OrbitalHub

We presented in a previous post the Lunar Reconnaissance Orbiter (LRO) mission. The goals of the LRO mission are to map the lunar resources and to create a detailed 3D map of the lunar surface in preparation for future NASA missions to the Moon. However, NASA is not the only space agency that has high hopes regarding the exploration of the Moon. The Indian Space Research Organization (ISRO) is another agency heavily involved in space activities.

Credits: ISRO

Interest in undertaking a lunar scientific mission was sparked at a meeting of the Indian Academy of Sciences in 1999. One year later, the Astronautical Society of India made a recommendation supporting the idea.

The ISRO formed a National Lunar Mission Task Force that involved leading Indian scientists. The Task Force provided an assessment on the feasibility of such a mission. The mission, called Chandrayaan-1, was approved in November 2003 for an estimated cost of $83 million USD.

The Chandrayaan-1 spacecraft is a 1.5 meter cube with a weight mass of approximately 523 kg. The spacecraft bus is based on an already developed meteorological satellite. Chandrayaan-1 will carry a 30 kg probe that will be released to penetrate the lunar surface. The power for the onboard systems is generated by a solar panel. The 750 watts generated by the solar panel will be stored by the rechargeable batteries onboard the spacecraft. Maneuvering in the lunar orbit is done using a bipropellant propulsion system.

Credits: ISRO

The scientific payload contains a diverse collection of instruments. The instruments were designed and developed by ISRO, ESA, NASA, and the Bulgarian Space Agency.

There are two instruments that will map the surface of the Moon: the Terrain Mapping Camera (TMC) will produce a 5 meter resolution map of the surface, and the Lunar Laser Ranging Instrument (LLRI) will scan the lunar surface and determine the surface topography.

The X-ray spectrometer onboard the spacecraft has three components: the Imaging X-ray Spectrometer (CIXS), the High Energy X-ray/gamma ray spectrometer (HEX), and the Solar X-ray Monitor (SXM). The X-ray spectrometer will measure the concentration of certain elements on the lunar surface as well as monitor the solar flux in order to normalize the results of the measurements taken.

The mineralogical configuration of the surface will be mapped by four instruments: the Hyper Spectral Imager (HySI), the Sub-keV Atom Reflecting Analyzer (SARA), the Moon Mineralogy Mapper (M3), and the Near-Infrared Spectrometer (SIR-2).

The Radiation Dose Monitor (RADOM-7) will record the radiation levels in the lunar orbit.

Credits: ISRO

ISRO has two operational launch vehicles: the Polar Satellite Launch Vehicle (PSLV) and the Geosynchronous Satellite Launch Vehicle (GSLV). For Chandrayaan-1, ISRO has chosen to use PSLV as a launch vehicle. The PSLV developmental flights were completed in 1996 and the rocket has had 12 successful missions since then. PSLV is 44.43 meters tall and has a weight of 294 tonnes at launch. It can inject a payload of 1,000 kg – 1,200 kg into a polar orbit.

The launch of the Chandrayaan-1 mission is scheduled for the end of October 2008. The PSLV rocket will take off from the Satish Dhawan Space Center in Sriharikota on the southeast coast of India. The transfer to the lunar orbit will take approximately five days and after additional maneuvers the spacecraft will reach its final polar orbit, 100 km above the surface. The spacecraft will be operational for two years.

The Chandrayaan-1 mission opens the door to future lunar missions. ISRO has already committed to a second Chandrayaan mission that will land a rover on the surface of the Moon. The rover will perform a number of experiments on the lunar surface and the results will be relayed to Earth by the Chandrayaan-2 orbiter.

We will come back with more details about the Chandrayaan-1 mission as the events unfold. Please stay tuned on the OrbitalHub frequency.

In the previous three posts we presented the most common solutions employed by spacecraft designers in order to generate the power needed by onboard systems during a space mission: the batteries, the solar panels, and the radioisotope thermoelectric generators. We will conclude by presenting two more existing solutions: the fuel cells and the nuclear reactors.

