Learning To Walk

It was at around this point that managers decided to postpone the TSS-1R deployment until 25 February to gain additional confidence-building and testing time with the Smartflex. Although, so far, it had remained stable, additional time was desirable to allow engineers to better understand its behaviour after several unexpected crashes and restarts. The 24-hour delay also gave them ample opportunity to develop work-around procedures should the Smartflex encounter difficulties during the deployment procedure. It was "like learning to walk before you run," according to Mission Scientist Nobie Stone.

One of these confidence-building tests involved 'mapping' Earth's charged-particle environment, which varied dramatically as Columbia circled the globe in periods of sunlight and darkness every 45 minutes. During this time, Nicollier also activated the Tether Optical Phenomena (TOP) experiment, developed by Stephen Mende of Lockheed Martin's Palo Alto Research Laboratory in California; this employed a hand-held, low-light-level television camera to provide visual data in support of questions concerning tether dynamics and the optical effects created by

Learning to walk 255

TSS-1R inches away from its mast at the start of deployment operations.

the satellite itself. Through the overhead flight deck windows, Nicollier acquired stunning views of atmospheric airglow and aurorae over the South Pole using TOP.

''At this point, all of the instruments are working fine and returning data,'' concluded Adam Drobot of the Science Applications International Corporation in McLean, Virginia, in a live televised interview on 24 February. ''I think everybody is excited that things are going very well." Drobot's own experiment, the Theory and Modelling in Support of Tether (TMST), provided theoretical electrodynamic assistance to TSS-1R. Meanwhile, as preparations got underway, other experiments were undergoing tests. One of these was an electron gun called the Shuttle Elec-trodynamic Tether System (SETS), which was designed to generate an electron beam in support of the science experiments.

It would also provide voltage and current readings from the tether throughout the deployment process. Multiple test beams were fired from the experiment's electron gun on 24 February to acquire data on Columbia's ionospheric environs. A day later than originally planned, but nevertheless right on cue at 8:45 pm, the deployment got underway and proceeded without a hitch under the watchful eyes of Hoffman and Gui-doni. The satellite pushed itself away from its docking ring on top of the mast using its own cold-gas thrusters and was expected to reach a maximum distance of 20.5 km over a five-and-a-half-hour period.

''The satellite is rock-solid,'' Hoffman excitedly told Mission Control. It was expected that, after reaching its maximum extent, the satellite and tether would remain extended for 22 more hours of studies of electrody-namic phenomena using onboard and payload bay-mounted instruments. A slow 'creep' back towards Columbia,

TSS-1R inches away from its mast at the start of deployment operations.

precisely choreographed from Mission Control in Houston, would then have resulted in a docking back onto the mast at around 6:43 pm on 26 February.

Orbital dynamics resulted in TSS-1R initially deploying 'upwards', at a rate of just 15 cm/min, and an angle 40 degrees 'behind' Columbia's path; its motion was carefully controlled by electric motors which reeled out the tether, and by the satellite's cold-gas thrusters. An hour into the deployment, TSS-1R eclipsed the 258-m limit of its predecessor. Gradually, the rate increased to 1.6 km/h, slowing briefly within a couple of kilometres of the Shuttle in order that the 40-degree angle could be reduced to just five degrees. This placed the satellite almost directly 'above' the Shuttle's cabin.

Throughout the deployment, Columbia's RCS thrusters were disabled by Allen and Horowitz to avoid causing unwanted oscillations in the tether. Beginning at a distance of 610 m, the satellite underwent a series of very slow rotations in support of its onboard and payload bay-mounted science investigations. The deployment rate of the tether, meanwhile, increased to a peak speed of 8.1 km/h around 1:00 am on 26 February, when at a distance of 15 km from Columbia. It was shortly after this point that things started to go wrong.

The intention was that, as the tether neared its 20.5-km maximum length, its deployment rate would have been gradually reduced; however, at 1:29:35 am on the 26th, at a tantalisingly close-to-target distance of 19.6 km from the Shuttle, the tether snapped! According to the shocked crew who recorded video footage of the incident, the break appeared to have occurred near the top of the mast in the payload bay. ''The tether has broken at the boom!'' Hoffman radioed urgently. ''It is going away from us'', accelerating away at about 670 km during every 90-minute orbit.

By mid-afternoon, it was trailing Columbia by 4,830 km, flying some 50 km 'above' the Shuttle. After winding in the remaining 10 metres or so of tether by 6:58 pm, the astronauts retracted the collapsible mast to its original configuration. Although nearly a day's worth of electrodynamic measurements had been lost, the $154-million reflight was not, however, a total failure. Already, when the satellite was less than 6 km from the mast, Carlo Bonifazi - Principal Investigator of the Deployable Core Equipment (DCORE) investigation - reported his first data.

0 0

Post a comment