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Risk in Conceptual Design Trades: A Crew Exploration Vehicle (CEV) Propulsion System Example (PSAM-0475)

Excerpt

No matter how much science and logic might be involved in its basis, design is and always will be, primarily a creative process. It is initiated by creative conceptualization on the part of the designers. The designer envisions, or more accurately “previsions”, concepts that he or she believes have the potential for satisfying the design requirements implied by the stated design objectives, which in turn are motivated by the needs of the ultimate user.

This set of design requirements must be met by any suggested design alternative. In this way the design requirements play the same role in the engineered design process as the role the medium chosen plays in the purely artistic design process. That is the requirements set separates the range of design concepts that are viable from those which are not. In the cases where the requirements can be satisfied by existing designs, the choice among “off-the-shelf” designs is determined by considering the performance provided for the cost expended.

Proposed developmental designs should be evaluated on a similar basis, but here the question of viability involves consideration of the risk of development as well. Specifically the benefits provided by the promised improved performance or reduced cost of developmental designs must be “discounted”, in the economic sense of the term, by the probability that it will be developed and deployed according to the required deployment date. The more creative the design, that is the scantier its heritage and the earlier the phase of its development, the riskier the development and therefore the more uncertain the promises of the design.

In the course of the Exploration Systems Architecture Study (ESAS) the designers were faced with many trades between the promises of new designs and greater uncertainty in their fulfillment and the lower performance but greater certainty of delivery in extant designs. One area of particular and continuing interest is the trade related to the choice of the service propulsion system for the CEV. The CEV is intended to be the first new manned vehicle developed for the exploration architecture, and yet it will likely be the US exploration vehicle for the next several decades. It must play an early role as a replacement for the space shuttle for near term International Space Station (ISS) missions, and yet be viable for longer-term missions to the moon and Mars.

The choice of propulsion system on the CEV allows for the opportunity to allow the near term ISS missions, where alternative modes for robust return are available, to buy down the risk for higher performance lunar propulsion systems, such as the ascent stage of the lunar lander where no such alternatives are available. Further the choice of fuel type with extensibility to in situ resource utilization on the moon and onto Mars offers valuable experience early on to allow for potentially significant future benefits. On the other hand, the use of heritage based systems from the Apollo and shuttle era offer significant early reliability benefits with little uncertainty.

In such a trade space environment it becomes essential that the decision maker understand fully the risks due to uncertainty, and the potential costs and benefits of buying down these uncertainties.

This paper discusses the process used in the ESAS study and in subsequent analyses to address these important risk issues in the development of a critical conceptual design.

  • Abstract
  • The Exploration Systems Architecture Study
  • Crew Exploration Vehicle (CEV)
  • Individual Trade Issues
  • ESAS Approach and Results
  • ESAS Insights
  • Conclusions
  • Reference

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