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.

[2] 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.

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.

[3] 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.

[2] 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.

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.

In addition, Atlas flights 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.

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.

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.

Thus, it was decided to install extra sensors in the engines to monitor combustion levels and the booster would also be held down on the pad for a few moments after ignition to ensure smooth thrust.

By late 1961, after a third missile (27E) had exploded on the pad from combustion instability, Convair developed a significantly upgraded propulsion system that featured baffled fuel injectors and a hypergolic igniter in place of the pyrotechnic method, but NASA were unwilling to jeopardize John Glenn's upcoming flight with these untested modifications and so declined to have them installed in Mercury-Atlas 6's booster.

Static testing of Rocketdyne engines had produced high-frequency combustion instability, in what was known as the "racetrack" effect where burning propellant would swirl around the injector head, eventually destroying it from shock waves.

The LOX fuel feed system received added wiring redundancy to ensure that the propellant valves would open in the proper sequence during engine start.

In early 1962, two static engine tests and one launch (Missile 11F) fell victim to LOX turbopump explosions caused by the impeller blades rubbing against the metal casing of the pump and creating a friction spark.

In addition Atlas 113D, the booster used for Wally Schirra's flight, was given a PFRT (Pre-Flight Readiness Test) to verify proper functionality of the propulsion system.

Mercury vehicles used a standard D-series Atlas pneumatic system, although studies were conducted over the cause of tank pressure fluctuation which was known to occur under certain payload conditions.

This change was made after Atlas 81D, an IOC test from VAFB, was destroyed in-flight due to a malfunction that caused the pressurization regulator to overpressurize the tank until it ruptured.

A hydraulic pressure switch on MA-7 was tripped and flagged an erroneous abort signal, so on subsequent vehicles additional insulation was added as cold temperatures from LOX lines were thought to have triggered it.

The vernier solo phase, which would be used on ICBMs to fine-tune the missile velocity after sustainer cutoff, was eliminated from the guidance program in the interest of simplicity as well as improved performance and lift capacity.

A common and normally harmless phenomenon on Atlas vehicles was the tendency of the booster to develop a slight roll in the first few seconds following liftoff due to the autopilot not kicking in yet.

This failure was due to a problem with the guidance system failing to execute pitch and roll commands, necessitating that the Range Safety Officer destroy the vehicle.