Atomic Insights Sept 1996

The Apollo missions to the moon are famous for heroic astronauts, exciting first steps and incredible pictures that fired the imaginations of a whole generation of scientists, engineers and school children.

Mixed in along with the hoopla about sending men into space on huge, fire spewing rockets, however, was some serious science. Each time the astronauts landed on the moon, they performed experiments and placed monitoring devices that beamed previously unknown information back to Earth to be analysed by a dedicated team of observers.

These monitoring devices measured seismic activity, they took samples of the moon’s surface rocks and they gathered images of the sky from the moon’s perspective. The data that the devices sent to Earth provided clues about the age of the moon, its cosmic origins, the nature of its composition, and its relationship to the origin of the earth.

Only One Choice

These experiments and measuring devices needed power supplies; engineers demanded a power source that could provide a reliable, steady current flow without any intervention. Because of the special nature of the mission, there was only a short list of alternatives.

The moon has no atmosphere; any energy source depending on chemical combustion was at a distinct disadvantage. Space and weight of the lunar module were severely limited, fuel cells needed too much bulky fuel.

The lunar night lasts for about 15 days; a solar panel with a battery back-up system would also be a bulky item. Sophisticated chemical storage batteries were considered, but quickly rejected based on their low energy density.

The chosen power source – the only one deemed capable of performing the required task – was a radioisotope thermal generator (RTG). The NASA designation for the devices that powered the Apollo Lunar Surface Experiments Package (ALSEP) for missions 12, 14, 15, 16, and 17 was
SNAP-27 (Systems for Nuclear Auxiliary Power model number 27).

SNAP-27 Characteristics

The SNAP-27 power supply weighed about 20 kilograms, was 46 cm long and 40.6 cm in diameter. It consisted of a central fuel capsule surrounded by concentric rings of thermocouples. Outside of the thermocouples was a set of fins to provide for heat rejection from the cold side of the thermocouple.

Each of the SNAP devices produced approximately 75 W of electrical power at 30 VDC. The energy source for each device was a rod of plutonium-238 weighing approximately 2.5 kilograms and providing a thermal power of approximately 1250 W.

Plutonium-238 is a non-fissile isotope of plutonium that decays by alpha particle emission with essentially zero associated gamma emissions. This characteristic was very important for the ALSEP powering application, both because the instruments would have been negatively affected by interference from a gamma emitter and because the devices required close handling by lunar astronauts.

Even though the only radiation from Pu-238 is alpha particles which require little shielding, it is necessary to use thick gloves when handling a 2.5 kilogram rod of Pu-238. The surface temperature will reach about 500 degrees C because of the energy being released by radioactive decay. After ten years of continuous power output, a Pu-238 based RTG will still produce 92% of its initial power.

One measure of performance that is often used for chemical storage batteries is the amp-hour. A modern battery might have a capacity of 1.5 amp-hrs/kg. The SNAP-27 power supplies demonstrated the ability to provide more than 4380 amp-hrs/kg during the four years that their performance was monitored. Similar RTGs have produced 24,000 amp-hrs/kg during a 20 year operating life and are still going strong.

What About Apollo 13?

The Apollo 13 lunar module carried a SNAP-27 as part of its payload, just like the rest of the lunar missions. As viewers of the recent movie know, the astronauts on that mission nearly lost their lives as a result of a power shortage caused by a leak in one of the fuel cell supply tanks. The ability of the crew to improvise in-flight to reduce power consumption to a minimum provided a gripping true life story that translated well to the big screen.

However, knowing that there was a high capacity, long life battery on board, one might wonder why it was not put to use. Having an extra 75 W of power that could not be depleted would have eased the astronauts’ fears considerably.

Unfortunately,the SNAP-27 RTGs were not shipped as complete units. The Pu-238 fuel rod was carried in a special transport case located on the outside of the descent stage of the lunar module. This location was inaccessible during flight operations; there was no way to assemble the power supply in an emergency.

For future space exploration, perhaps some consideration might be given to using any RTGs intended for surface experiments as back-up power supplies for the in-flight portion of the mission.