A recently concluded study by professors Joseph H. Saleh and Mitchell L. R. Walker could open up some value-enhancing design choices for the satellite industry.
Their findings, summarized in “Electric Propulsion Reliability: Statistical Analysis of on-Orbit Anomalies and Comparative Analysis of Electrical versus Chemical Propulsion Failure Rates,” give satellite manufacturers, insurers, and operators evidence-based support to pursue electric over chemical propulsion technology for future spacecraft.
The study is the result of a powerful collaboration between two experts. Saleh and Walker tackled some persistent spacecraft propulsion and reliability questions that had previously given chemical propulsion (CP) the undisputed edge.
“The work that I do - on reliability, with an epidemiological twist – and the work that Dr. Walker is nationally known for – electric propulsion – both touch on these questions. Our collaboration enabled us to address them and provide robust answers,” said Saleh, the author of Spacecraft Reliability and Multi-State Failures: A Statistical Approach (Wiley, 2012).
“The insights we gained that will, ultimately, benefit the industry, we found at the intersection of our work.”
Using 18 years worth of satellite data, Saleh and Walker conducted what amounted to an epidemiological study of the on-orbit anomalies and failures of electric propulsion (EP). They produced a comparative analysis of EP and (CP) reliability in spacecraft and examined data by orbit type, technology, severity of the anomalies experienced, and launch date. They also looked at the average time it took for anomalies to occur, the relationship of the time to event, and the anomaly rates.
Among the many results, their study revealed that, post-2005, EP technologies (Hall Thrusters and gridded ion engines) have consistently out-performed chemical propulsion in terms of reliability. This, alone, gives EP a clear advantage when it comes to space travel and transport, where reliability is paramount.
“With so much at stake, the space industry has been historically risk-averse,” said Saleh.
“You can’t go fix a satellite the way you can a car. No one wants to spend hundreds of millions of dollars to launch a satellite if there’s no guarantee of its reliability or that of its subsystems."
Their analysis also revealed that Hall thrusters exhibit minor anomalies very early (“infant anomalies”), a problem that they suggested could be addressed through improved ground testing and acceptance procedures. Gridded ion thrusters, on the other hand, exhibited both infant anomalies and wear-out failures – conditions that would benefit from a reliability growth program. The team found strong evidence that EP anomalies (onset and likelihood) and orbit type are dependent – a condition that is likely mediated by either the space environment or differences in thrusters’ duty cycles.
Historically, its reliability and superior thrust capabilities made CP the go-to technology for the space industry and other stakeholders. Electric propulsion was restricted to tasks that required little acceleration, such as small in-orbit maneuvers, station-keeping, and (with a chemical assist) some orbit raising.
An uptick in EP utilization over the past 20 years gave Saleh and Walker the raw data they needed to question that conclusion. Their findings make EP more competitive with CP for future satellites.
With less fuel to transport, an EP-propelled satellite starts out lighter than its chemically propelled counterpart. That means a typical EP satellite can carry more payload (e.g. communications transponders, scientific equipment, etc.) than the same-mass CP satellite. The cost savings in manufacture time, insurance, and materials cannot be overlooked.
“The bottom line is that we’ve developed a more detailed picture of EP satellite technology, one that enables satellite operators to make better reliability- and risk-informed decisions regarding the propulsion choice for their spacecraft,” said Walker. “This choice could also extend the lifetime of the spacecraft, and [consequently] the long-term value it produces.”
Saleh echoed his colleague, adding:
“It is thought-provoking to consider that perhaps the bell has begun to toll for spacecraft chemical propulsion. In the next decade, the market share of spacecraft chemical propulsion will continue to erode and is likely to become confined to a small niche market of missions with high thrust and high maneuverability requirements.”