Patent Application
20190277268
The present disclosure relates generally to operation of a spacecraft, and more particularly, to methods of
Satellite missions can be accomplished with simple spacecraft, which may or may not include propulsive capability. Some satellites do not have any on-board thrust capability due to the fact that most thrusters require large energy sources or consume large amounts of fuel. With propulsive capability, however, a range of missions that can be accomplished with a given size of spacecraft can be greatly enhanced since the spacecraft is able to maneuver.
One type of propulsion includes chemical rocket propulsion, in which propellant is given thermal energy by a violent chemical reaction. By expanding exhaust gases through a nozzle, a temperature and pressure of the gases is reduced, and energy is converted into kinetic energy of a jet.
Another type of propulsion includes electric propulsion, in which a propellant's kinetic energy is derived from electrical energy. Many existing electric thruster, however, do not produce large amounts of thrust due to lack of an amount of propellant that can be “pushed” through the electric thruster.
What is needed is a thruster that has a high specific impulse, while providing high amount of thrust for desired movement.
In an example, a method for producing thrust is described. The method comprises injecting a neutral gas into a cavity between an outer electrode and an inner electrode of a thruster, and the outer electrode is positioned coaxially about the inner electrode and an end of the outer electrode includes an exhaust orifice. The inner electrode includes an end facing the exhaust orifice. The method also comprises ionizing the neutral gas within the cavity into a plasma, causing the plasma to form into a plasma arc between the end of the inner electrode and the exhaust orifice of the outer electrode, generating a magnetic field that applies pressure on the plasma arc, maintaining stability of the plasma arc, and exhausting the plasma arc out of the exhaust orifice based on the applied pressure of the magnetic field, thereby producing thrust.
In another example, a thruster is described that comprises an inner electrode including an end, and an outer electrode positioned coaxially about the inner electrode. An end of the outer electrode includes an exhaust orifice and the end of the inner electrode faces the exhaust orifice. The thruster also comprises a gas injection valve positioned on one of the outer electrode or the inner electrode enabling injection of a neutral gas into a cavity between the outer electrode and the inner electrode, and at least one power source coupled to the outer electrode and the inner electrode. The thruster also comprises a control device having a processor and memory storing instructions executable by the processor for operating the at least one power source to (i) cause voltage to be applied between the outer electrode and the inner electrode resulting in ionization of the neutral gas within the cavity into a plasma and causing the plasma to form into a plasma arc between the end of the inner electrode and the exhaust orifice of the outer electrode, and to (ii) generate a magnetic field that applies pressure on the plasma arc; operating the gas injection valve to inject the neutral gas into the cavity so as to envelope an outer surface of the plasma arc and to generate a continuous sheared-flow of plasma around the plasma arc to maintain stability of the plasma arc within the cavity; and operating the exhaust orifice to exhaust the plasma arc out of the exhaust orifice based on the applied pressure of the magnetic field, thereby producing thrust.
The features, functions, and advantages that have been discussed can be achieved independently in various examples or may be combined in yet other examples further details of which can be seen with reference to the following description and drawings.
The novel features believed characteristic of the illustrative examples are set forth in the appended claims. The illustrative examples, however, as well as a preferred mode of use, further objectives and descriptions thereof, will best be understood by reference to the following detailed description of an illustrative example of the present disclosure when read in conjunction with the accompanying drawings, wherein:
Disclosed examples will now be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all of the disclosed examples are shown. Indeed, several different examples may be described and should not be construed as limited to the examples set forth herein. Rather, these examples are described so that this disclosure will be thorough and complete and will fully convey the scope of the disclosure to those skilled in the art.
An arcjet rocket or arcjet thruster is a form of electrically powered spacecraft propulsion, in which an electrical discharge (arc) is created in a flow of propellant. An example arcjet uses a z-pinch, also known as zeta pinch, in which electrical current in plasma is used to generate a magnetic field that compresses the plasma (e.g., “pinches” it). The name refers to the direction of the current in the devices being along the z-axis as referred to in Cylindrical geometric coordinates (r, theta, z). The arcjet can be described as a z-pinch configuration in which walls are used to provide stabilization of the z-pinch. Such a configuration works because the walls are sufficiently close to the plasma to prevent magnetic instabilities that would otherwise disrupt the z-pinch and cause the thruster to cease operating.
