Passively damped end fittings and brackets

A passively damped mechanical system is disclosed, for example for use in aerospace applications where vibration can adversely affect navigational and operational instruments. In one example, the passively damped mechanical system includes an end fitting of a strut used to connect a structural element to a payload. The end fitting may include outer and inner cylindrical hubs, with a space between the outer and inner cylindrical hub at least partially filled with a viscoelastic material. In a further example, the passively damped mechanical system includes legs used to connect a structural element to a bracket configured to support a payload. Each leg may include a hollow interior having a lattice structure to add strength and a viscoelastic material to provide passive damping.

BACKGROUND

Vibration and shock load suppression are critically important in aerospace applications, where such vibrations and loads can otherwise adversely affect navigational and operational instruments. It is known to employ passive vibration damping in aerospace applications to reduce vibrational amplitude at resonant frequencies. Passive vibration isolation is also used to prevent transmission of shock between structural elements. Vibration damping and isolation mechanisms are known, but at present, suffer several drawbacks. For example, at present, passive vibration and isolation mechanisms are designed and manufactured early in the design phase for spacecraft components. This makes it very difficult to modify and optimize a system for damping, strength and stiffness, as is often necessary when the spacecraft components are built and tested. Moreover, current passive damping and isolation mechanisms often add a significant amount of weight to the system, adversely adding to the load during liftoff and reentry.

SUMMARY

According to one aspect the present disclosure relates to a passively damped end fitting configured to be mounted on an end of a strut connected between a structural element and a payload. The end fitting in this aspect includes a proximal section configured to fit within an end of the strut and a distal section configured to be coupled to one of the structural element and payload. The proximal section in this aspect includes: an outer hub configured to connect to an end of the strut, an inner hub connected to the distal section, a flexure mount connected between the inner and outer hubs and defining a space between the inner and outer hubs, and a viscoelastic material within at least portions of the space and adhered to the at least portions of the inner and outer hubs.

In another aspect, the present technology relates to a passively damped mechanical structure. The passively damped mechanical structure in this aspect includes: a structural element; a payload; and a strut extending between and connecting the structural element to the payload. The strut may include an end fitting having a proximal section within an end of the strut and a distal section coupled to one of the structural element and payload. The proximal section may include: an outer hub connected to an end of the strut, an inner hub connected to the distal section, a flexure mount connected between the inner and outer hubs and defining a space between the inner and outer hubs; and a viscoelastic material (VEM) within at least portions of the space and adhered to the at least portions of the inner and outer hubs.

In a further aspect, the present technology relates to a passively damped mechanical structure. The passively damped mechanical structure of this aspect includes a structural element; a bracket configured to hold a payload; one or more legs each having a length extending between and connecting the structural element to the bracket, each leg having a wall and a hollow interior. A leg of the one or more legs may include: a lattice structure provided within the hollow interior along at least a portion of the leg; and a viscoelastic material (VEM) provided within the hollow interior along at least a portion of the lattice structure, the VEM adhering to the wall and the lattice structure.

DETAILED DESCRIPTION

In one aspect, technology is described for isolating a first structural element from a second structural element using a passive damping system. In a first embodiment, the passive damping system may be formed within an end fitting of a strut. The strut includes a first end, formed by the end fitting, connected to the first structural element, and a second end connected to the second structural element, which may or may not be formed with a damped end fitting.

The end fitting may have a proximal section affixed within the end of the strut, and a distal section extending from the end of the strut for coupling the end fitting to the second structural element. The proximal section is formed of an outer cylindrical hub and an inner cylindrical hub within and concentric with the outer cylindrical hub. The inner and outer cylindrical hubs are connected to each other by a castellated flexure mount, though other embodiments are possible to create a flexure feature. The space between the first and second cylindrical hubs is filled with a viscoelastic material (VEM) which passively damps vibrations from the strut and provides a second load transmission path from the proximal section to the distal section.

In a second embodiment, the passive damping system may be formed within a bracket for isolating one or more structural elements on the bracket from vibration and shock. The bracket may include support legs affixed to a second structural member. The support legs may each include an internal lattice structure imparting strength and stiffness to the support legs. A VEM may be injected into the support legs, around the lattice structure, to passively damp vibrations within the support legs and to isolate the first structural element from shock and vibration within the support legs.

