a novel railgun launcher design

IEEE TRANSACTIONS ON MAGNETICS, VOL. 31, NO. 1, JANUARY 1995 261

A Novel Railgun Launcher Design

David P. Bauer IAP Research, Inc., 2763 Culver Avenue, Dayton OH 45429-3723

Abstmct-Higher efficiency, and smaller power supplies are needed for most electromagnetic railgun applications. This paper describes the feasibility evaluation of a novel railgun barrel c o n f i i t i o n that addresses these issues. The Hypervelocity High Efficiency (HYPE) railgun concept uses a distributed power feed railgun Mi, but eliminates the need for many separate distributed power supplies. Nested, segmented main rails are individually conneded to a single pair of augmenting secondary rails. The augmenting secondary rails are powered from a single energy source. A 15 mm square bore, 300 mm long HYPE railgun was built and tested to evaluate concept feasibility. Analytical and experimental results show that the HYPE confiiiuration is feasible, at least for low velocity.

INTRODUCITON

Electromagnetic railgun systems must be made more compact and energy efficient for application outside the laboratory. Railgun systems for mobile applications are especially hampered by lack of high energy efficiency and small sized power supplies.

Railgun efficiency (defined as the ratio of launch package muzzle kinetic energy to initial stored electrical energy) has a first order impact on system size because losses in the railgun are magnified as inefficiencies in each component of the power supply train. To compound the problem, most of the wasted energy is dissipated resistively and must be removed from the conductors by thermal management system. Low railgun efficiency therefore, leads to sizable power supplies and large t h e m 1 management systems.

One important source of wasted energy in a railgun shot, results from residual magnetic flux stored in the railgun barrel at projectile launch. This magnetic flux is difficult to recover. Existing systems usually end up dissipating this energy resistively in the rails and in an arc at the muzzle. Conventional breech fed railguns often must have residual magnetic energy to achieve the desired launch velocity with a fixed barrel length. Significantly higher performance efficiency is possible if residual magnetic energy can be re- duced.

Distributed energy store (DES) railguns have been pro- posed as a means of achieving higher efficiency[l)[2][3]. In a DES powered railgun, energy is supplied to the rails at multiple intervals from multiple power supplies over the

Manuscript received April 29, 1994

length of the barrel. Only the power supply (or supplies) feeding power near the muzzle of the gun are active at projectile launch. Considerably less residual magnetic energy is stored and dissipated at projectile launch, leading to improved railgun efficiency.

A DES power system has multiple individual power supplies one for each barrel power feed connection. These individual power supplies are normally physically located near the barrel feed point and each must be discharged in concert with projectile passage. In contrast, the power supply for a breech fed railgun is a single unit, with simple discharge timing. The DES configuration therefore, tends to be noncompact and more complex to operate.

A railgun launcher configuration which simultaneously achieves minimal residual magnetic flux characteristic of a DES railgun, with the inherent compactness and simplicity of a breech fed railgun power supply is desirable. A railgun concept devised to meet these goals is shown schematically in Figure 1, and will be referred to as the "HYPE" railgun.

( I 1092)

Fig. 1. (a) The HYPE railgun is comprised of a pair of augmenting rails, nested segmented main rails, and stationary current crossovers; @) power supply energy is initially discharged into the augmenting turn inductance (shading shows where energy is stored); (c) as the projectile travels along the gun, current is supplied to the armature by more than one rail segment; no energy remains in inactive segments behind armature; (d) as projectile exits barrel there is very little residual energy.

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268

HYPE CONCEPT

The HYPE railgun consists of a pair of augmenting conductors located alongside (and outside of) a pair of nested, segmented main rails. The nested, segmented main rails are actually many conducting segments (each segment will normally be less than 100 cm long) embedded in insulator as shown in Figure 2 and illustrated in an overall concept in Figure 3. Multiple current crossovers, located along the length of the gun, connect the segmented rails to the augmenting rails as indicated in Figure 1 and Figure 3. Each nested, segmented rail is connected through these current crossovers to the augmenting rail situated on the opposite side of the gun, thus obtaining an augmenting magnetic field effect.

