![TATAKE et al: RAILGuN
4. DATA ACQUISITION AND SIMULATION
4.1 Data Acquisition
A computei-based data acquisition system was set
up to monitor important parameters that affect the
performance of the railgun. Current transformers and
Rogowski coils were used to measure the rail currents
in the range6of 100 to 500 kA. Magnetic probes were
used to get the position-time profile of the projectile
inside the bore of the gun and railgun current
distribution 7.These probes help detect plasma leakage
and formation of secondary arc. The velocity outsid6
the bore of the gun was measured using shorting
screens8.A high-speed camera was set up to measure
the velocity of the projectile and establish the intt;;grity
of the projectile at the muzzle end. This is a non-contact
method and is free from electromagnetic pickups.
Figure 3. Construction details of the railgun.
4.2 Simulation
A simulation code was developed to predict the
performance of the railgun. The performance of the
model was evaluated by monitoring different
parameters. Table 1 gives the equation used in the
simulation and analysis. The current measured is used
to derive other significant parameters like the
displacement, velocity and acceleration of the projectile
calculated from Eqns (1) -(3).
Table I. t:quations used for the analysis of the railgun system
(I)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
(11)
[LI(2.m)] .f fI(tf. dt
[LI(2.m)] .f I(tf.dt
[LI(2.m)] .I(tf
[L.f2] .I(t}2/area
[L.f2] .I(tf
Vb.(t. I(t).dt
E,(t)/E,olal
Epoj (t)/EI (t)
v muz( t) .I ( t) .dt
(1f2) .m. V(t)2
Eproj (t)/E,otaJ
=
=
=
The launchers with the last two design modifications
proved more reliable and durable tha.n the launchers
based on the first design. The details of construction of
the railgun are shown in Fig. 3.
Thermal energy transfer from the rails leads to
ablation and the melting of the bore materials. Such
ablation degrades the performance of the railgun by
adding parasitic massto tHeplasma. The bore materials
should have a high melting point and superior erosion
and ablation resistance. High rail conductivity
necessitated the use of copper rails. Polycarbonate and
fibreglass were most suitable as bore materials. Loose
bore to projectile tolerances or variation in bore
dimensions can result in plasma leakage. Most of the
launchers showed marked deterioration after a few
shots. The deterioration could be attributed to changes
in the bore dimensions due to the rail insulator ablation.
Substantial deposits of carbon were obseIVed inside the
bore of the gun and needed cleaning.
Projectile: The projectiles are made of Perspex or
polycarbonate cubes of 12 mm length. Perspex
projectiles tended to shatter. Polycarbonate projectiles
survived the high plasma pressures.The plasma and the
solid armature were both used for carrying the high
currents. Most firings were carried out using plasma
armature. A plasma armature is formed when AllCu
foil melts/explodes on the passageof high currents. The
foil vaporises by joule heating to produce a plasma to
drive the armature. A neoprene obturator was placed
at the rear of the projectile to seal the bore against
plasma leakage around the projectile. As a deviation,
a solid metallic projectile acting as an armature was
also used to carry the current.
=
5(t)
V(t)
A(t)
P(t)
F(t)
Et(t)
PLdI(t)
LdI(t)
E-(t)
Eproj (t)
5d1(t)
=
=
=
5. ANALYSIS
All measurements were supported by appropriate
software developed to analyse the entire performance
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EM RAilgun
- 1. Defence Science Journal, Val 44, No 3, July 1994, pp 257-262 @ 1994, DESIDOC Railgun S.G. Tatake, K.J. Daniel, K.R. Rao, A.A. Ghosh, and I.I. Khan ArmamentResearch& DevelopmentEStabliShment,Pune-411021 ABSTRACT A railgun using electromagnetic propulsion was developed to launch hypervelocity projectiles. A 240kJ, low inductance capacitor bank operating at 5 kV powered the railgun. Launchers and proj~ctiles were designed and developed for this purpose. The currents producing the launch forces are of the order of hundreds of kA. Even very low impedances for the current through the railgun circuit are substantial sources of energy losses. A simulation code was developed to optimise the performance of the railgun. Control and instrumentation facilities were set up along with a computer-based data acquisition system for measurement and analysis. The capacity to launch projectiles of3-3.5 9 weight to a velocity of more than 2.00 km!s was demonstrated. . NOMENCLAruRE I. INTRODUCTION A facility was devel6ped to launch hypervelocity projectiles using electromagnetic energy. The projectiles were launched using a railgui1. The railgun consi~ts of two parallel rails and a conducting metanic foil placed behind the insulating projectile. When a high current flpws through the rails, the foil explOdes and forms a plasma armature. The force acting on the armature is given by F(t) = 0.50 * L * I (t)2 The railgun currents are in the region of. hundreds of kA. This Lorentz force accelerates the projectilel-3. The railgun set-up is shown in Fig. 1. 