Project 1 - I‑16 Tailpipe Re-heat: This would involve designing, engineering, manufacturing and testing at least two full-size tailpipe configurations, six full-size burners of varying design and two full-size variable nozzles to different design with controllable adjustment capability. The General Electric I‑16 would be furnished to Solar by the U.S. Navy. Flight testing would be with a U.S. Navy provided and maintained airplane in the San Diego area. Solar personnel would direct and assist in the installation of the equipment. Based on test results, Solar would manufacture and deliver to BuAer four re-heat tailpipe assemblies incorporating the best design features previously developed and which fit an aircraft designated by BuAer for service tests. Contract completion would be twelve months from date of contract. Proposed price was $145,000.00.

Project 2 - Westinghouse 24-C Combustion Chamber Development: Develop a combustion chamber for the Westinghouse 24C jet engine which will operate with the required performance up to 35,000 feet and have a service life of 150 hours. To accomplish that, design and manufacture at least six full-size combustion chambers of different designs with various modifications and test all six up to 35,000 feet altitude in simulated conditions. Upon completion of development, manufacture and deliver four combustion chambers incorporating the best features and which are interchangeable with the standard Westinghouse combustion chamber for the 24C engine. Contract completion would be twelve months from date of contract. Proposed price was $158,000.00

The detailed price breakdowns for both projects were submitted on 28 February 1946 and for the combustion chamber work added the assumption that one standard combustion chamber designed for the 24C would be furnished by the Bureau so that comparative data could be established.

In the end, BuAer decided to contract only for the tailpipe burning contract and lowered the material deliverables at contract end to be four tailpipe assemblies built to the final design. Contract NOa(s) 8203 was put in place. All Solar testing would be done on the ground with technically qualified personnel to be furnished to assist, at a location designated by BuAer, in flight tests of the Solar tailpipe burning assembly if requested. The cost plus fixed-fee BuAer contract NOa(s) 8203 was dated 28 June 1946. All reports, drawings, final report and the four assemblies were to be submitted not later than one year from the date of the contract. Solar was to use their best efforts to develop a tailpipe assembly which was capable of producing a minimum thrust value of 150% of the military thrust of the I‑16 furnished versus its normal tailpipe. The estimated cost with fixed fee was $128,399.26.

An I‑16 that was overhauled and flight ready was ordered shipped to Solar on 1 August 1946. Shortly thereafter, Solar published their mathematical analysis for reheat based on the assumed performance of the I‑16. Using their derived formula, they could calculate the estimated performance of reheat on the I‑16 across a range of reheat temperatures. Note: Celsius° = Rankin° – 491.67

7 August 1946: First Progress Report for July 1946

It was estimated the preliminary calculations for the tailpipe reheat system for the I‑16 were about 15% complete. These (see Figures 1 and 2) were based on unofficial and estimated performance of the I‑16 as information was still forthcoming from the BuAer Representative (BARR). When compared to the procedure listed in BARR Report M-54, discrepancies were appreciably less than one percent. Based on the results, it was thought probable that an actual application reheat combustion or diffuser exit velocity of more than 300 feet per second would be difficult to achieve.

It was anticipated the basic design of the reheat tailpipe would be completed in the next four or five weeks. Two tailpipe configurations would be made, these utilizing two different diffuser divergencies in order to ascertain the effect of the wide-angle divergence on diffuser efficiency. If a high degree of divergence could be used in the diffuser section, the overall tailpipe could be made considerably shorter or the effective combustion space increased.

Small scale combustion tests were underway to investigate some of the problems involved in developing a suitable burner. No usable results were available as yet but tests were continuing.

Specific heat and enthalpy for combustion gases expected in the reheat tailpipe were calculated from the best available data and were included in the calculations data used to generate Figures 1 and 2. (Note: Enthalpy is a thermodynamic quantity equivalent to the total heat content of a system. It is equal to the internal energy of the system plus the product of pressure and volume.)

