- Email: [email protected]
Heaters
13
Heaters Abstract: In general, spacecraft and their parts and components must be kept within precise operational temperature ranges to assure the fulfilment of the expected performances. In some cases this implies the warming of the item under consideration by using appropriate heaters. When the power needed for this task is compatible with the power budget of the vehicle, electrical heaters are a suitable option. Otherwise other heat sources, such as a radioisotope heater, are needed. The main characteristics of these different heating systems are presented in this chapter. Key words: electrical heaters, radioisotope heat sources, heat switches.
13.1 Introduction Reliable long-term performance of most spacecraft components takes place at a specified temperature range. The attainment of some temperature range requires, in many instances, the generation of heat within the spacecraft. In these cases, heaters are sometimes required to fulfil specific requirements such as the protection of components from low temperatures, to provide precise temperature control for devices or components, or to warm up equipment to its operating temperature. For example, heaters are used to warm overboard dump valves for liquids; to keep constant structural temperature on space
225
Spacecraft thermal control
telescopes in order to prevent optical misalignment; to maintain the temperature of sensitive gyroscopes and accelerometer guidance platforms; to control the temperature of pressure transducers, other electronics components, and infrared reference sources; to prevent condensation on viewing windows; to keep solar panels warm at night; on catalyst beds of hydrazine thruster engines for spacecraft attitude control; and as shunt resistors, mounted on the skin of a satellite, to dump the excess power of overcharged batteries which would otherwise overheat delicate instruments. When a local uniform heat source or a profiled heating area is needed, electrical heaters can provide it efficiently due to their versatility, although other types of heaters (chemical or nuclear) are also used in spacecraft. Obviously, the use of electrical heaters requires the availability of a power source. For near-Earth applications, power provided by photovoltaic devices is the preferred option because of the relative proximity of the Sun. However, when spacecraft missions include distances far from the Sun, or in harsh environments (such as the surface of Mars or in certain lunar locations), reliable, long-lived, electrical and thermal power sources independent of the Sun are needed. In the following sections the main features of electrical heaters, as well as of the auxiliary devices needed for control and regulation, are presented; basic concepts of nuclear heaters are described; and, finally, thermal switches are presented.
13.2 Electrical heaters Electrical heaters are based on Ohm’s and Joule’s laws. Ohm’s law states that the steady electric current, I, flowing through an electrical conductor is proportional to the constant voltage, V, and to the inverse of the electrical
226
Heaters
resistance of the conductor, R, that is, I = V/R. According to . Joule’s law, the heat released per unit time, Q , by an electrical current, I, is equal to the square of the electrical current, . multiplied by the electrical resistance, R: Q = I2 R. Two types of heaters typically used on spacecraft are film heaters (or patch heaters) and cartridge heaters. By far the most commonly used type is the film heater, as due to its flexibility it can be installed on both flat and curved surfaces. These are made of electrical resistance filaments sandwiched between two layers of electrically insulating material, such as Kapton, attached to the leads. The heaters come in various sizes to fit any applications on the spacecraft. They can be custom-shaped to adapt them to specific applications or be simply rectangles of standard dimensions, whereas cartridge heaters consist of a wire-wound resistor enclosed in a cylindrical metallic case. Typically, the heating power density on the film heaters is limited to less than 0.8 W/cm2. These are installed on the surface of the particular equipment that needs to be heated using a pressure sensitive adhesive. For temperature requirements less than 473 K, heaters constructed of Kapton film and FEP Teflon are used, whereas in the case of electrical heaters working in hard conditions, for example temperatures over 473 K, they must be sheathed in metal. Nearly all heated systems have some type of temperature controller. Unregulated heaters can exhibit large temperature fluctuations when heat demands or ambient temperatures vary. Often, when the temperature is high, less heating is needed, and when the temperature is low, more is desirable. Heating power can be made to adjust automatically without complicated controls by choosing the heating element material. Nevertheless, the performance of a thermal system is improved by the use of sensors and controllers. Some of
227
Spacecraft thermal control
the most significant improvements are the accuracy of the system to maintain the temperature near the set point, the heater life, and the power rating. The temperature control typically involves a relay that is actuated from the ground (the local control) to enable or disable the power supplied to the heater, a fuse to protect the spacecraft from a short circuit, and usually a thermostat or solid-state controller to turn the heater on and off at predetermined temperatures. The most common type of control is thermostatic control using a bimetallic mechanical thermostat which opens or closes the heater circuit at a preset temperature. The standard thermostat is a hermetically sealed can containing a switch driven by a snap-action bimetallic actuator (Figure 13.1). The set point at which the thermostat opens can be selected to suit a given application; however, there is a dead band within which the thermostats will close. The dead band chosen for thermostats typically range from 5 °C to 10 °C.
