Gas core reactor rocket
They may be capable of creating specific impulses of 3,000–5,000 s (30 to 50 kN·s/kg, effective exhaust velocities 30 to 50 km/s) and thrust which is enough for relatively fast interplanetary travel.
It may also be possible to use partially ionized plasma from the gas core to generate electricity magnetohydrodynamically, subsequently negating the need for an additional power supply.
Therefore, in most gas core reactor rocket concepts, some sort of seeding of the propellant by opaque solid or liquid particles is considered necessary.
These particles would make up up to 4% of the mass of the exhaust gas, which would considerably increase the cost of propellant and slightly lower the rocket's specific impulse.
This would preclude the use of a nuclear thermal rocket with this high of a specific impulse, unless some other means of seeding or heat transfer to the propellant is found.
The counter flow toroid is the most promising because it has the best stability and theoretically prevents mixing of the fissile fuel and propellant more effectively than the aforementioned concepts.
However, previous cold flow tests have shown that hydrodynamic containment is more easily achieved with a spherical internal wall geometry design.
One must find a medium that is transparent to a wide range of gamma energies, but can withstand the radiation environment present in the reactor, specifically particle bombardment from the nearby fission reactions.
Because the situation is entirely unlike that of the confinement of a fusion plasma in vacuum, the required strength of a magnetic field for fission fuel retention must be estimated based on magnetohydrodynamic considerations (in particular, the suppression of turbulent mixing).
Neutrons born in the fuel region travel relatively unimpeded to the external moderator where some are thermalized and sent back into the gas core.
Due to the high core temperatures, however, on the return trip the neutrons are up scattered in the fuel region, which leads to a significant negative reactor worth.
To achieve criticality, this reactor is operated at very high pressure and the exterior radial wall is made up of a moderator of some sort, generally beryllium oxide.
The open-cycle gas-core rocket has many unique design attributes that make it a serious challenger to other proposed propulsion for interplanetary missions.
The high specific impulse and large thrust possible for the OCGCR correspond to shorter mission times and higher payload fractions.
Additionally, any test of the system performed on earth would be under a gravity field of 1 g, which would bring buoyancy effects into play inside the gaseous core.
Due to the inability to perform live testing on earth, research is focused primarily on computational modeling of such a system.
It was previously mentioned that the specific impulse could be as high as or higher than 3000 s. However, results of computational modeling point towards this number being somewhat optimistic.
When thermal hydraulics were modeled more completely for a typical base injection stabilized recirculation bubble gas core rocket by D. Poston, the specific impulse dropped from >3000 s to <1500 s. In the base injection stabilized recirculation bubble gas core rocket concept, it is thought that some additional method of fuel confinement will be beneficial.
