Project Icarus and the Motivation Behind Fusion Propulsion
by Kelvin F. Long
There are many proposed schemes for interstellar travel. These range from experienced based chemical fuels to highly speculative proposals such as the space drive. So when Project Icarus was put together, why did the team settle on ‘mainly fusion based propulsion’ in the Terms of Reference for the study? It is useful to spend some time justifying the motivation behind this decision. Firstly, it is necessary to understand some of the history of how Project Daedalus came about. It is generally accepted that the nuclear pulse propulsion scheme as proposed by Project Orion was demonstrated to work in principle. In other words, the Orion team produced a credible engineering design with most of the physics problems solved. What prevents something like Orion from becoming reality of course is the existence of several international treaties. In his autobiography ‘Disturbing the Universe’ the physicist Freeman Dyson clearly argues that today he does not support the propulsion scheme as proposed by Orion: “Sometimes I am asked by friends who shared the joys and sorrows of Orion whether I would revise the project if by some miracle the necessary funds were suddenly to become available. My answer is an emphatic no…..By its very nature, the Orion ship is a filthy creature and leaves its radioactive mess behind it wherever it goes…..many things that were acceptable in 1958 are no longer acceptable today. My own standards have changed too. History has passed Orion by. There will be no going back.” Other than Orion, there are several other propulsion systems which are potential candidates for the first missions to the stars. However, when one examines the potential performance and practicality of these different options objectively very few emerge as credible in the near term. This includes nuclear pulse propulsion and in particular either the continuous fusion or the pulsed fusion drive is desirable solutions. The pulsed fusion drive was used in the Daedalus design and uses a high intensity laser beam or electron driver to produce repeated detonation of fusion fuel pellets for thrust generation. The concept has been investigated thoroughly over the years from the work done on Project Orion through to extensive research on pulsed micro-explosions in the early 1970s. Hence, the physics of this type of propulsion scheme is understood sufficiently to enable confidence in any performance estimates for space applications. It is just the technology that is not yet mature. An ideal requirement for a deep space propulsive engine is the use of lightweight but energetic (high yield per unit mass) fuels. The fusion reaction of hydrogen isotopes comes out on top. A fission rocket is also credible but produces mass-energy conversion with a lower efficiency than for fusion reactions, as well as producing substantial radioactive products. Similarly, antimatter rockets also offer potential with a much greater mass-energy conversion than fusion, but the reaction products are difficult to direct for thrust and the production and storage of large quantities of antimatter is still a technical challenge. The ideal rocket would be a pure photon rocket. Fusion propulsion (and the more generic nuclear pulse scheme) offers advantages in performance that far outweigh other propulsion schemes. This includes a range of T/W ratio where in particular a low T/W of around .0001 is possible with very high exhaust velocities of around 10,000 km/s. Also, low mass ratios with very high specific impulse of up to a million seconds appear credible. The use of D/He3 reactions would seem to be the most promising fuel although other fuels such as D/T have potential. This sort of performance level would be required in order to reach any of the nearest stars within an approximately twelve light year radius in under a century. A fusion rocket which didn’t carry its own fuel would be even better and this has been proposed historically in the guise of the Bussard interstellar ramjet. The problem with this scheme however is achieving the fusion ignition of interstellar high flux ions or atomic hydrogen which has a smaller reaction cross section than hydrogen isotopes. Also, the drag of the spacecraft is proportional to the velocity, and a high velocity is required in order to collect sufficient matter using the magnetic scoop. Ensuring that the overall drag remains less than the thrust of the spacecraft is a technical challenge too. Most space propulsion systems can be classed into two categories. The first is those that are power limited and although can produce a high specific impulse and high exhaust velocity it is at low thrust. Electric rockets come under this category. The second is those that are energy limited and although can produce a high thrust with high exhaust velocity it is at the cost of a short specific impulse. Both Chemical and nuclear rockets come under this category and this is mainly because the fuel will get burned too quickly. Fusion propulsion offers the advantages of both categories with good propellant