Figure 13.1
Sketch of a typical thermostat used on spacecraft
Key: 1 – electric contacts; 2 – electric insulations; 3 – housing; 4 – actuator.
228
Heaters
Dead bands less than 4 °C are not recommended because small dead bands have been known to increase the working state in which the thermostat rapidly cycles on and off.
13.3 Radioisotope heat sources For an Earth-orbiting spacecraft, options for available thermal energy may extend beyond the onboard source (e.g., solar energy), whereas for a deep space mission, these options become more limited. Because the size of solar panels must increase as a function of the distance from the Sun to generate the same power, when designing for a deep space or planetary surface mission, an alternate power source such as a radioisotope power source is generally more effective at generating electrical power than solar energy. A radioisotope power generator consists of a heat source and a thermoelectric generator (Figure 13.2). The heat is produced from the natural decay of the radioisotope (alpha particles in the case of plutonium-238). With a half-life of 87.7 years, plutonium-238 is an excellent choice for powering long-lived spacecraft. The emitted alpha particles are easily absorbed in the heat source containment system. The converter in a radioisotope thermoelectric generator consists of an arrangement of thermoelectric couples or elements composed of two legs: a positive-type leg and a negative leg. Electricity generated at the radioisotope power source can be used to supply power to electrical heaters, although this process of converting the heat in the radioisotope thermoelectric generator to electricity and then back into heat in an electrical-resistance heater is extremely inefficient.
229
Spacecraft thermal control
Figure 13.2
Sketch of a generic radioisotope thermoelectric generator
Key: 1 – fuel capsule; 2 – thermoelectric elements; 3 – insulation. Source: After Bennett (2008).
Since 1961, 41 radioisotope thermoelectric generators have been successfully launched on 23 space missions.These devices have powered the Apollo Lunar Surface Experiments Packages; the Pioneer flybys of Jupiter and Saturn; the Viking Mars Landers; the Voyager flybys of Jupiter, Saturn, Uranus, and Neptune; the Galileo orbital exploration of Jupiter; the Ulysses solar polar explorer; the Cassini orbital exploration of Saturn; and, most recently, the New Horizons mission to the Pluto/Charon system. Increasingly powerful devices have been created since the first flight launched 50 years ago, as illustrated in Figure 13.3. In addition to the 41 radioisotope thermoelectric generators flown in US spacecraft, one space nuclear reactor was launched in 1965. Furthermore, it is believed that the former Soviet Union has successfully flown radioisotope
230
Heaters
.
Figure 13.3
Electric power, Q E, supplied by radioisotope thermoelectric generators flown in US spacecraft versus launch date, t
Source: Data are from Rinehart (2001) and Bennett (2008).
thermoelectric generators on two Earth-orbiting spacecraft and on two lunar rovers. Also, most of the Soviet effort was devoted to space nuclear reactors with about 31 (and perhaps 33) thermoelectric reactors used for marine radar observations. The former Soviet Union also launched two experimental thermionic reactors on Earth-orbiting satellites (Bennett, 2008). Efficiency is improved if the waste heat from a radioisotope power source is recovered by the spacecraft to provide additional thermal control for the avionics and instrumentation without resorting to additional electrical heaters. This leads to the concept of radioisotope heater units. These are devices that place the heat of radioactive decay directly where it is needed. A sketch of this concept is shown in Figure 13.4 (Rinehart, 2001). The heat power of a radioisotope heater unit is around 1 W. There are hundreds of these heating devices flying in US spacecraft.
