Hazardous Hydrazine

NASA managers, though, had already postponed Leestma and Sullivan's three and a half hour spacewalk from October 9th to the 11th to enable the Earth resources instruments to acquire additional data. Moreover, they would attempt a repair job on the Ku-band antenna to enable it to be properly stowed for re-entry. In spite of the problems, the crew's attitude was "good" and Crippen's absence during early training had no detrimental impact. In fact, the agency declared that, as long as Commanders were experienced, there did not seem to be a problem with them joining crews at a relatively late stage.

One of the objectives of Leestma and Sullivan's excursion was to test hardware for the refuelling of satellites in low-Earth orbit. Already, the Department of Defense had expressed interest in having the Landsat-4 Earth resources platform refuelled and repaired in 1987. NASA, too, hoped to refill its Gamma Ray Observatory (GRO) with station-keeping propellant in 1990. Mounted at the rear end of Challenger's payload bay for STS-41G was the Orbital Refuelling System (ORS), containing highly toxic hydrazine fuel, some of which the spacewalkers would transfer between two spherical tanks.

"Satellites have standard refuelling ports that engineers connect up when they're on the ground," explained Leestma. "One at a time, you very carefully have to handle the hypergolic fuels that go into it, because they're pretty dangerous. Hydrazine is very much like water, but it's got different properties, one of which is that it blows up if it's not handled right! Crip and the safety folks were very concerned that we shouldn't do this with hydrazine; we should just do it with water. The heat transfer properties of water and hydrazine are very similar and that's what we really wanted to know. NASA was worried about 'adiabatic detonation', which is - there's no convection in space -as fluids flow through ducts and into tanks, there's no real mixing of the temperature. As a tank is starting to fill up, if you're refuelling a tank in a constrained volume, there's less volume, so the pressure goes up inside the tank. As the pressure goes up, the temperature also goes up. There's really no big deal on Earth, because there's convection and this heat mixes around. In space, there's no convection. It is possible that all that heat will go into one very minute area - just a few molecules get heated -so that they could very rapidly get very hot and reach the detonation point of blowing the whole thing up. You want to be very careful as you flow the fluid, in that you don't get to this adiabatic point and detonate the fuel. I think Crip thought I was a little too cavalier, because I insisted that we should do it with hydrazine. Crip sent me to White Sands [Test Facility in New Mexico], so I spent about ten days there, watching them do adiabatic detonation tests, watching all kinds of things blow up. I came back with a real appreciation for the capabilities of this deadly stuff. You can't breathe it. If you get it on your skin, you can get poisoned. There were concerns that if we used hydrazine and it sprung a leak or even got on our suits, how are we going to get back in the airlock? We didn't want to bring this stuff back in."

Bob Crippen was not at all happy with using 'real' hydrazine in the ORS tests. He knew that the volatile substance could explode at temperatures above 230 degrees Celsius; temperatures which could easily be reached in the intense sunlight of orbital daytime. On the other hand, in orbital darkness, it could freeze, contract and then flow back, over-pressurising and rupturing its fuel lines. If Leestma and Sullivan got hydrazine on their spacesuits, Crippen and McBride would have had to reorient Challenger's payload bay towards the Sun so they could 'bake' it out.

In such a dire eventuality, they would have had to scrub their suits with towels and detergent, seal them in airtight bags, purge the airlock's atmosphere and pipe in fresh air, then remove their helmets. The effects on the spacewalkers were not Crippen's only concern. In an interview with Henry Cooper, he felt that 80 kg of hydrazine was enough "to take off the back end of the vehicle" if it exploded. Although he wanted to use water for the demonstration, his suggestion was rejected because, said NASA, using the real thing would permit tests of 'real' safety procedures.

Crippen was also assured that, although the ORS tanks and fuel lines had not undergone shaking tests equivalent to the stresses of launch and maximum aerodynamic pressure, they were designed with stiffness and robustness in mind. His four NASA crewmates, though, had already become comfortable with the experiment during their months of training without him and, at length, he was won over. In fact, Leestma and Sullivan had already successfully argued for a manual system to control temperatures and pressures in the tanks, on the grounds that no-one knew the exact parameters of hydrazine under different conditions.

"Crip finally agreed to have us do it with hydrazine," said Leestma, "because he had watched me several times in the WETF, doing the whole procedure and how careful we were. We had triple containment of all the liquids at all times. It's a very tedious task, using small tools and lots of arm and hand manipulation that you had to do to do this task." To achieve 'triple containment', three independent valves were placed in each of the coupling 'halves' and three seals were provided at the interface between the fluid path and the astronauts during the refuelling operation.

Before the flight, they had encountered problems - narrowly averted - with the tools they would use. Leestma insisted on testing them before they were sent to Florida for packing, although the engineers were not anxious for him to do this. When he did conduct his tests, a 'ball valve' - a pipe with a rotating valve that he would have screwed into an ORS fuel pipe and worked through it to open other valves - had an extra component fitted which made it two centimetres too long! Had it flown unchecked, the tool would have not have worked correctly.

Leestma also recalled problems with a new type of grease applied to several tools which, he was assured, was slightly different to that used in previous tests, but which should work in the same way. Leestma took the tools home that night and put them in his freezer; by next morning, the grease had frozen solid, the test was cancelled and the 'original' grease applied instead. "But that was still on my mind," he said, "that if something changes, you'd better make sure that those people know that they've looked at all the different things that can go wrong. I don't think it was so smart on my part. It was just the training that they put into you to kind of question everything. A lot of people don't like the astronauts, because they're always asking those silly, dumb questions, but sometimes those silly, dumb questions are appropriate and that one turned out to be okay."

