Design Considerations for an Electromagnetic

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Oct 13, 2014 - target is that of a typical anti-ship missile, its velocity ranges from subsonic ... rounds per second or 75 Hz. This high rate is achieved thanks ... detection of the launch of a projectile and the automatic launch of ... At 0 m, the gun's position, the SSHP is 100 %. .... Reinforced Plastic (GRP) bars, is 3 meters long.
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Design Considerations for an Electromagnetic Railgun Firing Intelligent Bursts to be Used Against Anti-Ship Missiles Johan Gallant, Tom Vancaeyzeele, Ben Lauwens, Barbara Wild, Farid Alouahabi, and Markus Schneider © 2015 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.

Abstract— Railguns can reach higher muzzle velocities and fire rates than conventional guns. Muzzle velocities up to 2400 m/s and fires rates of more than 50 Hz have already been demonstrated with projectiles having a mass of 140 g and a square calibre of 25 mm. We investigated if a Close In Weapon System (CIWS) based on a railgun performs better against incoming anti-ship missiles than a conventional CIWS such as the Goalkeeper and propose solutions to optimize the performance of such a railgun. CIWS are operational systems that defend a ship against incoming subsonic anti-ship missiles. However, future anti-ship missiles will be supersonic and more difficult to defeat with conventional gun systems. Railguns are expected to perform better against these future threats thanks to their higher muzzle velocity and fire rate. Furthermore, the muzzle velocity within a single burst can be varied easily from shot to shot, generating a so-called intelligent burst. It allows varying the velocity of each projectile such that all projectiles arrive on the target at the same time. The number of projectiles and thus the electrical energy required to achieve a target kill with an intelligent burst is expected to be lower than for railguns firing at constant muzzle velocity. In the first part, the performance of an electromagnetic CIWS is discussed using simulation models calculating the hit probability of a burst of projectiles fired with muzzle velocities ranging from 1200 m/s to 2400 m/s and fire rates ranging from 75 rounds per second to 300 rounds per second. The geometry of the target is that of a typical anti-ship missile, its velocity ranges from subsonic (300 m/s) to supersonic (600 m/s). The influence of the projectile mass on the performance of the system and the required electric energy was also investigated. We confirmed that the concept of intelligent burst reduces the required electric energy, especially against supersonic targets. The second part deals with some technical aspects of high fire rate railguns. We have shown experimentally that an automatic loading system

Published in IEEE Transactions on Plasma Science, vol. 43(5), pp. 1179 – 1184. DOI: 10.1109/TPS.2015.2416774. Manuscript submitted October 13, 2014. This work was supported by the European Pulsed Power Laboratories. J. Gallant is with the Royal Military Academy, Brussels 1000, Belgium, and also with European Pulsed Power Laboratories, Saint-Louis 68301, France (e- mail: [email protected]). T. Vancaeyzeele was with the Royal Military Academy, Brussels 1000, Belgium (e-mail: [email protected]). B. Wild, F. Alouahabi, and M. Schneider are with the French-German Research Institute Saint-Louis, Saint-Louis 70034, France, and also with European Pulsed Power Laboratories, Saint-Louis 68301, France (e-mail: [email protected]; [email protected]; [email protected])

allows increasing the fire rate of a medium calibre railgun from 50 Hz to 75 Hz. Index Terms— Railgun, parallel augmented railgun, system study, pulsed power, hypervelocity, simulation.

I. INTRODUCTION

C

LOSE In Weapons Systems (CIWS) are a ship’s last defense line against incoming anti-ship missiles. They are effective against subsonic missiles (maximum velocity: 300 m/s) and have a range of 2000 m. A typical CIWS is the Goalkeeper, in use on Belgian Navy ships. It will be used in this paper as a reference. The Goalkeeper fires armor piercing projectiles with a mass of 234 g and a muzzle velocity of 1200 m/s, typically in bursts of 300 projectiles with a fire rate of 75 rounds per second or 75 Hz. This high rate is achieved thanks to its seven-barrel Gatling gun. We have developed a simulation model in order to evaluate the performance of the Goalkeeper against anti-ship missiles. Simulations confirmed that it is effective against subsonic missiles (v = 300 m/s). However, we have shown that a Goalkeeper is not able to defeat supersonic missiles (v = 600 m/s). A parametric analysis indicated that higher fire rates (300 Hz) are required to intercept these missiles [1]. Railguns have the potential to realize such fire rates. However, the energy required to fire effective bursts against supersonic targets is too high (a kinetic energy of at least 120 MJ at the muzzle). Therefore, we studied a new fire strategy for the electromagnetic CIWS, the intelligent burst. In such a burst, the projectiles are fired with a varying muzzle velocity, so that they all impact on the target at the same moment and at the shortest possible distance. Each projectile then has the maximal efficiency and thus, less projectiles and energy are required to achieve a kill probability of 95 %. We evaluate in the first part of the paper the efficiency of the intelligent burst against subsonic and supersonic targets. In the second part of the paper, we present a method that allows increasing the fire rate of a railgun. It is based on the detection of the launch of a projectile and the automatic launch of the following projectile [2], thus reducing the idle time between two shots and increasing the fire rate.