6 August 1946: Solar wrote to BuAer stating they wished to contract with General Electric (based on discussions with GE) to develop an afterburner for the higher thrust I-40 engine if the Bureau had no objection. They pointed out the benefits of dual development to the Navy, as expenses were likely to be lower overall given common tasks between the current contract and a new one, lowered delays as a result of more efficient use of the labor between the two contracts and the objective of developing a basic afterburner design that could be applied to other engines would more likely be achieved.

16 August 1946: Solar requested a standard tailpipe of the Ryan FR-1 type be supplied for use in calibrating the I‑16 engine provided. This was necessary to validate assumptions and the design of a reheat system. In addition, a standard tachometer indicator and starter control box were needed based on the engine installation intended. The parts were needed ASAP to facilitate the test stand work to obtain data necessary to proceed with the reheat design.

23 August 1946: BuAer responded to Solar’s memo in a telegram stating they had no objections to such a contract with General Electric Allison Division. They asked Solar to consider changing the objective of NOa(s) 8203 from developing an afterburner for the I‑16 to that of one for the Westinghouse Model 24C using the I-40 engine to develop the necessary higher airflow. If acceptable, Solar was asked to submit a proposal based an amendment to NOa(s) 8203 after a contract with Allison was executed. Solar would continue to be able to use the I‑16 engine under the amended contract.

27 August 1947: Solar wrote to the Bureau restating their understanding of BuAer’s intent as stated in the memo and asking that if a contract with Allison was or was not concluded, should Solar submit a proposal to include development of an afterburner for the 24C as well as one for the I‑16 engine.

9 September 1946: Progress Report for August 1946

Design work was now 45% complete based on the I‑16. A proposed design would use a 15° diffuser with a 10 ft overall tailpipe length. The combustion chamber would be three feet in diameter including a burner element of 16" diameter and the combustion chamber section diameter would be 23-3/4". Diffuser exit velocities of 300 feet per second (fps) were planned and thought to be the maximum obtainable for practical for efficient combustion. High temperature materials were being investigated if high temperatures presented a problem. Normal type 347 stainless steel was to be used on the first tailpipes due to its immediate availability. A shroud would be used, beginning after the diffuser section, with cooling air circulated through the shroud from the pumping action of the tailpipe jet.

A burner element intended to eliminate secondary flame propagating devices such as flame holders was to be designed following completion of small-scale tests. Burner development was proceeding in the direction of providing small shielded pilot flames that would propagate a flame front at the fuel orifices. Pre-vaporizing of the fuel would be necessary. A vaporization chamber in the form of an annulus in which the fuel orifices were contained was planned. Early tests appeared promising. Altitude performance could not be calculated until the necessary engine data was forwarded from BuAer.

A scale combustion test stand had been completed and was in use testing burner parts. Spray nozzles from three different manufacturers and of varying capacities were undergoing tests. It was anticipated that testing and results would allow full-sized burner element designs to be completed in September.

18 September 1946: Solar complained in a letter that the thermodynamic and other data on the I‑16 engine which had been requested had not yet been received from the Air Material Command at Wright Field. The data was urgently needed. Since the I‑16 engine furnished was a rebuilt unit, it would be fully calibrated by Solar. The BARR sent a telegram the same day to the Chief of BuAer stating progress on the contract was being delayed for the reasons stated in Solar’s letter.

20 September 1946: Aviation Supply ordered a control box, GE – CR2781-C104B3, to the Vultee Aircraft BARR in San Diego for use on the Solar contract NOa(s) 8203 for I‑16 engine reheat tailpipe development.

3 October 1946: BuAer transferred contract inspection cognizance from the Vultee division of Chance Vought Aircraft Company BARR to the Ryan BARR at Ryan Aeronautical Company in San Diego. All documents and papers related to NOa(s) 8203 were also transferred.

16 October 1946: Solar submitted a suggested contract change to cover converting I‑16 data to a suitable afterburner (AB) design for the 24C. Price would be $8,550.00 plus fixed fee calculated as part of the total modified contract cost.

