NASA Procedures and Guidelines
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NPR 8705.2 Eff. Date: June 19, 2003 Cancellation Date: February 07, 2005	Human-Rating Requirements and Guidelines for Space Flight Systems w/Change 2 (6/25/04)
| TOC | Change | Preface | Chapter1 | Chapter2 | Chapter3 | AppendixA | AppendixB | AppendixC | AppendixD | AppendixE | ALL |

Appendix D: Implementation of Reliability, Abort, and Escape Requirements

D.1 Earth-to-Orbit (ETO) Space Flight Systems

The nature of the ETO space flight system exposes it to the risk of numerous missions over its lifetime but permits the use of aborts and crew escape systems to increase the probability of crew survival. Experience has shown that for the foreseeable future the reliability of the main propulsion system and other critical systems will limit the overall reliability of the ETO space flight systems. Although benign (contained) failures of engines and other systems during ascent can be dealt with through the use of abort modes, the relatively high probability of catastrophic failure of the main propulsion system utilizing current and expected near-term technology requires the inclusion of a safe and effective crew escape system.

D.2 Beyond Earth Orbit (BEO)

D.2.1 BEO missions require unique abort and survival modes. Missions designed for BEO require sufficient power, consumables, and trajectory design to maximize abort capabilities to ensure crew survivability. These abort modes include, but are not limited to, powered return, free return, pre-positioning capabilities, and safe haven. In general, this mission profile requires the space flight systems and its propulsion system to have sufficient propellant to fly off-nominal trajectories. Critical systems should also be designed so that failures do not result in a catastrophic event. The design should provide time for other systems or the crew to recover from a critical system failure. As a last resort, when abort modes are not feasible, a safe haven capability should be provided to ensure that survival capability and consumables exist to return the crew to a position from which a normal recovery or rescue can be conducted. Consideration should be given to pre-positioning consumables, spare parts, and other critical logistics and services to improve abort and safe haven capabilities.

D.2.2 The BEO mission must meet a high probability of safe crew return over the life of the program. However, the higher mission complexity and length is offset by the fact that there may be only a few missions conducted at that level of technical and safety risk. As experience with the mission grows and the possibility of establishing a permanent outpost or colony arises, the reliability goal for each individual mission must rise to account for the increased flight rate and consequent exposure. Autonomy, functional redundancy, and tools to deal with the unexpected are a critical part of the design for safety. Technology will likely pace the schedule for accomplishing this.

D.3 Crew Rescue

D.3.1 The crew rescue mission achieves its reliability through appropriate system design for reliability, simplicity of hardware, and failure tolerance. Flight experience has shown that it is likely to be used at least once during the life of an SS program, most likely due to a medical contingency. Since it may be attached to the SS for extended periods of time and is essential to the SS mission, it should be designed for operational availability on demand and high reliability throughout its on-orbit life. To achieve acceptable levels of reliability and availability, on-orbit checkout and maintenance capabilities may be required.

D.3.2 Since crew rescue vehicles provide emergency escape, traditional abort and escape modes are not applicable. This space flight system must be able to transport severely injured or ill crewmembers, in need of medical evacuation, safely to Earth.

D.4 Crew Transfer

D.4.1 The main function of a crew transfer system is to ferry crewmembers to or from space flight systems. Since life support systems aboard a crew transfer vehicle may be limited, abort modes must be provided to allow for the safe recovery of crewmembers.

D.4.2 When transferring crewmembers to or from space flight systems, there may be multiple options for abort modes (such as return to origin, abort to destination, and station- keeping). The abort mode provided, for any given failure, should ultimately result in the safe accommodation of the crew.

D.5 Non-Crewed Systems

When a space flight system is used without crew or passengers aboard and in proximity operations to a crewed vehicle, an abort mode to separate a safe distance from the crewed vehicle should be provided.

D.6 Space Station (SS)

D.6.1 An extended SS mission duration increases the probability that some emergencies will arise. This requires that the means be provided to manage these emergencies to successful resolution rather than evacuating at the first indication of system malfunction, crew illness, or crew injury. This can be accomplished through resilient core system design, including high degrees of failure tolerance, maintainability, skip cycle logistics stores on orbit, a robust logistics chain, and the provision of emergency medical facilities on board. However, the capability to evacuate and return to Earth should be provided at all times through some type of escape vehicle (such as Soyuz or permanently docked ETO space flight systems).

D.6.2 For SS missions, abort and crew escape requirements are functionally the same. Therefore, the program requires an escape vehicle and/or a safe haven, which provides for safe and timely crew return.

D.7 Planetary Surface Systems (PSS)

D.8 Extravehicular Mobility Unit (EMU)

D.8.1 EMU's operate in the vicinity of a larger space system. Therefore, the minimum reliability of the EMU must provide for enough reserve capacity to allow the crewmember to safely return to the larger space flight systems. This reliability must be allocated over the number of required missions of the EMU.

D.8.2 EMU's should include crew self rescue devices worn by each EVA crewmember during all periods when there is no vehicle to credibly rescue an inadvertently detached EVA crewmember. This device could be the Simplified Aid for EVA Rescue or an equivalent capability.

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