main propulsion power take-off configuration for an etc gun pulsed power generator

KENNETH E. PETTERSEN, CHARLES L. BIELITZ & JOHN CIANCI

Main Propulsion Power Take-08 Configuration for an ETC Gun Pulsed Power Generator

THE AUTHORS

Kenneth E. Pettersen graduated from the University of Maryland, Baltimore County in 1978. He is a mechanical engineer in the power systems directorate of the Annapolis Detachment, Carde- rock Division, Naval Surface Warfare Center (formerly David Tay- lor Research Center) and has been with the Department of the Navy since 1981. Mr. Pettersen worked for the Naval Sea Systems Command from May 1981 until June 1985. He has been employed with NSWC Annapolis Detachment since June 1985.

Charles L. Bielitz is a mechanical engineer in the mechanical transmissions branch of Annapolis Detachment, Carderock Divi- sion, Naval Surface Warfnre Center, where he has worked for the past six years. He received his BSME from Virginia Polytechnic Institute in 1981. Prior to joining NSWC Annapolis. he was employed by the General Electric Company in the medium steam turbine department.

John Cianci received his ASME from Tufts University in 1958 and his BSME from Tufts University in 1962. Mr. Cianci joined Gener- al Electric in 1954. He has thirty-seven years of experience in design, manufacturing, testing, installation and customer support of high speed, high power, industrial, marine and Navy gearing, bearings, couplings and lube systems.

His experience includes positions of both engineer and manager in the areas of gear design, gear products quality control and gear manufacturing engineering. he is currently technical leader, Gear Development Programs. He is a member of the American Gear Manufacturers Association, and is on the Marine and High Speed Gearing committees.

ABSTRACT

Electro-Thermal Chemical (ETC) Gun technology will increase the range and capabilities of existing CIWS and 5 inch guns. Because of their faster, yet more controllable acceleration, ETC guns will allow for the utilization of smart munitions and, in the case of the 5 inch guns, increase range to beyond 50 nautical miles.

This paper examines five power take-off (PTO) gear configurations from a surface ship class main reduction gears (MRG’s) which drive a nominal 10 Megawatt (MW) Pulsed Power (PP) generator. The PP generator is utilized to charge a

capacitor-based pulsed forming network (PFN) which, in turn, provides an electrical pulse to the cartridge of a munition at the breech of an ETC gun upon demand. The PTO gear and resultant placement of the generator will be the major focus of this paper.

Major issues that will be examined will include space con- straints within the main engine and auxiliary machinery rooms, gear sizing, survivability, shock and vibration, the penetration of watertight bulkheads with high speed shafting and equipment accessibility. Also, design guidelines for the PP generator will be provided. Special emphasis will be placed on the connection of the PTO gear to the MRGs.

INTRODUCTION

T h e Navy has been considering advanced weapon systems that operate by means of a pulsed electrical power source for a number of years now. The term “pulsed” describes high peak power (tens of Megawatts) delivered over short (mili-seconds) periods of time. The only existing shipboard power source capable of delivering power of this magnitude is the main propulsion system. A gas turbine surface combatant, with four typical LM 2500 turbines, is capable of delivering approximately 80 Megawatts of power. For this rea- son and in order to optimize shipboard integration, NSWC Annapolis Detachment has chosen the propulsion system as the source of their pulsed power architecture [ 1,2] versus in- stalling alternate power systems for which there is little room.

ETC guns are fired by means of the electrical pulse delivered directly to the breech of the ETC gun. A probe at the breech is positioned to mate into the back of the munition cartridge. Upon delivery of the pulsed load to the breech, expansion of the propellant within the cartridge propels the munition down the barrel at speeds considerably higher ( 1 S k d s e c ) than that of conventional guns ( 1.2km/sec). The increased projectile acceleration will allow 5 inch gun ranges to exceed 50 nautical miles. This acceleration is more controlled and at lower pressures than that of existing powder guns. This lack of peak pressure that is typical of powder munitions also permits the use of smart munitions that can be guided to targets, thus increasing accuracy.

