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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 1983 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, 1986.
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.".,
the Orbiter Maintenance Facility--appear adequate to handle flight
rates up to 12 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 (SEC) 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 12 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, 1983.
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With Orbiter landings at Edwards Air Force Base for the
foreseeable future, and a projected 18-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 1992 according to
Secretary of the Air Force Edward Aidridge.~ Use of this complex for
shuttle launches in the years beyond 1992 remains an open question.
Turnaround time
Shuttle turnaround time consists of 4 main elements: (1)
processing time at the Kennedy Space Center, (~) mission duration, (~)
transit time to ferry the Orbiter to KSC (if it lands elsewhere), and
(4) 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 KS~(: 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 tine. 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-1 .
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) of ~ 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 31, 1986.
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I OFF
'
(43) VAB
(7) PAD 75 workdays
(25)
r OFF |
Figure B-1 Shuttle Processing Time.
Conservative
Real istic/Optimistic
60 workdays
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.
Plight 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 10. For a 4-Orbiter fleet the sustainable rate becomes
11 to 13 per year.
The above estimates apply to the KSC. Should the Vandenberg
Launch Site be utilized for DoD missions after 1992, 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 10-12 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.
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Representative terms from entire chapter:
flight rates
