Patent Application
20160216075
1. Field of the Invention
This invention relates to ballistic munitions, and more particularly to gun-launched ballistically-stable spinning laser-guided munitions.
2. Description of the Related Art
Ballistic projectiles such as gun-launched munitions or tube-launched rockets are projectiles that are briefly powered at launch (e.g. rocket motor burn or blasting powder) and whose trajectory is subsequently governed by the laws of classical mechanics. These projectiles may have an additional rocket motor that is fired mid-course to maintain the ballistic trajectory. A remote control system computes a firing solution for a ballistic trajectory to intercept a target based on the information available at the time. By comparison, non-ballistic projectiles such as cruise missiles are aerodynamically guided in powered flight. If these projectiles lose power to control the forces of drag and lift, then they would follow classical aerodynamic laws and result in a ballistic path, most likely immediately falling to the ground under the influence of gravity.
Gun-launched munitions achieve ballistic stability by spinning at a high rate. The rifling of the gun barrel imparts a spin to the munition when fired. The spin rate must exceed a threshold that is determined by the mass and velocity of the munition. The shorter the length or lower the velocity of the munition the lower the threshold spin rate to achieve stability. These munitions have an angle of attack (AOA) between the munition's axis and the oncoming airflow of nominally zero degrees. The Phalanx close-in weapon system for defense against anti-ship missiles includes a radar-guided 20 mm Gatling gun mounted on a swiveling base. A land based variant, know as C-RAM, has been deployed in a short range missile defense role to counter incoming rockets and artillery fire.
Tube-launched rockets are either spin-stabilized (no spin), slowly spinning (rate of less than a few tens (10s) of Hertz) or de-rolled (control actuation system (CAS) is mounted on a bearing so that although the rocket may spin, the CAS is spin-stabilized). These types of rockets achieve ballistic stability via aerodynamic control surfaces (such as fins, canards or wings). These rockets typically have a non-zero AOA.
These weapon systems are “fire and forget”. The system computes a firing solution based on a ballistic trajectory to intercept the target. The firing solution is based on the best information available about the target (e.g. range, speed, direction), the environment (e.g. temperature, wind conditions etc.) and the projectile itself. The accuracy of such systems is limited by this knowledge and environmental stability.
In certain tube-launched rocket systems, post-launch guidance may be built into the rocket to adjust the trajectory and guide the rocket to the target. Some rockets use GPS and inertial guidance while other use semi-active laser (SAL) guidance to produce a guidance signal. In a SAL system, a laser designator illuminates the target with pulsed laser energy. An optical system and quad-cell detector on the projectile detects the laser spot and produces a guidance signal to remove the error. The CAS uses the guidance signal to control the aerodynamic surfaces to adjust the rocket's AOA and maneuver the rocket to the target. This approach is affective to increase the accuracy of spin-stabilized and slowly spinning rockets that are only marginally ballistically stable. The control surfaces can change the rocket's AOA without destabilizing the rocket.
This approach is not effective for gun-launched munitions. These munitions have a high spin rate, and thus are highly ballistically stable. It is very difficult to change the AOA of such a highly stable munition. Furthermore, if sufficient control force is applied to adjust the AOA it is more likely that the munition will nutate, destabilize, tumble and fall out of its ballistic trajectory. At these high spin rates, the control system simply does not have adequate bandwidth to control the AOA.
The following is a summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description and the defining claims that are presented later.
The present invention provides a gun-launched ballistically-stable spinning laser-guided munition comprising a bullet and a firing cartridge. The bullet is backward compatible with unguided bullets of the same caliber in that the guided bullet may be fired from existing guns with rifled barrels as-deployed, and will follow the same ballistic trajectory. This allows the guided bullets to be used with the existing base and logistics of weapons systems.
