A brief history
In the 1930’s the U.S. Department of Defense developed the original specifications for a military grade Circular Connector to meet the demands of severe aeronautical and tactical applications. These were known as Type "AN" (Army-Navy) and they set the standard for the range of military circular connectors that followed.
The AN5015 connector (now known as MIL-DTL-5015) and their evolutionary derivatives are often called Military Standard, "MIL-STD", "MIL-SPEC" or "MS" connectors and are now used extensively in demanding aerospace, industrial, marine, automotive and commercial applications.
The 5015 range is a very capable connector for supporting the reliable transmission of AC and DC power supply but as the smallest contact size available is size 16 it is not suitable for higher frequency signals where a smaller size 20 contact and lower wire gauges (AWG) are needed. Therefore, to overcome this limitation MIL-C 26482 and MIL-C 83273 qualified connectors were developed in the 1950’s to introduce size 20 contacts. This supported lower power applications and also enabled robust transmission of higher frequency signals between systems. The MIL-C-26482 specification was, at the time, driven by the U.S. Navy and this was followed soon after in the 1960’s by MIL-C-83723 supporting the differing requirements of the U.S. Air Force, but both being similar connectors.
By the mid-1960’s the rapid development of computer-controlled equipment meant that a specification supporting higher density insert layouts and smaller contacts was at the top of everyone’s lists. In the 1970’s more signal capability and less power capacity in the input/output connectors of systems was a priority. To meet this need the MIL-DTL-38999 standard was developed along with MIL-STD-1560 to determine the interface and insert layouts. These developments supported contacts down to size 22 enabling higher speed data transmission along smaller gauge wire.
Smaller contacts and improved couplings
The introduction of MIL-DTL-38999 (commonly known as ‘38999’ or ‘d38999’) also brought about the ability to operate reliably in higher vibration environments due to more robust coupling mechanisms and mating threads. The 38999 series I & II have a traditional bayonet style coupling (as per 26482) and the series III has a ‘Tri-Star’ threaded coupling with an anti-vibration system in the coupling nut enabling both rapid coupling / de-coupling and high vibration resistance.
Series I & III 38999 connectors are referred to as “scoop proof”, a design feature whereby exposed contacts of the connector cannot be touched or damaged by any portion of the mating connector. Series II, due to their low profile, space and weight saving construction are ‘non-scoop proof’, but are ideal for use in low-vibration, high moisture environments and where space is an issue.
These developments allow a higher density contact arrangement of up to 128 contacts within a smaller space envelope and with the capability to meet higher vibration requirements and achieve improved EMI/RFI shielding effectiveness, fluid resistance and environmental sealing with increased durability.
Development of shell materials
Initial design developments for 38999 were focused on meeting higher vibration requirements, improving EMI shielding, and increasing durability in fluid resistance and environmental sealing. Later in the 1980’s and ‘90’s when weight and durability became a higher priority the ability to offer lighter weight material alternatives to aluminium and stainless steel became paramount.
Advances in materials made it possible to offer a lighter-weight alternative, a composite shell on MIL-DTL-38999 III. The composite series III connectors employ military-qualified thermoplastic shells. These composite shells are up to 40 percent lighter than aluminium and have four times greater corrosion resistance, with durability that extends to 1,500 mating cycles.
MIL-DTL-38999 connectors are now available with Signal, Power, Co-Ax, Tri-Ax, Quadrax and Fibre optic contacts in various shell styles, with the main body (Shell) of the connector in Aluminium, Stainless Steel and Composite material with various plating finishes, including RoHS compatible finishes. A Nickle Aluminium Bronze shell is also available to the CECC-75-201-002 specification and there now a wide verity of commercial variant also available.