Acta Astronautica Vol. 25, No. 5/6, pp. 301-307, 1991 0094-5765/91 $3.00 + 0.00 Printed in Great Britain Pergamon Press plc

THE CASE FOR THE EVOLUTION OF THE SHUTTLE SYSTEMt

AARON COHEN

NASA Johnson Space Center, Houston, TX 77058, U.S.A.

(Received 29 June 1990; received for publication 7 February 1991)

Abstract--Examining the expendable and reusable space transportation systems currently in use, it appears that the service life of a launch system, with periodic upgrades, can be 30~0 years. It is also evident that both reliability and performance increase with flight experience. With a life expectancy of at least 30 years, the Shuttle system is today only in the first third of its potential service life. This paper addresses the issues of why Shuttle evolution is warranted, how the evolutionary process is managed, and how that process can improve system performance and reduce costs. Specific examples of enhancements currently under consideration are presented and discussed in terms of their relationship to the achievement of greater inherent reliability in the Shuttle system.

1. INTRODUCTION

The primary issue in the operation of space transpor- tation systems is reliability. Operations costs are important, but the cost of failure is more significant. This is particularly true in partially reusable systems such as the Shuttle and the one-of-a-kind high value spacecraft that characterize its payloads, such as Galileo, the Hubble Space Telescope, and elements of the Space Station Freedom.

Figure 1 provides an instructive way of looking at the significance of reliability to cost. The figure relates expected life cycle cost to the launch reliability of the transportation system. From the beginning, the success of criterion for the Shuttle has been a 0.995 probability of not losing the orbiter or its payload. At this level of reliability, if a family of 200 Shuttle missions is flown, the loss of one orbiter is "expected", with a direct operations costs for all 200 missions. If the success criterion were set at a lower reliability value, the "expected", orbiter and payload losses would be greater.

A value of $28,000 U.S. 89/kg may seem high, but it is representative of the cost of replacing an orbiter and one of its payloads, considering both out-of- pocket costs and program delays. The figure illus- trates that with launch success reliability at lower values, the expected life cycle cost of any given program can be estimated to be two or more times as large as the specified value.

Reusable systems are generally more reliable than expendable systems because the operator has the opportunity to learn from experience with the system, test it in ways that are not available with expendable systems, fix the components or subsystems that are

tPaper IAF-89-200 presented at the 40th Congress of the International Astronautical Federation, Malaga, Spain, 7 13 October 1989.

troublesome or lack adequate margin, and exploit unexpected capability in robust subsystems. Some comparable' learning takes place with the use of expendable systems, but the opportunity to learn from success is not as great because systems are not recovered and cannot be assessed after flight. Also, the incentive to enhance reliability is greater in par- tially or fully reusable systems because a significant transportation asset is at risk with each launch.

Examining the systems currently in use and fore- cast to be operating for the rest of this century, it is clear that the bulk of the launches will be made by systems that have been in service for 20 years or longer, i.e. Delta, Titan, Atlas, SL 4, Proton, and others. Figure 2 illustrates the life cycle history of three of the systems now in use. A noteworthy aspect of the two most commonly used U.S. launch systems is that both payload delivery and reliability have improved as the systems have matured. It would appear that the service life of a launch system, with periodic upgrades, can be 30-40 years and that both reliability and performance increase with flight experience. This history suggests that each system has a far longer operational life than its custodians are generally allowed to advocate at any early point in its life cycle.

2. SPACE SHUTTLE AVAILABILITY AND RELIABILITY

If the Shuttle is to be in service for 30 years, it is today only in the first third of its potential service life. Therefore, investment in enhancing reliability and performance is warranted since there is time to exploit the benefits of such enhancements. Con- versely, such enhancements are required to reduce the cost of ownership because many of the systems used in the development of the Shuttle are increasingly difficult to maintain as they obsolesce. In selecting the technology base for enhancements to the Shuttle, as

301

302 AARON COHEN

5 -

~o 4 -

~ 3 0~

Cos t o f Failure Considerations

Life Cycle Cost = Direct Op~rations Cost + Cost of Failure (asset loss/replacement) for any given

I I I I I .95 .96 .97 .98 .99 .995 1.0

System Reliability Fig. 1. The value of the assets at risk and system reliability

are the major determinants of cost.

in selecting the technology base for its original devel- opment, it is important to choose systems that are mature enough to be brought to flight qualification readily and that will be used widely enough to provide an economic supplier base, For a reusable system such as the Shuttle, there is an additional need: that ground processing systems for both hard- ware and software receive attention equal to that given to flight systems.

