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Written by Keith Harrison on 23/04/2019

How Military Power Supply Contracts Have Changed in the 21st Century

 

In the second half of the last Century, the combination of huge global technological advances and the Cold War contributed to a very different style of military contracting from that which we see today.

Those were the days of “Cost Plus” contracts where, in essence, a specification and outline control drawing would be handed to you, along with a long list of military standards and you were pretty much expected to come back eighteen months later with a fully compliant and qualified product. That’s a bit of an over-simplification admittedly but not that much!

Proposals to change the specification during the course of a programme were possible, of course, but much harder to negotiate.

 

Documentation issues

A vast amount of documentation would also be required; 100% Component Stress tables, followed by Parts-Stress MTBF, Component-Level FMEA (or even FMECA) Nuclear Hardness Analysis and so on… Design Assurance Plans, Test Plans, Qualification Plans and then the subsequent reports added to this workload. Hugely expensive, but with the cost-plus approach it didn’t matter.

 

MTBF

In case you were wondering; MTBF is Mean Time Between Failures. Generally using MIL HDBK 217 (nowadays frequently still referring to Iss F Notice 2); this gave all contractors a level playing field to produce reliability estimates for their product which the prime contractor could then combine to assess the reliability of the final product. Parts-Stress calculations need every component in the design to be assessed for its’ worst-case stress (voltage, current, power, temperature) and these figures to be entered into the calculation sheet, along with a base failure rate and various factors to do with temperature, operating environment, procurement quality and so on.

 

FMECA

FME[C]A (Failure Mode Effect [and Criticality] Analysis goes further; once you know the likelihood of each component failing, you then assess the likely failure modes – a capacitor might fail 50% of the time short-circuit, 30% open-circuit and 20% change of value for example – and you assess for each type of failure what the effect will be.

A hierarchical structure is created where, for example, our capacitor might cause loss of a power supply output (short circuit), increased output voltage ripple (open circuit) or no detectable effect (change of value). At a higher level, this might cause loss of multiple power supply outputs, increased EMI or no detectable effect. At the highest level, this might translate to mission abort, erroneous mission data or nothing. The criticality is more difficult for a simple power supply designer to evaluate; if a head-up display starts wobbling or showing incorrect data is that more critical than if it goes out and the pilot turns tail and heads for home?

SP2140 custom power supply

Military power suppliers post 1989

After the fall of the Berlin Wall in 1989 and the subsequent thawing of the Cold War things changed quite rapidly. The relationship between Tier 1 and Tier 2 companies and their suppliers became ever closer and a mutual desire to cut costs and timescales grew in prominence. Some early indications of this was a general move to Parts-Count MTBF calculations which did not require the comprehensive stress analysis of every component.

Block-Level rather than Component-Level FMEA was used and joint qualification programmes were developed with the agreement that the supplier (us) would take responsibility for getting involved in finding a solution for, say, an EMC problem when their unit (in our case a power supply) was tested as part of the complete system.

These changes helped to save a huge amount of money in their own right but better still was the fact that the open working relationships which could now be developed allowed a more technically polished final product. A “before and after” example of this follows: In the 1990s we embarked on an avionics power supply design program where one of the four outputs was specified to be 5.5V ±3%, along with remote sensing and both local and remote monitoring. Now 3% is fine, but it requires fairly precise reference voltages (expensive) and separate, more precise references for both channels of the monitoring (more expense again). It was not until the mid-2000s that we got access to the customer’s product and found that this output was feeding a 5V linear regulator which could have handled a slightly higher input with 10 or even 15% tolerance.

 

Naval power supplies

In contrast, a naval radar power supply programme started in the mid-2000s where mass was an absolute priority; We had advised that we were struggling to get our mass estimate down to the customer’s requirement and were asked to go to a review to discuss how we could achieve the requirement.

During the course of the visit we were taken into the Research and Development area where we observed that during assembly of the radar, one of the customer’s cable sets which connected to the power supply could be fitted at the same time as the power supply. We took on the cable as part of the power supply design eliminating a large D-Type mated connector and associated locking hardware. With the number of power supplies in the whole radar system, this reduced the total mass by ~1kg so we came away from that meeting with an increase in our mass target rather than a reduction – and smiles all round!

 

Celab can handle complex military and defence contracts

Celab has a wealth of history working alongside the military and navy on a variety of complex projects. We understand how military contracts work and work effectively with our clients to ensure projects are delivered on time, to spec and within strict budgets.

We also create military power supplies and converters with the future in mind. We understand the requirement to develop products that last – especially within military contracts. Contact us via our online form or by telephone on 01420 477 011 today to confidentially discuss your military power supply requirements.