Tuesday, February 19, 2019
Satellites in space
Jeremy Curtis is an engineer and business development manager for lay science at the Rutherford Appleton laboratory (RAL) in Oxfordshire. His job includes on the joint European telescope for X-ray astronomy (JET-X), due to train been launched in 1999 on the Russian Spectrum-X posecraft.He says I trained as a mechanical engineer, exclusively I find space engineer exciting because I pee to work with all kinds of experts such as astronomers, physicists, designers, programmers and technicians workings around the existence.He was sponsored by RAL during his university degree and then spent several old age on designs for a large proton synchrotron (a machine for accelerating protons to very high energies) out front moving over to space instrument design. In the following enactment he describes some of the aspects of space engineering.Why satellites?Getting spacecraft into field of force is a very expensive activity with typical launch be generally steps in tens of thousand s per kilogram. So what entertains it worth the b early(a)? There atomic number 18 three key reasons.First, a satellite is a good reward point for studying the balls surface and atmosphere just think how many mailcrafts would be needed to photograph the whole of the earth, or how many ships to monitor the temperature of the oceans.Second, if we want to study most of the radiation coming for distant separate of the universe we have to get above the atmosphere. The earths atmosphere absorbs almost everything that tries to go through it from X-rays to ultraviolet and from infr ared to millimetre waves. Only microscopic prosperous and intercommunicate waves feces get through it. In fact, even visible uncontaminating suffers convection in the earths atmosphere makes stars translatem to jump round or twinkle, blurring telescope images, so a telescope in space haves cardsharp images than possible from earth.Finally, and not least, a communications satellite can tool TV pictures across the globe and link telephone users from different continents.The puzzle with spaceOnce youve got through the huge trouble of expense of intromission your satellite, a new set of problems confront you in space.First, a typical spacecraft may need several kilowatts of powerfulness however where do you plug in? The only convenient renewable source of power is the sun, so most spacecrafts are equipped with panels of solar cells. You can see these on the Infrared space observatory (ISO). Unlike earth in that respect is no worry about what to do on cloudy days, but batteries are still needed for periods when the satellite is in the earths nighttime (usually up to an hour or two per land) and the satellite has to be continually steered to time lag the panels pointing at the sun.So now we have our spacecraft floating in orbit and pointing to face the sun all the time. Although the solar cells propose disperseial derivative shade from sunlight the surface still st arts to heat up, and with no air to convect the heat away the temperature can rise dramatically.To add to the difficulties, the separate side of the spacecraft faces cold space (at about 3k or -270C) and so begins to cool down, unchecked this would distort the structure, wreck the electronics and decompose the materials that make up the spacecraft. So most surfaces of the spacecraft are covered in space blanket multilayer insulation made of metallised plastic which reflects the radiation away and insulates the spacecraft. This is uneven shiny material.1.2 Studying with satellitesThe UoSAT satellites are very abject, relatively low-cost, spacecraft whose objective is to test and evaluate new systems and space technology and to enable students and recreational scientists to study the near-earth environment. They are designed and built by the university of Surrey spacecraft engineering research unit. UoSAT, also known as Oscar 11 has sensors to record the treetopical anaesthetic magnetic field, providing in variateation about solar and geomagnetic disturbances and there affects on radio communications at various frequencies.Instruments on board also measure some 60 items relating to the satellites operation. These include the temperature of its faces, its batteries and other electronic devices the current provided by its solar arrays and the battery voltages. It can also receive store and ravish messages to simple radio receivers anywhere in the world. UoSATs orbit takes it over two poles at a height of about 650km above the earths surface, and the spinning of the earth allows it to receive data about six times a day. each(prenominal) UoSAT spacecraft is designed to last about 7 years.Even small spacecrafts such as these need electricity to run all onboard systems, form the computer that controls it all, to the radio transmitters and receivers that send and receive all data to and from understanding stations on the earths surface. UoSATs are small, eac h with a mass of typically 50kg and about 0.5m across. For comparison, JET-X is about 540kg in mass and about 4.5m long. Communications satellites are larger still, with masses of typically 2 to 5 tonnes.At the top en of the scale is the proposed International Space Station (ISS) a co-operative conjecture between 13 nations, including the UK. Construction and testing started in1995 and completion is due in 2002. The completed station will have a mass of about 470 tonnes, measure 110m from tip to tip of its solar arrays, and have pressured living and working space for its crew of six almost equal to the passenger space on two 747 jet airliners. It will have a imply of about 110kw.1.3 Spacecraft power systemsSchematic diagram of a spacecraft power systemThe below figure shows three main elements in a spacecraft power system. The primary source involves the use of provide to produce electric power. Primary sources include open fire cells in which a chemical reaction between hydrog en and oxygen produces electricity (with boozing water as a useful by-product), and radioisotope thermoelectric generators (RTGs) in which a radioactive decay process produces heating in a thermoelectric module that generates electricity. In spacecraft, the most common primary source s the photovoltaic cell, powered by solar radiation here the initial fuel is protons in the sun, which undergo thermonuclear fusion.The alternate source is the free energy storage system usually a set of batteries. Sometimes regenerative fuel cells are utilize in which power from solar arrays electrolyses water to produce hydrogen and oxygen gases during the take down cycle, followed by hydrogen and oxygen recombining to make water during the discharge cycle. n electronic power control and dispersion unit controls and adjusts the voltage and current inputs and output signals, often using primary and secondary sources together to boost the overall output power.There are other systems available a nd these are shown in figure 8 in the textbook, on page 69. Here are some listed* Chemically fuelled turbines and reciprocating engines.* Chemical turbines and batteries.* Batteries.* cryogenic hydrogen/oxygen expansion engines.* Cryogenic engines and fuel cells.* Fuel cells.* nuclear dynamic systems.* Solar and nuclear dynamic systems.* Photovoltaic and radioisotope thermoelectric systems.A useful link to research this further is http//spacelink.msfc.nasa.gov/Question 1, Page 70Using figure 8 on page 69, decide which would be the most suitable power source(s) for a spacecraft needing(a) 1kw power output for just one week.Cryogenic engines and fuel cells.(b) 10kw for 10 years.Solar and nuclear dynamic systems.The most common primary source of energy used in satellites is the photovoltaic cell or solar cell. Hundreds of thousands of such cells are connected together to make up solar arrays. UoSAT 2 and the ISS have many arrays of solar arrays attached to them. Solar cells have one g reat characteristic they only generate electricity when illuminated. Orbiting satellites undergo between 90 and 5500 eclipses, moving into the shadow of the earth, each year.The former is typical of a geostationary telecommunications satellite, the latter of a satellite is in a low orbit like UoSAT 2. The ISS will have sixteen thirty minute periods of shadow each day. The secondary power supply is therefore vital, because during eclipse electrical power has to be supplied by batteries. There are also do when batteries are needed to provide power in addition to that of the solar panels.The spacecrafts solar panels are used to recharge its batteries when it emerges into sunlight. To do this they essential provide a high enough voltage higher than the batteries own voltage. (A courser for a 12v car battery provides about 30v.) The power system must therefore be carefully designed to ensure that the solar panels can charge the batteries and that the batteries can operate the electric al equipment on-board.So what voltage does a solar cell provide? How does this voltage vary with the brightness of the light? How can we connect up solar cells in order to charge batteries and operate equipment? These are questions I will explore in part two of this unit.