Patent Publication Number: US-2021171223-A1

Title: Propulsion Systems Utilizing Gas Generated Via An Exothermically Decomposable Chemical Blowing Agent, and Spacecraft Incorporating Same

Description:
RELATED APPLICATION DATA 
     This application claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 62/519,917, filed on Jun. 15, 2017, and titled “PROPULSION SYSTEMS UTILIZING GAS GENERATED VIA AN EXOTHERMICALLY DECOMPOSED CHEMICAL BLOWING AGENT, AND SPACECRAFT INCORPORATING SAME”, which is incorporated by reference herein in its entirety. 
    
    
     GOVERNMENT RIGHTS 
     This invention was made with government support under contract number NNX15AP86H awarded by the National Aeronautics and Space Administration. The government has certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of propulsion systems. In particular, the present invention is directed to propulsion systems utilizing gas generated via an exothermically decomposed chemical blowing agent, and spacecraft incorporating same. 
     BACKGROUND 
     As the capabilities of small satellites (&lt;180 kg) have matured, mission designers have begun to consider them for formation flying missions, such as multi-point Earth observations or synthetic aperture arrays for deep space exploration, that would be cost-prohibitive to perform with larger satellites. Small satellites are well-suited for these types of missions, as all of the elements can be launched simultaneously and for a fraction of the cost of traditional missions. There has been particular interest in missions designed around the CubeSat platform, as the supporting infrastructure for launching and deploying satellites built to this standard are already in place. Indeed, a number of CubeS at-based missions are in development, demonstrating the potential of the platform and the demand for continued improvement of the supporting technologies. 
     While current small satellite designs are considerably more capable than previous generations, to support these new mission concepts there is additional subsystem development required. Perhaps the most critical of these subsystems are propulsion systems capable of providing the relative position and orientation control necessary to enable on-orbit formation flying. Small satellite attitude control thrusters are particularly challenging as they must provide reliable, low impulse-bit operation while conforming to the size, weight, power, and cost constraints of the form factor. In addition to the technical challenges these propulsion systems must address, many of them face regulatory hurdles that will limit their adoption for small satellites. These regulations include propulsion systems that must meet range safety and secondary payload requirements that limit the storage tank pressurization, amount of stored chemical energy, and toxicity of the propellant. Those requirements immediately eliminate many propulsion options, and limit the efficacy of others. 
     SUMMARY OF THE DISCLOSURE 
     In one implementation, the present disclosure is directed to a propulsion system that includes a chemical-blowing-agent chamber containing a predetermined amount of a chemical blowing agent, wherein the chemical blowing agent is in solid form and decomposes exothermically in response to an initial application of heat to a portion of the chemical blowing agent so as to initiate thermal decomposition of the portion; a heating element in thermal communication with the portion of the chemical blowing agent for initiating the thermal decomposition of the chemical blowing agent so as to form a propelling gas during operation of the propulsion system; and an exhaust region in fluid communication with the chemical-blowing-agent chamber, wherein, during operation of the propulsion system, the exhaust region exhausts the propelling gas so as to provide thrust. 
     In another implementation, the present disclosure is directed to a method of propelling a spacecraft. The method includes initiating, aboard the spacecraft and with an initial application of heat, thermal decomposition of a portion of a chemical blowing agent so as to generate a gas, wherein the chemical blowing agent is in solid form and decomposes exothermically in response to the initial application of heat to the portion of the chemical blowing agent; stopping the initial application of heat to the chemical blowing agent before all of the chemical blowing agent has exothermally decomposed; allowing the chemical blowing agent to continue to thermally decompose after stopping the initial application of heat so as to generate pressurized gas; and directing the pressurized gas offboard of the spacecraft so as to provide thrust to the spacecraft. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1  is a schematic diagram of a chemical-blowing-agent (CBA) based propulsion system in accordance with aspects of the present invention; 