However, while such arcjet configurations may operate well for some applications, it is limited in performance due to low temperature of the plasma. Since the walls used to stabilize the plasma are close to the plasma, the temperature is limited by the materials used for the walls. In some examples, tungsten is used for materials for the wall due to its high melting point. In those examples, arcjets operate at between 0.1N and 1N of thrust with specific impulse around 1000s. However, even with tungsten it may not be possible to generate high temperatures of the plasma necessary to increase efficiency and thrust of the device due to thermal losses of input power to the walls.
In one example, the temperature of the plasma can be increased by moving the walls further away. However, this would increase effects of magnetic instabilities and cause plasma disruptions.
Within examples described herein, sheared-flow of plasma is provided around the plasma arc to shed instabilities before they disrupt the plasma. The sheared-flow of plasma stabilizes the z-pinch thruster to produce large increases in thrust and specific impulse over traditional electric propulsion devices. The sheared-flow of plasma further allows the chamber walls to be farther away from plasma arc, thereby reducing thermal losses, which in turn allows the plasma arc to attain even higher temperatures. Higher operating temperatures result in higher thrust output, which can enable the electric engine to be more than just a tool for attitude adjustment, but also used for main engine thrust capabilities (e.g., enabling next generation space craft and long range/duration missions (i.e., rapid transfer to Mars)). Within examples, example thrusters described herein can reduce a Mars mission time from months to weeks, and LEO-GEO transfer time from months to days, compared to conventional thrusters.
Referring now to the figures,
In some examples, the inner electrode 102 is about 5 cm to about 15 cm in diameter, and the outer electrode 106 is placed coaxially around the inner electrode 102. The outer electrode 106 is about 10 cm to about 20 cm in diameter, for example. Within examples, the use of the term “about” herein refers to tolerances or variances of +/−5% of the measurement unit.
An end 108 of the outer electrode 106 includes an exhaust orifice 110 and the end 104 of the inner electrode 102 faces the exhaust orifice 110. The thruster 100 further includes a gas injection valve 112 positioned on one of the outer electrode 106 or the inner electrode 102 (shown in
The thruster 100 also includes at least one power source 120 coupled to the outer electrode 106 and the inner electrode 102. The power source 120 may be configured to provide a range of voltages, and example power sources may be used that can provide voltages in a range of between about 500V to 10,000V, for example. In some examples, the power source 120 is capable of causing a current of between about 5,000 A to about 2,000,000 A to flow between the outer electrode 106 and the inner electrode 102. A size of the power source 120 and amount of voltage provided can depend upon a physical size of the thruster 100 and an amount of thrust desired.
The thruster 100 further includes a control device 122 having a processor 124 and memory 126, storing instructions 128 executable by the processor 124 for operating the thruster 100, as well as a communication interface 130 and an output interface 132 each connected to a communication bus 134. The control device 122 may also include hardware to enable communication within other computing devices (not shown). The hardware may include transmitters, receivers, and antennas, for example.
The communication interface 130 may be a wireless interface and/or one or more wireline interfaces that allow for both short-range communication and long-range communication to one or more networks or to one or more remote devices. Such wireless interfaces may provide for communication under one or more wireless communication protocols, Bluetooth, WiFi (e.g., an institute of electrical and electronic engineers (IEEE) 802.11 protocol), Long-Term Evolution (LTE), cellular communications, near-field communication (NFC), and/or other wireless communication protocols. Such wireline interfaces may include an Ethernet interface, a Universal Serial Bus (USB) interface, or similar interface to communicate via a wire, a twisted pair of wires, a coaxial cable, an optical link, a fiber-optic link, or other physical connection to a wireline network. Thus, the communication interface 130 may be configured to receive input data from one or more devices, and may also be configured to send output data to other devices.
