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TECH SPACE
Challenges and Practices for Space Mechanisms - Part 2
by Bill Purdy for Launchspace
Bethesda MD (SPX) Aug 14, 2013


As with all flight hardware, attention to detail by everyone in the process is a key to success. Particular emphasis should be placed on the proper application of lubricants and the verification of their performance. Mechanism design must prevent lubricants from migrating out of the critical area and must keep them within their successful operating regimes and temperatures.

Release and Deployment Mechanism Challenges. Challenges associated with release and deployment systems often include the impracticality of complete and independent redundancy and difficulties in simulating operation in zero G for large deployable systems.

Redundancy in electronic systems is usually as easy as having duplicates of an electronic box, with one box being a redundant spare to be switched on in the event of a problem with the primary box. For the moving parts of a deployment or release mechanism to be redundant, one has to consider schemes such as nested bearings, that is, one bearing mounted inside another bearing so that should one bearing seize, the second bearing provides the needed rotation.

While this scheme does provide redundancy, this large and complex design may not be able to meet stiffness, freeplay, size, mass or other critical requirements.

This example shows one way in which one runs into difficulties in providing redundancy without degrading performance or adding unacceptable complexity. The resulting non-redundant critical moving parts present challenges that must be met primarily through good design, and thorough well-thought-out testing.

Deployable systems, including large solar arrays and large antenna reflectors, enable tremendous capability for satellites, but present significant testing challenges. It is simply not practical to size such a deployment system to be self-supporting in Earth's gravity.

Therefore, the success of these systems depends on clever testing apparatus to simulate zero G for the deployable. Further, a redundant solar array or redundant antenna is the only path to true redundancy, if affordable, and the complex interrelationships of all the moving parts cannot be considered redundant.

Long Life Mechanism Challenges. Challenges associated with long-life mechanisms include wear and lubrication breakdown, coupled with the impracticality of service or re-lubrication, the difficulties with completing life testing prior to launch, and the limited choice of lubricants that are compatible with the space environment.

As the reliability of solid-state electronics has drastically improved over the past two decades, satellite lifetimes can now be at the mercy of mechanisms lasting a very long time, as is the case with the reaction wheels for the Kepler space telescope.

Lubricant failure in bearings and gears is one of the most common life limiters for mechanisms. There are only a few very capable lubricants available for space that, if applied properly, can provide outstanding lifetimes.

However, the selection of space-qualified lubricants is limited and considerable expertise is required for their proper use in a given application. Life testing is most accurate when it is conducted at the same speed of operation as in flight.

The downside of testing at the flight speed is that when the flight operating regime is continuous operation, then the life test would take the duration of the mission to be completed, making it unlikely that the test would be finished prior to launch.

The alternative of testing at a higher mechanism speed to accelerate the life test has the drawback of changing the critical physical behavior of the lubricant under test. Hence, there are no perfect answers and engineers do the best they can through life testing and examination of flight heritage.

Meeting These Challenges. Mechanisms have a good on orbit record despite these challenges. Key practices include heavy dependence on thorough testing, large functional margins, and attention to detail in design, manufacturing and test.

Analytical techniques for mechanisms have limited accuracy and limited applicability for qualification because the physics of wear and friction are not understood well enough to support accurate predictions. Therefore, the world of mechanisms, both for long-life mechanisms and release mechanisms, must emphasize testing. This includes testing of prototypes, dedicated qualification units, and testing of each and every flight mechanism.

The testing should verify that each and every mechanism has proper, in family, performance, should verify functional margins and, of course, should verify performance in all operating environments and regimes. This sounds like motherhood, but mechanisms testing must be a practice that receives extra scrutiny if one wants to ensure mission success.

Mechanism reliability greatly improves with the selection of large functional margins. Margins should be considerably larger than those of other subsystems because many variables, among them coefficients of friction that are subject to considerable variation over life and suffer from poor predictability.

The selection of large margins for driving force/torque greatly reduces the likelihood of a single unexpected behavior stopping all moving parts. The often catastrophic consequence of a mechanism failure dictates that these large margins are warranted and worth the associated cost and/or impacts to power and mass.

The basic rule regarding functional margins for mechanisms is that a lot is good, too much is better and way too much is just right. Excess horsepower is great at resolving unexpected friction behaviors.

As with all flight hardware, attention to detail by everyone in the process is a key to success. Particular emphasis should be placed on the proper application of lubricants and the verification of their performance. Mechanism design must prevent lubricants from migrating out of the critical area and must keep them within their successful operating regimes and temperatures.

Expertise in lubrication is required and should be sought out for critical applications. Since it is often tough to finish life tests prior to launch, we must depend upon, and be aware of, flight heritage for similar mechanisms. The flight record is far and away the best life-data available for long-life mechanisms and their lubrication.

In summary, meeting the challenge of building space mechanisms is a critical component of mission success, and success is achievable with a proper and thoughtful approach.

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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TECH SPACE
Mission Criticality of Space Mechanisms - Part 1
Bethesda MD (SPX) Aug 07, 2013
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 fro ... read more


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