         FIG. 2  is an isometric view of a microsatellite that includes a CBA-based propulsion system having pressurized propulsion gas storage tank; 
         FIG. 3A  is a partially transparent isometric view of the CBA-based propulsion system of  FIG. 2 ; 
         FIG. 3B  is a partially transparent end view of the CBA-based propulsion system of  FIG. 2 ; 
         FIG. 3C  is a side view of the CBA-based propulsion system of  FIG. 2 ; 
         FIG. 4  is an isometric view of a microsatellite that includes a CBA-based propulsion system that includes a plurality of thruster-array devices; 
         FIG. 5  is an exploded side view of a CBA-based propulsion system that includes a burst disc for releasing propulsion gas in a burst impulse; 
         FIG. 6  is an isometric view of another example CBA-based propulsion system of the present disclosure; 
         FIG. 7  is a cross-sectional view of a tank portion of a CBA storage unit, showing an example heater arrangement; 
         FIG. 8  is an enlarged end view of the heater of  FIG. 7 ; and 
         FIG. 9  is an enlarged cross-sectional partial view of one of the microthrusters of one of the thruster-array devices of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION 
     In some aspects, the present invention is directed to propulsion systems that use a solid-form (e.g., powder) chemical blowing agent (CBA) that decomposes exothermically to produce pressurized gas that is then exhausted to produce thrust for a vehicle, such as a spacecraft, and especially, but not limited to, microsatellites and nano-satellites. For the sake of convenience, the term “spacecraft” as used herein and in the appended claims, includes a satellite of any size and any other object put into space by humans and that includes one or more onboard propulsion systems. CBAs are a family of chemical compounds that produce a significant volume of gas as a result of thermally induced decomposition. They are commonly used as additives in polymer manufacturing and food production to reduce the density of the surrounding medium. The present inventors have found that CBAs are highly attractive as a propellant for space applications, as they are non-toxic, inert at temperatures below their activation temperature, and low cost due to their use in other industries. Examples of CBAs having these properties include azodicarbonamide, isocyanate, titanium hydride, and zirconium hydride, among others. A benefit of using a CBA is that it does not need to be stored under pressure during launch or any other phase of a mission; this is a tremendous safety benefit. As a detailed example, azodicarbonamide, C 2 H 4 N 4 O 2 , decomposes into a mixture of nitrogen (N2), carbon monoxide (CO), and carbon dioxide (CO2) in a ratio of 65:32:3. The residual solids are made up of urazole, biurea, cyanuric acid, urea, and ammonia salt. 
       FIG. 1  illustrates some general components of a propulsion system made in accordance with aspects of the present invention.  FIG. 1  shows an example propulsion system  100 , which includes at least one CBA chamber, here a single CBA chamber  104 , that initially contains a CBA  108  and functions as a decomposition chamber once the exothermic decomposition of the CBA has been initiated. As noted above, CBA  108  is in solid form, such as a powder, and has the property that it decomposes exothermically and produces a gas  112  (hereinafter call a “propulsion gas” based on its ultimate function) as a product of that decomposition. It is desirable that CBA  108  be non-combustible, non-toxic, and produce non-toxic gas upon thermal decomposition. Depending on the construction, CBA chamber  104  may be sealed, for example, using any suitable means, such as welding (e.g., electron beam, laser, ultrasonic, MIG, TIG, etc.), polymer O-rings, or metal seals, among others. 
     Propulsion system  100  also includes a heater  116  for providing initial heat  116 A to at least a portion of CBA  108  to initiate thermal decomposition of the CBA. An important aspect of using an exothermic CBA is that only a relatively small amount of input energy is needed to produce a relatively large amount of output energy that can be used directly for propulsion. Heat from heater  116  only needs to be provided to initiate thermal decomposition of CBA  108 . After thermal decomposition has started, the decomposition is self-sustaining, meaning the thermal decomposition continues, without the need for heat input from heater  116 , until the original amount of CBA  108  has thermally decomposed into propulsion gas  112  and byproducts. Heater  116  can be any suitable heater that can raise the temperature of CBA  108 , or a portion thereof, to the appropriate thermal decomposition temperature. Heater  116  includes, among other things, a heating element (not shown), such as an electric heating element, that may be placed inside or outside CBA chamber  104 . CBA chamber  104  may be in a vessel  120 , which, depending on the design of propulsion system  100 , may be a pressure vessel or non-pressure vessel. In some embodiments, vessel  120  may include internal metallic features (not shown) that improve heating a surface area of the vessel exposed to CBA  108  within CBA chamber  104 . 
