How We Can Reach The Stars Within The Next Two Centuries If Not Much Sooner: Case 4, Antimatter Catalyzed Fusion Reaction Powered Propulsion Systems.

This fourth part of the to be continued series on manned interstellar travel systems that are potentially practical and doable this century, perhaps during its first half, or by next century is on the topic of antimatter catalyzed nuclear fusion powered space craft.

 Nuclear fusion reactors can in theory be used to energize electron, ion, or photon rockets as well as electrodynamic-hydrodynamic-plasma drive systems.

In addition, nuclear fusion rockets can be used to directly propel manned starships.

 In order to improve system efficiency, a fusion reactor can be surrounded by multi-cycle fluid cooling systems that operate lower temperature turbine systems, and the final waste heat left over and needing to be exhausted from the system in order to provide a thermodynamic gradient for the system to operate can be radiated out into space behind extremely infrared reflective mirrors that are optimally designed  to provide unidirectional backward thermal radiation reflection in order to increase the effective specific impulse of the fusion fuel.

  For a traditional form of nuclear fusion powered propulsion, we discussed a perhaps   reasonable, M0/M1 = 100, we have delta V = C tanh [(0.119C/C) ln 100]=  0.49899 C, and for a very reasonable M0/M1, we have delta V = C tanh [(0.119C/C) ln 10] = 0.2673 C.. Note that Mo is the total fueled rest mass of the propulsion system whereas M1 is the dry weight or final payload mass. The mathematical form of the above equation is delta V = C tanh [(Isp/C) ln (Mo/M1), where Isp is the specific impulse of the fuel expressed in units of C. The best performing fusion fuel reaction yields a maximum theoretical specific impulse of about 0.119 C when expressed in units of C.

 But what about the possibility of utilizing a relatively small mass of antiprotons carried on board the craft wherein the antiprotons could be used to initiate nuclear fusion of small fusion fuel pellets? Such a mechanism might entail carrying 0.7 percent of the rest mass of the total fuel in the form of antiprotons. The anti-protons would interact with the protons in the normal matter fusion fuel with the result that from matter /antimatter annihilations alone, about 1.4 percent of the total fuel rest mass would be converted into energy. Add to this, the maximum 0.7 percent mass conversion into energy as a result of the fusion of protons into Helium and one obtains a conversion efficiency of mass into energy which can be used to propel the space craft of a whopping 2.1 percent or [(2 x 1.4) + 0.7] percent.

The above concept is not trivial in its scope as it permits significant increases in maximum theoretical Isp performance of the fuel over the case of traditional nuclear fusion schemes.

Note that the rest mass of the proton 938.272013(23) MeV/[C EXP 2], so as a result, of a single proton antiproton fusion, about 1,879 MeV is produced.

The antimatter might be stored in a neutral form as anti-hydrogen ice, either in bulk form or in the form of ready to annihilate tiny pellets that would be fired into or otherwise mixed with the normal matter hydrogen fuel, which may also take on a frozen form of hydrogen ice. Alternatively, the anti-hydrogen ice could be ionized whereupon beams of anti-protons would be fired into the portions of hydrogen fuel to be fused thereby causing mini thermonuclear explosions wherein the heat generated by the matter antimatter annihilation would cause the hydrogen fuel pellets to fusion and explode.

As mentioned in the previous article in this series, the problem of how to store all of the required nuclear fuel in proximity to the ship without creating a critical or supercritical mass is completely alleviated through the use of fusion power propulsion systems instead of fission powered systems. 

Fusion fuel is cheap and the most mass wise abundant baryonic matter fuel in the known universe. Locked up within the Gas Giant planets of our solar system, and within the Kuiper Belt and Oort cloud, there may exist a total of one solar mass or more of hydrogen and/or helium fuel. Assuming an averaged manned star ship uses 1 million metric tons of fusion fuel per mission that involves a 10,000 metric ton, or a 100,000 metric tons space craft, this is enough fuel to launch 10 EXP (27 – 6) missions or 10 EXP 21 missions. This works out to be about 100 billion missions per year over a period of 10 billion years. This yearly flight rate is far more than the total number of airline commercial passenger flights per year on Earth.

