The propulsion plant of the Zheng He is a Barr Fusion Industries type 702 milspec helium-3/deuterium fusion tandem mirror variable-thrust mass-augmented stardrive.

History

In the late 2470s, The Allied Worlds Space Force desired a new high-performance stardrive (a term indicating any high-efficiency propulsion system capable of a 10% g acceleration) to improve its capabilities to respond rapidly to distant emergencies without keeping large numbers of ships deployed throughout the Allied Worlds' jurisdiction, an expensive proposition.

The BFI-702-M stardrive was designed to the AWSF's specifications by Barr Fusion Industries of Heidelberg, a leader in the fusion sector, the contract for its development being awarded after a competitive process which concluded in 2481 AE. Legendary chief designer Shi-tsen Jiang worked with the synthetic intelligence Heidelberg Q37 'Quint' and a team of engineers for six years on the design, simulations and engineering prototypes. The design of the direct conversion electrical generator was subcontracted to Al-Qawiyy Fleet Systems of Thane per a contractual requirement.

Construction (in orbit) of the full prototype began in orbit in 2487 and was completed in early 2489, achieving first ignition later in the year after extensive testing. AWSF's Materiel Command engineering inspectors deemed the prototype to meet the space force's specifications and officially accepted it on behalf of the Allied Worlds on 6/2490 AE. A production line was established and the first regular production drive was delivered to the orbital shipyard where the future AWS Buraq was being constructed in 2492, with the capacity being four per year.

In 2500, the AWSF cut its order from four drives per year to two. In 2505, the AWSF's cut its order entirely, but paid BFI a lump sum for its remaining parts inventory and to preserve the production tooling and documentation for future reactivation. The tooling remains in a large warehouse owned by BFI. Spare parts inventory was taken by the AWSF into its maintenance and logistics system.

The BFI-702-M was used in the Buraq class of rapid response ships, for which it was designed, and the Miri class of fast transport ships. A light drone carrier was planned, and drives were purchased in anticipation of it, but the project was canceled. In total, 33 of the production drives BFI produced were used in AWSF ships, out of 48 built. (10 were intended for the light drone carrier that failed to materialize; the other 6 were for replacements or additional ships.)

31 BFI-702-M drives remain either in active service or mothballed in the reserve fleet. One was destroyed with the fast transport ship using it, the AWS Ridge Ten, in the Dirac civil war intervention, by an antisatellite rocket launched by the Empire of Dirac. Another was deemed beyond economical repair after an accident in which a metal object, thought to have been a trim weight and associated structural parts, found its way inside the drive of the AWS Senrima while the ship was docked to an orbital station around Rosmerta for an overhaul. The shipyard's reactor operators tried to start the drive unsuccessfully, and did not investigate the cause of the failure until after they had repeatedly attempted startup (on shipyard power.) The result was that high-Z elements from the debris were sputtered onto the plasma-facing material, heavily contaminating the drive with long-lived radioisotopes and creating a persistent plasma contamination problem. A technician sent to inspect the drive after the shipyard operators finally gave up received a lifetime radiation dose and was forced into early retirement. The incident resulted in the cancellation of Rosmertine Space Yards' maintenance contract with the AWSF, with the space force citing the incident as evidence of poor safety practices and operator training. There were several lawsuits and firings, and a manager was criminally prosecuted for negligence in relation to the technician's radiation exposure, but found not guilty. RSY filed for bankruptcy six years later. Some former RSY employees claim that the debris was placed in the drive as an act of sabotage by a disgruntled former shipyard worker. The Senrima had a new drive installed and was returned to service, before being removed from active service in 2507.

The prototype propulsion plant was used for training purposes until 2502, at which point in was sold to the United Cities of Heidelberg for use in a new Presidential Starship. The President found the fueling costs prohibitive and the planetary government paid BFI to convert it for deuterium/deuterium fusion, which was successfully accomplished, but at a severe performance cost, because it was limited by the increased neutron radiation of D/D fusion and could only be operated at fractional power.

The Space Force Oversight Committee, in its Deccenial Report, considered the Advanced Helium-3 Stardrive project of which the BFI-702-M was the main product, to have been a qualified success. The AWSF was criticized for having ordered too many of them, however.