2 Experiments with the rapid-fire railgun (RAFIRA) at the French-German Research Institute Saint-Louis (ISL) [3], [4] are presented. An increase of the fire rate from 50 Hz to 75 Hz has been demonstrated. II. OPERATIONAL ANALYSIS OF A CIWS An incoming anti-ship missile must be destroyed at a certain range before the ship, indicated by Rkill (kill range) on Fig. 1. This distance is 428 m for subsonic missiles, and 856 m for supersonic missiles and guarantees that the ship will not be hit by the debris of the exploding missiles on the assumption that the debris follows a parabolic trajectory. Since a high number of projectiles (up to 300) are required to achieve a kill probability of 95 %, the gun starts firing at a distance Ropen fire (open fire range) before the ship. However, the hit probability of a projectile (SSHP, single shot hit probability) decreases rapidly with the range, as shown in Fig. 2. This curve has been calculated with the model described in detail in [1] for a projectile with a muzzle velocity of 1200 m/s and a mass of 234 g. At 0 m, the gun’s position, the SSHP is 100 %. However, the SSHP drops to 10 % at 285 m, and to only 1 % at 885 m. It is therefore important that the projectiles strike the target as close as possible to the kill range, which can be obtained by increasing the fire rate or by varying the muzzle velocity.

Fig. 1. Open fire range and kill range

Fig. 2. Single shot hit probability (SSHP) as a function of the range for a projectile with a muzzle velocity of 1200 m/s and a mass of 234 g.

Fig. 3. Comparison of bursts of ten projectiles: conventional burst with

fire rate 75 Hz (A), conventional burst with fire rate 300 Hz (B), and intelligent burst (C).

Three different bursts of ten projectiles are shown on Fig. 3. Burst A is a conventional burst: all projectiles are fired with the same muzzle velocity. The first projectile of the burst will hit the target far before the ship; the last projectile will impact at the kill range. Burst B is also a conventional burst, but with a higher fire rate. These projectiles have a higher SSHP and fewer projectiles are required to achieve a cumulative hit probability of 95 %. Burst C is a so-called intelligent burst. Each projectile of the burst is launched with a greater muzzle velocity than the preceding projectile, such that all projectiles arrive at the kill range together. Each projectile has then the maximal SSHP. In order to investigate the performance of the intelligent burst in comparison with the conventional burst, we will first analyze the influence of the fire rate and the muzzle velocity on the total kinetic energy at the muzzle for the conventional burst. This quantity can be easily calculated and is a measure for the required electrical energy. Table I shows the required number of rounds and the total kinetic energy at the muzzle for different muzzle velocities and fires rates. The number of rounds in a burst is relevant since the volume of the ammunition drum is limited. The actual Goalkeeper drum contains some thousand rounds, allowing three bursts of 300 rounds. In a typical naval encounter, several anti-ship missiles will be fired. Therefore, a sufficient number of rounds allowing several bursts must be available in the ammunition drum. By consequence, it is important to reduce as much as possible the required number of shots in a burst. As expected, the results in Table I show that a higher fire rate reduces the required number of rounds. A higher muzzle velocity also contributes to a lower ammunition consumption, since a lower flight time increases the SSHP. However, even if the number of rounds is lower, the results show clearly that a higher muzzle velocity leads to a higher energy requirement. The optimal solution against subsonic missiles is therefore a muzzle velocity of 1200 m/s and a fire rate of 300 Hz. Table II presents only the acceptable scenarios against supersonic missiles. Indeed, if the fire rate is too low for a certain muzzle velocity, it is impossible to obtain a cumulative hit probability of 95 %. The number of rounds is much higher when engaging supersonic targets then when subsonic targets are encountered. This is due to the fact that the kill range against supersonic target is higher (856 m versus 428 m) and that the SSHP is much lower at that distance. A first conclusion is that a powder-based CIWS, such as the Goalkeeper, firing at 1200 m/s and 75 Hz, is not effective against supersonic targets. A second conclusion is that the total kinetic energy at the muzzle is at least 120 MJ. In this case (1200 m/s, 300 Hz) the required number of rounds is 711. A more acceptable number of rounds (286) is required while firing at 2400 m/s and at a rate of 300 Hz, but the muzzle energy is then 193 MJ, which is not a realistic value.

