Direct Fusion Drive (DFD) is a conceptual low radioactivity, nuclear- fusion rocket engine designed to produce both

thrust Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that sys ...

and electric power for interplanetary spacecraft. The concept is based on the Princeton field-reversed configuration reactor invented in 2002 by Samuel A. Cohen and is being modeled and experimentally tested at

Princeton Plasma Physics Laboratory Princeton Plasma Physics Laboratory (PPPL) is a United States Department of Energy national laboratory for plasma physics and nuclear fusion science. Its primary mission is research into and development of fusion as an energy source. It is known ...

, a

US Department of Energy The United States Department of Energy (DOE) is an executive department of the U.S. federal government that oversees U.S. national energy policy and manages the research and development of nuclear power and nuclear weapons in the United States. ...

facility. It is also modeled and evaluated by Princeton Satellite Systems. As of 2018, the concept entered Phase II, a simulation phase, to further advance the design.

Principle

The Direct Fusion Drive (DFD) is a conceptual fusion-powered spacecraft engine named for its ability to produce thrust from

nuclear fusion Nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles ( neutrons or protons). The difference in mass between the reactants and products is manife ...

without going through an intermediary electricity-generating step. The DFD uses a magnetic confinement and heating system, fueled with a mixture of helium-3 () and

deuterium Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two stable isotopes of hydrogen (the other being protium, or hydrogen-1). The nucleus of a deuterium atom, called a deuteron, contains one proton and one ...

(D or ), to produce a high specific power, variable thrust and specific impulse, and a low-radiation spacecraft propulsion system. Nuclear fusion happens when two or more atomic nuclei, combine to form one or more atomic nuclei or sub-atomic particles. This process releases energy due to the mass of the products being lower than the initial mass of the reacting nuclei. In the DFD a hot (100 keV ) plasma, a collection of electrically charged particles that includes electrons and ions, fu together, releasing enormous amounts of energy. In the DFD system, the plasma is confined in a torus-like magnetic field inside of a linear solenoidal coil and is heated by a rotating magnetic field to relevant fusion temperatures.s. Bremsstrahlung and synchrotron radiation emitted from the plasma are captured and converted to electricity for communications, spacecraft station-keeping, and maintaining the plasma's temperature. This design uses a specially shaped

radio frequency Radio frequency (RF) is the oscillation rate of an alternating electric current or voltage or of a magnetic, electric or electromagnetic field or mechanical system in the frequency range from around to around . This is roughly between the ...

(RF) "antenna" to heat the plasma.Direct Fusion Drive Quad Chart
Princeton Satellite Systems. Accessed: 18 July 2018. The design also includes a rechargeable battery or a

-oxygen auxiliary power unit to startup or restart DFD. The captured radiated energy heats to a He-Xe fluid that flows outside the plasma in a boron-containing structure. That energy is put through a closed-loop

Brayton cycle The Brayton cycle is a thermodynamic cycle that describes the operation of certain heat engines that have air or some other gas as their working fluid. The original Brayton engines used a piston compressor and piston expander, but modern gas tu ...

generator to transform it into electricity for use in energizing the coils, powering the RF heater, charging the battery, communications, and station-keeping functions.

Thrust generation

Adding propellant to the edge plasma flow results in a variable

and specific impulse when channeled and accelerated through a

magnetic nozzle A magnetic nozzle is a convergent-divergent magnetic field that guides, expands and accelerates a plasma jet into vacuum for the purpose of space propulsion. The magnetic field in a magnetic nozzle plays a similar role to the convergent-divergent ...

; this flow of momentum past the nozzle is predominantly carried by the

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s as they expand through the magnetic nozzle and beyond, and thus, function as an

ion thruster An ion thruster, ion drive, or ion engine is a form of electric propulsion used for spacecraft propulsion. It creates thrust by accelerating ions using electricity. An ion thruster ionizes a neutral gas by extracting some electrons out of ...

Development

The construction of the experimental research device and most of its early operations were funded by the US Department of Energy. The recent studies—Phase I and Phase II—were funded by the

NASA Institute for Advanced Concepts The NASA Institute for Advanced Concepts (NIAC) is a NASA program for development of far reaching, long term advanced concepts by "creating breakthroughs, radically better or entirely new aerospace concepts". The program operated under the name ...

(NIAC) program. A series of articles on the concept were published between 2001 and 2008; the first experimental results were reported in 2007. Numerous studies of spacecraft missions (Phase I) were published, beginning in 2012. In 2017 Princeton Satellite Systems reported that "Studies of electron heating with this method have surpassed theoretical predictions, and experiments to measure ion heating in the second-generation machine are ongoing." As of 2018, the concept has moved to Phase II, a simulation phase, to further advance the design. The full-size unit would measure approximately 2 m in diameter and 10 m long. PSS reported that electron heating in PFRC-2 surpassed theoretical predictions, reaching 500 eV with pulse lengths of 300 ms. Ion heating experiments are ongoing as of 2020. Stephanie Thomas is vice president of Princeton Satellite Systems and the principal investigator for the Direct Fusion Drive.

Projected performance

Princeton Satellite Systems estimate that the Direct Fusion Drive could be capable of producing between 5-10 Newtons thrust per each MW of generated fusion power, with a

specific impulse Specific impulse (usually abbreviated ) is a measure of how efficiently a reaction mass engine (a rocket using propellant or a jet engine using fuel) creates thrust. For engines whose reaction mass is only the fuel they carry, specific impulse is ...

(I_sp) of about 10,000 seconds and 200 kW available as electrical power. Approximately 35% of the fusion power goes to thrust, 30% to electric power, 25% lost to heat, and 10% is recirculated for the RF heating. The company's modeling shows that this technology could propel a spacecraft with a mass of about to

Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest ...

in four years, enabling deep space missions. Since DFD provides power as well as propulsion in one integrated device, it would also provide as much as 2 MW of power to the payloads upon arrival, expanding options for instrument selection and laser/optical communications, and even transfer up to 50 kW of power from the orbiter to the lander through a

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...

beam operating at 1080 nm wavelength. Princeton Satellite Systems believes that this technology can expand the science capability of planetary missions. This dual power/propulsion technology has been suggested to be used on a

orbiter and lander mission, as well as integration on the

Orion spacecraft Orion (officially Orion Multi-Purpose Crew Vehicle or Orion MPCV) is a partially reusable crewed spacecraft used in NASA's Artemis program. The spacecraft consists of a Crew Module (CM) space capsule designed by Lockheed Martin and the Euro ...

to transport a

crewed mission to Mars The idea of sending humans to Mars has been the subject of aerospace engineering and scientific studies since the late 1940s as part of the broader exploration of Mars. Some have also considered exploring the Martian moons of Phobos and Deimo ...

in a faster timeframe (4 months instead of 9 with current technology). DFD is projected to deliver scientific payloads to Titan in 2.6 years.

References

External links