Debunking the Manrating Myth

[Me2]

NASA documented their human rating requirements in NPR 8705.2A “Human-Rating Requirements for Space Systems” was released on 10/15/04. This was the version that was used during ESAS. NASA updated the document to 8705.2B on 5/6/08, which is the version being used by Constellation.

There were a number of changes between the two documents. Most changes eliminated specific, verifiable requirements, opting instead for more of a philosophical approach to human rating. Here are two primary and often-used examples where NASA Human Rating requirements were significantly changed between 8705.2A and 8705.2B.

1.4 Factor of Safety:
NPR 8705.2A originally included "NASA Standard 5001, Structural Design and Test Factors of Safety for Spaceflight Hardware" as an Applicable Document. In this document you can discover the 1.4 FS requirement in paragraph 5.1 "Design and Test Factors of Safety."

NPR 8705.2B no longer includes NASA Standard 5001 as an Applicable Document. No other requirement specifies or refers to any similar design and test factor of safety.

The 1.4 Factor of Safety Requirement was originally included in 8705.2A when NASA wanted a reason to exclude EELVs from the ESAS Trade. The requirement was removed in 8705.2B because Ares couldn't meet it.

Fault Tolerance:
8705.2A Paragraph 3.1.1 stated "Space systems shall be designed so that no two failures result in crew or passenger fatality or permanent disability."

This was commonly understood as the dual fault tolerance requirement. It was succinctly written in such a manner as to be verifiable.

8705.2B Paragraph 3.2.2 states: “The space system shall provide failure tolerance to catastrophic events (minimum of one failure tolerant), with the specific level of failure tolerance (one, two or more) and implementation (similar or dissimilar redundancy) derived from an integrated design and safety analysis (per the requirement in paragraph 2.3.7.1)."

Paragraph 2.3.7.1 states “The Program Manager shall perform an integrated safety and design analysis to determine the following:

a. The requirements for additional levels of failure tolerance (above the minimum of 1 failure tolerant per 3.2.2) for the space system.

b. The appropriate implementation of failure tolerance for the space system, to include an evaluation of dissimilar redundancy and backup systems.”

This means that the Program Manager develops a process and determines what the proper level of Fault Tolerance needs to be for a specific application. The PM could determine that it's single, dual or triple redundant. But clearly it's no longer two fault tolerant.

Once again, NASA originally included a dual-fault tolerant requirement (8705.2A) for ESAS to exclude EELVs. When it was clear that Ares/Orion could not meet the original dual fault tolerance requirement, NASA changed it (8705.2B) to allow the PM to determine the proper level of fault tolerance.

8705.2A paragraph 3.7.3 that states “The system shall provide the capability for the crew to select between independent flight control processing systems during flight”?

The backup flight software requirement is gone in 8705.2B.

http://nodis3.gsfc.nasa.gov/displayD...PR&c=8705&s=2B

[Andrew]

That is MOST interesting! Thanks!

-Andrew