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Allison J71
Allison J71
Allison J71
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J71
J71 engine change in progress on a F3H-2 Demon, 1963.
Type Turbojet
Manufacturer Allison Engine Company
First run 1950
Major applications B-66 Destroyer
F3H Demon
Developed from Allison J35

The Allison J71 was a single spool turbojet engine, designed and built in the United States. It began development in 1948 as a much modified J35, originally designated J35-A-23.[1]

Operational history

[edit]

The Allison J71 turbojet powered the Douglas B-66 Destroyer and the McDonnell F3H-2 Demon after the failed Westinghouse J40 proved unworkable. The prototype P6M-1 SeaMasters were also fitted with the engine.

Variants

[edit]

Data from: Aircraft engines of the World 1953[2]

J71-A-1
J71-A-2
Powered the McDonnell F3H Demon
J71-A-2B
J71-A-2E
9,700 lbf (43.15 kN) thrust (14,000 lbf (62.28 kN) thrust with afterburner), for the McDonnell F3H-2 Demon.
YJ71-A-3
7,000 lbf (31.14 kN) thrust (9,500 lbf (42.26 kN) thrust with afterburner)
J71-A-4
Afterburning turbojet engines for the Martin XP6M-1 Seamaster flying boat prototypes.
J71-A-6
Afterburning turbojet engines for the Martin YP6M-1 Seamaster pre-production flying boats.
J71-A-7
14,000 lbf (62.28 kN) thrust with afterburner
J71-A-9
Powered the Douglas RB-66 Destroyer
J71-A-11
10,200 lbf (45.37 kN) thrust
J71-A-13

Specifications (Allison J71-A-2)

[edit]

Data from Aircraft engines of the World 1957[3]

General characteristics

  • Type: afterburning turbojet
  • Length: 284.5 in (7,230 mm)
  • Diameter: 39.5 in (1,000 mm)
  • Frontal area: 8.5 sq ft (0.79 m2)
  • Dry weight: 4,890 lb (2,220 kg)

Components

  • Compressor: 16-stage axial compressor
  • Combustors: cannular with 10 flame tubes
  • Turbine: 3-stage axial
  • Fuel type: JP-4 / aviation kerosene
  • Oil system: pressure spray with scavenge at 10–60 psi (69–414 kPa)

Performance

See also

[edit]

Related development

Related lists

References

[edit]

Further reading

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Allison J71

View on Grokipedia
from Grokipedia
The Allison J71 was an American single-spool axial-flow turbojet engine developed by the Allison Engine Company, featuring a 16-stage compressor, single annular combustor, and three-stage turbine, with production totaling 1,707 units before the program ended in the late 1950s due to market competition. Initiated in 1949 as the company's first turbojet designed entirely in-house—drawing on prior experience with the General Electric J33 and J35 engines—the J71 originated as a redesign of the J35 (initially designated J35-A-23) to offer similar dimensions for retrofit compatibility while competing against rivals like General Electric and Pratt & Whitney. Key specifications included a maximum thrust of 13,000 lbf (57.8 kN) at 6,100 rpm in its YJ71-A-4 variant, a dry weight of approximately 4,476 lb (2,030 kg), a length of 512 cm, and a of 100 cm; variant thrust ranged from 9,700 lbf dry (with up to 14,000 lbf using in the J71-A-2E model) to 10,200 lbf in the J71-A-11. The engine powered several military aircraft and projects during the early Cold War era, including the fighter, bomber, Martin XP6M-1 Seamaster flying boat (in prototype testing), and the early developmental version of the Northrop missile, though limited orders curtailed broader adoption.

Development

Background and Origins

The development of the turbojet engine was initiated in the late 1940s as a heavily modified version of the , with early work focusing on scaling up its design for greater performance. Originally designated as the J35-A-23, the project involved significant redesign efforts, including an increase in stages from the J35's 11 to 16 axial stages, to handle higher airflow rates. This redesignation to J71 occurred as the engine evolved into a distinct model under Allison's lead. The primary motivation behind the J71 was to produce a larger, more powerful successor to the J35, meeting the U.S. military's post-World War II demand for advanced turbojets capable of powering emerging high-performance aircraft programs. In the immediate aftermath of the war, the need for engines with substantially higher thrust became critical as the U.S. shifted focus to for both and applications, addressing limitations in range, speed, and of earlier designs. Allison aimed to deliver an engine that could support these requirements while improving overall efficiency and reliability. This project marked Allison's transition from primarily producing licensed designs—such as the General Electric J33 and J35, which originated from GE but were manufactured and uprated by Allison—to developing a wholly original . Drawing on extensive experience gained from enhancing the power output of the J33 and J35 through modifications like improved and afterburners, Allison leveraged this expertise to pursue independent innovation. A key early decision was retaining a single-spool architecture, which simplified the design while allowing the 16-stage axial to achieve the desired airflow and thrust gains over the J35.

