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The Daedalus Propulsion System
by Adam Crowl
The idea of rocket propulsion is somewhat familiar to most people, in the sense that it is understood that gases are ejected at high pressure from the rear of a spacecraft, and that this ejection generates thrust. Rocket propulsion is ideal for launching spacecraft from the Earth, because the amount of force generated by the fuel is huge. However, one of the drawbacks of conventional rockets is their low exhaust velocity; that is, only a relatively small amount of net energy can be liberated from the fuel, when compared with, say, nuclear fuel. This drawback means that to accelerate an object to very high speeds, say a reasonable fraction of the speed of light, that a huge amount of fuel would be required. In fact, some fairly simple calculations using the Tsiolkovsky rocket equation demonstrate that to accelerate a spacecraft to about 10% the speed of light would require more propellant than is available in the known universe! Clearly chemical rocket fuels are inadequate for the task of interstellar propulsion if travel times are to be on the order of a human lifespan. Fusion processes liberate, on average, about a million times more energy per unit mass than chemical processes. This makes them ideal for propulsion purposes, where minimization of the total mass of the spacecraft and fuel is of paramount importance. Although we have not yet mastered the controlled release of fusion energy, the progress to date indicates that it is simply a matter of time until this new technology can be harnessed. The Daedalus spacecraft, which was a 1970’s effort to design and interstellar spacecraft, was to be powered by a process known as pulsed fusion whereby small pellets of fusion fuel would be injected at a 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, where 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. These ejecta would be responsible for an overall momentum transfer mediated by magnetic fields interacting with the reaction chamber. In this article we examine the major elements of the propulsion system, which can be broken down into the following:
- Propellant Storage
- Pellet Injection Gun
- Electron Beams
- Reaction Chamber
- Magnetic Nozzle Coils
Schematic of the Daedalus Propulsion sytem, showing all major components. (Image courtesy of Bond and Martin – Project Daedalus Final Reports)
Propellant Storage Daedalus was a two stage spacecraft, with stage one carrying 46,000 tonnes of fuel, and stage two carrying 4000 tonnes. The fuel pellets had a mean radius of 19.7 mm for the first stage, and 9.16 mm for the second stage. The core of the pellet, was a Deuterium Tritium (DT) trigger, which was required to generate a thermonuclear burn wave which would ignite the Deuterium Helium-3 fuel. This core was surrounded by the actual Deuterium Helium-3 fuel which made-up only 10% to 15% of the overall mass of the pellet. The entire pellet was coated with a superconducting shell which was required so that the pellet could be electromagnetically accelerated from the storage tanks to the reaction chamber by a magnetic gun. A DT trigger was chosen, because it is the easiest fusion reaction to ignite, and ignite the reaction of the Deuterium Helium-3 fuel. The storage tanks were spherical in shape, and cryogenically cooled to a temperature of 3 K while being maintained at a constant pressure of 0.812 atm. This temperature and pressure combination was required so that the Helium-3 would be in a liquid phase rather than a gaseous phase. This was a requirement since the fusion rate increases with the density of the fuel. Pellet Injection Gun The fuel pellet injection gun was designed to channel the fuel pellets into the main reaction chamber at a rate of 250 per second by using a travelling magnetic wave, which is a wave of magnetic energy that is capable of imparting momentum to the pellet. The magnetic gun consisted of a number of field coils, in series, where each imparted an equal amount of kinetic energy into the fuel pellet. Both stage one and stage two injection guns were to have a field strength of 15 Tesla, and would accelerate the pellets at a rate of 3.83 x 107 m/s2 (1st stage) and 8.21 x 107 m/s2 (2nd stage). To minimize energy dissipation in the coils, they were to be cryogenically cooled to about 20 K. The pellet injection system was a particularly massive component in the overall propulsion system, with the bulk of the mass being in the cooling system mass (40 tonnes 1st stage, 4 tonnes 2nd stage) and the capacitor mass (29.6 tonnes for stage 1, 0.79 tonnes for stage 2).
