Radioisotope thermoelectric generators (RTGs) have been the power plants of interplanetary spacecraft for fifty years. However, one major problem is that they are relatively heavy. Modern RTGs may weigh hundreds of pounds. They are fine for large-scale missions, but they are prohibitively big for small scale missions to the outer planets.
A group of engineers from the Aerospace Corporation and the US Department of Energy's Oak Ridge National Lab are working on a way to shrink RTGs dramatically so they can be used on much smaller missions.
Their technology is referred to as the Atomic Planar Power for Lightweight Exploration (APPLE) project. It has three main goals. The first goal is to generate power, the second to store that power and the third to provide heat for other components.
The second goal has a problem because RTGs start at peak power and release declining energy as time goes by. An RTG system must be designed with the length of the mission in mind. If a mission is intended to last five years, the RTG must be able to produce sufficient power for the whole five years, despite diminishing capability. APPLE solves the problem by providing energy generation and storage in the same package. It can either radiate heat away or direct it to other necessary components.
APPLE is designed as a tile that outputs and stores a specific amount of power. The tile can be single sided and used to coat the outside of the spacecraft so that heat can be radiated away. Or the tile can be dual sided and isolated on an external strut. The tiles can be strung together so multiples tiles can be used to archive a specific energy output to support a mission.
One of the primary decisions in the design of APPLE is the selection of the radioactive isotope to be used. The RTG design needs a reasonable mix of heat generation while not needing too much radiation shielding. The researchers settled on Plutonium-238 which is primarily used in nuclear weapons. Plutonium-238 has a half-life of eighty-eight years and a high-power density. The U.S. produced its own Plutonium-238 from 1964 to 1988. Since halting production, the U.S. has sourced Plutonium-238 from Russia. However, Russian supplies are running out and a new source of Plutonium-238 must be found.
Radiation shielding was another major issue. This is related to the location of the tiles and their material composition. Dr. Joseph Nemanick and his co-authors carried out numerous radiation simulations to find answers to both of these questions. They configured each tile so that the biggest radiation source affecting battery materials for fifteen to fifty year missions would be received from cosmic rays and not from the highly fissile material in the tile itself. Other positioning considerations were investigated. Models of such systems have improved greatly over the past few decades. Engineers have some idea of the best configuration before even making parts.
The APPLE team did make some parts including battery components and a radiation test setup. It is unclear whether the technology will be adopted by the myriad of small interplanetary missions planned for the big space agencies.