The memory 126 may include data storage, such as one or more computer-readable storage media that can be read or accessed by the processor(s) 124. The computer-readable storage media can include volatile and/or non-volatile storage components, such as optical, magnetic, organic or other memory or disc storage, which can be integrated in whole or in part with the processor(s) 124. The memory 126 is considered non-transitory computer readable media. In some examples, the memory 126 can be implemented using a single physical device (e.g., one optical, magnetic, organic or other memory or disc storage unit), while in other examples, the memory 126 can be implemented using two or more physical devices. The memory 126 thus is a non-transitory computer readable storage medium, and executable instructions 128 are stored thereon. The executable instructions 128 include computer executable code.
The processor(s) 124 may be a general-purpose processor or special purpose processor (e.g., a digital signal processor, application specific integrated circuit, etc.). The processor(s) 124 may receive inputs from the communication interface 130, and process the inputs to generate outputs that are stored in the memory 126 and used to control operation of the thruster 100. The processor(s) 124 can be configured to execute the executable instructions 128 (e.g., computer-readable program instructions) that are stored in the memory 126 and are executable to provide the functionality of the control device 122 described herein.
The output interface 132 may be similar to the communication interface 130 and can be a wireless interface (e.g., transmitter) or a wired interface as well.
Within one example, in operation, when the executable instructions 128 are executed by the processor(s) 124 of the control device 122, the processor(s) 124 is caused to perform functions including to operate the at least one power source 120 to (i) cause voltage to be applied between the outer electrode 106 and the inner electrode 102 resulting in ionization of the neutral gas 114 within the cavity 116 into a plasma and causing the plasma to form into a plasma arc between the end of the inner electrode 102 and the exhaust orifice 110 of the outer electrode 106, and to (ii) generate a magnetic field that applies pressure on the plasma arc; operating the gas injection valve 112 to inject the neutral gas 114 into the cavity 116 causing a continuous sheared-flow of plasma to envelope an outer surface of the plasma arc to maintain stability of the plasma arc; and operating the exhaust orifice 110 to exhaust the plasma arc out of the exhaust orifice 110 based on the applied pressure of the magnetic field, thereby producing thrust.
In one example, the thruster includes a nozzle 138 that is positioned at the exhaust orifice 110. The nozzle 138 directs the plasma arc out of the exhaust orifice 110. In addition, different or additional nozzles may be included instead of or in addition to the nozzle 138 to exhaust the plasma arc out of the exhaust orifice 110 to produce thrust.
Referring to
Subsequent to or concurrent with providing the neutral gas 114 within the cavity 116, the power source 120 is activated, causing the power source 120 to apply a high voltage differential across the outer electrode 106 and the inner electrode 102. In response to application of the high voltage differential, an electric arc 140 forms between the outer electrode 106 and the inner electrode 102, causing the neutral gas 114 to ionize into a plasma 142 that is capable of conducting current. At this point, the plasma 142 is a low pressure/temperature plasma causing the current to flow between the inner electrode 102 and the outer electrode 106. The current flow pushes the plasma 142 to the right, due to the Lorentz forces (F).
In one example, about 5,000V are needed to initiate the electric arc 140 and cause the neutral gas 114 to ionize into the plasma 142. However, other voltage levels may be used as well depending upon an application and amount of thrust desired, such as between about 500V to 10,000V, for example.
Referring to
Current flowing through the plasma 142 (shown flowing from left to right in the figures) creates a magnetic field 144 within the cavity 116 that squeezes or compresses portions of the plasma 142. The magnetic field 144 increases a density of the plasma 142 causing an increase in pressure and temperature of the plasma 142.
Referring to
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Referring to
In some examples, the thruster 100 can be operated to provide the plasma arc 146 as a sheared-flow stabilized z-pinch. A sheared-flow stabilized z-pinch is a z-pinch that is stabilized by a flow (e.g., a continuous flow) of plasma outside (e.g., immediately outside of or proximate to) the plasma arc 146. In these examples, the control device 122 operates the gas injection valve 112 to inject the neutral gas 114 into the cavity 116 causing a continuous sheared-flow of plasma to envelope an outer surface of the plasma arc 146 to maintain stability of the plasma arc 146 (or at least increase a time of stability). Injection of the neutral gas 114 into the cavity 116 along with application of the voltage causes a flow of plasma proximate to the z-pinch configured plasma arc 146, and in some examples surrounding the plasma arc 146, inducing a sheared flow of the plasma 142 that stabilizes the z-pinch configured plasma arc 146 without using close fitting walls or axial magnetic fields, thereby enabling the z-pinch to remain stable.
