Space Connectors Get Tough for the Harshest Environment
Heat, shock, vibration, and radiation are all in a day’s work for the special components that explore worlds beyond our own.
by Ann R. Thryft
The most mission-critical of military/aerospace systems are the ones that go into space, where they encounter the toughest environments. Mil/aero systems must be tough enough on Earth to withstand the demands of harsh environments, such as shock and exposure to temperature cycling, and foreign substances like dust, water, and chemicals.
But when it leaves the Earth’s surface, a spacecraft is bombarded by radiation levels that increase the farther out it flies. It must also survive even greater temperature extremes, potential collisions with all kinds of space junk, and the likelihood that Earthlings won’t be able to repair it. All that makes durability and reliability critical features.
On top of those requirements, newer small satellite designs, such as the compact CubeSats, are demanding miniaturized, high-density parts, and that includes connectors. The two Mars Cube One satellites launched in spring of 2018 were the first CubeSats to fly beyond Earth’s orbit, traveling behind NASA’s InSight Mars lander on its journey to the Red Planet. They will test new miniaturized communications and navigation technologies, which will relay data about InSight’s entry, descent, and landing back to Earth, as well as demonstrate how CubeSats can be used in deep space.
The Cube Ones use non-custom, commercial off-the-shelf (COTS) components.
As its name implies, the InSight — Interior Exploration using Seismic Investigations, Geodesy and Heat Transport — lander will study the interior of Mars, its crust, mantle, and core. It was sent on its way to Mars by an Atlas V-401 launch vehicle, a huge rocket standing about the height of a 19-story building at 188 feet (57.3 meters) tall.
The rise of CubeSats is one indication of the rapid changes in space technology and the space business. The industry is no longer dominated by government-controlled space agencies like NASA and the European Space Agency (ESA), often called Old Space due to their caution, extended testing requirements, high level of specifications, and more research-based cultures, said Bob Stanton, director of Omnetics Corporation. In contrast, recent New Space groups like Virgin Orbit and SpaceX are competitive, market-based companies, not government-controlled. “They move faster, depend more on proven examples of reliability, are looking for ROI from the space business, and thrive on challenge and competition,” he said.
Mission assignments have also changed. Many new space applications are aimed at expanding earth observation technologies, and in massive data collection, transmission, and cloud storage, as well as responding to instant demand during emergencies, said Stanton.
The general connector needs of aerospace electronics closer to Earth, such as launchers, are somewhat similar to those of onboard electronics in deep space satellites, said Aidan Doran, Carlisle Interconnect Technologies’ product manager for space. “The main difference is in materials for use in deep space. Due to our high level of quality controls, we make the connector and cable the same regardless of the application. What changes is the testing a specific market requires.”
Two of the most important basic requirements for electronics and connectors are the ability to operate successfully in a vacuum, and to withstand radiation effects. In a vacuum, outgassing from some plastics can deposit material on electronics, degrading their performance over time, said David Koenig, vice president of AirBorn’s space business unit. NASA’s outgassing test standards for materials for space, aerospace, and other low-outgassing applications, ASTM-E595, focuses on total mass loss (TML) and collected volatile condensable materials (CVCM). To pass NASA outgassing standards, materials can’t have a TML of more than 1.0% and a CVCM of more than 0.1%.
The effects of radiation on spacecraft and their components is also related to plastics. “AirBorn’s connectors use a polyphenylene sulfide [PPS] as an insulator material, which resists those effects and also has very low outgassing levels,” said Koenig.
To fight radiation, both cable and connectors on spacecraft often require stainless steel shells and polyether etherkeytone (PEEK) insulators instead of liquid crystal polymer (LCP), said Stanton. This is called radiation hardening, or rad hard. But connectors don’t always have to be rad-hard if they’re inside the rad-hard package.
Certain metals such as pure tin, cadmium, and zinc are prohibited in space due to their mechanical reactions. Any non-metal materials, such as plastics, are at risk for damage from radiation exposure. “Of course, dielectric materials in the connector or cable must be rated for radiation resistance, as well as outgassing,” said Doran.
Other considerations include extreme operating temperatures of -55°C to +125°C, and the shock and vibration of launch. Since the connector’s basis is its pin and socket, that contact system needs to withstand shock and vibration, plus maintain normal forces to keep constant contact between pin and spring so there’s no loss of the electrical signal, said Koenig.
For connectors, reliability can include multiple points of contact for redundancy, said Jason Smith, AirBorn’s senior director of technology development. Reliability can also include durability, such as a high mating cycle count, even though connectors won’t be mated and re-mated in space. For both reliability and durability, AirBorn maintains a minimum of 50μin of gold on contact engagement areas of its space connectors, per MIL-DTL-55302, MIL-DTL-83513, and MIL-DTL-32139, said Koenig.
Each orbit and deep space mission presents its own challenges, said Doran. For low Earth orbit applications, the effects of atomic oxygen exposure in Earth’s atmosphere must be considered on top of radiation, while an external flying connector or cable in geosynchronous Earth orbit must consider high radiation along with extreme thermal changes.
Although miniaturization and multi-connection solutions are driving the industry, there’s always a compromise between miniaturization and a connector’s ruggedness, said Doran. The ideal for an interconnecting RF coaxial cable assembly would be the smallest connector possible on the largest, lowest-loss cable. “But that’s not practical because of the miniature interface’s potential mechanical failure due to the forces from a too-large cable, and the connector’s reduced centerline-to-centerline positioning lost due to cable diameter,” he said.
Omnetics sees two big changes in connectors for space. First is a general increased emphasis on mixed-signal, hybrid connectors, such as power and signal or RF and signal, for more compact size. Second, there’s growing interest in nano-coax cables for transmitting microwave or millimeter-wave communications and Earth-based control and monitoring. “They’re not as robust as big coax cables, in either frequency range or power, but they’re small,” said Stanton.
Communications connectors and cable for space are typically controlled by industry standards: MIL-PRF-39012 or ESCC 3402 for a coaxial connector and MIL-DTL-17 and ESCC 3902 for coaxial cable.
“Due to the long mission life, satellite integrators like to mitigate risk and reduce connector choices to a select few: sub-miniature version A [SMA], sub-miniature version K [SMK], and threaded Neill-Concelman [TNC] — with a few others chosen for specific equipment,” said Doran.
AirBorn is developing a customized cable assembly — a space-rated, space active optic cable (SAOC) — for a specific customer. It replaces the typical heavy and bulky copper cable coming from a board-mount connector and, in the cable’s backshell, converts electrical signals from that board-mount connector to light signals over optical fiber. This saves space and weight, and also extends signal reach while minimizing signal loss and reducing EMI, said Koenig. This will be the first SAOC cable designed and rated for space, and will be available later this year.
Ann R. Thryft has been writing about manufacturing- and electronics-related technologies for 30 years, covering interconnect, single-board computers, robotics, machine vision, embedded devices, manufacturing materials, and processes, and all kinds of datacom and telecom. She’s written for EE Times, Design News, COTS Journal, RTC Magazine, EDN, Test & Measurement World, and Nikkei Electronics Asia.
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