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. MODEL 105

In late 1956 I was assigned to Curt Bradley’s Model 105 Project. Why it was identified asthe Model 105, I do not know; but I think it was Helmut Schelp’s designation for a project thathad top security requirements which he wished to control. The unit was later designated ModelGTCP100-50. This was the development and qualification configuration.

I was very pleased, for this was my first opportunity to work on a unit from the verybeginning, i.e., I was there for laying down the center line! Many would wonder why the unit hadto be within a twenty-two inch cylinder. It wasn’t until we reached the mock-up stage that welearned the Model GTCP100 was to be the ground service unit for the North American A-5, asupersonic US Navy attack nuclear bomber. The aircraft was not a successful bomber, so allaircraft were ultimately converted into reconnaissance aircraft with the designation RA-5. TheGTCP100, with all of its equipment (bleed air hose, electrical cable, controls, etc.) was requiredto be a self contained system, for it was to be packaged in a flyable pod similar to the old GTC85Douglas Pod. Because of supersonic aerodynamic requirements, the pod diameter was held to aminimum; but even with the 22 inch diameter APU the pod length exceeded 20 feet!

The general configuration of the GTCP100, Figure 1, was very similar to the GTCP85;but it was much larger. The two stage compressor was composed of a double entry first stageradial impeller which discharged via crossover ducts into a single entry second stage radialimpeller. The pressure ratio was approximately 5.2 and the through air flow was approximately660 pounds per minute. The turbine was composed of a single radial inflow turbine wheelfollowed by a single axial turbine wheel (for some reason we called this a stage and a halfturbine).

FIGURE 1 Cross Section of the Model 105

Jack Haasis and Bill Caan were responsible for the combustion system design. Theyresolved the restricted diameter requirement by using six can-annular reverse flow combustion

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chambers mounted axially on the aft end of the turbine plenum. The chambers were internallyconnected by crossfire ducts and were retained, within a V-band clamp mounted dome, by theduel orifice fuel nozzle mounting system.

Bleed air, which required the development of a broad range compressor, was tapped offaxially at the top center of the turbine plenum between the two upper combustion chambers. Thegearbox provided mounting pads for all unit accessories and for a 60 KVA, 400 cycle ACGenerator.

Helmet imposed some requirements that he considered desirable. The gearbox,compressor and turbine had to be modules permitting ease of removal and maintenance. He alsoinsisted that all fuel system components, with the exception of the flow divider and fuel nozzles, be incorporated into the pad mounted fuel control to reduce plumbing clutter. This resulted inwhat became known as the “Barrel Fuel Control” it was quite large and heavy for the standardAN accessory mount pad!

I was assigned the gear box, bearings, seals, lubrication system, plumbing, ignition systemand coordination of the of the combustion system (I had some crossover duct experience from myJ47 work at General Electric).

Curt was very aware of potential critical speed problems, for he had worked on theGTCP185 Project. The GTCP185 utilized Dr. Stanitz’s mixed flow aerodynamics, which was acombination of radial and axial aerodynamics for both the compressor and turbine design. Thisresulted in approximately a 45 degree compressor discharge and a similar entry into the turbine. The GTCP185 rotor consisted of one very large compressor wheel and one very large turbinewheel. The rotating group weighed approximately 130 pounds.

The GTCP185 was designed long before the "tie bolt-curvic coupling" arrangementbecame available for rotor assemblies. Therefore close tolerance pilot diameters andperpendicular mating surfaces along with a series of bolts were utilized to mount the compressorwheel and turbine wheel on a large diameter shaft. After group balance it was necessary todisassemble the balanced group to incorporate it into the unit. This contributed to the criticalspeed problem.

The rotating group was supported by two bearings, a ball bearing located in thecompressor inlet housing and a roller bearing located in the turbine stator housing. Unfortunatelythis arrangement, along with the rigid bearing mount system, resulted in two very severe criticalspeeds, one just prior to starter cutout (about 12000 rpm) and the second just prior to governedspeed (approximately 35000 rpm). There were occasions when the GTCP185 could notaccelerate through the second critical. This condition could potentially result in unit selfdestruction due to the very high vibratory loads (in excess of 35G)! Therefore Curt instructed meto investigate resilient bearing mount arrangements and to utilize the bearing test rig that wasavailable from the GTCP185 program. The test rig consisted of complete external structure witha representative rotor mass supported by the actual bearing arrangement. An air turbine starterwas used to drive the rig.

