Frequently Asked Questions

What are circular connectors?

Circular connectors are cylindrical interconnect devices that join electrical, RF, or fiber-optic circuits between systems. Their round shell geometry provides uniform mechanical strength, simplified sealing against dust and moisture, and compact packaging for high contact density.

Available with power, signal, coaxial, and fiber contacts, circular connectors are designed for fast, reliable mating and unmating using bayonet, threaded, or push-pull couplings. This makes them the preferred choice for aerospace, defense, industrial, transportation, and medical applications where ruggedness and versatility are essential.

What is contact resistance, and why does it matter?

Contact resistance is the small electrical resistance that exists at the interface between two mated contacts. It depends on the contact material, plating, surface finish, and spring force that maintain metal-to-metal contact. Even a few milliohms of added resistance can cause power loss, local heating, or voltage drop in high-current or precision-signal circuits.

Qualified MIL-DTL circular connectors are designed to maintain low and stable contact resistance—typically below 6 mΩ for size 22 contacts—even after hundreds of mating cycles and environmental stress testing.

What is mating cycle durability?

Mating cycle durability defines how many times a connector can be mated and unmated before its electrical or mechanical performance falls outside specification. For MIL-DTL-38999 QPL connectors, the standard requirement is a minimum of 500 mating cycles with no significant change in contact resistance or coupling torque.

Some commercial, miniaturized 38999-style families (such as Cinch MD801 or Amphenol 2M) are designed for up to 1,500–2,000 cycles, providing extended life for applications that require frequent maintenance or reconfiguration.

Why is EMI shielding important?

Electromagnetic interference (EMI) can disrupt power, data, or control signals by inducing unwanted voltages in conductors. Circular connectors prevent this through continuous conductive paths that act as a Faraday cage around the signal lines.

Effective EMI control depends on:

• Conductive shells and platings (e.g., cadmium or zinc-nickel).
• 360° cable-braid termination within the backshell to maintain ground continuity.
• Tight metal-to-metal coupling and conductive gaskets that close potential leakage paths.

Together, these elements maintain signal integrity even in high-density avionics or military systems exposed to severe electrical noise.

What is scoop-proof design, and why is it used?

Scoop-proof connectors feature extended front shells that recess the male contacts, preventing the pins from being damaged if the connector is inserted at an angle or during blind mating. The shell takes any initial impact, not the contacts themselves.

This design minimizes the risk of bent pins, electrical shorts, and field maintenance failures—a key advantage in aerospace and military systems where connectors are often mated in confined or low-visibility locations. MIL-DTL-38999 Series I (bayonet) and Series III (threaded) connectors are scoop-proof; Series II is not due to its lower-profile shell.

What’s the difference between QPL and COTS connectors?

• QPL (Qualified Products List) connectors are tested and certified by the U.S. Department of Defense to meet all requirements of a MIL-DTL specification (such as MIL-DTL-38999). Manufacturers must pass rigorous qualification, periodic audits, and lot testing to remain listed. QPL parts provide traceable, verified compliance for defense and aerospace programs.
• COTS (Commercial Off-the-Shelf) connectors are commercially designed and may be ruggedized to meet similar performance goals but are not formally qualified under a MIL-DTL spec. COTS designs allow faster deployment, lower cost, and greater flexibility for emerging or commercial markets, including UAVs, eVTOLs, and small satellites.

Why are MIL-Spec connectors preferred in harsh environments?

MIL-Spec circular connectors are engineered to maintain reliable electrical and mechanical performance under extreme conditions. Qualified to standards such as MIL-DTL-38999, MIL-DTL-26482, and MIL-DTL-5015, they undergo rigorous testing for temperature, vibration, shock, humidity, and corrosion.

Typical capabilities include:

• Operating temperature range of −65 °C to +200 °C
• Sealing equivalent to IP67 when mated
• Corrosion resistance up to 500–1,000 hours salt-spray
• EMI shielding through conductive shells and backshells
• Mechanical robustness verified through shock and vibration tests

These attributes make MIL-Spec connectors the standard choice for aerospace, defense, and other mission-critical environments where reliability cannot be compromised.

Are commercial connectors used in defense or space?