utilization. The concept of a fusion based drive is not limited to theoretical studies. Already an engine is in the process of being developed for Mars missions called the Variable Specific Impulse Magnetoplasma Rocket (VASIMR) which is essentially a scaled down fusion demonstrator engine if improvements in the power, shielding and field control were made. This engine offers the potential for bridging the gap between high thrust-low specific impulse technology as used in conventional rockets and low thrust-high specific impulse technology as used in electric ion engines and it can function in either mode. Similarly, for a full-up pulsed fusion engine, the employment of different pellet sizes also allows a tailoring to the amount of thrust needed for each pulse cycle, with some of the emitted energy being bootstrapped to run the next cycle. The physics of fusion research has moved forward dramatically in recent years with the US National Ignition Facility now operational and others such as Laser MegaJoule in France under construction. Fast ignition proposals such as HiPER are also under consideration. The chances for scientists finally solving the ‘fusion problem’ are greatly increased. With this in mind, thinking about the implications to a deep space missions is timely. It is quite possible that the demands of a fusion based drive will necessitate a sophisticated space based infrastructure for resource acquisition, processing, manufacture and construction. Especially if He3 mining of the gas giant Jupiter or even the Moon is considered. However, as a theoretical exercise in the application of science and engineering Project Icarus has a large amount of intrinsic worth. There is another reason why fusion was chosen as the main propulsion engine. Daedalus was a historical design study that changed perceptions about what was possible with interstellar flight. When thinking about performing a design study for an interstellar craft, instead of starting from scratch with an unfocussed ‘anything goes’ design philosophy, it is arguably more useful to build upon the good work that has already been done, essentially standing on the shoulders of the previous generation of designers. Hence, a re-examination of Daedalus seemed an obvious way to go. This would allow a complete re-evaluation of the original assumptions as well as hopefully improve the design. Ultimately, the aim would be to improve the Technological Readiness Level for this sort of engine design type. If other teams used the same approach, and say built upon historical projects like Vista, Longshot, TAU or Starwisp it is a personal belief that the credibility of engineering designs for interstellar missions would be vastly improved. The historical link with both Orion and Daedalus also captured the hearts of the Icarus team and made for a strong support base upon which to galvanize both academic and public interest; a necessary condition to inspire people that this design study is worth doing. Although it is also true that after having questioned the original assumptions of Daedalus, the final Icarus design may look very different with technology not envisaged in the 1970s. Of course, it is also worth pointing out that the Project Icarus Terms of Reference actually stipulate ‘mainly fusion based propulsion’. This allows for the potential for high gain enhancements such as by using Antimatter Catalyzed Fusion techniques. Similarly, the main engine can be supplemented by a secondary engine for part of the mission trajectory, such as by using a nuclear-electric engine. It is generally the consensus within the interstellar community that the two strongest candidates for interstellar flight and which are a balance between performance and near term technology readiness is arguably solar sails and nuclear pulse propulsion. The large interest today in using solar sails for interstellar missions came about as a result of a publication in 1984 titled ‘World Ships: Concept, Cause, Cost, Constructions, and Colonization”, written by several members of the Daedalus Study Group who had spent many years studying fusion based propulsion. Thus demonstrating that the act of completing an interstellar design study opens up opportunities perhaps not before envisioned. If designers concentrate on trying to advance these two schemes, solar sailing and nuclear pulse, the prospects for robotic missions to the Kuiper belt, Oort cloud and beyond will become ever more likely. Project Icarus hopes to contribute towards this end objective. It was the vision of Arthur C Clarke that humans should expand out into space as soon as possible, a necessary step for the continued survival and advancement of our species. Project Icarus is aimed at working towards this ultimate goal. So while from time to time we may have to justify to a sceptical public the motivations behind the assumptions of Project Icarus such as the choice of a fusion engine, the design team will remain focussed on achieving our objectives and continue to dream about the challenge of the spaceship.
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