18 October 1946: Solar requested authorization for four Solar personnel to visit the personnel and test laboratories of the University of Southern California that were doing combustion work in connection with other BARR cognizance activities.

01 December 1946: Progress Report for November 1946

(No progress reports for September or October 1946 were found.) Task 1 was now 80% complete. Design work on Combustion Chambers I, II and III had been completed and released to the shop. The designs of type I and type II were included in the report as pencil drawings.

Chamber III would be a pilot type burner that would not depend on high turbulence. (The drawing is referenced as Type 3 in the report but is missing from the attachments.)

Two tailpipe sizes were planned. One with a combustion section velocity of 300 fps and the other 400 fps. These would investigate the minimum tailpipe diameter required to retain combustion efficiencies. Tailpipe length would be varied by changing the combustion sections of each size pipe to determine the minimum tailpipe length. The larger size pipe design was complete and released to the shop and the smaller pipe was built and already in test.

A variable nozzle design was complete for the larger pipe. It consisted of a movable bullet-type which was to be cooled by air drawn in through the supporting struts and circulated over the inner surfaces. The design used a chain and sprocket drive system chosen for its simplicity and its requirement for smaller strut sizes. Calculations indicated that the maximum skin temperatures should not exceed 1,400-1,500°F at a gas temperature of 3,000°F if a cooling air flow of 1,500 lb/hr was provided. A reversible 1/2 hp motor with a 600:1 gear ratio would be used for nozzle actuation during testing. This type of nozzle might not prove practical for the smaller pipe size due to high pressure losses. As a result, for the smaller pipe, an external nozzle type was being included in the studies and would be complete the following month.

Overall, the configuration was found to be satisfactory with ignition occurring at all RPMs without difficulty. The 16" nozzle, as calculated, should provide a rating in excess of 135% at Military power. Lack of fuel delivery ability required was being rectified to verify this. Combustion efficiency data was not yet obtained to determine a value, but observation and specific thrust values indicated that good results had been obtained.

18 October 1946: BuAer notified Solar that the amendment to add the 24C engine to the contract was in process and also that the contract was now classified as Restricted and that all correspondence should adhere to the requirements related to that classification.

6 January 1947: Amendment 1 was approved. It added to Item 3 the tailpipe design and drawings for the Westinghouse 24C engine to the deliverables of the contract. Item 5 was changed to specify the fabrication and furnishing of four (4) tailpipe afterburning assemblies for the 24C engine instead of the originally specified I‑16 engine. The contract’s estimated cost rose from $122,285.20 to $130,835.20 and the fixed fee from $6,114.26 to $6,541.76.

6 January 1947: Progress Report December 1946

Item 1 - The report and drawings for the preliminary design of the tailpipe afterburning assemblies for an I‑16 engine, was 100% complete. The report was forwarded to BuAer on 2 January 1947.

Item 2 – The tailpipes were in fabrication and assembly and were 30% complete. Tests with the first 300 fps No. 1 tailpipe had been conducted using burner I. The 400 fps tailpipe was almost ready for tests. The variable area nozzle of the bulb type was under construction and would be installed on the 400 fps tailpipe (confusingly referred to as the “No. 1 Tailpipe”. If successful results were obtained, it would be adapted to the smaller tailpipe “No. 2”. Tests would next be done using burners I, II, III and IV with both tailpipes Nos. 1 and 2. Testing had been discontinued for a short period due to making necessary revisions to the test area as a precaution against fire hazards. Testing was again in progress by the end of the reporting period. No substantial test results had been obtained to add to those reported in the November 1946 Progress Report.

17 February 1947: Solar wrote to clarify whether Item 4 of the contract, assisting in the flight tests of the afterburners, should be moved to specify the 24C engine instead of the I‑16. This would eliminate testing on an obsolete engine. The control system contemplated for the I‑16 engine afterburner would likely have to be modified for the 24C engine article. It was recommended that one of the afterburners constructed under Item 5 be tested prior to shipment to identify the changes prior to delivery. The requested changes would not increase the cost of the contract.

