CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present disclosure claims priority to United States Provisional Patent

Application No. 62/355,274, filed June 27, 2016.

BACKGROUND

[0002] Solid propellant rocket motors include a case, a throat, and a nozzle. The solid propellant is generally cast within the case. Solid propellant rocket motors burn at higher pressures at high temperature and lower pressures at low temperatures. The structure of the motor is designed for the high pressure operating conditions. Such structure includes thick walls and high mass, which debits performance.

SUMMARY

[0003] A solid rocket motor according to an example of the present disclosure includes a case, a throat, a nozzle, a nitinol ring at least partially surrounding the throat, and a spring-loaded pin to which the ring is attached. The spring-loaded pin abuts the throat.

[0004] In a further embodiment of any of the foregoing embodiments, the spring- loaded pin includes an inner tip that abuts the throat, and the inner tip has an aft side that faces toward the nozzle and a forward side that faces away from the nozzle. The forward and aft side meet at the throat to form an acute angle.

[0005] In a further embodiment of any of the foregoing embodiments, the spring- loaded pin includes an outer tip that does not abut the throat. The outer tip is attached with the nitinol ring.

[0006] A further embodiment of any of the other embodiments include a support structure in which the spring-loaded pin and the nitinol ring are mounted, The support structure has a groove in which at least a portion of the nitinol ring and at least a portion of the spring-loaded pin are disposed.

[0007] A solid rocket motor according to an example of the present disclosure includes a solid propellant section, and a throat section attached with the solid propellant section. The throat section has a throat that has a flow area there through, a nozzle attached with the throat section, spring-loaded pins that are moveable in the throat section, and a passive actuator connected with the spring-loaded pins. The passive actuator provides temperature-dependent actuation of the spring-loaded pins. The actuation causes the spring- loaded pins to move and thereby change the flow area of the throat.

[0008] In a further embodiment of any of the foregoing embodiments, the passive actuator is a shape memory alloy.

[0009] In a further embodiment of any of the foregoing embodiments, the passive actuator is a bimetallic actuator that has two joined strips of different metals.

[0010] In a further embodiment of any of the foregoing embodiments, the passive actuator is a ring.

[0011] In a further embodiment of any of the foregoing embodiments, the spring- loaded pins each include an outer tip opposite the throat, and the outer tip is interlocked with the ring.

[0012] In a further embodiment of any of the foregoing embodiments, the ring is a bimetallic actuator that has two joined strips of different metals.

[0013] In a further embodiment of any of the foregoing embodiments, the ring is a shape memory alloy.

[0014] In a further embodiment of any of the foregoing embodiments, the spring- loaded pins each include an inner tip that defines the throat, and the inner tip has an aft side that faces toward the nozzle and a forward side that faces away from the nozzle. The forward and aft side meet to form an acute angle.

[0015] A further embodiment of any of the other embodiments includes a support structure in which the spring-loaded pins and the ring are mounted. The support structure has one or more grooves in which at least a portion of the ring and at least portions of the spring- loaded pins are disposed.

[0016] In a further embodiment of any of the foregoing embodiments, the spring- loaded pin includes an outer tip opposite the throat, and the outer tip is interlocked with the passive actuator.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

[0018] Figure 1 illustrates an example solid rocket motor.

[0019] Figure 2 illustrates a sectioned view through a nozzle and throat section of the solid rocket motor. [0020] Figure 3 illustrates another examples in which the passive actuator is a nitinol ring.

[0021] Figure 4 illustrates an isolate view of a nitinol ring.

[0022] Figure 5A illustrates a throat in an open state.

[0023] Figure 5B illustrates a throat in a closed state.

[0024] Figure 6A illustrates a portion of a bimetallic strip element.

[0025] Figure 6B illustrates the bimetallic strip element in a deflected state.

DETAILED DESCRIPTION

[0026] Figure 1 schematically illustrates a cross-section of selected portions of an example solid rocket motor 20. The solid rocket motor 20 generally includes a nozzle 22, a throat section 24 having a throat 24a, and a solid propellant section 26. The solid propellant section 26 includes a forward end 26a and an aft end 26b. The nozzle 22 is attached at the aft end 26b. As will be appreciated, the solid rocket motor 20 may include additional components related to the operation thereof, which are generally known and thus not described herein.