Schematic of the Fuel Pellet Injection gun.(Image courtesy of Bond and Martin – Project Daedalus Final Reports)
Relativistic Electron Beams Once accelerated, the pellets would be ignited within the reaction chamber. A number of diodes capable of emitting beams of relativistic electrons were focused on to the specific location where the injected pellets would pass. At the instant the pellet reached the focus, the beams would be discharged and the outer layers of the pellet would be ablated as a consequence of the the electron beam collisions, which would heat it to a high temperature. This heating would result in a strong coupling of the pellet to the electron beams and lead to high heat transfer rates to the surface of the pellet. A convergent shock wave would result from the ablated material, focusing on the pellet center with sufficient pressure to ignite the thermonuclear trigger of DT. The driver energy for the first stage would be 2.7 GJ, and 400 MJ for the second. Due to the high gain of the system, each cycle would generate enough energy to power the subsequent cycle, and the process would continue as required. Reaction Chamber The ignited fusion fuel would reduce the pellet to an expanding plasma radiating from ignition point. The basic concept of the reaction chamber was to enclose the electromagnetic field of the plasma in a conducting shell. The hemispherical shell would perform as a shock absorber, which would absorb the momentum of the plasma and transmit it to the vehicle. The process would occur rapidly, over a few microseconds, and the rise and fall in magnetic pressure would be received by the shell as an impulse which set it in motion. The shell would then relax, and become set in motion once again, by the next pellet. The reaction chamber would experience a constant forced oscillatory motion for the duration of the acceleration. The reaction chamber would need to have a low density to conserve mass. It would also need a high temperature capability, and a high electrical conductivity. For the Daedalus spacecraft, Molybdenum was chosen since it met all the requirements. The first stage reaction chamber had a mass of 218.7 tonnes, and the second reaction chamber mass would be 25 tonnes. Magnetic Nozzle Coils External field coils were required since the reaction chamber was to have a weak magnetic field which was required for the plasma compression. The field was to be shaped such that it did not interfere with the propagation of the electron beams to the target. The field also had to run parallel to the reaction chamber walls, and diverge downstream, so as to guide the plasma for propulsive purposes. This is what is typically referred to as a ‘magnetic nozzle’. The Daedalus team selected a configuration of four field coils, which they believed provided the desired field configurations. The field coils would be superconducting with a peak field of 14 Tesla. The stage one coils had a total mass of 124.7 tonnes, and the stage two coils had a total mass of 43.6 tonnes. For comparison, the dry mass of the space shuttle orbiter is only 85 tonnes, so one immediately appreciates the sheer scale of the Daedalus. Conclusion The first stage engine cycle involved the ignition of 16 billion pellet cycles lasting a total of 2.05 years, and a second stage cycle lasting 1.76 years. This made for a total boost period of 3.81 years, and a top speed of 12.2% the speed of light. What is particularly fascinating about this propulsion system 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 a Daedalus class spacecraft would be easy, since the engineering and economical costs are quite staggering, but it is certainly encouraging that this design could theoretically be built, given sufficient ambition.
Staggering is to small a word for the project.
46000 tonnes of fuel.. that’s processed fuel mind, extracting the helium3 and other isotopes required from sources on earth or the lunar regolith requires a level of infrastructure that is unlikey to seen for another forty years.
Hi Cybrax
It’s quite a big task, but consider the mass of ocean going vessels and it’s not so gargantuan.
Is the Daedalus propulsion system feasible from our current point of view? If it is possible to extract enough energy from the reaction to run the propellant gun and the electron beams, shouldn’t this also work as a power plant here on Earth?
Hi Max
The induction power-return system is perfectly feasible for a power-system here on Earth. Something similar has been proposed for several different fusion reactor designs. But running a “Daedalus” engine as a power plant has the problem of being too much of a good thing. The power of the 1st Stage Engine is a massive ~44 terawatts, which is much more than all the energy currently used by technology on planet Earth. In theory the engine could set off a pulse once every second or so, thus producting power at lower levels, but practically power plants are typically 0.5-1.0 GW, so a 1st Stage main engine would need to run about ~44,000 times slower to make a generator. The problem then would be storing the energy released, because the release happens in mere microseconds. Power supplies need to supply power smoothly, not abruptly.
These are all issues that have solutions and pulsed fusion reactors are being actively researched. The original “Daedalus” design relied heavily on electron beams to implode the fuel to ignition conditions, but lasers or ion beams might prove more suitable, as acknowledged in the original reports. The USA’s National Ignition Facility uses a large number of powerful lasers and is expected to produce fusion ignition this year or next.
So a scaled-down version would be able to function as a power plant? Or is the induction power-return system so inefficient that it needs a large fusion pulse to produce relatively modest electrical power, just enough to run the pellet injection gun and the electron beams?
Related to this, how about igniting the fusion process with antiproton beams? That way you may be able to get rid of the heavy relativistic electron beams and eliminate the need for a induction power-return coil. The antiproton “guns” / penning traps could be powered by an onboard fission reactor. Is this an option being considered for Icarus?