24 February 1947: Contract Amendment 2 was issued to correct a typo in Item 4 of Amendment 1 which listed the Total Estimated Cost as $130,855.20. This was corrected to be $130,835.20.

28 February 1947: Contract Amendment 3 was put in process by the BuAer Power Plant Division (PPD). It would delete the current Item 4 entirely and substitute “Item 4 – Material and services for development and performance testing of afterburner as installed on the Westinghouse 24C engine.”  The Section H Government Furnished Material section would delete paragraph (a) and substitute language affirming the government furnishing of one I‑16 engine and one 24C engine. These, upon completion of Item 5, would be required to be preserved against corrosion, crated, and redelivered to the Naval inspector.

4 April 1947: The BARR asked BuAer to clarify which model of the 24C engine Item 5 referred to in the design, development and delivery of 4 tailpipe afterburning assemblies. (Note: The early 24C-2 had a much lower mass flow than the developed 24C-4 and, as a result, the tailpipe design would have to be different.)

11 April 1947: Progress Report for March 1947

Item 1 for the design for the I‑16 was 100% complete.

Delays were experienced in completing the variable nozzles and Tailpipe No. 3 assemblies while awaiting receipt of special high temperature alloys (N-155 and Inconel X) from the supplier. These were Type 347 and 321 stainless steel alloys. In lieu of those, two types of Iconel had been substituted but these had limited all high temperature testing. Further use of the inferior materials was not going to allow a satisfactory practical tailpipe design. The Tailpipe No. 3 was anticipated to be the final design. It would have a maximum normal diameter of 18-3/4" at the combustion end and be approximately 6-1/2' long including the diffuser. Burners No. VII and VIII were variations of the most successful prior designs and were being developed for the Tailpipe No. 3. This tailpipe was to include a short 6" removable section adjacent to the diffuser. This is where the burner element and fuel manifolds would be installed and would allow inspection and removal of the burner. The total estimated weight, including cooling shroud and simple nozzle, was 165 lbs. A clam-shell type variable nozzle had been designed to connect directly to the small tailpipes. These would be actuated via hydraulic cylinders acting on each half shell.

Investigation had concentrated on improving combustion characteristics and finding the source(s) of both low frequency pulsation and high frequency squeals encountered with Burner No. III and Tailpipe No. 2. Sight glasses were used to view the combustion process. No firm conclusions had been reached but the disturbances appeared to be a function of the geometric configuration and, to some extent, the fuel injection rates.

The I‑16 was found to have turbine blade rubs on the outer ring in two places after testing Burner No. III. Improper tip clearances were found. Since only a few hours of AB operation with the engine had been experienced, it was concluded the rubs were not a result of the testing. The ring was machined to the proper size and shape and no further rubbing had been experienced.

Burner No. V was tested in Tailpipe No. 2 and it achieved a corrected thrust of 2,305 lb, which was 137% of normal Military thrust. A skin temperature of 1,710°F was recorded at the 137% thrust value due to improper functioning of the cooling shroud. This would be investigated. The top rating of the burner was not reached due to the material’s temperature limitations. At this point, it appeared the thrust loss due to the AB pipe did not exceed 3%.

Testing procedure for maximum thrust with each fixed nozzle was to increase the reheat fuel flow until the turbine inlet temperature reached the values obtained in the calibration of the engine with the standard Ryan FR-1 tailpipe. Fuel consumption data had not been reliable but now fuel flow rotameters of sufficient capacity had been obtained. The bulb type variable nozzle (Fig. 14) was almost ready for test on the larger tailpipe. (Note: Report says Tailpipe No. 1 but might have meant No. 2 or 3.) Actuation would be by reversible electric motor.