2. POWER SUPPLY An electromagnetic propulsion system requires a storage device with an energy.density comparable to that of chemic(fl explosives. The most expensive and technologically difficult part of the system is the high-energy electric source. The power sources considered for electromagnetic propulsion are well 4 researched . A (t) projectile acceleration at time t Earc(t) energy dissipated in plasma arc at time t El (t) energy of launcher at time t Eproj( t) kinetic energy of projectile at time t Etolal total energy delivered by the power source F( t) force at time t I ( t) current through the launcher at time t L inductance per unit length of the launcher Leff (t) launcher efficiency at time t m mass of the projectile P(t) plasma pressure at time t PLe((( t) power source to launcher efficiency at time t S (t) displacement of the projectile at time t Set(( t) total railgun system efficiency at time t v( t) velocity of the projectile at time t Vbr(t) voltage at the breech end of the railgun at time t Velmz(t) vel()City at the muzzle end of the railgun at time t Vmvz(t) voltage at the muzzle end()f the railgun at time t Received 5 march 1993 257
- 2. DEF SCI VOL 44, NO 3, JULY 1994 PROJECTILE ARHAT~E ROLL CURRENT ~..-/,/"" (071) ouTA FIELD Basic configuration of the railgunFigure 1 2.1 Capacitor Bank and Charging Unit The capacitor bank was used as a power source owing to its availability and lower CU"I.despite its lower energy density5. A low-inductance, 240 kJ capacitor bank was set up to provide the basic power to the railgun. A high-voltage charging unit wasusedto charge the capacitor bank. Figure 2. Schematic circuit diagram or the railgun powered by the capacitor bank. 2.2 High Current Switches Launcher: A simple, single pulse driven railgun launcher was developed with a minimum of metal components in proximity to the bore to maximize the inductance of the launcher and to improve the launch effici.ency. The launcher has a 12 mm square bore cross-section. The launcher was fabricated with lengths ranging from 1 to 2 m. The following launcher designs were used for the firings: The capacitor energy is switched into the railgun by high-power ignitrons. When the peak current is reached, additionalhigh-power ignitrons are used to crowbar the capacitors out of the circuit to obtain a dc pulse. This minimises the stress on the capacitors, the launcher and the projectile. A schel1}atic diagram of the railgun powered by the capacitor bank is shown in Fig. 2. 2.3 Transmission Lines The copper rails and the insulater were rigidly contained within Perspex side plates and bolted. These launchers were found to be weak, getting damagedand cracking at higher plasmapressures. (a) Low-inductance transmission lines were made using sandwiched conducting plates to maximize the energy transfer to the load. The transmission lines are subjected to repulsive forces owing to the passage of current through them. These, forces were 'estimated to provide proper bolting and bracing to avoid deformation of the transmission lines. The copper rails and the insulator were rigidly contained using fibreglaS;sside plates and potted in an epoxy resin. The assembly was housed in a cylindrical mild steel jacket. (b) The copper rails and the 'insulator were rigidly contained using fibreglass side plates and wound with a fibregtass material. They were potted in an epoxy resin and the assembly was housed in a cylindrical nonmetallic jacket. (c)3. LAUNCHER AND PROJECTILE Launchers and projectiles are subjected to high plasma pressures. high magnetic fields and high temperatures. In the present railgun set-up. the plasma pressures generated varied between 100and 150 MPa. 258
- 3. TATAKE et al: RAILGuN 4. DATA ACQUISITION AND SIMULATION 4.1 Data Acquisition A computei-based data acquisition system was set up to monitor important parameters that affect the performance of the railgun. Current transformers and Rogowski coils were used to measure the rail currents in the range6of 100 to 500 kA. Magnetic probes were used to get the position-time profile of the projectile inside the bore of the gun and railgun current distribution 7.These probes help detect plasma leakage and formation of secondary arc. The velocity outsid6 the bore of the gun was measured using shorting screens8.A high-speed camera was set up to measure the velocity of the projectile and establish the intt;;grity of the projectile at the muzzle end. This is a non-contact method and is free from electromagnetic pickups. Figure 3. Construction details of the railgun. 