Patent Description:
Rocket propellant grains rely on a polymer binder for their structural integrity. Changes to structural integrity may be described by a change in mechanical properties that, in part, determines the propellant grain lifespan. While the chemical composition of a polymer type affects the way it ages, the changes in propellant grain mechanical properties due to polymer aging are a factor in determining propellant grain lifespan. One method of assessing the lifespan of a solid rocket motor is by destructively disassembling the solid rocket motor to measure mechanical properties of the propellant grain <CIT> relates to a relaxation modulus sensor.

A method as defined in claim <NUM> is provided.

In various embodiments, the method further comprises determining the lifespan of the solid rocket motor propellant grain based upon the pressure of the gas.

In various embodiments, the method further comprises calculating a value of a mechanical property of the solid rocket motor propellant grain based upon the deformation.

In various embodiments, the mechanical property comprises a bulk relaxation modulus (k) calculated using equation <MAT>, where P is the measured pressure, ΔV is a change in volume of the perforation, and Vinitial is a volume of the perforation before it expands in response to the gas.

In various embodiments, the method further comprises comparing the first value with the second value.

In various embodiments, the method further comprises predicting a future value of the relaxation modulus based on a trend between the first value and the second value.

In various embodiments, the method further comprises determining a remaining lifespan of the solid rocket motor propellant grain based on a comparison between the future value and a predetermined design threshold.

In various embodiments, the first value of the relaxation modulus is measured based upon an initial volume of the perforation and a second volume of the perforation.

In various embodiments, the first value of the relaxation modulus is calculated using an equation P<NUM>Vinitial = nRT, where R is a universal gas constant of the gas, T is a temperature of the gas, n is a number of moles of the gas, Vinitial is an initial volume of the perforation, and P<NUM> is a measured pressure of the gas.

A solid rocket motor propellant grain arrangement as defined in claim <NUM> is provided.

In various embodiments, the solid rocket motor propellant grain arrangement further comprises a conduit coupled to the port.

In various embodiments, the solid rocket motor propellant grain arrangement further comprises a pressure gauge in fluid communication with the perforation, via the port.

A more complete understanding of the present disclosure, however, may best be obtained by referring to the detailed description and claims when considered in connection with the drawing figures.

The detailed description of various embodiments herein makes reference to the accompanying drawings, which show various embodiments by way of illustration. While these various embodiments are described in sufficient detail to enable those skilled in the art to practice the disclosure, it should be understood that other embodiments may be realized and that logical, chemical, and mechanical changes may be made without departing from the scope of the disclosure.

With reference to <FIG>, a solid rocket motor <NUM> is illustrated, in accordance with various embodiments. Solid rocket motor <NUM> may comprise an aft end <NUM> and a forward end <NUM>. Solid rocket motor <NUM> may comprise a casing <NUM> extending between aft end <NUM> and forward end <NUM>. In various embodiments, casing <NUM> may comprise a cylindrical geometry. Solid rocket motor <NUM> may comprise a nozzle <NUM> disposed at aft end <NUM>. Nozzle <NUM> may be coupled to casing <NUM>. Solid rocket motor <NUM> may comprise a solid rocket motor propellant grain (propellant grain) <NUM> disposed within casing <NUM>. In various embodiments, propellant grain <NUM> may be comprised of a solid fuel, such as a pure fuel, inert without an oxidizer. For example, propellant grain <NUM> may comprise a hydroxyl-terminated polybutadiene (HTPB), a polymethyl methacrylate (PMMA), or a polyethylene (PE), among others. In various embodiments, propellant grain <NUM> may be comprised of a composite propellant comprising both a fuel and an oxidizer mixed and immobilized within a cured polymer-based binder. For example, propellant grain <NUM> may comprise an ammonium nitrate-based composite propellant (ANCP) or ammonium perchlorate-based composite propellant (APCP). Propellant grain <NUM> is a solid mass with an exposed inner surface area defining a perforation volume (also referred to herein as a perforation) in the interior of the solid rocket motor. In this regard, propellant grain <NUM> comprises a perforation <NUM>. Perforation <NUM> may be defined by a bore extending axially through propellant grain <NUM>.