The initial test established the base line data for rigid mounted bearings. The resultsconfirmed the very high loads imposed on the unit. Bob Van Nimwegen had presented a study ofresilient bearing arrangements in his Masters Thesis, so we decided, (over Bud Cain’s objection, who was very doubtful of soft mounts for his bearing design), to use the mechanical approach in

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our study. For the roller bearing, a radial wave spring contained in an annular ring, was utilized. The design permitted approximately five thousandth of an inch radial movement under the spring load. The ball bearing presented a more complicated design due to the thrust loads. Themechanical arrangement for the ball bearing resilient mount was a cylinder within a cylinder. Thecylinders were assembled to each other by twelve small cylindrical beams. The beams wereattached to the front of the inner cylinder and to the aft of the outer cylinder allowing the beamsto act as springs in the radial direction and to resisted thrust loads in tension. A five thousandthradial clearance between the cylinders was established for radial movement under the spring loadand the outer cylinder was slightly longer than the inner which permitted the inner to move freelywithin these limits.

The method of assembly for the ball bearing mount was Nicro braze, this arrangementproved to have problems because the gap between the cylinders was within brazing allowable gapdimensions! Herb Bergen fussed about it; but ultimately found a vender who could produce ourdesign. There was an easy method of inspection for a bad braze. All one had to do was hold theinner cylinder and hit the outer cylinder, if it had a bell like ring it was a good unit.

The GTCP185 test rig was assembled using mechanical resilient bearing mounts andcomparable test were conducted. The resulting elimination of critical speeds and reduction inbearing loads was spectacular! After issuing the test report Bud Cain became an advocate andsometime in the future (I believe it was after Armon Bosco joined his group) the hydrodynamic(oil dampening) resilient bearing mount became standard on all high speed rotating machinery.The high speed rotating group bearings were defined and placed on order from the variousbearing manufactures. After development bearings were received Bud Cain’s bearing engineersinitiated extensive bench testing, in which, the bearings were placed under realist loads,temperatures and rotational speeds.

The lubrication pump was a new design composed of a pressure pump and three scavenge elements (one to scavenge the gearbox and front compressor cavity, a second to scavenge the midcavity between the compressor and turbine and the third to scavenge the turbine cavity located inthe turbine exhaust). This new design was our first use of W.H. Nichols Gerotor pump elements.A very excellent design with better altitude performance, more efficient than the gear type pump,ease of manufacture and a good vender providing the elements. These qualities resulted in the Gerotor type pump becoming a standard design for most of our future units.

Most of the seal requirements were of standard designs and we had several vendors(Cartriseal, Chicago Rawhide, Parker, etc.) who could supply our needs. The shaft seal located inthe turbine exhaust was a twofold problem due to high temperatures and no leakage allowed!After receipt of development seals, Wally Silberschlag conducted extensive bench testing. He found all to be acceptable for the low temperature areas; but only two designs, both from ChicagoRawhide, were acceptable for the turbine exhaust. One used a convoluted brass bellows and thesecond utilized welded stainless steel bellows. Cartriseal lost out with their control gap carbonring seal. My white paper on the test results had me in much trouble, for I reported their seal tobe a controlled leak which would result in much turbine exhaust smoke.

As the design progressed and fabrication began, a mockup of the complete system,mounted in a pod mockup was completed. The mockup review was held in conference room ofthe Ramada Inn on Van Buren Street. Believe it or not, in those days this was one of the best

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motel/hotels in the area. Navy and AiResearch personnel in attendance were very impressed withthe arrangement of the equipment; but I think the size of the pod was somewhat of a surprise tothe Navy men.

As parts became available many component test were initiated. Bill Caan combustor rigtesting, Bob Borman compressor rig testing, etc., and finally the first unit was placed under test. Initially every thing progress very well. The shaft seal in the turbine exhaust worked so well thescavenge pump created a negative pressure within the bearing cavity during operation. Thenegative pressure was maintained even after long periods of shut down. The first failure was ofthis seal! We had utilized the brass bellows seal in the initial build because it was the leastexpensive of the two designs. Unfortunately the bellows was prone to fatigue failure, muchsmoke and even fire in the tailpipe! So the stainless steel bellows became the accepted design andproved to be a very good seal.

This initial testing taught me another lesson. The turbine exhaust bearing carrier wassupported by three beams that were arranged tangentially between the bearing carrier and thestructure outside of the turbine exhaust. This arrangement maintained proper location of thebearing centerline as temperature changes caused expansion and contraction of the carriermounting system. Some time during this initial test the heads of the bolts that retained the three beams snapped off. Failure analysis revealed hydrogen embrittlement due to cadmium plating. Inever used cad plated bolts in the hot zone again. Another bad design that was revealed duringour initial testing was the failure of the braze joint between the exducer and the radial turbinewheel.