It is understood that the present technology may be embodied in many different forms and should not be construed as being limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete and will fully convey the technology to those skilled in the art. Indeed, the technology is intended to cover alternatives, modifications and equivalents of these embodiments, which are included within the scope and spirit of the technology as defined by the appended claims. Furthermore, in the following detailed description of the present technology, numerous specific details are set forth in order to provide a thorough understanding of the present technology. However, it will be clear to those of ordinary skill in the art that the present technology may be practiced without such specific details.

The terms “longitudinal” and “transverse,” “top” and “bottom,” “upper” and “lower” and “vertical” and “horizontal,” and forms and synonyms thereof, as may be used herein are by way of example and illustrative purposes only, and are not meant to limit the description of the technology inasmuch as the referenced item can be exchanged in position and orientation.

For purposes of this disclosure, a connection may be a direct connection or an indirect connection (e.g., via one or more other parts). In some cases, when a first element is referred to as being connected, affixed or coupled to a second element, the first and second elements may be directly connected, affixed or coupled to each other or indirectly connected, affixed or coupled to each other. When a first element is referred to as being directly connected, affixed or coupled to a second element, then there are no intervening elements between the first and second elements.

Referring now toFIGS.1and2, there is shown a system100including a first structural element102being connected to a second structural element104by a strut106. Both structural elements102and104are shown schematically and may for example be any of a wide variety of components found in association with the spacecraft. In general, the structural element102may be a support surface subject to vibrational and other forces which are transmitted to strut106. In general, the structural element104may include instrumentation or other components which are ideally isolated from the vibrational and/or other forces exerted on structural element102and strut106. In embodiments, structural element104may also be referred to herein as payload104. As one of any of a wide variety of examples, structural element104may be a launch vehicle, and the payload104may be a spacecraft. In a further example, the payload104may be sensitive components or instrumentation mounted and structural element102may be a support surface on which the payload is mounted.

In embodiments, the strut106may initially be fixedly mounted to structural element102as shown inFIG.1, and thereafter affixed to payload104as shown inFIG.2. The strut106may be affixed initially to payload104and subsequently to structural element102in further embodiments. In general, strut106may be a rigid tubular member with a hollow interior along at least portions of its length. However, strut106may have cross-sectional shapes other than circular in further embodiments, including for example square, rectangular, triangular and oval. In one example, strut106may have a length of 36″, and a cross-sectional width of 1″. However, strut106may have a variety of lengths and cross-sectional widths. In one example, a 1″ strut may have a stiffness ˜100 kips/in, and strength capability in excess of 5000-lb in compression.

WhileFIGS.1and2show the end fitting110at an end of strut106adjacent the payload104, the end fitting110may be on the end of strut106adjacent the structural element102in further embodiments. Additional embodiments may have end fittings110at both ends of a strut110. Further, whileFIGS.1and2show a single strut106extending between structural element102and payload104, there may be two or more struts106extending between the structural element102and payload104in further embodiments.

In further embodiments, multiple struts106may be affixed to each other in series. Such struts may be affixed to each other using a pair of end fittings110connected to each other. In further embodiments, one strut may have an end fitting110which connects to a conventional coupling of the next attached strut. In further embodiments, the pair of struts may be affixed to each other using any known affixation scheme, including bolting, adhesive bonding, welding and pinning. The end of the last strut106in the series may be affixed to a payload104via an end fitting110.

The purpose of strut106is to fixedly mount the payload104to structural element102while isolating payload104from vibration and shock from structural element102using a passive damping system in the end fitting110as explained below. The end fitting110may be affixed to one end of strut106as also explained below. In embodiments, the end fitting110may be pinned to the payload104using a pin112, thus allowing one degree of rotational freedom of the payload104relative to the strut106. In further embodiments, an end fitting110may be affixed at both ends of strut106, so that both structural elements102and104are fixed to strut106by a passively damped end fitting110.