A mechanical property envelope describes the minimum and maximum performance values necessary for a propellant grain to function as designed. The calculated mechanical property envelope is typically derived from a series of tests to determine propellant failure limits under various loading conditions. When a propellant sample mechanical property falls outside of the calculated envelope, the propellant grain service life is at an end.

The mechanical properties of the propellant comprising the grain can be measured both immediately after curing and after an accelerated aging period. Typically, the measurements are performed on propellant samples produced simultaneously with the production of propellant grains. Accelerated aging of the propellant samples is usually achieved through exposure to high temperatures for a duration of time designed to mimic the passage of time. The mechanical properties of the propellant grain contained within the rocket motor are typically assumed to be represented by the simultaneously produced propellant samples. The service life of the propellant grain is then assumed to be represented by the performance of the propellant samples subjected to accelerated aging, with a conservative reduction to compensate for potential variation between propellant sample and propellant grain.

To validate the typical assumption that the propellant grain within the rocket motor is accurately represented by the propellant samples, it may be desirable to calculate mechanical properties of a propellant grain to determine the health of the corresponding solid rocket motor. Typically, in order to determine the health of a plurality of solid rocket motors, a sacrificial solid rocket motor is disassembled using destructive means to gain access to the propellant of the sacrificial solid rocket motor in order to take proper measurements. The sacrificial solid rocket motor would typically be similar to the plurality of solid rocket motors (e.g., same type, age, storage conditions, etc.). Stated differently, a solid rocket motor is sacrificed in order to estimate the health of a plurality of similar solid rocket motors.

The present disclosure, as described herein, provides systems and methods for non-destructively surveilling solid rocket motor propellant grains for predicting the lifespan and the remaining lifespan of the solid rocket motor.

With reference to <FIG>, a method <NUM> for non-destructively surveilling a mechanical property of a solid rocket motor propellant grain is illustrated, in accordance with various embodiments. Method <NUM> includes applying a first force to a surface of the propellant grain at a first time, wherein a first deformation is formed on the surface of the propellant grain in response to the first force (step <NUM>). Method <NUM> includes calculating a first value of the mechanical property of the propellant grain, based on the first deformation (step <NUM>). Method <NUM> includes applying a second force to the surface of the propellant grain at a second time, a second deformation formed on the surface of the propellant grain in response to the second force (step <NUM>). Method <NUM> includes calculating a second value of the mechanical property of the propellant grain, based on the second deformation (step <NUM>). Method <NUM> includes determining the remaining lifespan of the propellant grain, based on the first value and the second value (step <NUM>) and through comparison of their values with the modeled performance minima and/or maxima.

With combined reference to <FIG>, step <NUM> and step <NUM> includes applying a force to surface <NUM> of propellant grain <NUM>. The force is applied via a variety of devices and/or methods, as will be described with further detail herein. Surface <NUM> is an inner surface of propellant grain <NUM>. Surface <NUM> may be a radially displayed inner surface of propellant grain <NUM>. Surface <NUM> defines perforation <NUM>. Perforation <NUM> comprises a bore formed through propellant grain <NUM>. A deformation may be formed in propellant grain <NUM> in response to the force. For example, a deformation may be formed in surface <NUM> in response to the force. Step <NUM> and step <NUM> includes calculating a mechanical property of propellant grain <NUM>, based upon the respective deformations. For example, a mechanical property that may be calculated is the bulk relaxation modulus (k) of propellant grain <NUM>. As will be described with further detail herein, the amount of deformation of the propellant grain <NUM> in response to a given force, may indicate the magnitude of the bulk relaxation modulus (k) of propellant grain <NUM>.

