Patent Publication Number: US-10322826-B2

Title: Interplanetary spacecraft using fusion-powered thrust

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority under 35 U.S.C. 119(e) from prior U.S. provisional application 62/379,875, filed Aug. 26, 2016. 
    
    
     TECHNICAL FIELD 
     The present invention relates to providing thrust to interplanetary spacecraft, especially but not exclusively for deceleration, orbital capture and landing, and further relates to inducement of controlled muon-catalyzed nuclear micro-fusion, as well as micro-fusion obtained from fuel bombardment with high-energy particles (particle-target fusion). 
     BACKGROUND ART 
     For space travel, whether between the Earth and the Moon or when traveling among the planets (or any of its moons), it&#39;s a good idea to minimize the propellant mass needed by your spacecraft and its launch vehicle. When a spacecraft fires its rocket engines, it accelerates to reach some planned velocity. When those rocket engines are shut off, that spacecraft will then, in the absence of gravitational forces, continue under its own inertia with the same speed and trajectory it has attained. 
     When traveling between planets in the solar system it useful to consider the spacecraft as being in orbit around the Sun. The trajectory with minimal propellant usage is a transfer orbit around our Sun where the desired orbit&#39;s perihelion (closest approach to the sun) will be at the distance of Earth&#39;s orbit, and the aphelion (farthest distance from the sun) will be at the distance of Mars&#39; orbit or of some other planet. (Likewise, for journeys to the inner planets of Mercury or Venus, the aphelion will coincide with Earth&#39;s orbit and the perihelion will coincide with the destination planet&#39;s orbit. It uses its rocket to accelerate opposite the direction of Earth&#39;s revolution around the sun, thereby decreasing its orbital energy.) Most of the journey between the planets will then consist of coasting towards its destination with the engines turned off. 
     In order to be captured into the destination planet&#39;s orbit, the spacecraft must then decelerate relative to that planet using a retrograde rocket burn or some other means. In order to slow sufficiently to obtain orbital capture by the destination planet, the spacecraft must use about the same amount of fuel that it used to speed up originally. 
     In general, if we want to reduce the travel time between Earth and Mars or some other planet, the more fuel we will need in order to accelerate the spacecraft to a higher coasting velocity and consequently the more fuel will need upon arrival to slow down in order to enter the planet&#39;s orbit and then to land. Present spacecraft systems use liquid fuels that constitute a very large percentage of the overall mass. If one could save fuel in some way, while still achieving the desired acceleration, coasting velocity and deceleration, one could shorten travel time or carry more passengers and/or cargo. 
     Several projects have explored the possibility of nuclear spacecraft propulsion. The first of these was Project Orion from 1958-1963 built upon general proposals in the 1940s by Stanislaw Ulam and others, in which external atomic detonations would form the basis for a nuclear pulse drive. Later, between 1973 and 1978, Project Daedalus of the British Interplanetary Society considered a design using inertial confinement fusion triggered by electron beams directed against fuel pellets in a reaction chamber. From 1987 to 1988, Project Longshot by NASA in collaboration with the US Naval Academy developed a fusion engine concept also using inertial confinement fuel pellets but this time ignited using a number of lasers. Naturally, these last two projects depend upon successfully achieving nuclear fusion. 
     Muon-catalyzed fusion was observed by chance in late 1956 by Luis Alvarez and colleagues during evaluation of liquid-hydrogen bubble chamber images as part of accelerator-based particle decay studies. These were rare proton-deuteron fusion events that only occurred because of the natural presence of a tiny amount of deuterium (one part per 6000) in the liquid hydrogen. It was quickly recognized that fusion many orders of magnitude larger would occur with either pure deuterium or a deuterium-tritium mixture. However, John D. Jackson (Lawrence Berkeley Laboratory and Prof. Emeritus of Physics, Univ. of California, Berkeley) correctly noted that for useful power production there would need to be an energetically cheap way of producing muons. The energy expense of generating muons artificially in particle accelerators combined with their short lifetimes has limited its viability as an Earth-based fusion source, since it falls short of break-even potential. 