What is needed for the foreseeable future is assured Shuttle availability. To achieve this assured avail- ability, two significant conceptual elements are necessary that are not required in a maintenance/ enhancement program. They are:

(1) More basic assets than the minimum necessary to meet current flight commitments. Such "nomi- nally" surplus assets are needed to allow units to be taken "off line" for modification and upgrad- ing and to cope with attrition.

(2) A definition of future requirements beyond those specified for the basic system or beyond the de facto capability.

This set of requirements is generated by demands that were beyond the planning horizon during the

development of the Shuttle system either in concept or in our ability to define detailed attributes of those requirements so that they could be translated into specific subsystem requirements.

3. MANAGING THE EVOLUTIONARY PROCESS

To make Shuttle evolution a manageable process, we have established specific procedures for manage- ment review and defined set goals for assured Shuttle availability. These goals are:

(1) Improved safety/reliability. (2) Reduced cost--including obsolescence and cost

of maintenance. (3) Increased capability.

Obviously these objectives are not mutually exclu- sive. Any one action can impact all three priority considerations. The goals are listed here, however, in terms of the respective weight of importance allo- cated to each. They are highlighted further in Fig. 3.

The management review process we have estab- lished tests not only the absolute merits of each proposed enhancement to the Shuttle, but determines the relative contribution of this change to the overall objectives of the program. The process would have been costly and cumbersome if new procedures and personnel were added to normal change process management; however, by using the existing struc- ture, cost and effort are minimized. Also, to preclude resistance to change, the personnel in place are enlisted as the agents of change.

Figure 4 illustrates the assessment methods used to formulate and assess proposed enhancements to the Shuttle. These methods are essentially the same pro- cedures used by any program change control board but are broader in scope in terms of both content and time than has historically been the case.

In every instance we have studied, actions taken to improve inherent reliability have a/so improved per- formance and reduced costs. It is reasonable that this

Payload 6,600 Ib 40,000 Ib

I Learning / Modernize - Titan 34D / Titan IV ,~35 + Years

Titan II, Ill, IV >2000 1960

Payload 1 O0 Ib 11,000 Ib

[ Learning / Modernize - Delta 3000 / Delta II ,~35 + Years

Delta A-N, 2000, 3000, >2000 1962

Payload 50,000 Ib >100,000 Ib?

I -m,o0 ./..222L;i: io2. i i;2 22222222222 2222222222 , Shuttle >2020 1981

Fig, 2. Life cycle history of current space transportation systems.

Shuttle system 303

Relleblllty/Satsty • Reduces critl failure

nodes • Enhance crew escape capability

• Reduce hazardous turnaround OPS

• Increase margins/ robustness

Lower Recurring Cost Increased Capability • Vehicle processing • Launch probability

improvements • Flight rate/surge • Streamline flight • Enhance on-orbit

operations capability-stay time • Simplify payload • Crew transport

integration • WTR • Reduce sustaining • Cryogenic/storable engineering propellants

• counter obsolescence • Resiliency ~ = • Up/down payload

Priority

Fig. 3. Goals for assured Shuttle availability.

should be so because real reliability is a function of how well we understand the components and pro- cesses with which we work. Inherent reliability is margin in system design and a thorough understand- ing of the design. Redundancy is what we resort to when we do not have the capability to achieve desired reliability because our fundamental understanding is inadequate or our ability to control process variance is incomplete.

4. SPACE SHUTTLE EVOLUTION

What follows is a review of the options for Shuttle evolution that are currently under study. Also dis- cussed is the relationship of each of these proposed

enhancements to future program needs and to the achievement of greater inherent reliability in the Shuttle system.