In various embodiments, step <NUM> occurs at a first time and step <NUM> occurs at a second time. Similarly, step <NUM> may occur during the first time and step <NUM> may occur during the second time. For example, step <NUM> and step <NUM> may occur a year or more after step <NUM> and step <NUM>. In this regard, the health of solid rocket motor <NUM> may be surveilled over a period of time. With additional reference to <FIG>, a plot <NUM> of various bulk relaxation modulus (k) values calculated over time is illustrated, in accordance with various embodiments. For example, first value <NUM> may be calculated at a first time, second value <NUM> may be calculated at a second time, and third value <NUM> may be calculated at a third time. A trend (also referred to herein as a curve) <NUM> may be determined based on first value <NUM>, second value <NUM>, and third value <NUM>. For example, a curve of best fit (i.e., curve <NUM>) may be determined using any suitable method including, but not limited to, interpolation, polynomial interpolation, smoothing, line fitting, curve fitting, extrapolation, analytic models, etc. Although illustrated as having three separate values, it is contemplated that curve <NUM> may be determined using two or more values. For example, using solely first value <NUM> and second value <NUM>, or using more than three values.

Curve <NUM> may be used to determine a future value <NUM>. For example, curve <NUM> may be compared with a pre-determined threshold value <NUM> of bulk relaxation modulus (k) and a time <NUM> at which curve <NUM> will intersect with pre-determined threshold value <NUM> may be used to define future value <NUM>. In this regard, curve <NUM> may be extrapolated to estimate a time <NUM> at which the mechanical property (e.g., bulk relaxation modulus (k)) will reach the pre-determined threshold value <NUM>. Value <NUM> can be determined by modeling and calculation, through measurement of propellant samples subjected to accelerated aging, or by destructive testing of a sacrificial solid rocket motor. In this regard, it may be determined that solid rocket motor <NUM> has a lifespan of duration <NUM>. Duration <NUM> may be measured in units of time, such as years, months, or days, for example.

With reference to <FIG>, a plot <NUM> of various bulk relaxation modulus (k) values calculated over time is illustrated, in accordance with various embodiments. Plot <NUM> differs from plot <NUM> of <FIG> in that the propellant grain bulk relaxation modulus (k) of plot <NUM> increases over time. Thus, methods described herein may be suitable for propellant grains that have a bulk relaxation modulus (k) that increase or decrease over time. Stated differently, methods described herein may be suitable for propellant grains that soften or harden over time.

Having described a method for non-destructively surveilling a mechanical property of a solid rocket motor propellant grain using two measured values, it is contemplated herein that a method for non-destructively surveilling a mechanical property of a solid rocket motor propellant grain may be performed using only a single measured value. With reference to <FIG> a method <NUM> for non-destructively surveilling a mechanical property of a solid rocket motor propellant grain is illustrated, in accordance with various embodiments. Method <NUM> includes applying a force to a surface of the propellant grain, wherein a deformation is formed on the surface of the propellant grain in response to the force (step <NUM>). Method <NUM> includes calculating a value of a mechanical property of the propellant grain, based on the deformation (step <NUM>). Method <NUM> includes determining the remaining lifespan of the propellant grain, based on the calculated value and a predetermined trend (step <NUM>).

With combined reference to <FIG> and <FIG>, step <NUM> may include applying a force to surface <NUM> of propellant grain <NUM>. Step <NUM> may include calculating a mechanical property of propellant grain <NUM>, based upon the deformation. For example, a mechanical property that may be calculated is the bulk relaxation modulus (k) of propellant grain <NUM>. Step <NUM> may include comparing the calculated bulk relaxation modulus (k) of propellant grain <NUM> with a predetermined trend which represents that of propellant grain <NUM>, for example using a trend representing the performance (i.e., bulk relaxation modulus) of a propellant sample subjected to an accelerated aging process, or a trend calculated using a model produced by the structural analysis of the propellant grain. In various embodiments, the predetermined trend is determined by modeling and calculation, through measurement of propellant samples subjected to accelerated aging, and/or by destructive testing of a sacrificial solid rocket motor.