Fundamental Concepts

As has been known since antiquity, mass and energy have an equivalence, E = mc², where E is the energy, c is the speed of light, and m is the mass. This relation implies that a very small amount of mass represents an enormous quantity of energy. The atoms of helium-3 (consisting of two protons and one neutron) and deuterium (hydrogen-2, consisting of a proton and a neutron) can be fused to produce helium-4 (containing two protons and two neutrons) and protium (hydrogen-1, containing a proton.) The sum of the molecular masses of helium-3 and deuterium is, however, greater than the sum of the masses of protium and helium-4. When deuterium and helium-3 fuse, that energy is released, much of it as kinetic energy in the fusion products, i.e. they come away from the reaction going extremely fast and bearing a great deal of momentum in relation to their mass. The extremely fast particles are flung out the back of the drive, which causes the ship to accelerate in the opposite direction.

In a practical He3/D fusion drive, some of the particles are also used to power an electrical converter, which provides electrical power for the ship, and the machinery of the drive. The latter demand is quite substantial, because the conditions required to fuse atoms are extremely energetic.

The efficiency of a drive (that is, how much it can accelerate the ship using a given amount of propellant) depends on how fast the particles leaving it are going. The particles resulting from this fusion reaction are tremendously fast - almost 9% of the speed of light. However, because only relatively few particles are leaving, their momentum is small compared to that of the ship. if we add a propellant to the escaping fusion plasma, some of the fast-moving fusion products will collide with the propellant and transfer some of their kinetic energy to the propellant atoms. The exhaust velocity will go down. But a great deal more mass will be flung out the back of the ship, meaning that the force pushing the ship forward will be greater, i.e. the engine will have higher thrust. In this way, depending on the mission requirements and propellant availability, the operator of the drive can choose a higher or lower acceleration, possibly arriving at the destination sooner. Sufficiently mature developments on the basic fusion drive which use this concept are called variable-thrust mass-augmented stardrives.

Components

The drive consists of a magnetic nozzle, propellant injector, after magnetic mirror, coolant stream receiver, seven (7) solenoidal magnet sections, port and starboard neutral beam injector assemblies, forward magnetic mirror, direct conversion electrical generator, coolant stream projector, and startup system. Important auxiliaries include the cryogenic plant necessary for the superconducting magnets and for fusion fuel storage and the auxiliary cooling heat pump, and the control and operational monitoring systems.

Magnetic Nozzle

The magnetic nozzle directs the late-stage fusion plasma (containing mostly fusion products, some unexpended deuterium and helium 3, and trace quantities of carbon and other atoms sputtered from the plasma-facing surfaces of the drive), possibly mixed with secondary propellant for higher thrust, out the back of the ship, and makes the mechanical connection to the ship's structure to push it forward. The magnetic nozzle uses magnets and cryostats very similar to those of the solenoidal sections, but with different heat removal and radiation protection systems. This is possible because the nozzle is a "filigree" structure with much more open space than the solenoidal fusion containment sections, and is very far from the inhabited (forward) parts of the ship. Also, especially when operating with injected propellant, the radiation environment is less rigorous, as the plasma falls below ignition conditions as it expands in the nozzle. The plasma-facing surfaces are protected with high-chemical-resiliency coatings designed to resist ablation from nice things like highly ionized oxygen from water-based propellant.

The exhaust of the magnetic nozzle is diverges at a fairly small angle.

Propellant Injector

The propellant injector adds propellant, most typically water, to the fusion plasma after the plasma passes through the after tandem mirror. This decreases the temperature of the fusion plasma, but greatly increases its mass, allowing exhaust velocity (and hence efficiency / specific impulse) to be traded for increased thrust. The amount of propellant injected is adjustable by the operator, to fit maneuvering and delta-V budget needs.

The propellant injectors use superconducting-brushless-motor turbopumps to blast atomized streams of liquid (usually water, ammonia and hydrocarbons are also suitable) into the a magnetic containment chamber immediately preceding the nozzle.

The propellant is kept liquid from waste heat. The propellant injection system contains pressurized adjustable-volume accumulator tanks feeding the main injection turbopumps, which can provide a steady supply of propellant when the ship is not accelerating and there may not be liquid at the main propellant tank intakes due to null gravity conditions. This allows a long delay for ullage under the miligee accelerations (provided by propellant-free operation) to be avoided.

Magnetic Mirrors

The "tandem mirror" fusion reactor takes its name from the pair of of magnetic mirrors at each end of the cylindrical ("solenoidal") confinement section. Their curving magnetic field lines turn particles around for another pass through the solenoid. The mirrors do not reflect all particles, some pass through in a controlled manner. The forward mirror's strength (i.e. what fraction of charged particles it redirects) is used to control how much fusion plasma is released into the direct conversion generator. The after mirror's strength controls what fraction of the fusion plasma is released into the propellant mixing chamber at the throat of the magnetic nozzle.