     Another controlled fusion technique is particle-target fusion which comes from accelerating a particle to sufficient energy so as to overcome the Coulomb barrier and interact with target nuclei. To date, proposals in this area depend upon using some kind of particle accelerator. Although some fusion events can be observed with as little as 10 KeV acceleration, fusion cross-sections are sufficiently low that accelerator-based particle-target fusion are inefficient and fall short of break-even potential. 
     It is known that cosmic rays are abundant in interplanetary space. Cosmic rays are mainly high-energy protons (with some high-energy helium nuclei as well) with kinetic energies in excess of 300 MeV. Most cosmic rays have GeV energy levels, although some extremely energetic ones can exceed 10 18  eV.  FIG. 3  shows cosmic ray flux distribution at the Earth&#39;s surface. In near-Earth space, the alpha magnetic spectrometer (AMS-02) instrument aboard the International Space Station (ISS) since 2011 has recorded an average of 45 million fast cosmic ray particles daily (approx. 500 per second). The overall flux of galactic cosmic ray protons (above earth&#39;s atmosphere) can range from a minimum of 1200 m −2 s −1  sr −1  to as much as twice that amount. (The flux of galactic cosmic rays entering our solar system, while generally steady, has been observed to vary by a factor of about two over an 11-year cycle according to the magnetic strength of the heliosphere.) Outside of Earth&#39;s protective magnetic field (e.g. in interplanetary space), the cosmic ray flux is expected to be several orders of magnitude greater. As measured by the Martian Radiation Experiment (MARIE) aboard the Mars Odyssey spacecraft, average in-orbit cosmic ray doses were about 400-500 mSv per year, which is an order of magnitude higher than on Earth. 
     Cosmic rays are known to generate abundant muons from the decay of cosmic rays passing through Earth&#39;s atmosphere. Cosmic rays lose energy upon collisions with atmospheric dust, and to a lesser extent atoms or molecules, generating elementary particles, including pions and then muons, usually within a penetration distance of a few cm. Typically, hundreds of muons are generated per cosmic ray particle from successive collisions. Near sea level on Earth, the flux of muons generated by the cosmic rays&#39; interaction by the atmosphere averages about 70 m −2 s −1 sr −1 . The muon flux is even higher in the upper atmosphere. These relatively low flux levels on Earth reflect the fact that both Earth&#39;s atmosphere and geomagnetic field substantially shields our planet from cosmic ray radiation. Mars is a different story, having very little atmosphere (only 0.6% of Earth&#39;s pressure) and no magnetic field, so that muon generation at Mars&#39; surface is expected to be very much higher than on Earth&#39;s surface. Planetary moons, such as Phobos and Deimos around Mars, would experience similar high levels of cosmic ray flux. 
     In recent years, there have been proposals to send further spacecraft to Mars in 2018 and then manned space vehicles to Mars by 2025. One such development project is the Mars Colonial Transporter by the private U.S. company SpaceX with plans for a first launch in 2022 followed by flights with passengers in 2024. The United States has committed NASA to a long-term goal of human spaceflight and exploration beyond low-earth orbit, including crewed missions toward eventually achieving the extension of human presence throughout the solar system and potential human habitation on another celestial body (e.g., the Moon, Mars). Unmanned missions to Europa (a moon of Jupiter) and other bodies are also planned. 
     It is generally expected to take about nine months to travel to Mars. To get to Mars in less time would require that one burn the rocket engines longer to achieve a higher coasting velocity, but this uses more fuel and isn&#39;t feasible with current rocket technology planned for the Mars Colonial Transporter. The spacecraft has an overall payload of 100 metric tons, calling for a significant weight penalty in fuel for its liquid rocket engines. Once Mars orbit is reached, the vehicle is too massive to rely upon parachutes and/or a “sky crane” tethered system to descend to the Martian surface. Supersonic retro-propulsion using thrust from large rocket engines are expected to do the job. 
     The advancing of propulsion technologies would improve the efficiency of trips to Mars and could shorten travel time to Mars, reduce astronaut health risks, and reduce radiation exposure, consumables, and mass of materials required for the journey. Sustained investments in early stage innovation and fundamental research in propulsion technologies and an incremental buildup of capabilities is required to meet these goals. 
