Do the drawbacks of using commercial-off-the-shelf components for military and aerospace applications outweigh the benefits?
I got my first cell phone in the mid-’90s. It was one of those monolithic designs about the size and shape of a large potato. It didn't fit in my purse, let alone in my pocket (although I got a swingin’-looking belt holster for it). I probably should have known it was just a matter of time before the novelty wore off and I lost the danged thing. Indeed, it slipped off my belt in a cab one day as I was heading to the airport from a conference. Those were the days when mobile handsets actually cost serious money, so I spent a fair amount of time trying to get it back (unsuccessfully) before I finally gave up and headed to the store for replacement.
I figured I’d buy the same model, which would keep me from spending time I didn't have learning the ins and outs of the software. It shouldn't have surprised me that my previous phone was a good two generations out of date — technology moves on, after all, and it had been at least two weeks since I’d bought the other handset.
Okay, maybe that's an exaggeration, but it does serve to illustrate one of the primary challenges of using commercial off-the-shelf (COTS) components in military and aerospace applications. We all know the upside, of course — high availability, fast delivery, low cost — but the same market forces that create these advantages have also introduced serious disadvantages when the components are used in other markets. COTS components are subject to the consumer electronics regulatory environment. To comply with RoHS, for example, commercial electronics manufacturers have had to move to lead-free solder. That's good for the environment and human health, but it causes a thorny old problem called tin whiskers — spontaneous growth of small projections that can short out a component and that can occur at any time. Not exactly an issue you want to have crop up in mission- or safety-critical hardware. The longer the operational lifecycle of the component, the greater the chance that tin whiskers will occur.
Which brings up the second major drawback: lifecycle. My cell phone anecdote, while somewhat exaggerated, illustrates an important point: consumer electronics have shorter operational lifecycles, let alone production lifecycles, than military or aerospace systems. A consumer model might have a production lifetime of months, whereas a military or avionics program will need to be supported for decades. That creates problems for the latter set of products when a manufacturer discontinues a COTS component that has been engineered in.
That brings up the third drawback, which is that the sheer size of the consumer electronics market makes it the 10-ton gorilla compared to the mil/aero market. If a part is no longer selling to the consumer market at high volumes, a vendor whose revenues are primarily fueled by that market will very likely discontinue it, despite the need of a small group of its customers for maintenance, or even production, parts. What's a mil/aero engineer to do?
The more basic the COTS component, the more effective the model is; for parts like MCUs and other ICs, go custom if you can't. If that's not possible, the most obvious solution is to lay in a supply of parts for the long-term, although not all components have a long enough shelf life for this to be a viable technique. You can also work with a manufacturer who will guarantee long-term support and availability — some out there had been manufacturing the same parts for almost three decades. You may have to do some hunting and you may have to compromise somewhat on your performance demands, but if you don't spend the time there, you may well spend it hunting down a new source. Indeed, some manufacturers have made a cottage industry out of reengineering obsolete parts on requests, although that can add time for design, prototyping, fabrication, and qualification. Other manufacturers have begun releasing product lines of MIL-qualified basic components targeted at gaps left behind by discontinued parts. The other alternative is to avoid the use of COTS entirely.
Trade-offs are at the core of engineering. In some ways the COTS dilemma is new, in others it's the same old problem of weighing one option against another on a project-by-project basis. Is this issue of disappearing COTS parts one that you’ve encountered? If so, how do you deal with it? Inquiring minds want to know.
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