Radioisotope rocket
Encyclopedia
The radioisotope rocket is a type of rocket engine
that uses the heat generated by the decay of radioactive elements to heat a working fluid
, which is then exhausted through a rocket nozzle to produce thrust
. They are similar in nature to the nuclear thermal rocket
s such as NERVA
, but are considerably simpler and often have no moving parts.
The basic idea is a development of existing radioisotope thermoelectric generator
, or RTG, systems, in which the heat generated by decaying nuclear fuel is used to generate power. In the rocket application the generator is removed, and the working fluid is instead used to produce thrust directly. Temperatures of about 1500 to 2000°C are possible in this system, allowing for specific impulse
s of about 700 to 800 seconds (7 to 8 kN·s/kg), about double that of the best chemical engines such as the LH2-LOX
Space Shuttle Main Engine
.
However the amount of power generated by such systems is typically fairly low. Whereas the full "active" reactor system in a nuclear thermal rocket can be expected to generate over a gigawatt, a radioisotope generator might get 5 kW. This means that the design, while highly efficient, can produce thrust levels of perhaps 1.3 to 1.5 N, making them useful only for thrusters. In order to increase the power for medium-duration missions, engines would typically use fuels with a short half-life
such as Po 210
, as opposed to the typical RTG which would use a long half-life fuel such as plutonium
in order to produce more constant power over longer periods of time. The even shorter half-life element fermium
has also been suggested
Another drawback to the use of radioisotopes in rockets is an inability to change the operating power. The radioisotope constantly generates heat that must be safely dissipated when it is not heating a propellant. Reactors, on the other hand, can be throttled or shut down as desired.
TRW
maintained a fairly active development program known as Poodle from 1961 to 1965, and today the systems are still often known as Poodle thrusters. The name was a play on the larger systems being developed under Project Rover, which led to NERVA. In April 1965 they ran their testbed engine for 65 hours at about 1500°C, producing a specific impulse of 650 to 700 seconds (6.5 to 7 kN·s/kg).
The inadvertent construction of a radioisotope rocket is one of the suggested solutions to the Pioneer anomaly
. The Pioneer space probes are powered by radioisotope thermal generators located on the end of a long arm to keep their radiation away from the spacecraft electronics. In this position the back of the main radio dish is preferentially exposed, meaning that radiation (primarily infra-red) scattered by the dish would tend to be scattered to the rear of the spacecraft. This could lead to a tiny amount of net thrust being generated, although all calculations to date suggest it is not enough in itself to fully explain the effect.
A similar phenomenon occurred on the New Horizons
spacecraft; photons (thermal infrared) from the RTG, reflected from the spacecraft's antenna, produced a very small thrust which propelled the spacecraft slightly off course.
Rocket engine
A rocket engine, or simply "rocket", is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...
that uses the heat generated by the decay of radioactive elements to heat a working fluid
Working fluid
A working fluid is a pressurized gas or liquid that actuates a machine. Examples include steam in a steam engine, air in a hot air engine and hydraulic fluid in a hydraulic motor or hydraulic cylinder...
, which is then exhausted through a rocket nozzle to produce thrust
Thrust
Thrust is a reaction force described quantitatively by Newton's second and third laws. When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system....
. They are similar in nature to the nuclear thermal rocket
Nuclear thermal rocket
In a nuclear thermal rocket a working fluid, usually liquid hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. In this kind of thermal rocket, the nuclear reactor's energy replaces the chemical energy of the propellant's...
s such as NERVA
NERVA
NERVA is an acronym for Nuclear Engine for Rocket Vehicle Application, a joint program of the U.S. Atomic Energy Commission and NASA managed by the Space Nuclear Propulsion Office until both the program and the office ended at the end of 1972....
, but are considerably simpler and often have no moving parts.
The basic idea is a development of existing radioisotope thermoelectric generator
Radioisotope thermoelectric generator
A radioisotope thermoelectric generator is an electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.RTGs can be...
, or RTG, systems, in which the heat generated by decaying nuclear fuel is used to generate power. In the rocket application the generator is removed, and the working fluid is instead used to produce thrust directly. Temperatures of about 1500 to 2000°C are possible in this system, allowing for specific impulse
Specific impulse
Specific impulse is a way to describe the efficiency of rocket and jet engines. It represents the derivative of the impulse with respect to amount of propellant used, i.e., the thrust divided by the amount of propellant used per unit time. If the "amount" of propellant is given in terms of mass ,...
s of about 700 to 800 seconds (7 to 8 kN·s/kg), about double that of the best chemical engines such as the LH2-LOX
Lox
Lox is salmon fillet that has been cured. In its most popular form, it is thinly sliced—less than in thickness—and, typically, served on a bagel, often with cream cheese, onion, tomato, cucumber and capers...