15 April 1947: A Solar memo of 13 March 1947 reminded BuAer the contract only included manual controls on the four ABs being produced. BuAer responded stating that manual controls for actuating the variable area nozzle and for controlling the fuel flow would be acceptable for the four experimental ABs being procured under the contract. They went on to advise Solar that specification for future AB controls would require that AB operation be automatically controlled so that the maximum allowable turbine discharge temperature would not exceed the maximum allowable engine turbine discharge temperature under any and all conditions of operation. This required an automatic control for the fuel-air ratio or the variable area nozzle.

15 April 1947: BuAer memo stated that an engine model X24C-2 (no serial number) completed an overhaul and was shipped to Solar on 4 April for use in the AB testing program. Factory acceptance test showed a thrust at Standard Sea Level conditions of 2,625 lb at 12,000 rpm.

30 April 1947: BuAer instructed Solar to specifically design their AB for the 24C-4 engine. All information required for the completion of the design for a 24C-4 was forwarded to the BARR for their use. However, due to the unavailability of a 24C-4 engine, a 24C-2 had been shipped to them for AB testing.

11 May 1947: Progress Report for April 1947

Item 2 was now reported as 75% complete.

Components completed and in test now included the Variable Nozzle (Bulb type) and Burner No. VI. Components designed and in process of fabrication remained the same less the Bulb Type variable nozzle and Burner No. VI items.

The receipt of the awaited high temperature alloys had allowed fabrication of the last four components to begin.

The Bulb Type nozzle was positioned by energizing the reversible DC electric motor through a three-position switch on the control panel. The pointer type indicator in the picture was calibrated for the approximate equivalent diameter setting of the nozzle. Thermocouples imbedded in the supporting struts indicated maximum safe temperatures and determined the effectiveness of the open-ended bulb as a cooling air pump. The weight was 85 lb including the motor and part of the combustion section.

Tests during the month had included Burner No. IV as a vapor pilot burner, same modified as a simple gutter burner and tests on the bulb type Tailpipe No. 1. The early tests on Burner No. IV showed that the vapor pilot system would ignite easily and quickly and in turn smoothly ignite subsequent injections of the main (liquid) fuel. Burning was experienced inside the distribution manifold at Military speed. Adding leading edge fuel to cool the manifold indicated that leading edge injection alone would provide similar ignition ease with greater simplicity.

Additional tests were run with various modifications in an effort to determine the parameters of burner design with the aim of seeking the optimum performance and operation relationship between burner friction loss, tailpipe velocity and fuel injector location. Though most observations were general in nature, a point was reached in increasing the burner friction loss where additional burner friction loss adversely affected the thrust performance.

Initial tests of the bulb type nozzle were conducted in April. Initial test mechanical operation was satisfactory but upon shutdown the drive sprocket shaft seized in the bearing block. After repairs, satisfactory performance resumed without further difficulty. Future testing would aim to determine the performance characteristics and determination of the cooling requirements of the nozzle for reliable operation and reasonable service life.

Standard calibration tests were complete on the 12.5", 14.0", 15.0", and 16.0" diameter fixed nozzles. The results would be used for performance comparisons. Other tests results would be included in the next report. Burner No. IV-L had shown a 138% augmentation on the 16.0" with a fixed nozzle. Thrust loss with no burning was not expected to be greater than 2%.

Eleven (11) charts were attached showing summary of test runs versus engine speed using Burner IV-L.  The present system of nozzle cooling had held the strut temperatures below 1,540°F and no warpage or binding had been encountered. After initial tests, a 16" length of pumping tube was added to the bulb to obtain the present cooling performance. Air bleed from the compressor would be added to keep nozzle and strut temperatures within limits when higher values of burning were attempted. The control system was being studied and methods of control were pending further ignition and variable nozzle tests.

11 June 1947: Progress Report for May 1947

Item 2 was now 90% complete and the only item not completely fabricated and in test was the modified cooling shroud section.

During the month, the bulb type variable nozzle tests were completed and the I‑16 compressor was cleaned to restore thrust. Burner No. VI had produced a 143% rated augmented thrust and smoothly ignited at full engine speed with the 16.0" diameter nozzle. Nozzle skin temperatures were reduced to about 1,200°F during afterburning during the cooling shroud tests.