     Propulsion system  100  further includes an exhaust region  124  for ultimately exhausting propulsion gas  112  from the propulsion system and offboard of the spacecraft (not shown) of which the propulsion system is part, so as to produce thrust for the spacecraft. In some embodiments and as will become apparent from examples below, exhaust region  124  may be, fluidly, immediately downstream of CBA chamber  104 , whereas in some embodiments one or more other components, such as a pressure tank  128  and one or more valve assemblies, such as valve assemblies  132  (1) and  132  (2) are fluidly coupled between the CBA chamber and the exhaust region. Depending upon the design of propulsion system  100  and the needs of the spacecraft of which the propulsion system is a part, exhaust region  124  may be a simple exit orifice, a convergent nozzle, a divergent nozzle, or a convergent-divergent nozzle, among other things. Propulsion system  100  further includes a control system  136  that controls the operation of the propulsion system. One point of control that control system  136  may be programmed to perform is to control heater  116  so as to initiate thermal decomposition of CBA  108  at an appropriate time. For example, control system  136  may be suitably programmed to energize heater  116  for the time needed to initiate the thermal decomposition of a relatively small portion of CBA  108  within CBA chamber  104 . Once the activation of heater  116  has initiated thermal decomposition, thermal decomposition of the remaining CBA  108  continues by virtue of the exothermic nature of the CBA. A benefit of leveraging the exothermic nature of CBA  108  is that the amount of CBA provided to any given CBA chamber, such as CBA chamber  104 , can be scaled without needing to also scale the input energy needed to initiate the CBA&#39;s thermal decomposition. Once thermal decomposition of CBA  108  has been initiated with the initial input of energy, it continues without the need for additional energy input regardless of the amount of the CBA. Consequently, the total impulse (force multiplied by time) is likewise scalable generally with no change in input energy. This can results in significant savings of weight and cost relative to endothermic-reaction-based systems having similar total impulse outputs. As those skilled in the art will readily appreciate from reading this entire disclosure, control system  136  may control other aspects of propulsion system  100  and may base its control command(s) on input from one or more sensors (not shown), such as one or more pressure sensors, a position sensor, and an orientation sensor, among others. With these generalities in mind, following are descriptions of some example embodiments and experimental instantiations, along with some experimental results. 
       FIG. 2  illustrates a satellite, here, a microsatellite  200 , that includes a CBA-based propulsion system  204  made in accordance with aspects of the present invention. In this example, CBA-based propulsion system  204  is secured to a frame  208  of microsatellite  200  and provides primary thrust for the microsatellite. Other systems aboard microsatellite  200  are not shown for sake of clarity.  FIGS. 3A to 3C  illustrate components of CBA-based propulsion system  204 . 
     Referring now to  FIGS. 3A to 3C , in this example CBA-based propulsion system  204  includes four CBA storage units  300  (1) to  300  (4) each having an internal chamber (not seen) containing a suitable CBA (not shown) in solid form and capable of thermally decomposing to form a propulsion gas. In the embodiment shown, the internal chamber functions both as a CBA storage chamber and a CBA decomposition chamber once thermal decomposition has been initialed. CBA-based propulsion system  204  also includes a pressure tank  304  and a valve assembly  308  fluidly coupled between the pressure tank and each of the four CBA storage units  300  (1) to  300  (4). In this example and as described below, valve assembly  308  isolates pressure tank  304  from CBA storage units  300  (1) to  300  (4) and isolates each CBA container from the other CBA container so as to minimize backflow from the pressure tank and to minimize energy loss. CBA-based propulsion system  204  includes an exhaust region  312 , which may or may not include a nozzle (not shown). If a nozzle is included, it may be, for example, a monolithic and integrally-formed nozzle (formed, e.g., by 3D printing) or produced separately from pressure tank  304 . Depending on the design of the CBA-based propulsion system, the nozzle may or may not be mechanically coupled to pressure tank  304 . CBA-based propulsion system  204  may include a valve (not seen, but fluidly upstream of exhaust region  312 ) and/or a pressure regulator (not shown) fluidly coupled between pressure tank  304  and the exhaust region for controlling the flow of the propulsion gas through the exhaust region. Another example CBA-based propulsion system having such a valve and pressure regulator visible is shown in  FIG. 6  and described below. 