One way to contain the fuel is to store it within large tanks that  could be  made of highly refractive alloys, that would be largely immune from the cold of interstellar space and any partial exposure to the exhaust stream. Other materials for fuel tank construction might be carbon fiber epoxy, carbon nanotube materials, or carbon Hexane materials which in theory, can be stronger than carbon nanotube materials. The later two of these materials do not yet exist in commercial industrial scale bulk quantities.

 Some of the spent fusion fuel or perhaps all of it can be used as a reaction mass in ion rockets powered by electrical power generated from the nuclear fusion reactors. It may even be the case that portions of the fusion products can be collected and used again for yet another cycle of fusion, perhaps followed by additional cycles until the last remaining end product of the most stable form of Iron is produced at which point, no further energy can be extracted due to nuclear fusion. The iron can then be exhausted as an ionic reaction mass. The fusion of these higher atomic number initial fusion reaction products can also be catalyzed by antimatter.

In order to operate the reactors at high temperatures to achieve high reactor mass specific power densities, the nuclear reactors could be liquid molten metal cooled reactors wherein the molten metal that flows through piping deposits its heat into a water steam based heat exchanger system that drives turbo-electric generators. Regenerative counter-flow heat exchange mechanisms can be used to maintain the high temperatures of the primary thermodynamic cycles.

As part of  secondary, and tertiary, etc,  heat exchanging systems, lower boiling temperature liquids could be used to drive secondary and tertiary turbo-electric generators. At the final end stage of the cooling process before the residual waste heat is radiated to space, the waste heat from the final steam cycle could be used to heat thermo-electric cell materials for a final power extraction phase before the remaining energy is discarded to the vacuum of space.

 In the case of the pure non-antimatter based fusion rockets, say we have a space craft with a payload rest mass of 1 million metric tons, and an Mo/M1 ratio of ten. Then the required amount of fuel to obtain a velocity 0.2673 C is at least 9 million metric tons. To obtain a velocity of 0.49899 C, as stated above, the Mo/M1 value is at least 100. Thus, for the million metric ton payload, 99 million metric tons of fusion fuel would be required.  

 Now assuming that 0.2673 C can be reached in one year, the craft could simply cruise and travel about  26 light years in 100 years Earth time which is close to but not equal to 100 years ship time due to the slight relativistic time dilation experienced by the craft. The craft could be set on route to any star within  26 light years of Earth. Since the craft would necessarily be very large because of the mass of the nuclear reactors and the required shielding, the craft might best be designed as a multi-generation ship wherein successive generations of humans would be born enroute and trained to operate and maintain the craft on its heading.

Now assuming that 0.49899 C can be reached in one year, the craft could simply cruise and travel about  49 light years in 100 years Earth time which is close to but significantly less than the corresponding 86.7 years ship time due to the relativistic time dilation experienced by the craft which starts to become significant at velocities around 0.5 C. The craft could be set on route to any star within  49 light years of Earth. Since the craft would necessarily be very large because of the mass of the nuclear reactors and the required shielding, the craft might best be designed as a multi-generation ship wherein successive generations of humans would be born enroute and trained to operate and maintain the craft on its heading The relativistic gamma factor of a  craft traveling at 0.5 C is 1.155.

 Note that there are about 1,400 stars within 50 light years of Earth, about 133 which are visible to the naked eye. Most of the visible stars are similar to our Sun. Most of the non-visible stars are red dwarfs. Note that some stellar and planetary evolution numerical studies suggest that planets within the habitable zone of Sun like, or G class stars as well as cooler K and M class stars may be common. This is a huge amount of territory to explore.