[0027] The solid propellant section 26 includes a solid propellant material 28. In this example, the solid propellant material 28 defines an elongated bore 30. The geometry of the bore 30 may be cylindrical and may have radial fin slots or other features. Alternatively, the solid propellant material 28 may not have a bore. The solid propellant material 28 is generally disposed within a motor case 32 about a central axis A.

[0028] Upon ignition the solid propellant material 26 reacts to produce high temperature and high pressure gas (combustion gas). The combustion gas flows down the bore 30 (if present) and discharges out a flow passage 34 through the throat section 24 and nozzle 22 to produce thrust. Although the throat section 24 is shown schematically, it is to be understood that the throat 24a is where the flow passage 34 converges to a minimum cross- sectional area (in a plane perpendicular to the central axis A).

[0029] Figure 2 illustrates a sectioned view of the throat section 24 and the nozzle 22 of the solid rocket motor 20. As shown, the throat section 24 includes one or more compressively (spring)-loaded pins 36 ("pins"). In this example, each such pin 36 includes a shank 36a that joins an inner tip 36b and an outer tip 36c. The inner tip 36b abuts the throat 24a, and the outer tip 36c is located opposite the throat 24a. The terms "inner" and "outer" refer to the radial orientation relative to the central axis A. [0030] In this example, the inner tip 36b includes an aft side 37a that faces toward the nozzle 22 and a forward side 37b that faces away from the nozzle 22 (i.e., toward the solid propellant section 26. Here, the aft side 37a is substantially radially oriented and the forward side 37b is obliquely sloped relative to the radial direction. The aft side 37a and the forward side 37b meet to form an acute angle, represented at Q. The acute angle serves to provide a sloped surface for impingement and guidance of hot gas flow coming from the solid propellant section 26. The edge or point at which the sides 37a/37b meet defines, in part, the perimeter of a cross-sectional flow area, represented at T, of the throat 24a. The material of construction would be such that the pins would be resistant to erosion from the hot propellant gas.

[0031] As indicated by arrows M, the pins 36 are moveable in the throat section 24. Movement of the pins 36 changes the cross-sectional area (T) of the throat 24a. The nozzle 22 diverges from the throat 24a (or from a location slightly downstream of the throat 24a).

[0032] The solid rocket motor 20 further includes a passive actuator 38 that is attached with the pins 36. For instance, the attachment is a mechanical connection, an integral connection, or other connection that can transmit motion of the passive actuator 38 to the pins 36. The passive actuator 38 is temperature-responsive to the surrounding environmental temperature and thus provides temperature-dependent actuation of the pins 36. For instance, in one example, the passive actuator 38 provides an increment of actuation in relation to a threshold temperature, spontaneously actuating at or near the threshold temperature but not outside the temperature. In another example, the passive actuator 38 provides continuous actuation with increases and decreases of the surrounding environmental temperature.

[0033] The pins 36 and passive actuator 38 are mounted in a support structure 40 that is attached with the nozzle 22. For example, the support structure 40 may be an integral extension of the nozzle 22 or a separate component attached with the nozzle 22. The support structure 40 includes one or more grooves 42 that open radially outwards. In this example, at least portions of the pins 36 and at least a portion of the passive actuator 38 are disposed in the groove 42, which may facilitate stabilization of the pins 36 and passive actuator 38. The pins 36 extend through one or more radial openings 42a in the support structure 40. Each pin 36 includes a spring 36d that is engaged to react off of the support structure 40. That is, the pins 36 are moveable, but the springs 36d bias the pins 36 to a default position, such as a fully open throat position. [0034] The actuation of the passive actuator 38 causes the pins 36 to move (M) against the bias force of the springs 36d and thereby change the flow area (T) of the throat 24a. As used herein, the term "passive actuator" refers to an actuator that can move the pins 36 without drawing power from an external power supply, such as a battery, hydraulic system, or fueldraulic system, and without control signals from a programmable controller.

[0035] As examples, the passive actuator 38 is, or includes, a shape memory element, a bimetallic strip element, or other element that deflects in dependence of the surrounding environment temperature. Such deflection is not to be confused with mere thermal expansion/contraction and any nominal deflection therefrom. Rather, such deflection, although it is thermally induced, is an amplified displacement of the element beyond the volumetric change and nominal displacement that occurs from thermal expansion/contraction.

