Lifetime Testing, Redundancy, Reliability, and Mean Time to Failure

Abstract

The environment of outer space is quite hostile to the many spacecraft that are now deployed in Earth orbit and beyond. There are many hazards in terms of severe thermal gradients, space weather from the sun and beyond, and intense radiation from the Van Allen belts as well as strong magnetic forces. Today, application satellites also must plan to cope with man-made hazards that arise from space debris, RF interference (RFI), and other possible hazards such as spurious commands. There are also risks associated with the launch and deployment of satellites since there are strong “g forces” during launch and difficulties that can arise from the unfolding, roll-out, or explosive or spring-loaded extension of solar arrays, antennas, and other systems that must be deployed in space in response to remote command. This complex series of hazards requires extensive testing of application spacecraft that are deployed into Earth orbit with the hope of extended lifetime operation. These hazards and difficulties of space operations increase the importance of lifetime testing. It also demands the design of application satellites to be rugged and to have reasonable levels of redundancy so that service can be maintained if various components happen to fail. In the case of application satellites, rugged design, redundancy, and demanding lifetime testing of applications satellites and its subsystems and components are of utmost importance simply because there is little opportunity for repair or refurbishment operations in space. Without these precautions, a very expensive application satellite that requires perhaps an even larger investment to launch it into space could be lost to the satellite operator and thus require replacement at very high cost either to the satellite operator or to the companies that have insured the launch and operation of the satellite.

In recent years, there has been an alternative approach taken in terms of deployment of large constellations of small satellites in space as an alternative to a few large satellites designed and tested for long life. These small satellites have been built at much lower cost using off-the-shelf components and most frequently by advanced manufacturing techniques that include 3-D printing. These have frequently been launched as “piggyback” missions and thus at much lower cost.

Networks such as Skybox Imaging, Planet Labs, PlanetiQ, Dauria Aerospace, Tyvak Nano-Satellite Systems, NovaWurks, and GeoOptics have all emphasized this approach that involves miniaturization, low-cost satellites, and associated modest launch costs over larger and more capable satellites that subjected to extensive lifetime testing prior to launch. This new paradigm is also now being tested by new communications satellite operators such as OneWeb that proposed to nearly 800 mass-produced satellites plus spares to create a network optimized for Internet-based services, and a megaLEO constellation by SpaceX might ultimately involve thousands of small satellites. For this type of alternative design architecture, the replacement of failed satellites with a ready supply of spares is the key to achieving system reliability. This approach is seen as the alternative to stringent testing and flight-qualified components with proven long-life capabilities in a stringent space environment.

The following text discusses all of these strategies for coping with and minimizing risk for the satellite application industry although the much greater emphasis is on the stringent reliability and long-life design approach, since the ventures employing a constellation of small satellites largely depend on a robust sparing effort.