4.2 Simulation A simulation code was developed to predict the performance of the railgun. The performance of the model was evaluated by monitoring different parameters. Table 1 gives the equation used in the simulation and analysis. The current measured is used to derive other significant parameters like the displacement, velocity and acceleration of the projectile calculated from Eqns (1) -(3). Table I. t:quations used for the analysis of the railgun system (I) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) [LI(2.m)] .f fI(tf. dt [LI(2.m)] .f I(tf.dt [LI(2.m)] .I(tf [L.f2] .I(t}2/area [L.f2] .I(tf Vb.(t. I(t).dt E,(t)/E,olal Epoj (t)/EI (t) v muz( t) .I ( t) .dt (1f2) .m. V(t)2 Eproj (t)/E,otaJ = = = The launchers with the last two design modifications proved more reliable and durable tha.n the launchers based on the first design. The details of construction of the railgun are shown in Fig. 3. Thermal energy transfer from the rails leads to ablation and the melting of the bore materials. Such ablation degrades the performance of the railgun by adding parasitic massto tHeplasma. The bore materials should have a high melting point and superior erosion and ablation resistance. High rail conductivity necessitated the use of copper rails. Polycarbonate and fibreglass were most suitable as bore materials. Loose bore to projectile tolerances or variation in bore dimensions can result in plasma leakage. Most of the launchers showed marked deterioration after a few shots. The deterioration could be attributed to changes in the bore dimensions due to the rail insulator ablation. Substantial deposits of carbon were obseIVed inside the bore of the gun and needed cleaning. Projectile: The projectiles are made of Perspex or polycarbonate cubes of 12 mm length. Perspex projectiles tended to shatter. Polycarbonate projectiles survived the high plasma pressures.The plasma and the solid armature were both used for carrying the high currents. Most firings were carried out using plasma armature. A plasma armature is formed when AllCu foil melts/explodes on the passageof high currents. The foil vaporises by joule heating to produce a plasma to drive the armature. A neoprene obturator was placed at the rear of the projectile to seal the bore against plasma leakage around the projectile. As a deviation, a solid metallic projectile acting as an armature was also used to carry the current. = 5(t) V(t) A(t) P(t) F(t) Et(t) PLdI(t) LdI(t) E-(t) Eproj (t) 5d1(t) = = = 5. ANALYSIS All measurements were supported by appropriate software developed to analyse the entire performance 259
- 4. DEF SCI J, VOL 44, NO 3, JULY 1994 vnOOTY /IETAII:E ANJ PRO.ECTI.f [ISPlA(D.:NT vs TK 0 ~ NO 1200 1600 2000 2~ 2800 3200 3600 ~000 TM (115) 6. RESULTS Sometypical railgun trial results are given in Tabl~ 2. Projectile velocities greater than 2000m/swere obtained for trial nos 1 to 3. The efficiency varied between 4 to 5 per cent with railgun current in excessof 260 kA. Plasma leakage and formation 01 secondary arcs8 were responsible for the lower projectile velocities than expected from the computer model for trial nos 5 to 7. Trial no.7 was done using a solid conducting projectile made of aluminium. An armature was kept behind the projectile with no ablator. The armature vaporised and t;e plasma escaped ahead of the projectile. This led to a lower system efficiency and projectile velocity. A solid projectile made of Perspex and armature made of several copper foils were used in trial no.4. The massof each foil was kept around 100mg to avoid the melting of the armature owing to lne high railgun current. 7. CONCLUSION Our study has shown that projectiles attain hypervelocities by using a single small square bore railgun. In the existing railgun facility the efficiency varied between 4 to 5 per cent. Significant improvement in the efficiency of the railgun set-up is one of the key issues that will determine the use of railguns for various weapon applications. Hence we carried out detailed modelling and simulation of the entire railgun system. The re5ults from the simulation were validated with the measurements. Measurements made at high common mode voltages of around tOOOs of volts and high electromagnetic noise were exceptic;>nally good, providing reliable and repeatable records. Intact projectile launch and 2-3 m of free flight projectile were studied using high-speed photography when punctures in1he shorting screens were obsered. Using a high-speed camera the integl;ity of the projectile was established beyond doubt (Fig. 5). A 12 mrn cubical polycarbonate projectile weighing