The Mercury-Redstone Launch Vehicle, designed for NASA's Project Mercury, was the first American crewed space booster. It was used for six sub-orbital Mercury flights in 1960 and 1961, culminating with the launch of the first and, eleven weeks later, the second Americans (and the second and third humans) in space. The four subsequent Mercury human spaceflights used the more powerful Atlas booster to enter low Earth orbit. A member of the Redstone rocket family, it was derived from the U.S. Army's Redstone ballistic missile and the first stage of the related Jupiter-C launch vehicle; but to human-rate it, the structure and systems were modified to improve safety and reliability.
Modifications from the Redstone missile NASA chose the U.S. Army's Redstone liquid-fueled ballistic missile for its sub-orbital flights as it was the oldest one in the US fleet, having been active since 1953 and had many successful test flights. Though the standard military Redstone lacked sufficient thrust to lift a Mercury capsule into the ballistic suborbital trajectory needed for the project, the first stage of the Jupiter-C, a modified Redstone with lengthened tanks, could carry enough propellant to reach the desired trajectory. Therefore, the Jupiter-C first stage was used as the starting point for the Mercury-Redstone design. The Jupiter-C's engine, however, was being phased out by the Army, so, to avoid potential complications such as parts shortages or design revisions, the Mercury-Redstone designers chose the Rocketdyne A-7 engine used on the latest military Redstone. Hans Paul and William Davidson, propulsion engineers at the Army Ballistic Missile Agency (ABMA), were assigned the task of modifying the A-7 to be safe and reliable for crewed flights. During 1959, most of ABMA were preoccupied with the Saturn project, but those engineers who could find enough free time in their schedule were invited to work on man-rating the Jupiter-C. As a starting point, the most obvious step was getting rid of its staging capability as the Mercury-Redstone would not utilize upper stages. Many of the more advanced Jupiter-C components were also removed for reliability reasons or because they were not necessary for Project Mercury. The standard Redstone was fueled with 25% water–75% ethyl alcohol with liquid oxygen (LOX) used as the oxidizer, essentially the same propellants as the German V-2 missile, but the Jupiter-C first stage had used hydyne fuel, a blend of 60% unsymmetrical dimethylhydrazine (UDMH) and 40% diethylenetriamine (DETA). This was a more powerful fuel than ethyl alcohol, but it was also more toxic, which could be hazardous for an astronaut in a launch pad emergency. Furthermore, hydyne had never been used with the new A-7 engine. The Mercury-Redstone designers rejected hydyne and returned to the standard ethyl alcohol fuel. The lengthened propellant tanks were thus also necessary in lieu of using more powerful fuel. Use of alcohol created a problem with the Mercury-Redstone in that the graphite thrust vector vanes could be eroded due to the significantly longer burn time, so NASA put out a requirement that the launch vehicles be equipped with high-quality vanes. Because Mercury-Redstone had larger propellant tanks than the Redstone missile, an additional nitrogen bottle was added for tank pressurization, and an extra hydrogen peroxide tank for powering the turbopump due to the longer burn time.
The most important change in making the Mercury-Redstone suitable for an astronaut was the addition of an automatic in-flight abort sensing system. If a catastrophic failure of the rocket were imminent, the launch escape system attached to the Mercury capsule would be activated, rapidly lifting the capsule from the booster. Either the astronaut or the ground controllers could initiate an abort manually, but some potential failures during flight might lead to disaster before an abort could be manually triggered. The Mercury-Redstone's automatic in-flight abort sensing system solved this problem by monitoring the rocket's performance during flight. If it detected an anomaly which might threaten the astronaut, such as loss of flight control, engine thrust, or electrical power, it would automatically abort, shutting down the engine and activating the capsule's escape system. The abort system could not shut off the engine until at least 30 seconds after liftoff in order to prevent a malfunctioning launch vehicle from coming down on or near the pad; during the first 30 seconds, only the Range Safety Officer could terminate the flight. Review of flight data from the more than 60 Redstone and Jupiter C launches since 1953 was used to analyze the most likely failure modes of this launch vehicle family. The abort sensing system had to be kept as simple as possible, monitoring only those parameters that were vital to booster operation. An automatic abort could be triggered by the following conditions, any of which could indicate a catastrophic malfunction:
Pitch, yaw, or roll angle deviating too far from the programmed flight profile; Pitch or yaw angle changing too rapidly; Pressure in the engine's combustion chamber falling below a critical level; Loss of electrical power for the flight control system; or Loss of general electrical power (including power for the abort sensing system itself). Instant abort capability was important because certain failure modes such as loss of thrust upon liftoff (for example the third Redstone test flight in May 1954) could result in an immediate catastrophic situation. Other failure modes such as deviation from the proper flight path or a drop in engine chamber pressure during ascent did not necessarily present an immediate risk to the astronaut's safety. He could initiate a manual abort by pulling a lever in the capsule to activate the Launch Escape System, or ground control could send a command to activate it.
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