Space-grade connectors are generally spin-offs of military-grade designs. Compared with common commercial connectors, there’s a much smaller variety of space-grade connectors. However, commercial off-the-shelf (COTS) and COTS+ connectors are increasingly used in cost-sensitive missions such as CubeSats.

This FAQ begins with a look at the unique operating conditions in space and how those conditions impact connector requirements; it presents some techniques used to increase the reliability and efficiency of connections, briefly reviews the common types of space-grade connectors, and closes with an overview of EIA-364, electrical connector/socket test procedures including environmental classifications, for space-grade connectors.

Almost a vacuum
The pressure in outer space is so low that it’s often called a vacuum — the actual pressure is 1.322 × 10⁻¹¹ Pa; super low, but not a perfect vacuum. The lack of air brings a couple of problems. It affects a connector’s maximum voltage rating. Flashover between adjacent conductors can occur if the flashover voltage rating is exceeded. Connector flashover voltage needs to be specified for a vacuum or high altitude to be suitable for use in space.

A bigger concern is outgassing, sometimes called off-gassing. It’s the release of gas dissolved, trapped, or absorbed in a material. Outgassing introduces contaminants into the surrounding area and can affect the operation of other devices such as sensors, optics, or other sensitive instruments. It’s not specific to connectors, but connectors can be a prime source.

This is an area where space-rated connectors face stiffer requirements compared with military-grade components. MIL-DTL-38999 connectors can contain nonmetallic materials such as rubber, plastic, adhesives, and potting compounds that can produce outgassing under conditions of a vacuum or elevated temperatures (also found in space). ASTM E595 is used to evaluate outgassing properties of components intended for use in space; it’s not required for most military-grade components. Space-rated hermetic connectors are designed to prevent outgassing. Because of the importance of minimizing any possible outgassing, hermetic connectors for spacecraft are available in various configurations (Figure 1).

Radiation – thermal and cosmic
The effect of thermal radiation increases in direct sunlight with no atmosphere. In direct sunlight, the temperature of a satellite can climb to 120 °C, or higher, and on the side away from the sun, the temperature can drop to -100 °C. Depending on the size and design of a satellite, thermal cycling can present a significant challenge. Space-rated connectors have an extended operating temperature specification. They are usually tested for thermal soaking, also referred to as high-temperature, long-term, or temperature life testing, as well as thermal cycling.

Cosmic radiation is a concern at high altitudes and in space. It may not directly impact connector performance, but it can cause problems in system operation. The solution is adding shielding on cables, connectors, and circuit boards to mitigate problems from cosmic radiation.

Getting into orbit is stressful
To be useful in orbit, connectors need to survive the rigors of launch. Launching into orbit is a high acceleration, high vibration, and generally stressful event. Connectors and connections must survive so the satellite systems can function as planned. Sometimes, locking or latching connectors are specified. That can be a double-edged sword. On the one hand, it can help ensure connector survival during launch. But, on the other hand, every gram of weight is expensive to move from the surface into orbit, and adding locking or latching mechanisms can add to weight and cost.

As a result, it’s crucial to specify fully, but not over-specify, the mechanical ruggedness needed in connectors. And it’s different for crewed and non-crewed missions. The cost of failure in a CubeSat is high but relatively low compared to the failure of a life support system in a crewed space vehicle.

Failure is an option
Today, the cost to launch a small CubeSat can be far greater than the cost of the CubeSat itself. These small satellites aren’t exactly disposable, but they are built to be cost-effective and use COTS or COTS+ components, including connectors, wherever possible. Earlier satellite design processes were not cost-centric but focused on maximizing satellite reliability and reducing the possibility of failures. Value-centric design methodologies are increasingly being used for small low earth orbiting satellites. It can be more cost-effective to have a constellation of small redundant satellites to reduce the impact of a single satellite failure rather than maximizing the reliability of individual satellites regardless of the cost.