Yes. Many Commercial Off-The-Shelf (COTS) connectors are ruggedized to meet demanding electrical and mechanical requirements for defense and aerospace applications. They’re widely used in UAVs, CubeSats, portable equipment, and ground systems where cost, lead time, and SWaP-C performance outweigh the need for formal MIL-DTL qualification.

For spaceflight hardware, COTS connectors can be selected if they undergo additional screening—such as vibration, outgassing, and thermal-vacuum tests—to verify suitability for vacuum and launch environments.

How does Cinch support both needs?

Cinch supports both qualified military programs and next-generation commercial aerospace designs through a dual product strategy:

• QPL-qualified MIL-Spec circular connectors—such as BACC45 (MIL-DTL-26500) and BACC63 (MIL-DTL-83723)—provide proven reliability and traceable compliance for defense and aviation platforms.
• Innovative miniature commercial series, including MD801, deliver MIL-level performance in a smaller, lighter form factor, optimized for SWaP-sensitive systems like UAVs, eVTOL aircraft, and small satellites.

This combination allows engineers to choose between formally qualified and ruggedized commercial solutions from a single supplier—balancing compliance, performance, and development speed.

What coupling styles are available?

Circular connectors are offered with several coupling mechanisms, each suited to specific application needs:

• Bayonet: Quick, positive-locking action for fast connect/disconnect; common in MIL-DTL-26482 and MIL-DTL-38999 Series I and II.
• Threaded: Screw-type coupling for maximum vibration resistance and EMI shielding; used in MIL-DTL-38999 Series III and BACC63.
• Breech-Lock / Rapid-Advance: Quarter-turn mechanism combining bayonet speed with threaded strength; ideal for blind-mate or high-density systems (e.g., Series IV).
• Dual-Start / Tri-Start Threads: A multi-lead feature of threaded couplings that reduces turns-to-mate and improves usability without compromising EMI performance.

How do shell sizes and insert arrangements affect design?

The shell size defines a connector’s overall footprint, mounting dimensions, and contact capacity, while the insert arrangement determines the number, size, and layout of contacts inside that shell.

Larger shells accommodate more or higher-current contacts, while smaller shells save weight and space. Insert arrangements allow designers to mix power, signal, coaxial, and fiber contacts, or to optimize for keying and polarization.

Selecting the right combination ensures proper mechanical fit, electrical performance, and serviceability for the application.

What materials and finishes are available?

Circular connector shells are built from materials that balance strength, corrosion resistance, and weight:

• Aluminum: Lightweight, strong, and highly conductive—ideal for EMI shielding.
• Stainless steel: Provides maximum strength and corrosion resistance in marine or chemical environments.
• Composite: Extremely light and corrosion-proof; typically coated with a conductive layer for EMI protection.

Available finishes include:

• Olive drab cadmium: Traditional MIL finish offering excellent EMI and corrosion performance (non-RoHS).
• Electroless nickel: Smooth, conductive, and RoHS-compliant with moderate corrosion protection.
• Nickel-PTFE: Provides low coupling torque and improved corrosion resistance.
• Black zinc-nickel: Modern RoHS-compliant alternative to cadmium with 500–1,000 h salt-spray resistance and excellent shielding.

What makes the MD801 series unique?

The MD801 series delivers MIL-DTL-38999 Series III–level performance in a smaller, lighter, and faster-mating form factor. Up to 40% lighter than standard 38999 connectors, it combines dual-start ACME threads for quick, vibration-resistant coupling with IP67 environmental sealing and a wide –65 °C to +175 °C operating range.

Rated for 2,000+ mating cycles, MD801 supports signal, power, and coax contact options in compact shells, making it ideal for UAVs, eVTOL, avionics, and rugged embedded systems where SWaP and reliability are critical.

Where is MD801 used?

The MD801 series is deployed across a wide range of SWaP-sensitive aerospace and defense platforms where reliability and miniaturization are critical:

• UAVs and drones: Lightweight, vibration-resistant interconnects for flight control, sensors, and payload systems.
• eVTOL aircraft: Compact, EMI-shielded connectors supporting high-voltage propulsion and avionics networks.
• Avionics and flight computers: High contact density in reduced form factors for navigation and control modules.
• Rugged embedded systems and defense electronics: Withstand shock, vibration, and extreme temperature cycling in mobile or airborne systems.

