The propulsion plant of the Zheng He is a Barr Fusion Industries type 702 milspec helium-3/deuterium fusion tandem mirror 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 forward-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 fan 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, 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 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 station 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, 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.

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.

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.

Operational Aspects

Typical of ships using this drive, sixteen crew 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 cruise, three when at action stations.) The operators stand their watches in Propulsion Control, a null-gravity space outside the centrifuge rings.