SPACE SHUTTLE OPERATIONS EXPERIENCE

Charles R. Gunn Director, Unmanned Launch Vehicles and Upper Stages*

NASA Headquarters Washington, DC

Abstract

In twenty-five Space Shuttle f l ights, NASA has learned valuable lessons which wi l l improve future operations and designs of space launch vehicles. The major lessons learned from Shuttle operations are addressed. Flight operations experience has demonstrated the reusabi l i ty of the orbi ter, the solid rocket boosters, and main engines. However, orbi ter ground turnaround time is s igni f icant ly higher than projected due to expanded refurbishment of some systems and the many engineering modifications required to correct orbi ter system design deficiencies uncovered during f l i gh t experience. Although reduced by a factor of four since the early flows, orbiter turnaround remains the f l i gh t l imi t ing constraint that sets the Shuttle annual f l i gh t rate. Similarly, operations experience in f l i gh t design, payload integration, and f l i gh t control ler and astronaut training shows that i n i t i a l projections underestimated the complexity, time, and manpower required. The principal reasons for this are: (I) the demands of customizing each mission to maximize the science return in a shrinking Shuttle f l i gh t program; and (2) frequent changes to the cargo manifest to optimize the mix of science and applications payloads. Wh i l e experience has reduced the time and level of e f for t required for these operations tasks, the lesson is clear; performance margins must be preserved so missions do not have to be indiv idual ly tai lored and the cargo manifest must be stabil ized to reduce rework of f l i gh t and software products.

Introduction

The Space Shuttle is the f i r s t par t ia l l y reusable space launch vehicle. In twenty-five f l ights, NASA has learned valuable lessons that wi l l improve operations when the Shuttle returns to f l i gh t status in 1988 and wi l l improve future designs of new space launch vehicles. With the exception of the Challenger accident, caused by a design rather than operations deficiency, the overall Shuttle operations and mission performance have been extraordinari ly successful.

Since this paper focuses on the lessons learned from Shuttle operations, i t w i l l , of necessity, focus on the system's few deficiencies rather than i ts many achievements and successes in the advancement of technology and manned space f l i gh t operations. The lessons learned are categorized into f ive areas: (1) reusabi l i ty of f l i gh t systems; (2) orbi ter ground turnaround; (3) f l i gh t design; (4) payload integration; and (5) training. Where possible, the impact of the Challenger accident on Shuttle operations is noted.

* Previously Director, Shuttle Operations

Reusability of Fl ight Systems

The Shuttle's reusable f l i gh t systems are the orbi ter vehicle, the solid rocket boosters (SRBs), and the Space Shuttle main engines (SSMEs). A measure of the mission-to-mission reusabi l i ty of these systems is determined by the extent of refurbishment required after each mission. Refurbishment encompasses removal, rework, and replacement of damaged, fai led, or l imited l i f e items. The orbi ter 's overall f l i gh t performance and reusabi l i ty have been outstanding. The number of space f l ights made by each of the orbiters and the Space Transportation System (STS) mission designation and launch date of each orbi ter f l i gh t as shown in Figure I.

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Fig. 1 - Orbiter Flight Designations and Launch Dates

The reusabi l i ty of the orbi ter avionics systems, as expected, parallels experience on a i rcraf t with one signif icant difference. The Shuttle's avionics power-on usage is about 80 percent on the ground compared to commercial a i rcraf t where the principal usage is in f l igh t . The orbi ter mechanical systems reuse also parallels experience on a i rcraf t systems, with the single exception of the landing deceleration sytem. The landing deceleration system is essential ly a conventional a i rcraf t t r icyc le landing gear configuration. However, the orbi ter landing speed is nearly twice that of most a i rcraf t and the landing gear weight had to be minimal. To compound these design challenges, the orbi ter weight grew during development and the landing gear and brake systems were not sized to handle such an increase (188,000 to 212,000 pounds). The result is a brake system which is marginal for some missions and requires more refurbishment than expected. Consequently, the landing system is currently being strengthened to improve performance and reusabi l i ty.

The reusabi l i ty of the orbi ter thermal protection system (TPS) ceramic t i les was i n i t i a l l y a major concern, although the f l i gh t performance of the advanced technology system has been outstanding. Operationally, the TPS is

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extremely labor intensive. The t i les are glass coated, relat ively fragi le, and easily damaged i f impacted. On each orbiter there are over 24,000 t i les with a arge number of unique shapes and very few that are interchangeable. The surface of the mosaic of t i les over the orbiter airframe must be maintained aerodynamically smooth to measured dimensions in step and gap between each t i le . Also, the t i les must be rewaterproofed after each f l ight to prevent moisture absorption that could add as much as 500 pounds of undetected orbiter weight and fracture the t i les during on-orbit vacuum boil off. The number of t i les removed or repaired relating to operational f l igh t exposure has steadily decreased as shown in Figure 2.

