Stony Brook Web page (Mechanical Engineering) lists some of the radio-nuclides used.
Long-duration space missions invariably use plutonium-238 owing to its long half-life, although this is an extremely high-cost fuel amounting to many million dollars per RTG. For ground use, strontium-90 is preferred. Strontium-90 is, indeed, the radionuclide that powers the controversial 500 or so RTGs installed by the former Soviet Union along the coast of the Kola Peninsula (bordering on Finland and Norway).
The Radioisotope Thermal Generator (RTG) that powered the "Galileo" missions to the outer planets represented the state of the art for thermoelectric power sources in 1978. It used the then novel selenium-based semiconductors. The specifications for this RTG listed in Table 5.7 correspond to both beginning-of-life (BOL) and end-of-mission (EOM) conditions. The EOM conditions are not the same as end of life, which is many years longer. In this particular case, BOL is 1000 hours after fueling, and EOM is 59,000 hours (almost seven years) after fueling.
A AT of about 700 K was kept throughout the mission. The thermoelectric efficiency degraded only slightly in the seven years of operation.
The advantage of an RTG is that it is extremely light if one considers that it includes both the electrical generator and the fuel for many years of operation. Even the lightest possible gasoline engine would be orders of magnitude heavier. A large airplane gasoline engine may deliver 1500 W/kg, but one must add the mass of the fuel and oxygen needed for longtime operation. The specific consumption of a gasoline engine is about 0.2 kg hp_1h_M For each kilogram of gasoline, the engine uses 3.1 kg of O2. The specific consumption of fuel plus oxygen is 0.8 kg hp_1h_1. Since 200 W correspond to 0.27 hp, the hourly consumption of a gasoline-driven generator that delivers 200 W is 0.24 kg of consumables. During the 59,000 hours of the mission, 14,000 kg of consumables would be used up. Thus, these consumables alone would mass over 3000 times more than the whole RTG. The latter, having no moving parts, requires no maintenance, while, on the other hand, it is inconceivable that a gasoline engine could possibly operate unattended for seven long years.
The low efficiency of thermocouples (less than 10%) has prompted NASA to investigate other ways of converting the heat of radioactive decay into electrical energy. One solution being actively pursued is the use of a free-piston Sterling engine (see Chapter 3) that can be made to run for decades without maintenance, thanks to hermetically sealed, lubrication-free arrangements. The oscillating piston of the engine is directly attached to a linear alternator. Efficiency of 24% (500 watts of heat input converted to 120 W of electrical output) has been demonstrated. This is important because for a given amount of electric energy required by the spacecraft, the mass of the generator becomes much smaller, as does the (very high) cost of the fuel.
The device is, of course, not an RTG; it is an SRG—a Stirling Radioisotope Generator. Whereas the real RTG inherently is totally silent and vibrationless, the SRG has to be designed with great care to avoid unacceptable levels of mechanical noise.
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