     In this example, the manner of operation of CBA-based propulsion system  204  is to use the CBA in the four CBA storage units  300  (1) to  300  (4) to serially pressurize pressure tank  304  with the propulsion gas. For example, the thermal decomposition of the CBA in CBA storage unit  300  (1) may be used to initially pressurize pressure tank  304  from an unpressurized state, which may have been the state during launch of microsatellite  200  ( FIG. 2 ). As an example, propulsion system  204  may be designed so that the maximum operating pressure of pressure tank  304  is 2000 psi. In this case, the amount of CBA in each CBA storage unit  300  (1) to  300  (4) may be such that, when the entire amount is completely thermally decomposed, the propulsion gas generated pressurizes pressure tank  304  to 2000 psi. Then, once the pressurized propulsion gas in pressure tank  304  has been used and the pressure has therefore reduced to a certain level, thermal decomposition of the CBA in another CBA storage unit, such as CBA storage unit  300  (2), can be initiated so that the self-sustaining exothermic reaction continues and generates propulsion gas that refills and re-pressurizes the pressure tank. This sequence can be repeated for each of the remaining CBA storage units. 
     In this example, CBA-based propulsion system  204  may include a control system  316  that monitors pressure within pressure tank  304  via a suitable pressure sensor (not shown) and uses the resulting pressure readings, among other things, to determine when to activate a heater (not seen, but see, for example,  FIGS. 7 and 8 ) of each CBA storage unit  300  (1) to  300  (4) to initiate the thermal decomposition of the CBA in a corresponding one of the CBA storage units. Control system  316  may also include algorithms for controlling any valve(s) (such as valve  320  between pressure tank  304  and exhaust region  312 ), heaters, and/or pressure regulator(s) provided as a function of the need for thrust for microsatellite  200  ( FIG. 2 ). Not shown for ease of illustration are, among other things, a power source (e.g., battery(ies), solar panel(s), etc.) for powering control systems, heaters, and other electronics, such as valve solenoid(s) and wiring between control system  316  and the heaters, valve(s), sensor(s), and pressure regulator(s), among other things. 
     In one instantiation, microsatellite  200  ( FIG. 2 ) is a CubeSat, and CBA-based propulsion system  204  is configured to occupy 0.2U and to take advantage of the CubeS at 3U+ standard, which includes an allowance for a propulsion system to, for example, extend into the “tuna can” space created by a launch spring of a poly-picosatellite orbital deployer (P-POD). For larger systems (6U, 12U, etc.), the system can be configured with additional storage units to increase the performance of the system, increase the output of the system, and/or otherwise change the characteristics of the system. In this instantiation, CBA-based propulsion system  204  may be designed to be a bolt-on propulsion option that provides primary propulsion for orbit maintenance, hazard avoidance, and/or de-orbiting without a major impact on the design of the system. All the electronics necessary to interface with the CubeSat bus (not shown) may be included in the system package, and one or more optional batteries (not shown), such as one or more lithium-ion batteries, can be used to provide the power to fire CBA-based propulsion system  204 , i.e., heat the CBA in storage units  300  (1) to  300  (4), operate control system  316 , and operate valve assembly  308  ( FIG. 3A ) and valve  320 , making it a truly stand-alone propulsion option. 
     In this instantiation, CBA-based propulsion system  204  is designed to maximize the performance-to-cost ratio, which is particularly important for low-cost CubeSat missions. The low-cost is realized through extensive use of additive manufacturing and COTS parts and an inexpensive, non-toxic propellant that is safe to transport and handle. The performance characteristics of the 0.2U configuration are presented in the table below as an example. 
     