Fig. 2. Several individual conductor segments simultaneously deliver current to the armature.

Initially, a power supply (for example a capacitor bank) co~ec ted to the muzzle end of the augmenting rails is discharged into the HYPE railgun, with a projectile and armature located at the breech (as depicted in Figure l(b)). During the capacitor discharge, energy is transferred and stored in the augmenting tum with the current path being completed in the railgun by the armature. The armature "short circuits" the current path as it moves down the barrel and contacts the second main rail segment. Current is then shared between the rail segments as it begins to flow into the second segment. Current sharing occurs with more segments as the armature contacts, and shorts these segments (as depicted in Figure 1 (c)). As each new segment becomes active, less current flows in the previous segments due to lower impedance of the path through the segments closest to the armature. Between six and eight rail segments will be actively carrying current to the armature except at the breech and muzzle. As the armature moves past a segment, current will completely commutate out of that segment and into those in contact with the armature. Close mutual magnetic coupling between rail segments results in very low energy dissipation as each rail segment is disconnected from the

armature and de-energized. Energy stored in the magnetic field (represented by the

shaded region in Figures l(b), l(c), and l(d)) of the augmenting tum/energy store drives the current and hence accelerates the projectile. As the projectile accelerates along the barrel, part of the electrical circuit is "removed" as each rail segment is de-energized. Augmenter inductance decreases as the projectile moves along the barrel, tending to keep the current constant. When the projectile exits the muzzle, only the final rail segment is energized, limiting the residual stored energy to a very small quantity.

Physical implementation of a HYPE railgun concept can be accomplished as shown in Figure 3. The nested, segmented main rails are fabricated with multiple small conductors embedded in an insulator. Each segment conductor has a relatively small cross section because of the short duration in contact with the armature and hence short conduction time. The augmenting conductors have large cross section to minimize ohmic loses.

A R M A T U R E 7 \\

E D RAIL SEGMENTS MAIN

LAMINATE STRUCTURE-J I*'O

Fig. 3. The HYPE railgun concept design.

EXPERIMENTAL VALIDATION

A small scale HYPE railgun was built and tested to develop fabrication techniques, to validate models, and to prove feasibility of the HYPE concept. A gun with the physical parameters summarized in Table 1 was constructed.


Bore Size 15 mm

Bore Shape square

Segment Length 85 mm

Segment Spacing 11 mm

Overall Length 300 mm

No. of Simultaneously Active 7 Segments

Segment Conductor Size

Intersegment Insulator Spacing 0.4 mm

1.6 mm x 9.5 mm

Aunmenter Rail Size I 12.7 mm x 38.1 mm

Copper (CDA110 Half Hard) augmenting and main rail segments were used. The augmenting rails are copper bar 1.27 x 3.81 x 30.5 cm. A photograph of the nested main rail is shown in Figure 4, and a drawing shows the details in Figure 5.

10.8 mm U

Fig. 4. Nested HYPE rails with 22 segments were fabricated.

Each nested main rail is composed of copper strips 0.16 x 1.27 x 11.4 cm separated by strips of G-10 insulator 0.04 x 1.27 x 10 cm. These strips were laminated together at a 10 degree angle (with respect to the bore centerline) using a high-peel strength commercial epoxy. Tabs were bent at one end of the copper strips to form a vertical connection for attaching the crossovers. At the 10 degree angle and the 1.08 cm pitch, the rail is designed to have between seven and eight

"

m m

Llntersegment Insulator L C o p p e r Rail Segment

Fig. 5 The rail segments were on a 10 degree angle.

rail segments active at any given time. Note that a wedge of G-10 glued to the breech end of the rail (right side in Figure 4) provides a continuous sliding surface to the projectile. The current crossovers are 5.08 cm long strips of equal cross section as the rail segments and were soldered to the rail tabs. The crossovers were located above the bore insulator, and fit into slots machined into the tops of the augmenting rails. A photo of the complete HYPE prototype core (without structure), including the placement of the instrumentation leads is shown in Figure 6. The completely assembled HYPE gun is shown in Figure 7.


