Trompeter News
Space Rated Connectors
As space becomes increasingly commercialized, more and more satellites and manned space vehicles are being launched by a number of service providers, both commercial and government, to accommodate the increased demand for telecommunications, broadcast and scientific services. Space launches have now become a routine occurrence. While the technology used in putting unmanned and manned vehicles in space has advanced one hundred fold since the days of Sputnik and Gemini, it is still very expensive, and not totally fail-proof.
When designing space vehicles, care must be taken in the design and use of materials so that when a launch is successful, the vehicle will meet the mission’s expectation and exceed its intended life span. This is also true of RF connectors, especially those used for 1553 Data Bus applications. These connectors are integral to the function of space vehicles, yet are sometimes overlooked as a possible source of contamination which can hinder or cripple the space vehicle’s mission.
In the following paragraphs, I will present some of the issues that must be taken into consideration when selecting materials for use in space rated RF connectors, since they contain polymeric or organic materials. Also, since NASA has a well-established reputation for space flight, they provide much of the guidance regarding material identification and testing, regardless of whether it is a commercial or government application.
Of critical importance in manned and unmanned space vehicles is the number of optical sensors used. These sensors can be rendered inoperable from the contamination given off by materials in close proximity. This contamination is produced by the effect called "Outgassing." Outgassing occurs when a polymeric, or non-metallic material, is placed in a vacuum, subjected to heat, and some of its constituents are volatilized (that is evaporate, or go from the solid state into a gas). The contamination then collects on the sensitive optical or thermal control surfaces within an assembly, or effect sensitive surfaces in an adjacent area and renders it inoperable.
To qualify materials for use in spacecraft, NASA uses the specification (SP-R-0022A), Vacuum Stability Requirements of Polymeric Material for Space Application. This has also become the de-facto specification in the commercial space industry along with ASTM-595-90. This specification governs the "Outgassing" of non-metallic materials.
One of the bi-products of outgassing is that the material will lose a portion of its mass. This is referred to as Total Mass Loss, or TML. Under the NASA and ASTM specification, the TML of the material should not exceed 1.0% of the total specimen mass. This issue is very important because shrinkage may contribute to the failure of a connector, as the components will no longer fit properly. In some instances however, the TML may exceed 1.0% if proven that the material’s contribution to TML is due to absorbed water vapor.
Another bi-product of outgassing is Collectible Volatile Condensable Material (CVCM). This represents the quantity of outgassed matter that condenses, and collects on a surface. Under the NASA specification, the maximum condensable material allowed is 0.1% of the total specimen mass. This is very critical in a thermal/vacuum environment because it is the condensed material that will contaminate optical and sensory surfaces causing them to possibly fail.
Some polymeric materials meet the requirements of a TML less than 1.0% and CVCM of less than .10% through the normal manufacturing process. Other polymeric materials, while not within limits after their manufacture, can be brought within vacuum stability limits by vacuum baking for a specific period of time, which is outlined in both NASA test procedures as well as ASTM-595-90. If the material cannot be brought into compliance, SP-R-0022A states:
If a material cannot be vacuum baked and its exposed area is 13cm2 or less, and is out of sight of payload surfaces and other contamination critical surfaces, then total mass loss may be up 3.0%, and volatile condensable material up to 1.0%.
Another issue of importance, especially in manned space vehicles is "Offgassing." Offgassing is defined as, "The evolution of gaseous products from a liquid or solid material into the atmosphere." This is a concern because airborne contaminant gases can be introduced into a habitable spacecraft’s air and pose a hazard the crew. There are two types of offgassed products that are of significance in a habitable environment. The first is Odor and the second is Toxicity. In both cases, there can be a serious health risk to the person who is exposed to the gas. Each has its own criteria for meeting offgassing and this testing criterion is governed by NASA-STD-6001.
An Odiferous gas is the condition where an organic material emits an objectionable or revolting odor. In rating an odiferous gas, you must consider not only the smell, but also whether exposure to the gas causes nasal irritation or any other medical problems.