231
Spacecraft thermal control
Figure 13.4
Sketch of a generic radioisotope heater unit
Key: 1 – fuel capsule; 2 – insulator tube nest; 3 – insulation; 4 – housing. Source: After Rinehart (2001).
13.4 Heat switches Heat switches cannot exactly be classified as heaters, but their ability to adjust to variations in heat dissipation rates make them an attractive option for temperature control in modern satellites. If a heat switch connects an electronic component to a radiator (Figure 13.5), heat is removed
Figure 13.5
Heat switch between internal components and satellite frame
232
Heaters
from the device when it is generating large amounts of energy, and conserved when the device is not producing heat, allowing the device to remain in a desired temperature range. Therefore, heat switches can passively control the temperature of warm electronics or instrumentation without the use of thermostats and heaters, thereby reducing power requirements. A list of the relevant properties of available heat switch technologies used in spacecraft is shown in Table 13.1. In paraffin thermal switches the volume change of paraffin, which expands approximately 15% when it melts, facilitates heat switch operation. Under normal operating conditions, a paraffin heat switch contains a mixture of solid and liquid wax. In addition, a gap exists between the two
Table 13.1
Summary of available heat switch technologies
Type
Conductance Switching ratio time [min]
Paraffin
100:1
∼1
Large physical Good conductance size, large mass ratios; well developed
Gas-gap
700:1 to 2500:1
∼5 to 60
High Slow switching conductance time, large ratios mass
Differential 700:1 thermal expansion
∼5
High Requires large conductance temperature ratios differences
Variable thermal layer
Not available
Flexible approach, active control
70:1
Source: After Hengeveld et al. (2010).
233
Advantages
Disadvantages
Requires power
Spacecraft thermal control
devices connected by the paraffin heat switch. Due to the vacuum in the gap, heat transfer across the heat switch is limited to radiation across the gap and conduction through the support structure. When heat is added to the heat switch, it is absorbed as latent heat and melts the remaining paraffin solid. The melted paraffin expands and closes the gap that previously separated the hot and cold sides of the heat switch, enabling conduction across the entire heat switch surface. As more heat is added, more paraffin melts and the pressure at the contact between the hot and cold sides increases, causing an increase in conductivity. The gas-gap heat switch, primarily a cryogenic solution, uses a sorbent bed to control the amount of gas in a gap separating the hot and cold structures. When heat transfer across the switch is desired, the sorbent bed is heated with an electrical resistance heater to release hydrogen gas into the gap. The hydrogen gas enables conduction through the heat switch until the controlling heater is turned off and the hydrogen is again absorbed by the sorbent material. Differential thermal expansion heat switches provide a more reliable alternative to gas-gap heat switches. This type of heat switch uses two materials with different coefficients of thermal expansion to control contact between the cold and hot sides of the switch. A recent concept, the variable thermal layer consists of an array of thermoelectric devices embedded in an otherwise insulating matrix. In effect, thermoelectric devices are utilized as individually controllable, bi-directional heat pumps, which provide precise thermal control of the component base-plate (see Chapter 15).
234
Heaters
13.5 References Bennett, L.G. (2008) Mission interplanetary: using radioisotope power to explore the solar system, Energy Conversion and Management, 49: 382–92. Hengeveld, D.W., Mathison, M.M., Braun, J.E., Groll, E.A. and Williams, A.D. (2010) Review of modern spacecraft thermal control technologies, HVAC&R Research, 16: 189–220. Rinehart, G.H. (2001) Design characteristics and fabrication of radioisotope heat sources for space missions, Progress in Nuclear Energy, 39: 305–19.
