Daedalus was a British Interplanetary Society project for the design of a robotic Interstellar probe, led by Bond and Martin.  The project report(1), from 1978, is a fantastic document, describing in minute detail an Interplanetary Spaceship designed according to the best science available at the time.  The design has held up remarkably well.   In a way, it is difficult to improve upon until a real nuclear fusion drive is designed.  

Le projet Dédale de la BIS visait la conception d'une sonde Interplanétaire permettant de visiter une étoile proche dans une période qui ne dépassait pas la vie d'un chercheur.  La vitesse devait donc être très grande, près de 14% de la vitesse de la lumière.  La rapport du projet, produit en 1978 par Bond en Martin, est un document remarquable, qui n'a pas encore été surpassé.  En fait, il sera difficile de faire mieux tant que la fusion nucléaire n'aura pas été réalisée dans la pratique, ce qui n'est pas encore le cas.

Daedalus Design review

(Michel Lamontagne, James French, Adam Crowl)

Could Daedalus have been used for an exploration mission, rather than for a fly-by mission as originally envisioned?  After all, both stages added about the same delta-V, therefore the second stage might have been used to stop the ship at target.  The main problem with this scenario is the tritium trigger that was chosen for the Daedalus fusion reaction.  Tritium has a high rate of decay, with a half life of only 13 years.  Therefore, by the time the ship reached its deceleration phase, most of the tritium would have decayed into He3, making the drive unworkable.

Other problems have been identified with the Daedalus design.  Although some have technical solutions, some might require a major redesign, or even put the feasibility into question.  It is important to remember that the original Daedalus team never expected their design to remain unchanged.  It is a tribute to the quality of their work that so few problems have emerged for a design that was done in the earlier stages of fusion research. 

Fusion reaction

Of the few possible candidates for fusion material (2D-2D, 2D-3T, and 2D-3He), the latter was chosen in large part because it produces the fewest number of neutrons. (1). Either of the other two reactions is easier to initiate, but produces less energy per fusion and also a large number of neutrons. The issue is the destructive effect on the vehicle of an intense flux of highly energetic neutrons. Not only are the vehicle electronics system at risk of single event upset and radiation damage but the flux may be sufficiently intense to cause weakening of structural elements over time due to damage of the crystal structure of the material and possibly transmutation of the constituent elements.

The 2D- 3He reaction produces no neutrons and thus in theory avoids the problem. However, since 2D is present, some percentage of these atoms will react producing neutrons. There is no way to prevent this, so some amount of neutrons must be tolerated even with the 2D- 3He reaction but much less than with the other two choices. At the time of the study, it was expected that the neutrons and the X-rays produced by the side reactions would be absorbed in the pellet, since the mean free path of the neutrons was shorter than the pellet diameter.  This is one of the parameters that determined pellet dimensions.  

However, new simulations(2) have shown that the neutron absorption would be considerably lower than the 99.6 used for Daedalus, therefore neutrons fluxes would be higher than what the nozzle structure could tolerate, and some form of radiation shielding with active cooling would be needed.  This would change the configuration of the ship considerably, and is why the Icarus designs all have large radiators for this purpose.   An alternative exists with Boron/hydrogen reactions, but the Bremsstrahlung radiation is high and the energy available from the reaction considerably lower than for 2D-3He, therefore requiring much larger propellant quantities.  

Since the number of side reaction from D2-He3 is higher than expected, this reduces the value of using He3 as a fuel, making the use of D2-D2 fusion a possible prospect.  This in turn reduces the need for an interplanetary economy, as long as the neutron heating of the ship elements from the D2-D2 reactions can be controlled.

Pellet trajectory

Each fusion pellet is assumed to be accelerated to a substantial velocity by a traveling wave accelerator interacting with a thin superconducting coating on the pellet. While such an approach seems feasible as stated, it is critical that the pellet arrive precisely at the target point at precisely the right time for the electron beams to intercept it. This must occur at 200msec intervals. The pellet must be in flight to the target point soon after the detonation of its predecessor. Velocity control, timing of pellet release and of electron discharge are all critical. While it seems feasible to control these adequately, it is less clear whether the pellet will manage to fly its proper trajectory. The time period following the departure of the previous plasma ball is likely to be quite dynamic with expansion of the magnetic field, possible oscillations of the magnetic field, mechanical structure vibration, and possibly other effects which may disturb the pellet trajectory and arrival time. Note that the pellet is coated with a superconductor in order for the accelerator to work. Thus one might expect it to be especially sensitive to variations in field shape. More study of this issue is needed.  Not only does the combination of effects mentioned above affect the pellet but it must also affect the electron beams (perhaps even more so) thus making the probability of a successful ignition even less probable.

