Atlas LV-3B (Mercury-Atlas)

The Atlas airframe was nicknamed the gas bag. NASA’s history of Project Mercury describes stainless steel sections thinner than paper, held rigid by helium pressurisation to a differential of between 25 and 60 pounds per square inch, which brought the empty weight of the airframe below two percent of the propellant weight. NASA’s Mercury Project Summary puts the sheet thickness at 0.048 down to 0.015 of an inch. That is under four tenths of a millimetre. Undo the pressure and the rocket crumples.

It is also the only crewed launcher that was neither one stage nor two. NASA calls it a one and a half stage vehicle: all five engines light on the pad, two 154,000 pound booster engines plus a 57,000 pound sustainer and two 1,000 pound verniers, 367,000 pounds or 1,632 kilonewtons in total. About two minutes in, the two booster engines and their skirt are jettisoned as a unit while the sustainer keeps burning from the same tanks. There is no second set of tanks and nothing has to be lit in flight, which is why it counts as a half. It is recorded here as two stages because it does shed hardware on the way up, but NASA’s own word for it is one and a half.

The height given here, 29.06 metres, is the full stack with the Mercury capsule and escape tower, and it is the one figure on this record that does not come from an agency source. NASA’s Mercury Project Summary and the Mercury-Atlas 9 press kit give component dimensions and never state a stack height. The NASA Historical Data Book puts the Atlas itself at 23.2 metres. Thrust also drifted across the programme: the Atlas 130-D that flew Gordon Cooper ran 150,000 pound boosters and a 60,000 pound sustainer, 360,000 pounds rather than 367,000.

Nine Mercury-Atlas missions flew and seven succeeded. MA-1 broke up about sixty seconds after liftoff in July 1960 and MA-3 failed to follow its roll programme in April 1961, the capsule escape system firing as designed. On 20 February 1962 MA-6 put John Glenn in orbit, and on 15 May 1963 MA-9 kept Gordon Cooper up for 34 hours and 19 minutes, which was the first time an American spent a day in space and the last time one flew alone.