235
Heaters Abstract: In general, spacecraft and their parts and components must be kept within precise operational temperature ranges to assure the fulfilment of the expected performances. In some cases this implies the warming of the item under consideration by using appropriate heaters. When the power needed for this task is compatible with the power budget of the vehicle, electrical heaters are a suitable option. Otherwise other heat sources, such as a radioisotope heater, are needed. The main characteristics of these different heating systems are presented in this chapter. Key words: electrical heaters, radioisotope heat sources, heat switches.
13.1 Introduction Reliable long-term performance of most spacecraft components takes place at a specified temperature range. The attainment of some temperature range requires, in many instances, the generation of heat within the spacecraft. In these cases, heaters are sometimes required to fulfil specific requirements such as the protection of components from low temperatures, to provide precise temperature control for devices or components, or to warm up equipment to its operating temperature. For example, heaters are used to warm overboard dump valves for liquids; to keep constant structural temperature on space
225
Spacecraft thermal control
telescopes in order to prevent optical misalignment; to maintain the temperature of sensitive gyroscopes and accelerometer guidance platforms; to control the temperature of pressure transducers, other electronics components, and infrared reference sources; to prevent condensation on viewing windows; to keep solar panels warm at night; on catalyst beds of hydrazine thruster engines for spacecraft attitude control; and as shunt resistors, mounted on the skin of a satellite, to dump the excess power of overcharged batteries which would otherwise overheat delicate instruments. When a local uniform heat source or a profiled heating area is needed, electrical heaters can provide it efficiently due to their versatility, although other types of heaters (chemical or nuclear) are also used in spacecraft. Obviously, the use of electrical heaters requires the availability of a power source. For near-Earth applications, power provided by photovoltaic devices is the preferred option because of the relative proximity of the Sun. However, when spacecraft missions include distances far from the Sun, or in harsh environments (such as the surface of Mars or in certain lunar locations), reliable, long-lived, electrical and thermal power sources independent of the Sun are needed. In the following sections the main features of electrical heaters, as well as of the auxiliary devices needed for control and regulation, are presented; basic concepts of nuclear heaters are described; and, finally, thermal switches are presented.
13.2 Electrical heaters Electrical heaters are based on Ohm’s and Joule’s laws. Ohm’s law states that the steady electric current, I, flowing through an electrical conductor is proportional to the constant voltage, V, and to the inverse of the electrical
226
Heaters
resistance of the conductor, R, that is, I = V/R. According to . Joule’s law, the heat released per unit time, Q , by an electrical current, I, is equal to the square of the electrical current, . multiplied by the electrical resistance, R: Q = I2 R. Two types of heaters typically used on spacecraft are film heaters (or patch heaters) and cartridge heaters. By far the most commonly used type is the film heater, as due to its flexibility it can be installed on both flat and curved surfaces. These are made of electrical resistance filaments sandwiched between two layers of electrically insulating material, such as Kapton, attached to the leads. The heaters come in various sizes to fit any applications on the spacecraft. They can be custom-shaped to adapt them to specific applications or be simply rectangles of standard dimensions, whereas cartridge heaters consist of a wire-wound resistor enclosed in a cylindrical metallic case. Typically, the heating power density on the film heaters is limited to less than 0.8 W/cm2. These are installed on the surface of the particular equipment that needs to be heated using a pressure sensitive adhesive. For temperature requirements less than 473 K, heaters constructed of Kapton film and FEP Teflon are used, whereas in the case of electrical heaters working in hard conditions, for example temperatures over 473 K, they must be sheathed in metal. Nearly all heated systems have some type of temperature controller. Unregulated heaters can exhibit large temperature fluctuations when heat demands or ambient temperatures vary. Often, when the temperature is high, less heating is needed, and when the temperature is low, more is desirable. Heating power can be made to adjust automatically without complicated controls by choosing the heating element material. Nevertheless, the performance of a thermal system is improved by the use of sensors and controllers. Some of
227
Spacecraft thermal control
the most significant improvements are the accuracy of the system to maintain the temperature near the set point, the heater life, and the power rating. The temperature control typically involves a relay that is actuated from the ground (the local control) to enable or disable the power supplied to the heater, a fuse to protect the spacecraft from a short circuit, and usually a thermostat or solid-state controller to turn the heater on and off at predetermined temperatures. The most common type of control is thermostatic control using a bimetallic mechanical thermostat which opens or closes the heater circuit at a preset temperature. The standard thermostat is a hermetically sealed can containing a switch driven by a snap-action bimetallic actuator (Figure 13.1). The set point at which the thermostat opens can be selected to suit a given application; however, there is a dead band within which the thermostats will close. The dead band chosen for thermostats typically range from 5 °C to 10 °C.
