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Aviation History
1997
1997 - 2240.PDF
SPACEFLIGHT • Reaction and wheel ball-bearings require exquisite lubrication. MMS has developed ball-bearing technology in which the amount of lubrication re quired has been reduced to a level of a few molecules. This is the equivalent to the amount of body moisture transmit ted to an object by very briefly touching it with a finger. The accuracy of machining ball bearings has been increased to a 1 micron level. • Lightweight-materials technology in spacecraft propellant tanks has made a major contribution to maximising pay- load weight. The Dowty Aerospace pro pellant tanks for the European Space Agency's (ESA's) Cluster science satel lites weigh just 6kg, yet contain 130kg of propellant and sustain pressures of more than 30 atmospheres. • Engineers have been working on the concept of electric ion propulsion for over 30 years, but the technology has only been recently introduced on satel lites, not for primary propulsion, but for attitude control. As communications satellites have been able to generate increased electrical power, using larger and more efficient solar panels, more of this power has been available to operate ion thrusters, which offer a much higher specific impulse than that by chemical thrusters and thus smaller amounts of propellant: the mass saving converts into a longer life or more payload. European ion thrusters will be assessed opera tionally on the Artemis and Stentor com munications-technology satellites being developed by ESA and France, respec tively. Tiny electric thrusters developed by ESA, called Field Emission Electric Propulsion devices, will be used on new astronomy spacecraft, as the quest for higher spatial resolution continues and even higher pointing stabilities down to nano-radians are required. • In the quest to give spacecraft greater autonomy to reduce the size of ground operations staff, future spacecraft will use star sensors which are able to recognise any pattern of stars without ground inter vention. Working with Oxford Uni versity, MMS has demonstrated a system which can search a catalogue of 20,000 known stars in a fraction of a second and deduce the spacecraft's orientation with no previous knowledge. • Spacecraft will also have higher on board computing power to provide this automony, while planetary landers will incorporate vision-based control systems which extract feature information in real time from digital pictures. 56 attitude data. Other sensors provide images of the Sun for two axes of attitude infonnation. Simple silicon detectors can provide a rough attitude reference of 1 ° accuracy with a 90° field of view and are used by the AOCS initially to control the spacecraft as it separates from the final stage of its launcher, or after an anomalous event somewhere else in the system. PRECISE SENSORS "Once coarse attitude pointing is achieved, the system can hand over control to much more precise sensors, such as fine digital Sun sensors or star trackers," says Parker. The fine Sun-sen sor uses a mask consisting of a complex set of slits, covering a row of photocells, which work like the key in a lock. The angle between the Sun and the sensor - and therefore die spacecraft - determines through which particular slits the sunlight passes to illuminate the photocells. Star trackers are more complex cameras which use a charged-coupled-device imager to detect the tiny points of light corresponding to distant stars. They provide attitude-sensing information, and at present are found only in demandingscientificandremote-sensingappli- cations such as the MMS-built Envisat polar orbiter and the XMM. Gyroscopes are useful adjuncts to optical sen sors because theycan provide infonnation when the spacecraft is turning too quickly for optical sensors to "lock on", or when references such as the Sun are missing, for example during an eclipse. Since die information from the gyros inevitably suffers from drift because of mechan ical and electronic imperfections, data have to updated using one of die optical sensors. Most current gyros are mechanical devices and some failures have occurred over the long lifetimes required. Spare sensors are always pro vided because the AOCS must be able to cope with equipment or software failures in any part of its operation. Xew types of "solid-state" gyros such as fibre-optic, hemispherical, ones (which work like "pinging" a wine glass) and quartz rate sensors are now being introduced. To change the position of a spacecraft, a directional force must be applied.To change die attitude, a torque is needed. Available actuators include an on-board propulsion system using thrusters, reaction wheels (flywheels about the size of a small-car wheel, turning at up to a few thousand RPM), electromagnetic coils which can interact with the Earth's magnetic field to slowly turn the spacecraft and the solar array. MMS-built communications satellites have patented solar "sailing" systems with small flaps at the ends of the solar array, which interact with the solar radiation to produce a weak, but usable, control torque. On current spacecraft, only the propulsion system provides directional forces, although solar sailing has also been proposed for this application in the future. The thrusters used can provide tiny bursts of force lasting perhaps only 25 milliseconds, or sustained forces for up The XMM has one of the most demanding attitude requirements of all to an hour to change satellite orbits. The effec tiveness of a propulsion system is measured by its specific impulse: how much impulse is pro duced per unit mass of fuel. Simple monopropellant systems are used on many spacecraft. Pressurised liquid-hydrazine propellant is decomposed over an iridium cata lyst to produce a gas at about 1,000°C, which is expanded through small nozzles. For short-life, simple, spacecraft, compressed nitrogen or other cold gas can be used, but the specific impulse is only one-third as good as that of a hydrazine system. For large, long-life, spacecraft, more- efficient bi-propellant systems are used. These employ monomethyl-hydrazine fuel and nitro- gen-tetroxide oxidiser, which ignite when mixed together, in a hypergolic reaction to pro duce thrusts of up to 400kN (90lb). DIRECT ATTITUDE CONTROL Using propulsion systems for direct attitude control is usually not the most efficient solution. Instead, the AOCS may use reaction wheels to assimilate disturbances smoothly by absorbing in the flywheels the momentum so produced, causing them to accelerate or decelerate slowly. Similarly, by deliberately driving the wheels faster or slower, die spacecraft can be made to rotate, or slew. This method is being used to move the XMMs aim between different star targets. "Spacecraft are powerful demonstrators of many basic laws of physics," Parker says. The attitude-control computer's failure- detection and correction system is a constantly vigilant watchdog which monitors incipient or actual failures in other parts of the AOCS. "It is the most mission-specific part of the system because it has to be designed to cope with any thing that could go wrong, and correctly decide what to do about it," Harris says. It has to take over control and either contin ue the normal mission sequence or put the spacecraft into a safe condition until ground control can decide what has gone wrong. Providing safe operation of the AOCS depends on a combination of testing and computer simu lation. "You cannot do a short test flight as with an aircraft. Once you've launched, it's all or nodiing," says Parker. • .IGHT INTERNATIONAL 3 - 9 September 1997
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