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Related Tile Damage

The large number of t i les damaged on the early Shuttle f l ights was due to ice and insulation shedding from the External Tank (ET) at l i f t - o f f and during ascent. With improvements in application and bonding of the ET insulation, orbiter t i l e damage has decreased dramatically. Today, most t i l e damage occurs when debris is blown up off the lake bed or runway during landing rollout. With TPS replacement now averaging 50 t i les per f l ight , reusability is no longer a major concern. The Space Shuttle also pioneered the recovery, refurbishment, and reuse of solid rocket boosters (SRBs). On the in i t ia l f l ights, the SRBs were damaged when parachuted into the ocean at impact velocities of about 60 miles per hour. At water impact, the hydraulic cavitation loads around the base of the SRBs severely damaged the aft booster compartment structure and the thrust vector control components within the compartment. By employing larger parachutes and foaming in polyurethane around the components to streamline the compartment, the damage was reduced to within the in i t ia l program expectations. An SRB is composed of approximately 75,000 piece parts and components, of which about 5,000 are removed and refurbished between each reuse. A more cost effective refurbishment design would have fewer parts that must be disassembled, inspected, refurbished, and reassembled. Much of this work is related to the removal and reapplication of thermal protection ablators on the SRB structure. A permanent thermal protection technique would significantly reduce refurbishment costs. Nevertheless, operations experience shows the cost of reusable SRBs is s t i l l about one-half of new expendable units.

Experience on reusability of the Space Shuttle Main Engines (SSME) shows that the turbomachinery inspections and replacements between f l ights is significantly higher than in i t ia l projections. Inspection time is not decreasing and, on the average, the turbopumps are replaced every second or third f l i gh t because of concerns with bearing wear and turbine blade cracks. These inspections have at least a 5-day serial impact on orbiter ground turnaround time. An intensive SSME ground test program is under way to extend the re l i ab i l i t y and l i f e time of these components with the goal of mean-time- between-replacement of twenty-five f l ights.

In summary, the reusability of the Shuttle is demonstrated. The effectiveness of the supporting operations that sustain reusability are proven. Those few systems requiring more refurbishment effort than i n i t i a l l y planned are being gradually improved in design and durabil i ty.

Orbiter Ground Turnaround

The orbiter ground turnaround operations are the pacing element in the total Shuttle opera- tions and sets the annual f l ight rate of the f leet. This process begins when the orbiter lands and ends with i ts next l i f t - o f f , Figure 3. After landing at the Kennedy Space Center (KSC), the orbiter is towed from the landing f ie ld into an Orbiter Processing Faci l i ty (OPF) where i t is safed. The residual cryogenic fluids and the payloads airborne support equipment (ASE) are then removed, and refurbishment of the f l i gh t system ini t iated. After refurbishment is completed, the ASE for the next payloads are installed, i f the payloads are to be installed in the OPF; otherwise, the payloads are installed on the pad. The orbiter is then moved to the Vehicle Assembly Building (VAB) to be integrated to the SRB's and External Tank (ET). These two elements are assembled and integrated off l ine in the VAB in parallel with the refurbishment of the orbiter in the OPF. Finally, the integrated Shuttle stack is moved to the launch pad on a mobile launch platform and loaded with propel- lants and launched. Approximately one-quarter of the cost per f l igh t of a Space Shuttle mission is spent on the ground turnaround launch operations.

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Fig. 3 - Orbiter Ground Turnaround Operations

As discussed previously, the degree of reusabil ity of the Shuttle's TPS, landing systems, and SSME have signif icantly contributed to the lengthening of ground turnaround. Operations experience shows that an orbiter can be turned around up to three or four times annually whereas early projections were ten or twelve. This difference exists because several facets of the orbiter design and operations did not evolve as planned. The prime emphasis during the orbiter design was achievement of r e l i ab i l i t y and payload performance capabil ity. Design for operational reservice, maintainability and l i f e cycle cost did not play a major role in design concepts and decisions. As a result, too l i t t l e consideration was given to component accessibil i ty, interchangeability and commonality. On the operations side, in i t i a l projections (1975) of turnaround duration were highly optimistic. They assumed the orbiter would be autonomous with a capabil ity of self checkout both in- f l ight and on the ground. In addition, i t was assumed that there would be no launch constraints for payload launch windows, rain, lightning, upper atmosphere winds and that all landings would be back at the launch site. Moreover, projections assumed that the orbiter 's cargo on each f l igh t would have simple, standardized interfaces necessitating minimal verif ication and checkout and the cargo manifest would not change as the preparations for the mission progressed.