       
         
           
               
               
               
             
               
                   
                   
               
               
                   
                 Parameter 
                 Target Value 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Thrust 
                 5 
                 mN 
               
               
                   
                 Impulse-bit 
                 0.1 
                 mN · s 
               
               
                   
                 Total Impulse 
                 20 
                 N · s 
               
            
           
           
               
               
               
            
               
                   
                 Volume 
                 0.2 U + “tuna can” 
               
            
           
           
               
               
               
               
            
               
                   
                 Mass 
                 &lt;350 
                 g 
               
               
                   
                 Power 
                 &lt;2 
                 W (peak) 
               
               
                   
                   
               
            
           
         
       
     
       FIG. 4  illustrates a satellite  400  that includes a “digital” CBA-based propulsion system  404  that includes a plurality of digital thruster-array devices  404  (1) to  404  (24) (four on each of the six faces of the satellite; some not seen in  FIG. 4 ) made in accordance with aspects of the present invention. In this example, each thruster-array device  404  (1) to  404  (24) comprises an array of microthrusters (not seen in  FIG. 4 , but see microthruster  900  of  FIG. 9 ) that may be individually fireable to provide thrust in a digital manner. Depending on the thrust needs, one or more of the microthrusters can be fired at any particular time to create one or more “bits” of thrust. Each digital thruster-array device  404  (1) to  404  (24) may be formed as a micro-electromechanical system (MEMS) device  408  comprising, for example, a thermal reaction-initiation layer  408 A, a CBA-storage layer  408 B, and a micronozzle layer  408 C made using any suitable MEMS fabrication technologies. CBA-storage layer  408 B contains an array of storage+decomposition chambers  412  (only some labeled to avoid clutter) that function to store a CBA (not shown) until used and provide a space for containing the thermal decomposition of the CBA during firing of the corresponding microthruster. Each storage+decomposition chamber corresponds to a respective one of the microthrusters aboard MEMS device  408 . 
     Thermal-initiation layer  408 A includes an array of heaters  416  (only some labeled to avoid clutter), with each heater corresponding to one of the microthrusters aboard MEMS device  408 . As described below, in some embodiments each heater  416  is individually actuatable relative to the other heaters so that the microthrusters aboard each MEMS device are individually fireable. Micronozzle layer  408 C contains an array of micronozzles  420  (only some labeled to avoid clutter), with each micronozzle corresponding to one of the microthrusters aboard MEMS device  408 . Each storage+reaction chamber may be sealed between the storage+decomposition chamber and micronozzle  420  using a burst disc. 
     In some embodiments, micronozzle layer  408 C may be eliminated altogether or micronozzles  420  therein may be, for example, incorporated into the structure in which the storage+decomposition chambers are formed. As an example of the latter, a burst disc and a micronozzle may be integrated into a single structure (not shown) that may further be integrally formed with the structure of CBA-storage layer  408 B. 
     In this connection, CBA-based propulsion system  404  may include one or more controllers  424  (only one shown for convenience) for controlling the firing of the microthrusters aboard thruster-array devices  404  (1) to  404  (24). For example, satellite  400  may be provided with a single controller, which controls all aspects of operation of the satellite. As another example, satellite  400  may be provided with a high-level mission controller that communicates with one or more propulsion-system controllers. If multiple propulsion-systems controllers are provided, one may be provided for each thruster-array device  404  (1) to  404  (24) or, alternatively, for some subgroup of the thruster-array devices. Not shown are the communications links (wired, wireless, or combination thereof) that allow the one or more controllers  424  to communicate with thruster-array devices  404  (1) to  404  (24) and/or one another and/or offboard controller or other device. 
