A COMPULSATOR-DRIVEN RAPID FIRE RAILGUN SYSTEM

Prepared by

S.B. Pratap, M.L. Spann, W.G. Brinkman, M.D. Werst, M.R. Vaughn

Presented at

The 5th Pulsed Power Conference Arlington, VA

June 10-12, 1985

Publication PR-28

Center for Electromechanics The University of Texas at Austin Balcones Research Center Bldg. 133, EME 1.100 Austin, TX 78758 512/471-4496 A COMPULSATOR-DRIVEN RAPID- FIRE RAILGUN SYSTEM S. B. Pratap, M. L. Spann, W. G. Brinkman, M. D. Werst, and M. R. Vaughn Center for Electromechanics The University of Texas at Austin Taylor Hall 227 Austin, TX 78712 Summary current pulse in the gun be initiated near a specific phase angle of the open circuit voltage. The current A compensated pulsed alternator (compulsator) is pulse in the railgun is initiated by the armature on an attractive alternative to the homopolar-inductor the projectile. The high inductance gradient injector switch system as a power source for a rapid-fire correspondingly delivers the projectile at a specific railgun. The design of the compulsator and a few phase angle of the open circuit voltage because it is other components of the system are presented. Results triggered by the ignitron at a specific time. of the tests on various components of the system are also discussed. The system is designed to accelerate Throughout the design, emphasis has been placed on 80-g projectiles to 2 to 3 km/s at 60 Hz. minimizing the mass of the compulsator and making the system compact. Introduction Design of the Compulsator The rapid-fire gun system consists of several components. The compulsator is the power source and Considering the parameters in Table 1, a 2-ms provides current pulses to the injector and the wide, about 1-MA peak current pulse is required of the railgun. The injector, which is a high inductance compulsator. To obtain the required peak in a short gradient EM gun, accelerates the projectile from rest time interval and reasonable voltage levels, it is nec­ to an initial velocity of 200 m/s in order to reduce cessary to minimize the circuit inductance. It is damage at the breech of the rails. The railgun has a therefore neccessary to compensate the armature parallel rail geometry and accelerates the projectile winding inductance. This is possible to achieve using from the injection velocity to a final velocity of 2 passive compensation. to 3 km/s. The rails are directly connected to the compulsator without a switch, whereas the injector is This machine uses a rotating excitation field triggered with an ignitron. A schematic of the system winding on an iron rotor and a lap wound armature is shown in Figure 1. Table 1 gives the system para­ winding on a laminated iron stator. Compensation is meters. achieved with the use of a conductive shield on the periphery of the rotor. Figure 2 shows the cross sec­ tion of the machine.