Tailpipe No. 3 was 60" long (compared to the 147" of the Ryan FR-1 standard tailpipe, which weighed 117 lb.) Future weight reductions in the clamshell nozzle were to be expected as the first version underwent tests and design revisions. The alternate fixed nozzle section of the Tailpipe No. 3 was about 12 lb.

11 July 1947: Progress Report for June 1947

The BARR reported that he considered the progress on the contract to be satisfactory with the quality of the product being excellent. Afterburner No. VIII appeared to be especially practical from the standpoint of replacement, inspection and maintenance. It appeared to be the simplest design encountered thus far in the two (Ryan being the other) afterburner programs under the cognizance of his office.

Items 1 and 2 were reported as 100% complete with one addition from the prior month, Issue 3, Film cooling nozzle section. This last was tested on Tailpipe No. 2.

In a testing summary, Tailpipe No. 3 had been tested with Burner No. VIII and produced 138% rated AB thrust and only 1.5% thrust lost when the AB was not operating (dry). The clamshell variable nozzle was tested in three gas leakage conditions and with no leakage there was no loss in thrust. Smooth full speed (Military) (16,500 rpm) ignition was obtained with Burner No. VIII and the 16.0" nozzle. The possibilities of shorter combustion chamber length were investigated with combustion section lengths varying between 0 – 30".

The I‑16 engine was removed after 29 hours of operation, the Westinghouse 24C-2 was then installed and instrumentation of it begun. The cooling shroud tests were performed and the results were presented on the charts with the rpm corrected to 16,100 for standard conditions. Skin temperatures were reduced by only 50°F with the limited air available from the existing compressor taps and piping arrangement, but increased air flow through shorter combustion lengths was expected to improve the results.

Following the cooling shroud tests, Tailpipe No. 3 designed for 500 fps diffuser exit velocity was tested. The summarized significant performance results were:

Fig. 26. Burner No. VIII in June 1947

During some tests with the AB operating, roughness was found to have caused cracking of the small fuel manifold tubes. It was expected that high-temperature alloy tubes would eliminate the problem in future burners. Ignition tests with Burner No. VIII experienced smooth ignition at all speeds up to Military using the 16" nozzle.

Modified I‑16 spark plugs had been used in all tests to date except on Tailpipe No. 3 where a modified Champion C35S shield-type spark plug was used. The internal construction of this plug appeared to give better service life and reliability under high temperature conditions. The plug was also modified to protrude a minimum distance above the tailpipe surface to a wider variety of airplane installations without interference.

The clamshell nozzle actuated using two 1.0" compressed air piston type actuators operating at 135 psi and supplied through small 1/4" copper lines. Each piston took only 40-45 psi to operate. Actuation open or closed took less than 1.0 second. There was no binding or warpage. During wet operation, the inner conical skin, shrouded by the spherical ball, reached 1,800 – 2,000°F for short periods without damage to the N-155 high temperature alloy used for the part. Future designs would be two-position; sealing against gas leakage when in the closed position and permitting cooling air to flow over the nozzle skin when in the open position.

Best performance was obtained with the 30" combustion section (standard design), although assemblies as short as 6" gave good flame distribution and stable combustion without blowout. Results indicated a reduction in the overall combustion length to at least 1.5 diameters could be expected with burners designed specifically for reduced length conditions.

Burner No. VIII (Fig. 26) was designed as an integral part of a short compact section of the tailpipe. It consolidated the flame holder, fuel injection rings and spark plug within one removable unit. This was done to facilitate present development, future service, maintenance and repair.

15 July 1947: Solar wrote to BuAer and pointed out that because of the revision of the contract under Amendment 4, the development and tests of the 24C tailpipe were in Item 4 and these would be completed by 28 October (or 16 months after the date of contract). They asked that the delivery date of the Item 3 items (final design data and drawings) be moved out three months to coincide with the Item 4 delivery date. As design work progressed, drawings covering preliminary tailpipes for the 24C would be forwarded for approval.

