Stanton Friedman


The design and development of nuclear flight-propulsion systems requires the solution of very real problems associated with complex nuclear physics, sophisticated hardware operating at very high temperatures, and the lethal radiation produced by the fission process. Similar problems, although not as difficult, were solved first for nuclear weapons and then in the production of a large, relatively low-temperature submarine and
Project Orion Nuclear Rocket
Project Orion Nuclear Rocket NASA
stationary nuclear-power plants. The primary difficulty in employing fission for space or atmospheric propulsion systems is associated with the weight and high performance limitations of such systems. Large ships weight more than a hundred thousand tons. Airplanes weigh fewer than four hundred tons, and even the Saturn 5 rocket weighed only three thousand tons. Despite the problems, the NRX A-6 nuclear-rocket-reactor propulsion system was successfully tested in December 1967 by Westinghouse Astronuclear Laboratory at a power level of 1.1 billion watts in a package less than ten feet long and under five feet in diameter. In June 1968 the Los Alamos Scientific Laboratory successfully tested the Phoebus-2B at a power level of 4.4 billion watts; it had a diameter under six feet. The old Grand Coulee Dam produced 2.2 billion watts by comparison. All the NERVA (and preceding KIWI and Rover) systems used solid fuel, through which was pumped liquid hydrogen which changed to a gas and was exhausted through a nozzle. Because hydrogen has the lowest weight of any molecule, for the same energy expended it will achieve the highest exhaust velocity. The weight of the oxygen and its associated tankage is also eliminated. More advanced systems have been designed in which the U-235 is in a very high-temperature gas-plasma form and thus provides far higher exhaust temperatures for the hydrogen. Reactors actually have operated with the fuel in a gaseous form.

Of considerably greater interest from a long-term viewpoint would be fusion propulsion. Fusion is the nuclear process involving the combining of light nuclei to make heavier nuclei and, as in fission, convert a small amount of mass into a huge amount of energy. It is the primary process by which energy is produced in most stars and in so-called hydrogen bombs. Every civilization—even on distant stars—would become aware of the fusion process as it reached a minimal level of scientific maturity. There are many different reactions and processes which can be used in both fission and fusion devices. One of the most attractive for a space-propulsion system would be to cause the reaction of just those particles which, when made to fuse, produce only charged rather than neutral particles. These very high-energy particles then could be directed out the back of the rocket, using appropriate electric and magnetic fields. Neutral particles come off in all directions and cannot be directed or controlled, only slowed down and their heat absorbed . . . a very inefficient process. Using the right reactions in the right way, a space fusion-propulsion system could be designed to exhaust light ions having more than ten million times as much energy per particle as they can receive in a chemical rocket. A second advantage of considerable interest is that the fuel or propellant for a fusion rocket would be isotopes of hydrogen and helium, which are not only the lightest elements but are also by far the most abundant in the universe. Thus one could be certain of finding the raw materials for a fusion fuel stockpile in any star system to which one traveled.

There have been a number of studies published showing that staged fission and fusion deep-space propulsion systems are capable of round trips to nearby stars in a shorter time than an average life span. Chemical rockets would be used to launch starships into orbit or to the moon for relaunching from there because of the greatly reduced energy requirements on the moon. Clever design would be employed such as was used by the lunar landing program. Full advantage would be taken of every “free loading” possibility just as the Apollo vehicle takes advantage of the earth’s high rotation to the east near the equator and of the gravitational field of the moon and of staged rockets which fire in programmed succession on the way and by counting on the earth’s atmosphere to slow it down rather than carrying and firing retrorockets to slow it down on the way back. The final weight and cost depend almost entirely on the design assumptions rather than (as academic calculations so often assume) being independent of those design features. An early study of the required launch weight of a chemical rocket capable of sending a man to the moon and back concluded that the launch weight would have to be a million million tons. The launching was accomplished less than thirty years later with a chemical rocket weighing three hundred million times less.

Stars and planets along the way also would be used both for their fuel and solar energy and for gravitational assistance, just as the Pioneer spacecraft, which was without propulsion systems after leaving the vicinity of the earth, used the gravitational field of Jupiter to hurl itself past Saturn and eventually out of the solar system.

Earthlings are capable of building both fission and fusion deep-space propulsion systems if they are willing to spend the tens of billions of dollars required. However, these are not the only possibilities for interstellar travel. Other possibilities include:

  1. Lasers based on the earth, or in orbit, or on the moon, to be aimed at the back of the rocket, spilling off material which would exhaust toward the laser and push the rocket forward. This has the advantage of putting the power supply elsewhere than on board the rocket.

  2. Systems producing energy by some as yet unknown process power the strange stellar beasts known as quasars. Watts per gallon of fuel are enormously greater in a quasar than in a typical fusion-powered star like the sun.

  3. Systems utilizing whatever type of force holds subnuclear particles together are also a possibility. In the nucleus involved in fission and fusion the amount of energy per particle is much greater than in the larger atoms involved in chemical processes. Going inside the nucleus should also decrease the size of the particle but greatly increase the amount of energy available per particle.

  4. Systems using some means of bending space and time so as to “pop” from one place to another without having to really travel along the path between the points would do the trick. Picture a flat sheet of paper and then bend it so that diagonally opposite corners touch each other. Obviously travel between these touching corners would be more rapid than travel across the paper had it remained flat.

  5. We also must remember there undoubtedly are systems that we cannot yet imagine—just as fusion as the primary energy-producing process on the sun wasn’t understood until 1937 although it had been going on for five billion years. Any study of technological progress clearly shows us that progress comes from doing things in an unpredictable way. The future, technologically speaking, is not an extrapolation of the past.

An important aspect of the design of any interstellar propulsion system involves
taking full advantage of Albert Einstein’s theory of relativity. Theory and experiment have both clearly demonstrated that as things having mass such as people, particles, and starships approach the speed of light (c), time slows down for them as compared to those not moving so rapidly. The extent of the time slowdown depends on how close one approaches c, the speed of light. For example a one-way trip of thirty-seven years (the distance to Zeta 1 or 2 Reticuli) at 99.9 percent c would take only twenty months’ crew time; at 99.99 percent c it would take only six months’ crew time. Thus even a trip to a distant galaxy such as Andromeda, two million light-years away, would take under sixty years’ crew time if the intergalactic ship somehow could manage to keep accelerating at one G, using some yet unknown technique.

An important point to bear in mind in any discussion of interstellar travel is that it would be done in a systematic fashion. Observations would be made, unmanned craft would be sent, followed by orbiters, the installation of refueling stations, manned craft, colonizers, travelers, and all the rest. It took only twelve years from the time the first small satellite was launched before we accomplished a manned landing on the moon.

Considering that there are stars in our local neighborhood that are billions of years older than the sun, it would not be surprising if interstellar travel has been commonplace for billions of years. Several published papers have concluded that our Milky Way galaxy already has been colonized. Furthermore, it must be noted that travel between star systems is more likely to occur the closer the next system is. Zeta 1 and Zeta 2 Reticuli are both sunlike stars that are less than three light-weeks apart. Observers on a planet around one of them could easily observe planets around the other. One would certainly expect interstellar travel to develop earlier there than in our isolated corner of the neighborhood, where the nearest star to us is one hundred times farther away than the Zeta Reticulans are from each other.

Stanton Friedman

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