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Mission Criticality of Space Mechanisms - Part 1
by Bill Purdy for Launchspace
Bethesda MD (SPX) Aug 07, 2013

File image: Kepler space telescope.

The failure of a single space vehicle mechanism can cause a total loss of mission. This criticality is compounded by the fact that redundancy in mechanisms is often impractical.

Consider this fictitious scenario: After a successful launch and injection, one of the four release mechanisms holding satellite XYZ to the rocket fails to release, thus preventing the spacecraft from deploying from the rocket and resulting in a total loss of mission. Many other releases are usually required to fully deploy a satellite; the failure of any one can result in the total loss of mission.

So why not just make the release mechanisms redundant, like every other system on a high-reliability space vehicle? Well, in many types of release mechanisms, redundancy for one or more critical functions, such as a separation nut releasing its bolt, is just not practical. Hence, this critical moving part simply must work correctly the first and only time it is activated for mission success.

Consider this true event from recent news: Spacenews May 15, 2013: "The planet-hunting days of NASA's prolific Kepler space telescope, which has discovered more than 2,700 potential alien worlds to date, may be over.

The second of Kepler's four reaction wheels - devices that allow the observatory to maintain its position in space - has failed." NASA officials announced May 15.

It is a significant challenge to make a mechanism operate continuously for 5, 10 or even 15 years with no chance of service or re-lubrication and mechanisms have often been the life-limiting item on a spacecraft.

These two scenarios highlight the critical consequences that can be associated with mechanism failures. Yet mechanisms have been very successful at meeting these challenges through intelligent design, thorough testing and the diligence and attention to detail of the engineers and technicians involved. To be successful with mechanisms, one must deal appropriately with the vagaries of friction.

The coefficient of friction of the various moving parts is usually one of the key factors in the performance margins for mechanisms. The coefficient of friction can easily vary as much as +/- 25% based on life and unit-to-unit variation. This variation must be dealt with wisely in the design and testing process. A wise man once said "Friction is perverse by nature - when you want it not to be there, it will, when you want it to be there, it won't."

Mechanisms have a good track record on orbit, but that has not been easy to achieve. In Part 2 of this series, we will explore the challenges and practices necessary to meet these challenges to provide successful mechanisms.

Bill Purdy is Associate Editor of the text Space Vehicle Mechanisms: Elements of Successful Design (Wiley and Sons) and teaches the course on space mechanisms for Launchspace Training


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