Ignition method

Electron Beams

It was envisioned that the electron beams will be fired radially inward in the exit plane of the hemispherical chamber. Some of the same concerns as for the pellet might be applicable here. The magnetic field is intended to be shaped so that the radially inbound electrons do not have to cross magnetic field lines. There would seem to be some risk that the magnetic field will not be this cooperative given the dynamics of the situation. More disturbing however is the following. The electron beams are fired inward in a single plane and would therefore impinge upon the pellet in a single plane. This would cause the majority of the compression to occur around the “equator” of the pellet. We know from such things as plutonium bombs, which depend upon radial compression to achieve criticality, that the compression forces must be spherically uniform and perfectly timed in order for the compression to occur as required. This compression uniformity also tends to drive the design of essentially all current experiments using particle beams or lasers. It seems that the most probable result of the Daedalus scheme is for most of the material to “squirt out” in a direction normal to the impact plane resulting in little or no fusion occurring. The Daedalus design team dealt with this issue with vague statements to the effect that the “atmosphere” of the pellet (whatever that may be) would cause the electron beam to distribute uniformly upon arrival. This seems tantamount to hoping for a miracle. It is most likely that the multiple electron beams will be required to impinge upon the pellet as nearly uniformly as possible all around the sphere. It may be possible to design more complex pellets which can accept the incoming electrons and redistribute them uniformly by means of internal reflection (this might be better suited to laser compression) to converge upon a fusion core. But the pellets have now gone from simple spheres to much more complex (read expensive) structures and orientation of the pellet probably becomes very critical. Much more attention is required to this issue. This must include a very detailed look at the requirements for generating inertially confined fusion and an assessment as to whether the Daedalus concept can meet the requirements. If not, a considerable redesign of the concept is indicated.

It was also determined by Benford(3) that the electrons beams would not remain collimated over the required distances due to the inherent dispersal created by the uniform negative electrical charge of the beams, proving electron beams to be unworkable.

Lasers

The Daedalus team considered laser compression but rejected it in favor of relativistic electrons because of uncertainty as to the technical feasibility of lasers of sufficient power. In view of the enormous progress in high powered lasers and the application of this technology to fusion ignition and “ray-gun” type weapons, this decision begs for reconsideration. Use of lasers eliminates some of the concerns inherent in the use of electrons discussed above. In the discussion of lasers reference is again made to the “atmosphere” around the pellet distributing the energy uniformly. As with the electron beams, this seems improbable (and it is unclear just what is the atmosphere that is referred to). Any design for a compression system probably needs to be configured to provide 360 spherical degrees of uniform coverage.  The lack of success of the NIF facility(4) to achieve ignition, however, shows the application of lasers still needs a lot of development, if they can be made to work at all.  Lasers are still fairly inefficient, and at the levels of power required would probably need very important cooling.  An alternative, explored with the Zeus ship, is replacing beams with plasmoids,  ‘slugs’ of fusible materials in a plasma state held together with magnetic fields.  If these can be accelerated to ~2,000 km/s efficiently, then the power-to- mass ratio needed could be achieved.  Even though use of lasers eliminates beam deflection by magnetic fields as an issue, deflection of structures due to shock wave impact and thermal effects might be sufficient to demand some sort of active control of pointing.