Figure 13.1
Sketch of a typical thermostat used on spacecraft
Key: 1 – electric contacts; 2 – electric insulations; 3 – housing; 4 – actuator.
228
Heaters
Dead bands less than 4 °C are not recommended because small dead bands have been known to increase the working state in which the thermostat rapidly cycles on and off.
13.3 Radioisotope heat sources For an Earth-orbiting spacecraft, options for available thermal energy may extend beyond the onboard source (e.g., solar energy), whereas for a deep space mission, these options become more limited. Because the size of solar panels must increase as a function of the distance from the Sun to generate the same power, when designing for a deep space or planetary surface mission, an alternate power source such as a radioisotope power source is generally more effective at generating electrical power than solar energy. A radioisotope power generator consists of a heat source and a thermoelectric generator (Figure 13.2). The heat is produced from the natural decay of the radioisotope (alpha particles in the case of plutonium-238). With a half-life of 87.7 years, plutonium-238 is an excellent choice for powering long-lived spacecraft. The emitted alpha particles are easily absorbed in the heat source containment system. The converter in a radioisotope thermoelectric generator consists of an arrangement of thermoelectric couples or elements composed of two legs: a positive-type leg and a negative leg. Electricity generated at the radioisotope power source can be used to supply power to electrical heaters, although this process of converting the heat in the radioisotope thermoelectric generator to electricity and then back into heat in an electrical-resistance heater is extremely inefficient.
229
Spacecraft thermal control
Figure 13.2
Sketch of a generic radioisotope thermoelectric generator
Key: 1 – fuel capsule; 2 – thermoelectric elements; 3 – insulation. Source: After Bennett (2008).
Since 1961, 41 radioisotope thermoelectric generators have been successfully launched on 23 space missions.These devices have powered the Apollo Lunar Surface Experiments Packages; the Pioneer flybys of Jupiter and Saturn; the Viking Mars Landers; the Voyager flybys of Jupiter, Saturn, Uranus, and Neptune; the Galileo orbital exploration of Jupiter; the Ulysses solar polar explorer; the Cassini orbital exploration of Saturn; and, most recently, the New Horizons mission to the Pluto/Charon system. Increasingly powerful devices have been created since the first flight launched 50 years ago, as illustrated in Figure 13.3. In addition to the 41 radioisotope thermoelectric generators flown in US spacecraft, one space nuclear reactor was launched in 1965. Furthermore, it is believed that the former Soviet Union has successfully flown radioisotope
230
Heaters
.
Figure 13.3
Electric power, Q E, supplied by radioisotope thermoelectric generators flown in US spacecraft versus launch date, t
Source: Data are from Rinehart (2001) and Bennett (2008).
thermoelectric generators on two Earth-orbiting spacecraft and on two lunar rovers. Also, most of the Soviet effort was devoted to space nuclear reactors with about 31 (and perhaps 33) thermoelectric reactors used for marine radar observations. The former Soviet Union also launched two experimental thermionic reactors on Earth-orbiting satellites (Bennett, 2008). Efficiency is improved if the waste heat from a radioisotope power source is recovered by the spacecraft to provide additional thermal control for the avionics and instrumentation without resorting to additional electrical heaters. This leads to the concept of radioisotope heater units. These are devices that place the heat of radioactive decay directly where it is needed. A sketch of this concept is shown in Figure 13.4 (Rinehart, 2001). The heat power of a radioisotope heater unit is around 1 W. There are hundreds of these heating devices flying in US spacecraft.