The excursion began at 3:38 pm on October 11th, when Leestma pushed open the outer hatch and entered Challenger's payload bay. He would later tell Henry Cooper that the difference between being inside the Shuttle and outside on a spacewalk was "like the difference between sitting at a desk in a big room and sitting at a desk in the middle of a prairie - you can see so much more". Both astronauts needed about 30 minutes to fully acclimatise to their surroundings, learning how to move and how different their suits 'felt' in space, compared with the WETF pool.

"I grabbed the handhold and pulled out and when I first saw Earth, my heart rate went real high," Leestma said later, "and the docs later confirmed that, because my electrocardiogram reading went real high, 'This is when you came out of the hatch'. I said 'Yeah, no kidding!' I had a tumbling sensation - came out of the hatch and felt I was going to fall! I think my handprints are still in those payload bay handholds, because I just stopped for a short period of time and had to get my heart rate back down and then continue."

However, the excursion went perfectly and six hydrazine transfers were completed overall, without incident. Already, on October 6th, a series of automatic transfers of small quantities of the toxic propellant had been conducted between the two tanks, using controls on Challenger's aft flight deck. Then, during their spacewalk, Leestma and Sullivan modified the piping with the ball valve, successfully leak tested it and transferred around 50 kg more hydrazine through the fuel lines. In fact, because there was no weight on the thread, Leestma found it much easier to attach the ball valve in space than during pre-fiight training.

The overall procedure did take somewhat longer than on Earth - a full, 90-minute circuit of the globe, rather than an hour - because Leestma had to stop work periodically so that Sullivan could photo-document the task. Their work closely

Dave Leestma during his spacewalk.

mirrored what spacewalkers were expected to do on the Landsat-4 mission and, in the summer of 1990, on a flight to refill GRO with fresh reserves of hydrazine. The observatory, it was intended, would be launched in May 1988, loaded with 1,800 kg of the highly toxic fluid and fitted with a specially designed standardised refuelling coupling to support the procedure.

Refuelling GRO, therefore, would mark the first time a fully functional satellite had been refilled with propellant whilst in orbit. Contracts to develop the coupling mechanism for its hydrazine transfer unit were awarded by NASA in December 1984, for completion and delivery just 15 months later - barely six weeks after the Challenger tragedy. It was even optimistically envisaged that high-pressure helium and nitrogen, and even cryogenic fluids, would flow through refuelling lines on subsequent Shuttle missions. In spite of the threefold safety mechanisms, many astronauts breathed a sigh of relief when STS-51L terminated such plans.

With, arguably, the most hazardous portion of the spacewalk over, Leestma and Sullivan's next step was to tend to the Ku-band antenna to ensure it could be retracted and stowed for re-entry. To do this, they had to move it by hand, such that a 'pin', activated from the aft flight deck, locked it securely in place; if they could accomplish this, they would leave the dish open so it could continue relaying data from the Earth-watching instruments. If, on the other hand, they could not get the locking pins in place, they would manually close, deactivate and latch the antenna.

Obviously, for the sake of maximum data return from SIR-B, it was hoped that the second option could be averted. Moreover, if the antenna could not be retracted at all, the crew would be forced to jettison it overboard in order to close the payload bay doors for re-entry. That, said Leestma, was equally unthinkable. "The Ku-band assembly and digital avionics was worth a million dollars," he said, "so it would have been a very big loss to the programme if we had to jettison it."

The repair involved not only the spacewalkers, but also their colleagues inside the cabin. In fact, because of her role in the effort, Sally Ride 'missed' watching most of the three and a half hour excursion. After she and Jon McBride had unplugged the wire to the antenna's electrical motors on October 6th, they also disabled a mechanism that drove the pins to 'lock' the alpha and beta gimbal axes into place. Early on the day of the spacewalk, they rigged a 'jump wire' that would allow them to reconnect power to the pins, though not the motors.

Unfortunately, both plugs in the jump wire were 'female' and they had to quickly rig up a new, 36-pin 'adapter'. As Ride laboured in the middeck, Leestma manually moved the Ku-band in one axis, then the other, while Sullivan radioed her crewmates when the pins were correctly lined up with the holes they were meant to slot into. Crippen, meanwhile, told Ride when to plug in the two ends of the jumper. Working the current in pulses - plugging and unplugging the cable, such that the pins were 'hammered' into position - the attempt succeeded.

Difficulties with SIR-B's data gathering, though, continued.

"Now, that caused us problems, orbiter-wise," admitted Leestma, "because to use the Ku-band, which the SIR-B required, we had to reorient Challenger so the antenna was pointed towards TDRS-1 and make the orbiter rotate. We'd take data and then do data 'dumps' and point the orbiter at the TDRS; then we'd go back and do data 'writes', rather than being able to take data the whole time and point the antenna and dump it. The SIR-B scientists didn't get all the data that they wanted, but the mission was not a loss and they got almost everything."

Prior to returning inside Challenger's airlock at 7:05 pm, Sullivan took a long look at SIR-B, in an attempt to discover why it had proven so difficult to automatically latch into position. It looked, Henry Cooper wrote later, "like an overstuffed sandwich"; its thermal insulation having billowed in space to make it 'thicker' than it should have been. This pure white blanketing had thus frustrated previous efforts to close it. "The insulation is billowing enough," she told her crewmates, "to interfere with a single motor closing and you don't need to miss by much to keep the latch from shutting."

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