Currently I am working on a paper & presentation for the 100 YSS Symposium in Houston, to be presented by an Icarus colleague. I am examining the effectiveness of using a magnetic-sail to brake to low-speeds in the target system, but part of that is a comparison with a pure fusion rocket. As it is still the most detailed design for an interstellar fusion rocket I am using the performance characteristics of the “Project Daedalus” star-probe. The most economical use of propellant for pure-fusion is to boost up to cruise speed using the 1st Stage, drop the spent stage, then brake using the 2nd Stage after a period of cruising. “Daedalus”, due to its ignition system and the tricky physics of implosion ignited fusion, had two different exhaust velocities for the stages – 1st Stage was 10,600 km/s and 2nd Stage was 9,210 km/s.
A limiting variable on the possible mass-ratio was the mass of the cryogenic tankage required to keep helium-3/deuterium fuel at a chilly 3 K storage temperature. For the 1st Stage the tankage was 2.85% of the fuel mass stored and 4% for the 2nd Stage. As a critical mass-ratio is approached the required mass of propellant goes asymptotic – runs off to infinity. Thus there’s a maximum cruise speed for a single stage using “Daedalus” style storage systems. It works out as 0.1c for the 2nd Stage engine. To achieve that speed requires infinite propellant mass, so it’s not really practical.
A more practical question is the fastest trip to a given destination. Rockets are limited in how quickly they can burn their fuel – Stage 1 burns it at 0.72 kg/s and Stage 2 burns it at 0.0711 kg/s. To achieve higher speeds requires burn-times that are asymptotically rising, when the critical mass-ratio is factored in.
Alpha Centauri is 4.36 light-years away. A two-stage “Daedalus” vehicle can travel there in 68 years at a maximum speed of 0.075c and then brake to a halt at the destination. However the amount of fuel required is about 300,000 tonnes. Going a bit slower – arriving in 71 years – can reduce the fuel required to just 140,000 tonnes. “Daedalus” carried an immense payload by modern standards – 450 tonnes, the equivalent of the International Space Station. The recent paper on boot-strapping a robotic economy on the Moon only required delivery of 41 tonnes to kick-start things. A large exo-solar industrial base could be sent to other star systems in a decent time frame to build, in advance of human arrival, large laser or mass-beam facilities to decelerate a human-carrying star-ship. Such would allow much faster trip-times.
Adam, do you have a better ignition system than the original Daedalus one?
My point is that the original engine depended upon getting the fusion reaction started with tritium. This was OK, as the engines were only firing for 4 years or so, but tritium won’t keep for the duration of an interstellar journey, since it has a half-life of only about 12 years. So to restart the engines after several decades requires some other ignition system.
S.
Hi Stephen
Our “behind-the-scenes” email conversation answered this question partially. A more complicated answer is that the original triggering system of “Daedalus” used a small deuterium-tritium pellet inside the heart of each larger D-He3 fuel pellet – and this doesn’t work! Tritium is highly radioactive – admittedly very “mild” in terms of particle energy, but very hot in thermal terms. One kg of tritium releases about ~350 W of heat as it decays, meaning the fuel pellets would’ve boiled – helium-3’s boiling point is just above the storage temperature of 3 K – and thus the fuel tanks would’ve ruptured from all the gas pressure.
So “Daedalus”, to work, needs something different to trigger a fusion burn. The original electron-gun compression system also has electron-beam propagation issues, so a different triggering system would also be required. A newer concept is Plasma-Jet Magneto-Inertial Fusion, which uses “plasma bullets”, accelerated to ~1,000 km/s, to compress fuel targets. This has a lot of potential – a preliminary study by Milos Stanic gives very attractive performance results.
But to make my paper at all possible I had to start with something and “Daedalus” is a design I know very well. In spite of its flaws, its attention to detail is still noteworthy. The power/mass ratio I kept, as well as the associated mass-flow rate. I personally think we should try to develop pure deuterium fusion, due to the relative abundance of the fuel. Helium-3 is very difficult to access, requiring herculean industrial effort to extract from the gas giants, even a giant as mild as Uranus.
Pure deuterium ignition is rather difficult, as it’s not as willing to fuse as the D-T reaction. However the discovery of ultradense deuterium raises the possibility of using small triggers of the stuff which can ignite a much larger burn in a bigger fusion mass. Friedwardt Winterberg’s pure deuterium fusion pre-print from 2009 is worth referencing in this respect – he describes how to initiate the burn and confine neutrons for long enough in the plasma to thermalise their energy. In theory he computes an exhaust velocity of 0.0635c, from which I think we can assume exhaust velocities in the “Daedalus” performance range should be feasible.