Another question, why is the effective specific impulse “only” 1 Million sec, rather than closer to the theoretical limit of D-He3 fusion? Has it to do with the burn up fraction or the efficiency of the magnetic nozzle in channeling the fusion products out the back of the reaction chamber?
Hi Max
The induction system isn’t inefficient, just designed to tap a tiny fraction of the exhaust energy, else the induced EMF in the coils would generate a strong magnetic field that would interfere with the plasma flow. What energy is tapped from the exhaust is lost from the exhaust velocity thanks to conservation of energy!
Antiprotons sound sexy, but in reality they don’t transfer energy very easily to the fusion fuel, so they’re not as effective as first imagined. Proton or ion beams might be more effective than the electron beams, as the original report noted, but they went with e-beams due to the higher power reported from experiments with them in the 1970s. Later experiments using e-beams found other physics issues and so they’ve fallen from favour in Inertial Confinement Fusion research, but they may prove viable in future.
The “low” exhaust velocity is due partly to the inefficiency of the compression process, as about 25% of the pellet explodes outwards as plasma to compress the remainder. The fusion burn initiated in that remainder fuses only about ~25% of the 3He/D mix, thus reducing the overall energy added to the plasma by the reaction. Effectively 0.175 of the whole pellet is fused, meaning the million second Isp. The magnetic nozzle was assumed to direct the flow with about ~95% efficiency.
Higher burn-up fractions for such compression systems require higher energy inputs from the electron beams. There comes a point where the energy to drive the beams is greater than what is produced by fusion and before that point a maximum exhaust velocity is reached at a burn-up fraction of about 0.25 overall. That means a maximum exhaust velocity of about ~0.04 c for the “Daedalus” main engine, half the theoretical maximum of ~0.088c.
Thanks for your very insightful reply! Daedalus sure has an awesome engine!
On anti-proton triggered inertial confinement fusion, they don’t target the beams at the fusion fuel proper but at a layer of U-238 (or other heavy nuclei) coating the pellet. The anti-protons react with the protons in the U nucleus, the annihilation energy leading to a very energetic fission process and release of neutrons, which in turn compresses and heats a layer of D-T fuel, which then ignites another D-T or perhaps D-D or even D-He3 fusion reaction. If the anti-protons are not effective enough themselves as a driver for ICF, they may also be used as a “catalyst” together with electron/ion/laser beams, reducing ignition power requirements or enabling a higher burn-up fraction for the same power input. Perhaps such a hybrid system may be a key to achieving a higher gain and thus higher exhaust velocity or a less massive ignition system?
Is the Daedalus propulsion system itself a near-term option, meaning that the physics and engineering are relatively well understood? It would surely make for an awesome interplanetary propulsion system as well … perhaps a less demanding sub-scale version may be something to be added to the list of NASA’s flexible path tech developments … it would certainly beat any solar or nuclear electric option.
Max
Hi Max
For large applications fusion pulse drives have many advantages. The difficulty lies in the heavy mass of a fusion engine – even “Daedalus’s” second stage engine massed several hundred tonnes. That’s a lot more than space planners have traditionally imagined for early space missions.
Smaller fuel pellets are used by the VISTA fusion space-craft concept, but it is a much heavier vehicle due to the limitations of the laser ignition system and the choice of D-T fuel. A high neutron heating penalty means heavy waste heat handling machinery and shielding for payloads, plus heat from the tritium’s decay needs to be handled as well.
With NASA’s new HLV, which will hopefully be available when this decade is out, such an engine could be orbited in several pieces and then assembled in obit.
But surely the development alone would be an effort requiring tens of Billions of dollars and I believe ground-testing would be next to impossible, but it may be worth it, even before we fly missions to such places as Mars.
Hi Max
There might be better options. More on those in a future blog-post. Thanks for your input thus far.
Why not a fission engine? Couldn’t some sort of liquefied uranium be used to create a sustained nuclear explosion where two streams of liquified uranium reach critical mass where they intersect? This would be an improvement over the “Bomb Thrower” style space craft depicted in “If the Stars are Gods” by Benford & Eklund which was pretty much a giant shock absorber with living quarters. Since Nuclear Fusion is pretty much fantasy land anyway (otherwise, we would be replacing our power plants with fusion reactors right now!) Nuclear Fission seems like a more plausible, although politically incorrect method of long distance space travel.
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