Design and Testing Phase

The development of the Allison J71 turbojet engine progressed from initial prototyping in the late to production readiness in the early , marking Allison's first fully independent design after uprating earlier models like the J33 and J35. The prototypes featured a 16-stage and measured approximately 16 feet in length, emphasizing enhanced reliability through scaled-up airflow and structural improvements over the J35 to support higher levels exceeding 10,000 lbf. The first engine run occurred in , with the basic design essentially complete by that year, allowing for rapid iteration toward military applications. Early ground testing took place at Allison's facilities in , focusing on core performance and integration compatibility, followed by altitude simulations at NASA's Altitude to evaluate behavior under simulated flight conditions up to 40,000 feet and Mach 0.8. Flight integration trials began in the early , including test installations in a modified B-45C bomber at , where the engine demonstrated stable operation but revealed challenges such as compressor stalls during acceleration. Additional evaluations addressed environmental sensitivities, including water ingestion issues in that damaged and turbine blades, prompting refinements tested through 1960. Innovations during this phase centered on compressor refinements to improve efficiency and mitigate stall risks, such as increasing the first stator throat area by adjusting blade stagger angles from 97% to 132% of original design, which boosted weight flow by 13.6% and expanded the engine's operational temperature ratio by 27.5%. These changes enhanced acceleration and overall flexibility without altering the single-spool configuration. U.S. military acceptance came in the early following type testing that validated afterburner integration and durability, enabling initial production within roughly two years of the first run; the program transitioned to full-scale manufacturing for aircraft like the and Douglas B-66 by mid-decade.

Technical Design

Core Engine Components

The core of the Allison J71 engine consists of a single-spool axial-flow , integrating a high-pressure , annular , and on a common rotating shaft to deliver efficient generation for high-performance . This architecture, developed in the late , emphasized reliability and performance under demanding operational conditions, with the spool rotating at a rated speed of approximately 6,100 rpm. The is a 16-stage axial-flow unit, designed to achieve a suitable for applications while maintaining stable operation across a wide speed range. It features two-position guide vanes, which help optimize incidence and prevent surge during acceleration or low-speed conditions. Additionally, compressor acceleration bleeds, which open below 5,300 rpm and close above that threshold, further mitigate risks by bleeding off excess air to reduce backpressure. The handles an capacity of approximately 160 lb/s at rated conditions (6,175 rpm and 1,240°F turbine-outlet temperature, sea-level static). Downstream of the , the employs a cannular configuration with 10 circular through-flow inner liners arranged annularly, promoting even fuel-air mixing and stability. This design minimizes pressure losses while ensuring reliable ignition, with fuel injected via single inlets to each liner for controlled burning at high temperatures. The cannular setup balances the simplicity of individual flame tubes with the compactness of an annular chamber, contributing to the engine's overall efficiency and durability. The is a three-stage axial-flow assembly that extracts energy from the hot combustion gases to drive the compressor spool. Positioned at the rear of , it operates at elevated temperatures, with the design incorporating features to manage thermal loads for sustained performance. In the single-spool integration, the and are rigidly connected via a central shaft, enabling synchronized rotation and compact packaging. This configuration relies on precision-engineered bearings and seals to support the high-speed shaft (up to 6,100 rpm) while containing lubricants and minimizing losses, ensuring the core's mechanical during prolonged operation.

Afterburner and Accessory Systems

The afterburner system of the engine incorporated a variable-area tailpipe to facilitate reheat operation, enabling efficient expansion of the combustion gases for augmented . This design supported selective activation through dedicated nozzles that sprayed additional fuel into the turbine exhaust stream, where flame holders stabilized the combustion process. Ignition was achieved via integrated systems that allowed for on-demand reheat, contributing to a increase of approximately 40 percent over dry power ratings—from 10,000 lbf dry to 14,000 lbf with engaged. The J71 offered variants with long and short configurations, the former optimized for takeoff and low-altitude performance, while the latter suited high-altitude applications. Accessory systems were housed in a dedicated section forward of the , encompassing essential pumps and auxiliary components for sustained operation. Fuel pumps delivered metered flow to both the main and , while an integral oil lubrication circuit circulated fluid through bearings and gears to manage thermal loads and wear. Starter mechanisms employed an air-turbine motor, often powered by an external Solar gas-turbine trolley during ground operations, ensuring reliable engine spool-up. Electrical and hydraulic interfaces in the accessory drive provided power for engine controls and aircraft integration, including variable actuation. The engine's featured bifurcated annular air ducts with adjustable guide vanes to optimize airflow at varying flight conditions, complemented by automatic screens for anti-icing and foreign object protection. At the exhaust end, a continuously variable iris-type served both primary and secondary functions, adjusting area to match heat addition and minimize tailpipe losses, which were estimated at 6-8 percent under normal conditions. These systems were refined through extensive altitude testing, addressing integration challenges for like the .

