The four subsequent Mercury human spaceflights used the more powerful Atlas booster to enter low Earth orbit.

[4] 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.

The most important change in making the Mercury-Redstone a suitable vehicle for an astronaut was the addition of an automatic in-flight abort sensing system.

[11] In an emergency where the rocket was about to suffer a catastrophic failure, an abort would activate the launch escape system attached to the Mercury capsule, which would rapidly eject it from the booster.

[13]The Mercury-Redstone's automatic in-flight abort sensing system solved this problem by monitoring the rocket's performance during flight.

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 and he could either 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.

The range safety system was modified slightly in that a three-second delay would take place between engine cutoff and missile destruct so as to give the escape tower enough time to pull the capsule away.

The aft section held most of the Mercury-Redstone's electronics and instrumentation, including the guidance system, as well as the adapter for the Mercury capsule.

The LEV-3, whose design dated back to the German V-2, was not as sophisticated or as precise as the ST-80, but it was accurate enough for the Mercury mission and its simplicity made it more reliable.

[25] The fuel prevalves were deleted from the Mercury-Redstone in the interest of improved reliability, since if they closed during a launch, an abort condition could be triggered.

On the three uncrewed flights, it was discovered that the Mercury-Redstone exhibited a roll transient of 8° per second versus 4° for the Redstone missile.

The ASIS system activated and the escape tower yanked the capsule away, subjecting Ham, its chimpanzee passenger to high G loads.

The third flight, Mercury-Redstone BD, was designed as an engineering test to correct these problems before the booster could be considered man-rated.

This created a series of disputes between Von Braun's team at ABMA and NASA, as the former preferred simply making the abort system as foolproof as possible so as to guarantee that the astronaut would be bailed out of a malfunctioning launch vehicle, while the latter favored maximum booster reliability to minimize the chance of aborts happening at all.