Project Icarus: What else is “Daedalus” good for?
by Adam Crowl
“Project Daedalus” was inspired by advances in nuclear fusion technology as well as the 1960s “Orion” nuclear pulse rocket. The apparently rapid developments in laser and electron beam initiation of fusion in the early 1970s caused many to believe that interstellar travel was now at least possible, no longer merely in principle, but using real near-term technology. Thirty years later nuclear fusion is edging closer to “break-even” – when sufficient energy is generated to power the triggering of the reaction, at least if energy conversion machines were in place – and the possibility of interstellar travel has become more tangible. “Project Icarus” carries the torch of “Daedalus” further forward. Fusion will give us much more than interstellar travel, however. As the energy source of the Sun and the stars thus, indirectly of all life on Earth, fusion already has given us so much. Modern civilization became possible because of its use of chemical energy stored in the remains of living things once powered by the Sun, and in effect already uses (stored) fusion energy. While plenty of sunshine for our needs falls on planet Earth, it is spread very thinly and requires large areas to collect, and very large batteries or chemical fuels to store. Nuclear fusion can use the relatively abundant fuel of deuterium, found in any body of water in the Solar System, though some advocate rarer elements like boron and lithium, or even rarer isotopes like helium-3. Sufficient amounts of deuterium exist in the oceans to supply an energy-rich civilization on Earth for billions of years. With such abundant energy we could even mine the planet’s “rubbish” mountains for the resources we have discarded as “unusable” waste and begin reversing the damage wrought by 3 centuries of fossil-fuel powered industrial development. High-performance fusion rockets would also open up the Solar System to speedy access via large space-vehicles, as well as enabling star-probes. Assuming a 1,000 ton space vehicle and constant acceleration at 0.722 m/s2, which an exhaust velocity of 10,000 km/s with a mass-flow rate equal to “Daedalus’s” second-stage would produce, we get the following figures for travel times to various Solar System destinations. Table 1. Travel Times for Mean, Minimum and Maximum distances. Mean distance is the difference between the mean orbital radii of the destination and Earth, minimum uses the distance of the destination at its perihelion and the maximum places the destination at its aphelion on the opposite side of the Sun to the Earth.
As can be easily seen even distant Eris, the former Tenth Planet, can be reached in a little over 3 months for just 960 tons propellant. The most difficult flight to Mars needs a mere 60 tons of propellant to deliver 500 tons of space-vehicle and its 500 ton payload. For comparison, the nuclear-propelled Integrated Manned Interplanetary Spacecraft (ref. Encyclopedia Astronautica), flying to Mars and back on 420 day missions were extensively studied in the 1960s. They typically required 1226 tons of materials in Earth orbit, used 873 tons propellant, and carried only 110 tons of payload, accommodating 6 crew persons. Since the shutdown of the NERVA program in the early 1970s nuclear rocket technology has essentially stagnated. More recent VASIMR plasma-rocket technology requires 476 tons of propellant to deliver 124 tons of space-vehicle in 39 days to Mars, while requiring 200 megawatts of power from a Magnetohydrodynamic generator fed by a gas-core reactor, itself a technology almost as difficult to achieve as the “Daedalus” fusion engine. Another potential use for high-thrust, high-exhaust velocity fusion-rockets is the deflection and/or orbital management of comets and asteroids. Asteroids and comets more than 200 metres wide pose a long-term threat to human civilization, if not all life, on Earth. A fusion rocket using 100 tons of propellant and a 10,000 km/s exhaust velocity can produce a 0.1 km/s change in the orbital velocity of 200 metre wide asteroids that mass roughly 10 million tons. Even multi-billion ton asteroids, kilometres across, can be steered away from a planetary collision given a year or two of warning. Smaller asteroids can also be shepherded into more useful orbits and mined, their products returned to Earth-orbit via fusion rocket. “Daedalus”, and its successor “Icarus”, extend our fusion-making abilities by requiring greater energy efficiencies and harder to achieve fusion reactions. Regular deuterium fusion, for example, produces ~40% of its energy as neutrons, which produces large amounts of heat. On Earth this is very useful and we have advanced steam-era technology for turning heat into power, but in space heat must be radiated away and this complicates the energy conversion process. “Daedalus” reduced the neutron heat-load by using the deuterium-helium3 reaction which produces far fewer neutrons and its end products are charged particles that can be steered via electromagnetic fields in useful directions. “Icarus” hopes to do the same, but with a clearer view of what such reactions require from our fusion technology. On Earth such low-neutron fusion-power would reduce the heat being ‘dumped’ to the environment and reduce safety concerns from neutron-activation of containment vessel structure, a source of low-level radioactivity. The high-speed spaceflight enabled by high-efficiency, low-neutron fusion energy enables another desirable opening of possibilities – the mining of the gas-giants for helium-3. The interdependence of the two might seem a bottleneck, but there is a significant amount of helium-3 closer to Earth embedded in the crystal structure of the Moon’s regolith, deposited by the solar wind. As recently revealed the Moon’s regolith is also periodically coated by a thin layer of water, probably also produced by interaction with the solar wind. This may well provide a useful source of deuterium, thus allowing fusion rocket propellant to be extracted solely from the Moon, at least at first. The estimated helium-3 resource of the Moon is roughly 2.5 million tons, and the deuterium is probably similar. An estimated 4.1 million tons of fusion propellant as close as the Moon will be important in the early days of a developing solar economy. Long before we have disfigured the Moon we will have the means to mine the gas-giants, a resource many billions of times greater than the Moon’s. “Project Daedalus” was the vanguard of practical fusion propulsion technology, and “Project Icarus” will carry us all closer to that goal. A goal that once achieved will give us far more than the stars, nothing less than security for our planet and access to many more as well. “Icarus” aims for the stars, but will lift up all of us with it as well.
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