For 25 light year journeys, the travel time would be equal to the productive working lifetime of a healthy adult human and such missions could be accomplished in about 43.3 years time. There are 33 stars just within 12.5 light years of Earth and so such missions could be used to explore much territory.

 A medically enhanced life expectancy of 1,000 years could permit crews to arrive at stars that are 1,900 light-years distant yet still allow the crew members an average of most of a century to live out the rest of their lives on the new home.

 Even at 0.49899 C, the problem posed by cosmic rays seems manageable for such a large craft since the relative energies of the incident gas atoms and plasma particles would be within 1 1/2 orders of magnitude of that of radiation from nuclear fission reactors, which obviously is currently capable of being adequately absorbed via radiation shielding.

Now if we can obtain such relativistic velocities from the above M0/M1 examples using non-antimatter catalyzed fusion, imagine what we can achieve with antimatter catalyzed fusion wherein 0.7 percent of the total fuel is carried on board from the start. In the limit where the antihydrogen supply has the same mass as that of the carried on board fuel, for traditional matter antimatter rockets that utilize proton anti-proton annihilation, the maximum theoretical Isp is about 0.6 C. In cases where the entire energy produced in the reactions is converted to ship kinetic energy (i.e., with 100 % efficiency), then the effective Isp = 1C or simply C exactly.

Note that once Isp values of fuel begin to climb above  those of pure nuclear fuels, the ability to propel craft to high gamma factors exists wherein such gamma factors can reduce the effective travel time of the crew to stellar destinations by one half an order of magnitude or more for plausible and perhaps reasonable Mo/M1 rations. Even for modest gains in Isp over the 0.119 C maximum theoretical values for the best performing fusion fuel, the terminal gamma factor of a given craft grows dramatically.

Regarding radiation shielding for the craft, perhaps a permanent magnet or  electromagnet based “mini-magnetosphere” can be set up around the craft that diverts the incident plasma toward the poles of the magnetic field where they can be collected and absorbed, and possibly be used as reaction mass and/or fuel such as for nuclear fusion reactors that provide extra power for the space craft propulsion systems or maybe even as rocket fuel directly for nuclear fusion rockets.

The space craft could be slowed by electro-dynamic breaking mechanisms such as deployed magnetic induction breaking coils, a magnetic sail, and/or an electro-dynamic-hydrodynamic-plasma based breaking systems and the like electrical breaking mechanisms. Such systems would negate the need to use fuel or carry extra fuel as might otherwise be required to slow the craft down through reverse thrust.

As far as micrometeoroid sized interstellar dust particles and smaller dust particle are concerned, perhaps a layered or tiered combination of water shields, in conjunction with shields made of very high heat capacity materials such as isotopically optimized artificial diamond; and also vacancies between such layers where very strong permanent magnets could be judiciously arranged in order to deflect the plasma that would punch through the water and carbon shields, could permit space craft to travel at velocities of at least 0.2 to 0.3 C, and perhaps velocities with a gamma factor in the 10 EXP 3 to 10 EXP 4 range such as those that might be obtained with other forms of interstellar propulsion, which is within the range of the Tevatron at Fermilab at the lower end and the LHC at the higher end.

The larger the space craft, the larger the radius the magnetic field permeated vacancies between the layers can be. Any collected mass during the deflection and filtering process could perhaps be broken down into pure energy by some yet to be discovered means or at least used as fusion fuel and/or reaction mass. A mechanism to convert the kinetic energy of the incoming plasma to ship based kinetic energy would be great, also, in order to reduce the net drag on the spacecraft by the interstellar medium.

The idea of using a liquid water shield is appealing since the water within the shield would immediately flow back into position after the plasma dust particle left its track through the water. Alternatively, the use of hydrogen ice can be made to block the plasma trail produced by the dust particle with the added utility that the ice can be used at least in part as fusion fuel.

The fifth article in this series will be posted on this Friday or Saturday.

 

Regards;

 

Jim