Shuttle enhancement for reliability and per- formance can take many forms. Better-than-expected performance by a subsystem can be employed to simplify or eliminate components and failure modes. For example, we are considering an integrated aft Reaction Control Subsystem-Orbital Maneuvering Subsystem (RCS-OMS) tankage system. Such a sys- tem would exploit the successful experience with the 0-g propellant acquisition and retention system in the OMS tanks. Figure 5 illustrates the current OMS pod. In its current configuration, the RCS tankage uses complex propellant acquisition screens at both

Current STS Capability I ,~ • Performance I ~ 1 ~ • Fleet size/flight rate I ~ J • Recurring cost I v

"Lesson Learned"

KSC Ground Efficiencies Studies

~ ~ o u s Enhancement Studies

,vau=ons Sased On I l Seecte,.gh P.o./ I Assured I • Long-term ~ [Options ~ ! ~ 1 Shuttl e

requirements/ • Block changes I F ~ . L ~ 1 Availability goals ~ [ "Operat,ons . W I Strategies

• Cost/benefitSassessments I I ennancemems I I

Candidate Enhancements • Intergratod orbit

maneuvering system • Glass cockpit • Electromechanical

activators - high energy fuel cell

Future Programs Studies

Advanced Launch System Studies

Advanced Manned Launch System Studies

I Advanced Development I Recommendations

Lunar Base Studies

Fig. 4, Assessment methodology.

AA25/5/~-E

304 AARON COHEN

Fig. 5. The Shuttle OMS pod.

the forward and aft ends of the tank. The OMS propellant acquisition and retention system, shown schematically in Fig. 6, has performed so effectively in flight tests as to demonstrate that the orbiter cannot generate forces that displace propellants from within the acquisition system. Because we are now reasonably assured of a gas-free supply of propellants from the OMS tanks for the RCS, the RCS tankage

and the related valves and pressurization components can be removed.

Figure 7 illustrates schematically the configuration changes and Table 1 shows the reduction in part count that can be achieved with such a change. There are many benefits to eliminating so many components in this manner. The system is simpler and more reliable. It is lighter and easier to maintain and repair.

JVD

Gal (4 I:

discharge port Fig. 6. OMS propellant acquisition system.

Shuttle system 305

[]Deleted []Added

Fig. 7. Simplified APS schematic (left pod).

Cross leed

This enhancement is feasible only because of success- ful flight experience and developmental flight testing. We did not know enough about the flight perform- ance of this type of screen retention device to commit to such a design initially.

Further, early in the design phase of the program, we did not understand all the derivative require- ments; these could only be poorly defined. This is best illustrated by all the requirements generated by alternative abort scenarios. As these abort scenarios become more complex and subtle in order to cover all the performance regimes between Return to Launch Site (RTLS) and Abor t Once Around (AOA), or all possible Transatlantic landing (TAL) situations, the " t rue" demands on the system become known and quantified. Now, we can assess change to the system in the context of a comprehensive understanding of the requirements. This mature understanding of requirements and capability allows us to make the system simpler and more effective.

Another form of Shuttle evolution that can occur results f rom modifying subsystems to use new tech- nologies that both improve performance and reduce

the cost and effort of maintenance. For example, replacing the redesigned solid rocket boosters with the Advanced Solid Rocket Moto r boosters will provide the option to increase the lift capability of the Shuttle system and provide a new design that is less sensitive to critical process control variables.

This kind of evolutionary enhancement is also illustrated by the effort underway to convert the electromechanical displays now used in the orbiter cockpit to the electronic displays utilized in modern aircraft. The term "glass cockpit" is being used to characterize this major change to the crew compart- ment displays and controls.

Figure 8 shows the orbiter main display and con- trol console. It is characteristic of systems designed in the early 1970s. There are numerous electromechani- cal displays-- the flight director attitude indicator and the tape drives are the most prominent. The cathode ray tubes are monochrome data displays.

In the past 20 years there have been significant advances in the development of electronic devices for displaying information. Significant advances have also been made in the development of information

Table 1. Integrated OMS pod evaluation considerations Current pod Integrated pod

Failures* part count part count

Helium tank 12 6 2 Helium isolation valve 80 12 4 Helium regulator 76 12 4 Helium check valve 70 10 6 Relief valve/burst disc 31 l0 6 Propellant tank manual values 26 10 6 Disconnects 174 182 125 RCS propellant tanks - - 4 0 RCS sump tanks - - 0 4 *All failures to date including qualification, acceptance, test and flight. Elimination of 16 critical item list components. Reduction of 40 criticality 1 components.

Reduction in weight of 350 lb. Reduction in ground support equipment and in propellant loading time.

