Dhanireddy R. Reddy

Glenn Research Center, Cleveland, Ohio

70 Years of Aeropropulsion Research at 

NASA Glenn Research Center

NASA/TP—2013-216524

September 2013
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Dhanireddy R. Reddy

Glenn Research Center, Cleveland, Ohio

70 Years of Aeropropulsion Research at 

NASA Glenn Research Center

NASA/TP—2013-216524

September 2013

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Available from

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7115 Standard Drive

Hanover, MD 21076–1320

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Level of Review: This material has been technically reviewed by expert reviewer(s). 
NASA/TP—2013-216524 1 

70 Years of Aeropropulsion Research at NASA Glenn Research Center

Dhanireddy R. Reddy

National Aeronautics and Space Administration

Glenn Research Center

Cleveland, Ohio 44135

Summary

This report presents a brief overview of air-breathing 

propulsion research conducted at the NASA Glenn Research 

Center over the past 70 years. It includes a historical 

perspective of the Center and its various stages of propulsion 

research in response to the country’s different periods of crises 

and growth opportunities. Glenn’s research and technology

development covered a broad spectrum, from a short-term 

focus on improving the energy efficiency of aircraft engines to 

advancing the frontier technologies of high-speed aviation in 

the supersonic and hypersonic speed regimes. This report

highlights major research programs, showing their impact on 

industry and aircraft propulsion, and briefly discusses current 

research programs and future aeropropulsion technology 

trends in related areas. 

Introduction

Ground was broken for the NASA Glenn Research Center

on January 23, 1941, in Cleveland, adjacent to the Municipal 

Airport (now known as the Cleveland Hopkins International 

Airport), and the Center’s rich heritage in air-breathing 

propulsion research began as soon as Glenn began operations 

in 1942 as the National Advisory Committee for Aeronautics 

(NACA) Aircraft Engine Research Laboratory.1 This report 

gives an overview of Glenn’s significant contributions in the 

development of air-breathing propulsion over the last 70 years.

In the 1940s, research that Glenn conducted in support of

World War II included the turbocharging of reciprocating

engines, which enabled the high-altitude flight of the B–29 

Superfortress; the testing of the first U.S. turbojet, which gave

Glenn hands-on experience with early jet engines and made 

 

1

The Center’s name has changed several times over the last 70 

years. The Center began operation in 1942 as the NACA 

Aircraft Engine Research Laboratory (AERL). In 1947 it was 

renamed the NACA Flight Propulsion Laboratory to reflect 

the expansion of the research. In September 1948, following 

the death of the NACA’s Director of Aeronautics, George 

Lewis, the Center’s name was changed to the NACA Lewis 

Flight Propulsion Laboratory. On October 1, 1958, the Center

was incorporated into the new NASA space agency and it was 

renamed the NASA Lewis Research Center. On March 1, 

1999, following John Glenn’s flight on the space shuttle, the 

Center’s name was changed to its present name: the NASA 

Glenn Research Center. 

Glenn the Government’s expert in jet propulsion; and the 

addition of the Icing Research Tunnel (IRT) to significantly 

enhance aviation safety by advancing icing technology. In the 

next decade, Glenn advanced air-breathing propulsion technology through pioneering research in transonic compressors, 

cooled turbines, stable afterburning, and the use of closedcycle Brayton or Rankine conversion systems for space power 

conversion technology. In the 1960s, technologies were 

developed for noise and emission reductions—10-dB quieter 

and 60-percent cleaner—along with unique expertise in wind 

turbine design for power generation. In the next decade, technologies for high-efficiency turbofan engines were developed

under programs such as the Energy Efficient Engine (E3) 

Program. These technologies enabled designs for the highly 

efficient General Electric GE90 and Pratt & Whitney PW4000, 

which are now powering Boeing 777 aircraft. In the 1980s, 

technology advancement included ultraefficient high-speed

turboprops (developed under the Advanced Turboprop Project

(ATP)), with the potential to reduce fuel consumption by 

35 percent. Subsequently, and continuing until the present 

time, Glenn has been developing technologies for ultralow 

noise and emissions and for efficiency for subsonic and supersonic aircraft, including icing technologies for significantly 

increased aviation safety and efforts to overcome barriers to 

high-speed propulsion, such as hypersonic air-breathing 

propulsion applicable for access to space. 

Glenn’s status as the recognized leader in air-breathing 

propulsion technology is evidenced by the fact that it has 

received the prestigious Collier Trophy three different times: in 

1946 for the development of a thermal ice protection system, in 

1987 for the development of the ATP engine, and in 2009 for 

contributions to the Commercial Aviation Safety Team. In 

addition, Glenn researchers have received numerous Research 

and Development (R&D) 100 Awards and patents in recognition of their significant contributions to the advancement of 

aerospace propulsion technology. The following paragraphs 

outline a few highlights of these technology advancements.