52 Naval Engineers Journal, May 1994

PETTERSEN, BIELITZ & CIANCI MAIN PROPULSION POWER

The baseline platform, as discussed earlier, will be a surface combatant with two Close-in-Weapons Systems (CIWS) and a 5 incW.54 caliber forward gun. The ETC version of CIWS will be a 60mm gun versus the 20mm that now exists; however, the footprint and mount will remain basically the same. The larger projectile with guidance capability is designed for one hit-to kill versus the multi-hundred “cloud” that is fired now. For the CIWS, we will assume a 10 shot burst at 4 shots per second. There will be 10 bursts per engagement with with a five second rest between bursts and one hour between engagements.

The 5 inch gun will be the most unchanged with the main modification being the power input to the gun. Operation andor repetition rate for the 5 inch gun will be five shots in 15 seconds (one shot every three seconds) with a 20 second rest period before the next burst. The five shots will actually be fired within twelve seconds at this rate, however, three seconds are added to the front end of this scenario for charg- ing time. There will be six of these bursts with an hour between engagements.

POWER SYSTEM ARCHITECTURE

Now that the principle of ETC gun operation has been touched on, it is necessary here to first lay out the surface combatant pulsed power system architecture prior to discus- sion of the actual power take-off configuration. These archi- tectures were first presented in References 1 and 2.

The ship has twin main propulsion plants that each con- sist of two LM 2500 gas turbines (rated 26,250 hp at 3,600 rpm) driving a General Electric MRG. The MRG takes the turbine output of 3,600 rpm and reduces that to 168 rpm at the propulsion shaft and screw at full power. Power take-off for our pulsed power system originates at the first reduction pinion of the MRG, directly in line with the turbine output and terminates at the pulse forming networks (PFN) which are in close proximity to each ETC gun.

This architecture utilizes a power take-off capability designed into the ship MRGs but not previously used. The original specification for the MRG required provisions be made for retrofit of the Franco-Tosi Reversible Converter Coupling (RCC) to permit future conversion to a fixed pitch propeller. Power was to be transmitted to each RCC via a quill shaft run- ning through the high speed pinion and connected to the input (i.e. main turbine) side of a synchro-selfshifting (SSS) 160T automatic overrunning clutch with remote lockout capability. For the baseline controllable pitch propeller version a com- pletely automatic SSS 140T clutch with no quill shaft connec- tions was substituted for the 16OT clutch. Since conversion to a fixed pitch propeller system is not actively being considered, this power take off capability is available for use, requiring only a changeover to the 160T clutch and addition of a quill shaft through the high speed pinion. The quill shaft is coupled to the pinion of the PTO gear as depicted in Figure 1. The size and position of this PTO gear is the subject of this paper and will be discussed in the next section. The PTO gear is a speed increaser and is coupled to the PP generator which is driven at 8,OOO rpm full power (10 Megawatts). The generator is rated at 10 M W , 13.8 kV1-1, 3 phase, 533 Hz. The generator can operate at 10 MW with a minimum of 6,222 rpm.

C CIRCUIT BREAKER

10 YW GENERATOR

O N

GEAR

Figure 1. Machinery arrangements.

The next components downstream of the generator are a circuit breaker provided for generator fault protection and a full bridged phased controlled, silicon controlled rectifier (SCR) which converts the generator’s AC to 15 kV DC in the direct vicinity, or within, the main engineroom. Through switchgear, three coaxial cables (one for each gun) then tra- verse the ship to each PFN located at the guns.

The PFN’s are capacitor based. The capacitors are high energy, high density storage devices. The PFN takes charg- ing from the generator, stores it, then discharges the pulse to the gun upon demand.

The above architecture is for one engineroom. The same system is also located in the other engine room which provides redundancy and enhanced survivability. In case of a casualty in one engine room the guns can still be fired-from the other. This architecture is depicted in Figure 2.

POWER TAKE-OFF OPTIONS: ISSUES AND ARRANGEMENTS

General Electric Corporation has been tasked by NavSea to downselect from five potential PTO gear designs for the configurations described below and the generator character- istics given in Table 1. The purpose of this contract was to identify the optimum configuration of the PTO gear from the gear manufacturer’s standpoint. The results of this study are provided herein. It should be noted here that the footprint/size of the design generator is critical and a slight reduction would allow for a sixth configuration [3] that may be the overall most attractive. The PTO gear for Option 2 below would directly apply to this configuration. This arrangement will also be discussed at the end of this section.