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TABLE I. CONVENTIONAL BURST AGAINST SUBSONIC TARGETS (300 m/s). NUMBER OF ROUNDS AND TOTAL KINETIC ENERGY FOR DIFFERENT SCENARIOS. Scenarios

Requirements

Muzzle velocity (m/s)

Fire rate (Hz)

Number of rounds

Total kinetic energy at muzzle (MJ)

1200

75

153

25,8

1200

150

88

14,8

1200

225

78

13,1

1200

300

73

12,3

1800

75

85

32,2

1800

150

61

23,1

1800

225

56

21,2

1800

300

54

20,5

2400

75

70

47,2

2400

150

53

35,7

2400

225

49

25,0

2400

300

47

24,0

TABLE II. CONVENTIONAL BURST AGAINST SUPERSONIC TARGETS (600 m/s). NUMBER OF ROUNDS AND TOTAL KINETIC ENERGY FOR DIFFERENT SCENARIOS. Scenarios

Requirements

the projectiles, we will take the performance of the projectile at the maximum effective range of the Goalkeeper (2000 m) as a reference. The kinetic energy at impact against a subsonic target at 2000 m is 158 kJ. We assume that this energy is required to destroy the target if it is hit and thus neglect all other parameters that influence the lethality of the projectile. It is therefore possible to reduce the mass of the projectile and the muzzle velocity, as long as the energy at impact is equal or greater than 158 kJ. The variation of the hit probability with the fire rate is shown in Table III. If the fire rate is too low, a hit probability of 95 % cannot be obtained since not enough projectiles can be fired in the available time. The optimal fire rate is 300 Hz, with 50 rounds fired and a total kinetic energy at the muzzle of 18,0 MJ. A higher fire rate leads to a higher mean velocity. Fewer projectiles are then required, since each projectile will have a higher SSHP, but the total kinetic energy at the muzzle will be higher. TABLE III. VARIATION OF NUMBER OF ROUNDS AND HIT PROBABILITY WITH FIRE RATE (PROJECTILE MASS = 234 g, TARGET VELOCITY = 300 m/s). Fire rate (Hz)

Number of rounds

Hit probability (%)

Number of rounds for phit = 95 %

Total kinetic energy at muzzle for phit = 95 % (MJ)  

75

14

56,8

-

-­‐  

150

28

81,1

-

-­‐  

200

38

89,5

-

-­‐  

47

93,8

-

-­‐  

300

56

96,4

50

18,0

Muzzle velocity (m/s)

Fire rate (Hz)

Number of rounds

Total kinetic energy at muzzle (MJ)

250 350

66

98,0

49

19,0

1200

225

2900

488

400

75

98,8

48

19,8

1200

300

711

120

450

84

99,3

47

20,4

1800

225

501

190

500

94

99,6

47

21,2

1800

300

357

135

2400

150

917

618

2400

225

370

250

2400

300

286

193

A way to reduce the required energy is to fire lighter projectiles without reducing the kinetic energy on target. This is possible thanks to the higher muzzle velocity of the railgun. We will also investigate how the intelligent burst influences the required energy. III. PERFORMANCE OF ELECTROMAGNETIC CIWS WITH INTELLIGENT BURST AND REDUCED PROJECTILE MASS The muzzle velocity of the projectiles within a burst launched by a railgun can be easily varied. The maximum velocity is 2400 m/s in order to avoid excessive aerodynamic heating of the projectile during its flight. The minimum muzzle velocity is determined by the impact energy. Since we do not have detailed information on the terminal ballistics of

