National Aeronautics And Space Administration Nasa Perspective

NASA defines an architecture as [9.2]:

How functions are grouped together and interact with each other. Applies to the mission and to both inter- and intra-system, segment, element, and subsystem.

In their mission design activities, NASA emphasizes the following:

• Requirements

• Decision making and evaluation criteria

• Optimization and descope options

• Robustness and flexibility

• Cost and schedule as a trade parameter

• Developing the cost plan and schedule

• Risk assessment and mitigation

• Establishing design margins

• Analyses and trade studies

• Technical performance measurement

We will find some of these themes reiterated as we look at the formal process of system architecting.

As indicated in earlier chapters, NASA looks at mission design in terms of three early phases:

1. The conceptual design process: Prephase A

2. The mission analysis process: Phase A

3. The definition process: Phase B

If we examine closely the specifics for these phases, we see the following purpose for Phase A:

The purpose of the Phase A study is to refine the mission and systems(s) requirements, determine a baseline mission configuration and system architecture, identify risks and risk mitigation strategies, identify the ''best'' candidates, and select one.

Thus, it is clear that systems architecting and selection of a preferred architecture are carried out during Phase A. With respect to the Phase A study final report, NASA requires that the topics listed in Exhibit 9.1 be covered.

Exhibit 9.1: NASA Phase A Study Document Topics

• Technology needs and development plan

• Refined and validated mission requirements

• Final feasibility assessment

• Disposal requirements

• Functional/operational description

• Hardware/software distribution

• Design requirements

• Definition of top-level interfaces and responsibilities

• System/subsystem description

• Mission description

• Data handling requirements

• Launch vehicle requirements

• Mission operations

• Preliminary work breakdown structure

• Refined cost estimates and schedules

• Establishment of accountability for delivery of an end item and its performance

• Apportionment of technical resources, the distribution of margins, and allocation error budgets

• System-level block diagram, flight, and ground

• Maintenance and logistics requirements

• Top-level system architecture

This documentation must provide answers to the following types of questions:

1. Does the conceptual design meet the overall mission objectives?

2. Is the design technically feasible?

3. Is the level of risk acceptable?

4. Are the schedule and costs within the specified limits?

5. Do the study results show this option to be better than all others?

NASA is explicit regarding the use of ''appropriate and predetermined weighted and prioritized evaluation criteria to select the best of the candidate designs that were evaluated.'' Weighted evaluation criteria for this purpose come up again later in this chapter as part of the recommended system ar-chitecting process.

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