Precision manifold for an ion thruster using characterized flow restrictors

Precision flow restrictors and techniques for manufacturing the same for ion thruster manifolds are disclosed. Flow restricting features are moved out of the manifold base and into separate flow restrictors, allowing a wider range of manufacturing techniques and materials to be applied. Quality control can be performed at the level of the flow restrictors as subcomponents, ensuring that only good parts with ideal flow characteristics make it into the final assembly and improving the yield rate of the final manifold assembly.

FIELD

The present invention generally pertains to space propulsion, and more particularly, to ion thruster propellant manifolds and techniques for manufacturing the same using characterized flow restrictors.

BACKGROUND

Flow restricting features in an ion thruster propellant manifold assembly, which are typically precision-manufactured orifices in a common plate, can contribute to significant flow non-uniformity if tolerances on the features are not properly controlled during manufacturing. Such non-uniformity in flow distribution negatively impacts thruster performance. Furthermore, the manifold assembly is typically complex and expensive to manufacture, and ensuring flow uniformity of the assembly is difficult and expensive to verify.

In the specific case of a Hall-effect propellant manifold, current manufacturing techniques typically drill orifices in a manifold base, which is then embedded in an all-welded manifold assembly prior to testing. Such a conventional manifold base100is shown inFIG.1. As seen inFIG.1, manifold base100has precision machined orifices110drilled in a single solid piece. An example gas flow path is illustrated as well. Gas comes down orifices110and then gets distributed through channels120.

Any defect with manufacturing any of the orifices can affect the performance of the overall gas delivery system, and ultimately, of the thruster. Thus, if a manifold assembly incorporating such a single piece manifold base is discovered to have a substandard flow uniformity during this testing late in assembly, the entire all-welded manifold assembly must usually be scrapped, and manufacturing must start anew, incurring substantial scheduling delays and costs. Discovering that a manifold assembly has substandard flow performance at the final assembly level is thus a consistent risk for fabrication cost and schedule overrun. Accordingly, an improved manifold assembly for ion thrusters may be beneficial.

SUMMARY

Certain embodiments of the present invention may be implemented and provide solutions to the problems and needs in the art that have not yet been fully solved by conventional space vehicle propulsion technologies. For example, some embodiments of the present invention pertain to characterized flow restrictors and techniques for manufacturing the same for ion thruster manifolds. As used herein, “characterized” flow restrictors are selected for certain flow restricting properties pertinent to the manifold.

In an embodiment, an apparatus includes a plurality of flow restricting elements configured to restrict flow of a propellant and a manifold base including a plurality of holes. The plurality of flow restricting elements are located in respective holes of the plurality of holes of the manifold base. The plurality of flow restricting elements are separate subcomponents from the manifold base.

In another embodiment, a manifold assembly for an ion thruster includes a plurality of flow restricting elements configured to restrict flow of a propellant and a manifold base including a plurality of holes. The plurality of flow restricting elements are located in respective holes of the plurality of holes of the manifold base. The manifold assembly also includes a propellant delivery tube including an end that enters the manifold base and is configured to deliver propellant thereto. The plurality of flow restricting elements are separate subcomponents from the manifold base. Gas-dynamic flow restriction characteristics of the plurality of flow restricting elements are based on a proximity of the respective flow restricting element to the end of the propellant delivery tube.

In yet another embodiment, a flow restricting element for a manifold assembly for an ion thruster includes at least one orifice or a sintered porous structure configured to restrict a flow of propellant through the flow restricting element. The flow restricting element is a separate subcomponent from the manifold base. The flow restricting element is configured to be inserted into a respective hole in the manifold base.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Some embodiments of the present invention pertain to precision flow restrictors and techniques for manufacturing the same for ion thruster manifolds. Indeed, some embodiments may be used for any type of ion thruster using a gaseous propellant and having a uniform or non-uniform propellant distribution without deviating from the scope of the invention. Such precision restrictors and techniques may provide a lower cost and more reliable approach to propellant manifolds for use in Hall-effect thrusters, for example. For instance, some embodiments may reduce the cost of acceptance testing. Also, the higher confidence in manifold manufacturing quality provided by some embodiments may eliminate the need for verification tests on flight hardware where the manifold is placed within the discharge chamber of the ion thruster in a vacuum test facility and pressure measurements are taken around the manifold in the vacuum to determine flow characteristics. The techniques of some embodiments may be applied to any type of ion propulsion device without deviating from the scope of the invention.