The sheared-flow of the plasma 142 provided around the plasma arc 146 thereby stabilizes the plasma arc 146 enabling the plasma arc 146 to lengthen and be exhausted out of the exhaust orifice 110 without requiring a close proximity wall. For instance, the sheared-flow stabilization of the plasma arc 146 is used to maintain the outer electrode 106 walls distant from the plasma 142, which allows the plasma 142 to attain higher temperatures due to a decrease in thermal loss due to wall contact. The plasma 142 moves relative to the plasma arc 146 and conceptually operates as boundaries to confine and maintain a stability of the plasma arc 146.
It is desired to maintain stability of the plasma arc 146 in a substantially cylindrical form with no kinks or bends in the plasma arc 146 (e.g., a smooth outer surface). Without stability, as soon as ripples in an outer surface of the plasma arc 146 may form, instabilities cause the plasma arc 146 to break up resulting in a reduction or elimination of current flow and a reduction of possible thrust. Thus, using examples described herein enables the plasma arc 146 to be stabilized for extended periods of time, which enables thrust to be provided for longer periods of time. The stabilization of the plasma arc 146 further enables higher currents/voltages to be used, which in turn, enables generation of higher thrusts, for example.
In some examples, the flow of the plasma 142 has a sheared flow velocity profile in the sense that the plasma 142 flows at different velocity at an immediate edge of the plasma arc 146 than it does at radial distances farther from the plasma arc 146. For example, a velocity of the flow of plasma 142 over the plasma arc 146 increases based on a distance away from the plasma arc 146.
In some examples, the plasma arc 146 is stabilized during a duration of operation of the thruster 100. Thus, once at the stage of operation where the z-pinch configuration of the plasma arc 146 is formed, the plasma arc 146 will stay formed and be capable of producing thrust for as long as the sheared-flow of the plasma 142 is maintained (e.g., due to injection of the neutral gas 114 into the cavity 116 and power is continued to be applied by the power source 120 ).
Within the plasma arc 146 stabilized, a wall of the outer electrode 106 can be positioned further away from the plasma arc 146 enabling a temperature of the plasma arc 146 to be increased, resulting in higher output thrust capability. As an example, the wall of the outer electrode may be positioned a distance away from the plasma arc 146 of a distance equal to about ten times a diameter of the plasma arc 146. As an example, for a 1 cm diameter plasma arc, the wall of the outer electrode 106 may be about 10 cm away.
In other examples, the third power source 150 is not needed, and the second power source 148 can be operated to also generate the magnetic field that applies pressure on the plasma arc 146 to exhaust the plasma arc 146 out of the exhaust orifice 110.
In the example shown in
An amount of thrust produced by the thruster 100 depends on a size of the thruster 100 as well as an applied voltage/current by the power source 120. In one example, the thruster 100 generates an output thrust that is proportional to a square of the current I flowing between the outer electrode 106 and the inner electrode 102. As an example, if 10,000 A are applied, the thruster 100 may generate about 2N. In further examples, the thruster 100 may be capable of generating an output thrust of up to about 100,000N with an application of 2MA.
Below is an example table of different operating parameters for the thruster 100 to provide various output thrusts. Generally, the input current determines other operating parameters.
A thrust/power ratio of about ˜100 mN/kW is desirable for the thruster 100, for example, in some applications. As seen in the table, the operating parameters scale with the input current whereas the voltage and gas flow are specific parameters to a size and configuration of the thruster 100.