With IP67 sealing, dual-start threaded coupling, and –65 °C to +175 °C operation, MD801 enables reliable performance in the most space- and weight-constrained environments.

Why are circular connectors important in eVTOL?

eVTOL aircraft demand connectors that combine aerospace reliability with automotive-grade efficiency. Circular connectors provide the compact geometry, EMI shielding, and sealing needed to manage high-voltage power, data, and control signals within tight weight limits.

Cinch’s MD801 series is purpose-built for SWaP-optimized electric aircraft, offering dual-start threaded coupling, IP67 sealing, –65 °C to +175 °C operation, and 2,000+ mating cycles. These features ensure dependable performance across propulsion, battery, and avionics systems—critical for passenger safety and airworthiness certification.

How do Cinch connectors support aerospace?

Aerospace systems—from avionics and flight controls to sensors, power units, and communication networks—rely on Cinch circular connectors for proven electrical and mechanical reliability. Cinch’s products meet or exceed key aerospace standards, including MIL-DTL-26500, MIL-DTL-83723, MIL-DTL-38999, and Boeing’s BACC45 and BACC63 specifications.

With lightweight aluminum or composite shells, EMI-shielded finishes, and temperature capability from –65 °C to +200 °C, these connectors provide the strength, weight efficiency, and performance consistency required for flight-critical applications across commercial, military, and space platforms.

What about defense applications?

Cinch circular connectors are QPL-qualified, EMI-shielded, and ruggedized to meet the extreme mechanical and environmental demands of modern defense systems. Built to standards such as MIL-DTL-38999, MIL-DTL-83723, and MIL-DTL-26500, they combine secure coupling, corrosion-resistant finishes, and wide operating temperature ranges (–65 °C to +200 °C) for long-term reliability.

These connectors are deployed in ground vehicles, soldier systems, radar and communications equipment, and avionics, where resistance to shock, vibration, and EMI is critical to mission success.

Why are Cinch connectors used in space?

Space platforms demand interconnects that can endure vacuum, thermal extremes, high launch loads, radiation exposure, and strict reliability requirements. Cinch circular connector families—including models such as the MD801—are engineered for harsh-environment performance (low outgassing finishes, durable materials, high shock/vibration tolerance) and are used on satellite, CubeSat, launch, and avionics systems when qualified by the program.

Why does Boeing qualification matter?

Boeing qualification certifies that a connector meets Boeing’s internal BACC specification requirements, which are derived from corresponding MIL-DTL standards but controlled by Boeing for its aircraft programs. This qualification allows the connector to be used directly on Boeing platforms—such as commercial and defense aircraft—without additional requalification testing, simplifying procurement and ensuring configuration control.

It’s important to note that Boeing qualification is separate from U.S. Government QPL listing; some connectors may hold both approvals, while others are qualified only under Boeing’s BACC program.

What Boeing standards do Cinch connectors meet?

Cinch is qualified by Boeing to two primary connector standards:

• BACC45 – Based on MIL-DTL-26500, used in avionics, flight controls, and instrumentation systems across Boeing aircraft.
• BACC63 – Based on MIL-DTL-83723 Series III, featuring threaded, high-vibration-resistant coupling for engine, landing gear, and military applications.

These BACC (Boeing Airplane Company Connector) standards adapt proven MIL-DTL designs to Boeing’s specific configuration and quality requirements, ensuring interchangeability and direct approval for use on Boeing platforms.

How do BACC45 and BACC63 differ?

• BACC45 – Derived from MIL-DTL-26500, this series is used widely in avionics, control, and instrumentation systems on Boeing aircraft. It offers bayonet or threaded coupling options, reliable EMI performance, and easy maintenance for moderate-vibration zones.
• BACC63 – Based on MIL-DTL-83723 Series III, this series uses a tri-start threaded coupling that provides superior vibration and environmental resistance. It’s preferred in engine bays, landing gear, and military applications where durability under stress is critical.

Can I download datasheets and CAD models?

Yes. Cinch provides datasheets, specifications, and 3D CAD files on product pages.

Where can I buy Cinch circular connectors?

Through Digi-Key, Mouser, Newark, Air Electro, and Cinch’s “Where to Buy” tool.

How can I get design support or a custom solution?

Cinch engineers support unique insert layouts, shell finishes, and board-tail options.