Magnetic nozzle

The primary means of controlling and expelling the plasma is the magnetic field in and immediately outside the exit of the reaction chamber. It is desirable that the plasma be expelled as nearly in a parallel beam as possible in order to achieve maximum propulsive effectiveness. The hemispherical shape does not achieve a parallel beam. There is some divergence.  A possible option is a paraboloid chamber rather than hemispherical. This might give a more nearly parallel beam. The parabolic shape would probably be heavier but if higher performance is achieved this might trade favorably. A parabolic shape with the fusion occurring at the focus would probably be more convenient for placement of electron beams or lasers to provide uniform spherical coverage.  An additional concern in expelling the plasma is the shape of the external magnetic field. Probably this is actually more significant than the shape of the reflector for pure performance but, there are other reasons to need the physical shell as well. Analogous to a conventional rocket, the fusion rocket tries to get the momentum of the propulsion stream collimated and proceeding away from the vehicle as nearly uni-directionally as possible. For a highly ionized plasma, the field as the plasma exits the reflector will have a great deal of impact on this. Concern has been expressed that electromagnetic nozzles may not be able to release the exhaust stream as cleanly as would be desired(5).  Some of the stream might try to follow magnetic field lines back toward the front of the vehicle resulting in reduced propulsion efficiency and possible damage to the vehicle. A reliable means of analyzing such nozzles and some experimental data anchoring the analysis in reality is essential. The physics must be well understood before any attempt is made to design a flight nozzle.  Results obtained by the AdAstra company for the Vasimir thruster(6) as well as simulations with modern MHD(7) software have shown that the 96% nozzle efficiency suggested by the Daedalus team might not be achievable, and that values between 60 and 80%, at best, would be more likely.

Tritium decay in the fuel pellets

Tritium decay in the pellets is a significant heat load that was not taken into account in the Daedalus study.  The heat production rate of Tritium is 1.954 W/mol -i.e. 324 W/kg. The density of stoichiometric mix DT @ 3 K is 261 kg/m^3. Thus the total pellet heat is 861 kW for Stage 1 of Daedalus and 174 kW for Stage 2. This would need to be removed while keeping the overall temperature at 3 °K, so the maximum temperature difference available would be 1-2 °K.  This heat load in inherent to the design and would completely change the design of the fuel tanks, possible making the very concept of tritium triggered pellets unworkable.  To remove the heat about 20 MW of compression power would be required, and this power would add itself to the heat being radiated.  This problem was identifed by Adam Crowl during the Icarus Interstellar study.

Tank pressurization

The quantity of gas required to keep the tanks pressurized is higher than what was allowed for in the Daedalus design.  This might be corrected using collapsible tanks, or higher mass allowances.

Computers

This is an area where the predictions of the Daedalus team have proven to be both conservative and highly optimistic.  On one hand, the computing power of Daedalus was less than the power of a modern laptop computer. On the other hand, the operation of the Wardens, autonomous maintenance robots capable of diagnostics and troubleshooting, has proven to be a problem far beyond the capabilities of the best modern Artificial Intelligence.  The advent of self driving cars does show promise in this direction though, and progress in the field is rapid.  The answer here will probably be a highly redundant and reconfigurable system design which will lend itself to working around problems. The 40-year-old Voyager spacecrafts used a primitive version of this and are still functioning, although the present architecture bears little resemblance to the original.  For the few cases in which actual hardware reconfiguration is required, it will have to be carefully designed with that eventuality in mind so that relatively simple-minded robotics can handle the chore.   It will always be possible to send updated version of the software, as the early probes are still operating at a fraction of the speed of light and a transmission from Earth can reach the vehicle at all times.  

Destinations

The choice of Barnard’s star as a specific target in the original Daedalus study may have been wrong, since the data used has since proved to be in error().  However, the abundance of extrasolar planets, detected at an ever increasing pace, has shown that the idea of planets around nearby stars it correct.  Indeed, there is an abundance of possible targets today and the stretch goal that was Barnard’s star remains as a valid concept.  The development of laser sails offers a possible substitute to fusion probes for flyby exploration.

New Daedalus

With all the issues raised above, a redesign of Daedalus seems required.  Although some elements can be fixed, some require significant changes, and it could be argued that the original configuration cannot be maintained.  Project Icarus attempted to find a replacement design, but there are still issues with all fusion starship designs.  L'Espérance was an attempt at creating an update to Daedalus, but lacks technical rigor.  Firefly is a more complete solution, but still has problems. Solar sails with beamed laser propulsion offer an alternative path.

References

1- BOND, Alan et MARTIN, Anthony R. Project Daedalus. JBIS, 1978, vol. 31, p. S5-S7.      (See discussion in Reference 1 pg. S47)

2- Nozzle simulation papers

3- BENFORD, James. E-Beam ICF for Daedalus Reconsidered. 

4- NIF performance problems

5- Concerns with nozzle efficiency

6- Vasimir efficiencies

7- Simulation by Icarus paper

Next up: Firefly