Starflight on the Cheap II: Beating the Fuel Costs
by Adam Crowl
In Part 1 we looked at the cost of building and launching a “Daedalus” class star-probe, massing some 2,000 tonnes with empty tanks. The original “Project Daedalus” study fuelled the vehicle with a 50,000 tonne mixture of deuterium (worth $25,000/kg) and helium-3 (currently available only in tiny amounts on Planet Earth.) To solve the fuelling problem, the Design Team of “Daedalus” examined the different sources of helium-3, the lightest helium isotope, composed of just 2 protons and one neutron. What makes it rare is what makes it desirable as a star-probe fuel – it can be fused relatively easily. Not as easily as deuterium fusing with itself, or deuterium with tritium, but helium-3’s reaction with deuterium doesn’t make damaging high-energy neutrons like the former two reactions. So helium-3, like deuterium, is rare in the Universe because it fuses too easily relative to plain hydrogen and plain helium-4. Of deuterium there is no supply issue as it is typically present in almost all sources of water at the 150 parts per million level. That might not sound like much, but it’s enough to make it straight-forward to extract and concentrate. Heavy water – deuterium oxide – is used in a number of important nuclear power applications already and should not be difficult to supply. However it is expensive, as already noted. On Planet Earth there are just a few thousand tonnes of helium-3 in the atmosphere, which makes it far too hard to extract even for present day experiments. Fortunately for science it is a waste product of nuclear warheads, forming via the radioactive decay of tritium (hydrogen-3), and because it interferes with tritium’s role as a neutron-making booster of nuclear fission it has to be constantly removed. To supply 32,000 tonnes for a “Daedalus” class probe, preferably several probes, the source has to be more abundant. “Project Daedalus” examined several sources, like the Solar Wind and the Moon, which is believed to collect it from the Solar Wind, but settled on mining it from Jupiter, which has an abundance of helium. However there are a few problems with Jupiter as a fuelling station. Jupiter has a very large magnetic field which accumulates and accelerates high energy particles from the Sun. This makes operating near Jupiter very difficult for both machines and humans. Not impossible, but not easy either. Jupiter also has an immense gravitational field. To reach a low orbit around Jupiter requires a speed of 42 km/s – about 5.5 times the orbital speed in low Earth Orbit. Jupiter’s rapid rotation (it spins once every 10 hours) means a launch vehicle only needs to add 30 km/s to its motion with Jupiter’s rotation to reach orbit. That’s still very difficult, even for nuclear rockets and impossible with chemical rockets. Finally, thanks to direct sampling by the “Galileo” mission’s Entry Probe, we know Jupiter has somewhat less helium than expected. Let’s look further afield. Saturn is very attractive – it only needs a speed of 15 km/s added to its rotation speed to reach orbit – but it might be even more deficient in helium than Jupiter. A “Galileo” style Entry Probe is still needed to find out. About twice as far from the Sun is Uranus, which has the lowest speed to reach orbit of all the gas giants, just 12 km/s. Uranus, and Neptune, formed differently to Jupiter & Saturn, so they both might have enhanced levels of helium and deuterium, which would make Uranus doubly attractive. What would mining Uranus require? Ever since the clean fusion potential of Helium-3 became widely known, several different researchers have developed concepts to mine Uranus. A relatively detailed plan was developed by John Paniagua, James Powell, & George Maise (1999) which used a two-part set of nuclear propelled vehicles. A Uranus Transfer Vehicle would send a combined Earth Return Stage and Space Station Depot to Uranus, while a second would send a Lifting Body Shuttle to be used for servicing an Aerostat (Balloon) Mining platform. The Aerostat Mine, let’s call it, would be already set floating in Uranus’s atmosphere at the 10 bar pressure level, about 55 km deeper than the 1 bar reference level which defines the planet’s radius. Over a period of 180 days the Aerostat Mine would separate out the He-3 from the hydrogen and helium-4 via cooling the incoming gas mix to below the freezing point of hydrogen and below the point where helium-4 becomes a liquid, while helium-3 remains a gas. About 10 tonnes would be purified via this process, returned to orbit by the Lifting Body Shuttle and stored in the Space Station Depot. Once a full cargo of fuels was ready,then transferred from the Depot, the Earth Return Vehicle would launch it on a five year trip back to Earth. Having multiple ERVs being tanked up in staggered order would allow continuous delivery of helium-3, and deuterium if so desired, back to Earth. All