With additional reference to <FIG>, a plot <NUM> of a calculated bulk relaxation modulus (k) value with respect to a predetermined trend is illustrated, in accordance with various embodiments. For example, value <NUM> may be calculated and compared with a predetermined trend (also referred to herein as a curve) <NUM> representing the change in bulk relaxation modulus of the propellant grain with respect to time. Curve <NUM> may be used to determine a future value <NUM>. Curve <NUM> may be compared with a pre-determined threshold value <NUM> of bulk relaxation modulus (k) and a future time <NUM> at which curve <NUM> will intersect with pre-determined threshold value <NUM> may be used to define future value <NUM>. In this regard, calculated value <NUM> may be superimposed with curve <NUM> to estimate a time <NUM> at which the mechanical property (e.g., bulk relaxation modulus (k)) will reach the pre-determined threshold value <NUM>. Value <NUM> can be determined by modeling and calculation, through measurement of propellant samples subjected to accelerated aging, and/or by destructive testing of a sacrificial solid rocket motor. In this regard, it may be determined that solid rocket motor <NUM> has a remaining lifespan of duration <NUM>. Duration <NUM> may be measured in units of time, such as years, months, or days, for example.

Having described methods for non-destructively surveilling a mechanical property of a solid rocket motor propellant grain for determining a lifespan of a solid rocket motor, <FIG> illustrate various methods for applying a force to the propellant grain, as well as calculating the mechanical property.

With reference to <FIG>, step <NUM> and/or step <NUM> of method <NUM> of <FIG> and/or a step <NUM> of method <NUM> of <FIG> may include moving a gas into a perforation (sub-step <NUM>). Step <NUM> and/or step <NUM> of method <NUM> of <FIG> and/or a step <NUM> of method <NUM> of <FIG> may include measuring a pressure of the gas (sub-step <NUM>). Step <NUM> and/or step <NUM> of method <NUM> of <FIG> and/or a step <NUM> of method <NUM> of <FIG> may include calculating a mechanical property of the propellant grain, based on the pressure (sub-step <NUM>).

With respect to <FIG>, elements with like element numbering, as depicted in <FIG>, are intended to be the same and will not necessarily be repeated for the sake of clarity.

With reference to <FIG>, perforation <NUM> is hermetically sealed at both axial ends thereof. In this regard, a first end <NUM> of perforation <NUM> may be sealed via a seal <NUM>. Seal <NUM> may comprise any suitable hermetic seal, including rubber plugs, or glass-to-metal hermetic seals, among others. Seal <NUM> may be coupled to propellant grain <NUM> such that pressurized gas does not leak from perforation <NUM>. Furthermore, second end <NUM> of perforation <NUM> may be hermetically sealed. The forward end wall <NUM> of casing <NUM> at second end <NUM> may hermetically seal perforation <NUM>. In various embodiments, forward end wall <NUM> includes an ignitor for igniting propellant grain <NUM>.

In various embodiments, solid rocket motor <NUM> includes a port <NUM> in fluid communication with perforation <NUM>. With combined reference to FIG. 6A and <FIG>, sub-step <NUM> may include connecting a conduit <NUM>, such as a hose for example, to port <NUM>. Perforation <NUM> may comprise an initial volume (Vinitial). With combined reference to FIG. 6A and <FIG>, sub-step <NUM> may include moving a gas <NUM> into perforation <NUM>. Gas <NUM> may be any compressible gas including air, nitrogen, etc. A gas supply <NUM> may be connected to conduit <NUM> to supply the gas <NUM> to perforation <NUM>. In various embodiments, gas supply <NUM> may comprise a gas cylinder. In various embodiments, sub-step <NUM> may include moving a pre-determined number of moles of gas <NUM> into perforation <NUM>. Thus, gas <NUM> may be moved into perforation <NUM> in a controlled manner. Perforation <NUM> may expand in response to the gas <NUM> being moved into perforation <NUM>. In this manner, all radial expansion of perforation <NUM> may correspond to deformation of propellant grain <NUM>. For example, gas <NUM> may exert a force, depicted by arrows <NUM>, on surface <NUM> which may cause surface <NUM> to expand. Force <NUM> may be exerted onto surface <NUM> of propellant grain <NUM> in response to gas <NUM> being moved into perforation <NUM>. Thus, perforation <NUM> may comprise a volume (V<NUM>) in response to being filled with gas <NUM>. In this regard, a change in volume of perforation <NUM> may correspond to a volume of the deformation. The change in volume of perforation <NUM> may correspond to a mechanical property of propellant grain <NUM>, such as the bulk relaxation modulus (k) of propellant grain <NUM> for example. Force <NUM> may be a relatively small force, causing a relatively small deformation, such that the deformation does not damage the performance of propellant grain <NUM>.

