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6
Appendix C
LOGISTICS CONSIDERATIONS
The shuttle in 1986 has accomplished a long series of successful
operational flights. Nonetheless, from a technical point of view, it
is not yet out of the development phase. Design flaws, expected in a
system of this complexity, are still being corrected. The spares
complement is just being developed for main engines, solid rocket
boosters, other line replacement units (LKUs), and so forth. In that
respect, the past operational flights have been essential to
determining the real, as opposed to the hypothetical, logistics reeds
for various sustainable flight rates. Consequently, a more efficient
logistics program can now be set up than was possible some years ago.
By the l990s, most major development should be completed and the
logistics picture should have stabilized. This appendix endeavors to
estimate what that picture will be.
The panel accepts NASA's estimates of the immediate logistics
needs. However, NASA has not systematically examined the consequences
of the eventual loss, through use or accident, of Orbiters. Such
losses must be expected, as NASA itself has stated in testimony on the
Challenger loss. The complexity and uniqueness of the shuttle
(critical elements, design margins, "rebuild" for every flight, etc.)
reinforce that point. Well-recognized calculations relating system
reliability, confidence level in accomplishing the mission, fleet
size, and flight rate (equipment lifetime) have been used for years to
determine the buy rate for aircraft, satellites, and other fleets.
Figures C-l and C-2 show the relationships among these factors for the
range of parameters applicable to Orbiters. Figure C-1 shows the
situation assuming no Orbiter losses: the upper curve gives an upper
bound on flight rates per year so that there is a 50 percent chance of
no Orbiter loss over the period. The lower curve represents an upper
bound for a 90 percent chance of no loss. Figure C-2 shows the
situation assuming one loss, i.e., the upper bounds on flight rates
for no more than one Orbiter loss during the period. ("Loss" may be
through wear out, severe overstress, or any accident that precludes
further use.) An alternate way of showing the information in Figure
C-l is given in C-3. Based on all experience to date, one would have
to have unachievable reliability to have a high confidence manifest
without some planned backup, workarouno and/or replacement Orbiter.
33
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SHUTTLE MISSION SUCCESS RATE
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EQUIVALENT MISSION FAILURE RATE
The Chances of Losing Zero Orbiters
1988 through 2000 Time Period.
0.01
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EQUIVALENT MISSION FAILURE RATE
Figure C-2 The Chances of Losing No More than One Orbiter
1988 through 2000 Time Period.
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Figure C-3 (An Alternative View of the Information in Figure C-1.)
The shuttle system reliability to date is 0.96 (a failure rate of 4
percent). Given NASA's vigorous efforts at improved safety, this
failure rate might be cut by a factor of 4; reducing it by a factor
of 10 to a reliability of 0.99b is most unlikely in a short time or
for costs less than the development cost to date (based on aircraft
development experience).
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It is important here to distinguish between having sufficient LRU
spares and replacing a lost Orbiter.* The former is planned by NASA
with appropriate budget requests; it is crucial for maintaining an
acceptable Orbiter turnaround time. But, it assumes infinite life for
the airframe, major structural components, and fittings. A
noncatastrophic failure of an LBU--even if the cause were a design
problem that grounds the Orbiter fleet for a while--is unlikely to
affect the flight schedule for much more than a year. (The Challenger
solid rocket problem will take somewhat longer but past ELV experience
suggests that this is an exception.) Such schedule disruption can be
accommodated through increased satellite design life of more than 5
years, as well as spares on orbit; planetary and space station
programs clearly have more difficulty accommodating delays.
The loss of an Orbiter, however, as demonstrated by the Challenger
accident, has long-lasting effects on all missions. Missions are
placed in jeopardy. Mission and industrial teams are in danger of
breaking up, especially university scientific teams and subcontractor
teams. Time-critical satellites may be mothballed or discarded. The
collateral costs are already a major fraction of the cost of another
Orbiter. Yet this kind of disruption is inherent with small fleets of
reusable, fully-booked vehicles with ion& replacement times.
There are several possibilities for minimizing the severity of
impacts. First, a full-up spare Orbiter, with another ordered later,
would help alleviate the buildup of backlogs if the anomaly is not a
systems design-related one. Satellite constellations, for example,
frequently have spares on orbit, but such an alternative for the
shuttle may be too expensive. Another, albeit riskier, possibility is
a flight rate sufficiently below the maximum rate where a multiyear
"surge" could alleviate schedule conflicts until a replacement Orbiter
is brought on line. Shifting of "standard" aual-compatible satellites
to ELVs along with a sufficient inventory of "ready-to-go" vehicles,
might help.
In any case, without an agreed strategy to accommodate Orbiter
loss, long-term confidence in any shuttle manifest is certainly
limited. This could be crucial for the space station, planetary
launches, and some national security flights.
The Report of the Presidential Commission on the Space Shuttle
Challenger Accident and the statements of individuals who met with the
panel Identified a number of relatively short-term problems that the
panel believes NASA is likely to solve by the early l99O's:
o The cannibalization of LBUs frown one Orbiter for parts or
repair of other Orbiters (on the order of 50 percent on past
flights).
O Limitations on nondestructive testing.
~ ..
*The NASA spares program does not supply a full "ship set" of spares,
some of which have very long lead times. Consequently, a replacement
Orbiter cannot be built Just out of spares.
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o Main engine and landing gear replacement and repair provisions.
O Flight simulator limitations (technological and capacity).
O Critical skills, spares, and maintenance requirements at KSC.
O Critical skills, simulators, data storage and software
development at JSC.
0 Establishment of a long-range logistics support plan, updated
annually or as experience dictates. General concurrence in the
plan and its financing by the Executive and Legislative
Branches.
O Agreement by the NASA and DoD as to which payloads would be
compatible with both Orbiter and ELYs. In this connection, it
is not necessary that all payloads be dual compatible in order
to assure access to space. But those that are will require
advance planning of launch support if changeover is to be
practical.
The panel also notes the continuing trend toward the formation of
a Shuttle Operations Organization, which could be an important factor
in achieving confidence in any shuttle manifest.
A serious limitation to a reliable manifest is the maintenance of
an industrial base to support the spares and replacement needs of the
shuttle fleet. The shuttle fleet is a small one, with a low
replacement rate. But according to Rockwell International, to ensure
that a replacement Orbiter is available within, say, an 18-24 month
call-up would require a production capability of ~ Orbiters on order
all the time (or 1 every 2 years). However, an efficient industrial
base can be maintained with a production rate of one Orbiter about
every 3 years along with continued production of spare parts. But
with an Orbiter fleet operating at about 10 flights per year and a
loss rate of 1 to 2 percent, the replacement order rate would be one
every 5 to 10 years, too low to retain an efficient industrial base.
Increasing the shuttle's reliability to the point of not needing any
further replacements nor an industrial base is not practical (see
Figure C-1~.
A compromise would be a shuttle production rate of about one
Orbiter every 4 years along with a national commitment to aggressive
use of shuttle-unique capabilities.
6
Representative terms from entire chapter:
industrial base
