THERE HAVE been four previous attempts at tethered flight between space vehicles. The abortive TSS 1 attempt in 1992 took place 26 years after the previous attempt aboard the Gemini 12.

The first tethered flight took place during the sixth manned spaceflight, NASA's Mercury Atlas 7/Aurora 7, in May 1962, piloted by Scott Carpenter. He deployed, for a space-vision experiment, a multi-coloured balloon, which did not inflate properly and the movement of which at the end of its tether was unpredictable, even twining around the capsule's antenna canister before the balloon was abandoned.

In September 1966, the Gemini 11 crew, Pete Conrad (now working on the Delta Clipper, DC-XA programme with McDonnell Douglas) and Dick Gordon, flew a "gravity-gradient" experiment with a tethered Agena stage. The stage had been the target for a docking after one Earth orbit, to simulate the ascent of an Apollo lunar module to join the command module in Moon orbit. The 30m tether, attached during docked flight by the spacewalking Gordon, never became fully taut.

"This tether's doing something I never thought it would. It's like the Agena and I have a skipping rope between us. It's rotating and making a big loop...I can't get it straight. This a weird phenomenon," said Conrad at the time. Similar experiences on the Gemini 12 mission in November 1966, piloted by James Lovell and Buzz Aldrin, which left the crew exasperated and their spacecraft and its Agena target rocket at one time "wigwagging" through 360¡, forced NASA to admit that it had not got to grips with the "puzzling experiment".

The flight objectives of the STS46 Shuttle mission in 1992 were so ambitious that both NASA and the Italian space agency conceded that the chances "...were low that the system will achieve its major goals".

To the STS46 astronauts, the tether-reel system seemed to be baulky compared with when they worked with it on the ground. They experienced the predicted tether pendulum motion and use of the Shuttle's thrusters was required to regain stability. The satellite's own thruster-control system "...had extraordinary handling properties", says Hoffman.

Franco Malerba, the Italian payload specialist aboard the STS46, recalled in his book, La Vetta, that, "...just after the fly-away, the satellite seemed to lean to the right", very visibly, at an angle of 45¡ to the vertical, "approaching the limit", where "...the satellite could fall back on us". Apart from the two tether jams, the deployment was made in a constant stop-start process, as various problems were overcome and careful monitoring completed.

The tether reached a distance of 179m and suddenly stopped. "The satellite, jerked by the sudden stop, rebounded towards us," says Malerba, recalling the events. Later, it "...swayed back and forth". Unknown to the crew, there was a bolt protruding in the gearbox under the mast, interfering with the level-wind mechanism.

The irony is that the errant 6mm bolt added to the deployment-mast mechanism was part of a modification to strengthen the reel to increase safety. The tether snagged because it hung more loosely within the mast than had been anticipated because of "...some phenomenon we don't understand fully", said Martin Marietta at the time.

What Hoffman describes as his "very anxious moment" came when the tether was freed after its first jam and unreeled faster than anticipated, leaving a lot of slack, almost "outracing the satellite" at its end. Mission control had ordered a "running-start" redeployment at a speed of 600mm/s. "We would have transient slack tether," Malerba believed, correctly. The tether's guillotine was armed, in case. The unstable tether unwound and the satellite passed the 179m point. Then, at 256m, the cable stopped again.

Post-mission analysis revealed that, after the tether had made contact with the bolt, "...right up to 256m, the reel-drive mechanism was working against the level-wind mechanism to which the bolt was attached". The gearbox and gear chain in between the two mechanisms were "...popping out like the gear chain of a bicycle does when the wheel is blocked", Malerba says.

The tether's capacity to generate electricity was proved when induced voltage was measured at 60V and about 2mA of current drawn by the circuit. A camera recorded the satellite brightening up with intermittent luminosity, synchronised with the on-off cycles of its electron gun.

"I think that the TSS system basically works," says Chang-Diaz. "We did not get as far as we wanted to go, but the system is a very sound concept." Loren Shriver, the STS46 commander, says that the mission was a "...tremendous success from a test pilot's perspective because we learned so much about the hazardous phases of tethered flight". Allen's promotion to command the STS75 was therefore a natural move.

Source: Flight International