As stated, there were five alternative connection systems that were considered with the four of those most promising illustrated in Figure 3. These options are: 1) the Overhead Configuration which places the generator directly above the MRG; 2) the Through-Bulkhead Configuration places the generator in the adjacent space (Auxiliary Machinery Room # 2) from each engineroom; 3) the Side Mount Configura- tion which places the generator at the side of the MRG; 4)

Naval Engineers Journal, May 1994 53

MAIN PROPULSION POWER PETTERSEN, BIELITZ & CIANCI

Figure 2. Electric gun equipped surface combatant.

the Right Angle Drive Configuration which also places the generator at the MRG’s side; and 5) “Racer” pinion location with generator again mounted on top of MRG. The term “Racer” pinion is another unused option in the MRG design originally intended for input from a Rankin Cycle Energy

Table 1. Generator data.

Power Rating:

Speed Range: 6222-8000 rpm

Max Torque: 17450 Ft Ibs

IOMW over speed range

Weight: 2400Kg - 5280 Ib

Maximum envelope size: 30” diameter X 60” long plus shafl

extension and flange diameter as

required

Fault Multiplier: 3.2 - 4 times max. torque

Efficiency: 80%

Recovery (Racer) steam turbine. These configurations are made possible by accessing the original MRG. Redesign of the gear is not required.

Each configuration has advantages and disadvantages and each required close scrutiny and trade-off. The Overhead Mount Configuration allows the generator to be placed in space which is currently under utilized, thus minimizing the rearrangement of existing machinery. The additional weight of the generator and power take-off gear borne by the MRG casing and housing will require the redesign of the existing sound isolation mounts and reinforcement of the housing. This configuration allows the generator/auxiliary gear weight to be more evenly shared over the existing MRG mounts although the offset between the input and output centerlines required for the auxiliary gear could be as long as 9 ft (2.75m) as shown in Figure 1 . This long offset requires a power take-off gear with multiple idlers which is a more complicated and a heavier auxiliary gear train than other options. Reference 3 analysis indicated that this configuration would pose severe shock and vibration issues.

The Through-Bulkhead Configuration would not require the modifications to the sound isolation mounts and MRG casing and would also allow optimizing PTO gear size to a two element gear. This arrangement would, however, in-

~~



OVERHEAD CONFIGURATION

THROUGH-BULKHEAD CONFIGURATION

AWLIARY MACHINERY

ROOM

SIDE MOUNT CONFIGURATION

GEAR

RIGHT-ANGLE DRIVE CONFIGURATION

ROOM

SIDE MOUNT CONFIGURATION

GEAR

RIGHT-ANGLE DRIVE CONFIGURATION

10IIWGEUERATOR '

Figure 3. Power take-off options.

clude flexible coupling and seal requirements at the high speed shaft penetration of the watertight bulkhead. In addition, quill shaft length restrictions and lateral vibrations have to be addressed.

The Side Mount Configuration would require less modifications to the MRG casing than those required for the Over- head Mount Configuration. The support system for the side mount would also be less complicated which would also im-

prove shock and vibration issues associated with the Over- head Mount. Also, the PTO gear would be smaller and less complex. This configuration would also require the redesign and addition of shock mounts. Engine room catwalks would require relocation and careful attention must be paid to elim- inate interference with MRG inspection ports.

The Right Angle Drive Configuration provides enhanced arrangements flexibility. However, there is great concern



that this arrangement may be quite noisy at this power level. Spiral bevel gear applications at this power level are virtual- ly nonexistent.

The “Racer” mount configuration would extract power from a fifth second reduction pinion vice the first reduction pinion as in all other cases. This option would have mounting issues as discussed with the Overhead Configuration, although the offset would be much less allowing for a smaller PTO gear. This would also require that the propeller be tum- ing during the PP Generator utilization.