The bullet comprises a plurality of explosive divert elements (e.g., squibs, poppers) arranged around the bullet. Each divert element is configured to produce a force vector (a 1-shot fixed magnitude force vector applied over a brief rotational window) through the center of mass (Cm) of the bullet. The divert elements may be arranged perpendicular to the long axis of the bullet or at an angle thereto. The divert elements may be configured to all produce the same or different force magnitudes.
The bullet includes a semi-active laser (SAL) guidance system that includes an optical system configured to collect and focus reflected pulsed laser energy off of a target and a non-imaging detector (e.g., a quad-cell) configured to detect the focused laser energy and measure a sequence of roll and nod angle pairs (or their equivalent) to the target.
A processor is configured to derive roll and nod angles from one or more pulse measurements. The roll angle provides the direction and the nod angle the magnitude to compute a force vector to laterally displace the bullet to drive the nod angle to a prescribed value. This value may represent a non-zero lead angle initially and then change to a zero angle, or may be held at the calibrated zero angle throughout. The processor is configured to compute based on one or more roll angle measurements and an estimate of the spin rate a firing solution that specifies one or more of the divert elements and firing times to approximate the required force vector to minimize the lateral flight error. The estimate of the spin rate may be derived from a lookup table of spin rate vs. flight time or may be calculated from multiple roll angle measurements which would allow the spin rate and spin rate of change to be determined in-flight. A fire controller is configured, once its operational mode is initiated, to fire the one or more explosive divert elements according to the firing solution to laterally displace the bullet without affecting the bullet's angle of attack and destabilizing the bullet. The processor and fire controller may be configured to implement either a single or iterative firing solutions.
In an embodiment, the bullet has a caliber between 10 mm and 50 mm. Bullets of this caliber are spun up to at least 500 Hz spin rate by the rifling of the gun (and traditionally even above 2,000 Hz upon leaving the barrel).
In different embodiments, the bullet may initiate its operational mode under different criteria. In general, once fired the bullet starts to collect energy and look for a laser spot indicative of a target. The bullet may traverse a substantial portion of its ballistic trajectory based on the initial firing solution of the gun before detecting the target. At this point, the bullet may initiate operational mode immediately or may wait some number of received pulses to gather a measurement history to aid in computation of the firing solution. The bullet may initiate its operational mode based on an estimated time-to-target. A coarse estimate can be made based on the rate of change of the nod angle. A high rate of change, one exceeding a threshold, is indicative of a short time-to-target and the need to initiate the operational mode to divert the bullet. A finer estimate can be based on target data (range, speed, direction) from an external source that is downloaded to the bullet at firing (e.g. an inductive coupling inside the gun platform). The two sources may be fused to further refine the estimate of time-to-target.
In different embodiments, the computed force vector and the firing solution to approximate that force vector may be affected by the specific configuration of the bullet (configuration of explosive divert elements, mode of control, single or multi-pulse, predictive) and estimated time-to-target if available. A so-called “bang/bang” approach determines the direction of the force required to reduce the nod angle and computes a firing solution to approximate that force without concern for the magnitude of the lateral displacement and its cross-range distance at intercept. This approach relies on a high bandwidth and sufficient supply of divert elements to iteratively reduce the nod angle. Another approach uses the estimated time-to-target to configure the firing solution to control the lateral displacement to remove an approximate cross-range distance at intercept. Another approach uses the rate of change of the nod angle in a predictive control loop.
In an embodiment, the guided munition is fired from a weapons system comprising a gun, a laser designator to illuminate the target and a supply of guided munitions. The gun is configured to fire unguided bullets of the same caliber along a ballistic trajectory to intercept a target. The gun may be manually (e.g., a pilot) or automatically (e.g., Phalanx radar) controlled, for example. The laser designator may be slaved to the motion of the gun. In other embodiments, the laser designator may operate independently of the gun. Should the bullet's guidance system be unable to acquire the target, the bullet will follow the initial ballistic trajectory as if it were a normal unguided bullet. In addition, guided munitions can be interspersed with unguided munitions as required by the operation, facilitated by the backwards compatibility and common logistics for operation.
