* In dual element heater patches the local delamination will likely cause failure to propagate  between both elements, in both side-by-side and layered conditions.  A heater is typicaliy considered failed when it delaminates. The remaining steps lead to failure of the  element and an open circuit. There is no data available to determine a critical flaw size. As a result, any  visible delamination would be considered unacceptable.  Design and Testing Recommendations  Desian Recommendations  Due to their physical proximity, elements in a dual element heater patch are highly likely to fail  simultaneously, even if they are not layered directly on top of one another. They do provide system  redundancy for other failures that occur in the power system leading up to the heater patch, but any  failure within the patch itself will fail all elements. Multiple element heater patches do NOT provide fully  redundant thermal conditioning. Dual element heaters are inappropriate for situations that require true  redundancy and fault tolerance unless survivability is addressed.  Individually, high watt density, layered design, and installation difficulty would not necessarily be a  significant driver towards failure for a flexible heater; however, the combination of multiple higher risk  design elements into an individual heater patch creates sensitivity to conditions that increase the risk of  failure. Because layered design has been common across all discovered failures to date it should be  approached with a high level of caution. Each individual issue further erodes the margin for error.  Heater systems should be integrated as early in the design process as is possible. Earlier inclusion in the  design process would allow for specifications of system components that would permit a large, flat area  for heater installation.  Heater patch effective area should be maximized and watt density should be minimized within the bounds  of required heat generation: An ideal heater system will be as large, simple, flat and low-watt density as  possible. As watt density increases, the importance of effective heat transfer from the element to the heat  sink increases. Installation conditions must be carefully examined and fully understood to mitigate  overheating.  Testina Recommendations  Acceptance and qualification testing of thermofoil heaters is specified similarly to standard electronic  components; however, this approach is not sufficient to determine all three crucial criteria: Suitability of  the design for the intended use, Capability of the design to maintain the desired temperature under  minimum power conditions, or Survivability of the design under extended use and maximum power  conditions. Survivability verification under worst-case use conditions must be part of acceptance testing.  Survivability in particular has not been part of heater acceptance and qualification testing programs. ISS  voltages historically have been running near the upper end of the acceptable range of 11 3-126 volts.  Heaters were typically tested with the minimum voltage (or ambient equivalent) to verify they would  produce sufficient heating in a worst-case environment, but never tested to verify they could survive the  highest available voltages indefinitely. A 25% increase in heater power can lead to failures not found in  qualification testing at 11 3 V.  Testing under vacuum conditions in particular will create a more realistic environment for verifying  survivability of a heater system design. Ideally, all hardware would be fully tested in a thermal vacuum  environment, but cost and schedule constraints can prohibit this level of testing. When planning the  testing regime, additional planning and analysis should be spent to ensure that the heater has seen an  equivalent duty cycle. Because vacuum conditions drastically change the thermal environment, testing of  a heater design under vacuum attached to an appropriate heat sink would be a reasonable compromise.  131