Radioisotope Thermoelectric Generator

In the clean room at KSC’s Payload Hazardous Servicing Facility, technicians prepare the New Horizons spacecraft for a media event. The RTG seen in this picture is not the real flying unit and is only a mockup. The real RTG was installed shortly before launch.

Today's trivia looks at something I have been interested in for a while, the power supplies used on space probes.

The Radioisotope Thermoelectric Generator is a class of electric generator that operates using a thermocouple to generate electricity using the Seebeck effect.

Important details for this class of generator:

• No moving parts
• Powered by heat source and thermocouple
• Very low efficiencies
• Long operational life time
• Consistent steady output

These types of generators are useful for both space and terrestial usage. In space they have been used on a number of the recent space probes, and on Earth they have been used in unmanned remote light houses, beacons and other facilities.

Thermocouple

Before we go further we should of course take a moment to understand how a thermocouples works.

The thermocouple is based on the Seeback effect (Thomas Johann Seebeck, 1821) and occurs when two conductors (typically metals) come into contact and are at different temperatures. When this occurs a positive voltage can be detected at the cooler end.

This principle is widely used in thermometers, where the difference between a reference temperature (room temperature) and the substance being sampled generates an electrical difference into a temperature difference.

K-type thermocouple (chromel–alumel) in the standard thermocouple measurement configuration.

In the case of an RTG, the thermocouple connects the heat source to a heat sink which radiates the heat away, creating the temperature difference to generate energy.

Heat Source

In the case of the RTG used in the New Horizons mission the radioactive element of choice is Plutonium 238.

A pellet of 238PuO2 to be used in an RTG for either the Cassini or Galileo mission

With its half life of 87 years and its tendency to emit mostly alpha radiation it is ideal for the space probes. However by comparison to Plutonium 239 used in nuclear weapons, 238 is considerably more radioactive. Handle with care recommended.

The generator onboard the New Horizons Mission at launch provided 250W 30V DC which by the time it had reached Pluto 9.5 tetras later had dropped to 200W. Enough to power most of the instruments at the same time.

Other materials can be used as well. Strontium-90 for example has been used in Russia to power various terrestrial research stations. As a by product of nuclear power plants it is readily available.

By comparison, Plutonium 238 has to be manufactured specifically, and Russia and the US are the only countries that manufacture it.

Assembly/Design

A radioisotope thermoelectric generator (RTG) assembly and Ulysses undergo a fit-check in 1989.

Given that we are dealing with radioactive decay rather than nuclear fission, the design of a RTG is comparably very simple. A tube containing the heat source is connected to a thermocouple and heatsinking. The voltage is generated is then connected directly to the power distribution parts of the circuit being powered.

Diagram of an RTG used on the Cassini probe. The GPHS (General Purpose Heat Source) is powered by a pellet of Plutonium 238

Future

Will this power source be used on future space missions? Possibly, it is very reliable and easy to manufacture. However the efficiency for mass is a design issue with RTGs, most of which are around 3-7%. The demand on electric systems by scientific instruments will only increase and power output will become a limiting factor.

For example, New Horizons spent the majority of its mission time in hibernation. Only to be very active during the actual fly-by, which is of course a small time window in which all its instruments need to be highly active.

Time lapse photograph of the heater head of a Stirling radioisotope generator undergoing lifetime assessment.

NASA have investigated more efficient alternatives including the Stirling Radioisotope Generator which might offer up to four times the efficiency from the same Plutonium heat source.

Stirling Engines work by using two pistons which are connected together by a tube. The first piston is heated, the second is cooled. The rods of the pistons are connected via a rotary motion to cause the opposing piston to move.