Operational Use

Introduction and Primary Applications

The Allison J71 turbojet engine entered U.S. military service in the early , emerging as a reliable axial-flow powerplant to address deficiencies in competing engines like the , which had failed to deliver promised performance in key aircraft programs. Developed by the starting in 1949, the J71 provided approximately 10,200 pounds of thrust, enabling its selection for demanding carrier-based and strategic missions where consistent power output was essential. By the mid-, Allison had produced around 1,707 units before phasing out the program in the late . The J71's primary applications began with its integration into the McDonnell F3H-2 Demon, a U.S. Navy carrier-based all-weather fighter, where it replaced the J40 in later production models starting with the prototype's first flight in the summer of 1955; this redesign allowed the Demon to achieve operational viability for tactical interception and strike roles. Similarly, the engine powered the Douglas B-66 and RB-66 Destroyer series for the U.S. Air Force, with the reconnaissance variant's initial flight occurring on June 28, 1954, supporting light bombing and electronic reconnaissance in tactical scenarios. The J71 also equipped prototypes of the Martin XP6M-1 Seamaster, a U.S. Navy jet-powered flying boat intended for long-range and , whose first flight took place on July 14, 1955. These initial adoptions highlighted the J71's role in enhancing performance for U.S. Navy and operations, particularly in carrier operations and missions, where its thrust reliability proved advantageous over earlier underpowered alternatives.

Service History and Challenges

The Allison J71 turbojet engine entered operational service in the mid-1950s, powering key U.S. military aircraft during the transition from the era into the early period. It was primarily deployed in the fighter, which began fleet operations in March 1956 with squadrons like VF-14, conducting naval carrier-based missions until its phase-out in August 1964. The engine also equipped the family, including reconnaissance and electronic warfare variants, which supported intelligence gathering and saw extensive use in the , particularly the EB-66E for radar jamming missions over from 1966 onward. However, the J71's adoption was curtailed by the 1959 cancellation of the maritime patrol flying boat program, after only prototypes had incorporated the engine, due to shifting priorities toward ballistic missile submarines and budget constraints. Despite its deployment successes, the J71 faced notable reliability challenges, particularly in high-altitude operations where thrust reductions and stalls were common, necessitating frequent maintenance overhauls and inspections after just 100 hours of use. In the , the engine's integration—originally designed around the underperforming —triggered congressional investigations into naval procurement practices following multiple accidents and performance shortfalls with the J40, though the J71 retrofit improved overall stability and to 9,500 pounds dry (14,400 with ), mitigating some earlier issues but not eliminating and inlet problems. For the B-66, the J71-A-13 variant offered marginal reliability gains over predecessors but lagged behind alternatives like the , leading to serviceability limitations and early flutter/control difficulties during testing. These issues contributed to higher operational costs and delayed full for affected platforms. The J71 was gradually retired by the late as more advanced engines like the J57 and J79 superseded it in performance and reliability, with F3H squadrons transitioning to the F-4 Phantom by 1964 and B-66 bombers phased out around 1965, though EB-66 variants persisted in electronic warfare roles until their final missions in December 1973. Its legacy endured in research applications, including extensive testing in NASA's during the 1950s to evaluate maturation under simulated flight conditions up to 65,000 feet. Overall, while the J71 bridged critical gaps in early jet aviation, its challenges underscored the rapid evolution of technology during the era.

Variants

Production Models

The Allison J71 entered production in the early at the company's facilities in , with a total output of approximately 1,707 units across all variants before the program concluded in the late . These engines were primarily destined for U.S. Navy and aircraft, emphasizing reliability for carrier-based and reconnaissance operations. The J71-A-2 served as the baseline production model, selected to power the McDonnell F3H-2 carrier-based fighter after the original engine failed to meet performance requirements. It delivered 9,700 lbf (43.1 kN) of dry thrust and 14,000 lbf (62.3 kN) with , enabling the to achieve supersonic speeds in level flight. Approximately 300 J71-A-2 engines were produced to equip the 292 F3H-2 and F3H-2M Demons that entered service, with total J71-powered F3H variants exceeding 400 aircraft. Subsequent models included the J71-A-9, which powered early reconnaissance variants of the Douglas RB-66 Destroyer in the mid-1950s. This variant featured a thrust rating of approximately 9,600 lbf (42.7 kN) dry, with production focused on addressing operational reliability issues observed in initial flight tests. The J71-A-7 and J71-A-11 represented incremental enhancements for later F3H blocks and the RB-66B, incorporating minor modifications for improved durability and high-altitude operation; the A-11 achieved 10,200 lbf (45.4 kN) . These versions powered over 140 RB-66 aircraft, each requiring two engines, contributing significantly to the overall J71 production total.

