3.3 - The Operability Criteria - Listed

These criteria deal with increasing operability by providing reduced operations, maintenance, manpower, equipment in increased productivity. How much will the resulting reductions, increases and ultimately cost savings be affected by these criteria is dependent on the degree to which the criteria are met. The criteria present little quantitative values but provide great qualitative metrics with which to evaluate design concepts for capability of meeting the operations concept of the preceding section. The criteria have been split into those which endorse a minimization of specific attributes of vehicle design and those advocating maximization of characteristics as opposed to just a numerical listing.

RLV Design concepts should MINIMIZE:

• 1) Separate system/subsystems within the Space Transportation System.
• 2) Potential fluid leakage sources.
• 3) Hands-on activities required to handle, assemble, checkout, service, and launch.
• 4) Different fluids used in the Space Transportation System (Flight and Ground).
• 5) Active components required to function - - including flight operations (Flight and Ground).
• 7) Active systems required to maintain a safe vehicle.
• 8) Systems that require monitoring because of hazards.
• 9) Hazardous subsystems requiring corrective action to safe.
• 12) Element to element interfaces requiring engineering control.
• 13) Hours, special tools, and GSE for turnaround for re-flight.
• 15) Purges (Flight and Ground).
• 16) Toxic fluids.
• 17) Hours to refurbish the system.
• 19) Confined spaces on the vehicle requiring safety control.
• 21) Pollutive/toxic materials.
• 22) Physically difficult areas to access.
• 23) Active conditioning requirements for hardware function.
• 24) Checkout required.
• 25) Time to access a check point/points.
• 26) Inspection points, but easy access for those required.
• 27) Criticality one failure modes.
• 28) Quantity of pollutants (based on regulations).
• 29) Manufacturing/test/operations facilities (minimum total infrastructure).
• 30) Separate systems required for thrust vector control.
• 32) Countdown fluid servicing time with minimum complexity and emergency safing procedures.
• 34) Energy release from unplanned reaction of propellants for safety reasons.
• 35) Cost per day of delay of launch.

RLV Design concepts should MAXIMIZE:

• 6) Components with demonstrated high reliability (0.999...).
• 10) Percentage of totally automated systems.
• 11) Systems with BIT/BITE.
• 14) Systems with fault tolerance when reliability of hardware is not sufficient to support adequate safety of both hardware and personnel; however, emphasis should be placed on increased hardware reliability to provide this solution.
• 18) Mean time between major overhaul of system or its components.
• 20) Technologies with readiness demonstrated prior to program development start.
• 31) Thrust level margin.
• 33) Mass fraction margin.
• 36) Engine chamber pressure margin.

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Edgar Zapata, NASA Kennedy Space Center

Shuttle Process Engineering Directorate, Fluid Systems Division