Allocate adequate technical and programmatic margins or you will pay for it later.
Principle VI: Project Success is Determined by Life Cycle Considerations
This principle recognizes the importance of life cycle issues—its effect on design choices available, the large role that concept selection plays, recognition of how requirements have far-reaching, sometimes unintended consequences, and the need to achieve the best design for the entire life cycle.
Four lessons will be discussed:
14. The Design Space is Constrained Based on Where You Are in the Life Cycle 15. Concept Selection and Design Process
16. Requirements Drive the Design 17. Design for the –ilities and Cost
Lesson 14: The Design Space is Constrained Based on Where You Are in the Life Cycle
Three Life-Cycle Categories Conceptual/Initial Design
Open design space for choices and trades
Detail Design
Design space constrained by concept choice Verification/Operations
Design space constrained by the as-built system
Must make system work. Solution options limited
As we proceed through the system‘s life cycle, the range of choices available to affect the design and its operation become more constrained. Consider three timeframes within the life cycle: (1) at Conceptual/Initial Design, (2) at Detail Design, and (3) at
Verification/Operations. At Conceptual/Initial Design, the design space is open, allowing essentially free choice of design variables to meet the mission requirements. By the time of Detail Design, the concept has been selected and preliminary design decisions have been made. This severely limits the freedom of the designer to choose variables. The majority of the system performance and cost attributes are determined by the concept that has been selected, as will be addressed in Lesson 15. Once the system has been manufactured, at the time frame of Verification or Operations, the design space is constrained by the as-built system. Unless there is to be a major redesign cycle, the system must then be made to work by minor adjustments or by operational constraints. Solution options are very limited at this point. It is clear that early choices have a major constraining effect on the range of
downstream choices that can be made.
Examples:
International Space Station Rack Design
Replacement of SSME Baseline Turbopumps with Alternate Design Turbopumps
International Space Station Rack Design
In designing the International Space Station (ISS) science racks (Figure 14-1), four issues were paramount. (1) The racks needed to be lightweight. (2) Flexibility in mounting and operating various experiments was desired. (3) Because some of the experiments would need a low-gravity environment, the presence of crew motion and other disturbances meant that a vibration-isolation system should be incorporated into the racks. (4) The racks had to be sufficiently stiff to meet minimum frequency requirements as a Shuttle payload, to avoid dynamics and loads problems during launch. [Bookout, 1996]
Four issues were paramount in the design of the ISS Science Racks - Weight reduction
- Experiment operations and mounting flexibility in rack - Vibration isolation at zero g - Meeting Shuttle frequency
requirements
Figure 14-1. ISS Science Rack Design
Initial consideration of the design requirements and constraints led to a choice of composite material as a means of saving weight and achieving acceptable stiffness;
however, when all the constraints, requirements, and accommodations were met, the weight saving of composites as compared to aluminum was not realized. It would have been simpler and less expensive to have gone with metal construction from the start.
Also, design of the active vibration isolation system was difficult because of geometric constraints in addition to the other constraints and requirements listed above. It is clear that constraints create unwanted compromises of a design.
Another aspect of ISS rack design related to the need to use the racks for many different functions. A number of them are illustrated in Figure 14-2. Granting the flexibility of a single design to accomplish all these functions created design and verification problems.
However, in general, the advantage of commonality outweighed the design issues.
Figure 14-2. ISS Rack Functions and Configurations
The Space Shuttle Orbiter configuration is another example of this issue. The Shuttle was developed to a multi-agency set of requirements from both NASA and the Department of Defense. Military requirements dictated larger payloads and greater reentry cross-range than did NASA‘s requirements. DoD required a 1500 nautical mile cross-range to enable landing at the launch site after a once-around delivery orbit. NASA‘s cross-range requirement was less, being dictated by abort considerations. There were two competing Orbiter
configurations: a delta-wing design needed to produce sufficient hypersonic lift/drag ratio to achieve the DoD cross-range requirement, and a simpler straight-wing design that could satisfy the NASA requirement. Because the vehicle design had to envelope both
requirements, a delta wing was chosen.
Obviously, there is no way of determining how a matured straight wing design would have performed, although predictions at the time indicated it would have a simpler thermal protection system and a less-demanding landing system. Because of subsequent
developments, the DoD never made use of the Orbiter‘s cross-range capability, but the Shuttle program has continued to pay the operational costs associated with the delta wing configuration and its thermal protection system.
Flexibility and commonality are desirable goals, but they usually constrain the design space and can create design issues and challenges. Early in the project, make sure that commonality requirements are indeed firm, and that their downstream implications are understood.
Replacement of SSME Baseline Turbopumps with Alternate Design Turbopumps The replacement of the original Rocketdyne turbopumps by the Pratt and Whitney alternate turbopumps (Figure 14-3) drew on lessons learned from the original pumps to improve reliability and maintainability. The weight allowance for the alternate pumps was increased over that for the original pumps. The increased weight allowance accommodated
significant improvements in turbopump design and reliability; however, the alternate pump designs had to interface with the existing engine system, including the powerhead and ducts.
Geometric, pressure, and thermal interfaces had to be matched while meeting weight requirements. Because of these constraints, the alternate pumps couldn‘t make full use of the lessons learned and produce a fully optimized design.
Figure 14-3. Alternate High Pressure Turbopumps
In general, interfacing constraints required to use a new component in an existing system lead to a non-optimal design and higher costs.
So, the lesson is to recognize life cycle effects on the design space. As we progress through the design process, fidelity increases and uncertainty decreases, but the design choices decrease. Minimize constraints to increase design opportunities.
A key message from Lesson 14 is:
Recognize Life Cycle Effects on the Design Space - Fidelity increases
- Uncertainty reduces but
- Design choices decrease
Minimize Constraints to Increase Design Opportunities.