These and other features and advantages of the invention will be apparent to those skilled in the art from the following detailed description of preferred embodiments, taken together with the accompanying drawings, in which:
The present invention provides a gun-launched ballistically-stable spinning guided munition comprising a bullet and a firing cartridge. The bullet is backward compatible with unguided bullets of the same caliber in that the guided bullet may be fired from existing guns with rifled barrels as-deployed, and will follow the same ballistic trajectory. This allows the guided bullets to be used with the existing base and logistics of weapons systems such as the Phalanx system or a laser-guided weapons pod for aircraft.
Referring now to
To enable laser guidance, the Gatling gun is fitted with a SAL designator 22 that may be co-boresighted with the Gatling gun 14 or offset at a lag angle. As the Gatling gun slews on the turret to track the inbound target, typically at a non-zero lead angle, the SAL designator 22 transmits pulsed laser energy 24 to illuminate target 20.
Each laser-guided bullet 12 comprises a plurality of explosive divert elements (e.g., squibs, poppers) arranged around the bullet to produce a force vector through the center of mass (Cm) of the bullet, a SAL guidance system configured to received reflected laser pulse energy 26 scattered off of target 20 to measure a sequence of roll and nod angle pairs (or their equivalent) to the target, a processor configured to process the roll and nod angles to compute a firing solution for one or more of the divert elements to produce a force vector to laterally displace the bullet to drive the nod angle to a prescribed value and a fire controller configured, once its operational mode is initiated, to fire the one or more explosive divert elements according to the firing solution to produce a force vector 28 that approximates the desired force vector to laterally displace the bullet to a corrected aim-point 30 without affecting the bullet's angle of attack and destabilizing the bullet. The processor and fire controller may be configured to implement either a single or iterative firing solutions.
Referring now to
Referring now to
The bullet is provided with a SAL guidance system that includes an optical 62 system configured to collect and focus reflected pulsed laser energy off of a target and a non-imaging detector 64 (e.g., a silicon or InGaAs quad-cell) configured to detect the focused laser energy and measure a sequence of roll and nod angle (or their equivalent) pairs to the target. Optical system 62 includes a conformal window 65 that is substantially transparent to the spectral band of the laser (approximately 1.05 to 1.6 microns) and a lens(es) 66 that collect and focus the laser energy. The lens(es) 66 may comprise standard geometric lenses or a lenslet array.
Electronics 70 include a processor(s) configured to compute based on the nod angle from one or more angle pairs a force vector to drive the nod angle to a prescribed value. This value may represent a non-zero lead angle initially and then change to a zero angle, or may be held at a calibrated zero angle throughout. The processor is configured to compute based on one or more roll angle measurements and an estimate of the spin rate a firing solution that specifies one or more of the divert elements and firing times to approximate the required force vector to minimize the lateral flight error. The estimate of the spin rate may be derived from a lookup table of spin rate vs. flight time or may be calculated from multiple roll angle measurements that allows an estimate of the spin rate of change to be determined. Electronics 70 include a fire controller configured to produce the ignition pulses to fire the one or more of explosive divert elements according to the firing solution to laterally displace the bullet without affecting the bullet's angle of attack and destabilizing the bullet. A power source 72 such as a long-life (chemical) battery or a (super) capacitor charged via inductive coupling at firing is used to power the detector and electronics.
In different embodiments, the bullet may initiate its operational mode under different criteria. In general, once fired the bullet starts to collect energy and look for a laser spot indicative of a target. The bullet may traverse a substantial portion of its ballistic trajectory based on the initial firing solution of the gun before detecting the target. At this point, the bullet may initiate the operational mode immediately or may wait some number of received pulses to gather a measurement history to aid in computation of the firing solution. The bullet may initiate its operational mode based on an estimated time-to-target. A coarse estimate can be made based on the rate of change of the nod angle. A high rate of change, one exceeding a threshold, is indicative of a short time-to-target and the need to initiate the operational mode to divert the bullet. A finer estimate can be based on target data (range, speed, direction) from an external source that is downloaded to the bullet at firing (e.g. an inductive coupling inside the gun barrel or platform). The two sources may be fused to further refine the estimate of time-to-target.