     This research and development activity is expected to proceed in several general stages, beginning with an Earth-reliant stage with research and testing on the ISS of concepts and systems that could enable deep space, long-duration crewed missions, followed by a proving ground stage in cis-lunar space to test and validate complex operations and components before moving on to largely Earth-independent stages. Such a proving ground stage would field one or more in-space propulsion systems capable of reaching Mars to undergo a series of shakedown tests to demonstrate their capabilities, select a final architecture, and make needed upgrades revealed by the shakedown tests. While systems already in development for the initial Earth-reliant missions largely make use of existing technologies, investment in the development of newer technologies will be needed to meet the longer-term deep space challenges. 
     SUMMARY DISCLOSURE 
     The present invention is a propulsion technology that takes advantage of the abundance of cosmic rays available for free in interplanetary space and the abundance of muons generated on Mars or other planets (and their moons) with a thin (or no) atmosphere and weak (or no) magnetic field in order to catalyze fusion events sufficient to produce thrust for orbital capture and then descent to the planet surface. The cosmic rays and muons are available here for free and do not need to be generated artificially in an accelerator. Since the amount of energy needed for thrust is generally much less than the multi-kiloton yields of atomic weapons, “micro-fusion” is the term used here to refer to fusion energy outputs of not more than 10 gigajoules per second (2.5 tons of TNT equivalent per second), to thereby exclude macro-fusion type explosions. 
     Micro-fusion fuel targets when shot externally ahead (i.e. in the flight path) of the spacecraft or descent vehicle will interact with the flux of cosmic rays and muons such that some combination of particle-target micro-fusion and/or muon-catalyzed micro-fusion will take place, generating a decelerating or braking thrust against the vehicle. A similar concept can also be used for the initial acceleration, with the micro-fusion fuel pellets being shot behind the spacecraft. An external pusher configuration similar to that proposed for Projects Orion, Daedalus or Longshot could be used to receive the thrust, except that here it is billions of controlled, non-contaminating micro-fusion events, not atomic explosions, are the source of that thrust. 
     The deuterium “fuel” for the particle-target and/or muon-catalyzed micro-fusion may be supplied in the form of solid LiD, or even heavy water (D 2 O) or liquid deuterium (D 2 ). Muon-created muonic deuterium can come much closer to the nucleus of a similar neighboring atom with a probability of fusing deuterium nuclei, releasing energy. Once a muonic molecule is formed, fusion proceeds extremely rapidly (on the order of 10 −10  sec). One cosmic ray particle can generate hundreds of muons, and each muon can typically catalyze about 100 micro-fusion reactions before it decays (the exact number depending on the muon “sticking” cross-section to any helium fusion products). 
     Other types of micro-fusion reactions besides D-D are also possible depending upon the target material. For example, another reaction is Li 6 +D→2He 4 +22.4 MeV, where much of the useful excess energy is carried as kinetic energy of the two helium nuclei (alpha particles). Additionally, any remaining cosmic rays can themselves directly stimulate a micro-fusion event by particle-target fusion, wherein the high energy cosmic ray particles (mostly protons, but also helium nuclei) bombard relatively stationary target material. When bombarded directly with cosmic rays, the lithium may be transmuted into tritium which could form the basis for some D-T micro-fusion reactions. Although D-D micro-fusion reactions occur at a rate only 1% of D-T fusion, and produce only 20% of the energy by comparison, the freely available flux of cosmic rays and their generated muons should be sufficient to yield sufficient micro-fusion energy output for practical use. 
     The present invention achieves nuclear micro-fusion using deuterium-containing target material, and the ambient flux of cosmic rays and generated muons that are already naturally present. The optimum concentration of the target material for the particle-target and muon-catalyzed fusion may be determined experimentally based on the particular abundance of cosmic rays with a view to maintaining billions of micro-fusion events for producing adequate thrust for the specified application, while avoiding any possibility of a runaway macro-fusion event. 