The MRG casing extension configuration (Option 6), de- tailed in Reference 3, has three of the most attractive features which pertain to the most critical issues. The first of these features is the PTO gear itself which would be only a two element gear as described in Option 2. This gear would be the smallest and lightest of all options. Also, since the generator and PTO gear would be mounted into an extension of the existing MRG casing as one contiguous unit, the mechanical dynamics of this configuration would also be the most optimum. Third, survivability would be at the maximum potential since the generator unit/PTO gear would be located in the same location as the power source (i.e. one target vice two for separate locations). However, as mentioned earlier, the generator size will have to decrease in order for this configuration to be viable. Without this decrease in generator size, the watertight bulkhead bordering AMR #2 will be impacted which will induce unacceptable naval architecture practices. The generator at it’s current size would require that the watertight bulkhead be “notched” while naval architecture guidelines require that the bulkhead remain in a plane.

GEAR RATING AND SIZING

Specific gear sizing for each of the above options, as provided by the gear manufacturer, will be discussed next along with details of the generator to be mounted. Generator data, Table 1, and gear rating data, Reference 2, was provided to the MRG gear manufacturer.

Gear rating is based upon a peak generator load resulting in a torque of 38,740 ft-lb at the gas turbine and a speed increasing ratio 2.22. Peak torque at the generator is therefore

Gear rating is 26,580 hp at 3,600/8,000 rpm. Loading of the gear will be at 20 percent of peak torque as a steady state base load with a cyclical load reaching peak torque applied at intervals of three seconds during firing of the 5 inch gun.

The life expectancy of the ship’s MRG is 150,000 hours or 6,250 days. This life expectancy is operational use. At 720 shots per day, total shots fired is 4.5 X 106. Since the gun will not be fired every day and the speed and torque will vary during a firing cycle, using this number of shots (4.5 X 106) as the number of cycles to determine the allowable tooth stress levels seems appropriate.

17,450 ft-lb.

GEAR DATA

Tooth loading on this PTO gear does not need to be held to propulsion gearing linlits because it sees fewer cycles during its life and because the teeth on all of the gear ele-

ments can be surface hardened by carburization, whereas the large fabricated gears in an MRG can only be nitrided which cannot handle the high compressive stresses that carburized gears can.

For carburized, hardened and ground teeth and 4.5 X 106 cycles, the allowable root stress is 61,000 psi and the allowable compressive stress is 198,000 psi (or about 1000 K Factor). The K Factor is a tooth pitting index number which reflects the capacity of gear teeth to resist pitting. The compressive stress (SC) on the tooth flanks is proportional to the square root of the K Factor.

S ,=C dK

Where: C is a constant based on the gear materials and geometry

K=W,(M,+ 1)

DPM,

Where: W, = Tangential force at tooth pitch diameter, lb. F = Tooth face width, in.

D, = Pitch diameter of pinion, in. M, = Gear ratio

A single offset drive, such as Option 2, at the 1000 K Factor load level would result in a center distance of 11.2 inches and a pinion pitch diameter of 6.96 inches. In order to achieve a good balance between bearing loading and pro- portions, and shaft stresses, it is desirable to increase the pinion pitch diameter. A 7.75 inch pitch diameter was chosen as a minimum for this option. This allows a root diameter which is just large enough to clear a 6.75 inch journal diameter with eight module teeth. With a 6.75 X 9.00 inch pinion bearing, the unit load will be about 475 psi.

Looking at the pinion deflection and tooth scoring potential, however, we find that this would require a lead modification of .002 inches in a 5 inch helix width and the ,scoring risk is high unless the teeth are super-finished to at least 14 RA or silver plated.

Consequently, it makes good sense to design to a larger center distance which will improve both of these conditions. In other words, the Through-Bulkhead Configuration (Op- tion 2) PTO gear design will be optimized by increasing the center distance slightly above the smallest possible (i.e.

Table 2. Gear data. Q H X a 1 2 9

Center Distance (in.) 76.557 14.205 70.00

Pinion Diameter (in.) 9.63 0.01 0.01

Gear Diameter (,in.) 21.37 19.54 19.54

Idler Diameter (in.) 30.53 NIA 27.91

No. Idlers 2 0 2

Face Width (in.) 6.75 8.00 8.00

56 Naval EnQlneOr8 Journal, May 1994


Table 3. Gear sizing.