about 3 g could defeat a 6 mm aluminium sheet at 2 m from the muzzle end of the gun (Fig. 6). The complete railgun system was also placed in a 5 m long vacuum chamber to study the railgun performance. Our studies are as yet i,nconclusive. Owing to the failure of some odd capacitors in the capacitor bank, repetitive trials could not be carried using the full energy of the bank. The energy extracted from the Ii1gure 4. Projectilevelocityand displacementcomputedfrom the currentsignalrecordedonscopes.Theinboreandshorting screenpulseswith respectto timeare alsoshown. of the railgun. The current-time data are used to predict the displacement, velocity and acceleration of the projectile and the plasma pressure. Figure 4 shows the graphical display of the results derived from the measurements. N9t only were the measured currents and the simulated currents compared, but als9 the results derived from these equations, to validate the computer model. The breech and the muzzle voltages are monitored and along with the current signals are used to estimate the arc voltage and resistance. The energy of the launcher and the gun efficiency are als~ computed from Eqns (6) and (8). The projectile exit at the muzzle is also indicated on the muzzle voltage signal. The breech and the muzzle voltage signals aiong with the in-bore flux probes detected plasma leakages. attributed to low projectile velocities. The magnetic flux probes were used to obtain the displacement of the plasma armature which leads to in-bore velocity and the acceleration of the projectile from Eqns (2) -(3). The pl~sma position-time history and the current signal help estimate the inductance per unit length of the railgun, a useful input to the simulation code (Eqn 1).The pulses from the shorting screens were used to monitor the velocity, acceleration and the kinetic energy of the projectile outside the bore of the railgun. There was a substantial agre~ment between the results predicted from the computer model and the actual measurements, t.hus validating the simulated model. 260
- 5. TATAKE et sI: RAILGUN Table 2. Some typical railgun results -5 6 71 2 3 4Para!J1eter 3.7 3.0 7.0 3.03.0 3.0 3.0 Solid PlasmaPlasma Plasma Solid PlasmaPlasma 155.5 138.24 138.24 138.24138.24 138.24 138.24 267.0 18 271.0 18 287.0 20 234.0 14 298.0 22 296.0 21 290.0 21 740 730 700 491 624 300 466 123 128 143 95154 152 146 1250 22002250 2250 2250 1730 21S0 17()() 588 13002080 2050 2(100 1550 2.566.49 6.30 6.00 4.44 4.35 1.20 4.34 2.58 3.13 0.87 .874.69 4.56 Massof projectile, 9 Type of armature Energy of bank,kJ Peak,kA Peak force on the projectile, kN Peak acceleration on the projectile, k'g' Peak pressure,MPa Theoretical projectile velocity,m/s Measured projectile velocity. m/s Kinetic energy of the projectile, kJ System efficiency. % Figure 5. A polycarbonate projectile photographed through an Imacon high speed camera (framing speed : 500,000 frames/s; exposure time: 400 ns; flash duration: 500 JIS;and aperture: 4). 261
- 6. DEF SCI J, VOL 44, NO 3, JUL y 1994 Pune. for giving us the permission to publish this p,,per. We are also indebted to him for the facilities that were made available to us in this Establishment for the present study. REFERENCES 1. Rashleigh, S.C. & Marshall, R.A. Electromagnetic acceleration of macroparticles to high velocities. I. App. Phy., 198q49,2540-45. 2. Weeks, D.A.; Weldon, W.F. & Zowraka Jr, R.C. Plasma armature railgun launcher simulations at University of Texas at Austin. IEEE Trans. Magnetics, 1989, 25, 580-86. 3. ARDE Closure Report No.892. To establish technique for launching hypervelocity projectiles, March 1991. 4. Gully. J.H. Power supply technologies for electric guns. IEEE Trans. Magnetics, 1991,27,329-34. 5. Rose, M.F. Compact capacitor power railgun systems.IEEE Trans. Magnetics, 1986,22,1717-21. 6. Pellinen, D.G.; Di Cpana, M.S.; Sampayan, S.E.; Gerbracht, H. & Wang, M. Rogowskii. Coil for measuring fast high level pulsed currents. Rev. Sci. Instrum., 1980,51, 1535-39. FIgure 6. PenetratIon of the poIycarbonate cuhe through a 6 mm aluminium sheet at 2 m from the muzzle of the railgun. Bauer, David P. & Braker, John P. In-bore railgun p;rojectile velocity. IEEE Trans. Magnetics, 1986, 22, 1395-98. 7-: capacitor bank varied between 120 and 160 kJ. The kinetic energy of the projectiles can be incrcascd substantially by using a higher-energy capacitor bank as a power source, .- T~take, S.G.; Daniel, K.J.; Ghosh, A.A.; Rao, K.R.; Tokekar, K.P, & Khan, I.I. Diagnostic for an electromagnetic propulsion system, All India Conference on Applied Instrumentation, 14-15 February 1992, Roorkee, 275-78. 8. ACKNOWLEDGEMENTS We thank Major General D Kapil. Director . Armament Research and Development Establishment. 262