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Gas leak led to Telstar 402 explosion

THE LOSS OF the Telstar 402 communications satellite after launch, aboard an Ariane 4 on 8 September 1994, was caused by an explosion, Martin Marietta Astro Space has confirmed (Flight International, 11-17 January).

Although the official accident report had not been released as Flight International went to press, the company admitted that the explosion "...occurred in the spacecraft-propulsion system and was caused when leaking hot gases from the firing of a pyro-valve reacted with hydrazine in the valve".

Nathan Lindsay, vice-president of mission success and product assurance at Martin Marietta Astro Space says: "We're confident we've identified the root cause of the failure."

Over 50 realistic test firings of the pyro-valves have confirmed that excessive "blow-by" of combustion products, is produced occasionally and will react with hydrazine.

"That explosion will rupture the fuel line," the company says, adding that the failure scenario matches conditions reconstructed from telemetry.

Two immediate recommendations have been made. The first is to assure that a vacuum instead of hydrazine is present on the downstream side of the valve. Tests show that this will prevent an explosion if there is excessive blow-by.

The second recommendation is to change the pyro-valve to limit blow-up in the fraction of a second after the valve is fired.

"We are prepared to implement these recommendations and move forward with an aggressive plan for delivering space craft to our Series 7000 customers," says Russell McFall, vice-president of commercial programmes.

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