Fission Fragment Nuclear Propulsion Variants and Technical Details

The problem with space nuclear propulsion is NOT raw power, but how to eliminate waste heat. The more efficiently we can generate thrust, the less waste heat produced.

Can we have our cake and eat it too? Can we have a non-thermal nuclear propulsion minimizing waste heat?
* Yes. By making the fuel into dust.

There is the original dusty fission fragment version. These systems can have ISP of 500,000 to 1.5 million. However multi-gigawatt systems would have only a few dozen newtons of thrust. The systems could potentially reach 5% of the speed of light.

There is also afterburner versions that trade specific impulse for thrust.

Increasing thrust by 10 times at the expense of a reduction by a factor of 10 in specific impulse brings about an interesting tradeoff between the mission duration and the propellant expended. An ―afterburne, the physical implementation of this thrust increase, injects an inert gas into the FFRE exhaust beam. This concept is proposed for a future NIAC study. The figure shows one example of how the afterburner engine would be used in which thrusting is terminated early so that the deceleration needed to match the Jupiter orbit is minimized. This means that an Earth Departure requires 4 days rather than 55 days and introduces a long coast period. The result is that the mission duration nearly matches that of the hypothetical HOPE NEP mission using only 16.5 mT of propellant (vice 400 mT of LH2 for HOPE). Of the fuel used, about 0.25 mT would be the expensive nuclear fuel. This represents only a five percent increase in vehicle size mass. If the same mission was optimized instead for minimum mission time, the vehicle would be accelerating roughly half the way and decelerating into Jovian orbit the other half. With the afterburner engine attributes the same, this would result in Jupiter missions on the order of a year and a half each way and a total round trip propellant expenditure of about 90 mT, including less than 1 mT of nuclear fuel.

The dusty fuel is nanometer sized particles of slightly critical plutonium carbide, suspended and contained in an electric field. A moderator of deuterated polyethylene reflects enough neutrons to keep the plutonium critical, while control rods adjust the reaction levels. The moderator is protected from reaction chamber heat by a heat shield, an inner layer composed of carbon-carbon to reflect infrared radiation back into the core. The heat shield coolant passes through a Brayton cycle power generator to create some electricty, then the coolant is sent to the heat radiator.

The details of Werka’s initial generation FFRE can be found in the diagram below. The reaction chamber is about 5.4 meters in diameter by 2.8 meters long. The magnetic nozzle brings the length to 11.5 meters. The fuel is uranium dioxide dust which melts at 3000 K, allowing a reactor power of 1.0 GW. It consume about 29 grams of uranium dioxide dust per hour (not per second). Of the 1.0 GW of reactor power, about 0.7 GW of that is dumped as waste heat through the very large radiators required.

The second most massive component is the magnetic mirror at the “top” of the reaction chamber. Its purpose is to reflect the fission fragments going the wrong way so they turn around and travel out the exhaust nozzle. Surrounding the “sides” of the reaction chamber is the collimating magnet which directs any remaining wrong-way fragments towards the exhaust nozzle. The exhaust beam would cause near-instantaneous erosion of any material object (since it is electrically charged, relativistic, radioactive grit). It is kept in bounds and electrically neutralized by the magnetic nozzle cage.

Engine	Isp	Thrust
FFRE	527,000 sec	43 Newtons
AFFRE	32,000 sec	4,651 Newtons

Dusty-plasma fission-fragment nuclear rocket that can provide both thrust and electricity for a mission to Mars, substantially improving over the 40+ year old NERVA. It is able to achieve higher power (~5GW) than NERVA (~1GW) through its innovative dusty core that cools very efficiently by radiation. It is able to achieve higher specific impulse (~100,000s) than NERVA (~800s) or DS1 (~10,000s) by emitting fission fragments at a few percent of the speed of light where the charged dust is confined by strong magnetic and electric fields, which also transfer the thrust. It uses modern neutron moderators that are about 100 times more effective and lighter weight than NERVA, for a “wet” mass of a few tons. It can produce electricity directly from the charged fission fragments at about 85% efficiency, with less thermal radiators than the corresponding Carnot process of “nuclear-electric”. The environmental impact of radioactive exhaust for starting the rocket in low-earth orbit amounts to approximately one years worth of natural C14 production, depending on space weather. And finally, it uses proven HEU or Pu reactor fuel, which other than its processing as dust, is readily available.