The Atlas LV-3B, Atlas D Mercury Launch Vehicle or Mercury-Atlas Launch Vehicle, was a human-rated expendable launch system used as part of the United States Project Mercury to send astronauts into low Earth orbit. Manufactured by Convair, it was derived from the SM-65D Atlas missile and was a member of the Atlas family of rockets. With the Atlas having been originally designed as a weapon system, testing and design changes were made to the missile to make it a safe and reliable launch vehicle. After the changes were made and approved, the US launched the LV-3B nine times, four of which had crewed Mercury spacecraft.
Design The Atlas LV-3B was a human-rated expendable launch system used as part of the United States Project Mercury to send astronauts into low Earth orbit. Manufactured by American aircraft manufacturing company Convair, it was derived from the SM-65D Atlas missile and was a member of the Atlas family of rockets. The Atlas D missile was the natural choice for Project Mercury, as it was the only launch vehicle in the US arsenal that could put the spacecraft into orbit and also had many flights from which to gather data.
The Atlas had been originally designed as a weapon system. Thus, its design and reliability did not need to necessarily be 100% perfect, which led to Atlas launches frequently ending in explosions. As such, significant steps had to be taken to human-rate the missile to make it safe and reliable, unless NASA wished to spend several years developing a dedicated launch vehicle for crewed programs or else wait for the next-generation Titan II ICBM to become operational. Atlas's stage-and-a-half configuration was seen as preferable to the two-stage Titan in that all engines were ignited at liftoff, making it easier to test for hardware problems during pre-launch checks.
Shortly after being chosen for the program in early 1959, the Mercury astronauts were taken to watch the second D-series Atlas test, which exploded a minute into launch. This was the fifth straight complete or partial Atlas failure, and the booster was at this point nowhere near reliable enough to carry a nuclear warhead or an uncrewed satellite, let alone a human passenger. Plans to human-rate Atlas were effectively still on the drawing board, and Convair estimated that 75% reliability would be achieved by early 1961 and 85% reliability by the end of the year. Despite the Atlas' developmental problems, NASA had the benefit of conducting Project Mercury simultaneously with the Atlas R&D program, which gave plenty of test flights to draw data from, as well as test modified equipment for Mercury.
Quality assurance
Aside from the modifications described below, Convair set aside a separate assembly line dedicated to Mercury-Atlas vehicles which was staffed by personnel who received special orientation and training on the importance of the crewed space program and the need for as high a degree of top-quality workmanship as possible. Components used in the Mercury-Atlas vehicles were given thorough testing to ensure proper manufacturing quality and operating condition. In addition, components and subsystems with excessive operating hours, out-of-specification performance, and questionable inspection records would be rejected. All components approved for the Mercury program were earmarked and stored separately from hardware intended for other Atlas programs and special handling procedures were taken to protect them from damage. The factory inspection of Mercury vehicles was performed by Convair personnel specially chosen for their experience, familiarity with the Atlas hardware, and who had demonstrated a favorable disposition and work ethic.
Propulsion systems used for the Mercury vehicles would be limited to standard D-series Atlas models of the Rocketdyne MA-2 engines which had been tested and found to have performance parameters closely matching NASA's specifications. NASA decided that the best choice of engines would be units with roughly medium-tier performance. Engines with higher than average performance were not considered acceptable because it could not be determined exactly why a given set of engines performed the way it did, and so it was considered safest to use medium-performance ones.
For the most part, NASA preferred to stay conservative with the Mercury vehicles and avoid modifying them any more than necessary. Modifications to the Atlas would largely be limited to those that improved pilot safety, and the standard D-series Atlas configuration was to be retained as much as possible, so assorted enhancements made to the latest Atlas missiles would not be used. Various equipment and procedures used with Mercury vehicles, although outdated and often not the best or latest, were preferred because they were proven and well-understood. Any new equipment or hardware changes made to Mercury vehicles had to be flown on at least three Atlas R&D tests before NASA would approve them for use. Despite the conservatism and caution taken with the design of Mercury vehicles, a large number of changes did take place over the years of the program from lessons learned, and the emphasis on quality control got tighter as time went along; the last two Mercury flights were given a level of testing and pre-flight inspection that was unheard of when Big Joe flew in 1959.
All launch vehicles would have to be complete and fully flight-ready at delivery to Cape Canaveral with no missing components or unscheduled modifications/upgrades. After delivery, a comprehensive inspection of the booster would be undertaken, and prior to launch, a flight review board would convene to approve each booster as flight-ready. The review board would conduct an overview of all pre-launch checks and hardware repairs/modifications. In addition, Atlas flights that had taken place over the past few months in both NASA and Air Force programs would be reviewed to make sure no failures occurred involving any components or procedures relevant to Project Mercury.