By removing the flow restricting elements from the propellant manifold structure and making them separate and insertable components, more reliable and repeatable precision manufacturing techniques can be applied. Furthermore, the resulting components can be tested, characterized, and sorted for acceptance before being installed into the larger manifold assembly. Poor performing parts can be discarded before installation. Performing quality control at the component level can lead to increased performance of the final assembly as well as improving yield rate of the more expensive final propellant manifold assembly.

Controlling the flow characteristics of the flow restricting features is typically done by using precision manufacturing techniques. By moving the flow restricting features out of the manifold base and into a separate subcomponent, a wider range of manufacturing techniques and materials can be applied. Additionally, quality control (e.g., testing, performance characterization, and rejection) can be performed on the subcomponent, ensuring that only good parts with ideal flow characteristics make it into the final assembly. This improves the yield rate of the final manifold assembly.

FIG.2is a partially transparent perspective cutaway view illustrating a manifold assembly200for an ion thruster, according to an embodiment of the present invention. Propellant delivered to manifold assembly200in this embodiment occurs at one azimuthal location in the manifold ring—i.e., via a propellant delivery tube210having an end212that delivers propellant through a manifold base220. However, in certain embodiments, multiple propellant delivery tubes may be used. The thicker gray arrows inFIG.2indicate the directions of propellant flow. A manifold cover230forms a plenum240together with manifold base220. The propellant flows through orifice plugs250(also called “flow restrictors”, “flow elements”, “flow restricting elements”, or “subcomponents” herein) and out of manifold assembly200via corresponding exit channels260.

In this embodiment, orifice plugs250are shown as having a converging section with a funnel-like shape, as shown inFIG.4. However, any desired orifice shape(s), size(s), location(s) and/or number of orifices may be used without deviating from the scope of the invention. In certain embodiments, orifice plugs250are surrounded by a carrier (not shown) that helps to secure them in place. See, for example,FIGS.5A and5B. Indeed, any of the orifice plug designs shown inFIGS.3-6or combinations thereof may be used in manifold assembly200without deviating from the scope of the invention.

As the propellant delivered to manifold assembly200flows azimuthally around plenum240, delivering propellant to each orifice plug250, a drop in pressure occurs in plenum240. Whereas all orifices in conventional propellant manifolds are manufactured to an identical specification, in some embodiments, orifice plugs may have flow characteristics designed for their position in manifold assembly200. This may be to compensate for the fact that, assuming similar flow restricting characteristics, orifice plugs250closest to end212of propellant delivery tube210, where the pressure in plenum240is the highest, would eject the highest mass flow of propellant, while orifice plugs250further from end212of propellant delivery tube210, where the propellant pressure is lower, would eject decreasing amounts of propellant based on their distance from end212. This effect can result in significant azimuthal flow nonuniformity in an ion thruster.

By characterizing precision orifices for their unique gas-dynamic properties, orifice plugs250at each location may be appropriately selected to counter the negative effect of the pressure drop in plenum240. Orifice plugs250with a lower predisposition to gas flow can be installed closest to end212of propellant delivery tube210, while orifice plugs250with a higher predisposition to propellant flow may be installed away from end212of propellant delivery tube210. This may essentially cancel out the effect of pressure drop, where all orifice plugs250may deliver equal or near-equal flow rates (e.g., within a few percent). While attempting such an outcome with machined holes in a single plate would be challenging and expensive, doing so with characterized orifice plugs, as is done in some embodiments, is much more practical and cost effective. The resulting flow distribution in a propellant manifold fabricated using this approach can be applied not only to achieve uniform flow fields, but also to achieve non-uniform flow fields based on the implementation.

FIG.3is a perspective view illustrating a portion of a manifold base300, according to an embodiment of the present invention. Manifold base300includes a flow element310(also called a “flow restrictor”, “flow restricting element”, or “subcomponent” herein) with an orifice320. It should be appreciated that while one orifice is shown here, any number, size(s), shape(s), and/or orientation(s) of orifices may be used without deviating from the scope of the invention. In this embodiment, flow element310has a cylinder shape, but any suitable shape may be used without deviating from the scope of the invention (e.g., cube, truncated pyramid, truncated cone, irregular shapes, etc.). Orifice320may be precision machined in some embodiments. Flow element310may be press fit into a respective hole330(or plenum) drilled in manifold base300, soldered or welded in place, etc. Additionally or alternatively, the outer surface of flow element310and an inner surface of hole330may be counter-threaded with respect to one another such that flow element310screws into hole330.