     At a minimum, since both particle-target micro-fusion and muon-catalyzed micro-fusion, while recognized, are still experimentally immature technologies (since measurements have only been conducted to date on Earth using artificially accelerated particles and generated muons from particle accelerators), various embodiments of the present invention can have research utility to demonstrate feasibility in environments beyond Earth&#39;s protective atmosphere and/or geomagnetic field, initially above Earth&#39;s atmosphere (e.g. on satellite platforms) for trial purposes, and then on the Moon or the surface of Mars, in order to determine optimum parameters for various utilities in those environments. For example, the actual number of fusion reactions for various types of micro-fusion fuel sources and target configurations, and the amount of rocket thrust that can be derived from such reactions, are still unknown and need to be fully quantified in order to improve the technology. 
     The fusion-enhanced space vehicle requires strong cosmic ray flux to create sufficient nuclear micro-fusion for thrust purposes. Therefore, it applies primarily to the final stage of a multistage rocket fired from Earth or any stage fired from Mars or the Moon. A chemical-based rocket may be used to first lift the spacecraft from Earth into orbit (and possibly also insertion from Earth orbit into an initial trajectory headed toward its deep-space destination), and then the micro-fusion propulsion is used for the spacecraft&#39;s flight to its destination as well as for deceleration to obtain orbit at the destination. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side plan view of a spacecraft shooting projectiles along a trajectory that disperse a cloud of micro-fusion fuel material for reaction with cosmic rays and muons according to the present invention. 
         FIG. 2  is a front-end view of the spacecraft of  FIG. 1  that illustrates an arrangement of projectile guns housed around a circumference of the spacecraft. 
         FIG. 3  is a graph of cosmic ray flux at the Earth surface versus cosmic ray energy. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to  FIG. 1 , one technique is to project the fusion target material outward from a spacecraft  11 . A supply of deuterium-containing micro-fusion “fuel” material is provided, which can be solid Li 6 D in powder form, D-D or D-T inertial-confinement-fusion-type pellets, D 2 O ice crystals, or droplets of (initially liquid) D 2 . One shoots “fuel” packages as a series of projectiles  13 , e.g. once every minute or once per second, which can then disperse the micro-fusion material as a localized cloud  15 , much like fireworks or artillery from an antiaircraft gun. The spacecraft propulsion system works in the presence of an ambient flux  16  of cosmic rays and/or muons which interact with the cloud and trigger the nuclear micro-fusion of the particle target material, either by particle-target fusion or muon-catalyzed fusion or both. Fusion products having significant kinetic energy are generated and are received at some portion of the spacecraft (e.g. the flat nose  17 , a much larger diameter disc on the flat nose  17 , the larger diameter forward surface of the gun  23  mounted around the spacecraft, or some other pusher arrangement, like those described in, Projects Orion, Daedalus, or Longshot) to produce thrust upon the spacecraft. 
     Stored fuel packages will be shielded to reduce or eliminate premature fusion events until they are to be delivered in front of (or in the case of acceleration, behind) the spacecraft. An inter-planetary astronaut crew will itself need shielding from radiation (which can cause brain damage and other adverse health effects). Therefore, the crew&#39;s shielding could double as a shield for the fuel packages. One important source of such shielding will be the spacecraft&#39;s water supply, which should be adequate for the task. One need not eliminate cosmic rays or their secondary particles (pions, muons, etc.) to zero, but merely reduce their numbers and energies sufficiently to keep them from catalyzing sufficiently large numbers of fusion events in the stored target particle material. Additionally, since the use of micro-fusion fuel is expected to reduce the required amount of chemical rocket propellant by at least a factor of two, one can easily afford the extra weight of some small amount of metal for shielding, if needed. (For example, the Juno spacecraft to Jupiter contains radiation vaults of 1 cm thick titanium to shield its electronics from external radiation. A similar type of vault might be used in this case for the shielding the stored fuel.) After being shot from the spacecraft, the casing of the projectiles themselves will continue to provide some shielding until dispersal of the target particle material as a cloud. 
