Other changes for production F-20's can be traced back to a comment made by an Air Force logistics general at a logistics meeting. Northrop was touting, as usual, the high reliability and low maintenance of the F-20 compared to other aircraft. The general finally burst out in frustration "You just don't get it. I have to man squadrons according to mil standards. The only way for your low maintenance to translate to savings for me is to eliminate airman specialties". This meant going through the official USAF specialty codes and looking for what unique specialties were hardly required and could be eliminated through changes in the aircraft. Therefore the liquid oxygen supply used for the environmental control system was eliminated, and with it the oxygen cart and its associated technician and logistics tail . So another high-tech, but higher-maintenance, more complex, and more expensive device replaced the liquid oxygen tank and regulator - an OBOGS, or On-Board Oxygen Generating System. This was a molecular sieve, that sucked in the thin air of high altitudes, screened out the harmful ozone, and concentrated the oxygen for use by the pilot.
Official Description:The F-20A used a state-of-the-art onboard oxygen generating system (OBOGS) that eliminated the need for a liquid oxygen (LOX) system by concentrating oxygen from compressor bleed air. The OBOGS system eliminated the need for LOX support equipment and its associated logistics problems, thus reducing F-20A turnaround time.
The performance and simplified support requirements of OBOGS were verified by evaluation on the AV-8B Harrier. Man-rating testing of the OBOGS system were completed by Brooks Air Force Base. The prime component of the OBOGS system was the oxygen concentrator. The concentrator, which was driven by aircraft ac power, worked on the molecular sieve absorption (surface adhesion) principle, separating a small percentage of oxygen, along with inert gases, primarily argon, from the airflow on each cycle. The concentrator contained two canisters of zeolite, one canister of which was purged while the other was in the oxygen-generating cycle. The cycle time for operation on each canister was about 5 seconds with in-let air alternately directed to each canister by a motor- driven valve. This valve was equipped with a sensor connected to a caution light in the cockpit, which alerted the pilot of any mechanical or electrical power-supply failure.
Concentrated oxygen-enriched air entered a storage plenum in the concentrator. The plenum contained as large a volume as practical, maintained at approximately 80 psi. The storage plenum automatically provided the pilot with up to 16 minutes of oxygen during short-term high demands or low-output periods, i.e., air system/electrical system failures or engine flameouts. The plenum also prevented multiple short-term use of the emergency gaseous oxygen (GOX) system and thereby eliminated subsequent added servicing requirements to replenish the GOX supply.
A low pressure switch, installed in the supply line immediately upstream from the oxygen regulator, was connected to the caution light which alerted the pilot if oxygen pressure dropped to 15 psig. OBOGS had a negligible effect on the F-20A environmental control system (ECS). It used less than 2 pounds of air per minute, which translated to less than 3 percent of available air from the ECS refrigeration package.
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