PROJECTILE

Figure 1. Schematic of compulsator-driven rapid-fire system

Table 1. System parameters Parameter Units Nominal Value Figure 2. Sectional view of a passively compensated Projectile mass kg 0.08 rotating field machine Projectile velocity km/s 2.0 Projectile kinetic kJ 160.0 The excitation winding is wound of 0.64- x 0.64-cm energy solid copper wire which is coated with insulation Repetition rate pps 60.0 later. The wire is also wound with fiberglass tape Pulses/burst 10.0 which is epoxy impregnated, this provides the winding Barrel length m 3.0 with mechanical integrity under centrifugal loading. Peak current kA 940.0 The excitation winding is pulse rated and carries Acceleration time ms 2.0 1.2 kA for 6 s. The total on time of the field coil is 10 s to allow the current to ramp up and to allow time for the eddy currents in the poles to decay. Heat dissipation in the rails requires the use of three gun barrels and correspondingly three injectors The armature winding is made of six lap wound and autoloaders. Each autoloader in this case needs coils which are connected in parallel. Each coil is to operate at 20 Hz. In order to minimize rail ero­ made of 7 x 7 x 16 x #24 AWG copper litz wire which is sion at the muzzle, the projectile must exit the gun fully transposed. The litz wire is wound with a half barrel near zero current. This requires that the lap wrap of fiberglass tape and is epoxy impregnated, this provides the turn to turn and part of the ground obtained from the circuit simulation are presented in plane insulation. The ground plane insulation is Table 3. Figures 4 and 5 show the compulsator current further enhanced with an epoxy impregnated fiberglass and projectile velocity as a function of time. mat. The end turns of the armature winding are embedded in slots machined in G-10 rings on both ends of the compulsator. Table 3. Circuit simulation results The shield which provides passive compensation is Parameter Units Nominal Value an essential part of the machine. The primary purpose of the shield is to provide a highly conductive sur­ Mass of projectile kg 0.08 face, however mechanical constraints limit the conduc­ Peak current in tivity that is possible. The shield undergoes the injector kA 260 following types of loading. Final velocity in injector m/s 200 1) centrifugal loading due to rotation Peak current in 2) azimuthal loading during discharge rai lgun kA 944 3) compressive loading during discharge Final velocity in 4) a hoop stress due to shrink fit on the rotor railgun km/s 2.4 5) radial growth due to heating by the flow of Pulse width of eddy currents injector pulse ms 0.9 Pulse width of Centrifugal loading can be minimized by using a railgun pulse ms 2.2 light material like aluminum. In order to counteract Angular velocity of the thermal and centrifugal radial growth and still be machine at start R/s 492 able to take the discharge torque, the shield is Angular velocity of shrunk onto the rotor with an interference fit which machine after 10 shots R/s 420 results in a significant hoop stress even after com­ Total temp. rise in pensating for the compressive loading. Aluminum 7050 armature winding •c 52 was selected as the shield material which compromises Total temp. rise in •c 47 the conductivity to some extent, but satisfies all the shield mechanical criteria. Kinetic energy in kJ 230 projectile Due to the transient nature of the compulsator Change in inertial MJ 1.2 duty cycle, subcritical rotor dynamic behavior is energy of machine desirable. The radial vibration mode is most excited Energy transfer % 19 and hydrostatic bearings have been selected which pro­ efficiency vide the requisite radial stiffness. The thrust bearings are also hydrostatic. Table 2 gives the various machine parameters. Current• 104 120 Table 2. Machine parameters

Parameter Nominal Value 80

Rotor radius m 0.40 80 Outer radius m 0.60 of machine •o Length of machine m 1.52 20 Total mass kg 11,000 Polar moment of J-s2 432 inertia -20 ~ Machine inductance 0.45 --IN.IICTOJI PUI.II J.tH I -40 L Machine resistance J,tll 462 o .o~ .I .15 .2 .25 .3 .35 .4 .45 .5 Peak open circuit kV 2.0 Time (SEC) • IO·l voltage Rotor speed rpm 4,700 Figure 4. Compulsator current vs time Predicted Performance

VeloC'Ily • 10 A schematic of the compulsator, injector, railgun 3>0 connection scheme is shown in Figure 3. Results 300 .._ ! 250 r

200 '~

J&o L !

100 ~ •o ~ \'"7 rINJICTOO PULOI

0 ~ I , _ , _J o .os - .1 .1s .z .Z& .3 .35 .. .-15 .5 1 Time (SEC) • 10' Figure 3. Schematic representation of compulsator­ injector-railgun system Figure 5. Velocity vs time Test Model of the Compulsator

The shield is one of the important components of the compulsator from the point of view of the electrical design. From the mechanical standpoint it is the component which undergoes the most severe loading. In order to simulate the conditions in the shield a small test model of the compulsator was . built. This machine has a rotor diameter of 10 ems and a length of 30 ems. The machine stores 53 kJ at the test speed of 5,000 rpm. This machine has four poles and a wave wound armature winding made of litz wire. The shield is made of 6061-T6 aluminum which is 1.27-cms thick. Due to the low mass and energy of the rotor, rolling element bearings were used on this test model. Figure 6 shows the machine with the experimen­ tal setup for the short circuit test.

Figure 8. Run #14, machine terminal voltage

Run #25: In this ~est the machine was connected to a 1.91-cm bore, 15~cm long railgun. The projectile mass was 1 g. The fo.llowing results were obtained.

