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Aviation History
1992
1992 - 1344.PDF
capability, relatively expensive though this is. The crew of the newest Shuttle, the Endeavour, on its recent satellite rescue mission (Flight International, 20-27 May, P 5), faced problems in the delicate task of capturing and fixing a booster to a stranded communications satellite so as to send it into its correct orbit. It eventually suc ceeded, however, where remotely operated mechanical equipment could not have done. The nerve-wracking haste with which the crew had to replan and execute the difficult and delicate task considerably increased the risk to the men carrying out the extra vehicular manual work, however; they would have been glad of the extra few days in orbit that an EDO ca pability can bestow. EVOLUTION As the programme has evolved, the Shuttle has changed from a space truck on nomi nal seven-day sorties to a versatile vehicle which can act as a space laboratory. The longer it can stay up and the more power it can provide to some payloads, the more ef fective the research and the happier the scientists will be. The EDO mission kit will be offered as an optional service to Shuttle clients. Among the fields that could benefit from additional on-orbit time is materials-processing re search — in semiconductors, glasses, ce ramics, biologicals, alloys, catalysts and ultra-pure materials — which is the re search being conducted during the USML 1 mission. Other disciplines which would benefit from longer in-orbit time are life- sciences research in basic physiology and microgravity horticulture; advanced space operations development and space-station support, including assembly work and on- orbit servicing and maintenance; and scien tific observation missions in such areas as astrophysics and Earth remote sensing. Missions already booked for the EDO after STS50 are Spacelab Life Sciences (SLS2), to be launched aboard the Columbia on mission STS58 in July 1993; Interna tional Microgravity Laboratory (IML 2) on the Columbia's STS66 in May 1994; USML 2 in 1995 and SLS3 in 1996. There are also discussions about flying a second Astro astronomy mission, lasting 28 days, after the success of the STS35 seven-day Astro 1 sortie in December 1990. NASA hopes that the EDO programme will allow the Shuttle to make 16-day missions, gradually building up to 28 or 45 days on independent flights. Even 60 days on missions docked to the Freedom Space Station may be possible, with the Shuttle's power and life-support systems being bol stered by those of the Freedom. The Colum bia will be able to fly only 16-day missions because, being the heaviest orbiter, the payload and amount of consumables which can be carried are affected. The Endeavour has been built to support 28-day missions with the EDO, with the potential for more. The other orbiters, the Discovery and the The EDO capability makes the Shuttle a more flexible space vehicle Atlantis, would need to be modified exten sively to support EDO flights. The heart of the EDO is in the provision of more electrical power and life support supplies and equipment. The first EDO mission kit comprises mainly a cryogenic pallet of liquid-oxygen/liquid-hydrogen (LOX/LH) tanks to feed the existing fuel- cell system to provide power for a longer duration; a regenerative carbon-dioxide re moval system; waste-collection systems, in cluding a unique trash compactor; additional nitrogen tanks; and crew-cabin improvements for better equipment storage and increased habitable volume. POWER FOR LONGER All Space Shuttle orbiters operate three independent 27-32V electrical buses, sup plied by three 92kg, 7kW Pratt & Whitney LO/LH fuel cells, each providing the equiva lent of 18kW of electricity for two days. They provide an average of 14kW continu ous power with a peak of 24kW possible. The cells are part of the Shuttle's power- reactant storage and distribution system. The cryogenic LO/LH reactants, stored in up to five pairs of spherical tanks — depending on mission length and crew number — under the forward payload bay, provide the reactants for the fuel cells and also oxygen for the environmental-control system, to be mixed with nitrogen from gas stored in four 23kg tanks in the forward bay in the proportion 21:79, to make breathea- ble air. Two main ll.OOOrpm fans circulate the air both in the cabin and as coolant over the avionics-bay equipment and the inertial measurement unit. The LOX/LH fuel-cell reactants are stored in double-walled, vac uum-jacketed, Dewar- type spherical tanks, the oxygen at 97K and the hydrogen at 22K. The tanks contain ei ther 354kg of LO or 42kg of LH. These cold, dense, high-pressure gases are provided to the cells, which gener ate electricity through chemical reaction. A bi- product of the reaction is drinking and cooling water, produced at a rate of llkg/h. AUTOMATIC Automatic controls, ac tivated by pressure, en ergise the tank heaters and thus add heat to the reactants to main tain the essential pres sure during depletion. Each tank has relief valves to prevent over- pressurisation from ab normal operating conditions. The dis tribution system consists of filters, check valves and shut-off valves. Fuel cells are well-proven: they were first flown on a manned spacecraft, the Gemini 5, in August 1965, and, apart from the battery-powered Gemini 6, have flown on every US mission since. Former Soviet manned spacecraft were powered by batter ies until the flight of the Soyuz 1, which had two solar panels. Like the Gemini 5's fuel cells, which misbehaved and nearly cut short an attempted record eight-day mis sion, the Soyuz l's solar panels did not work as planned. One failed to deploy, and other problems caused the craft's demise and the first space fatality, that of Vladimir Komarov, in April 1967. An oxygen-tank explosion in the Apollo 13's fuel-cell pack resulted in the infamous "lost in space" saga in April 1970, which thankfully ended in glory (the crew was rescued by the Apollo lunar module in a "lifeboat" role). The EDO is the next step forward in manned-spacecraft electrical generation, but it is a simple affair — basically more oxygen 44 FLIGHT INTERNATIONAL 27 May - 2 June, 1992
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