In some embodiments, the orifice through which the propellant gas (e.g., xenon) flows may be non-cylindrical. Such an embodiment is shown inFIG.4. LikeFIG.3, manifold base400includes a flow element410with an orifice420. However, inFIG.4, a converging section430is machined into flow element410and leads to orifice420.

In certain embodiments, it may be desirable to make the flow element from a different material than the manifold base, such as from corundum (e.g., sapphire or ruby) or a ceramic. It is typically possible to achieve higher precision ceramic orifices than with a metal or alloy. For instance, high precision techniques used for watch making or water jets may be employed to create precise orifices in sapphire. Sapphire orifices are used in water jet nozzles. Likewise, watch movements sometimes use sapphire movements. This means that there is a preexisting manufacturing base that can make these parts to tight tolerances and for a reasonable price. As such, these components can be made at a low cost and can achieve tolerances on the order of ten thousandths of an inch.

However, such materials may not be amenable to press fitting or being secured in a hole machined in the manifold base by themselves. For instance, sapphire cannot be welded into an Inconel® manifold. In such embodiments, a different carrier material, such as an austenitic nickel-chromium-based superalloy with a low coefficient of thermal expansion (e.g., Kovar®, Invar®, etc.) or another metal alloy, may be used to secure the flow element in the manifold base. In certain embodiments, the carrier material may be the same material as the manifold base. Such an embodiment is shown inFIG.5A.

Unlike manifold bases300,400ofFIGS.3and4, respectively, manifold base500includes a flow element520that is a laminar flow element with relatively small precision orifices530. In certain embodiments, flow element520may be constructed from a sintered porous material. Flow element520is located within a carrier510that allows flow element520to be secured within hole540of anode base500.

Some embodiments use a sintered porous material flow element in place of flow element520ofFIG.5A. Such a flow element550is shown inFIG.5B. Flow element550has small pores560formed in the sintered porous material (e.g., stainless steel, titanium, alloys such as Hastelloy® or Inconel®, etc.) that increase the surface area of flow element550(i.e., a restricting element) such that pores560of flow element550are less prone to clogging than precision orifices530of flow element520. As with flow elements310,410, carrier510may be press fit, screwed, welded, soldered, etc. to secure carrier510and flow element520or550in hole540of manifold base500. Regardless of the technique(s) used to install and secure flow elements within respective holes of the manifold base, hermetic seals should be created. A press fit relying on plastic deformation of flow element520or550, carrier510, or manifold base500to provide a hermetic seal is one option. Another is to tap manifold base500and machine threads on carrier510, enabling one to thread carrier510with flow element520or550therein into manifold base500. Yet another option is sealing carrier510by brazing or crimping. Carrier510could then be laser welded on the top surface of manifold base500to provide a robust hermetic seal. However, in certain embodiments, flow elements may be extracted and replaced after initial insertion.

A unique feature of some embodiments is the ability to use materials that are not traditionally used in ion thruster manifolds. Since the manifold base is conventionally one piece, the material in conventional manifold bases must be uniform. However, by breaking the manifold base into multiple components, different materials from those of the manifold base could be used for the flow elements of some embodiments. Broader material selection opportunities could allow the use of alternative manufacturing techniques to achieve the desired flow control performance. Additionally, access to additional techniques of quality control, potentially more accurate and used earlier in the process, could boost final yield rate numbers for manifold assemblies.

Some embodiments allow a statistical distribution of flow elements to be fabricated with less stringent tolerances than if machined in a single plate. Flow elements could be pre-screened for their flow performance characteristics, sorted, and binned with flow elements having similar characteristics. While each manifold assembly that is manufactured typically requires some number of flow restricting elements with sufficiently similar (or appropriately dissimilar) flow characteristics to achieve uniform or non-uniform flow distribution for a given application, each manifold assembly does not need to have flow restricting elements with the exact same characteristics as another manifold assembly. Thus, sorting the flow elements can be used to reduce manufacturing tolerances rather than actually reducing manufacturing variance, which is typically far more expensive.

