NUCLEAR FISSION ROCKETS:

Nuclear rockets based on fission or fusion ideally could far exceed the limitations of rockets using chemical energy. The potential usable energy content per unit mass of fuel is 5.5X10⁶ times greater than chemical energy and for fusion it is 2.6X 10⁷ greater. In a chemical reaction, an insignificant amount of mass is converted to energy as electrons rearrange themselves among the electron clouds of combining atoms. In U fission, a significant fraction of the nuclear fuel mass,(e) is potentially convertible to direct energy of the exhaust, namely 7.9X10^-4,which represents complete fuel fissioning or "burnup" (in practice, contemporary nuclear reactors fission only about 1% of the atoms in their fuel, but 7.9X10^-4 is the fraction of the fissioned mass converted to energy) In fusion rockets based on deuterium reactions, e=4x10^-3 .
NERVA (Nuclear Energy For Rocket Vehicle Application) demonstrated the application of small compact mobile reactors could work, but none of these systems were allowed to fly. These experiments were in fact an outgrowth of a period that saw development in reactor development for large aircraft. that could maintain strategic airborne presence for long periods without the need to refuel With the advent of KIWI and ROVER projects the applications were for an anticipated human mars mission several billion dollars were spent before it's untimely demise in the early 1970's. While the project's demise may have been attributed to over zealous environmental arguments over it's perceived pollution it none-the -less lost direction and in the end violated the justification in spending billions of dollars without ever putting the technology to use, for a propulsion system that after all was designed to be used in space. Only recently has a major rocket company been willing to revisit the nuclear thermal rocket legacy left by project NERVA but with major changes to materials, design and function.
The basic principle is simple fission-powered rocket engine consisting of a compact nuclear reactor through which propellant usually hydrogen is passed and thereby heated. The energy of Uranium fissioning products and neutrons appears as thermal energy in the reactor. the associative contact of the hydrogen gas with parts of the reactor brings the gas to high temperature.
The super hot gas is then allowed to expand through a nozzle forming a high velocity stream.

VARIATIONS OF THE NUCLEAR ROCKET ENGINE

There are three generic variants of the fission rocket:

Solid core (Isp 500-1000 sec.): Contains solid fuel elements; nuclear fuel clad with high temperature alloys and does not permit the loss of nuclear fission product.
Liquid Core (Isp 1100-1600 sec.): These designs relax the containment of nuclear fuel. In theory they can achieve higher temperature thus provide a more efficient Isp. In typical liquid core design, hydrogen gas is forced through a spinning annulus of microscopic liquid molten nuclear fuel droplets. The purpose of spinning being to retain the nuclear fuel and sustain the fission reaction.
Gas Core (Isp 3000-7000 sec.): The gas-core fission rocket spins gaseous nuclear fuel in a high temperature vortex to permit the loss of as little nuclear fuel as possible. Another advantage these higher temperature system have is a higher operating temperatures which in space makes them more efficient at heat rejection than reactor cores do not have this advantage thus they require less cooling surface area toward the 'heat sink' (space) than other systems.
Nuclear rockets achieve their high specific impulse in part because of the low molecular weight of their exhaust products, that is diatomic hydrogen gas that has been significantly broken down into individual hydrogen atoms and ions. Fission rockets like their chemical cousins (Isp 450 sec.). They also have high thrust/weight ratios T/W 0.1-1.0.
Short of using antimatter, the highest reactor core temperature in a nuclear rocket can be achieved by using gaseous fissionable material. In the gas core rocket, radiant energy is transferred from a high-temperature fissioning plasma to a hydrogen propellant. In this design propellant temperature can be significantly higher than the engine structure temperature. In some designs; the propellant stream is seeded with submicron particles to enhance heat transfer. Limitations are noted for radioactive fuel loss and its negative effects on performance.
Open-cycle Gas Core Engine: Relies on flow dynamics to control fuel loss.
Closed-cycle Gas Core Engine: The engine design avoids the nuclear fuel loss evident in the open-cycle gas core engine by containing the nuclear plasma under special high temperature quartz capsule 'glass'. Thermal radiation from the plasma passes through the quartz wall are regeneratively cooled by the hydrogen propellant.

OTHER FISSION SPACE NUCLEAR REACTORS:
For POWER-or-PROPULSION with NUCLEAR ELECTRICAL PROPULSION CONCEPTS LIST.


1.  SP-100: Scalable, fast reactor pin type fuel
2.  ENABLER: (NERVA based) closed Brayton cycle graphite moderated
3.  PARTICLE BED REACTOR:
4.  PELLET BED REACTOR:
5.  10 MWe NUCLEAR RANKINE SYSTEM:
6.  POTASSIUM RANKINE SYSTEM:
7.  TORCHLITE: Thermionic power conversion
8.  IN-CORE THERMIONIC SYSTEM:
9.  VAPOR CORE REACTOR: UF4 fuel magnetohydrodynamic (MHD)
10.NEPTUNE: NERVA based: Thermal-to-electric
11.RMBLR: (Rotating multi-megawatt boiling liquid-metal reactor)
12.TRITON: tungsten cermet NTR/NEP hybrid Brayton cycle

ELECTRICAL ENGINE PROPULSION CONCEPTS:
-MAGNETOPLASMADYNAMIC (MPD)
-NEPTUNE (NERVA-BASED)
-DEFLAGRATION THRUSTER (PLASMA GUN)
-PULSED PLASMOID THRUSTER
-PULSED ELECTROTHERMAL THRUSTER
-ION CYCLOTRON RESONANCE THRUSTER (ECR engine)
-PULSED INDUCTIVE THRUSTER
-VARIABLE Isp PLASMA ROCKET ENGINE

EPP (EXTERNAL PULSE PROPULSION) CONCEPTS:

-ORION: Nuclear explosive pulses with pusher plate technology (fission/fusion)
-Mini-magORION (MMO): Gigajoules scaled pulse nuclear fission device where low mass criticality is accomplished by electromagnetic compression of individual explosive fission pellets.




BASIC SAFETY STRATEGIES:
1.Launch with 'clean' core (fuel kept separate from core).
2.Design to prevent criticality under launch and ascent accident conditions.
3.Provide secure command & control communication (prevent unauthorized startup and operations).
4.Start operations only after successful attainment of orbit.
5.Select operating orbits (insure radioactive decay to negligible levels before reentry or provide highly reliable boost capacity).
6.Design to assure intact reentry (prompt, secure retrieval and burial or reuse even though this event was an anomaly).

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