Presidential Commission
Flight Rate. The nation's reliance on the Shuttle as its principal space launch capability created a relentless pressure on NASA to increase the flight rate. Such reliance on a single launch capability should be avoided in the future.
NASA must establish a flight rate that is consistent with its resources. A firm payload assignment policy should be established. The policy should include rigorous controls on cargo manifest changes to limit the pressures such changes exert on schedules and crew training.
FLIGHT RATE REQUIREMENTS
Several major actions taken in the last year have reduced the overall requirements for NSTS launches. Each of these actions reduces the reliance on the Space Shuttle as this nation's single launch capability and maximizes its availability for missions that require the unique capability of the vehicle and its crew.
The addition of a fourth orbiter to the fleet will significantly improve the current program launch capability. However, a reduction in NSTS requirements must be achieved to ensure a launch rate consistent with the available resources.
NASA and the Department of Defense (DOD) have jointly established, and are implementing, a mixed-fleet concept of expendable launch vehicles (ELV's) and the Shuttle to meet national requirements for access to space. Many of the DOD payloads previously scheduled on the NSTS can be launched on ELV's. NASA and DOD have identified these payloads and replanned the overall launch strategy to provide for their launches on ELV's.
The initial step in this effort resulted in the identification of requirements for more than twice the number of Titan IV launch vehicles (10 to 23) planned for DOD payloads in the near term (through 1992). The Shuttle and the Titan IV are nearly equivalent in launch capability; therefore each additional Titan IV launch reduces the DOD requirements for NSTS launches by one flight.
The medium launch vehicle (MLV) being developed by DOD will be used to launch Navstar Global Positioning System satellites. Some 20 of these DOD satellites, previously scheduled for deployment from the NSTS, are now planned for the MLV. As part of the budget and manifest planning exercises currently under way, NASA and DOD are evaluating options for additional offloading of payloads from the Shuttle to ELV's.
The presidential decision to limit use of the NSTS for launch of communication satellites to those with national security or foreign policy implications has resulted in more than 20 of these satellites, previously scheduled on the NSTS, being reassigned to commercial ELV's. NASA has worked actively with the United States commercial ELV industry and the commercial satellite owners and operators to ensure an orderly transition.
The NASA Office of Space Flight conducted a study to determine the civil payload  launch requirements that could be satisfied with a mixed fleet. This study concluded that approximately 25 percent of the NASA and National Oceanic and Atmospheric Administration payloads currently scheduled for launch on the NSTS could potentially be launched on ELV's.
NASA has initiated the overall planning required to implement a mixed fleet, to define the required near- and far-term launch requirements, and to identify the number, type, and cost of the launch vehicles required to satisfy the requirements.
For payloads in the post-1992 time period, the mixed-fleet study recommended further Shuttle offfloading through the use of an unmanned Shuttle-derived vehicle (SDV). NASA is vigorously exploring SDV concepts as a means of satisfying future payload requirements.
FLIGHT RATE CONSISTENT WITH RESOURCES
In March 1986, Admiral Truly directed that a "bottoms-up" Shuttle flight rate capability assessment be conducted. To accomplish this, a Flight Rate Capability Working Group was established. Representatives from each NSTS Program element that affects the flight rate participated in the group.
Ground rules were developed to ensure that projected flight rates were realistic. These ground rules addressed such items as overall staffing of the launch process work...
....force, work shifting, overtime, astronaut training, and maintenance/inspection requirements for the orbiter, main engine, solid rocket motor, and other critical systems.
Based on these ground rules, a careful assessment was made of the vehicle processing cycle (Figure 37), the payload preparation process (Figure 38), and the mission planning process (Figure 39) to determine their capacity to support the flight rate. The actual flight rate that can be achieved at any time is dependent on a well-defined and stable set of requirements that allow all the activities portrayed in these figures to be accomplished on a carefully planned basis.
The working group identified enhancements required for the Shuttle mission simulator, the Mission Control Center, the Orbiter Processing Facility, and other areas such as training aircraft and provisioning of spares. With these enhancements and the replacement orbiter, NASA projects a maximum capability of 14 flights per year (Figure 40). This capacity, considering lead time constraints, learning curves, and budget limitations, is expected to be achieved by 1994. The experience gained after flights are resumed will be used to adjust future flight rate projections.
Two independent assessments of Shuttle flight rate capability have been made. A National Research Council (NRC) report published in October states in part:
"Three Orbiters can sustain a rate of 8 to 10 flights per year after an initial buildup period of approximately 2 years providing: (1) no Orbiter is lost or becomes inoperable, (2) adequate logistics support exists, and (3) no problems exist that require extensive downtime. A surge rate of 12 flights per year should be possible...
....for short periods of time for simple payloads and flight plans.
"With a 4-Orbiter fleet the sustainable flight rate would be 11-13 per year with a surge rate of 15 flights per year only if appropriate ground support facilities are acquired.
"In order to sustain such rates and take account of possible contingencies, the Shuttle scheduling should be based upon fewer vehicles than are actually in the inventory by almost one Orbiter."
The other independent assessment was made by the Aerospace Safety Advisory Panel. At the conclusion of their study, the panel concurred with the NRC report. Since these assessments are in close agreement with the NASA assessment, it is felt that the capability projections are realistic.
MANIFESTING POLICY AND RIGOROUS CONTROLS
The manifesting and scheduling of payloads on the NSTS will be consistent with the flight rate projections defined above.
To ensure the stability of future cargo manifests, firm policies have been established and a formal control process has been implemented. The control process provides for a series of "freeze points" (Figure 39), at specified intervals over the year and a half required to prepare for each flight, that rigidly define the vehicle, payload, and mission characteristics.
Approximately 18 months before launch, the flight production process begins with a flight/cargo freeze point that baselines the primary payload assignments and defines the orbiter vehicle configuration. Only manda....
...tory changes required to ensure crew or vehicle safety, or the accomplishment of primary mission objectives, will be made after this point.
Eleven months before launch, the cargo integration freeze point baselines the detailed flight design, orbiter hardware and software, payload specialists assignments, if any, and secondary payloads.
At 7 months, the flight planning and stowage freeze point baselines crew activities and crew compartment stowage. This is the last opportunity to add orbiter mid deck or small payload bay self-contained payloads that meet standard NSTS interface requirements. One month later, the ascent flight design-launch site flow review freeze point baselines the ascent trajectory and the launch site work plan.
This freeze point process is conducted under the rigorous rules of the NSTS change control process. This ensures that changes receive the proper senior program management review and are acceptable in terms of their effect on crew training, work schedules, and other elements that could adversely impact mission safety.
Other facts that strengthen manifest stability include:
The actions discussed above are being implemented, and the flight rate is now set at an achievable level, consistent with program resources, the four-orbiter fleet, and an agency-wide commitment to safety. With the mixed-fleet policy, many payloads have been reassigned to other launch vehicles. Firm manifesting policies have been established, and rigorous change control procedures have been implemented. Any required manifest change that is not consistent with these defined policies will cause that payload to be scheduled on a later mission. The resumption of flight is approached with confidence that these measures will prove effective.