The mirrors use specially shaped superconducting electromagnets similar to the ones in the solenoidal sections, and have similar provisions for shielding.

Solenoidal Sections

Most of the fusion takes place in the seven solenoidal sections, which use extremely powerful magnets to contain the fusion plasma while the reaction takes place. Each section contains interchangeable magnet assemblies; assemblies from different sections are not interchangeable because their geometry varies along the length of the drive.

The magnets are electromagnets which use wire made of silver and advanced high-field superconductors (the silver is for mechanical properties and for conduction in case of a magnet quench.) The magnet coils are kept in a cryostat, surrounded by mechanically-cooled superfluid Helium 4. The cryostat is insulated by vacuum, with surfaces being coated with ultra-low-emissivity films and supported by low-conductivity helical spacers. An outer layer of liquid nitrogen further reduces thermal transfer. On the plasma-facing side (interior) of the magnet, there is a layer of actively cooled radiation shielding including a particle shield of borated polymers optimized for the radiation profile of the D/He3 fusion reaction and a graded-Z shield. Interior to this is another vacuum gap and the plasma-facing inner wall.

The inner wall is made of diamond, fabricated as an outer part containing gallium coolant channels, and an inner part which contains radial filaments of a functional nanomaterial designed for ballistic thermal conduction into the coolant. The channels also contain electrodes for the distributed magnetohydrodynamic pumping of the (conductive) coolant through the system, and feeder wires to the (superconducting) power distribution system. The innermost surface is lined with an extremely refractory transparent anti-sputtering coating and x-ray scattering resonant nanoparticles tailored to the spectral distribution of the drive at full power, designed to redirect bremsstrahlung x-rays back into the plasma to some extent.

The magnet assemblies also contain smaller shaping coils to optimize the magnetic containment and scavenge escaped ions.

Between magnet assemblies, neutron reflectors are placed to attempt to reflect as much neutron radiation as possible into space, away from the forward sections of the ship and the droplet radiators.

Neutral Beam Injector Assemblies

The neutral beam injectors serve two functions - to raise the temperature during reactor startup (i.e. to ignite the fusion reaction, providing heat until it becomes self-sustaining) and to supply fusion fuel (deuterium and helium-3) to the operating reactor.

There are port and starboard NBI clusters, each with multiple beamlines. All beamlines are used to ignite the fusion reaction, though ignition can be achieved with 75% of them operational. Just four beamlines are sufficient to supply the reactor in operation at maximum power. The reactor is started with some tritium in the plasma to make it easier to ignite the fusion reaction; once ignition is achieved, the operator quickly transitions it to helium-3/deuterium, because the reactor is not sufficiently shielded for long-term T/D fusion.

Each neutral beam injectors contains a negative ion generator with piping routing it to helium-3, deuterium, or tritium supplies. Next follows a linear accelerator which raises the energy of the negative ion beam into the 10s of MeV. The ions pass into a photodetachment neutralizer, losing their electrical charge but retaining their momentum. A magnetic deflector steers remaining ions into a diversion, where they are recycled to the ion source via a turbomolecular vacuum pump. The beam of neutral atoms goes straight through, passing through windows in the reaction chamber. It enters the long solenoidal magnetic containment at an shallow angle, allowing ample opportunity for the neutral atoms to deposit their energy in the fusion plasma despite their high speed.

Direct Conversion Electrical Generator

The direct converter uses the moving charged particle stream to create an electric current from the particles' kinetic energy. It provides power for the reactor itself (cooling, startup, neutral beam injection, etc) and for the ship's other electrical needs, which are extensive. The current from the device feeds into a DC-DC converter to provide the various voltages required for distribution to the loads. There is also a substantial bank of ultracapacitors, which provide the ship with power when the reactor is not operating, and power for reactor startup. The latter is, unsurprisingly, extremely energy intensive.

The system is a high-power inverse-cyclotron converter.

Operational Aspects

Personnel

Typical of ships using this drive, sixteen spacers and their division officer comprise the Propulsion Division aboard the Zheng He and are primarily responsible for the plant's condition. Operation requires two watchstanders, a reactor operator and a power distribution operator. (One crewperson when on long cruising stations, three when at action stations.) At action stations, operators stand their watches in Propulsion Control, a null-gravity space outside the centrifuge rings. Ordinarily and at long cruising stations, they are in the main control room. (Obviously, all of the machinery is teleoperated under normal circumstances, and having them in Propulsion Control during action stations is more about the centrifuge's vulnerability to damage than a desire to keep them close to the machinery they operate, the vast majority of which they cannot manually access without spacesuits anyway.)