Note that the whole assembly has to have a 20 million hour MTBF. If there are 10 components that means 200 million hour MTBF each. If there are 100 components that means 2 billion hour MTBF each (assuming the MTBFs are all equal). Now this is strictly BOE but it gives you a rough idea.bwang wrote:Thanks M Simon for the reliability analysis. I knew that was key but did not take the time to quantify it.
http://en.wikipedia.org/wiki/Mean_time_between_failure
MTBF is Mean Time Between Failures, which is described at the wikipedia link. The reliability of the pistons is a key aspect.
Although having a system for monitoring the pistons and system for wear and having an easy method to rapidly swap out pistons could maintain a sufficiently high operating reliability. Keeping 90% uptime throughout a year with an average of one piston or some other component replaced every day with a 15 minute hot swap would be doable. Quality and cost of components would need to be balanced against overall costs.
http://ntrs.nasa.gov/archive/nasa/casi. ... 015833.pdf
Nasa has stirling free piston cryocoolers with mean time between failure of over 500,000 hours. Aircraft engines wiith 40,000 hours of mean time between failure are common and microturbines have 14,000 hours of mean time time between failure. Twice weekly and even daily maintenance can be doable if the maintenance can be fast (using a robotic arm to pop a piston assembly out and replace it). Faulty units can be refurbished and put back into service if that keeps costs down without sacrificing overall reliability.
Maintenance doesn't kill you. Unscheduled maintenance kills you.