Control system for a microwave electrothermal thruster

A microwave electrothermal thruster (MET) and its control system is disclosed and claimed. The MET control system uses a dielectric resonator oscillator (DRO) in series with a GaN MMIC-based Solid State Power Amplifier (SSPA) to generate microwave energy, transfer it to a thrust chamber, and heat a propellant that exits a nozzle, providing thrust. The control system uses feedback to provide autonomous control of the MET. A wide variety of propellants may be used, including, for example, hydrazine, ammonia, and water.

BACKGROUND

The invention relates generally to advanced propulsion technologies and more particularly to control systems for microwave electrothermal thrusters.

Launches of small and miniaturized satellites such as CubeSat are occuring more and more frequently. A CubeSat is made of up multiples of 10×10×10 cm cubic units that have a mass of no more than 1.33 kg per unit. Because of their small size, propulsion systems that rely on large portions of a space vehicle's size, weight, and power allowances are not suitable for this market. Solid rockets, for example, are less efficient and lack an ability to relight. Chemical rockets have complex plumbing and delicate components, which are generally not suited for small missions. Cold-gas thrusters are inefficient and do not provide enough change in velocity (delta-V) for extended missions. Hall-effect thrusters (HET) require large power levels whle arcjets have electrodes in the flow path of the propellant that can erode.

Advanced propulsion technologies, such as electric propulsion, are commonly used for station keeping on commercial communications satellites and for primary propulsion on some scientific missions because they have significantly higher values of specific impulse (Isp). Station-keeping refers to maneuvers that are taken by an object in orbit to maintain the orbit or its relative position to one or more other orbiting objects. For example, small impulses from the propulsion system are used for precise positioning of spacecraft relative to each other, such as a space telescope that is formation flying with an occulting disk for a planet finding mission.

A microwave electrothermal thruster (MET) uses microwave energy to heat up a gaseous propellent to convert it into plasma. The plasma expands within a resonate cavity in the MET. As the pressure increases, the plasma is expelled through a nozzle, creating thrust. Previously developed versions of microwave propulsion have relied upon one of two methods for introducing the microwave energy to the propellant: 1) beaming the microwaves from ground stations to the flying propulsion system, or 2) use of an onboard klystron, magnetron, or traveling wave tube amplifier (TWTA) connected to the thrust chamber. These solutions do not meet the size, weight and power limitations of small and miniturized satellites. Generally, these other solutions are incredibly inefficient and not worth the cost to implement.

Thus, there is a need for an improved MET having a small packaging volume and mass. There is a further need for a MET control system that provides high values of specific impulse (Isp) with low power requirements and precise and automatic control.

SUMMARY

The invention in one implementation encompasses a control system for a simplified microwave electrothermal thruster for a small or miniaturized satellite that provides automatic control of microwave generation and propellant flow. An extremely efficient transfer of energy from microwaves to propellant allows small impulses to provide precise control of the satellite. A high specific impulse power results in propellant mass savings at low power levels.

In an embodiment, the invention encompasses a control system for a microwave electrothermal thruster (MET) including a propellant system for providing propellant to the MET at a selected pressure and flow rate; a microwave generation system for providing microwave energy to the MET at a selected power; and an electronics control board for controlling the propellant and microwave generation systems based on system parameters and feedback from the propellant and microwave generation systems.

In a further embodiment, the invention encompasses a control system for a microwave electrothermal thruster (MET) including a propellant system having a propellant tank and a flow meter coupled to the propellant tank for measuring the flow rate of propellant as it is supplied to the MET; a microwave generation system having an oscillator for generating microwaves, an integrated microwave assembly (IMA) for increasing power of the microwaves from the oscillator then sending the microwaves to the MET and a sensor for monitoring microwave power sent to the MET; and an electronics control board for monitoring the flow meter and the sensor to control the operation of the MET.

In either of the above embodiments, the propellant system may include a proportional valve for controlling a flow rate of the propellant according to a control signal from the electronics control board and a flow meter for measuring the flow rate of the propellant and providing a control signal to the electronics control board.

In any of the above embodiments, the control system is located on a satellite and the electronics control board provides autonomous control of the MET to adjust a position of the satellite.

In any of the above embodiments, the propellant system may include a pressure regulator and a proportional valve for controlling the rate of flow of the propellant between the tank and the flow meter.

In any of the above embodiments, the microwave generation system may include a variable attenuator; a driver amp; and an attenuator.

In any of the above embodiments, the microwave generation system includes a circulator for receiving microwave energy from the IMA and transferring it to the MET and for receiving reflected power from the MET and transferring it to the sensor.

In any of the above embodiments, the oscillator is a dielectric resonator oscillator (DRO).

In any of the above embodiments, the DRO generates microwaves having a frequency of approximately 20 GHz.

