The Technology of Discovery

7End‐to‐End Assembly and Pre‐flight Operations for RTGs

Shad E. Davis

Idaho National Laboratory, Idaho Falls, Idaho

7.1 GPHS Assembly

The core of a Radioisotope Thermoelectric Generator (RTG) is a radioisotopic heat source. The US uses General Purpose Heat Sources (GPHSs) as the essential building blocks of a radioisotopic heat source for RTGs. Early design goals and attributes of the GPHS included modularity to allow for flexibility and compatibility with static and dynamic systems, high power density to produce 165.5 thermal watts per kg (75 W_th/lb), improved safety backed by extensive safety testing, and lower cost.[1]

The first iteration of a GPHS, designated Step‐0, was used to fuel the GPHS‐RTGs used by the Galileo, Ulysses, and Cassini spacecraft; see Figure 7.1.

A small but critical design modification that added a “webbing” between the cavities for the graphite impact shells (GISs) enhanced the Step‐0 GPHS. This increased the strength of the aeroshell and earned it the name Step‐1 GPHS. The GPHS‐RTG for the Pluto New Horizons utilized the Step‐1 GPHSs. Today, the Step‐2 GPHS is in use. The thickness of the top and bottom surfaces were increased to strengthen those elements and add margin against ablation induced by aerodynamic heating experienced during an inadvertant re‐entry scenario. Step‐2 GPHS modules were used to fuel the Multi‐Mission Radioisotope Thermoelectric Generator (MMRTG) aboard the Curiosity and Perseverance rovers.

A Step‐2 GPHS is a module or block ...