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Re-entry into the atmosphere

Re-entry into the atmosphere

  • On the Apollo missions, the craft re-entered the atmosphere after at least a partial orbit, and after discarding the larger part of what still remained of the spacecraft leaving only the small command module, a mere 5 tonnes of the
  • 3500 tonnes or more of the mass at launch.
  • From re-entry to splashdown was one of the most difficult, and in some ways the crudest part of the whole procedure.
  • Once again extreme accuracy was needed because the craft, by final use of the rocket power still available, must enter the atmosphere, at some 400 000 feet, through a ‘window’, as it is called, only 8 kilometres wide, and at an
  • angle of between 5.6° and 7.2° to the top of the atmosphere – if it entered too steeply it would have burned up,
  •  if too shallowly it would have bounced off again. Not only is the angle of entry very critical, but the craft also had to be manoeuvred into such a position that it encountered maximum drag (from drag rather than skin friction) and so maximum retardation of about 6 g.
  •  Even so, the speed was so high, and the skin friction so great, that the heat generated was quite alarming, the surface of the craft was burnt and scarred, and the air ionised so that radio communication between the earth and the crew was temporarily interrupted.
  • When denser air was reached, first a drogue parachute was released, followed by at least three large parachutes, and these reduced the velocity sufficiently for any surplus fuel to be jettisoned, and finally for a reasonably soft splash-down in the sea, again with reasonable accuracy of position.

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