P

B

Fig. 6. A 30 cm long HYPE core was assembled.

The gun inductance characteristics were measured with an Hewlett Packard model HP 4912A Impedance Analyzer. The inductance was measured as a metal armature was moved along the length of the rail

The self and mutual inductances of the augmenting turn and main segments were measured independently. The measured self inductance gradient of the augmenting rail ( 0 . 5 ~ H/m) and of the rail segments ( 0 . 5 6 ~ H/m) compared to within 8% of the values computed from Kerrisk[4]. The mutual inductance between an adjacent rail segment pair and a rail segment pair separated by one segment were measured. Combined with the measured segment self inductance, the mutual coupling between adjacent segments was computed to be 0.95 with the armature positioned at the end of the segment. The coupling coefficient between a pair of segments separated by an intermediate segment is 0.75. This high coupling leads to efficient energy transfer between segments.

ARMATURE PULL TEST RESULTS

The first tests were performed to measure segment currents under controlled current and velocity conditions. We therefore built an experiment in which a metal armature was pulled (by a pneumatic actuator) through the HYPE prototype barrel. Nearly constant current at 2000A was supplied via a battery/inductor power supply, made with eight 12 volts batteries connected to a 30 tum welding cable inductor

The experimental setup included Rogowski coils, muzzle and breech voltage leads, and B-dot loops. To measure currents in the individual rail segments, four 0.3cm diameter Rogowski coils were made, and wrapped around the crossovers c o ~ e ~ t e d to four consecutive rail segments. The instrumented segments were located in the middle of the main rail and were chosen to find a steady-state response. A fifth Rogowski coil was used to measured the total power supply current.

A trailing arm type, metal armature &de of 7075T6 alu$num was used in the experiments. The armature had an overall length of 2.5 cm with a contact patch on each arm of dimensions 3 mm x 15 mm. A pneumatic piston assembly pulled the armature along the bore via a G-10 rod. The

(-250 pH).

Fig. 7. The HYPE core was installed in a bolted clamp structure. velocity was consistently 20 m/s. Individual segment current data are shown in Figure 8 for

the four consecutive segments instrumented with Rogowski coils. These data show that essentially, each segment carried most of the current consecutively. For example, during the period from about 0.0 to 0.7 ms the second (instrumented) segment carried a substantial fraction of the total current. At about 0.6 ms, when the third segment became active, the current in the second segment dropped rapidly. The segment current had almost retumed to zero by the time the armature reaches about 50% to 60% of the segment length. The impedance through the other segments is apparently much


271

2-

1-

smaller by that point. The same basic circuit behavior was predicted by an electrical circuit model at similar velocity conditions[5].

I W " W 4 __

/ / / ,

Fig. 8. Each segment in-turn, camed the total current.

The voltage lead records from this test show that at 4.392 ms the armature left the main rails, resulting in an l00V arc, as shown in Figure 9. Prior to exit the armature voltage was only 1 to 2V. Examination of the voltage records prior to armature exit shows a nearly constant voltage. This lack of voltage spikes indicates that no arcs occurred during the time in which the armature was in the bore, confirming the arcless segment-to-segment current shunting.

LAUNCH TEST RESULTS

Launch tests were performed with the 15 mm HYPE barrel to evaluate its electrical behavior at higher (compared to the pulled armature tests) velocities. In particular, the goals were to evaluate acceleration performance and to measure main rail segment current sharing.

The HYPE barrel was powered by a capacitive discharge supply consisting of a 300pF capacitor connected to a 2pH inductor. A triggered ignitron switch initiated the discharge, and a silicon diode crowbarred the circuit at peak current to obtain a decaying exponential waveform. A one meter long helium gas preaccelerator was used to give the projectile a velocity of about 200 m/s. The projectile was a trailing arm metallic armature attached to a nylon forebody. The total launch mass was 17.8 g, and its overall length was 40 mm. A pair of electrically conductive wires, located betyeen the preaccelerator and HYPE barrel, were sheared by the moving projectile. These break wires provided position-time data and a triggering mechanism for the capacitive power supply. Projectile position-time data down range from the HYPE muzzle was measured with MAVIS[6] style magnetic detec- tors.