To facilitate inductive coupling for power and data transfer, an inductive coil 74 is formed inside the bullet casing and coupled to the electronics and power source. The induction coil is configured to produce a current in response to passing through an external magnetic field in or near the gun platform. The current comprises a DC term to charge a power supply to power the bullet and an AC term that includes target information being passed to the bullet to compute the firing solution.
In different embodiments, the computed force vector and the firing solution to approximate that force vector may be affected by the specific configuration of the bullet (configuration of explosive divert elements, mode of control, single or multi-pulse, predictive) and estimated time-to-target if available. A so-called “bang/bang” approach determines the direction of the force required to reduce the nod angle and computes a firing solution to approximate that force without concern for the magnitude of the lateral displacement and its cross-range distance at intercept. This approach relies on a high bandwidth and sufficient supply of divert elements to iteratively reduce the nod angle. Another approach uses the estimated time-to-target to configure the firing solution to control the lateral displacement to remove an approximate cross-range distance at intercept. Another approach uses the rate of change of the nod angle in a predictive control loop.
In this embodiment, the bullet also includes a counter-weight/impact mass 76. The impact mass serves to improve the lethality of the bullet upon impact, by allowing the impact tip to be harder or stronger metal, or include features to increase the resulting impact pressure to improve penetration. In addition, this mass feature and location allows a deterministic approach to adjust the center of mass (Cm) location in any given bullet, by removing/adding small metal quantity on any individual bullet. This aids the bullet behavior by ensuring the Cm location is within a prescribed tolerance zone at the divert/squib axis location(s).
Referring now to
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For a single divert maneuver to completely eliminate the cross-range error, all of the computed and produced force vector and the timing of the divert maneuver must be perfect, which is extremely unlikely. However, perfection is not the goal of the laser-guidance of the bullet. The goal is to reduce the error and to increase the probability that the bullet will intercept the target along the ballistic trajectory with minor course modifications. Based on time-to-target once the target is acquired and the divert capability of the explosive divert elements, the cross-range displacement is typically less than 100 meters, which is sufficient for the missions performed by these types of guns.
The accuracy of the desired force vector can be improved in a number of ways. First, the electronics can process multiple pulse measurements to reduce noise. Second, the electronics can compute the rate of change of the nod angle measurement to predict nod angle measurements (assuming no further divert maneuvers) and use that prediction to compute the desired force vector. For example, the current nod angle estimate may be 1 degree. Absent the rate of change information a divert maneuver to remove the 1 degree nod may be computed and executed. However, if the rate of change information indicated that the nod angle was trending towards zero, a smaller divert maneuver or no divert maneuver may be executed.
The firing solution to approximate the desired force vector similarly has a number of variables. The system must produce both the roll angle (timing of ignition of the divert elements) and the magnitude (force produced by the ignited elements). The accuracy of the ignition timing depends on both the accuracy of the roll angle measurement and the accuracy of the current spin rate of the bullet. Processing of multiple pulse measurements will reduce the noise of the roll angle. The current spin-rate can be estimated in different ways. For a given bullet and gun, the spin-rate of the bullet as it exits the gun barrel can be estimated and stored in the bullet electronics fairly accurately. As shown in
The magnitude of the force vector produced by the ignition of the explosive divert elements can be controlled in a number of ways. First, the bullet may be provided with explosive divert elements with different explosive charges to produce force vectors with different magnitudes. Second, the firing solution may be computed to ignite multiple divert elements to increase or decrease the effective magnitude of the force vector.
