Experimental study of Stagnation Pressure Reaction Control for mid-calibre non-spinning projectiles

Abstract

Increased precision of gun-launched ammunition can reduce collateral damage and increase stand-off distance. It can maximize the tactical advantage of each shot and reduce the number of rounds needed to achieve mission success. Worldwide, technology for controlled smart munitions is being researched and demonstrated in various implementations. Although the use of aerodynamic control fins is an effective control means, such fins are complex to integrate with the projectile, they increase radar reflectivity (with associated risk of counter battery fire) and will reduce payload. The Netherlands Organisation for Applied Scientific Research has been developing a Stagnation Pressure Reaction Control technology without aerodynamic fins, for aerodynamically unstable projectiles. The on-off character of this control technology requires a tuned control algorithm in order to achieve stable and controllable projectile behaviour. This article discusses simulated projectile behaviour under different control algorithms and presents achieved test object performance in a supersonic wind tunnel experiment performed in 2018. The experiment demonstrated feasibility of the combination of the Stagnation Pressure Reaction Control Technology and the tuned control algorithm for a non-spinning, aerodynamically unstable test object, hinged through its centre of mass. In a Mach 2 wind tunnel flow, this test object was stabilized and put under a controlled angle of incidence up to 1.5 degrees with angular error bandwidths up to 0.3 degrees, requiring and realising actuator response times on the order of 2 ms. For higher stable angles of incidence, required for correcting disturbances such as wind gusts, the SPRC technology could be integrated into projectiles with a reduced margin of instability. The short response time would provide such a projectile with a high level of (end game) manoeuvrability, for instance against moving targets.

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.