Experimental study of Stagnation Pressure Reaction Control for mid-calibre non-spinning projectiles
Introduction
Current gun systems can select optimal gun parameters for minimizing target miss distances, based on modelling, simulation and experimental data [1], [2], [3]. However, perturbations exist that cannot be corrected for, such as balloting of the projectile during gun in-bore travel, limited repeatability in projectile geometrical and internal characteristics, aiming errors, influence of wind, and target manoeuvring, requiring an increased level of projectile delivery precision. The increasing need for affordable projectile precision, together with the emergence of new and miniaturized technology, positioning means, and (additive) manufacturing techniques, has enabled many new avenues of technology development and concept studies for the application of controllable gun-launched ammunition [4], [5]. Notable examples of such ammunitions of significant TRL are DARPA's EXACTO projectile [6] and the self-guided 0.50 calibre Sandia Labs projectile [7].
A projectile can be controlled by traditional aerodynamic surfaces, examples of which are seen in the Excalibur projectile [8] and the Precision Guidance Kit [9], [10]. Alternatively, the projectile boundary layer may be actively affected, creating a differential drag or differential base flow [11], [12], [13] affecting the projectile's flight path. A boundary layer can be affected for instance by surface spoilers [11], [12], [13], [22], [23], gas injection over a coanda surface at the back [14], [15], creating a plasma in the boundary layer by discharging surface electrodes [24], [25], or using ram-air to asymmetrically modify the flow around a projectile and generate pressure differences for control [33]. Three other means to control a projectile are: i) Creating thrust in a direction perpendicular to the direction of flight [14], [15], [26], ii) Morphing the projectile's surface to alter its surface pressure distribution, e.g. by body articulation [16], [17], [18] and iii) Shifting internal mass [19], [20], [21].
The control means discussed in this article is generation of lateral thrust. This is a very effective means to generate a control moment around the projectile's centre of mass without relying on aerodynamic surfaces and has been well researched in terms of technology implementation [27] and operating and guidance principles [12], [28], [29], [30]. If placed sufficiently far away from the centre of mass, a gas jet may put a projectile under a significant angle of incidence, providing substantial control authority. Such gas jets may consist of small, single-use energetic charges [31]. Alternatively, a high-pressure chamber, fed by a high pressure gas source, may feed several controlled exit nozzles.
Similar to [26], the technology discussed in this paper uses ram air as a propellant, and expels it radially through ejection nozzles located downstream of the stagnation point of an aerodynamically unstable projectile. The aim of this paper is to clarify the technical challenge for this use-case and to present the practical implementation and demonstration of this innovative technology, which may provide a control capability to mid-calibre projectiles, without the technical complexity of aerodynamic stability- or control fins.
This paper describes the Stagnation Pressure Reaction Control (SPRC) technology, it demonstrates, by simulation, the most suitable control algorithm to use with this technology in the presented use-case. It furthermore shows an experimental setup and the performance of this setup in a supersonic wind tunnel experiment, thereby giving a proof-of-concept of SPRC. It furthermore puts the obtained results into the context of practical use.
To the best of our knowledge, SPRC technology in the described current use-case and the successful demonstration of this technology and its controlling algorithm have not been reported elsewhere.
This article is built up as follows. Section 2 describes the SPRC control technology that was developed by TNO. Section 3 presents the experimental setup that was developed to demonstrate the practical feasibility of this technology. Section 4 provides theoretical background and describes the expected behaviour of different control algorithms in combination with the developed hardware. Section 5 presents the results of the wind tunnel experiments and illustrates the feasibility of the concept and the performance of the control hardware and selected control algorithm for the selected use-case. Section 7 illustrates how SPRC may be employed in practice, as an innovative control technology for smart, course-correcting munitions. Section 7 presents the conclusions that can be drawn based on the wind tunnel experiments.
Section snippets
Principle
The control method discussed in this work is the use of stagnation point, or ‘ram’ air as a source of high pressure gas to feed radial nozzles, generating side thrust and, therewith, a control moment around the centre of mass of a non-spinning, 30 mm calibre projectile. Using ram air to generate side thrust greatly increases the control authority of the projectile over existing ram-air solutions [26]. Earlier research into the use of stagnation point air for control [32] aimed at controlling a
Description of the control technology
Fig. 2 displays a drawing of the nose of the projectile with the air intake in the nose and four, 3.5 mm diameter nozzles in the conical part, located 10 mm away from the stagnation air intake, measured along the axis. To facilitate vertical plane wind tunnel demonstration four nozzles were chosen, rather than the minimum number of three that would allow horizontal and vertical control.
Four separate Valve Actuator Assemblies (VAA) were developed which are capable of up to 400 Hz opening and
Algorithm design
The SPRC nozzles are operated in a discrete manner: they are either open or closed, with a known delay time and transfer function between the opened and closed position, giving a non-linear response. In such a situation, proportional control is difficult to implement and was therefore not considered. For SPRC, Bang-Bang control [34] was selected and modified to suit this application as will be explained hereafter. This section discusses the different control options considered and the option
Experiment objectives
The objectives of the wind tunnel experiments were to: i) Keep an aerodynamically representative and highly unstable, projectile-like test object stable around zero angle of incidence using SPRC technology; ii) Determining the maximum stable angle of incidence of the test object. For a similar projectile case, reported in [35], angles of incidence of tenths of a degree are useful for providing a dispersion-reduction capability. For the tests described in this paper, a pass-fail benchmark of 1
Evaluation of the results
As mentioned in the objectives, based on angles of incidence found in [35], the benchmark of the controllable angle of incidence was set at 1 degree. The stable angle of incidence obtained from the experiment shows that this benchmark is well surpassed, underlining the potential of the demonstrated control technology in reducing dispersion of direct-fire munitions. However, under realistic free-flight conditions, a projectile can experience disturbances, such as wind gusts, that can increase
Conclusions
Stagnation Pressure Reaction Control technology was developed and integrated into a 30 mm calibre projectile-like non-spinning, fin-less, aerodynamically unstable test object. Experiments performed with this test article at Mach 2 conditions demonstrated the ability of the technology to stabilize this aerodynamically unstable object around desired angles of incidence up to 1.5 degrees. Being significantly higher than the set pass-fail benchmark of 1 degree for providing effective dispersion
Acknowledgements
The authors would like to express their thanks to Mr. Henry Tol of TNO, for his upbuilding criticism in writing this article. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Operational feasibility study of stagnation pressure reaction control for a mid-caliber non-spinning projectile
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Systems Engineer and Innovator at TNO, Dpt. of Weapon Systems.