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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.

 

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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.

 

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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.

 

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This post is the first in a series discussing how power is generated onboard spacecrafts.

 

Credits: NASA

 

One major component of a spacecraft is the power system. All systems onboard a spacecraft need electricity in order to run. From the early days of space flight, development of this essential system has been a challenge for spacecraft designers.

 

There are a number of factors that spacecraft designers must take into account: the size, the accessibility, and some operational constraints that can limit the options available. A good example of operational constraints is a spacecraft operating in the Van Allen radiation belts, where radiation exposure can contribute to the rapid degradation of the solar arrays. Other important factors that designers need to consider are the lifetime required by the mission, the orbital parameters, and the attitude control concept employed.

 

Probably some space geeks (especially science fiction fans) would think of M/AM reactors onboard spacecrafts, but the reality is not as glamorous. Maybe future generations of spacecrafts will use that type of technology, but the present generation is using what some might call primitive technology by comparison.

 

Credits: NASA

 

As a general requirement, power systems must control, condition, and process the power received in order to comply with the needs of the systems onboard the spacecraft. The power is received from the primary source, which can be a battery, a solar array, etc. For the duration of the mission, the power system must supply stable and uninterrupted power. If not, the mission is lost.

 

In the second part of this series, we will take a look at the options available to spacecraft engineers when designing the power system for space missions.

 

 

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