Megawatt Class Direct Fusion Drive For 25X ISP and 3 Times Faster to Pluto

Princeton Satellite has had various NASA, SBIR and IR&D grants to develop a multi- megawatt-class nuclear fusion propulsion and space power system. This funding has enabled them to precisely simulate their designs and performed experiments. They have stated they would need $100 million and five years to actually make a full megawatt propulsion system. They have not received the level of funding needed to proceed with the main development.

Studies of electron heating in PFRC-2 surpassed theoretical predictions and recently reaching 500 eV with pulse lengths of 300 ms, and experiments to measure ion heating with input power up to 200 kW are ongoing. When scaled up to achieve fusion parameters, PFRC would result in a 4-8 m long, 1.5 m diameter reactor producing 1 to 10 MW.

They have published various journal papers that review their computer models and simulations of the system. The Direct Fusion Drive concept is an extension of ongoing fusion research at Princeton Plasma Physics Laboratory dating to 2002.

The NASA NIAC project included analysis of the following subsystems: the superconducting coils, heat extraction system, startup system, radiators, and shielding.

Direct Fusion Drive (DFD) would produce between 5-10 Newtons thrust per each MW of generated fusion power, with a specific impulse (Isp) of about 10,000 seconds (chemical is 300-400ISP) and 200 kW available as electrical power. These would be first-generation capabilities.

The DFD system would have
* 35% of the fusion power for thrust
* 30% to electric power,
* 25% lost to heat, and
* 10% is recirculated for the RF heating.

Modeling shows that this technology can potentially propel a spacecraft with a mass of about 1,000 kg (2,200 lb) to Pluto in 4 years. The Pluto Express flyby mission took 9.5 years to reach Pluto. The DFD system would be slowing down and going into orbit around Pluto.

The modeled system would have 2 MW of power for use at Pluto. It could transfer up to 50 kW of power from the orbiter to the lander through a laser beam operating at 1080 nm wavelength.

Better Superconductors Would Make This System More Feasible and Have Higher Performance

Superconducting coils are a major portion of the engine mass. In order to estimate this mass, they reviewed published data on both low-temperature and high-temperature superconductors. Current generation Amperium 12 mm high-temperature wire has a current capacity of 350 A at 77 K, but 700 A at 30 K. This wire has a linear density of 0.2 g/cm. They counted the number of turns to produce 3 MA (3e6 A) in a 0.5 m radius coil: 8572 turns at 77 K and 4286 turns at 30 K, or 579 kg and 298.5 kg, respectively. Considering a single-engine will require 6 to 8 such coils plus the nozzle shaping coils, producing a 1 MW engine on the order of 1000 kg is clearly driven by superconductor mass. High-temperature superconductor companies are working to make thinner tapes with less cladding, but also consider low-temperature superconductors. They need to be cooled to 4.2 K, limiting the choice of coolant and increasing cryostat mass, however, a 1.04 mm NbTi wire has a linear density of just 0.063 g/cm and a capacity of 700 A. The same number of turns of this wire would has a mass of only 84.6 kg. This huge range in available properties is one reason they are still working on superconductor designs.

Nextbigfuture has covered the Princeton Satellite Direct Fusion Drive system several times. The June 2019 article was the last coverage.

Nuclear and Future Flight Propulsion – Modeling the Thrust of the Direct Fusion Drive

Direct Fusion Drive for Interstellar Exploration is paper than covers various possible space missions.

DFD uses an innovative radiofrequency (RF) plasma heating system. The thrust augmentation method is described along with results of multi-fluid simulations that give an envelope of expected thrust and specific impulse. The power balance is described and the subsystems needed to support the fusion core are reviewed. The paper gives the latest results for the system design of the engine, including just-completed work done under a NASA NIAC study. A mass budget is presented for the subsystems.

The paper presents potential interstellar missions.

One is the proposed 550-AU mission that would use the Sun as a gravitational lens for exoplanet research. This mission can be done without a deceleration phase.

Future flyby missions that would need major technological advances and a mature version of the technology would flyby the nearest star.

They sketch a mission to orbit a planet in either the Alpha Centauri A or Alpha Centauri B systems. The mission analyses include a communications system link budget. DFD can operate in an electric-power-only mode, allowing a large fraction of the fusion power to be used for the payload and communications, enhancing the scientific return. All of the missions start in low Earth orbit.

2014 Paper on Space Rapid Transit

Space Rapid Transit (SRT), is a horizontal-takeoff launch vehicle that would revolutionize both the space launch and flight transportation industries. To break the cycle of escalating space launch systems cost, it is necessary to consider concepts that are drastically different from current launch options. SRT is a fully reusable two stage to orbit vehicle. The Ferry Stage is powered by a dual fuel coaxial turbofan ramjet. The turbofan stage uses jet fuel while the ramjet uses hydrogen. The Orbiter uses liquid hydrogen/liquid oxygen engines. Stage separation is at Mach 6.5 at 40 km. The full system, Ferry with reusable Orbiter, is expected to deliver payloads to low earth orbit for less than $300 USD/kg.

NOTE: The SpaceX Falcon Heavy has pricing of about $2200 per kilogram and with a lot of flights and majority reused could get to $1000 per kilogram. The upcoming fully reusable SpaceX Super Heavy Starship could reach $100-1000 per kilogram pricing.

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