     Not seen in  FIG. 4  are the pressure-release devices, such as burst discs, that are present in the plurality of microthrusters aboard MEMS device  408 . In this embodiment, each pres sure-release device is designed and configured to allow pressure to build within each CBA storage chamber  412  to a predetermined release pressure as the CBA contained therein thermally decomposes to form pressurized propulsion gas (not shown). When the propulsion gas reaches the release pressure, the pressure-release device opens, allowing the propulsion gas to exit MEMS device  408  through the corresponding micronozzle  420 . If a burst disc is used, it may be provided to MEMS device  408  in any suitable manner, such as part of CBA-storage layer  408 B (e.g., as a thinned wall) or part of micronozzle layer  408 C (e.g., as a membrane covering the inlet of micronozzle  420 ) or in a separate layer sandwiched between the CBA-storage layer and the micronozzle layer, among other possibilities. While burst discs are simple, another type of pressure-release device, such as a valve, can be used. When pressure released from CBA-storage layer  408 B is sudden, it creates a burst impulse. It is noted that the design of each micronozzle  420  may need to be tuned to minimize undesirable transient conditions within that micronozzle caused by the burst impulse. 
     Each MEMS device  408  may be any size and containing any number of microthrusters suitable for a given application. As one example, it is noted that current MEMS manufacturing capabilities allows for the creation of a 7 mm square array of microthrusters (see  FIG. 9 ) with spacing for structural and thermal integrity, consisting of 19,600 individual microthrusters, each designed to provide about 1 micro-Newton of thrust. In some embodiments, materials for MEMS device  408  may include ceramic polymer compositions, polymer composites, and polymer metal composites, among others. 
       FIG. 5  illustrates a CBA-based propulsion system  500  that is similar in design to each of the microthrusters described above in connection with  FIG. 4 . However, in this example, CBA-based propulsion system  500  is embodied as a standalone device. In the example of  FIG. 5 , CBA-based propulsion system  500  includes a base portion  504  that includes a CBA chamber  508  and a heating element  512  extending into the CBA chamber. Though not shown, an exothermically decomposable CBA is placed in CBA chamber  508  prior to use of CBA-based propulsion system  500 . Heating element  512  would then be energized to initiate the thermal decomposition of the CBA to generate propulsion gas. Heating element  512  may be, for example, a nichrome heating element, among others. In some embodiments in which heating element  512  is provided inside CBA chamber  508 , the heating element may be surrounded by a tube (not shown) that is in fluid contact with the CBA but the inside of the tube is not exposed to the CBA-containing environment outside the tube. 
     CBA-based propulsion system  500  also includes a nozzle  516  and a burst disc  520  for optimizing the conversion of energy in the pressurized propelling gas into thrust. In this example, nozzle  516  is a C-D nozzle, but it could be a divergent nozzle, or it could be simply replaced by a plain orifice. Burst disc  520  is designed to rupture at a predetermined pressure, allowing the propelling gas formed within CBA chamber  508  to rapidly release in a burst impulse into nozzle  516 . As those skilled in the art will readily appreciate, such a burst impulse may cause undesirable transient conditions within nozzle  516 . Consequently, nozzle  516  may need to be specifically designed to minimize such transient conditions. In this example, burst disc  520  is separate and distinct from both of base portion  504  and nozzle  516  and is sealed between the base portion and nozzle using a pair of O-rings  524 (1) and  524 (2). In other embodiments, burst disc  520  may be integrated with either, or both, of base portion  504  and nozzle  516 . Burst disc  520  may be made of any suitable material, such as metal, polymer, or ceramic, among others. In one example, burst disc  520  is composed a multiple layers of metal foil. In alternative embodiments, burst disc  520  may be replaced by a filter having a filtration size smaller than the size of particles of the CBA. 
     It is noted that while exothermically decomposable CBAs are used in the foregoing embodiments, some embodiments may use an endothermically decomposable CBA. However, exothermically decomposable CBAs are generally more desirable for many applications because of the lower input energy requirements and simplicity of operation. With exothermically decomposable CBAs, generally only enough energy to initiate thermal decomposition need be provided. Once the thermal decomposition starts, further decomposition is self-sustaining. In contrast, with endothermically decomposable CBAs, thermal decomposition occurs only while input energy is provided. Those skilled in the art can readily appreciate the positive impacts on spacecraft design and weight that the lower input energy requirements of exothermically decomposable CBAs have relative to endothermically decomposable CBAs. 
