A lot of people have asked what it means to "human rate" a rocket -- to put people on top of a rocket and send them into space.  How does an agency like NASA take on this challenge?  And what considerations do engineers give human rating as they design Ares to deliver astronauts to the International Space Station by 2015 and for future trips to the moon and beyond?

In a nutshell, human-rating a rocket means that we take our understanding of how the rocket can fail to a higher level of fidelity (than for a non-human rated rocket), and then take steps to prevent failures or have it fail in such a way that the crew can survive the failure (e.g. crew abort).

For NASA, the Ares I rocket is being designed from the outset to fly humans as its primary role vs. modifying an existing system.

To get a little deeper into the subject, I talked with some senior NASA Marshall Space Flight Center engineers, Neil Otte and Gary Langford, who work this challenge for NASA and here’s how they explain it:

A human-rated system accommodates human needs, effectively utilizes human capabilities, controls hazards and manages safety risks associated with human spaceflight, and provides, to the maximum extent practical, the capability to safely recover the crew from hazardous situations. This statement makes up the basic three tenets of human rating:  assuring the total system can safely conduct the mission, incorporate design features that accommodate human interaction with the system, and incorporate design features and capabilities to enable safe recovery of the crew from hazardous situations.

Simply put, human rating is a thorough process that consists of many variables being taken into account to safely design, build and launch a crewed spacecraft and return that spacecraft, and its crew safely to the earth. The process begins at program inception and continues throughout the life cycle of the program and includes: design and development; test and verification; program management and control; flight readiness certification; mission operations; sustaining engineering; maintenance, upgrades, and disposal.

We can now look closer at human rating. The first tenet is to safely conduct the planned mission. To accomplish this requires a very careful design. This design is accomplished by a careful examination of the hazards and design features that prevent the hazard known as hazard controls. In the design, the first step would be to try eliminating the hazard; if that is not possible then hazard controls can be put into place to prevent the occurrence of the hazard. Hazard controls can take many forms such as failure tolerance by incorporating redundant or backup systems and components, application of system margins to assure function of the system even under the most extreme conditions, and quality assurance from early material and component selection through final assembly and checkout operations. If applied to a simple example of say a home heating system, the hazard would be that the house is too cold for the health and safety of the occupants. Moving to a warmer climate, however, could eliminate the hazard, if not possible then hazard controls are put into place. Use of redundant systems or components can be applied.  For example, many heat pump systems have backup electrical or gas systems to provide heat in the event of a compressor failure or the inability of the compressor to meet the needed heat requirements. The system is carefully sized to provide adequate heat under the most extreme expected winter temperatures for the local climate, and the equipment manufacturer and the installation contractor control quality.  

For the Ares I rocket the foundations of the first basic tenet in developing a human rated system have been carefully laid out. Factors such as hazard elimination and hazard controls have been carefully thought out and placed as requirements in the system design. In addition, program management and control places additional requirements on the development to assure adequate system margins, proper test and verification, and safety and mission assurance practices to further minimize the risk to the flight crew.

Even with all the care that goes into the system design and development, the system design must accommodate failure. Sometimes failure is dealt with by simple redundancy that allows mission continuation. In some cases, however, mission continuation is no longer possible and steps must be taken to safely return the crew. For the example of the house, for extreme cold and total system failure, the occupants could choose to leave, go stay with family or friends, or stay in a hotel until repairs are made. In short you remove the humans from the hazard. Ares accomplishes this by incorporation of the launch abort system (LAS). The LAS allows the spacecraft to be lifted away from a failing launch vehicle and allows for spacecraft reentry and rescue of the crew by search and rescue forces.

Launch of a crew to low earth orbit is an energetic process that inherently has significant associated risk. The process of human rating attempts to eliminate hazards, control the hazards that remain, and provide for crew survival even in the presence of failures that expose the crew to the hazards. The Ares Projects team was assigned the task of designing a launch vehicle capable of carrying the Orion crew exploration vehicle, with a crew of four to six astronauts, to the International Space Station AND later support lunar missions.  NASA's top priority is to design and build a vehicle that supports the crew with the safest design possible given real external constraints. The Ares design is a culmination of years of studying the best attributes for a human rated launch system. Every aspect of human rating has been taken into account in the Ares design, therefore the Ares I rocket will be fully human rated, something only achieved by a small fraction of launch vehicles.

The Ares I rocket is three years into its development process and has successfully passed every major design review. Ares is being designed with human rating in mind as the primary requirement vs. modifying an existing rocket. Human rating has been an integral part of the Ares I development since day one.

How did NASA select the Ares family of rockets as America's new space transportation system?

Since the 1980’s, NASA has evaluated thousands of studies relating to space transportation. It has been said we could “pave” the way to the moon with all the studies that have been conducted. These studies looked at thousands of combinations and variations of how to send humans beyond low Earth orbit, back to the moon and on to Mars. 

NASA looked at a wide variety of launch concepts -- from the Evolved Expendable Launch Vehicle (Atlas V, Delta IV), Space Shuttle (including Shuttle C, Direct type approaches and other solid and liquid rocket booster propelled systems) combinations, foreign systems and clean sheet designs.

The Exploration Systems Architecture Study (ESAS) was chartered in the spring of 2005 to recommend a fundamental architecture for supporting International Space Station, Lunar and Mars transportation.

Using data from previous and ongoing studies (several hundred vehicles), and consisting of a team of knowledgeable experts from inside and outside NASA, this study compared many launch and staging options for safety, effectiveness, performance, flexibility, risk and affordability.

ESAS concluded that NASA should adopt and pursue a Shuttle-derived architecture as the next-generation launch system, using a smaller vehicle for crew missions and a dedicated, heavy-lift launcher for cargo missions. This approach was selected due to several significant advantages, particularly safety, reliability and cost.

NASA continued to refine its launch recommendations post-ESAS. Since early 2006 NASA has made the following major modifications to the initial designs.

Upgraded from the shuttle's four-segment reusable solid rocket booster design (RSRB) to a five-segment RSRB design -- forming a common basis for Ares I and V Eliminated the space shuttle main engine (SSME) in favor of a newly designed J-2X engine for the Ares I upper stage. The Ares V upgraded from a five-segment RSRB with expendable SSME Core to a derivative of the Ares I 1st stage with a six-engine RS-68 Core and the J-2X engine for the earth departure stage (EDS).

Developing the new J-2X engine for both the Ares I upper stage and the Ares V Earth departure stage solves several potential problems including starting the SSME at altitude and the major expense of using it for the first stage engine. For additional cost savings Ares will use the expendable RS-68 engine which is “off the shelf” technology that meets both Department of Defense and NASA needs.

These combined changes represent a projected savings of over $5 billion in life cycle costs below the initial ESAS recommendations.

The shuttle heritage design offers years of proven flight concepts with a very strong technical and safety foundation for next-generation vehicle. Since ESAS, NASA has continued to assess options -- over 1,700 to date. After a thorough analysis of all the exploration architecture requirements, other solutions were ultimately determined to be less safe, less reliable, and more costly than Ares I and Ares V.

Throughout the selection process for its launch vehicles, NASA has been thorough, transparent, subject to regular independent reviews, open to alternative ideas, and has made all of its decisions based on hard data.

Ares is a solid foundation for America's future in space.