FIGS.1and2illustrate strut106as being a single straight tube extending between structural elements102and104. Strut106may have other configurations in further embodiments. For example,FIG.3shows a strut106having multiple legs106a-106cconnected to multiple structural elements102a-102c. The legs106a-106ccome together at some point along their length so that a single strut having a single and fitting110affixes to payload104. The legs106a-106cmay reside in the same or different planes. As a further example,FIG.4shows a strut106having multiple arms106d-106fconnected to multiple payloads104a-104cby multiple end fittings110. The arms106d-106fstem from a single strut106connected to structural element102. The arms106d-106fmay reside in the same or different planes.

FIG.5shows a further example of a multi-tube strut with a central nucleus off of which extend a number of tubes. The tubes may be affixed at the nucleus by a variety of affixation methods, including for example welding. Although not shown, each of the tubes may be connected to a structural element102(subject to forces) or a payload (isolated from forces). Some or all of the tubes may include an end fitting110(not shown inFIG.5), as explained above and below, at the exposed end of the tube for affixation to a structural element102or payload104. In embodiments, the end fitting110may be printed in an additive manufacturing process so that the end fitting110is built into one or more of the tubes. The end fitting110may additionally be printed into one or both ends of strut106in any of the embodiments described herein. The example shown inFIG.5is by way of illustration only, and there may be more or less tubes, and each tube may be shorter or longer, in further embodiment. Other examples of further configurations of strut106are contemplated.

FIGS.6and7illustrate perspective and cross-sectional views of an end fitting110affixed at one end of a strut106. In general, the end fitting110may have a proximal section114affixed within the end of the strut106, and a distal section116extending from an end of the strut106for coupling the end fitting110to the payload104. The proximal section is formed of an outer cylindrical hub120and an inner cylindrical hub122within and concentric with the outer cylindrical hub120. The inner and outer cylindrical hubs are connected to each other by a castellated flexure mount126as shown. The distal section116includes a neck128and a pin mount130for receiving a pin when coupled to a payload104.

The spaces between the castellations in flexure mount126may be about ½ the arc length of the castellations, though these spaces may be smaller or larger in further embodiments. The circular portion of the flexure mount126is sized with a radius to provide a cylindrical space134between the inner and outer cylindrical hubs120,122. In accordance with aspects of the present technology, at least portions of the cylindrical space134may be filled with a viscoelastic material (VEM)136. As explained below, VEM136may be provided to dampen vibration and shock loads exerted on the end fitting from the strut106, as well as providing a second load transmission path from the proximal section114to the distal section116.

In one embodiment, the VEM may be Appli-Thane® 7125 from Appli-Tec, Inc., Salem, N.H., but other thermosetting polyurethane, nylon or plastic viscoelastic materials may for example be used. The VEM136may be injected into space128as an A-stage liquid or foam, and thereafter cured to a C-stage solid, where the VEM136at least partially fills space128and adheres to both the inner and outer cylindrical hubs120,122. As noted below, the properties of the VEM136may be selected to get the optimal damping response. However, in one embodiment, once cured, the VEM136may have a tensile modulus of 1646 MPa, a density of 1002 Kg/m3and a Poisson ratio of 0.34. These values are by way of example only and may vary in further embodiments. As explained hereinafter, the selection of the VEM, as well as the pattern with which the VEM is applied may be selected to tune the damping and load response of the end fitting110as needed or desired during testing and implementation of the system100.

The outer cylindrical hub120may be fixedly attached to an inner diameter of the strut106end, as by welding, bolting, high strength adhesive and/or other adhering mechanism. Vibrational, shock, torsional and compressive loads exerted axially on strut106are transmitted to the outer cylindrical hub120. From there, some of these loads (vibration and shock) are damped by both the flexure mount126and the VEM136. As explained below, the properties of the VEM136may be selected to optimize damping at resonant frequencies of the end fitting110and/or system100. Vibrational and shock energy is transmitted from the outer cylindrical hub120as shear forces into the VEM136, which dissipates the energy as heat.

Some of the loads on the outer cylindrical hub120(e.g., tensile and compressive loads) are transmitted through the proximal section114to the distal section116of the end fitting110. In accordance with a further aspect of the present technology, provision of the VEM136provides a second load transmission path for transferring these loads. As shown inFIG.8, a load on the strut106(in this case a compressive load) is transmitted along a first path138athrough the outer cylindrical hub120, flexure mount126and inner cylindrical hub122to the distal section116. This same load on strut106is also transmitted along a second path138b, through the outer cylindrical hub120, VEM136and inner cylindrical hub122to the distal section116.

