During 1954 in the U.S. work begun on a lightweight, pressure-breathing oxygen mask for long-range, high-altitude flying.
Hope was that the new mask would eliminate the fitting and comfort problems encountered with the MS-22001 (A-13A). Initially, a one-size-fits- all design (MC-1 mask) was investigated but eventually a four-size mask (MBU-5/P) was developed.
Under different contracts, one-size face pieces produced for 156 test masks. Several manufactured by using the technique of “latex dipping”, a process during which the mask mold is repeatedly immersed in liquid latex and withdrawn at a controlled rate to "build up" the face piece on the mold.
Silicone rubber, used for 56 examples of the masks, has properties far superior to any natural rubber compound. It shows better resistance to high (108°C / 225°F) and low (-73°C /-100° F) temperatures, eliminates skin sensitization, perspiration and skin oils, has practically no deteriorating effects if exposed to ozone and can be worn many hours longer than any latex product.
Reports indicate that no deformation or deterioration of the silicone face piece occurred after more than 400 and, in some cases, up to 1200 flight hours. Nevertheless, latex pieces used to connect the face piece to the mask shell showed some deterioration.
During the first years of the mask development, the silicone material showed some reduced strength, in particular resulting in lower tear resistance. This disadvantage overcomed by suitable indoctrination of aviators on the caretaking of the mask. The silicone industry has since continuously increased the strength and durability of the material.
To simplify and reduce size and weight, the three valves used in the MS-22001 mask (two for inhalation and one for exhalation) combined into one pressure-compensated, inhalation-exhalation valve for use in the MC-1 mask.
The location of the valve within the mask and the freezing of condensed water within the valve has been a problem while flying in cold temperatures. The MC-1 mask structure was designed with a non-freezing chamber in which the valve is imbedded and not to be exposed to freezing temperatures. The pressure-compensated, inhalation-exhalation valve is located in the lower part of the mask structure and is constantly in contact with warm exhalation air.
The MC-1 oxygen mask is satisfactory at all altitudes up to 43’000 feet, provided that a suitable facial seal obtained.
Since the single-sized mask was not adaptable to any facial contours, adding to that the tendency to slip down on the face during high G-force manoeuvers, various mask sizes and an improved harness were needed to correct these deficiencies. The mask caused no appreciable interference with vision while being worn and the noise-cancelling microphone was compatible with the communication systems of all aircraft in which the mask was tested.
The mask required none unscheduled maintenance, neither during the normal conduct of the test, nor at extreme temperatures, except for the removal of the slight excess of adhesive, which had formed around the inhalation-exhalation valve receptacle.
The MC-1 oxygen mask was more comfortable than the standard MS-22001 oxygen mask because of its lighter weight, smaller facial-covering area, and because it is not necessary to fit the mask tightly to obtain a good seal.
Knowledge and skills gained during the MC-1 mask program were used to develop the MBU-5/P mask.
The basic face piece, shell, pressure-compensation, inhalation-exhalation valve, harness, and other components of the MC-1 mask incorporated into the MBU-5/P. The difference between the two was primarily in the sizing. The MC-1 mask had a one-size face piece and a one-size shell for all pilots. The MBU-5/P had four sizes of each.
It became apparent during the testing of the MC-1 mask that one size would not be suitable for all pilots. Using the statistical data obtained during anthropometric sizing and fit testing of the MC-1 mask, six facial forms representing the Air Force’s flying population were constructed.
Further studies revealed that four forms would be adequate for MBU-5/P mask sizing. Molds were produced by using various facial forms, followed by silicone-dipping of the face pieces. After stripping the face pieces from the molds and subsequently curing them, the mask components - shell, valves, harness, etc. - assembled; a specific mask shell, made of semi-rigid plastic material, provided for each of the four sizes of face pieces.