MIL-DTL stands for “Military detail specification”. The MIL-DTL specifications states requirements for design such as approved materials, how a requirement is to be achieved, or how an item is to be fabricated or constructed. The specification can both contain performance requirements and detail requirements.

The connectors according to MIL-DTL 38999, also called D38999 or MIL-C-38999, is a popular and a widely used circular hermetic connector for military and aerospace applications where secure and high reliability are needed. The connectors are designed for cable-to-panel I/O applications and are always male/female connectors. The 38999 have nine different shell sizes (9, 11, 13, 15, 17, 19, 21, 23, 25) and a wide variety of contact arrangements. The connectors are available in four sub-series, with different coupling types. The primary differences between the series lie in the coupling mechanisms, materials, and intended applications.

Key differences between Series I, II, III, and IV

Series I

Coupling mechanism: Bayonet, quick disconnect.

Applications: Often used in applications where quick-disconnect coupling system is required for blind mating or difficult-to-access areas. They have a scoop-proof design to prevent damage on the contacts. Ideal for applications with risk of high vibration, high-wind and moisture environments.

Materials: Conductive and non-conductive finishes, electroless nickel, Cadmium/Olive, and anodic.


Series II

Coupling mechanism: Low-profile Bayonet, quick disconnect, captive.

Applications: Ideal in applications where a low weight and small size is desirable for example aviation, medtech and communication. The connector is non-scoop-proof.

Materials: Conductive and non-conductive finishes, electroless nickel, Cadmium/Olive, and anodic.

Series III

Coupling mechanism: Threaded coupling design, rapid advance, self-locking

Applications: Used in applications with high-vibration and high-temperature where fast and secure mating is needed. Scoop-proof design to prevent damage on the contacts. Most common in military and aerospace applications.

Materials: Conductive and non-conductive finishes; Cadmium/Olive, electroless nickel, anodic and stainless steel. Conductive composite shell; cadmium/olive drab over electroless nickel, and electroless nickel.


Series IV

Coupling mechanism: Breech lock coupling design, rapid advance, self-locking

Applications: Used in applications where robust mating and extreme resistance to vibration and shock is required, such as missile applications and aerospace. Scoop-proof design to prevent damage on the contacts.

Materials: Conductive and non-conductive finishes; Cadmium/Olive, electroless nickel, anodic, and stainless steel.

The benefits of MIL-DTL-38999 fiber connector

In applications for the military and aerospace, disturbance caused by electromagnetic fields can lead to critical situations and one way to avoid that type of disturbance is to use fiber optic. The MIL-DTL-38999 connectors with fiber optic are not impacted by electromagnetic interference and can therefore be used in applications where EMI can be a risk factor.

Originally the MIL-DTL 38999 was a physical contact and designed with copper, but because of the need for higher speed and data transmissions, and secure and reliable transmissions, the technology have evolved to also be equipped with fiber optic. The 38999 fiber connectors are today available as physical contact with copper or fiber optic, or with the fiber optic expanded beam contact.

Fiber optic have a higher transmissions capacity compared to the conventional copper and can therefore transfer more data in higher speed. It is also impossible to wire tap the fiber optic without breaking the link and connect to the wire. On copper cable and radio signals the information can be shared unwillingly.

The benefits of MIL-DTL-38999 expanded beam connector

Foto: Antonia Sehlstedt/Försvarsmakten
Foto: Antonia Sehlstedt/Försvarsmakten

The MIL-DTL-38999 expanded beam connector is well suited for aerospace and military applications where secure and reliable communications is essential.

In the aerospace application the connector must endure extremes in temperature, variations in pressure and vibrations. The connector is a no-contact connector with lenses that expand and collimate the light into the mating contact. The no-contact makes the connector less vulnerable for wear and tear from the mating and unmating, cleaning and vibrations. With higher mating cycles the Expanded beam 38999 has a long product lifecycle and are reliable.

The MIL-DTL-38999 connector with physical mating is more often used in applications where the mating is for longer periods or in stationary installations, compared to the expanded beam version which is perfect for frequent mating cycles. 

When to use what connector?

The expanded beam 38999 are often recommended for applications where mating and unmating is typical use, when the physical contacts are more used in fixed applications. Since expanded beam connector has an “air gap” in the connector one risk by using the expanded beam technology in aerospace is it that it might cause problem with condensation which can disturb the transmission.

Five short facts about MIL-38999

The military standard

All the circular connectors according to the standard MIL-DTL-38999 is designed to meet the detail specification. The standard is set by the U.S. Department of defense.

Coupling options 

The 38999 connectors have three coupling options such as: bayonet, breech, and threaded options. The coupling options is suitable for blind-mate applications.

Material options

The first MIL-DTL-38999 connectors were designed in aluminum but are today available in nickel, composite, stainless steel, and hermetic options. It’s common with a plating on the connectors and even there are a variety of choices such as: space-grade electroless nickel, black zinc nickel, nickel PTFE, olive drab cadmium, and passivated stainless steel.

Designed for harsh environments

The MIL-DTL 38999 are designed to resist vibrations, shock, changing weather conditions with extreme temperatures and have a hermetic sealing to protects the contact from moisture and debris.

How do you pronounce it?

The most common pronunciation is “38-triple-nine”.

Related subjects

What is expanded beam?

The need for accurate and reliable communication have never been more important. The fiber optic technology allows for large amount of data to be transferred with low insertions loss. Read more about the expanded beam technology.

Learn more about MIL-DTL-83526 20/21

All the Micropol expanded beam connectors are according to the military detail specification MIL-DTL-83526 20 & 21 to be reliable in harsh environments and in crucial situations. We have since the start been supplier of choice regarding passive fiber optic to the Swedish Armed Forces, but what does the specification mean and in which application are the standard needed?