The VEM136takes the load from the outer cylindrical hub120as shear load, and transfers at least part of the load to the distal section116via the inner cylindrical hub122. Part of the load may be dissipated as heat. While the hub120, hub122and space134are referred to herein “cylindrical,” it is understood that these components may be referred to more generally as the outer hub120, inner hub122and space134, for example in embodiments where the cross-sectional shape of the end fitting is not circular.

Having a dual load transmission path for load transfer provides benefits of reducing stress and strain on the flexure mount126and the inner cylindrical hub122. A further benefit of the dual load transmission path is that it allows tuning of the stiffness vs. damping of the system. In other words, in order to get damping, there needs to be shear loads in the VEM136. This results in some loss of stiffness. The flexures help to tune the stiffness loss to remain above a required stiffness while still allowing some load to go through the VEM and provide damping.

FIG.9shows the mechanical elements of the end fitting110modeled as springs and a damper. The loading elements are modeled as springs having a spring coefficient, k. The VEM damper is modeled as a damper with a damping coefficient, c. The strut (kstrut) loads the outer cylindrical hub (khub(o)), which in turn loads the coupling mount (kcoupling) and the inner cylindrical hub (khub(i)). The spring response of these elements is damped by VEM (cVEM). The resultant forces are communicated the neck (kneck) and then to the pin mount (kpin), where the end fitting transfers the load to the payload104. Loads can also flow along the dual pathway in the opposite direction.

In addition to the advantages of the damping and dual load path, use of the VEM136provides a further advantage in that the VEM136can be added to the cylindrical space134during assembly or test of system100. In particular, when strut106is connected between the structural element102and payload104in system100, it may turn out that, contrary to design indications, the payload104is subject to non-optimal loads or levels of vibration at certain frequencies. At that point, the end fitting110can be redesigned using for example additive manufacturing and/or a particular VEM136(having the desired damping properties) can be injected into and/or removed from the cylindrical space134in the end fitting110to tune the vibration damping and/or response to shock loads.

In one embodiment, the VEM136may be injected around the entire circumference of the cylindrical space134. In further embodiments, it may be found that optimal damping and/or shock load reduction is achieved by injecting the VEM136in strips around the circumference of the cylindrical space134(as shown inFIG.6). The strips may be parallel to a central axis of the end fitting, or wrapped around the inner hub122at an angle (including up to 90°). Again, these features further allows technicians to tune the vibration and load response of system100during assembly or test. As noted in the Background section, this is a significant advantage over conventional systems in which vibration and shock load concerns are addressed early in the design phase process, without a feasible method of making changes during the later assembly and test phases.

FIGS.10-12relate to a further embodiment of the present technology for managing vibration and/or shock load on a bracket200. Bracket200(which may be an example of a payload104) may include sensitive components and/or instrumentation which are desirably isolated from vibration and/or shock loads. In one example, bracket200may include a number of surfaces202(four in the example shown) to which momentum wheels (not shown) may be mounted. Other components and instrumentation are contemplated. The bracket200may also have an upper surface206to which components and/or instrumentation may be mounted. Bracket200may have other surfaces, and other configurations, for receiving sensitive components and/or instrumentation.

The bracket200may be mounted to a support surface (not shown), such as for example structural element102described above, using a number of legs204. In the embodiment shown, there may be for cylindrical legs204affixing the bracket200to the structural element102. There may be more or less than four legs204in further embodiments, and legs204may have other cross-sectional shapes in further embodiments, including for example square, rectangular, triangular and oval.

FIG.11is a cross-sectional view through bracket200, including through a pair of the opposed legs204. As shown, the legs204may extend up into the bracket200, near to or at the upper surface206of the bracket. The cross-sectional view shows a hollow interior of legs204that is filled with a lattice structure208and a VEM210injected and cured around the lattice structure208. Greater detail of the lattice structure208and VEM210in one section of a leg204is shown inFIG.12.

