Pad abort and a trio of satellites

Discovery's debut mission, as STS-41D, the twelfth flight of the programme, started as an exercise in frustration. After a perfect Flight Readiness Firing on 2 June 1984, NASA set the launch for 22 June, postponed it to 25 June, and then scrubbed it at T-9 minutes due to a fault in one of the general-purpose computers. The next day, the count ran smoothly to the point at which the three hydrogen burning engines were to ignite at 120-millisecond intervals, whereupon the hydraulically activated fuel valve of the first engine failed to open. The second engine was up to 20 per cent thrust before this misfire was diagnosed by the computer, which immediately ordered a shut-down, inhibiting the third engine. The flame fizzled out and the billowing cloud of steam rapidly dispersed. This was the first time that a launch had been scrubbed following engine-start. Although spectacular, it was not an unprecedented situation because the vehicle was in essentially the same state as after an engine test firing. As the engines that had fired required to be refurbished, the launch had to be pushed back by a month. When launched on 30 August, this became the first Shuttle to deploy a trio of satellites. Two rode PAM motors which this time fired properly. The third satellite was the first of a new type. In 1978, the Navy had ordered a series of advanced Syncom satellites. The resulting HS-381 exploited the fact that an early cost-benefit analysis had envisaged charging a fee based on how much of the payload bay's length a satellite occupied. In contrast to the HS-376, which was a 2.8-metre-tall drum of just over 2 metres diameter in order to fit the shroud of a vehicle such as the Delta, the marginally taller drum of the HS-381 was fully 4 metres across. Whereas the HS-376 satellites were mounted on a perigee-kick motor and set upright in the bay, the HS-381 was carried on its side. Although the HS-381 was 7 tonnes, half of this mass was propellant for the in-built motor that was to enable it to attain its operating station. Consequently, this series was not The deployment °f Lea^ 3. delayed by the grounding of the

IUS. As the spring-loaded cradle ejected the satellite 'frisbee style', it set it rotating at 2 revolutions per minute for stability and flipped a lever to activate its sequencer, which immediately elevated the omni antenna to enable Hughes to verify the systems before the satellite set off for geostationary orbit. Since these satellites were to remain the property of Hughes and be leased by the Navy only after they were installed on-station, they were referred to as Leasats.

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