[0036] As an example, a shape memory element is an element, made of a shape memory material (e.g., a shape memory metal alloy), that when deformed returns to its pre- deformed shape when heated above a transition temperature because of a solid state phase transition (e.g., martensite/austenite transition) at the transition temperature. A shape memory element can thus recover apparent permanent strains when heated above the transition temperature. Shape memory alloys may include copper-aluminum-nickel alloys, nickel- titanium alloys (e.g., nitinol), or zinc-copper-gold-iron alloys. The compositions, processing, and shape "training" of such alloys is known and thus not discussed further herein.

[0037] A bimetallic strip element is an element that has two or more strips of different metals that thermally expand at different rates as they are heated. The strips are interfacially joined. The differing expansions force the element to deflect when heated. Although there is thermal expansion/contraction in the bimetallic strip, the joining of the strips and differential thermal expansion/contraction amplifies deflection. A bimetallic strip may include, but is not limited to, a steel strip joined with a copper strip.

[0038] A solid propellant material burns at higher pressures at high operational temperatures and lower pressures at low operational temperatures. That is, at elevated surrounding environment temperatures, as might be found in a desert region, a solid propellant material produces high pressures at the nozzle exit. In cool surrounding environment temperatures, as might be found in a polar region, a solid propellant material produces lower pressures at the nozzle exit. However, with the variable throat arrangement provided by the passive actuator 38 and pins 36, the flow area (T) of the throat 24a passively adjusts with differences in the surrounding environmental temperature, thereby passively controlling the pressure. This also potentially reduces the maximum pressure, reducing the need thicker structural walls that add mass and thus there can be an improved performance element when using this invention.

[0039] Figure 3 illustrates another example that is similar to Figure 2 but with a passive actuator 138. In this example, the passive actuator 138 is a ring 138a, also shown in an isolated view in Figure 4. In this example, the ring 138a is made of nitinol. Nitinol is a nickel-titanium alloy that has approximately equal atomic percentages of nickel and titanium. Here, the ring 138a has a substantially circular cross-section (in a plane perpendicular to the central axis A), although it is to be appreciated that the cross-section could be elliptical or other geometric shape. The nitinol transition temperature is selected so-as to provide the maximum enhancement for throat area adjustment and effect on motor performance.

[0040] The ring 138a is attached with the pins 36. For example, the outer tips or ends 36c of the pins 36 have a recess R that interlocks with the ring 138a. The geometry of the recess R may correspond to the geometry of the ring 138a. For instance, the recess may have a semi-circular geometry to match the circular geometry of the ring 138a. The interlocking between the ring 138a and the pins 36 facilitates keeping the ring 138a in stable position against vibrations.

[0041] The ring 138a is temperature -responsive to the surrounding environmental temperature and thus provides temperature-dependent actuation of the pins 36. For instance, the ring 138a provides an increment of actuation in relation to a threshold (transition) temperature, spontaneously actuating at or near the threshold temperature but not outside the temperature. For example, the ring 138a is deformed from an initial shape, and once heated above the threshold temperature, returns to the initial shape. This behavior is used to provide the passive actuating motion to move the pins 36. For instance, as shown in Figure 5 A, at a low temperature under the threshold temperature, the nitinol ring 138a has an initial shape. In this initial shape the pins 36 are in a retracted, open position. The cross-sectional area (T) of the throat 24a is thus open. When the surrounding environmental temperature heats the ring 138a above the threshold temperature, the ring 138a deflects, as represented at D (Figure 3). The deflection actuates the pins 36, moving the pins 36 radially inwards, as shown in Figure 5B. The inward movement of the pins 36 reduces the cross-sectional area (T), thus closing the throat 24a. Likewise, when the surrounding environment temperature cools the ring 138a below the threshold temperature, the ring 138a retracts the pins 36, thereby increasing the cross-sectional area (T) and opening the throat 24a.

[0042] Figure 6A illustrates a representative portion of a bimetallic strip element 238b that can be used in the ring 138a rather than nitinol. The bimetallic strip element 238b includes a first strip 239a and a second strip 239b that is interfacially joined with the first strip 239a. The strips 239a/239b are made of different metals that have different coefficients of thermal expansion. Thus, as the surrounding environment temperature increases the temperature of the bimetallic strip element 238b, the differential expansions of the strips 239a/239b cause the element 238b to deflect, as shown in Figure 6B. The deflection moves the pins 36 radially inwards, as in Figure 5B, to reduce the cross-sectional area (T) of the throat 24a. Upon cooling, the element 238b retracts toward its undeflected state, thereby retracting the pins 36 to increase the cross-sectional area (T) of the throat 24a.

[0043] Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments.

[0044] The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.