What Does It Mean to 'Human Rate' a Rocket?


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:

Let’s clearly define some of the primary human rated attributes and what they mean. First, human safety is the measure of risk of injury, or loss of life, to any spaceflight personnel. NASA’s policy is to protect the health and safety of humans involved in or exposed to space activities, specifically the public, crew, passengers, and ground personnel. Specifically human rating is involved with the risk to the flight crew. Risks to ground crew and the public are covered under other NASA policy directives and are inherent in all missions regardless of the presence of a flight crew.

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.

5 thoughts on “What Does It Mean to 'Human Rate' a Rocket?”

  1. I guy from france did it a few months ago, flew across the channel.
    I think he’s a nutter

  2. Here is an idea that will get us to Mars alot quicker then by using the Constellation Program. It is called the Pilyhas-1 and Pilyhas-11
    Interplantary Exploration Program or PIEP. The conceptual design that I have conceived of incorporates various I.S.S. modules into a working starship. It begins by re-engineering the ORION CM to be used the command deck for the ship. At the fore and aft centerline’s of the CM will be docking mechanism’s currently used to connect the OCM
    to the I.S.S. One docking mechanism will be an active design the other will be a passive design. The OCM will be reworked to house three crew members, the pilot, co-pilot and systems monitoring engineer. The remaining area that would have been used for the other three crew members would be replaced with systems montiors and expanded storage space for the command crew’s personal items along with additional space for cargo to be stored.

    The next module in line of the centerline would be a Unity Module. This module would give a more rigid form to the ship along with adding the properties of the module to the overall design of the starship. The module that would come next would be the Harmony Module, meant for life support, cargo storage and life support. The next in-line module would be another Unity module then another Harmony Module. Another Unity module would then follow the second Harmony Module with the final Harmony Module being connected to the Unity Module. A specialized Jules Verne Module would then be connected to the last Harmony Module. The Jules Verne Module would be re-worked to incorporate a docking mechanism to allow for EVA outside of the ship in case of emergency access to any of the modules, fuel storage module or the PDR Baseline engine sled. The JV would be used for either primary electrical power output, secondary electrical power sharing to augment the Harmony Modules or emergency electrical power back-up in case of Harmony Module failure. A reworked Ares style fuel storage module would be connected to the linear ship the same way that the other modules are connected. The final stage would be the uprated PDR Baseline engine sled that was to have been used on the I.S.S. More powerfull engines would need to be added to provide the thrust necessary to propell the ship based on the total amount of modules connected.


    This concept is the same as the Pilyhas-1 concept except that instead of sending single modules up at a time and connecting them seperately, the modules would be sent up as two connected units. The first stage would be the OCM as described above.Instead of using the Unity Modules to inter-connect the Harmony Modules or other modules used, a secondary hull would be placed around the modules. The starship would actually look like the current delivery vehicle used to send the ORION into space. The modules would sit atop the fuel tank. This increased weight would necessitate more storage volume for the fuel tank in order to launch the starship. The second stage would the aft section of the starship, two more Harmony modules, Jules Verne, uprated and the engine sled.

    The first Pilyhas-1 design is meant to send more then one module to a particular region of space that will be used to build a mini space station for emergency docking of the Pilyhas-11 or cargo storage of equipment meant to be picked up for the journey to Mars. The secondary use of the Pilyhas-1 design would be a modile science lab for projects to be conducted further away from the Earth then I.S.S. is.

    The Pilyhas-11’s primary use would be that of transporting crew and members in a larger capacity to the Moon or Mars or secondary orbital facility. This would eleviate the need to send two launch vehicles into orbit when one could be launched carrying seven to ten crew members.

  3. The article neglected to mention the human rating intent when the time comes to safely return the crew from the lunar surface to an orbiting command vehicle. The Lunar Lander ascent configuration only has one engine to do this, and if it fails, it is bad news for anyone aboard because everyone knows parachutes don’t work on the moon.

  4. To many things could go wrong and the problem is the astronauts are going to be to far out for us to be able to do anything in time.

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