 

Credits: NASA

 

Fuel cells are devices that convert chemical energy into electricity. Even if using the same type of energy conversion, fuel cells are more efficient than the batteries. Electricity is produced directly from an oxidation reaction. The fuel and an oxidant react in the presence of a catalytic material inside the cell. By eliminating the reaction products and maintaining the input flows, fuel cells can operate continuously. For space applications, only hydrogen and oxygen have been used as reactants. Other sources rich in hydrogen that can be used as fuel are methane, methanol, and ethanol.

Regenerative fuel cells are a viable option for energy storage in large space systems. They could successfully replace the secondary batteries. The regenerative fuel cells would use stored hydrogen and oxygen to generate electricity during eclipse periods and would use solar arrays to generate electricity to recharge the fuel cells during the illuminated portion of the orbit. The generated electricity would be used to produce oxygen and hydrogen by electrolyzing the water produced by the fuel cells during normal operation. As far as we know, there are no direct applications of regenerative fuel cells in the space industry to date.

 

The nuclear reactors used by spacecrafts for power generation are smaller versions of the nuclear reactors used onboard nuclear submarines or nuclear aircraft carriers. They are the only compact solution for large power levels, hundreds of kilowatts to megawatts. In principle, the controlled nuclear reaction generates the heat, while an agent carries the heat away and is used to generate steam. The steam is used to drive a turbine that generates electricity. I was not able to find any technical details on the cooling agent and the liquid (or gas) used to drive the turbine (or even if a turbine is used) for the SP-100. SP-100 is the only nuclear reactor destined to power space systems built by the US. I was able to dig up the information that the Russian-built nuclear reactors that operated on the RORSAT reconnaissance satellites used NaK-78 as cooling agent (NaK-78 is a sodium and potassium fusible alloy with a low melting point).

 

SP-100 initially was supposed to have a mass of approximately 3,000 kg and generate 100 kWe. The SP-100 program was eventually canceled due to the fact that as the design matured the weight exceeded the acceptable limit.

 

Credits: NASA/JPL

 

The RORSAT Russian satellites have an interesting story… they had active radars onboard and had to be placed in low Earth orbits in order to have the surveillance equipment work effectively. Orbiting in LEOs, RORSAT missions had a shorter lifespan and had to perform a destructive re-entry in the atmosphere. In order to avoid the re-entry of any radioactive material, the nuclear reactor’s core was ejected in a so-called disposal orbit (a high orbit that would postpone the re-entry of the core for a couple of hundreds of years).

Failures were recorded. Most notably, in 1978, a RORSAT mission failed to boost the radioactive core into the disposal orbit and radioactive material entered the atmosphere above the Northwest Territories in Canada. The affected area had over 124,000 square kilometers.

 

A major disadvantage of the deployment of nuclear reactors is that for manned missions, heavy shielding is required. The shield mass can be reduced by employing designs that use geometric separation, but this is attainable only for large configurations. Other disadvantages are the reduction in reliability due to the moving parts and the possible mechanical interferences due to the vibrations that any dynamic system generates. Despite all these drawbacks, nuclear reactors offer a considerable promise for the future.

 

Power systems are essential for a space mission and, due to the challenges raised by the space environment, finding the right solution for a space mission requires careful consideration of many factors. Each solution comes with its own advantages and disadvantages making the work of space systems design engineers hard and rewarding in the same time.

 

We hope you enjoyed reading this series of posts and that you found them interesting. We are looking forward to your feedback and welcome your comments.

 

Some of the most important questions asked in modern science are: how did the Universe begin, how did it evolve to its present state, and how will it continue to evolve in the future? To ask these questions, the remnant radiation that filled the Universe immediately after the Big Bang must be analyzed. This remnant radiation is known as the Cosmic Microwave Background (CMB).