Rather than using a highly specialized and advanced vacuum chamber used for ion thruster acceptance testing that requires thousands of dollars of testing time, flow elements may be tested using a less complex, less expensive, and more widely available device. For instance, flow elements could be placed into a flow testing device that includes a few valves, a couple of pressure sensors, a vacuum pump, and a propellant source. By passing a known flow of propellant through a flow restricting element, where the pressure drop across the device is held consistent with the intended application, and measuring the pressure upstream and downstream of the device, the predisposition of the flow restricting element to pass propellant can be characterized. One technique to characterize the orifice is determining Cd*A, where Cd is the discharge coefficient and A is the orifice area. The flow devices may also be characterized for a range of Reynolds numbers by varying the propellant mass flow, propellant temperature, or the type of propellant. Flow elements may then be binned based on the results of this testing.

A mass flow controller that regulates and measures the propellant flow may be used to make such measurements. The output of the mass flow controller goes past a pressure sensor, through the flow restrictor element, then past another pressure sensor, and finally, the propellant is transported away by a pump. The pump maintains a pressure drop across the orifice sufficient to ensure a choked flow condition.

In certain embodiments, orifice sizes, pore sizes, or other flow restriction characteristics may be selected based on distance from locations where propellant is fed into the manifold, per the above. For instance, a flow element that is closer to a gas feed tube would experience a higher gas pressure and may have smaller orifices or pores, whereas a flow element that is further from a gas feed tube would experience a lower gas pressure due to pressure drop in the channel and may thus have larger orifices or pores to ensure that it has a similar gas throughput to other flow elements, regardless of their distance from the gas supplies. This approach could also be applied to a large single piece manifold base where holes are drilled.

In the case of a sintered porous materials, the pores may be 10 thousandths of an inch, 15 thousandths of an inch, 20 thousandths of an inch, etc. If manufacturing variability is 1/1000thof an inch, this creates substantial variability by percentage. Thus, binning flow elements with similar flow characteristics may provide a mechanism for achieving desired performance characteristics without reducing manufacturing variability, which may be expensive.

Certain manufacturing techniques are suitable for manufacturing small cylindrical objects that are not suitable for creating monolithic manifold bases. For instance, precision grinding, sintering of porous material, and/or laser drilling may be used.

In some embodiments, stacked flow restrictor devices may be used, where multiple flow restricting elements are stacked on top of one another. To create a compact restriction, thin plates with channels may be stacked or other configurations may be used to create complex flow paths that result in the desired flow restriction characteristics.

Per the above, flow elements may include any number, size(s), shape(s), and/or orientation(s) of orifices without deviating from the scope of the invention. Additionally or alternatively, the flow elements may be made from a sintered and/or porous material with or without additional orifices therein. Some example flow element configurations600,610,620,630,640are shown inFIG.6. For instance, flow elements may have a central orifice that is larger than peripheral orifices (e.g.,600,610,630), have different shapes (e.g.,630), be configured in a ring (e.g.,620), increase in size in a direction further from a gas supply (e.g.,640), etc. It should be noted that orifice sizes may not be to scale.

FIG.7is a flowchart illustrating a process700for manufacturing, testing, and installing precision flow restrictors for ion thruster manifolds, according to an embodiment of the present invention. The process begins with precision manufacturing multiple flow restrictors at710. The precision machining may include, but is not limited to, precision grinding, sintering of porous material, laser drilling, any combination thereof, etc. The precision machining may also include machining orifices in the flow restrictors with desired sizes, shapes, locations, and/or orientations.

The flow restrictors are then tested to determine their flow characteristics and binned based on performance at720. Flow restrictors may be sorted and binned based on pore sizes, machined orifice sizes, flow restriction qualities, etc. In embodiments where carriers are used in the manifold base, the carriers are created at730. In some embodiments, the carriers are the same material as the manifold base.

The manifold base and holes therein for the flow restrictors are machined at740. This may be performed before, after, or during machining of the flow restrictors. Flow restrictors are selected for insertion into the respective manifold holes at750. For instance, flow restrictors closer to a gas plenum providing gas to the manifold base may restrict gas flow more than flow restrictors that are further from the gas plenum.

In embodiments using carriers, the flow restrictors are inserted into their respective carriers at760. The flow restrictors or carriers housing flow restrictors are then secured into the holes at770. This may be accomplished by press fitting, screwing in, brazing, crimping, welding, soldering, etc. The manifold base is then incorporated into the manifold assembly at780.