The timing of the divert maneuver greatly affects both the lateral displacement produced by a given force vector and the resultant cross-range distance at target intercept. The lateral displacement produced by a given force vector depends on the velocity of the bullet. The higher the velocity the more ballistically stable the bullet, hence the harder it is to displace the bullet. The same force applied to a bullet later in flight (at a slower velocity) will laterally displace the bullet further than the force applied early in flight at a higher velocity. However, the same lateral displacement produced earlier in flight will have a greater effect on cross-range distance at target intercept.
The issue is that based solely on the roll/nod angle measurements, the bullet does not know the range or time-to-target intercept. This is one reason that the goal of the laser-guidance system is to reduce, not eliminate all error, and that the preferred implantation is iterative such that the bullet can continually gather guidance information and perform divert maneuvers to reduce the lateral error until intercept.
Acknowledging these limitations, there are different approaches to initiate the operational mode to start performing the divert maneuvers. A first approach is to simply wait a fixed period after the bullet starts to detect target pulses. This can be based on apriori knowledge of the gun and bullets and the expected threat. A second approach is monitor the rate of change of the nod angle. If the rate of change is low, indicating a stable nod angle, the time-to-target is relatively long. As the bullet gets close to the target the lead angle of the bullet should rapidly change from its non-zero value towards zero. Once the rate of change exceeds a threshold the bullet can initiate its operational mode. Another approach is to use inductive coupling between the gun and the bullet to download the most up to date target information, from the radar tracking system for example, into the bullet as it is fired. The current range, speed and direction of the target plus environmental factors such as wind speed and direction can be downloaded to the bullet. This information, perhaps augmented by the measured rate of change of the nod angle, can be used to initiate the operational mode such that the lateral displacement of the bullet can more accurately remove the cross-range error at target intercept.
Referring now to
This can be achieved though either an angular rate sensor (most likely a high-rate MEMS device) that measures the spin-rate, or after correlating the spin-down rate through the dual-pulse approach (where the time constant between the two ˜20 nsec pulses are known, and can be approximately 100 microseconds).
Referring now to
Referring now to
Upon firing, the power source is activated and the electronics/detector are powered on (within first fraction of second) and local count time is initiated (step 204), (with any programmed offset to predict total flight time). The detector signal is analyzed (step 206) and if a signal of interest (e.g. a laser spot) is detected it is compared to a threshold (step 208). This repeats until a target signal is detected.
The roll/nod angles are captured from the signal and time stamped (step 210). The detector is reset for the next pulse (step 212). If a pulse history is available (step 214), the history is used to improve the roll/nod angle estimates, predict the nod angle and estimate the spin-rate and rate of change of the spin-rate (step 216). If not, control passes directly to step 218 to determine whether knowledge of the time-to-target exists, either from a coarse estimate of the rate of change of the nod angle or from external information downloaded to the bullet. If such knowledge exists, the system determines whether immediate action to correct the flight path is required given the remaining time-to-target (step 220). If such knowledge does not exists but exists and does not require immediate action, the system determines whether immediate action to correct the flight path is required to avoid loss of signal (e.g., is the laser spot moving outside the linear region of the quad-cell detector) (step 222).
If immediate action is not required, the system stands by for a next pulse (step 224) and returns to step 206 to analyze the detector signal. If immediate action is required, the system calculates the force vector and firing solution (step 226) and sets the timing trigger for the selected explosive divert elements (step 228). Equivalently the system could calculate the force vector and firing solution in step 226 for all detected pulses and only set the timing triggers once immediate action was required. The system triggers the divert elements (step 230) to produce a force vector through the Cm of the bullet to laterally displace the bullet and reduce lateral flight error.
Control returns to step 206 to analyze the detector signal and repeat the process to iteratively reduce the nod error until the bullet intercepts the target.
While several illustrative embodiments of the invention have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the invention as defined in the appended claims.