231
Spacecraft thermal control
Figure 13.4
Sketch of a generic radioisotope heater unit
Key: 1 – fuel capsule; 2 – insulator tube nest; 3 – insulation; 4 – housing. Source: After Rinehart (2001).
13.4 Heat switches Heat switches cannot exactly be classified as heaters, but their ability to adjust to variations in heat dissipation rates make them an attractive option for temperature control in modern satellites. If a heat switch connects an electronic component to a radiator (Figure 13.5), heat is removed
Figure 13.5
Heat switch between internal components and satellite frame
232
Heaters
from the device when it is generating large amounts of energy, and conserved when the device is not producing heat, allowing the device to remain in a desired temperature range. Therefore, heat switches can passively control the temperature of warm electronics or instrumentation without the use of thermostats and heaters, thereby reducing power requirements. A list of the relevant properties of available heat switch technologies used in spacecraft is shown in Table 13.1. In paraffin thermal switches the volume change of paraffin, which expands approximately 15% when it melts, facilitates heat switch operation. Under normal operating conditions, a paraffin heat switch contains a mixture of solid and liquid wax. In addition, a gap exists between the two
Table 13.1
Summary of available heat switch technologies
Type
Conductance Switching ratio time [min]
Paraffin
100:1
∼1
Large physical Good conductance size, large mass ratios; well developed
Gas-gap
700:1 to 2500:1
∼5 to 60
High Slow switching conductance time, large ratios mass
Differential 700:1 thermal expansion
∼5
High Requires large conductance temperature ratios differences
Variable thermal layer
Not available
Flexible approach, active control
70:1
Source: After Hengeveld et al. (2010).
233
Advantages
Disadvantages
Requires power
Spacecraft thermal control
devices connected by the paraffin heat switch. Due to the vacuum in the gap, heat transfer across the heat switch is limited to radiation across the gap and conduction through the support structure. When heat is added to the heat switch, it is absorbed as latent heat and melts the remaining paraffin solid. The melted paraffin expands and closes the gap that previously separated the hot and cold sides of the heat switch, enabling conduction across the entire heat switch surface. As more heat is added, more paraffin melts and the pressure at the contact between the hot and cold sides increases, causing an increase in conductivity. The gas-gap heat switch, primarily a cryogenic solution, uses a sorbent bed to control the amount of gas in a gap separating the hot and cold structures. When heat transfer across the switch is desired, the sorbent bed is heated with an electrical resistance heater to release hydrogen gas into the gap. The hydrogen gas enables conduction through the heat switch until the controlling heater is turned off and the hydrogen is again absorbed by the sorbent material. Differential thermal expansion heat switches provide a more reliable alternative to gas-gap heat switches. This type of heat switch uses two materials with different coefficients of thermal expansion to control contact between the cold and hot sides of the switch. A recent concept, the variable thermal layer consists of an array of thermoelectric devices embedded in an otherwise insulating matrix. In effect, thermoelectric devices are utilized as individually controllable, bi-directional heat pumps, which provide precise thermal control of the component base-plate (see Chapter 15).
234
Heaters
13.5 References Bennett, L.G. (2008) Mission interplanetary: using radioisotope power to explore the solar system, Energy Conversion and Management, 49: 382–92. Hengeveld, D.W., Mathison, M.M., Braun, J.E., Groll, E.A. and Williams, A.D. (2010) Review of modern spacecraft thermal control technologies, HVAC&R Research, 16: 189–220. Rinehart, G.H. (2001) Design characteristics and fabrication of radioisotope heat sources for space missions, Progress in Nuclear Energy, 39: 305–19.
235