Guest contributor Richard Obousy is a member of the Tau Zero Foundation, a non-profit group of scientists dedicated to the incremental advancement of interstellar spaceflight. Richard is the current leader of Project Icarus, one of the Foundation's key initiatives and a project that builds on a landmark nuclear pulse propulsion study from the 1970s: Project Daedalus.
Project Daedalus was a feasibility study for an interstellar mission, using 1970s capabilities and credible extrapolations for near-future technology.
One of the major objectives was to establish whether interstellar flight could be realized within established science and technology. The conclusion was that it was possible, but that it would be very difficult.
The potential of fission/fusion power as a propulsion mechanism that would allow for interstellar flight has been recognized since the first half of the 20th century. The idea was initially proposed by Stanislaw Ulam at Los Alamos in 1947, and then in 1958 Ted Taylor initiated Project Orion.
Just over a decade later, Alan Bond of the British Interplanetary Society (BIS) believed that the time was right to investigate the feasibility of fusion for an interstellar mission. He discussed the idea with other members of the Society, and Project Daedalus was born.
Project Daedalus took just over 5 years — the project began on January 10th, 1973 and the final reports were published May 15th, 1978. Approximately 100,000 person hours were invested into the project by 13 core designers and numerous consultants.
The heart of Daedalus was the fusion pulse propulsion engine, in which small pellets of fusion fuel would be injected at high velocity into a reaction chamber and ignited by high-energy electron beams. Conceptually this is not vastly different from a conventional internal combustion engine, in which small droplets of gasoline are injected into a combustion chamber and ignited.
The resulting fusion reaction products in the Daedalus reaction chamber would be channeled axially rearward from the main vehicle by a number of field coils acting as a magnetic nozzle. This ejecta would be responsible for an overall momentum transfer to the vehicle — much like the exhaust from a rocket engine propels a space vehicle forwards — mediated by magnetic fields interacting with the reaction chamber.
Daedalus was to be a two stage spacecraft, with stage one carrying 46,000 tonnes of fuel and stage two carrying 4000 tonnes. After a total boost phase of nearly four years, it would be traveling at its top speed of 12.2 percent the speed of light, and would reach its target (Barnard's Star, located about six light years away) in 50 years.
Daedalus was to be an unmanned fly-by probe, and so would only stay in the target solar system for a relatively short period of time (about two days to cross the equivalent of the solar system), during which it would gather important scientific data from the target solar system.
One of the notable features of the Daedalus design was its use of Helium-3 in the fuel pellets. Helium-3 is one of the most difficult of the fusion fuels to ignite, requiring a higher ignition temperature when compared to other fusion fuels. However, its energy release is among the highest of the various fusion fuels, and thus leads to the greatest thrust.
Helium-3 is incredibly rare on Earth; however, there is strong evidence for concentrations of between 0.01-0.05 ppm (parts-per-million) on the lunar surface. Additionally, vast quantities of He3 are known to exist in the atmosphere of the gas giants.
The Daedalus mission involved a plan to mine the atmosphere of Jupiter. This requirement in itself indicates the need for a vast solar system-wide civilization with abundant capabilities and a massive space-based infrastructure, and so makes the challenge of building a 'Daedalus Class' spacecraft great.
Despite these difficulties, what is particularly enticing about the Daedalus design is that it is within the realms of credible science, since no new physics is required. This in itself does not imply that the task of building Daedalus would be easy, as the engineering and economical costs are quite staggering, but it is certainly encouraging that this design could be built, given sufficient ambition.
Project Icarus, which began on September 30th, 2009, was inspired by Daedalus, and is a 21st century attempt to re-examine the problem of interstellar propulsion with the benefit of over thirty years of scientific progress and understanding since the original project. Broadly stated, the purpose of Project Icarus is as follows:
1. To design a credible interstellar probe that is a concept design for a potential mission in the coming centuries.
2. To allow a direct technology comparison with Daedalus and provide an assessment of the maturity of fusion-based space propulsion for future precursor missions.
3. To generate greater interest in the real term prospects for interstellar precursor missions that are based on credible science.
4. To motivate a new generation of scientists to be interested in designing space missions that go beyond our solar system.
Of course, Icarus is remembered from Greek mythology as the figure who flew too close to the sun, melting his wings. At first glance, this name might be considered a peculiar choice for a spacecraft. However, it makes much more sense when considering the following quote, which describes an important aspect of the spirit of Project Icarus:
In ancient days two aviators procured to themselves wings. Daedalus flew safely through the middle air and was duly honoured on his landing. Icarus soared upwards to the sun till the wax melted which bound his wings and his flight ended in fiasco. In weighing their achievements, there is something to be said for Icarus. The classical authorities tell us that he was only "doing a stunt", but I prefer to think of him as the man who brought to light a serious constructional defect in the flying-machines of his day. So, too, in Science. Cautious Daedalus will apply his theories where he feels confident they will safely go; but by his excess of caution their hidden weaknesses remain undiscovered. Icarus will strain his theories to the breaking-point till the weak joints gape. For the mere adventure? Perhaps partly; this is human nature. But if he is destined not yet to reach the sun and solve finally the riddle of its constitution, we may at least hope to learn from his journey some hints to build a better machine.
--From Stars and Atoms, by Sir Arthur Eddington (Oxford University Press, 1927, p. 41)