       FIG. 2  shows the front-end view of a set of fuel projectile guns  21  (here four in number, labeled A-D, as an example, although a spacecraft like the Mars Colonial Transporter could house  100  of them) located in a housing  23  surrounding a circumference of the shell  25  of the spacecraft  11 . The flat nose  17 , a large disc covering it, or other mechanism of the spacecraft for receiving the kinetic-energy-containing fusion products can also be seen. 
     Soon after the projectile has reached the desired distance from the spacecraft the fuel package releases its particle target material. For example, a chemical explosion can be used to locally disperse the micro-fusion material. The dispersed cloud of target material will be exposed to both cosmic rays and, especially during landing, to their generated muons. As cosmic rays collide with micro-fusion targets and dust, they form muons that are captured by the deuterium and that catalyze fusion. Likewise, the cosmic ray collisions themselves can directly trigger particle-target micro-fusion. In order to assist muon formation, especially when D 2 O or D 2  is used, the target package may contain up to 20% by weight of added particles of fine sand or dust. (This is particularly important if one desires to create a similar fusion reaction in interplanetary space, or over the Moon, which has no atmosphere.) 
     Besides D-D micro-fusion reactions, other types of micro-fusion reactions may also occur (e.g. D-T, using tritium generated by cosmic rays impacting the lithium-6; as well as Li 6 -D reactions from direct cosmic ray collisions). For this latter reaction, it should be noted that naturally occurring lithium can have an isotopic composition ranging anywhere from as little as 1.899% to about 7.794% Li 6 , with most samples falling around 7.4% to 7.6% Li 6 . Although LiD that has been made from natural lithium sources can be used in lower thrust applications or to inhibit runaway macro-fusion events, fuel material that has been enriched with greater proportions of Li 6  is preferable for achieving greater thrust and efficiency. 
     The micro-fusion reaction creates successive miniature suns, a kind of “external” combustion that will provide thrust against the spacecraft for braking or accelerating. Even the photon radiation applies pressure to help decelerate the spacecraft. However, the amount of energy generated depends upon the quantity of fuel released and the quantity of available cosmic rays and muons. Assuming most of the energy can be captured and made available for thrust, an estimated 10 15  individual micro-fusion reactions (less than lag of fuel consumed) per second would be required for 1 kW output. But as each cosmic ray can create hundreds of muons and each muon can catalyze about 100 reactions, the available cosmic ray flux in interplanetary space is believed to be sufficient for this rocket thrust purpose following research, development, and engineering efforts. 
     A piston area extension may be supplied around the perimeter of the spacecraft for increased thrust during accelerating and braking, and for storage and delivery of the fusion fuel projectiles or “shells” using a set of four or more “guns” that fire the projectiles forward or backward from the vehicle. The spacecraft effectively acts as the equivalent of a piston in an external combustion engine and the volume of the continuous slow micro-fusion creates high velocity fusion products (alpha particles, etc.) that bombard the front of the spacecraft and its piston area extensions. The needed of firing depends on the amount of deceleration required, the amount of fusion obtained from the ambient cosmic ray and/or muon flux, the dispersal rate of the fuel cloud from in front of the craft, and the efficiency of the transfer of the fusion products into thrust, but could be expected to be as much as one shell every few seconds for some spacecraft and once per second for the largest spacecraft for the duration of the accelerating or braking period. A large diameter flat disc can be mounted on the nose cap  17  at the front of the spacecraft to increase the efficiency of thrusting for braking and accelerating. 
     Additionally, it may be possible to generate electrical or magnetic fields, e.g. by charging the piston area extensions or large diameter flat disc, or by magnetizing the same or the spacecraft as a whole, to help steer cosmic rays toward the fusion fuel particle cloud (and away from astronaut crew areas) or to focus the electrically charged, high velocity helium nuclei fusion products onto the spacecraft&#39;s thrusting surfaces. This will increase thrust efficiency by capturing a greater portion of the kinetic-energy-bearing fusion products. 
     While the embodiment of the present invention described herein only utilizes thrust created by the kinetic energy of helium nuclei micro-fusion products that directly bombard the spacecraft, other embodiments may create thrust via the helium nuclei micro-fusion products impacting outboard parachutes or sails connected to the craft, thereby capturing kinetic energy of micro-fusion products moving away from the spacecraft.