Fig. 27. Nozzle Size Versus Temperature.	Fig. 28. I‑16 Reheat Performance

24 July 1947: Solar submitted their “Report on Design Data and Performance Calculations of Proposed Afterburning Reheat Tailpipe Assembly for I‑16 Turbo Jet Engine” in fulfillment of Item 1 of the contract in accordance with the original 12 month delivery date. The report was 74 pages long and consisted of an Introduction, Performance Calculations: Engine Data and Assumptions, Procedure of Calculation, and Analysis at Sea Level and Altitude. Seven tables and 15 figures were included (several appear here).

As a result of their calculations they “concluded that the degree of diffusion, otherwise stated as combustion section approach velocity, affected the reheat thrust only slightly at the same reheat temperatures, not considering the effect on combustion. It was further concluded that in order to stay within reasonable diffuser lengths, the rate of diffusion should be within 10° to 15° included angle, with which reasonably good diffusion could be expected.”

“Regardless of the tailpipe configuration chosen, it is obvious from Figure 11 (See Fig. 27) that a variable area jet nozzle is mandatory. The latitude of adjustment required is little affected by diffuser velocity, but is mainly a function of reheat temperature.”

The altitude performance charts were done for sea level, 20,000 and 40,000 feet. A single diffuser discharge velocity of 300 fps was used and a reheat thrust of 130% of the Military rating of the I‑16 engine. These were considered the probable minimums limits obtainable. All of the calculated data and assumptions for each performance point were included to support the tables.

Two reheat designs (Tailpipes 1 & 2) were proposed, one with a 23.75" combustion section velocity of 300 fps. The second used a 20.75" combustion section with a velocity of 400 fps. The first was 10 feet long and the second somewhat shorter due to use of a diffuser somewhat shorter in length because of the higher velocities in that design.

The nozzles were chosen to be a series of a plain, detachable type that would be easy to manufacture. The sizes selected and their corresponding reheat conditions were:

Tailpipe surface distortion had been effectively prevented using rods inserted through streamlined vanes to the inner cone. A rigid connection was made to the outer cone.

Overall heat protection for the airframe was provided by lightweight stainless steel sheet surrounding the tailpipe with cooling air drawn in by the augmenting action of the jet nozzle. The calculated data were considered conservative. It was considered that about 2 lb/sec of air would provide ample cooling to prevent excessive shroud and tailpipe temperatures of 3,000°R adjacent to the skin boundary layer.

As reported earlier, Inconel was not used in the first tailpipes due to its lack of availability. Inconel 22 and Type 25-20 with silicon were planned to be used for the variable jet nozzle which would have to endure the most severe service.

The Variable Nozzle design was complicated by the pressures in the tailpipe being considerably above atmospheric (about 5 psi at sea level), this caused leakage at sliding or ordinary hinged areas. Sealing would be complicated further by the wide range of temperatures that needed to be accommodated. Two approaches recommended themselves. The first was to provide a means of changing the external perimeter of the jet opening and the second was to position a streamlined body internal to the jet opening. The first offered minimum working temperatures of the parts and the second avoided sliding and hinged joints but had to tolerate very high temperatures as well as needing support members that added internal drag and would cause thrust lost when the AB was not operating. Figures 29 – 34 depict possible variable nozzle drawings that were included of possible solutions.

Fig. 35. Jumo 109-004 Bulb Nozzle Drive. It is not known if the Solar engineers were aware of the design of the 109-004 when their report was written. (Photo by author.)