Higher value and longer-range missions, such as sending a rover to Mars, can justify a higher-cost connector solution. It’s ironic, but that can be truer for non-crewed missions. In the case of some crewed missions, such as the International Space Station (ISS), repair can be possible. In these cases, the criticality of each system relative to life support needs to be considered, and connector and other component choices made accordingly.

For example, the Bartolomeo module was recently added to the ISS and experienced connector problems (Figure 2).  Bartolomeo is the first commercial module attached to ISS. It’s operated by Airbus and can house small science experiments designed for microgravity. During a spacewalk, the crew attempted to mate the four Bartolomeo cables to the main Columbus module but could not lock the connectors fully. According to NASA: The crew then mated, and wire-tied two of the four cables in place and placed protective caps on the other cables. The connection checkout was successful with the tie-down configuration. Due to time constraints, the crew had to make a second spacewalk to mate the remaining cables.

Connector contacts and efficiency
Connector efficiency is important in all applications, but it’s even more important in systems subject to high levels of shock, vibration, and mechanical stress, which may impair the reliability of interconnect systems. Contacts are the critical element in determining connector efficiency. Low and stable contact resistance is important and is challenging to maintain in high-vibration environments. Several factors are considered in the design of space-rated connectors to ensure highly stable connections.

Use high-conductivity alloys. Contacts should be fabricated using materials that offer conductivity levels near 100% as defined by the International Annealed Copper Standard (IACS) (Table 1). Use of some materials such as brass or beryllium copper is not recommended since they have lower conductivities. And, some types of metals are prohibited for spacecraft. According to NASA EEEINST-002 instructions for parts selection, screening, qualification, and derating, cadmium, zinc, chemically coated cadmium, zinc, or silver cannot be used as a connector or contact finish. NASA also recommends passivated stainless steel, electroless nickel, or gold finish on connector shells and contacts.

Maximize the electrical contact surface area. Connectors with larger electrical contact areas provide more consistent electrical performance and are more resistant to shock and vibration.

Maximize the force between the contacts. Springs or spring contact structures can help maintain firm and consistent connections between the contacts, maximizing the effective surface area and producing a higher current density.

What connectors are available?
With all the above considerations and challenges, the types of connectors qualified for use in space are generally limited to the following (more types of COTS and COTS+ connectors are available):

• Rack and panel, rectangular (Type A)
• Circular, power (Type C)
• D-Subminiature Rectangular (Type D)
• Hermetic (Type H)
• Microminiature Rectangular (Type M)
• Printed circuit, Rectangular (Type P)
• Coaxial (Type RF)

Space connectors are held to higher standards
A range of military, NASA, and other standards govern space-rated connectors. One of the most general and important is EIA-364 Electrical Connector/Socket Test Procedures, Including Environmental Classifications from the Electronic Components Industry Association that defines specific testing requirements for space-grade connectors (Table 2). EIA-364 is a broad and comprehensive standard. It covers a range of environmental conditions starting with controlled indoor environments and progressing to more severe environments, including outer space.

Table 2: These are the basic tests required for all connector types. Some connector types or application classes require additional testing not delineated above. (Table: Tempo)

Summary
Connectors for space are usually derivations from military connectors. But that’s not always the case, COTS and COTS+ connectors are also used in a growing number of orbital platforms. Performance expectations for space-rated connectors are often high but not so high to prevent all possible sources of failure. In some crewed missions and certain non-life support systems, connectors can be repaired or replaced, and value-centric design methodologies can be used. The range of actual space-rated connectors is limited compared with commercial or military-rated connectors, and to be space-rated, connectors have to be tested to EIA-364 and other standards.

References

Efficiency in Power Connectors for Space Applications, Air Electro
EIA-364 Revision G, July 2021
ESA books two payload missions on the Airbus Bartolomeo platform, European Space Agency
Factors Affecting Interconnects in Space, TE Connectivity
How to Choose Connectors for Space, Harwin
ISS Daily Summary Report – 1/27/2021, NASA
What are the Requirements for Space Grade Connectors?, Temp
What Kind of Hermetic Connectors Are Used in Space?, Hermetic Seal