From Jan Maciejowski, University of Cambridge
Helmut Zarzycki (Letters, 17 August, p 49) is a little unfair about the
British failure to protect the Europa 2 ELDO launch vehicle’s inertial guidance
system against electrical earthing problems. It is not true that nothing was
done to protect against a known failure mode.
In fact, an automatic restart capability was added (in hardware not in
software), but it did not provide sufficient protection. The design of the
protection circuitry assumed—probably unintentionally—that any
pulses resulting from such a failure would last at least as long as the
switching time of the logic gates, which at that time was about 10 microseconds
(using 1960s technology).
Post-flight analysis revealed that several current
surges occurred in the flight computer’s earthing strap as a result of the arc
discharge mentioned by Zarzycki, but each of these surges lasted considerably
less than 10 microseconds. Consequently, the logic circuitry was hung up in an
unanticipated state.