Proposed Design: It was concluded that the simplest approach to the nozzle design was to use a bulb type design (Fig. 32) provided it could be properly cooled. It was appreciated that for high velocity tailpipes of small diameter it might be impractical because of high internal drag. It was considered that the struts might be eliminated and the bulb suspended (Fig. 34) from above. Cooling in the first instance would be through the hollow struts with air produced by the augmenting action of the nozzle. If this was insufficient, compressor air could be tapped. This air would be discharged out of the opening in the bottom of the bulb in the plane of the nozzle end. Movement of the bulb in and out would be facilitated by three pairs of rollers riding on the tubular supports. Several packing layers of braided Inconel would be used to seal the forward end of the bulb from leakage of the hot gases. Actuation, as illustrated in Figure 32 or would be by sprocket and chain with a drive shaft extension (see Fig. 5) through one of the supports. This drive was selected to minimize strut size for minimal internal drag. (Note: German engineers at Junkers used such a bulb type variable nozzle in the Jumo 109-004 engine with the drive mechanism being almost identical, the main variance being that it was driven from the engine control via a shaft running parallel to the outside of the engine through a 90° gearbox to a rack and pinion instead of using a sprocket and chain (Fig. 35). A weakness was that the sheet metal bulb itself could (and occasionally did) break away from the drive shaft and block the nozzle exit causing a sudden flameout and very high drag. At low level this was lethal to the pilot.)

The material in the design would be stainless steel Type 25-20 with silicon modification. Pressures against the bulb would be in the order of 1,100 lb. For a total travel within five seconds, a 1/3 hp motor would be needed. A standard 10,000 rpm, 24-volt electric motor that could provide a gear reduction of 600:1 would suffice.

Burners: Tests showed that simple burners generally performed better than complex designs. Two successful approaches to burner performance were found to work. The first was to use a conventional flame holder utilizing turbulence producing devices in which flame is continuously propagated from the local stagnated areas of eddy current. Such zones could be created by various modification of U-shaped gutters placed in the air stream. This shape produced the lowest pressure loss (low drag) for a given cross section area of the flame holder. High combustion rates were practical but incurred pressure losses of excessive magnitude. Pressure losses could be reduced by reducing the burner solidity, but this increased the degree of fuel stratification and, more critically, increased the fuel rate per square inch of burner cross section. Other penalties were a longer flame length, lower combustion efficiency, and a greater blowout tendency.

The second was to use a pilot flame generated with ideal fuel/air mixtures within a shielded burner such that it was virtually unaffected by surrounding conditions and was able to provide a stable, constant ignition source to fuel/air mix as it passed the burner element. Lower solidities were possible with a reduction in pressure loss and allowing a reduction in possible tailpipe diameter.

Fig. 41. Reheat Combustion Model Test Stand

Fuel Injection: It was noted that at the combustion temperatures in the vicinity of 1,200°F the time to vaporize liquid fuel after injection is almost negligible, providing that the orifices are small and the fuel was sprayed in such a manner as to take advantage of the natural atomizing effects of the air or impingement against hot surfaces. Full vaporization was important from the standpoint of fundamental considerations of the combustion process. Model tests with an identical burner element using both pre-vaporized and solid fuel injection found little difference in combustion efficiencies and flame lengths at maximum fuel capacities of the burner. Pre–vaporization had resulted in the cracking of the fuel which rapidly formed thick, heavy deposits or coke, these reducing capacity and effectiveness of the vaporizer. Since pre-vaporization of the fuel involved complications of providing heat transfer surfaces, Solar proposed to use liquid fuel injection throughout the reheat system unless early full-scale development proved it to be unsuitable.
Blowout: This was found to be caused by the interrelation of two effects. First, the local velocities exceeded the propagation velocities of the flame, which in turn is dependent on the fuel/air mixture. Second, most apparent in shielded zones, high fuel rates appeared to cause a quenching action on the flame, which again is dependent on the fuel/air mixture.
Fuel: In the same burner, kerosene was generally found to burn as effectively as gasoline, but with a longer flame length.

24 July 1947: Solar submitted their Final Summary Report on Design, Development, and Ground Testing of a Tailpipe Afterburning Assembly to Boost the Military Thrust of an I‑16 Turbo-Jet Engine. This was to fulfill the requirement of Item 2 of the contract on the original 12 month schedule. It encapsulated all the results of testing to date on the I‑16.