In any of the above embodiments, the IMA includes a plurality of GaN MMIC-based (Monolithic Microwave Integrated Circuit) Solid State Power Amplifiers (SSPAs).

In any of the above embodiments, the IMA includes a first SSPA for receiving an and providing an amplified output; a first Wilkinson combiner receiving an input from the first SSPA and providing two outputs; second and third SSPAs for receiving inputs from the first Wilkinson combiner and providing amplified outputs; and a second Wilkinson combiner for receiving inputs from the second and third SSPAs and providing an amplified output.

In any of the above embodiments, the IMA increases the power of the microwaves to approximately 40 dBm.

In any of the above embodiments, the IMA is cooled by propellant as it exits the propellant tank.

In any of the above embodiments, the propellant is a pressurized gas. Further, the propellant may be hydrazine, ammonia, water or helium.

In an embodiment, the invention encompasses a microwave electrothermal thruster (MET) including an upper chamber further comprising a plurality of propellant inlets and a nozzle; a lower chamber separated from the upper chamber by a separation plate and further comprising a microwave energy inlet; and any of the control systems as discussed above.

In another embodiment, the invention encompasses a satellite including a microwave electrothermal thruster (MET) and any of the control systems as discussed above.

DETAILED DESCRIPTION

Reference will now be made in detail to one or more embodiments of the invention. While the invention will be described with respect to these embodiments, it should be understood that the invention is not limited to any particular embodiment. On the contrary, the invention includes alternatives, modifications, and equivalents as may come within the spirit and scope of the appended claims. Furthermore, in the following description, numerous specific details are set forth to provide a thorough understanding of the invention. The invention may be practiced without some or all of these specific details. In other instances, well-known structures and principles of operation have not been described in detail to avoid obscuring the invention. Although specific dimensions of various features have been given, these are representative so as to illustrate aspects of the invention.

Advanced propulsion technologies, such as electric propulsion, are commonly used for station keeping on commercial communications satellites and for primary propulsion on some scientific missions because they have significantly higher values of specific impulse (Isp).

The inventive microwave electrothermal thruster (MET) operates on a different principle from most electric propulsion systems. In the MET, microwaves are used to heat the propellant in a resonant cavity. The propellant is then expelled through a nozzle as in a traditional rocket to create thrust. The extremely efficient transfer of energy from microwaves to the propellant, as well as the high temperatures involved, produces a specific impulse much larger than for conventional chemical thrusters. The higher specific impulse for MET propulsion provides a propellant mass savings when used for typical maneuvers, such as station-keeping. It does not require the large power levels associated with Hall-effect thrusters (HETs), and can be added on to an existing hydrazine system. Many different propellants may be used in a MET, including hydrazine, ammonia, and water.

A schematic diagram of a MET10illustrating the principles of operation is shown inFIG. 1. A cavity12is divided into an upper chamber14and a lower chamber16divided by a separation plate18. Plasma20is created in upper chamber14using microwave power generated by antenna22in lower chamber16. Propellant is pumped tangentially into upper cavity14through propellant inlet24. This creates a vortex flow as represented by arrow26. The microwave energy creates an electric field having a resonant frequency concentrated near nozzle26. As propellant is injected into upper chamber14, the plasma ignites next to the nozzle. In appropriate pressure conditions, the plasma is self-sustaining and stabilized by vortex flow26caused by the continued injection of propellant. This turns the cavity into a pressure chamber and thrust is created when propellant exits from nozzle26.

FIG. 2depicts a satellite for use with MET10. MET10is mounted at one end of satellite housing30so that nozzle26provides thrust. Additional thrusters can be installed in any location where additional thrust might be needed. Furthermore, thrusters could optionally be attached with a gimbal such that the thrust direction can be modified in flight. Satellite housing30is provided with solar panels, representatively indicated at32and34, that are used to collect solar energy to recharge an onboard battery. Propellant inlet interfaces, indicated at36, are used to allow the propellant to enter the thrust chamber.

A more detailed diagram of an embodiment of a MET control system according to the invention is shown inFIGS. 3A and 3B. In order to clearly show the connections between the elements,FIG. 3Adepicts a perspective view whileFIG. 3Bdepicts a top view. Reference numbers for like elements are the same in both figures.FIGS. 3A and 3Bdepict a MET control system configured with a number of user interface elements that would be used during development and testing of the system.

Tank50holds propellant at approximately 3000 PSI (pounds per square inch). Fill valve52allows the tank to be filled with propellant from an external gas source. From there propellant passes through a pressure regulator54which decreases the pressure to approximately 100 PSI. Proportional valve56controls the flow rate of the propellant which then passes through flow meter58after which it is injected into MET10through propellant inlets24on either side as shown. For clarity of illustration, a propellant line from flow meter58to MET10is only shown connected to one of inlets24. Although two inlets24are shown, any number of propellant inlets could be provided.

