While the night skies and color photos from NASA may present great beauty, the near-earth space environment where satellites orbit presents difficulties for man and equipment. Unique product designs, processing, and testing are required.
“Space” represents a thriving industry with almost unlimited possibilities. The non-profit Space Foundation estimates that the space economy was $330 billion worldwide in 2014, 76% of which involved commercial activities. The industry had a compound annual growth rate (CAGR) of 7% from 2005 to 2014, almost doubling in size over the decade. The Goddard Space Flight Center (GSFC) lists 2,271 active satellites currently in orbit (Russia has 1,324 and the US has 658), plus there are more than 200,000 pieces of manmade items in orbit, from “dead” satellites and launch vehicles the size of a school bus to metal pieces a few millimeters in diameter.
While the night skies and color photos from NASA may present great beauty, the near-earth space environment (up to 20K nautical miles) where satellites orbit presents difficulties for man and equipment. Many of the considerations for getting into space, such as high shock and vibration, are replaced by other complications once in orbit or when continuing into deep space, and require uniquely qualified standard- and application-specific connectors, cables, and other interconnect components.
System Problems Cause Failures
Success is often measured in terms of “reliability” in meeting mission objectives. A study by the US Air Force in 2003 found that “45% of all satellites experienced one or more mission-critical failures” with root-cause analysis determining that this was due to “loss of systems engineering (SE) discipline in program execution.” SE includes applicable specifications and standards for design, materials selection, and testing. The loss of millions of dollars of “space assets” was an unintended result of procurement simplification started by Secretary of Defense William Perry, which eliminated many mil specs from Department of Defense (DoD) procurement.
A USAF follow-up report dated June 15, 2010, advised that “elimination of fundamental engineering practices (is) likely to occur with the pressures of competition in today’s acquisition (processes)” unless standards are properly defined, since suppliers are not going to include costs for unspecified work in their competitive bidding. This has begun to be corrected, starting with the reinstatement in 2008 of MIL-STD-1547B covering “Electronic Parts, Materials, and Processes for Space Vehicles.” NASA followed with its EEE-INST-002 “Instructions for EEE Parts Selection, Screening, Qualification” (latest revision dated May 2008). These requirements provide the basis for increased component reliability and mission success.
Materials Create and Solve Problems
Instrumentation sensitivity requires use of non-magnetic connectors that are tested to ensure that residual magnetism is ≤200 gamma (reference specifications NASA-S-311-* and GMSF 40M*, plus ESA-SCC-3401-* for D-Subminiature, D38999, and other connectors). While low PIM requirements are becoming increasingly important for terrestrial communications equipment, historically they have been fundamental for space-qualified connectors. Problems caused by material outgassing (per ASTM E595 and ECSS-Q-ST-70-02C) are well known, as are the temperature extremes that may affect components in space. While individual connectors, cables, and other components are uniquely procured to meet these parameters, assembled satellites also must undergo similar evaluations to ensure performance via testing as shown below.
Problems caused by tin whisker (TW) growths have been documented since the 1940s, but they can be exceptionally ruinous in the zero-gravity conditions of space where they may break off and float free, only to be attracted to parts with different electrical potentials. Tin whiskers are electrically conductive, crystalline structures of tin that sometimes grow from surfaces where tin (especially electroplated tin) is used as a final finish, such as tin-plated connector pins.
The Space Shuttle program was almost shut down in March 2008 when NASA confirmed that multiple Shuttle control boxes exhibited TW growths that could cause electrical shorts and system failures. Extensive inspections, testing, and maintenance were required before equipment could fly. NASA and ESA specifications now prohibit use of pure tin (and also zinc) for connector pins, PCB surfaces, etc. A common solution is to alloy with a minimum (typically) of 2 – 4% lead. These platings further separate components intended for space from potentially lower-cost COTS and commercial components with lead restrictions required to meet RoHS (Restriction of Hazardous Substances Directive 2002/95/EC) standards. The continued pressure to reduce weight and size has resulted in many connectors, backshells, and related parts made with composite materials. Housings and insulators may be molded from LCP, PEEK, or other materials (standard fluorosilicone elastomers and bonding agents, even if unique, normally require vacuum baking). Grounding and EMI shielding are provided by conductive fillers or metallized plating. Composites have different QPLs than the standard sources and lead times are usually longer.
Most space connectors with removable crimp contacts undergo 100% X-ray to confirm positioning of contact retention clips. Production and qualification testing usually take place on a single-lot basis involving raw materials’ traceability and destructive physical analysis (DPA) in accordance with specifications MIL-STD-1580B (March 14, 2014), NASA-SSQ-25000 (for Space Station components), MIL-STD-883K (April 25, 2016) for inspection of microcircuits including the RF connectors installed in MMIC packages, plus the ESA’s ECSS-Q-ST-60C Rev. 2 (February 7, 2013). All these special processes, tests, single-lot controls, and extensive reporting requirements help explain why “paperwork” costs usually far outweigh hardware expenses.
Changes in Microwave Connectors
Requirements for greater bandwidth and data rates are resulting in use of higher frequencies and applicable connectors. Newer satellites, such as the Advanced Extremely High Frequency (AEHF) Satellite for USAF secure communications, have 44GHz uplinks. Wideband Global SATCOM Satellites, currently being launched to provide the backbone for tactical combat control, can work simultaneously at 20GHz and 40GHz. Until recently, connectors with air-dielectric insulation (such as 2.92mm), were not used due to concerns about multipaction (related to ionization breakdown in vacuum conditions). However, recent studies show that problems are limited to high power at relatively low frequency, such as 1GHz, and are not a factor at higher frequencies, which opens up applications for microwave-frequency air-dielectric connectors.
Further growth of 2.92mm connectors also results when satellite producers change from intermateable SMA (with PTFE insulators) to 2.92mm to take advantage of the better electrical loss offered by 2.92mm connectors at lower frequencies where SMA were previously installed. The 2.92mm connectors are used aboard the ISS for the Ku-band HRCS (high-rate communication system). Satellite-to-satellite communications at 77GHz and new E-Band downlink requirements extending to 95GHz, plus the continued need to reduce weight and increase performance, should result in future use of 1.85mm and 1mm connectors for these applications.
Sometimes new connectors are created for space needs. For example, the ESA ’s Advanced Research In Telecommunications Systems (ARTES) Program recently funded a four-company consortium to develop new small-sized, higher-power coaxial connectors. The new connectors have envelope dimensions similar to SMA, but are superior in many ways. Called power sub-miniature (PSM), they can withstand more than 1500 watts in the P- and L-Band for a pulsed 2% duty-cycle signal. This is a 50% gain over other “power” connectors such as TNC, and mass is reduced by more than 60% versus TNC. PSM is optimized for RF-breakdown, corona, multipaction, and thermal power dissipation.Factors driving the satellite industry include the continued trend for miniaturization involving weight and size and the demand for high-speed connectivity networks. Projects are long-duration due to multiple-year lead times for planning through design, actual procurement, and extended testing. As a “business,” space has higher growth than most industries. Once approved, connectors and other components usually enjoy long-term business success due to restraints for change that also are entry barriers for start-ups. Users are adverse to risk and inherently resist using different connectors or suppliers unless benefits are significant. The globally expanding demand for cables and connectors for space flight and ground-support will continue to generate new opportunities for years to come.
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