Radioisotope rocket

The radioisotope rocket is a type of rocket engine

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

The working fluid in a machine is the pressurized gas or liquid which actuates the machine. Examples include steam in a steam engine, air in a hot air engine and hydraulic fluid in a hydraulic motor or hydraulic cylinder. More generally, a thermodynamic system consists of a system boundary across...

, 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 proportional but opposite force on that system.-Examples:...

. They are similar in nature to the nuclear thermal rocket

Nuclear thermal rocket

In a nuclear thermal rocket a working fluid, usually hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. The nuclear reactor's energy replaces the chemical energy of the reactive chemicals in a traditional rocket engine...

s such as NERVA

NERVA

NERVA is an acronym for Nuclear Engine for Rocket Vehicle Application. It was a design for a nuclear thermal rocket engine.-History:...

, 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 which 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.

The radioisotope rocket is a type of rocket engine

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

The working fluid in a machine is the pressurized gas or liquid which actuates the machine. Examples include steam in a steam engine, air in a hot air engine and hydraulic fluid in a hydraulic motor or hydraulic cylinder. More generally, a thermodynamic system consists of a system boundary across...

, 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 proportional but opposite force on that system.-Examples:...

. They are similar in nature to the nuclear thermal rocket

Nuclear thermal rocket

In a nuclear thermal rocket a working fluid, usually hydrogen, is heated to a high temperature in a nuclear reactor, and then expands through a rocket nozzle to create thrust. The nuclear reactor's energy replaces the chemical energy of the reactive chemicals in a traditional rocket engine...

s such as NERVA

NERVA

NERVA is an acronym for Nuclear Engine for Rocket Vehicle Application. It was a design for a nuclear thermal rocket engine.-History:...

, 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 which 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 impulse per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum...

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 and capers....

 SSME.

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 is the period of time, for a substance undergoing decay, to decrease by half. The name originally was used to describe a characteristic of unstable atoms , but may apply to any quantity which follows a set-rate decay....

 such as Po 210

Polonium

Polonium is a chemical element with the symbol Po and atomic number 84, discovered in 1898 by Marie and Pierre Curie. A rare and highly radioactive metalloid, polonium is chemically similar to bismuth and tellurium, and it occurs in uranium ores. Polonium has been studied for possible use in...

, as opposed to the typical RTG which would use a long half-life fuel such as plutonium

Plutonium

Plutonium is a rare transuranic radioactive element. It is an actinide metal of silvery-white appearance that tarnishes when exposed to air, forming a dull coating when oxidized. The element normally exhibits six allotropes and four oxidation states. It reacts with carbon, halogens, nitrogen and...

 in order to produce more constant power over longer periods of time.

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 Incorporated was an American corporation involved in a number of businesses, mostly defense-related, but including automotive, aerospace and credit reporting.The credit reporting business, spun off in 1996, is now called Experian...

 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 trajectories and velocities of various unmanned spacecraft visiting the outer solar system, most notably Pioneer 10 and Pioneer 11....

. 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 to explain the effect.

Patents

• 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.