together, the system suggested by Paniagua et.al. would cost about $15 billion to develop and deploy for the first 10 tonne cargo returned to Earth. The energy value of 10 tonnes of He-3 is roughly ~$150 billion, so it would be a profitable enterprise. However at $1.5 billion/tonne for He-3, the cost of tanking up “Daedalus” with 30,000 tonnes would be $45 trillion dollars. After many loads the price per load would approach ~$1.5 billion per load. Thus the cost would be more like ~$4.5 trillion, but that’s still an incredible price-tag. Extracted over the 20 years assumed by the “Daedalus” study, the fuel cost is ~$225 billion/year. How can we make the process more economical? All the moving components use a small nuclear rocket design only able to be reused a limited number of times, due to radiation and cooling limitations. The Uranus Transfer Stages and ERVs are single use stages, for example. Longer reuse for such nuclear engines requires a significantly heavier system, driving up initial development costs. For fuelling “Daedalus” the ERV system could be eliminated by flying a partially filled vehicle out to Uranus and tanking up in situ. Only a small fuel mass would be required to propel both empty stages to Uranus at a relatively high speed. In fact modified “Daedalus” 2nd Stages could be used to supply D/He3 directly back to Earth, with a far greater capacity than the 10 tonne cargo of the ERV. In that way the development of the “Daedalus” propulsion system could pay for itself by driving the costs of extracting He-3 downwards. Thus if we eliminate the ERV, we might slash the fuel cost for “Daedalus”, and by using modified 2nd Stage tankers, we might lower direct delivery costs for He-3 back on Earth 10 to 50-fold. Another factor is the Lifting-Body Shuttle, which is designed for limited reuse due to the limitations of its nuclear propulsion system. A more durable power source, perhaps fusing D/He-3, would allow larger Shuttles with longer reuse lifetimes. Larger Shuttles were seen to be undesirable in Paniagua et.al.’s system because they were arguing for a small cargo delivery back to Earth, but there’s nothing inherently infeasible about them. Uranus might help in the design too. If the Shuttles employ a ramjet system, scooping propellant from the air, they might be able to be reused many more times, with a larger cargo fraction. As the speed of sound is much higher in the outer atmosphere of Uranus, than Earth, the ramjet should work better, perhaps almost reaching the 12 km/s needed for low orbit. Let’s assume we get the price down to just 1% of the initial estimate. At $450 billion that seems incredible and perhaps a major “waste” if Earth’s economy is strained, like in recent years. However a side benefit – a very valuable one – is that a fusion-fuel supply system has been developed which could easily supply Earth’s ever-growing needs for clean energy. Assuming the program begins in 2100 and the demand for energy has risen 2.6% per year as it has for decades, on average, then the 9 billion people of planet Earth will need ~10 times the 20 terawatt-years presently used, some 200 TW-yr (or 6310 exajoules.) If it all came from D/He-3, then about 17,500 tonnes will be needed per year. The 50,000 tonnes used by “Daedalus” would represent several years of global energy supply. Depending on the importance of the mission, that might be acceptable. However we will probably want to do it cheaper. The original “Daedalus” design was partially limited by the needs of the fusion ignition system – very large electron beam generating diodes, and the harsh operating conditions of the engine. Smaller, lighter ignition devices may make smaller fusion engines burning D/He3 practical. The payload of “Daedalus” was an armada of sub-probes to be pre-deployed before the very high-speed encounter with the target star system. “Icarus” is being designed to decelerate in the target system and explore over a more leisurely time-frame. If the sub-probes and Main vehicle can have a weight reduction from 450 tonnes to 45 tonnes, and suitable fusion engines designed to match, then the overall mass of fuel needed might plummet to just ~5,000 tonnes. Assuming that, then the star-probe might have a more modest price tag, acceptable in a 2100 when fusion-propelled cargo ships are supplying the World’s energy from the bounty of the Outer Planets. The question is, for the next few years ahead, is whether fusion space propulsion can be developed to enable that energy-rich future. That’s a question that the “Project Icarus” team hopes to be a part of answering in the affirmative. References: Paniagua, J., Powell, J. & Maise, G. “A Cost-Effective Space Infrastructure for Retrieval of Helium-3 from Uranus for Earth-Based Fusion Power Systems utilizing the MITEE Nuclear Propulsion System”, Paper No. IAA-99-R.3.10, 50th International Astronautical Congress, 1999, Amsterdam, The Netherlands.