Dielectric resonator oscillator (DRO)60generates microwaves at a frequency of approximately 20 GHz with 7 dBm (decibel-milliwatts) of power. Although the invention is disclosed with a DRO, any oscillator capable of generating radio waves at microwave frequencies could be used. The DRO is connected to a variable attenuator62which controls the RF power with 3-30 dB range of attenuation. Variable attenuator62is connected to driver amp64which is generally used to increase the power of the signal from the DRO. Integrated microwave assembly (IMA)68, a GaN MMIC-based Solid State Power Amplifier (S SPA) receives the microwaves from attenuator66for additional amplification. From IMA68, the microwaves pass through circulator78before entering lower chamber16of MET10through antenna22as shown inFIG. 1. Reflective power meter70measures microwave power to provide control input.

Several electronics boards72provide voltage and power regulation as well as other control features for the MET10as described in more detail in connection withFIG. 4. Although multiple boards are shown, any number could be used as needed. Power is supplied by battery74. The electronic display boards, shown at76, provide feedback to the operator during ground testing. Display boards76are shown with four displays as described below in connection withFIG. 4, but any number of displays could be used. In an embodiment, the four displays76show battery voltage, IMA operating temperature, reflected power, and input power.

A schematic diagram of a MET control system for use in a satellite is shown inFIG. 4. Corresponding elements to those shown inFIGS. 3A and 3Bhave the same reference numbers. A propellant system includes tank50supplies propellant at approximately 3000 PSI, Pressure regulator54which reduces the pressure to 100 PSI, proportional valve56which controls the flow rate of the propellant and flow meter58. In an embodiment, pressure regulator is a compact, piston-sensing pressure regulator with a short stroke manufactured by Swagelok® but any suitable pressure regulator could be used. In an embodiment, flow meter58is an XFM digital mass flow meter from Aalborg Instruments® but any suitable flow meter could be used. Flow meter58provides a control signal to electronics72as will be explained in further detail in connection withFIG. 5.

A microwave generation system several components for generating and increasing the power of microwaves provided to MET10. As explained above forFIG. 3A, DRO60generates microwaves at approximately 20 GHz frequency and approximately 7 dBm power. The microwaves enter variable attenuator62which controls the RF power with 3-30 dB range of attenuation. In an embodiment, variable attenuator62is a GaAs MMIC Voltage-Variable Attenuator, 10-40 GHz from Analog Devices® but any suitable variable attenuator that meets the performance requirements could be used. Next, driver amp64increases the power to a desired power level before sending the microwave signal to a second attenuator66which protects the IMA from power levels that are too high. From attenuator66, the signal goes to IMA68, which provides additional amplification and is described in more detail in connection withFIGS. 7 and 8. Finally, microwaves at approximately 20 GHz and approximately 40 dBm are sent to circulator78which protects IMA68from any reflected power coming off of the thruster. Circulator78sends the microwaves to the antenna (not shown) in MET10. Any microwaves reflected back from MET10are sent through attenuator79and power detector80which provides a control signal to electronics72as will be explained in further detail in connection withFIG. 5.

One or more batteries72provide power to the apparatus ofFIG. 4. In an embodiment, one or more Lithium Ion 14.8V batteries from Tenergy are provided but any suitable battery could be used. Batteries72are connected to boost converter82, which increases the voltage from batteries72to approximately +30 V. LDO (low-dropout) regulator84provides a variety of voltages to various components of the control system as needed, for example −5V, +5V, +15V and +20V. Electronics board72controls the operation of various components within the control system, including proportional valve56, variable attenuator62through inverter85and trimmer potentiometer84, which is used to control the operation of DRO60. Electronics board72also includes a processor88for receiving, processing and generating control signals from components of the control system to provide autonomous control of the MET as described in connection withFIG. 5.

FIG. 5illustrates the control flow for autonomous firing of the MET ofFIGS. 3A, 3B and 4. In general, two feedback loops, one for the propellant system and one for the microwave generation system, provide autonomous control of the MET. InFIG. 5, power supply100is connected to firing control board101, part of electronics boards72. Firing control board101supplies control voltage the microwave generation system, represented as RF driver102, IMA103and reflected power sensor104. Microwaves from IMA103enter circulator78(FIG. 4) which passes microwave power into MET10. If there is reflected power from MET10, it goes back to circulator78and then into a 20 dB load in reflected power sensor104. A power sensor is at the end of the load and detects a very small amount of power. Reflected power sensor104creates a voltage which is read by firing control board101. In an embodiment, firing control board101incorporates processor88ofFIG. 4or other logic processing hardware.