Charlie Mulkin, one of our best metallurgist, solved this problem. His findings are quoted:“During the development of the Model 105 engine, the exducer attachment method

attempted to use a brazed joint, which consistently failed.It occurred to me that it might be possible to use a “blind rivet” approach using a piece of

metal inserted into mating machined cavities in both the exducer and the turbine wheel and presstogether to form an upset rivet shape holding the two parts together.

Engineering agreed that this approach was possible and took this basic concept andchanged it to provide an annular ring as the rivet material that is placed in circular mating groovesin the center hub portion of both parts. The edges of this ring are then upset when the parts arepressed together to form a rivet shape to hold the two parts together. I believe that Curt Bradleywas involved in the final design approach. (See sketches)

I selected the rivet ring material

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to be Inconel X750. The ring is machined to the annulus cavity dimensions then preaged toprovide the strength to resist deformation during the pressing operation. It is then locallyinduction annealed at both end edges to allow upsetting to fill the annular cavity. The assembly ispressed together in a hydraulic press with the ring edges forming the blind rivet. The assembly isthen aged to provide joint strength.

This attachment method proved to be extremely reliable and was used for many APUturbine wheel designs.”

Manufacturing had problems accomplishing the pressed joint. The following is GeorgeHutchinson’s report concerning this:

“One aspect of the process required achieving the “press.” We did not have a press in-house which would exert the pressure required to achieve the required extrusion of the ring intothe mating annuluses (the actual method used to machine the annulus in with the wheel andexducer is the subject of another discussion) since we had long since gone from the use of punchpresses for the forming of metal parts to the Hydroform method. It was therefore necessary totake the parts and the necessary tooling to the Arizona Highway Department testing laboratorydowntown where they had a unit which could handle the pressure requirements. This particularunit was normally used for the testing samples taken from cement pours on highway constructionto ensure that the proper mix was provided. On one of the first uses of the press to extrude thering, the AiResearch operator failed to notice a chunk of concrete which was stuck on the ram ofthe hydraulic press, causing something of a mess when the pressure was applied to the fixture inthe in a lopsided manner. The M.O.T. (Manufacturing Operations and Tooling) was then changedto add an inspection of the press for unwanted concrete chunks prior to inserting the fixture.”

Another feature unique to the Model 105 was a remote located cooling fan, for there wasno room for a gear driven fan on the gearbox due to the pod envelope restrictions. Our sisterdivision in Los Angeles had developed a 13 inch diameter tip turbine driven fan for a commercialaircraft application. We requested they design and developed a 6 inch diameter fan, for ourapplication, utilizing the tip turbine concept. The new unit utilized a partial emission nozzle todirect bleed air, from the Model 105 turbine plenum, to the turbine located on the outer shroud ofthe fan. Pressurized air required for operation was delivered to the fan by one inch diameterstainless steel tubing without valving or controls, i.e., the fan was in operation during anyoperation of the Model 105 and its speed and output varied freely with bleed air energy. Thesystem worked very well and permitted various locations and arrangements of the coolingequipment.

The development and qualification testing was completed without any great problems. The GTCP100 progressed into production and continued to be delivered in moderate quantities inseveral different versions. Aldo Romanin’s summary of the total program is included as anaddendum.

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NOTES

1. Figure 1 is a reproduction of cross sectional drawing 375818, dated June 25, 1958. It wassigned by: R. H. Singer, Draftsman; E. L. Gammill, Curt Bradley and Don Furst, Engineering; andG. E. Colburn, Nomenclature.

2. I apologize for the brevity, for I know there are many more who contributed to this excellentprogram. Unfortunately memory becomes cloudy with time. Further my design notes andcalculations were lost when our modern managers decided the Engineering Library was no longerneeded.

3. The innovational items created and/or utilized for the first time at AiResearch are: resilientmounting of high speed bearings, curvic couplings for the high speed rotating elements, definedcavities with a cylindrical “rivet” for assembling parts, multiple cannular combustor with crossover ducts, molded fiber glass inlet plenum and Gerotor type pump elements. Many of thesefeatures became standard design practices on future units. It was a very gratifying experience.

4. I wish to thank Kyle Holquist and Kathy Phoenix for their help in obtaining photographs. Ionly wish that the photo file was readily available to folks who wish to do research on the historyof many projects that will ultimately be lost.

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