: A Layman's Guide to Ultra-Reliability for Interstellar Missions Henry Garrett, 30 July 2012
Antimatter production Cost The cost of one gram of antimatter during the fall of 2003 (producer unknown) was 62.5 trillion dollars. Energy expenditure during 2011 for CERN 1g was stated as 25 million billion kWh costing more than 1 million billion Euros. In 2019 within CERN 1g was stated as being $2700 trillion. Potentially viable
Cost of feasible (nuclear propulsive) quantities of available antimatter was estimated in 1999 as both $60 million per mission and $6.4 million (max.).
Rate The production rate of antimatter at CERN during 2009 was from 0.000000001 (1 billionth) to 0.00000001 (10 billionths) of a gram per year. Non-viable The antimatter requirement for a beamed-core power source for transit to the nearest star — Proxima Centuri (4.2 light years away) — is approximately 40 metric tonnes.
Propulsion design System The propulsion system is: 1. Premade and, or, an onboard antimatter generator 2. Storage 3. A way to separate or extract a certain amount of antimatter from the storage mass at the necessary rate 4. Generation of motion of antimatter as transferal to the annihilation location 5. An annihilation chamber 6. Control or channel of antimatter products as thrust
Methods Theoretical antimatter inclusion propellant methods: Beam: annihilation product only Nuclear hybrid: with fusion and or fission: antiparticles to create or catalyse a reaction radioisotope Thermal: plasma core solid core annihilation particles contained within the propulsion generator and controlled for heating a rocket working fluid using conventional propellants: carbon dioxide, hydrogen, methane, water electricity generation using electro-thruster types: arcjet, gridded ion, Hall thruster
The alternatives to direct antimatter annihilation propulsion offer the possibility of feasible vehicles with, in some cases, vastly smaller amounts of antimatter but require a lot more matter propellant.
Beam antimatter rocket: direct use of reaction products Charged pions can be channelled by a magnetic nozzle, producing thrust. This type of antimatter rocket is a pion rocket or beamed core configuration.
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.Thermal Analysis of a Tungsten Radiation Shield for Beamed Core Antimatter Rocketry Jonathan A. Webb
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.
A new annihilation process has purportedly been developed by researchers from the University of Gothenburg, Sweden. Several annihilation reactors have been constructed in the past years which attempted to convert hydrogen or deuterium into relativistic particles through laser annihilation. The technology was explored by research groups led by Prof. Leif Holmlid and Sindre Zeiner-Gundersen, and a third relativistic particle reactor is currently being built at the University of Iceland. In theory, emitted particles from hydrogen annihilation processes could reach 0.94c and can be used in space propulsion. However the veracity of Holmlid's research is under dispute and no successful implementations have been peer reviewed or replicated. Nuclear catalyzed fission/fusion or spiked fusion
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.
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.
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 Isp~105s.
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.
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.
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.