Credits: ESA

ESA plans to answer these questions with Planck: the first European mission to study the birth of the Universe. The Planck Mission will collect CMB radiation measurements using highly sensitive sensors that are operating at very low temperatures. The measurements will be used to map the smallest variations of the CMB detected to date.

The Planck spacecraft will weigh around 1,900 kg at launch. It is 4.2 m high and has a maximum diameter of 4.2 m. There are two modules that comprise the spacecraft: a service module and the payload module.

The service module contains the systems for power generation, attitude control, data handling and communication, and the warm components of the scientific payload. The payload module contains the telescope, the optical bench, the detectors, and the cooling system (which is of critical importance, as we shall see).

The telescope is an important onboard component. The Planck telescope is a Gregorian telescope with an off-axis parabolic primary mirror 1.75 x 1.5 meters in size. A secondary mirror focuses the incoming microwave radiation on two sets of highly sensitive detectors: the Low Frequency Instrument (LFI) and the High Frequency Instrument (HFI). The Gregorian design offers two key advantages: it is compact and it does not block the optical path.

Credits: ESA/Thales

The LFI will be operating at –253 degrees Celsius. The array of twenty-two tuned radio receivers that comprise LFI will produce high-sensitivity, multi-frequency measurements of the microwave sky in the frequency range of 27 GHz to 77 GHz.

The HFI has to be cooled to –272.9 degrees Celsius in order to operate (one tenth of one degree above the absolute zero!). HFI’s fifty-two bolometric detectors will produce high-sensitivity, multi-frequency measurements of the diffuse sky radiation in the frequency range of 84 GHz to 1 THz.

A baffle surrounds the telescope and instruments. The baffle prevents light from the Sun and the Moon from altering the measurements. A complex system of refrigerators is used onboard the spacecraft in order to achieve the temperatures needed for nominal operation. The detectors have to work at temperatures close to the absolute zero, otherwise their own emissions can alter the measurements.

The two instruments will be used to measure the small variations of the CMB across the sky. By combining the measurements, a full sky map of unprecedented precision will be produced. The map will help astronomers decide which theories on the birth and the evolution of the Universe are correct. Questions like ‘what is the age of the Universe?’ or ‘what is the nature of the dark-matter?’ will be answered.

The mission was initially designed as COBRAS/SAMBA (Cosmic Background Radiation Anisotropy Satellite and Satellite for Measurement of Background Anisotropies) because it grew out of two mission proposals that had similar objectives. When the mission was approved in 1996, it was also renamed as Planck in honor of the German scientist Max Planck (1858 – 1947). Max Planck was awarded the Nobel Prize for Physics in 1918.

Credits: ESA

The mission is a collaborative effort. The Planck spacecraft was designed and built by a consortium led by Alcatel Alenia Space (Cannes, France). The telescope mirrors are manufactured by EADS Astrium (Friedrichshafen, Germany). The Low Frequency Instrument (LFI) was designed and built by a consortium led by the Instituto di Astrofisica Spaziale e Fisica Cosmica (IASF) in Bologna, Italy. The High Frequency Instrument (HFI) was designed and built by a consortium led by the Institut d’Astrophysique Spatiale (CNRS) in Orsay, France.

The Planck Mission has two predecessors: the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP).

Credits: ESA

The Planck spacecraft will be launched in early 2009 from Kourou, French Guiana. An Ariane 5 booster will place the spacecraft in a trajectory towards the L2 point. The L2 point stands for Second Lagrangian Point and it is located around 1.5 million kilometers away from Earth in a direction diametrically opposite the Sun. It will be a dual launch configuration, as the Herschel spacecraft will be launched together with Planck.

Between four to six months after the launch, Planck will reach its final position. It will take six more months before Planck will be declared operational.

Planck will perform scientific measurements for fifteen months, allowing two complete sky surveys. The spacecraft will be operational as along as there are resources for the cooling systems onboard.

For more details on the ESA’s Planck Mission you can visit the mission’s home page on the ESA website.

