Human rating of launch vehicles: Historical and potential future risk

Abstract

The history of launching human beings on launch vehicles began in 1961 with the Soviet Union's orbital launch of Yuri Alekseyevich Gagarin on April 12th and then the US suborbital flight of Alan Shepard on May 12th of that year. The launch of Shepard had been planned originally in October of 1960, and finally scheduled initially for 6 March 1961 would have made the first human in space an American, but for the questionable reliability of the Redstone booster and the ma ny failures of a previous flight which almost caused the death of Ham a chimpanzee passenger, caused Von Braun to de mand an additional test flight of the Redstone-Mercury system before agreeing to a human launch [1]. In both cases, that is both for Gargarin and for Shepard, the launch vehicles chosen were derived from military ballistic missiles. In the American case, the Army Redstone Intermediate Range Ballistic Missile (IRBM), was chosen for the first launches over the orbital capable Atlas Intercontinental Ballistic Missile (ICBM), because the reliability of the latter at the time was barely 50% [2], and the human rating of the missile included litt le more than the addition of an escape tower and some redundancy to the guidance system. This adaptation of an existing launch vehicle (a ballistic missile in this case) continued through the Mercury flights when the Atlas was considered to have matured enough, however the two crewed Gemini the paradigm changed. In addition to an escape system, a separation system was incorporated rather than a tower because of the Titan launch vehicle's propellant, the National Aeronautics and Space Administration (NASA) de manded modifications to the Titan production line and more intimate oversight into the production process over the strong ob jections of the US Air Force [3]. The subsequent Saturn launch vehicles were exclusively NASA designed with detailed NASA insight into their production and, for the most part, human rated from the start achieving a remarkable level of reliability, especially in launch to Low Earth Orbit (LEO) as compared to alternative launch vehicles of the era [4]. This paradigm of NASA design and NASA oversight was continued into the current shuttle era and had been proposed to continue into the next era of human exploration for the US space program [5]. All that changed in 2010. With the proposed cancellation of the Constellation program NASA was directed to consider existing launch vehicles, either previously produced or under development, as alternative crew launchers. This implied, as a minimum, that the design would not be a NASA design and the detailed insight might again be reduced to oversight, and in the case of US Air Force existing alternatives, might be returned to the era of Mercury with the use of “white tail” launchers that would be produced the same for both payload and crew use, but with crew safety additions added after production [6]. “White tail” is meant to imply that all vehicles coming off the factory line would be identical with additional desired features added post production. This paper reviews the history of human rating in the US space program, discusses the changes in the paradigm from Mercury to Saturn and the potential risk implications of returning to a “white tail” launcher approach for crew launch.