Launches at peak currents up to 92 kA were performed with this setup. Figure 10 shows the current record for the highest current test. The projectile exited the muzzle at a time of 1 ms on this test. Armature contact arcing initiated at about 850 us, changed the slope of the current decay as indicated in Figure 10.

80 i

0- lime (us)

Fig. 10. The highest current employed in the HYPEgun had a 93kA peak.

Fig. 9. The current transferred from segment-to-segment with no arcing. The projectile entrance and exit velocities were computed

as 192 m/s and 239 m/s respectively, for the 92 kA peak current test. This short length HYPE gun therefore, provided a momentum increase to the projectile of 0.84 kg-m/s. An overall assessment of the HYPE railgun performance can be


212

20-

15-

10-

5-

determined by computing an equivalent effective inductance gradient determined by dividing two times this momentum change (i.e., 1.68) by the electrical action during transit in the barrel. The action numerically computed from the current record shown in Figure 11 is 1.9 X 106 A's, which results in an effective equivalent inductance gradient of about 0 . 9 ~ H/m. This is within about 10 percent of the value computed (1 .Op H/m) based on the measured self and mutual inductances described above.

Rogowski loops were inserted around crossover conductors in order to measure three consecutive main rail segment currents. The measured currents for the 92 kA peak current test, are displayed in Figure 11. The three currents do not sum to the total current shown in Figure 10 because in this 15 mm HYPE gun, there are at least seven rail egments actively conducting current to the armature at any instant. Current in each segment in-turn rises to a peak and then falls as down bore segments create a lower impedance current path through the advancing projectile. As the figure shows, current in the first two segments (number 11 and number 12 counted from the breech) falls rapidly to zero, while the third segment (number 13) carried a low level of current even after the projectile exited the gun. It appears that the armature contact transition to arcing created a stationary secondary current path across the bore. Except for this characteristic, the measured currents display similar behavior to circuit model predictions[3).

Additional tests at even higher velocities and with a longer gun are required to make comprehensive comparisons between experimental and analytical results. Eddy currents in the current crossovers and segment-to-segment commutation losses may become important at higher velocity. Higher velocity experiments with a longer gun will allow this measurements/evaluations to be accomplished.

O----

-5 ' 650 760 760 aao 850 goo 960 1000 io50 1

Time (us) (1 3fW

CO

Fig. 11. advances.

Current transfers to successive rail segments as the projectile

CONCLUSIONS

The high efficiency of DES railguns, and the simplicity and compactness of breech fed power supplies are both desirable for mobile railgun applications. The HYPE railgun design provides a potential means of achieving a marriage between the two approaches. The cost is a more complex railgun core. Initial testing shows that the HYPE design provides projectile acceleration at the velocities tested so far. Efficient performance of the HYPE design must be proven at higher velocity. Adverse transient electromagnetic interactions between the complex trail segments, may significantly affect efficiency.

REFERENCES

R.A. Marshall, "The Use of Nested Chevron Rails in a Distributed Energy Store Railgun, " IEEE Trans. Mag., MAG-20, pp. 389-90, March 1984. J.V. Parker, "Electromagnetic Projectile Acceleration Utilizing Distributed Energy Sources, :I. Appl. Phys.53 (lo), pp. 6710-6723 (1 982). R.A. Marshall, "Distributed Energy Store Railgun: The Limiting Case, "IEEE Trans. Mag., Vo1.27, No.1, pp. 136-138, January 1981. Kerrisk, J.F., "Current Distribution and Inductance Calculations for Rail-Gun Conductors, " LA=9092-MS, Los Alamos National Laboratory; November 1981. Bauer, D.P., Geers, S., Daugherty C., Sumner, B., Final Report "High Efticiency Hypervelocity Railgun Launcher - Phase I, IAP-TR- 91-07, April 1992. Moody, R.L., Konrad, C.H., "Magnetic Induction System for Two- Stage Gun Projectile Velocity Measurement," Sandia Report, SAND84-0638, May 1984.


a novel railgun launcher design

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