In a previous post we presented the most common solution for power generation onboard unmanned spacecrafts (solar cells and secondary batteries) and mentioned some of the manned spacecrafts that employ this solution.

Credits: NASA

However, solar cells, in combination with secondary batteries, are not usable for missions beyond the asteroid belt, because there the sun’s energy becomes too diffuse. As deep space missions were designed, a new power source was required. The radioisotope thermoelectric generators, also known as RTGs, met the requirements for this kind of mission. RTGs are proven, compact, and reliable power sources that can produce up to several kilowatts of power and operate under severe conditions for many years.

RTGs convert the heat generated by a decay of radioactive fuel into electricity. There are two major components that RTGs consist of: the heat source that contains the radioactive material and a set of solid-state thermocouples that convert the heat energy to electricity. The principle that RTGs rely on is not a new discovery. In 1821, Thomas Johann Seebeck discovered the effect that bears his name and that allows us to convert heat directly into electricity using a simple and robust device. An electrical current is generated when two dissimilar electrically conductive materials are connected in a closed circuit and their junctions are kept at different temperatures. The heat generated by the radioactive decay is used to heat the hot junction of the thermocouple, and exposure to the cold outer space is used to maintain the temperature of the cold junction.

Credits: NASA

Over the years, RTGs have been used safely and reliably on many missions. Among these missions: some of the Apollo flights to the moon, the Pioneer spacecrafts, the Viking landers, the Voyager missions, the Galileo mission, the Ulysses mission, and the Cassini-Huygens mission. The present conversion efficiency achieved by the thermocouples is around 10%, and research continues in order to improve it. Because of the internal resistance and other losses, the overall RTG efficiency is typically 6-7%, which means that the amount of waste heat for every unit of electrical energy produced is quite large.

Even if the radioisotopes used present a loss in energy with time, the half-life of the radioisotopes is not a major life-limiting factor of current RTGs. Major life-limiting factors include the degradation of the thermoelectric elements and the breakdown of insulators because of temperature and radiation.

The radiation emissions from RTGs can damage the electronics onboard spacecrafts. This is why it is necessary to mount the units on booms at some distance from the body of the spacecraft or, at least, provide shielding of some sort in the on orbit configuration. One important thing to mention is that in the launch configuration, when the RTG boom cannot be deployed, the radiation exposure cannot exceed the inherent radiation tolerance of the onboard electronics.

There are two more solutions spacecraft engineers employ to generate power onboard spacecrafts and we will present them in our next post. Please come back later to read our conclusion to the series.

NASA’s return to the Moon requires careful preparation. Finding safe landing sites, locating potential resources, and taking measurements of the radiation environment are some of the tasks the Lunar Reconnaissance Orbiter (LRO) spacecraft will perform while in lunar orbit. LRO is an unmanned mission that will create a comprehensive atlas of the moon’s surface and resources.

The data gathered by LRO will be crucial in designing and building a permanent lunar outpost. The data will also be used to reduce the risk and increase the productivity of the future manned missions to the Moon.

The launch of LRO is scheduled for February 2009. An Atlas V rocket launched from the Kennedy Space Center will place the LRO on a transfer trajectory. After 4 days, the spacecraft will reach the Moon and after performing additional orbital maneuvers, it will move into its final orbit. The LRO’s final orbit will be a circular polar orbit 50 kilometers above the lunar surface.

Credits: NASA

The mission is designed to last for one year, with a possible extension. The total mass of the spacecraft is around 1,000 kilograms, of which 500 to 700 kilograms will be the fuel. The power is supplied by articulated solar arrays, and for the peak and eclipse periods a Li-Ion battery is used. The bandwidth of the communication link will be approximately 100-300 Mbps.

The LRO payload is comprised of six scientific instruments and one technology demonstration.

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) was built and developed by Boston University and the Massachusetts Institute of Technology in Boston. CRaTER will help explore the lunar radiation environment. The data gathered by measurements will help in the development of protective technologies that will keep future lunar crews safe.