       FIG. 6  illustrates another example CBA-based propulsion system  600  that is generally similar to CBA-based propulsion system  204  of  FIGS. 3A-3D . In this example, CBA-based propulsion system  600  includes a pressure tank  604 , eight CBA storage units  608  (only six units  608  (1) to  608  (7) seen), a manifold  612 , a gas conduit  616 , a nozzle  620 , a pressure regulator  624 , a shutoff valve  628 , and electronics  632 . In this example, pressure tank  604  is toroidal in shape and includes a central opening  604 A in which nozzle  620  is mounted. Each CBA storage unit  608  is fluidly connected to manifold  612 , and the manifold is fluidly coupled to pressure tank  604  via gas conduit  616 . In this example, pressure regulator  624  is fluidly coupled to pressure tank  604  downstream of the pressure tank via a conduit  636 , shutoff valve  628  is fluidly coupled to the pressure regulator downstream of the pressure regulator via a conduit  640 , and nozzle  620  is fluidly coupled to the shutoff valve downstream of the shutoff valve via a conduit  644 . In this embodiment, each CBA storage unit  608  include a heater recess  608 B (only a few visible in  FIG. 6 ) for receiving a suitable heater (not shown, but see, e.g.,  FIGS. 7 and 8  for an example). Electronics  632  includes a control system (not shown) and other electronics for powering the components of CBA-based propulsion system  600  (e.g., heaters, shutoff valve  628 , sensors (not shown), etc.) and controlling the operation of the CBA-based propulsion system. Not shown for clarity include the heaters, sensors (e.g., pressure sensors upstream and downstream of pressure regulator  624 ), and wiring to electrically connect-together electronics  632  and the various electrically powered components of CBA-based propulsion system  600 . The control system includes appropriate algorithms for controlling the heaters, shutoff valve  628 , pressure regulator  624 , as a function of the state of CBA-based propulsion system  600  (e.g., pressures, number of unused CBA storage units, etc.) and the propulsion needs of the satellite or spacecraft to which the CBA-based propulsion system is coupled. 
     As a non-limiting example, in one instantiation pressure tank  604  and each of CBA storage units  608  are designed to be pressurized to a working pressure of 1000 PSI and nozzle  620  is designed to operate at 50 PSI, with pressure regulator  624  providing the appropriate stepdown in pressure. In this instantiation, each CBA storage unit is provided with an amount of CBA (not shown) needed to, upon ignition and thermal decomposition, raise the pressure within the pressure tank to 1000 PSI, or there-about, from a depleted pressure level. In the embodiment shown, there are no valves between CBA storage units  608  and pressure tank  604 , and the pressure tank is in constant fluid communication with the CBA storage units via manifold  612  and gas conduit  616  such that the total volume pressurized by thermal decomposition of the CBA in any given CBA storage unit is composed of the volume of the pressure tank, the volume of the CBA storage unit in which the CBA was just decomposed (less solid reaction byproducts), the unoccupied volume of any other CBA storage unit, the volume of the manifold, the volume of the gas conduit, the volume of conduit  636 , and any higher-pressure-side volume of pressure regulator  624 . Consequently, unless the amounts of CBA in CBA storage units  608  are precisely tuned to the varying total volume and are thermally decomposed in a specific sequence, the actual pressure within pressure tank  604 , and indeed in the entire volume, will vary from 1000 PSI. In one instantiation, some or all of the major components of CBA-based propulsion system  600 , such as pressure tank  604 , CBA storage units  608 , manifold  612 , gas conduit  616 , and nozzle  620  may be 3D printed using suitable 3D-printing techniques. 
       FIG. 7  illustrates a portion of a CBA storage unit  700  that can be used as a CBA storage unit of any suitable embodiment of a CBA-based propulsion system of the present disclosure, such as CBA-based propulsion systems  204 ,  600  of  FIGS. 2 and 6 , respectively.  FIG. 7  shows CBA storage unit  700  as including a storage tank  704  and a heater  708 . In this example, storage tank  704  is designed for a working pressure of 3000 PSI or more, such pressure resulting from thermal decomposition of a CBA (not shown) contained within the storage tank and, if forming part of a larger volume with one or more other CBA storage units (not shown, but see  FIG. 6 ), thermal decomposition of the CBA contained within such other storage unit(s). 
