Mitigating the Impact of Outgassing

Author : Scott Miller, Cinch

05 March 2024

Figure 1: Schematic explaining outgassing
Figure 1: Schematic explaining outgassing

There are a well-known set of factors to balance when choosing connectors - most of them involving electrical characteristics and mechanical capabilities.

When the end system being designed must operate in extreme environments, then other considerations come into play though. Components used in low-pressure applications, for example, can be subject to outgassing. It is important to be aware of this phenomenon and have familiarity with testing standards designed to control it. 

There are many low-pressure commercial and industrial applications where outgassing can be witnessed, but the phenomenon is most commonly an issue for systems operating at high-altitudes or in the vacuum of space. Here, products are subject to elevated levels of radiation and perpetual mechanical stress, as materials expand and contract with the constant swinging between temperature extremes.

Defining outgassing
Outgassing is very simply the release of gases trapped inside a material. It occurs when there is a large difference in pressure between the internal elements of a material and the environment that it is operating in. 

Where do the gases come from? Well, it is not uncommon for gases to get trapped inside materials as they are being manufactured. While this is especially true for molten plastics, some metals can also trap gases. Microscopic bubbles may become trapped while material is being poured, injected, or otherwise formed, of course - but it is also entirely possible for bubbles to form internally. Gas bubbles can arise from chemical reactions, such as the solidification of thermoplastics, or during the impregnation of dopants into metals. 

Regardless of the material or the formation process, manufacturing occurs at ground level. The pressure inside these bubbles will consequently be one atmosphere. At ground level, the pressure outside the material is equal to that of the outward pressure within the gas bubbles. In other words, there is no net force on the gas bubbles. However, the moment that material is introduced to a low-pressure environment, such as in space, the internal pressure within the gas bubble grows larger than the inward pressure. With a sufficient differential, gas bubbles will find their way to the surface of the material and escape - i.e. they are outgassed. 

The problems associated with outgassing
The amount of gas likely to be released is almost immeasurably small, and yet outgassing has the potential to ruin multi-billion dollar space missions in any number of ways. 

Figure 2: Before and after outgassing formed a thin film on Cassini’s CCD image sensor array
Figure 2: Before and after outgassing formed a thin film on Cassini’s CCD image sensor array

One of these is creepage. As gas bubbles force their way out of the material of a connector, small cracks and fissures can form, naturally reducing the mechanical strength of the connector. Any component part is prone to experience temperature variations, but the temperature range is especially wide in space, where satellites and other systems get exposed to endless cycles of intense sunlight followed by shadow. Such stresses can cause any fissure to increase in size. The degradation of connectors can thus occur relatively quickly. 

Another major challenge is that even trace amounts of certain gases can damage some sensors. Once in a vacuum, the gas diffusing from connectors can coat nearby surfaces - this is literally how physical vapour deposition (PVD) works. Creating a thin film on a CCD imaging device or a gas sensor is likely to diminish its effectiveness and could even be catastrophic.

The risks might seem minimal, but the question is: how much risk should simple connectors be allowed to pose to an expensive mission-critical instrument? It’s not an idle query. During a mission to send a probe to Saturn’s moon Titan, the NASA Cassini spacecraft experienced outgassing of some on-board parts, affecting a navigational camera as a result. The coating induced significant flaring. Something similar impeded the Stardust space probe that was responsible for collecting samples from the passing Wild 2 comet.  

To lower, if not eliminate, such problems in subsequent missions, NASA introduced a testing standard. Referred to as SP-R-002A, it specifies materials and describes precisely how to test them. 

Mitigating outgassing
The prevention of outgassing is certainly a challenge, but there are methods and procedures that can be adopted to allay the problem. An obvious and relatively simple technique for minimising bubble formation in molten plastic materials is to create parts in a mild vacuum. The single most effective practice to prevent outgassing, however, is to pre-bake parts. Manufacturers can raise the temperature of a part, which increases the internal pressure of microscopic gas bubbles, thus encouraging them to diffuse from the part’s materials. Furthermore, applying heat can actually help reduce the formation of fissures from expanding gas, especially in plastics. 

Parts can also be placed in a vacuum chamber to encourage outgassing prior to their actual installation. This is recommended especially for applications that incorporate sensors known to be vulnerable to outgassing. Various institutions, including NASA and ESA, have compiled long lists of materials suitable for use in space, thus making the selection process more straightforward. 

Figure 3: An example of a space-qualified micro-D connector from Cinch
Figure 3: An example of a space-qualified micro-D connector from Cinch

Of course, it is easy to source components from manufacturers who routinely manufacture, test and verify their parts for low outgassing properties. A number of testing standards are applicable. Among the most prominent are NASA SSP 30426, NASA SP-R-0022A, ASTM E595-07 and IPC-1601.

Ready for space 
For space applications, NASA advises choosing parts that have already been tried and tested. The designated products are known to be reliable, although the process of getting on NASA’s list is arduous, which explains why all hardware sent into space by the agency is often a decade or so behind. 

With over 6 decades of experience, Cinch’s range of space-ready connectors have seen widespread usage in satellites and spacecraft. These components are routinely selected for major NASA projects. Among the most high-profile are the Apollo missions that took humans to the Moon, the Voyager probes that continue to explore the edge of the solar system (even after decades of continuous operation), the sentinel satellites which actively provide high-resolution images of the Earth’s surface and the Beagle 2 Mars lander. The Cinch space-ready RF, fibre, power and signal connectors are all compliant with ASTM E595 testing standards, making them suitable for applications where outgassing is a critical concern.

Conclusion
Outgassing can be extremely problematic, especially when dealing with multi-billion dollar projects that require high degrees of accuracy from their constituent sensor systems. The rigour and expense of recommended testing procedures can be daunting, making it far more economical and efficient for engineers to rely on parts that have already been tried and tested, manufactured by experts with extensive track records in this sector. Just as Cinch was with Neil Armstrong as he first stepped on the Moon, and has been on board probes exploring the vastness of our solar system, the company will be involved in future launches into the depths of space, helping to unravel the mysteries of the universe. 


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