How can a 200 year old technology enhance future space missions?
Patented in 1816 by Robert Stirling to rival the steam engine, the closed cycle air engine which later became known as the Stirling engine first found use pumping water in a quarry in 1818. A revised Stirling engine was used in the 1840’s at a Foundry in Dundee Scotland and was in service for three years until it was replaced by a steam engine due to reliability issues and a high repair cost.
Some claims suggest the development of the Stirling engine were to reduce fuel consumption and to lower the risk of boiler explosions which steam engines were renowned for at the time causing injuries and fatalities. Over time boiler technology improved while steam engine efficiency increased and the Stirling engine become less viable as a large scale power plant. It did however continue to be used as small scale power plant due to its ease of operation until the early 1900’s when domestic use began to be replaced by electric motors and small internal combustion engines.
A Stirling engine requires an input of heat energy to run, however the heat energy can be supplied in many forms including concentrated sunlight which means they can be a source of renewable energy in some configurations. Through a cycle of the trapped fluid (usually air) being continuously heated and cooled motion is provided to a displacer which can then be used to recover kinetic energy.
Deep space exploration poses some serious problems for scientists and engineers due to the long timeframes and physical distances travelled and when it comes to powering of spacecraft solar power seems like an obvious choice. Yet at the extremes of our solar system the sun is far too distant to provide enough energy unless solar panels were excessively large. Therefore another source of energy is required.
Enter the radioisotope thermoelectric generator (RTG). This type of energy source uses a decaying radioactive material to warm a thermocouple array which then produces the electricity required for the spacecraft. The problem is this material is in short supply and RTG’s are quite inefficient (conversion efficiency of only 6% to 7%) along with being heavy.
In recent years the principles of the Stirling engine have been used to create generators which are far more efficient. The Stirling radioisotope generator (SRG) has been in development for several years, and despite NASA reducing funding for its research, in 2020 more than 200 years after the original Stirling patent, a prototype called the Technology Demonstration Convertor (TDC) #13 reached the milestone of operating maintenance free for 14 years with no degradation of performance.
SRG’s are able to operate at a conversion efficiency of 20% to 30%, while having half the mass of an RTG. They use less radioactive material, and the reciprocating parts use frictionless bearings to move back and forth in a linear motion creating an alternating current power source. It is interesting to see that such a technology has the potential to be resurrected for use in a system the original designer would have never imagined.