NASA Office of Logic Design

NASA Office of Logic Design

A scientific study of the problems of digital engineering for space flight systems,
with a view to their practical solution.


4. RECOMMENDED PRACTICES

The design of a spaceborne digital computer entails a series of decisions involving interacting disciplines including stabilization and control, guidance and navigation, power, data processing, display, telemetry, and testing. A satisfactory configuration can best be achieved if consideration is given to all interacting aspects of design throughout the design process, so that the merits of various options may be assessed and their full impact on all systems identified. Therefore, it is recommended that personnel from the computer design group and all concerned groups work closely together throughout development and participate in all tradeoff decisions.

The experience, inventiveness, and general ability of a designer can greatly influence the number of failure-prone components, close tolerance items, interconnections, power requirements, and general simplification for a given system. However, to specify design methodology presupposes a priori knowledge and may result in unduly restricting a highly qualified designer. The difficulties in specifying how a system is to be designed, or who is to accomplish the effort, have led to a general practice of accepting the resultant design and restricting reliability discussion to the areas of component testing, environmental system testing, and mean time to failure statements. Notable exceptions have been made by the NASA Flight Research Center (FRC) and the NASA Lewis Research Center (LeRC) during recent procurements wherein, noting that a better grade of designers were on proposal staffs, they made it a contract requirement that these, same engineers participate in the actual design. Such a practice is recommended in future procurements.

The mission requirements should be established as soon as possible in the program so that computer functions, operations, and architecture can be identified. The consideration of the computer as a part of the overall system is the first step in formulating the computer requirements and is the primary factor in most design decisions. Care should be taken to insure that the computer system requirements are as liberal as possible since specifying tight requirements or choosing the lowest priced computer that barely meets the requirements usually results in a more costly system over the range of the program (ref. 82). Design freedom should be encouraged by not over specifying computer requirements, particularly in the areas of technology or architectural design. Standardization should be sought wherever possible - especially on interfaces, languages, and sample problems - in order to facilitate checkout, repair, and evaluation. To insure a reasonable performance match, it is important to specify the software operating system in some detail before "freezing" the hardware design.

If mission requirements are not firmly established, as is typical of long-range programs, the computer design should be approached with flexibility and foresight. The designer should anticipate the potential growth in use of the computer by carefully reviewing previous programs, particularly those which are similar to the present program (e.g., table 1 in section 2). The use of reference 83 is encouraged where more specific guidelines are unavailable.

The designer should consider the use of advanced technology to meet performance objectives or to reduce cost, especially if the requirements are difficult to fulfill within state-of-the-art technology and where the development program is long. The selection of advanced technology should be based on as much developmental testing as is currently available. If possible, its use should be restricted to minimize potential harm in the event of failure. Where a high design risk is involved, it is recommended that a competitive development be initiated, utilizing more proven technologies but converging on one design as soon as possible.


Home - NASA Office of Logic Design
Last Revised: February 03, 2010
Digital Engineering Institute
Web Grunt: Richard Katz
NACA Seal