Sub-step <NUM> may include measuring a pressure (P<NUM>) of gas <NUM>. A pressure gauge <NUM> may be used to measure pressure (P<NUM>). In this regard, pressure gauge <NUM> may be in fluid communication with perforation <NUM>. Pressure (P<NUM>) is the pressure of a pre-determined number of moles of gas <NUM> in perforation <NUM>. The pressure (P<NUM>) may vary in response to the bulk relaxation modulus (k) of propellant grain <NUM>. For example, bulk relaxation modulus (k) is defined per equation <NUM> below: <MAT> where k is the bulk relaxation modulus, P is the pressure applied by gas <NUM> to propellant grain <NUM>, ΔV is the change in volume of perforation <NUM>, and Vinitial is the initial volume of perforation <NUM> before expansion, such as is shown in <FIG> for example. In this regard, force <NUM> may be the product of pressure (P) and the area of the perforation <NUM> in contact with surface <NUM>.

In this regard, ΔV may be defined by equation <NUM> below: <MAT> where V<NUM> is the volume of perforation <NUM> after being filled with the pre-determined number of moles of gas <NUM>.

In various embodiments, the initial volume (Vinitial) is a known value. In various embodiments, the initial volume (Vinitial) is calculated by moving a known number of moles of gas (n), at a known temperature (T) into perforation <NUM> at a low, measured pressure (Pmeas) (i.e., low enough such that the volume of perforation <NUM> does not increase in response to the low pressure), and calculating V using equation <NUM> below.

Using the known initial volume, the expected pressure (Pcalc) of gas <NUM> can be determined using equation <NUM>: <MAT> where R is the universal gas constant of gas <NUM>, T is the temperature of gas <NUM>, n is the number of moles of gas <NUM>, and V is the initial volume (Vinitial) of perforation <NUM>. The expected pressure (Pcalc) is then compared with the measured pressure (P<NUM>) and the difference in pressures (i.e., Pcalc - P<NUM>) is noted. The difference from ideality is due to the compression of the propellant grain <NUM> under pressure of gas <NUM>. Stated differently, the calculated pressure (Pcalc) and the measured pressure (P<NUM>) may be slightly different for controlled additions of gas (varying n) in response to the propellant grain being compressed which in turn will create a slightly larger volume (and reduced pressure) for the gas <NUM> to occupy. The difference between calculated pressure (Pcalc) and the measured pressure (P<NUM>) may be used to determine the remaining lifespan of propellant grain <NUM>. In this regard, the Y-axis of <FIG>, and <FIG> may be "pressure (P<NUM>)" instead of "modulus" or pressure difference (Pcalc - P<NUM>) instead of "modulus. " However, it is understood herein that, in various embodiments, the pressure (P<NUM>) corresponds to the modulus of propellant grain <NUM>. In this regard, the modulus of propellant grain <NUM> may be calculated based upon the measured pressure (P<NUM>).

Claim 1:
A method (<NUM>) for non-destructively determining a lifespan of a solid rocket motor propellant grain (<NUM>), wherein the solid rocket motor propellant grain is a solid mass with an exposed inner surface area defining a perforation in the interior of the solid rocket motor propellant grain, comprising:
moving a gas (<NUM>) into the perforation, thereby applying a force (<NUM>) to a surface of the solid rocket motor propellant grain with a pressure of the gas, wherein a deformation is formed on the surface of the solid rocket motor propellant grain in response to the application of the force;
wherein the gas is pressurized in response to moving a pre-determined number of moles of gas into the perforation, wherein the deformation is formed in response to the gas being pressurized; and
measuring (<NUM>) a pressure of the gas.