Satellites in Space

Jeremy Curtis is an engineer and business development manager for space 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 have been launched in 1999 on the Russian Spectrum-X spacecraft.

He says "I trained as a mechanical engineer, but I find space engineering exciting because I have to work with all kinds of experts such as astronomers, physicists, designers, programmers and technicians working around the world".

He was sponsored by RAL during his university degree and then spent several years on designs for a large proton synchrotron (a machine for accelerating protons to very high energies) before moving over to space instrument design. In the following passage he describes some of the aspects of space engineering.

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Why Satellites?

Getting spacecraft into orbit is a very expensive activity with typical launch costs generally measures in tens of thousands per kilogram. So what makes it worth the bother? There are three key reasons.

First, a satellite is a good vantage point for studying the earth's surface and atmosphere - just think how many aircrafts 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 parts of the universe we have to get above the atmosphere. The earth's atmosphere absorbs almost everything that tries to go through it - from X-rays to ultraviolet and from infrared to millimetre waves. Only visible light and radio waves can get through it. In fact, even visible light suffers - convection in the earth's atmosphere makes stars seem to jump about or twinkle, blurring telescope images, so a telescope in space produces sharper images than possible from earth.

Finally, and not least, a communications satellite can beam TV pictures across the globe and link telephone users from different continents.

The Problem With Space

Once you've got through the huge trouble of expense of launching your satellite, a new set of problems confront you in space.

First, a typical spacecraft may need several kilowatts of power - but 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 there is no worry about what to do on cloudy days, but batteries are still needed for periods when the satellite is in the earth's shadow (usually up to an hour or two per orbit) and the satellite has to be continually steered to keep 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 provide partial shade from sunlight the surface still starts to heat up, and with no air to convect the heat away the temperature can rise dramatically.

To add to the difficulties, the other side of the spacecraft faces cold space (at about 3k or -270�C) 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 crinkly shiny material.

Studying With Satellites

The UoSAT satellites are very small, relatively low-cost, spacecraft whose purpose is to test and evaluate new systems and space technology and to enable students and amateur 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 local magnetic field, providing information 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 transmit messages to simple radio receivers anywhere in the world. UoSAT's orbit takes it over both poles at a height of about 650km above the earth's surface, and the spinning of the earth allows it to receive data about six times a day. Each 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 ground stations on the earths surface. UoSAT's are small, each 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 venture 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 demand of about 110kw.

Spacecraft power systems

The below figure shows three main elements in a spacecraft power system. The primary source involves the use of fuel to produce electrical power. Primary sources include fuel cells in which a chemical reaction between hydrogen and oxygen produces electricity (with drinking water as a useful by-product), and radioisotope thermoelectric generators (RTG's) 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 nuclear fusion.

The secondary source is the energy storage system - usually a set of batteries. Sometimes regenerative fuel cells are used in which power from solar arrays electrolyses water to produce hydrogen and oxygen gases during the "charge" cycle, followed by hydrogen and oxygen recombining to make water during the "discharge" cycle. n electronic power control and distribution unit controls and adjusts the voltage and current inputs and outputs, often using primary and secondary sources together to boost the overall output power.

There are other systems available and 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.

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 important 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 occasions when batteries are needed to provide power in addition to that of the solar panels.

The spacecraft's solar panels are used to recharge its batteries when it emerges into sunlight. To do this they must provide a high enough voltage - higher than the batteries own voltage. (A charger 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 electrical 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.

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