Few of the assumptions on Shuttle operations proved valid. Compounded with the previously noted shortfalls in reusability of some orbiter systems, the actual turnaround duration stretched from weeks into months. The lesson learned here is that maintenance and operations engineering must have a high pr ior i ty in designing future reusable space systems. As in every sytems development program, preliminary and cr i t ica l design review milestones are established from the outset. A similar emphasis and set of milestones for review of the operations aspects of the system should be established. We have learned from orbiter turnaround experience that in order to reduce cycle time and increase the orbiter f leet f l igh t rate capabil ity, the removal, refurbishment, and replacement time for a number of systems must be improved. Designing more rugged TPS t i les in order to withstand debris kicked up when the orbiter lands on the lake bed or when i t encounters rain during aircraf t ferry back to the launch site would fac i l i ta te turnaround; further, a TPS waterproofing technique that does not require reapplication after each f l igh t should be implemented. Improving thermal barriers and gap f i l l e r materials which are composed of various types of fabric and soft insulation would also fac i l i ta te turnaround. Indeed, a simpler thermal sealing system around the t i les , access doors, elevon, and other control areas is needed. For example, the elevon sealing system has more than 3300 parts per orbiter and is very d i f f i cu l t to rework. Accessibility to the SSME's turbopumps for torque testing and inspection without removal of the heat shields at the base of the orbiter would save significant time, but not nearly as much as increasing the mean-time-between replacement of the turbomachinery. In twenty- five Shuttle f l ights involving 150 turbopump

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exposures, 63 have been replaced in turnaround. When opportunities for systems redesign arise, the use of common parts needs to be emphasized. For example, the orbiter electromechanlcal actuation mechanisms ut i l izes seventy-six electr ic motors, but none has a common interface that w o u l d allow the motors to be interchangeable.

The largest single processing time driver affecting turnaround is the nonstandard tasks imposed on the flow. Nonstandard tasks include development and operations engineering modifications to the orbiter that generally are introduced to improve performance, correct design deficiencies, or add special instrumentation for design diagnostics. Also included are tasks to resolve in- f l ight and ground checkout anomalies and special inspections and systems recertif ications. Experience has shown that nonstandard tasks, never anticipated in in i t ia l turnaround time projections, account for more than 50 percent of an orbiter 's turnaround time. In i t ia l turnaround estimates, therefore, should factor in time to allow the engineering community to correct systems weaknesses identif ied during early f l ight experience.

Adverse weather at both launch and landing sites has expanded turnaround. Thunderstorms, cold temperatures, rain, (or the forecast of rain) and upper atmosphere high winds have delayed launches and caused landings planned for KSC to be waved off to land at Edwards Air Force Base (EAFB), California. Weather related delays in turnaround have averaged about one day each mission.

The orbiter cannot f ly through precipitation because the TPS t i les wi l l be damaged, or launched in high electric f ie ld potential because the Shuttle's exhaust plume can trigger lightning to the vehicle. Further, at launch the weather must be acceptable for an Orbiter landing back at the launch site and at each of the transoceanic abort landing sites in Europe and Africa.

A landing diverted from KSC to EAFB because of bad weather or an uncertain forecast adds at least five days to turnaround. I f en route precipitation is encountered during the across- country aircraf t ferry of the orbiter back to KSC, the turnaround delay wil l be longer. To increase the likelihood of landing at KSC and reduce weather related turnaround delays, the short-term weather forecasting (nowcasting) of the dynamic conditions at KSC must be improved both in accuracy and re l i ab i l i t y . S ince the deorbit decision and commit to a KSC landing is made about 90 minutes before touchdown, a high confidence 90-minute "nowcast" is needed. A high confidence "nowcast" that would allow several additional landings at KSC each year could aggregate into an additional annual orbiter turnaround and f l ight .