OPTlON 1 2 9

Est. wt. of Gear, Ib. 23550 5250 19850

Gear Losses, hp 400 260 430

Gear Oil Flow, GPM 100 70 100

14.285 inches versus 12.5 inches). Table 2 shows the optimum gear design for Option 2, which is also the most attractive overall (PTO gear only) at these power levels. Since 14.285 inches is the optimum center distance (between pinion and gear) for a two element gear (i.e. single offset) at these power levels, this offset arrangement was considered for all options. Idlers are then sized and added to make the required offset for a particular option. Table 2 shows acceptable gear designs for Options 1,2 and 3.

Rotor sizes for Options 1 and 3 were determined by pro- portioning the required idler gear size to the output pinion and input gear size for each required center distance and se- lecting a face width that would result in stresses that are within allowable levels.

Table 3 shows Option 1-3 PTO gear weight estimations and other data for the gear sizes provided in Table 2.

The gear manufacturer evaluated Option 4 utilizing a right angle spiral bevel gear to drive the generator. Since there is sufficient space at the MRG inboard corners oppo- site the gas turbines, it makes sense to mount the generator axis athwartships instead of forward and aft along the side of the MRG.

A single speed increasing spiral bevel gear mesh was considered as Option 4. Gear data is summarized in Table 4.

The Option 4A bevel gear of 30.24 inch diameter puts it in a size where available spiral bevel gear grinders are very limited. The gear most likely would have to be hard cut. Accura- cy levels of AGMA quality level 12-13 are attainable by this method. To maintain mesh alignment, antifriction rolling contact bearings are required. Bearing loads of about 28,000 lb. thrust and 6,000 lb. radial must be carried at the 8,000 rpm shaft speed and will have an unacceptable, short life.

Option 4B uses a dual torque path to reduce the required gear size to one that may be ground more readily. Bearing loads will be decreased but bearing life will still be limited. A parallel shaft gear stage is needed to combine the two branches to drive the generator.

There is limited test data for spiral bevel gears greater than 20 inches in diameter, and not a lot of experience in high power, high pitchline velocity applications.

Considering the above problems with right angle gears and the attractiveness of the gears in Options 1-3, right angle gears will no longer be considered for this study. Thus, Option 4 is hereby eliminated.

Option 5 considers the possibility of extracting power from the “Racer” location at the top of the MRG through a pinion meshed with the low speed gear. The maximum load at the LS gear mesh is a tangential driving force of 105,838

Table 4. Right angle drive gear data.

OPtlM 48 48

Material

Bevel Pinion Pitch Dia., in

Bevel Gear Pitch Dia., In.

Shaft Angle

Face Width, in.

Spiral Angle

Pressure Angle

Diarnetral Pitch, in.

Pitchline Velocity, Ft/M

@ 8000 pinion rprn

Carburized

12.0

30.24

900

5.0

350

200

2.4

251 33

Carburized

10.0

24.07

900

4.0

350

200

2.7

20245

lb. (346 K Factor), because of the nitrided teeth of the second reduction gear. If power is extracted at this load level and at rated LS gear speed of 168 rpm, the maximum available horsepower is 15,804 hp. As determined previously, the required power is 22,156 hp at 8,000 rpm of the generator. Therefore, Option 5 is not a viable option and was not considered further.

GAS TURBINE POWER EXTRACTION

The generator drive will be accomplished by extracting power from the inboard gas turbine. The SSS-140T clutch in the MRG HS line is replaced with a SSS-160T clutch which has provisions for attaching a quill shaft that will pass through the bore of the high speed pinion. During the operation of the pulsed power generator, the clutch will be disen- gaged allowing the high speed pinion to move freely as dic- tated by the MRG being driven by the outboard gas turbine.

Power will be delivered through the quill to the various speed increasing gear options. The performance characteris- tics provided by NSWC Annapolis (dynamic simulations) were reviewed by GE gas turbine engineering and found to be acceptable for the LM 2500 engine. Due to the cyclical loading, however, there is some concern that the hydrome- chanical fuel control variable fuel vanes may experience a phasing problem and there may be some control issues to deal with.

