Criticality-One is
NASA-speak for a
Space Shuttle point of failure that is
single point catastrophic. In other words, a failure for which there is (and perhaps can be) no
backup or emergency plan. For example, a
Solid Rocket Booster explosive bolt failing to fire leaves one of the
SRBs attached (likely badly) to the
External Tank. This will unbalance the Shuttle at the least, and the small
thrusters on the nose of the
SRB may fire their plumes into the
External Tank, damaging it and/or detonating the
hydrogen and
oxygen stores inside.
In other words, Criticality-One is bad.
One step below this is Crit-One-R.