The orbiter ground turnaround experience for the f i r s t twenty-five f l ights is shown in Figure 4. The f i r s t turnaround, between the f i r s t and second f l igh t (STS-I and STS-2), took nearly 200 workdays (3 shifts per day). As the orbiters matured, the number of in- f l ight component and system failures diminished following each successive mission, Figure 5. As ground processing experience developed, the orbiter turnaround time declined along a steep improvement curve. As new orbiters were

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introduced into the f leet (STS-6, 12, and 21), similar improvements in turnaround were experienced. The aggregate turnaround improvement followed a 75 percent "experience" curve, where "experience" includes a combination of learning, enhanced handling and checkout aids; fewer f l ight and ground checkout problems as systems matured; improved logistics support; and overall standardization, streamlining, and simplification of procedures and processes.

Fig. 4 - Orbiter Ground Turnaround

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Before the Challenger tragedy (STS-25), the Shuttle turnaround was reduced to about 55 workdays. We do not know whether this pace could have been sustained since the missions were becoming increasingly complex and demanding. The post-Challenger projections are now for turnaround i n i t i a l l y to start at about 150 workdays and after 4 years decrease to about 75 workdays, 3 shifts per day. For a four-orbiter f leet, this turnaround duration supports an annual f l ight rate between 12 to 14. To reduce turnaround at this stage in the Shuttle's evolution wil l require improving supplies of orbiter spares and reducing reliance on the original equipment manufacturers by establishing a central repair depot near the turnaround operations to shorten repair cycles. In addition, the ubiquitous, complex and massive

engineering, quality and operations paperwork systems must be standardized and automated on electronic systems. The multiple Shuttle information systems that are now incompatible and unlinked must be unified to fac i l i ta te rapid pedigree searches on all parts and systems. And f ina l ly , operations need to adopt an approach where successful systems operations on the last f l igh t is a sufficient test to verify f l ight readiness for the next f l ight , provided that system has not been disturbed during ground turnaround.

In summary, the orbiter ground turnaround is the f l ight rate pacing constraint in the program. Past experience demonstrates the capability for effective operations learning. In the aftermath of the Challenger accident, turnaround wil l increase i n i t i a l l y due to new processing work content, added safety precautions, and new contingencies for weather delays, irregular launch window opportunities, landings at EAFB as well as allowances for unplanned/nonstandard tasks.

Flight Design

During early planning of the Shuttle f l ight design, a number of assumptions were made. I t was assumed that all ascent and descent f l ight design trajectories and crew procedures would be standardized and that f l ight planning would not require optimization for any mission. Extra- vehicular act iv i t ies (EVA) would be planned only as a contingency and no real-time procedures or work-arounds would be developed to allow continuation of f l ight in the event of on-orbit problems. Finally, i t was assumed that the spacecraft customer required no interactive training with the launch, mission, or f l ight crews and would be fu l ly capable of developing his own detailed mission requirements without aid from the Shuttle program.

Few of these assumptions reflect actual experience. At the outset, too l i t t l e allowance was made for Shuttle weight growth as the design matured. As a result, the Shuttle performance margins decreased to the point where i t was essential to customize nearly every mission to preserve minimum reserves for final orbiter weight and day of launch environmental conditions. In addition, the planned Shuttle f l ight rate was scaled back and ref l ight opportunities were dropped to stay within available budgets, attendant with postponements of some missions. As a result, each remaining f l ight became more precious and was f i l l ed to capacity in order to maximize the science return of the mission. This had repercussions in every facet of f l ight design, mission planning, and crew training. Every aspect of the mission was fine tuned at great manpower cost to maximize performance and results. The f l ight design of one-third of the missions has had EVA operations to support payload objectives, where i n i t i a l l y EVA was to be used only for contingency purposes.

The complexities of carrying a mix of payloads on the Shuttle f l ight design were underestimated. The demand of a mixed cargo to satisfy a multiple and/or conflicting number of cargo requirements drove the manifest. The manifest was frequently revised when all the payloads didn't f i t or the readiness of a payload

was delayed. Often the f l i gh t designs and training products for a single mission would be redesigned three to sixteen times because the cargo manifest was revised that often in attempts to accommodate shif t ing payload pr io r i t ies , readiness, and interactive compatibi l i t ies with companion payloads. From this experience, we learned that the cargo manifest assignments must be stabil ized at least sixteen months before launch and even ear l ier , i f possible, to fac i l i t a te f ina l iz ing the f l i gh t design and to increase the f i de l i t y of f l i gh t control ler and astronaut training. Also, the program must hold back suff icient f l i gh t performance reserves to allow for late changes in propulsion tag values, launch window constraints, implementation of mandatory (safety of f l igh t ) design changes and pr ior i ty payload changes. We learned also that the payload command, control, and communications should not be linked into the orbi ter data systems. Imbedding the cargo data streams into the orbiters required the orbi ter f l igh t and data management software design to be revalidated and reveri f ied with each payload change. To avoid this, future designs should isolate the payload from the orbi ter systems and, wherever possible, minimize and standardize data interfaces.