Example experiments were performed using the thruster 100 that showed that the thruster 100 can be stabilized on the 100 μs time scale (e.g., due to limit of the power supply used) with a 1 cm radius plasma arc with electrode walls 10 cm away from the plasma arc. With this configuration, it was possible to increase a temperature of plasma to about 250 eV (e.g., equivalent to more than 2.5 million degrees Kelvin). Based on these results, experiments were performed to measure a thrust out of the thruster 100, and results were favorable with peak thrust levels of up to about 1,000N.
In some examples, the thruster 100 can be operated at 100 μs and can be used in a pulsed mode for pulsed output thrusts. In a pulsed mode, a high voltage power source strikes up the electric arc 140 and the thruster 100 is operated for a threshold time and then shut off, and the process is repeated to generate pulsed thrusts.
In other examples, the thruster 100 can be operated in a continuous fashion or in a continuous mode for substantially continuous output thrust as long as the power and the sheared-flow of plasma is continuously provided. In a continuous mode, the electric arc 140 is created, and the thruster 100 then switches for use of a long duration power supply to provide a steady current along with a steady application of gas for the shear-flow. The thruster 100 will reach a steady-state operation to provide a substantially continuous amount of thrust. In addition, because the thruster 100 is stable, the continuous mode can be achieved, in contrast to unstable z-pinches that would only be able to provide very short pulses on the order of a few microseconds.
Within examples, substantially continuous thrust output means that thrust is provided on a continuous uninterrupted basis, or in instances in which an interruption may occur, the interruption is trivial or minor and does not adversely affect operation of the thruster 100, and the thruster 100 may be restarted for continuous operation.
It should be understood that for this and other processes and methods disclosed herein, flowcharts show functionality and operation of one possible implementation of present examples. In this regard, some or all blocks (or portions of some or all blocks) may represent a module, a segment, or a portion of program code, which includes one or more instructions executable by a processor for implementing specific logical functions or steps in the process. The program code may be stored on any type of computer readable medium or data storage, for example, such as a storage device including a disk or hard drive. Further, the program code can be encoded on a computer-readable storage media in a machine-readable format, or on other non-transitory media or articles of manufacture. The computer readable medium may include non-transitory computer readable medium or memory, for example, such as computer-readable media that stores data for short periods of time like register memory, processor cache and Random Access Memory (RAM). The computer readable medium may also include non-transitory media, such as secondary or persistent long term storage, like read only memory (ROM), optical or magnetic disks, compact-disc read only memory (CD-ROM), for example. The computer readable media may also be any other volatile or non-volatile storage systems. The computer readable medium may be considered a tangible computer readable storage medium, for example.
In addition, some or all blocks (or portions of some or all blocks) in
At block 202, the method 200 includes injecting the neutral gas 114 into the cavity 116 between the outer electrode 106 and the inner electrode 102 of the thruster 100. The outer electrode 106 is positioned coaxially about the inner electrode 102 and the end 108 of the outer electrode includes the exhaust orifice 110, and the inner electrode 102 includes the end 104 facing the exhaust orifice 110.
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By the term “substantially” and “about” used herein, it is meant that the recited characteristic, parameter, or value need not be achieved exactly, but that deviations or variations, including for example, tolerances, measurement error, measurement accuracy limitations and other factors known to skill in the art, may occur in amounts that do not preclude the effect the characteristic was intended to provide.
By the term “continuous” used herein, it is meant that the recited characteristic, parameter, or value need not be achieved exactly without interruption, but that instances of interruption may occur which are trivial or minor and do not adversely affect operation as intended.
Different examples of the system(s), device(s), and method(s) disclosed herein include a variety of components, features, and functionalities. It should be understood that the various examples of the system(s), device(s), and method(s) disclosed herein may include any of the components, features, and functionalities of any of the other examples of the system(s), device(s), and method(s) disclosed herein in any combination or any sub-combination, and all of such possibilities are intended to be within the scope of the disclosure.
The description of the different advantageous arrangements has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the examples in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. Further, different advantageous examples may describe different advantages as compared to other advantageous examples. The example or examples selected are chosen and described in order to best explain the principles of the examples, the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various examples with various modifications as are suited to the particular use contemplated.
This invention was made with government support under Grant No. DE-FG02-04ER54756, awarded by the Department of Energy. The government has certain rights in the invention.
