DEPARTMENT OF THE AIR FORCE SMC REGULATION 800-18
Headquarters Space and Missile Systems Center (AFMC)
Los Angeles Air Force Base, California 90009-2960
"B" Nuts and Other
Separable Fluid Fittings
This regulation establishes policies and procedures and assigns responsibilities for SMC program office acquisitions that contain plumbing connections for pressurized systems in SMC launch and space vehicle programs.
1. Policy Separable fluid fittings for SMC space and launch vehicle program applications shall not use "B" nuts. Only permissible fitting assemblies as defined in this policy shall be used.
2. Background In numerous aerospace programs, standard AN - ARMY/NAVY, MS - MILITARY STANDARD and MC - MILITARY COMMITTEE (NATO) tubular flare fittings are used as separable plumbing joints. These have been found to be a potential source of single point failures. Metal-to-metal seal integrity depends upon maintaining locally high clamping stress. Required force is generated and transmitted across the mated parts by application of torque to the threaded elements. Tests have shown that some threaded fittings torqued to a proper sealing value tend to undergo strain deformation and that leaks may develop with time, temperature cycling, or vibration. Preflight procedures such as torque and leak checks have helped to maintain past mission reliability. However, torque corrections are not always feasible and fluid-leaks have occurred in systems employing "B" nut flare fittings on SMC launch and space vehicles.
Permanent plumbing connection -
A plumbing connection designed to provide non-separable connection between two components of a pressurized fluid system (usually a welded or brazed union).
Permissible fitting assemblies -
A plumbing connection designed to provide separability of pressurized fluid systems, and which meets all of the following constraints:
(1) The fittings shall be comprised of one or more compressed or internally pressure-energized members which maintain a seal even if stress relaxation occurs in any of the other components.
(2) The fittings shall have redundant seals in series.
(3) The fittings shall be lockwired to prevent any rotation between the fitting and the nut.
4. Responsibilities and Procedures
a. For SMC launch and space vehicle programs each program office shall ensure that permanent plumbing connections are specified unless functionality requirements demand separability.
b. For SMC launch and space vehicle programs each program office shall ensure that all separable plumbing connections use permissible fitting assemblies, as defined in this policy.
c. For SMC launch and space vehicle programs each program office shall require contractor compliance with paragraphs 4.a and 4.b during design, manufacture and test of all plumbing connections.
d. For SMC launch and space vehicle programs each program office shall review contractor compliance with paragraph 4.c from proposal evaluation through delivery of the vehicle. Evidence of the contractor's compliance to paragraph 4.c shall be provided to the system program office as part of design reviews and contractual audits.
e. For SMC launch and space vehicle programs each program office shall require contractor notification of any departure from this regulation. The program office will evaluate any deviations/non-compliance and recommend corrective action.
f. Instructions for applying and executing this regulation are provided by SMC Regulation 540-15 (Systems Engineering Policy). Any program director who does not apply and execute this regulation, as required by SMCR 540-15, shall immediately request a waiver from the Commander.
Edward P. Barry, Jr.
Lieutenant General, USAF