Lattice structure208may be formed of lightweight components assembled into a lattice of repeating truss structures extending through a portion or all of each of the legs204. Each truss structure may for example have a tetrahedral core of joined crosspieces, but other assemblies are contemplated including for example 3D Kagome, octahedral, hexagonal or pyramidal truss structures. The crosspieces may for example be formed of titanium, but other materials are possible. Such lattice structures are lightweight but impart a relatively high stiffness and yield strength to legs204. The lattice structure108, by itself, may have poor vibration damping at resonant frequencies.

In order to address this, VEM210may be injected into each of the legs, for example through ports212in surface206. The VEM210may be injected as an A-stage liquid or foam, along a length of a leg204. The length of the column of injected VEM210may be the entire length of a leg204, or a portion of the length of a leg204. The VEM210in embodiments is injected to take up the entire cross-sectional area along the column length, surrounding and engaging the repeating truss structures of the lattice structure208. After injection, the VEM210may be cured to a C-stage solid. VEM210may have the same or different properties as the VEM136discussed above.

In addition to the advantages of the damping, use of the VEM210provides a further advantage in that the VEM210can be selectively added to the interior of legs204during assembly or test of the components or instrumentation mounted to bracket200. In particular, when the components/instrumentation are mounted on bracket200, and bracket200is mounted to the structural element102, it may turn out that, contrary to design indications, the bracket200is subject to non-optimal loads or levels of vibration at certain frequencies. At that point, the legs204can be replaced with other legs with a different lattice structure208and/or VEM (having the desired damping properties). Instead of replacing legs204, VEM210can be injected into and/or removed from one or more legs204to optimally tune the vibration damping and/or response to shock loads.

The legs204including lattice structure208and VEM210are designed to balance optimal load bearing and vibration damping requirements with minimum weight requirements. The legs204and lattice structure208within the legs are designed with a high strength and modulus of elasticity to carry tensile, compressive, torsional and bending (moment) forces exerted on legs204by the structural element to which the legs are mounted. The VEM210absorbs vibration and shock, exerted as shear within the VEM210, and dissipates this energy as heat.

Various methods are known for optimizing the load bearing and damping response within bracket200including legs204with lattice structure208and VEM210. Such methods are described for example in Wang, R., Shang, J., Li, X., Luo, Z. and Wu, W., “Vibration And Damping Characteristics of 3D Printed Kagome Lattice With Viscoelastic Material Filling,”Sci Rep8, 9604 (2018)), which publication is incorporated herein by reference in its entirety. The methods described in the above-incorporated publications may be also be used.

When optimizing the load and vibration damping response of bracket200, various parameters may be controlled and adjusted as needed, using for example additive manufacturing, including the following:the number of legs204;the length and cross-sectional area of legs204;the wall thickness and type of material used for legs204;the configuration of the repeating truss structures in the lattice structure208(tetrahedral, 3D Kagome, octahedral, hexagonal, etc.);the length and cross-sectional area of the crosspieces in each truss structure of lattice structure208;the angle of the crosspieces in each truss structure relative to the central axis of the leg204;the type and properties of VEM210;the pattern in which the VEM210is applied;the length of the VEM210column in leg204.

In embodiments, the VEM210may be isotropic, exhibiting a uniform damping response in all directions. However, in further embodiments, the properties of the VEM and/or the pattern with which the VEM is applied, may result in an anisotropic response of VEM210, exhibiting greater damping for vibrations applied in a given direction than for vibrations applied in other directions. Again, the damping response of VEM can be tuned to optimize anisotropic damping in both end fitting110and within legs204, for example where vibration occurs more prevalently along one or more specific axes.

FIG.13is a graph of vibration amplification factor (Q) for different vibrational frequencies for the end fitting110and/or a leg204for both damped (including VEM) and undamped (not including VEM) structures. In this example vibrational resonance occurs at around 35 Hz. As shown, the VEM reduces the amplification factor Q by about half This is a significant reduction in vibration.

The foregoing detailed description has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the subject matter claimed herein to the precise form(s) disclosed. Many modifications and variations are possible in light of the above teachings. The described embodiments were chosen in order to best explain the principles of the disclosed technology and its practical application to thereby enable others skilled in the art to best utilize the technology in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope be defined by the claims appended hereto.