     In this embodiment, heater  708  includes a heating element  712 , for example, a ceramic heating element, at least partially located in a recess  716  formed in a wall  720  of storage tank  704 . In this example, recess  716  is cylindrical and is partially defined by a protrusion  724  that extends into the interior  728  of storage tank  704 . In the example shown, protrusion  724  is defined by a sidewall  732  and an end wall  736 . Tank wall  720  and side and end walls  732 ,  736  of protrusion  724  may be monolithically formed with one another, such as by 3D printing, casting, machining, etc., to provide robustness and pressure-tightness. When storage tank  704  is charged with a suitable amount of CBA, such CBA is in direct contact with protrusion  724  to maximize the heat transfer from heater  708 , through the protrusion, and into the CBA to initiate the thermal decomposition of CBA efficiently. 
     As seen in  FIG. 8 , in this example, heater  708  includes an electrically resistive element  800 , such as a resistive ceramic element, and a pair of electrical conductors  804  and  808  electrically connected to a suitable electrical power source (not shown). Electrically resistive element  800  may be made of any suitable material and may be any suitable shape. In some embodiments, the material(s) selected for protrusion  724  and, as desired, tank wall  720  in general, should have a relatively high thermal conductivity to maximize efficiency. While heater  708  is shown at one end of storage tank  704 , it is noted that it may be located at a different location on the storage tank and that the location of the heater may affect overall performance of the CBA-based propulsion system. 
       FIG. 9  illustrates an example microthruster  900  that can be a microthruster of any digital CBA-based propulsion system made in accordance with the present disclosure, such as digital CBA-based propulsion system  404  of  FIG. 4 . For the sake of convenience, microthruster  900  is described in the context of each thruster-array device  404  (1) to  404  (24) of  FIG. 4 . Referring to  FIG. 9 , and also to  FIG. 4 , microthruster  900  comprises thermal reaction-initiation layer  408 A, CBA-storage layer  408 B, and micronozzle layer  408 C. Together, layers  408 A,  408 B, and  408 C define a CBA-storage chamber  904  that contains a suitable CBA  908 . In this example, thermal reaction-initiation layer  408 A includes an electrically resistive element  912  that is individually powerable using any suitable addressing scheme to initiate the thermal decomposition of CBA  908 . Not shown are electrical conductors that supply electrical current to resistive element  912  during use, as well as the addressing circuitry, among other things. In this example, micronozzle layer  408 C may be considered to include both a nozzle  916  and a burst disc  920  located at the throat  916 A of the nozzle. Burst disc  920  may have any suitable design and be made of any suitable material that allows it to rupture at a desired pressure caused by the decomposition of CBA  908  within storage chamber  904 . Although nozzle  916  is shown as being a divergent nozzle, it may be a convergent-divergent nozzle or a convergent nozzle, among others. 
     The foregoing has been a detailed description of illustrative embodiments of the invention. It is noted that in the present specification and claims appended hereto, conjunctive language such as is used in the phrases “at least one of X, Y and Z” and “one or more of X, Y, and Z,” unless specifically stated or indicated otherwise, shall be taken to mean that each item in the conjunctive list can be present in any number exclusive of every other item in the list or in any number in combination with any or all other item(s) in the conjunctive list, each of which may also be present in any number. Applying this general rule, the conjunctive phrases in the foregoing examples in which the conjunctive list consists of X, Y, and Z shall each encompass: one or more of X; one or more of Y; one or more of Z; one or more of X and one or more of Y; one or more of Y and one or more of Z; one or more of X and one or more of Z; and one or more of X, one or more of Y and one or more of Z. 
     Various modifications and additions can be made without departing from the spirit and scope of this invention. Features of each of the various embodiments described above may be combined with features of other described embodiments as appropriate in order to provide a multiplicity of feature combinations in associated new embodiments. Furthermore, while the foregoing describes a number of separate embodiments, what has been described herein is merely illustrative of the application of the principles of the present invention. Additionally, although particular methods herein may be illustrated and/or described as being performed in a specific order, the ordering is highly variable within ordinary skill to achieve aspects of the present disclosure. Accordingly, this description is meant to be taken only by way of example, and not to otherwise limit the scope of this invention. The original appended claims form part of the original disclosure as if they appear in this Detailed Description section. 
     Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the spirit and scope of the present invention.