CONCLUSIONS

As stated previously, Options 4 (Right Angle Drive Con- figuration) and 5 (“Racer” Location Configuration) are not considered feasible options for the installation of a pulsed power generator aboard a surface combatant.

As depicted in Tables 2 and 3, Options 1-3 all have acceptable PTO gear designs which, from a gear designer

Naval Engineers Journal, May 1994 57


point of view, would support the installation of the PP generator. These tables also show that the most optimum gear, again from a gear designer standpoint, would be the Option 2 (Through-Bulkhead) arrangement. This Option 2 PTO gear would be the smallest and lightest, utilize the least amount of cooling oil and expend the least amount of horsepower of all options. However, other factors have to be taken into account before reaching a conclusion on an actual installation configuration.

Shock and vibration issues associated with the extended drivetrain into the adjoining space and the penetration of the watertight bulkhead into that space are major issues that have to be evaluated further. The gear manufacturer has had past successes with the penetration of watertight bulkheads with high speed shafting.

Since the power source and generator are located in separate spaces, survivability is a major issue. Reference 3 ad- dresses these survivability issues and confirms that other options are more attractive regarding survivability. The Reference 3 procedure to score the arrangements was de- rived from a Techmatics Inc. survivability evaluation tool and from investigations previously conducted by the Office of Naval Research, Reference 4. This technique provides a method for the evaluation of how survivable a subsystem is in terms of how well its 12 Critical Components are protect- ed against Battle Damage Mechanisms, Reference 5.

Of the two remaining options, both within the main engineering space, the Side Mount Configuration (Option 3) would be the most viable. The mass of the generator located above the MRG in Option 1 with regard to shock and vibration is a major deterrent in the consideration of this option. Also, the PTO gear is the least attractive of the three acceptable gear designs.

When taking all issues into consideration it appears that the Side Mount Configuration, with the generator at its current design size, is the optimum overall configuration. The PTO gear design is not the smallest and lightest but is acceptable. Survivability is as good or better than any other options. The exact mounting, addressing shock and vibration requires further study. This configuration also allows personnel access to

the MRG whereas the Overhead Option does not. In summary, the Through Bulkhead arrangement (Option

2) and Sidemount Configuration (Option 3) both appear to be viable arrangements for the shipboard installation of a PP generator. While the optimum gear design is held by the Op- tion 2 configuration, other shipboard installation issues may give a slight edge to the Option 3 configuration. Further study will be required for the exact downselect, particularly since the generator size (footprint and power) may be dimin- ishing. Should that be the case, an in-depth study of the previously mentioned Casing Extension Configuration 3 would have tm be conducted. This could be the ideal configuration because all shock and vibration issues would be minimized, the PTO gear would be as efficient as the best in Tables 2 and 3 (Option 2) and survivability would be at least as high as the other options.

ACKNOWLEDGEMENTS

Dr. Scott Fish, Institute of Advanced Technology, Austin, Texas (formerly of NSWC Annapolis).

REFERENCES

Doyle, T. J. and G. F. Grater, “Propulsion Powered Electric Guns - A Comparison of Power System Archi- tectures,” DTRCPAS-9 1-3 I , (July 199 1 ). Fish, S. and K. Pettersen, J. Cherry, S. Fagan, J. Dentler, G. Grater, “Preliminary Impact Assessment of Electro- Thermal Chemical Gun Outfit Aboard the DDG-5 1 with Mechanical Drive,” DTRCFAS-9153. (Feb 1992). Techmatics, Inc. Technical Report, “Electro-Thermal Chemical Gun Ship Integration Study,” NavSea Con- tract N00024-89-C-5683, (30 July 93). Methodology, Volume IV, “Advanced Naval Tactical Command and Control Study,” Office of Naval Re- search, dtd 1965. “The PASTA Analysis Technique Applied to BFC 20 10,” Techmatics, Inc., Arlington, Va., dtd January 1990. @


main propulsion power take-off configuration for an etc gun pulsed power generator

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