Similar simulations have been made with a lighter projectile (117 g instead of 234 g) and a supersonic target. The optimal solutions are presented in Table IV (case 4 to 7). Case 1 is the existing Goalkeeper, case 2 and 3 are the optimal solutions with a conventional burst against subsonic and supersonic targets respectively (Table I and II) and are added for comparison. Case 5 (400 Hz, 117 g) is the optimal solution against subsonic targets. The influence of the lighter projectile is clear. Case 4 shows that the intelligent burst with projectiles of 234 g requires more energy than the conventional burst. This is due to the fact that the kill range against subsonic targets is close to the gun and that a mean muzzle velocity higher than 1200 m/s is required to assure that all projectiles impact the target at the kill range. Case 7 (300 Hz, 117 Hz) is the optimal solution against supersonic targets. An important reduction in total kinetic energy is observed (from 120 MJ in case 3 to 17,1 MJ). This is due to a drastic reduction in required number of rounds, because all projectiles impact at the kill range, and

4 consequently have a higher SSHP. TABLE IV. COMPARISON OF OPTIMAL SOLUTIONS FOR CONVENTIONAL BURST (CASES 1 AND 3) AND INTELLIGENT BURST (CASES 4 TO 7). Case

Target velocity (m/s)

Mass (g)

Fire rate (Hz)

Number of rounds

Total kinetic energy at muzzle (MJ)

TABLE V. TECHNICAL PARAMETERS OF THE RAFIRA RAILGUN Mechanical Setup

Steel + GRP

Calibre

25 x 25 mm²

Inductance per bank

4 µH

1

300

234

75

153

25,8

Rail length (main accelerator)

3175 mm

2

300

234

300

73

12,3

Rail length (pre-accelerator)

350 mm

3

600

234

300

711

120

Rail material

Dural

4

300

234

300

50

18,0

Inductance gradient

0.45 µH/m at 100 kHz

5

300

117

400

48

9,94

Inductance per bank

4 µH

6

600

234

200

109

30,8

7

600

117

300

88

17,1

In conclusion, the higher fire rate and the variable muzzle velocity for each projectile of the burst, made possible by the use of a railgun, allows reducing significantly the required electrical energy of the railgun. In the next paragraphs, we will show that a fire rate of 75 Hz has been demonstrated. It is important to note that a conventional CIWS is based on a Gatling gun, a rotating multibarrel gun, in order to achieve a fire rate of 75 Hz. The Gatling gun of the Goalkeeper uses seven barrels, while the railgun described in the next paragraphs, uses only one barrel ! IV. RAFIRA FIRE RATE RAFIRA (Rapid Fire Railgun) is a multishot railgun, developed at ISL [3], [4], with which it is possible to investigate the potential of railgun technology for anti-ship missile scenarios. It is a linear electromagnetic accelerator (rectangular caliber of 25 mm x 25 mm) with which up to three shots in a row can be performed [3], [4]. The main accelerator, whose external housing consists of Glass-fiber Reinforced Plastic (GRP) bars, is 3 meters long. Up to two pre-accelerators can be mounted to the main accelerator. These pre-accelerators with a length of 0.35 m each are a copy of the main accelerator from the operational principle point of view but with rails turned by 90°. The purpose of the pre accelerators is to supply the main accelerator with the projectiles [3], [4] required in case of a multishot. More technical details of RAFIRA are shown in Table V. Typical projectiles equipped with brush armatures which are used with RAFIRA can be seen in Fig. 4. Ten brush armatures consisting of many metal fibers (CuCd) are incorporated into a sabot made of GRP. These brush armatures serve as the current carrying element in the main accelerator. For the pre-accelerator one brush armature is mounted perpendicular to the ten other brush armatures (see Fig. 4). The total mass of one projectile is about 120 g.

Fig. 4. Photograph of two typical projectiles used in the experiments. Ten brush armatures are used as current carrying element in the main accelerator. One brush armature mounted perpendicular to the other armatures is used for the acceleration in the pre-accelerator.

So far the fire rate of RAFIRA was limited by the loadingshooting process (Fig. 5). In case of a multishot, the first projectile is directly placed in the main accelerator and the second projectile is placed in the pre accelerator. After the discharge of the capacitors bank providing energy for the first projectile and a waiting time of 10 ms, the second projectile is pushed into the main accelerator, where it completely stops. After another waiting time of 10 ms, the capacitors banks providing energy for the acceleration of the second projectile are discharged and the second projectile is fired out. Because of the waiting times the fire rate is limited to 50 Hz. In order to increase the fire rate, we developed an automatic loading shooting process (Fig. 6). The first projectile is placed in the main accelerator and the second projectile is placed in the preaccelerator. Once the first projectile is accelerated and leaves the main accelerator it is detected by a B-dot sensor, whose signal is used as a trigger to push the second projectile into the main accelerator, where it is detected by a brush detection sensor developed at ISL [2]. This sensor is basically an electrical switch which closes when the projectile passes leading to a sudden rise of voltage across a built-in resistor [2]. The voltage across the resistor is filtered electronically to avoid erroneous measurements due to plasma development in

5 the barrel. The signal of the brush detection sensor, in turn, is used as a trigger to discharge the capacitor banks providing energy for the acceleration of the second projectile. It is important to note that the second projectile does not need to be stopped in the main accelerator, which increases the fire rate.