306 AARON COHEN

Fig. 8. The electronic display and control console in the orbiter cockpit.

processing equipment that uses the new electronic devices to generate integrated displays of infor- mation. This technology has contributed significantly to enhanced crew productivity and flight safety in commercial aircraft operations.

The display systems used in the orbiter are showing evidence of wear and will become increasingly costly to maintain. Because all the information is processed through the orbiter General Purpose Computers (GPCs) it is feasible to change out the display devices and preserve the data interface at the GPC data buss.

The "glass cockpit" currently under study is shown in Fig. 9. It uses a single active matrix liquid crystal display replicated nine times. On these devices current display formats can be preserved, or they can be modified to include color and enhanced graphics capabilities. Because any display device can carry any set of displays, the system is more tolerant of display device failure, and because the displays proximate to each crewmember can be configured to his or her immediate mission phase needs, the crewmember is more productive.

Figure 10 lists a number of the considerations in selecting a change such as this one. The primary incentive is to reduce costs, but in doing so, the new technology improves reliability, safety, and pro- ductivity. A phased program is required for an enhancement which involves so many components. As a result, this change will be installed in the mid 1990s during scheduled block modification and periodic maintenance downtime on each orbiter.

Fig. 9. The electronic display and control console (glass cockpit) currently being considered for use in the Shuttle.

Display Upgrade Program

Improve system fault tolerance ]

Provide reversion capability for display unit failures ]

Be transparent to GPC hardware and software I

Have Space Station compatibility I

Allow expanded capability for additional functions I

Offload display processing from GPC's I

Include an electronic flight data file I

Reduce maintenance and spare cost I

Fig. 10. Considerations for upgrading the Shuttle display systems.

The Office of Technology Assessment (OTA) of the U.S. Congress has recently issued its report "Round Trip to Orbit". In this document, OTA assesses what is required for assured Shuttle availability and con- cludes that continued production of orbiter vehicles is required to enhance fleet size, allow systematic maintenance and modification, assure meeting mani- fest capacity and schedule demands, and to cope with attrition.

The production of configurations capable of un- manned operation for simple missions with recovery of the orbiter and expendable configurations which replace the orbiter airframe and crew compartment with an aerodynamic shroud needs to be assessed against long-term commitments. The most immediate need, however, is to extend the basic fleet capability of the Shuttle program. NASA has proposed an initiation of OV-106 on an appropriate schedule to follow OV-105, Endeavour, the replacement for Challenger.

5. SPACE TRANSPORTATION NEEDS FOR THE NEXT CENTURY

We are still early in the activity of assessing how manned space transportation needs of the next century should be met. There is a case to be made by historical analogy and by rational analysis that incremental improvements to current systems can be both efficient and effective.

Our preliminary studies in support of a lunar base and planetary exploration indicate the need for a substantial heavy lift capability. Depending upon the development strategy elected, there are several poten- tial strategies for meeting this requirement. In such an enterprise, it is important to note that over half of the mass required in low Earth orbit is propellant while the other half is high value spacecraft and personnel. It may be prudent to use Shuttle-C, with Space

Shuttle system 307

Transportation System reliability, for the high value elements but some less costly alternative for the bulk-propellants whose value derives from being de- livered to orbit.

Launch systems are among the most complex and demanding systems ever built. A "new" system may eventually fulfill its promises of "better" and "cheaper", but the record shows that achieving such goals is difficult. The early Ariane experience is not unlike the early Delta and Atlas experience. In the long term, Ariane promises to be a good system, but it too is evolving as a result of gained experience.

World-wide experience in aeronautics has shown that incremental improvement is an effective path to safer, more efficient, more productive systems. It is

time to embody such a strategy in the formulation of our future space program plans.

REFERENCES

1. C. Teixeira and C. Mallini, A Shuttle evolution strategy. 27th Aerospace Sciences Meeting, Reno, Nev. (1989).

2. Office of Technology Assessment, Round trip to orbit: human spaceflight alternative~Speeial Report, OTA- ISC-419. Congress of the United States, U.S. Govern- ment Printing Office, Washington, D.C. (1989).

3. D. Branscome, United States space transportation survey. Second European Aerospace Conference, Bonn Bad-Godesberg (1989).

4. C. Teixeira, U.S. Space Shuttle evolution. Second Euro- pean Aerospace Conference, Bonn Bad-Godesberg (1989).