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Concept
Ariane flight V88
Summary
Ariane flight V88 was the failed maiden flight of the Arianespace Ariane 5 rocket, vehicle no. 501, on 4 June 1996. It carried the Cluster spacecraft, a constellation of four European Space Agency research satellites.
The launch ended in failure due to multiple errors in the software design: dead code, intended only for Ariane 4, with inadequate protection against integer overflow led to an exception handled inappropriately, halting the whole otherwise unaffected inertial navigation system. This caused the rocket to veer off its flight path 37 seconds after launch, beginning to disintegrate under high aerodynamic forces, and finally self-destructing via its automated flight termination system. The failure has become known as one of the most infamous and expensive software bugs in history. The failure resulted in a loss of more than US$370 million.
The Ariane 5 reused the code from the inertial reference platform from the Ariane 4, but the early part of the Ariane 5's flight path differed from the Ariane 4 in having higher horizontal velocity values. This caused an internal value BH (Horizontal Bias) calculated in the alignment function to be unexpectedly high. The alignment function was operative for approximately 40 seconds of flight, which was based on a requirement of Ariane 4, but served no purpose after lift-off on the Ariane 5. The greater values of BH caused a data conversion from a 64-bit floating point number to a 16-bit signed integer value to overflow and cause a hardware exception. The programmers had protected only four out of seven critical variables against overflow to keep within a required maximum workload target of 80% for the on-board Inertial Reference System computer, and relied on assumptions which were correct for the Ariane 4, but not Ariane 5, trajectory about the possible range of values for the three unprotected variables. The exception halted both of the inertial reference system modules, although they were intended to be redundant.
Official source
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