Materials that are hermetically sealed in containers do not have to meet the criteria because the odiferous gas is not exposed to breathing gases.
Toxicity is the condition where harmful or toxic gases encounter breathable gases. When making allowances for toxic gases, there are two different criteria: Long and short term SMAC (Spacecraft Maximum Allowable Concentrations). JSC 20584 states that SMACs are guidance concentrations that are designed to protect personnel from the adverse health effects of being exposed to toxic gases. Long-term exposure is considered to be 7 to 180 days. Short-term exposure is considered 1 to 24 hours and is designed to protect personnel from, "All adverse effects except mild irritation, mild headache, or other effects during accidental exposure." No level of exposure for any SMAC is acceptable if it is great enough to degrade mission effectiveness. A 0.01% risk of cancer is considered the maximum acceptable risk from exposure to airborne toxic gases.
The measurement of toxic gases in an atmosphere is the Toxic Hazard Index (T). The "T" value is determined by, "Calculating the ratio of the projected concentration of each offgassed product to its SMAC value and summing the ratios for all offgassed products without separation into toxicological categories." The toxicological equation for the combined airborne pollutants is:
T = C1/SMAC1 + C2/SMAC2 + …….. Cn/SMACn
If the combined "T" is less than 1 for all pollutants or for each group of pollutants, then the atmosphere is considered safe. While NASA categorizes their toxicological groupings according to the affected organ and nature of the adverse effect, it is understood that most toxins have a wide range of effects that cross the groupings.
Another area of concern is the use of polymeric materials in environments that support combustion. These are environments where the materials are exposed to Liquid Oxygen (LOX), Gaseous Oxygen (GOX), breathing gases or any other reactive fluid. There are three habitable environments of concern regarding combustible materials: Habitable flight compartments, areas other than habitable flight compartments, and vented and sealed containers. Flammability testing is outlined in both NASA-STD-6001, as well as ASTM-D635.
The issue of flammability is whether the material, once ignited, will continue to burn and spread the fire to an area beyond the initial combustion point. Containment can be easily obtained by selecting only materials that have burn characteristics that preclude this phenomena. Should it be necessary to use materials that have failed that criterion, then you must use other means to control the spread of fire.
When designing into an application a material that fails flammability requirements, you must take into account the risk that the fire will propagate. This risk is referred as a "Single Barrier Failure." A single barrier failure is defined as:
"Leaks through a barrier within a component that permit the fluid to contact the materials directly behind the barrier. Single barriers include mechanical joints (e.g., B-nuts); o-rings, gaskets, and bladders; and metallic and nonmettalic diaphragms. Structural parts, such as pressure lines and tanks, welded or brazed joints, and redundant seals in series that have been pressure-tested individually prior use are not considered to be single barriers.
A firebreak, which is defined as encasing the non-compliant material in compliant material, removing potential ignition sources, or reducing to the minimum the amount of non-compliant material are all ways to reduce this hazard. Additionally, if a material is located in a hermetically sealed container with a maximum leak rate of less than 1 x 10-4 cc/sec and contains an inert gas, then the material does not have to meet the NASA testing requirements for flammability.
NASA does allow for materials that do not meet the requirements for outgassing, offgassing or flammability. MSFC-PROC-1301 (Guidelines for the Implementation of Required Materials Control Procedures) details how to justify and document non-qualified materials using a Material Usage Document. Commercial space projects no doubt use similar selection processes when confronted with the same problems. Lists of NASA tested materials and their test results can be found in MSFC-HDBK-527/JSC 09604 (Materials Selection List for Space Hardware Systems). While the handbook lists specific manufacturers and material types, it is an excellent reference guide for materials in general.
In conclusion, when designing space rated RF connectors, care should be taken in the selection of polymeric or organic materials which are contained in RF connectors. As discussed, outgassing, odor, toxicity and flammability, are all considerations that must be addressed. Material selection can have a direct effect on the life span of space hardware, as well as the safety of personnel when used in habitable space vehicles. While RF connectors may be a minute portion of the cost of a satellite or manned space vehicle, they have the potential to influence a high cost failure.