Space Shuttle Main Engine
Space Shuttle main engine
The RS-25, otherwise known as the Space Shuttle Main Engine , is a reusable liquid-fuel rocket engine built by Pratt & Whitney Rocketdyne for the Space Shuttle, running on liquid hydrogen and oxygen. Each Space Shuttle was propelled by three SSMEs mated to one powerhead...
.
However the amount of power generated by such systems is typically fairly low. Whereas the full "active" reactor system in a nuclear thermal rocket can be expected to generate over a gigawatt, a radioisotope generator might get 5 kW. This means that the design, while highly efficient, can produce thrust levels of perhaps 1.3 to 1.5 N, making them useful only for thrusters. In order to increase the power for medium-duration missions, engines would typically use fuels with a short half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
such as Po 210
Polonium
Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie Skłodowska-Curie and Pierre Curie. A rare and highly radioactive element, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for...
, as opposed to the typical RTG which would use a long half-life fuel such as plutonium
Plutonium
Plutonium is a transuranic radioactive chemical element with the chemical symbol Pu and atomic number 94. It is an actinide metal of silvery-gray appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation...
in order to produce more constant power over longer periods of time. The even shorter half-life element fermium
Fermium
Fermium is a synthetic element with the symbol Fm. It is the 100th element in the periodic table and a member of the actinide series. It is the heaviest element that can be formed by neutron bombardment of lighter elements, and hence the last element that can be prepared in macroscopic quantities,...
has also been suggested
Another drawback to the use of radioisotopes in rockets is an inability to change the operating power. The radioisotope constantly generates heat that must be safely dissipated when it is not heating a propellant. Reactors, on the other hand, can be throttled or shut down as desired.
TRW
TRW
TRW Inc. was an American corporation involved in a variety of businesses, mainly aerospace, automotive, and credit reporting. It was a pioneer in multiple fields including electronic components, integrated circuits, computers, software and systems engineering. TRW built many spacecraft,...
maintained a fairly active development program known as Poodle from 1961 to 1965, and today the systems are still often known as Poodle thrusters. The name was a play on the larger systems being developed under Project Rover, which led to NERVA. In April 1965 they ran their testbed engine for 65 hours at about 1500°C, producing a specific impulse of 650 to 700 seconds (6.5 to 7 kN·s/kg).
The inadvertent construction of a radioisotope rocket is one of the suggested solutions to the Pioneer anomaly
Pioneer anomaly
The Pioneer anomaly or Pioneer effect is the observed deviation from predicted accelerations of the Pioneer 10 and Pioneer 11 spacecraft after they passed about on their trajectories out of the Solar System....
. The Pioneer space probes are powered by radioisotope thermal generators located on the end of a long arm to keep their radiation away from the spacecraft electronics. In this position the back of the main radio dish is preferentially exposed, meaning that radiation (primarily infra-red) scattered by the dish would tend to be scattered to the rear of the spacecraft. This could lead to a tiny amount of net thrust being generated, although all calculations to date suggest it is not enough in itself to fully explain the effect.
A similar phenomenon occurred on the New Horizons
New Horizons
New Horizons is a NASA robotic spacecraft mission currently en route to the dwarf planet Pluto. It is expected to be the first spacecraft to fly by and study Pluto and its moons, Charon, Nix, Hydra and S/2011 P 1. Its estimated arrival date at the Pluto-Charon system is July 14th, 2015...
spacecraft; photons (thermal infrared) from the RTG, reflected from the spacecraft's antenna, produced a very small thrust which propelled the spacecraft slightly off course.
External links
- AIAA meeting paper study comparing Poodle thrusters to a chemical (hydrogen/fluorine) option for upper stage propulsion
- United States Patent 3315471; Direct cycle radioisotope rocket engine; 1967; Lee, Dailey Charles, Verdes, Estates Palos
- United States Patent 3306045; Radioisotope rocket; 1967; Buford Jr., William H. Thomas Jr., Arthur N