The NASA Quality Assurance Program meant that each Mercury-Atlas vehicle took twice as long to manufacture and assemble as an Atlas designed for uncrewed missions and three times as long to test and verify for flight.
Systems modified
Abort sensor Central to these efforts was the development of the Abort Sensing and Implementation System (ASIS), which would detect malfunctions in the Atlas's various components and trigger a launch abort if necessary. Added redundancy was built in; if ASIS itself failed, the loss of power would also trigger an abort. The ASIS system was first carried on a few Atlas missile R&D flights, then flown open loop on Mercury-Atlas 1, meaning the ASIS could generate an abort signal but not send a cutoff command to the propulsion system. It was operated closed-loop on MA-3 for the first time.
The Mercury launch escape system (LES) used on Redstone and Atlas launches was identical, but the ASIS system varied considerably between the two boosters as Atlas was a much larger, more complex vehicle with five engines, two of which were jettisoned during flight, a more sophisticated guidance system, and inflated balloon tanks that required constant pressure to not collapse.
Big Joe and MA-1 had no escape tower, the latter's in-flight failure was possibly due to the lack of the LES negatively affecting its aerodynamic profile and so MA-2 carried a dummy tower. A live LES was carried for the first time on MA-3 (and ended up proving its functionality in an unplanned test).
Atlas flight test data was used to draw up a list of the most likely failure modes for the D-series vehicles, however simplicity reasons dictated that only a limited number of booster parameters could be monitored. An abort could be triggered by the following conditions, all of which could be indicative of a catastrophic failure: The booster flight path deviated too far from the planned trajectory Engine thrust or hydraulic pressure dropped below a certain level Propellant tank pressure dropped below a certain level The intermediate tank bulkhead showed signs of losing structural integrity The booster electrical system ceased operating The ASIS system ceased operating
The ASIS system was deemed necessary because some flight failures of Atlas vehicles (for instance, Atlas 6B) occurred so fast that it would be nearly impossible for the astronaut to react in time to manually activate the LES. Other failure modes such as a deviation from the correct flight trajectory did not necessarily pose an immediate danger to the astronaut's safety, and the flight could be aborted manually.
Not all of the modifications listed below were carried on every Mercury flight and numerous changes were made along the way in the interest of improvement or as a result of flight data obtained from failed Atlas launches. Quality control and checkout procedures also improved and became more detailed over the course of the program.
Rate gyros The rate gyro package was placed much closer to the forward section of the LOX tank due to the Mercury/LES combination being considerably longer than a warhead and thus producing different aerodynamic characteristics (the standard Atlas D gyro package was still retained on the vehicle for the use of the ASIS). Mercury-Atlas 5 also added a new reliability feature—motion sensors to ensure proper operation of the gyroscopes prior to launch. This idea had originally been conceived when the first Atlas B launch in 1958 went out of control and destroyed itself after being launched with a non-functioning yaw gyro, but it was phased into Atlas vehicles only gradually. One other Atlas missile test in 1961 also destroyed itself during launch, in that case because the gyroscope motor speed was too low. The motion sensors would thus eliminate this failure mode.
Range safety The range safety system was also modified for the Mercury program. There would be a three-second delay between engine cutoff and activation of the destruct charges so as to give the LES time to pull the capsule to safety. More specifically, if the Range Safety destruct command was sent, the ASIS system would allow the engine cutoff signal to go through, while blocking the destruct signal for three seconds. The decrease in engine performance would then be sensed by the ASIS, which would activate the LES, after which the destruct signal would be unblocked and destroy the launch vehicle. Engine cutoff and destruct commands were also blocked for the first 30 seconds of launch to prevent a malfunctioning vehicle from coming down on or around the pad.
Autopilot D-series Atlas missiles as well as early SLV variants carried an old-fashioned electromechanical autopilot (known as the "round" autopilot due to the shape of the containers its major components were housed in), but on Mercury vehicles, it was decided to use the newer transistorized "square" autopilot developed for the E and F-series missiles, and for the upcoming Atlas-Centaur vehicle. The first three Mercury-Atlas vehicles still had the round autopilot and it was flown for the first time on Mercury-Atlas 3, but failed disastrously when the booster did not perform the programmed pitchover maneuver and had to be destroyed by Range Safety action. Afterwards, the missile programmer was recovered and examined. While the exact cause of the failure was not identified, several causes were proposed and a number of modifications made to the programmer. On Mercury-Atlas 4, high vibration levels in flight resulted in more modifications and it worked successfully on Mercury-Atlas 5.
Telemetry Beginning on MA-3, a newer transistorized telemetry system replaced the old vacuum tube-based unit, which was heavy, had high power consumption, and tended to suffer from signal fade as vehicle altitude increased. As with most SLV configurations of Atlas, Mercury vehicles carried only one telemetry package while R&D missile tests had three.
Antenna