Payload Integration

As customers became famil iar with the orb i ter 's broad range of capabil i t ies and services available to support their mission objectives, the demand for services expanded. We learned that customers needed extensive support in selecting among the range of services, alternatives, and approaches available to them. In response, NASA signi f icant ly expanded i ts customer support staff. New customers found that the process of integrating their f i r s t payload into the Shuttle was a long, complex, and arduous task compared to integrating a payload on an expendable launch vehicle. Generally, this was because customers were not famil iar with manned space f l igh t safety requirements for ver i f iable redundance and fa i l -operat ional / fa i l -safe system designs. Also, the requirement to consider the potential influence of their payload on the other payloads in the cargo bay was new and the need to plan and train with the mission control and f l i gh t crews for contingency intervention in the event of i n - f l i gh t checkout or deployment problems was new. However, experience shows that after the i n i t i a l payload integration cycle, subsequent cycles through the process were re la t ive ly easy.

Ultimately, the time required for payload integration was reduced from 34 to 24 months. This reduction was achieved by inst i tu t ing a standardized and rigorous payload integration process supported by dedicated, experienced integration specialists. We have learned to reduce the number of integration and safety reviews, to improve the documentation so al l information required to successfully integrate the f l i gh t is captured in one place, and to emphasize early def in i t ion of al l mission requirements.

Training

Experience in training Shuttle mission

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controllers and astronauts crews for f l i gh t operations shows that because most missions were tai lored to maximize the science return, the complexity and level of mission unique training became signi f icant ly higher than or ig inal ly anticipated. However, since the early Shuttle f l ights , the training hours per mission have decreased steadily. The standalone training hours on the Shuttle Mission Simulator (SMS), for example, have been reduced by more than one-half, Figure 6. The SMS integrated training has also decreased. Surprisingly, experience shows that nearly two-thirds of the integrated training in the SMS and in the Mission Control Center is for the benefit of the payload customer--training never anticipated at the outset of the program.

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Fig. 6 - Shuttle Mission Simulator Standalone Training Support

Shuttle astronaut crews have sometimes been assigned to missions without regard for past training. To a large extent this required the entire crew to train as i f al l were inexperienced. Now crews are chosen, when possible, to match training to the f l i gh t manifest. Previously, the training teams were assigned blocks of training time and took whatever crew was scheduled. Now training teams are assigned crews when the crew begins training. This provides more consistent training and fosters a strong feeling of personal involvement among team members. Experience also shows that much of the crew training can be accomplished on part task trainers. This provides for more hands-on experience without involving ful l -up simulations and has proven to be very effective. Part task training now is being extended to ascent/entry trainers and payloads. Experience in training Shuttle commanders and pi lots to land the orbi ter has shown that the Shuttle Training Aircraft (STA), a modified Gulfstream I I , provides high f i de l i t y simulations of an orbi ter landing and is unmatched by other simulators. On an average, the Shuttle commander, who controls the orbiter during landing, practices over 700 STA landings before his f i r s t actual orbi ter landing; the Shuttle p i lo t , in the r ight seat next to the commander, experiences over 450 STA landings, Figure 7. Shuttle commanders and pi lots believe that continued training at these levels is essential to safe, rel iable landing performance. The overall training demands on astronauts have

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expanded as the requirement for customizing missions expanded. Experience now shows that a commander or p i lo t wi l l f ly only about two missions annually while mission specialists who are trained to conduct unique experiments aboard wi l l average only about one mission annually.

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In summary, the operations experience in f l igh t design, payload integration, and training has provided the insight and basis to reduce operations support, training, and payload integration ef fort . The NASA direct operations support for each mission which includes f l igh t planning, payload integration support, systems support, f l i gh t training, and mission operations, has been reduced by about two-thirds compared to early STS f l ights, Figure 8. The costs of these tasks represent about 22 percent of the overall Space Shuttle cost per f l igh t . Further, improvements in these areas wi l l come about principal ly by stabi l iz ing the Shuttle manifest, setting aside suff icient performance reserves to preclude changing the f l i gh t design and orbiter f l i gh t software when there are late changes and by developing families of prepackaged standardized mission segments that can be assembled to encompass most customer requirements.

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