The Diviner Lunar Radiometer Experiment (DLRE) was built and developed by the University of California, Los Angeles and the Jet Propulsion Laboratory in Pasadena, California. DLRE is capable of measuring surface and subsurface temperatures from orbit.

The Lyman-Alpha Mapping Project (LAMP) was built and developed at the Southwest Research Institute in San Antonio. LAMP will be used to map the entire lunar surface in the far ultraviolet spectrum.

Credits: NASA

The Lunar Exploration Neutron Detector (LEND) was developed at the Institute for Space Research in Moscow. This detector will create high-resolution maps of the hydrogen distribution and gather data about the neutron component of the lunar radiation.

The Lunar Orbiter Laser Altimeter (LOLA) was conceived and built at NASA’s Goddard Space Flight Center. LOLA will generate high-resolution three-dimensional maps of the moon’s surface.

The Lunar Reconnaissance Orbiter Camera (LROC), developed at Arizona State University at Tempe, will image the lunar surface in color and ultraviolet. LROC will be able to capture 1 m resolution images of the lunar poles.

The technology demonstration is called Mini-RF Technology Demonstration. The primary goal of this demonstration is to locate subsurface water ice deposits. The advanced single aperture radar (SAR) that will be used is capable of taking high-resolution imagery of the permanently shadowed regions on the lunar surface.

The data gathered by LRO will help us develop a better understanding of the lunar environment. This understanding is essential for a safe human return to the Moon and for the future exploration of our solar system.

After a short introduction to the power systems requirements and design factors, we will continue by covering the first solutions adopted by spacecraft designers: the batteries and the solar arrays (aka solar cells).

Credits: NASA

Batteries were used as a primary source of power onboard early spacecrafts. The obvious limitation is that batteries have limited energy storage capabilities and could not keep spacecrafts operational for more then a few days. Most space missions require a reliable power source running for a longer period of time.

Batteries remain the primary means of energy storage onboard spacecrafts. Batteries are divided into two major categories: primary batteries and secondary batteries.

Primary batteries offer higher energy and power densities but are not rechargeable. They are useful for one-time events such as expendable launch vehicle stages. Secondary batteries are rechargeable batteries.

Solar arrays are very well suited for long missions in space. The life expectancy of a solar cell power system is limited only by the degradation of its components. Spacecrafts operating for extended periods of time become feasible with the development of solar arrays. However, if only solar cells are used for generating power, spacecrafts that enter eclipse periods cannot employ only solar cells for power generation.

Credits: NASA

The first low-powered spacecraft designs were using the spacecraft skin for the solar cell deployment. In the case of drum-shaped spacecrafts, only about 40% of the arrays were illuminated by the Sun at any time. Because most of the time the available area on the fixed spacecraft structure is not enough from the standpoint of power requirements, deployable solar arrays are now used. The solar arrays of this type are deployed from the main structure after the spacecraft is injected into orbit.

The deployable panels are designed as extremely lightweight structures due to the fact that they are firmly locked to the spacecraft during the launch. In order to optimize the generation of power, these panels are designed to allow sun tracking.

Credits: NASA

Considering the limitations of the solar arrays, a reliable solution can be reached by employing solar cells and batteries at the same time. Solar arrays can generate power when direct sunlight is available in orbit, while rechargeable batteries can handle peak loads and provide power during eclipse periods. Solar panels and batteries in combination are a common solution used for the unmanned spacecrafts launched to date. The most notable exception is the deep space mission probes using radioisotope thermoelectric generators (we will cover them in a future post).

The early manned spacecrafts, including Mercury, some of the Gemini, and the Russian Vostok /Voshkod vehicles, used batteries. The Russian Soyuz employs solar cells and batteries similar to a typical unmanned spacecraft. The space stations built so far, Salyut, Skylab, Mir, and the International Space Station, have all used solar cells as the primary power source, having secondary batteries for load leveling and eclipse periods.

In the following posts we will see what solutions are available for missions that cannot rely on solar power as a primary source of energy.