Consequently, according to Fig. 8 the first projectile leaves at tout_1 = 4.78 ms and the second projectile leaves at tout_2 = 17.89 ms implying that we were able to increase the fire rate from 50 Hz to 75 Hz. This corresponds to an increase of 50 % of the fire rate compared to earlier experiments [4]. A good sliding contact behavior is characterized by a muzzle voltage below 15-20 V. Higher voltages indicate a loss of contact accompanied by an electric arc. For both shots the muzzle voltage is well below 15 V indicating excellent sliding contact behavior for both shots. It should be noted that the muzzle voltage of the second shot is significantly lower (Uaverage_1 ≈ 2.31 V) than the muzzle voltage of the first shot (Uaverage_1 ≈ 4.3 V). These values mean that the ohmic losses at the sliding contact interfaces are significantly lower for the second shot. Further experimental data are required to confirm this interesting finding. The small peaks between t = 3 ms and t = 4 ms for the first shot and between t = 16.4 ms and t = 17.1 ms for the second shot could be attributed to current switching processes between different armatures of the projectile [5], [6].

Fig. 5. Loading-shooting process without brush detection sensor; here the shot frequency is limited by the time steps (2) and (3).

Fig. 6. Automatic loading-shooting process, the shot frequency is determined by trigger signals of time steps (2) and (3).

V. RAFIRA EXPERIMENTS We used the described automatic loading process to accelerate two projectiles (m1 = 123.47 g and m2 = 123.85 g) with a primary energy of 0.8 MJ for each projectile. The applied current pulse is shown in Fig. 7 and as a rail material Dural was used. Fig. 8 shows the muzzle voltage recorded for both shots which is an important indicator of the quality of the sliding contact behavior of the armature(s). The five narrow peaks at the beginning of the recorded data are due to the discharge of the five capacitor units which were used for the acceleration of each projectile. The large peak at the end of each voltage profile U > 50 V corresponds to the muzzle arc which is formed when the projectiles exit the railgun.

Fig. 7. Typical current pulse used for the acceleration of the projectile for an applied primary energy of 0.81 MJ.

6 corresponding value of the action integral in the above mentioned formula, we obtain values of 4.23 109 A2s/g and 3.77 109 A2s/g for the first and second shot, respectively. These values indicate that in the case of the second shot, probably better sliding contact conditions between the projectile and the rails exist leading to a slightly higher velocity. Further experiments are needed to confirm our results and to understand the relevant effects in such multishot experiments. Nevertheless, with respect to multishot experiments this is a very promising result. VI. CONCLUSIONS We have shown that railguns have better performance against anti-ship missiles thanks to a higher muzzle velocity, a higher fire rate and the ability to vary the muzzle velocity during a burst, allowing an intelligent burst. All projectiles impact then on the target at the minimal interception distance and thus with the maximal hit probability. By consequence, the required number of rounds and energy is significantly lower than in the case of a conventional burst (all projectiles have the same muzzle velocity). The technologic solutions to increase the fire rate of a railgun are discussed. We have experimentally shown that the ISL-gun RAFIRA is capable of firing at 75 Hz. This is as good as the existing powder-based CIWS guns. However, these systems use multibarrel Gatling guns to achieve this fire rate. RAFIRA demonstrated a fire rate of 75 Hz with a single barrel.

Fig. 8. Muzzle voltage of the first and second shot.

REFERENCES [1]

[2] Fig. 9. Velocity of the first and second shot recorded with a radar system.

[3]

The velocity for each projectile, measured by a Doppler radar system installed at the muzzle of the railgun [7], was 862 m/s and 870 m/s for the first and second shot, respectively (Fig. 9). Please note that the measurement of the second velocity is much noisier due to plasma development and small particles disturbing the radar system. One interesting thing to point out is that the velocity of the second projectile is slightly greater than the velocity of the first projectile. According to the formula

v∝

1 m

t

∫ 0 I 2 dt

(1)

the velocity of the projectiles depends on the mass and the action integral t

∫ 0 I 2 dt .

(2)

However, when inserting the actual mass and the

[4] [5] [6] [7]

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