Similarly, firing control board101uses voltage to control pressure regulator106which controls sending gas through proportional valve107(corresponding56ofFIG. 3B), then flow meter108(corresponding to70ofFIG. 3A) and then to thruster10. Data collected from flow meter108is sent back to firing control board101through feedback loop109.

Feedback loops105and109provide autonomous control of thruster10. In an embodiment, firing control board101maintains a condition wherein as the propellant flow increases, the reflected power decreases, which increases the overall efficiency. In other words, firing control board82monitors and adjusts both the propellant flow and the RF power so that a steady state can be achieved. Once the system is at steady state, firing control board101slowly increases both RF power and propellant flow to improve efficiency and or thrust. A sudden increase in either variable (RF power or propellant flow) would disrupt the steady state and cause the thruster to fail.

FIG. 6Ashows a perspective view of a prior art thruster. This thruster has a 5 piece design including RF Feedthru plate110, bottom cavity plate112, top cavity plate114, viewport plate116and nozzle plate118. Each of plates110and112include mounting holes122and124which are used to attach the thruster to a spacecraft bus or a test stand. Top cavity plate114also includes propellant inlet120. In an embodiment, a corresponding propellant inlet120and mounting holes122and124are located on the opposite side of their respective plate (not shown inFIG. 6A). As noted forFIG. 3A, although two propellant inlets are shown, any number of inlets could be provided depending on a required propellant flow.

FIG. 6Bshows a perspective view of a MET according to the present invention. This design only has three pieces, lower chamber plate130, upper chamber plate132and nozzle plate134. This design holds a vacuum better than the design ofFIG. 6Aand allows for more efficient RF energy consumption.

FIG. 6Cshows a perspective cross-section view of the MET ofFIG. 6B. Lower chamber16fromFIG. 1is inside body130, upper chamber14is shown inside body132, nozzle26is located in nozzle plate134. Separation plate18ofFIG. 1is not shown but would be located in slot136. Antenna22ofFIG. 1enters lower chamber16through port138.

FIG. 7shows a more detailed view of one embodiment of an IMA68ofFIG. 3. After propellant from propellant tank50passes through pressure regulator54and proportional valve56ofFIG. 4, it is routed through tubes150on the bottom of IMA68before passing through flow meter58. This simultaneously heats the propellant for improved ignition, and dissipates heat generated by the IMA. The heat is concentrated on the bottom of the IMA housing152where the heat pipes move the heat away. In an embodiment, IMA68is a SSPA (Solid State Power Amplifier) that includes a plurality of GaN (gallium arsenide) MIMIC (Monolithic Microwave Integrated Circuit) amplifiers. IMA68also consists of DC power feedthrus158to provide power to the MIMIC devices. RF Feedthru156allows the RF energy to exit the IMA and enter the thruster. IMA Cap160is held down by sixteen screws154.

FIG. 8shows a schematic view of IMA68ofFIG. 7. Microwaves from attenuator66(FIG. 4) enter IMA68at RF feedthru170. From here they travel through a transmission line to a first MIMIC172which increases the microwave power, then to Wilkinson combiner174. To provide increased power at a reduced footprint, Wilkinson combiner174splits the microwaves between MMICs176and178for further amplification. A second Wilkinson combiner180merges the output of MMICs176and178then sends it to RF feedthru182which is connected to circulator78(FIG. 4). Capacitors184and resistors186are used to control the operation of the MIMIC devices. DC power feedthrus158on either side of IMA68provide power to the MMIC devices. In an embodiment, MMIC172increases the power to approximately 30 dBm, Wilkinson combiner174provides microwaves at approximately 26 dBm to each of MMICs176and178, which amplify the microwaves to approximately 41 dBm. Wilkinson combiner180outputs microwaves at approximately 43 dBm.

Due to its small size and low power requirements, the MET is a practical solution to providing robust propulsion capability for small satellites. Many different propellants may be used in a MET, including hydrazine, ammonia, water, helium and many other well-known substances. In an embodiment, the propellant is provided in the form of a pressurized gas. A MET can be configured into a 1U cubesat providing reasonable delta-V (change in velocity). Each “U” defines the size of the cubesat. A 1U cubesat is 10×10×11.35 mm in size while a 3U cubesat is three 1U cubes stacked together. The METs versatility regarding propellants is a further advantage, and in particular, the ability to utilize “green” propellants such as water, for example. The MET will also enable hitherto difficult to achieve capabilities in small satellites such as six degree-of-freedom agility, collision avoidance, accurate pointing, deorbiting, and atmospheric drag compensation which will extend mission life. The benefits of implementing a small, scalable propulsion system on a small satellite could eventually transfer into large spacecraft, perhaps even rendering reaction wheels obsolete.

The steps or operations described herein are just for example. There may be many variations to these steps or operations without departing from the spirit of the invention. For instance, the steps may be performed in a differing order, or steps may be added, deleted, or modified.