THE HIERARCHY OF FAILURE

All pigs are equal, George Orwell told us in Animal Farm, but some pigs are more equal than others. All three of the triple constraints are constraining—they limit our options and there are consequences for violating or exceeding the constraints placed on the project. But those constraints are not equal, and we can choose strategically among our ways to fail. We can exploit the constraints that are more flexible and even accept the failures that are less damaging to ensure that we do not fail where failure is not an option.

What is the order of acceptable failure for our CO2 filter project? Interestingly, the widest options for failure lie within the performance criteria. We can accept something as marginal as partial performance as long as it extends the time constraint. Second in exploitability is the cost constraint. We can exercise our creativity to the utmost and improvise within the constraint. A pair of socks, a report cover, a bungee cord—anything we can draft into service—becomes a resource. But failure is not an option where time is concerned. We solve the problem before they run out of breathable air. Period.

In other words, we define the project by listing the triple constraints in the order of flexibility, from least to most. We must succeed with the driver to call the project a success, and may exploit the flexibility within the other constraints—however much flexibility there may be—to do so. The weak constraint is the one with the greatest capacity for exploitation or flexibility, and the middle, well, is the one in the middle.

Triple Constraints for CO2 Filter Project

PROJECT: Build CO2 filter adapter to allow square CM filters to be used in cylindrical LEM sockets.

—DRIVER: Time Constraint (before the CO2 levels overwhelm the astronauts)

—MIDDLE: Cost Constraint (using only what’s available on the spacecraft)

—WEAK: Performance Criteria (that works well enough to allow the astronauts to live and continue performing their duties for the mission duration)