Rotor speed 5,820 rpm Field current 1,800 A Open circuit voltage 408 Vp-p Sho.rt circuit current 21.32 kA Pulse width 3.15 ms Projectile velocity 200 m/s Figure 9 shows the machine terminal voltage and current pulse. Figure 6. Machine with experimental setup This machine has been successfully tested and the test results are presented below. These test resul'ts agree well with the analytical models.

Test Results Run #14: Short Circuit Test Rotor speed 5,000 rpm Field current 2,200 A Open circuit voltage 400 Vp-p Short circuit current peak 46 kA Pulse width 4.6 ms

Figure 7 shows the trace of the current pulse obtained during the test. Figure 8 shows the machine voltage,. Figure 9. Run #25, machine voltage and current

Test Model of the Injector The injector is a high inductance gradient EM gun. The high inductance gradient is obtained by two inde­ pendent techniques. One of which is introducing an extra turn around the active rails. The second is enclosing the rails and augmenting turn in laminated iron. Both these techniques enhance the accelerating magnetic field. The injector is designed to have a round bore. In order to reduce damage to the rails at the breech, the injector is operated at relatively low current levels, the high inductance gradient also helps to reduce t,he rail damage. Figure 10 shows the injector assembly. The inductance gradient varies with the current Figure 7. Run #14, short ci~cuit current pulse through the rails d~e to the presence of the iron IICTIOII A-A

A t------e.o----_,·~11.0 ~ MUZZLE END Figure 10. Injector assembly

laminations. Figure 11 indicates this variation with Other Research Efforts current. The armature conductors are bonded to the inner surface of the stack of laminations of the compulsator pHim with fiberglass reinforced epoxy. Since the armature conductors provide the complete discharge torque which 2.0~------~~~·~U~N~S~A~T~UR~A~T~E~D~I~R~O~N--- is transmitted to the framework through the epoxy bond, an extensive program to test various compositions of the epoxy was developed and executed. This was THEORETICAL MAXIMUM ~ coupled with a detailed finite element analysis of the 1.11 ------(WITHOUT LEAKAGE FLUX) (WITHOUT -----IRON) stresses in the epoxy bond. 1.7 At present, work is being done to develop an auto­ 1.11 - loader system which will operate at 60 Hz. Work is also being done to develop the main railgun. Various 1.5 materials are being investigated for the spacers and the banding structure. The compulsator is being 1 1.4 L INCLUDING SATURATION fabricated at present, with testing scheudled for late 1985. 1.3 -

1.2 - Acknowledgements

1 1 · ~------~~~W~IT~H~O~U~T~I~R~O~N _____ The author wishes to acknowledge the invaluable 1.0 l___ __.l. ___ ..L._ __ __. ______contribution of W. F. Weldon, J. H. Gully, and 100 200 300 kA M. D. Driga. This research is supported by DARPA/ARDC under Figure 11. Inductance gradient vs current contract No. DAAK10-83-C-0126. for the injector

References Injector Test Results 1. M. L. Spann, s. B. Pratap, and M. R. Vaughn, The injector has been built and tested. An 80-g "Research and Development of a Compensated Pulsed projectile was accelerated to 320 m/s with a 300-kA Alternator for a Rapid-Fire Electromagnetic Gun," peak, 1.2-ms current pulse over 15 ems. This test was Interim Report #RF45, USARDC Contract No. repeated several times and the variation in the velo­ DAAK10-83-C-0126, July, 1984. city of the 80-g projectile was within 10 percent. Some rail damage at the breech was observed, however 2. s. B. Pratap, W. L. Bird, G. L. Godwin, and w. F. at least four consecutive shots were obtained from the Weldon "A Compulsator Driven Rapid-Fire EM Gun," injector before the bore was remachined. The injector Second Symposium on Electromagnetic Launch has been tested 16 times to date and of these 12 shots Technology, proceedings, Boston, MA, were at the 300-kA peak level. October 10-13, 1983.