Developmental and Special Variants

The developmental variants of the Allison J71 turbojet engine were primarily adapted for prototype aircraft and research applications, focusing on modifications for specific integrations and performance evaluations. The YJ71-A-4 served as a pre-production test variant, closely resembling the J71-A-2 but incorporating additional instrumentation for flight trials and a single air intake with a short exhaust duct to accommodate the engine's installation in the Martin XP6M-1 Seamaster flying boat . This configuration powered one of the four engines in the XP6M-1, which conducted its first flight on July 14, 1955, during early testing phases in the mid-1950s. Subsequent adaptations included the J71-A-4 for the XP6M-1 Seamaster prototypes, featuring single modifications aligned parallel to the and buried in wing roots to optimize airflow, though these led to challenges such as excessive heat and vibration from exhaust. The J71-A-6 variant was developed for the follow-on YP6M-1 pre-production Seamaster models, with afterburning capability and nacelles angled five degrees outward to mitigate exhaust scorching on the ; these engines underwent starting in January 1958. Both Seamaster programs were ultimately canceled on August 21, 1959, after extensive 1950s testing revealed technical issues and shifting priorities toward missile-based deterrence, with no transition to full production. Earlier developmental models, such as the J71-A-1 and J71-A-3, featured varying configurations to explore and augmentation, with some employed in ground test stands for component validation. The J71-A-5 was considered for upgrades to the B-47 bomber, including conversions like the YB-47C (also designated XB-56), but was not adopted due to engine reliability concerns and the selection of alternative powerplants. Special uses of J71 variants extended to missile applications, where a modified version powered the Northrop S-62 in prototype testing, and to NASA research, where engines like the J71-A-11 were subjected to altitude testing at the in the 1950s to assess large performance under simulated high-altitude conditions, including airflow, risks, and efficiency. These evaluations, documented in 1952 and 1955, contributed to broader advancements in axial-flow technology during a period of rapid maturation.

Specifications

General Characteristics

The Allison J71-A-2 is an afterburning single-spool turbojet engine featuring an axial-flow design, developed as a high-thrust powerplant for in the mid-20th century. Its physical dimensions include a length of 284.5 inches (7.23 m) and a of 39.5 inches (1.00 m), making it a relatively compact yet powerful unit suitable for integration into fighter and airframes. The dry weight stands at 4,890 lb (2,218 kg), encompassing the core engine, afterburner, and essential accessories without fuel or operational fluids. The engine operates on kerosene-based , a standard military specification for the era that provided reliable ignition and combustion stability under varying flight conditions. Key internal parameters include a pressure ratio of 8:1, which balanced and structural integrity in the single-spool .

Performance Data

The Allison J71-A-2 produced 9,700 lbf (43.15 kN) of dry thrust and 14,000 lbf (62.28 kN) of thrust with under sea-level static conditions, representing a typical augmentation of approximately 44% from the system. Specific consumption for the was approximately 1.0 lb/(lbf·h) in dry mode, reflecting efficient operation at rated conditions, while with it increased to around 2.2 lb/(lbf·h) due to the additional injection for thrust augmentation. Operational limits included a maximum engine speed of 6,100 rpm at military power. Afterburner augmentation efficiency ranged from 80-85%, as derived from combustion temperature rise analyses in test conditions. These performance figures were established through NACA altitude chamber tests and static thrust measurements, highlighting the J71-A-2's balance of thrust and for 1950s-era fighter and bomber applications.

References

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  2. http://www.pilotfriend.com/aero_engines/engine_specs/Allison/4.htm
  3. https://www.fundinguniverse.com/company-histories/allison-gas-turbine-division-history/
  4. https://www.designation-systems.net/dusrm/app1/sm-62.html
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  28. https://www.historynet.com/eb-66-destroyer/
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  34. https://www.globalsecurity.org/intell/systems/rb-66b.htm
  35. http://www.aviastar.org/air/usa/douglas_b-66.php?p=1
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