Post-Challenger Assessment of Space Shuttle Flight Rates and Utilization (1986)

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Appendix B SHUTTLE LAUNCH RATES A detailed assessment of space shuttle launch rates was provided in an earlier report issued in April l983 under the Committee on NASA Scientific and Technological Program Reviews, Assessment of Constraints on Space Shuttle Launch Rates. The present study takes into account lessons learned over the past 3 years, including circumstances following the loss of the Challenger on January 28, l986. During its short passage through the sensible atmosphere, the shuttle is stressed far nearer its design limits on every flight than is the case for transport aircraft, for which such an occurence is a rarity. Also, there are considerable differences between individual shuttle missions, unlike the repetitiveness of airline or more routine military operations. A shuttle flight manifest utilizing maximum possible turnaround rates must not compromise safety standards, since each flight is working so closely to the vehicle performance limits. The need for adequate safety reviews, logistics support (see Appendix C), spare parts, extensive inspection and maintenance, and extreme care in every facet of the turnaround operation cannot be overemphasized. In addition, sufficient crew-training facilities—including simulators and training aircraft—are needed to enable adequate training flowthrough to meet projected flight rates. Present ground facilities at KSC with planned augmentation—e.g., the Orbiter Maintenance Facility—appear adequate to handle flight rates up to l2 per year. However, manpower at Johnson Space Center and spare parts production would have to be increased for such flight rates. The single Shuttle Processing Contractor (SPC) concept presently in place appears to be advantageous from an administrative point of view. While the SPC has improved in technical competence over the past 2 years, it still requires further improvement in the handling of individual systems—(Space Shuttle Main Engine, Solid Rocket Booster, External Tank, and Orbiter). Crew training facilities and software development need improvement for flight rates up to l2 per year. Needed are an upgrading of existing facilities and an increase in the number of simulators as well as a fourth shuttle training aircraft.* *See Assessment of Constraints on Space Shuttle Launch Rates, National Research Council, l983. 29

30 With Orbiter landings at Edwards Air Force Base for the foreseeable future, and a projected l8-month delay in procurement, an additional Shuttle Carrier Aircraft would be prudent. Experience gained in the STS program, and from the accident, suggests that a sustained flight rate per Orbiter on a long-term basis should be more conservative than was thought necessary no more than 2 years ago. The estimates given below are based on launch from KSC. The DoD shuttle launch complex at Vandenberg Air Force Base on the West Coast is being placed in "caretaker" status until l992 according to Secretary of the Air Force Edward Aldridge.* Use of this complex for shuttle launches in the years beyond l992 remains an open question. Turnaround time Shuttle turnaround time consists of 4 main elements: (l) processing time at the Kennedy Space Center, (2) mission duration, (3) transit time to ferry the Orbiter to KSC (if it lands elsewhere), and (A) planned periodic inspection and maintenance. These elements can be quantified on an average basis. A fifth element that cannot be quantified consists of contingencies—major damage to, or loss of, an Orbiter; diverted landing; weather delays; late manifest and/or flight plan changes; unforeseen payload delays; facility or support system downtime; lack of timely availability of spares/logistic support. Shuttle processing at the KSC takes place successively in the Orbiter Processing Facility (OFF), the Vehicle Assembly Building (VAB), and on the launch pad (PAD). To be added in the near term is an Orbiter Maintenance Refurbishment Facility (OMRF) through which the Orbiters will flow on their way to the OPFs. The average shuttle processing time to date at the KSC has been 75 work days; the shortest was 46 work days. The latter is viewed as representing a surge condition and not a sustainable one. The former is viewed as an average conservative sustainable processing time. A more optimistic, but not unrealistic sustainable processing time would be 20 percent shorter, or 60 working days. Both are shown in breakdown in Figure B-l. Average mission duration is estimated to be 7 days. Since Orbiter flights will be landing at Edwards Air Force Base for the foreseeable future, account is taken of ferrying time (including one day for weather uncertainties) ot b days. Planned structural inspections and major maintenance are expected to require on average the equivalent of 2 weeks of downtime for every flight. *"Air Force Secretary Describes Defense Department Space Recovery Plans," Press release dated July 3l, l986.

3l OPF (43) Conservative VAB (7) (25) OPF (34) 75 workdays Realistic/Optimistic VAB (6) PAD 60 workdays (20) Figure B-l Shuttle Processing Time. With regard to the work week, bearing in mind the emphasis on safety and efficiency, it is postulated that ground crews work a 3-shift per day, 5-day week. The panel regarded a scheduled 7-day work week as undesirable over the long term because of the possible impact on flight safety. Flight Rates Based on the estimates above, the 75-work-day processing time results in an average number of flights per year per Orbiter of 2.7. With an improved processing time of 60 work days, the average rate per year per Orbiter becomes 3.3 (assuming a normal 3-shift, 5-day work week). Thus, the average yearly sustainable flight rate for a 3-Orbiter fleet is 8 to l0. For a 4-Orbiter fleet the sustainable rate becomes ll to l3 per year. The above estimates apply to the KSC. Should the Vandenberg Launch Site be utilized for DoD missions after l992, the U.S. Air Force expects to have one to 3 shuttle launches per year from the site. It is also expected that one Orbiter equivalent would be dedicated to Vandenberg. The effect would be to lower the flight rates noted above by approximately one flight/year—i.e., 7-9 for a 3-Orbiter fleet and l0-l2 for a 4-Orbiter fleet. It should be noted that these estimates do not account for contingencies noted earlier. While there is no way of predicting possible time lost due to some contingency or other, the experience of airlines and of U.S. Air Force aircraft operations shows that backup vehicles are needed to allow schedules to be maintained when an unforeseen event puts a vehicle out of commission.