Aircraft Engines for World War II 

Information in this report about Glenn’s aeropropulsion 

research from World War II through around 1990 (this section 

through the Emissions section) is based on Dawson (1991). At 

the start of World War II, the primary engines used for aircraft

propulsion were radial reciprocating engines. These rugged 

engines were known for their reliability and fuel conservation, 
NASA/TP—2013-216524 2 

and American aircraft engine companies were reluctant to 

accept radical changes to the existing engine designs. 

However, Europeans, particularly Germans, had begun to 

develop jet propulsion engines as early as 1935 that could fly 

at greater speeds and higher altitudes. To keep up with the 

European technology, the U.S. Government decided to invest 

in aeronautical research at a significant level during World 

War II. By assuming the cost of research and testing, 

Government engine research reduced the risk of innovation, 

determining the technical feasibility of an engine design or 

component before handing the design over to the engine 

companies for development. 

The U.S. Aeronautics Research program was directed by an 

advisory committee, the NACA, which was established by

Congress on March 3, 1915 (Gray, 1948). The NACA 

consisted of 12 prominent members of the U.S. aeronautical 

community who had been selected for their expertise in 

aeronautical science or engineering. 

The respect that the NACA’s careful financial management 

and its significant role in the development of aeronautics 

technology in the 1920s and 1930s had earned among Federal

Government funding authorities essentially ensured the 

organization’s survival through budget fluctuations.

The credit for the NACA’s reputation belongs to George 

William Lewis (1882 to 1948), who served as the Director of 

Aeronautical Research from 1923 to 1947. Under Lewis’

direction in the 1920s and 1930s, the NACA made significant 

contributions to aeronautics. Some of the notable accomplishments include the development of the NACA cowling to 

reduce drag and allow engines to cool more efficiently, 

retractable landing gear, and studies of the effects of

streamlining.

However, by the late 1930s, the NACA seemed to be 

lagging behind Europe in aircraft technology development. A 

special committee on aeronautical research facilities concluded that the United States needed better aircraft engines 

and strongly recommended that an engine research laboratory 

be built in a location accessible to the engine companies. The 

laboratory was to include an altitude wind tunnel (AWT) for 

the altitude testing of aircraft engines, because only three such 

facilities existed in the United States at that time, and none of 

those facilities could test engines at the high flight altitudes 

predicted for the new aircraft. In January 1940, the NACA

Power Plants Planning Committee recommended that the 

research laboratory include (1) an AWT capable of testing at a 

simulated altitude of 30 000 ft (9.144 km) and a wind tunnel 

speed of about 805 km/h (500 mph) and (2) a propeller 

research laboratory. Both proposals were accepted. The added 

capability would enable a full-scale test of the engine, 

supercharger, and propeller—separately and as a unit—which 

could eliminate months of flight testing.

During congressional appropriation testimonies in early 1940,

NACA officials described the role of the proposed new research

laboratory. They responded to engine companies’ concerns that 

large amounts of funding would go to the new Government 

laboratory instead of their own research by stressing that the 

Government would not compete with private industry. The 

NACA further clarified that the charter of a Government

research laboratory was to address fundamental research problems common to the entire industry and to ensure that the 

information was available to all companies in a given field. The 

NACA emphasized that competition within the industry 

prevented an exchange of information, because each company 

had to work independently to solve problems of common 

interest, resulting in an unnecessary duplication of research 

effort. If the engine companies were funded directly by the 

Government, they would focus their research on short-term 

development problems and would neglect fundamental 

problems.

In June 1940, Congress approved the funding for the 

proposed new research laboratory (NASA Glenn) as part of 

the First National Defense Appropriations Act, and the NACA 

formed a blue ribbon committee to select a site for the 

laboratory. The criteria for the new laboratory included the

need for 0.405 km2 (100 acres) of land adjacent to an airport to 

accommodate flight testing as well as ample water for cooling 

and adequate power for the large wind tunnel. In addition, the 

site would need to be accessible to engine companies throughout the country. In July 1940, the Cleveland Chamber of 

Commerce submitted a formal bid. In spite of stiff competition 

from 72 sites in 62 cities, Cleveland was selected on 

November 25, 1940, after negotiations with the electric company resulted in favorable power rates and logistics issues 

were resolved.

On January 23, 1941, ground was broken (Fig. 1). Almost a 

year later (on Dec. 31, 1941), a contract was signed for the 

construction of the Engine Research Building. The building 

was to contain numerous research laboratories to provide a

broad range of research capabilities that could advance 

Figure 1.—Groundbreaking for the NASA Glenn Research 

Center (Dawson, 1991).
NASA/TP—2013-216524 3 

U.S. aircraft engine technology to the level of European 

engines. The planned facilities included multicylinder and 

single-cylinder test facilities, supercharger rigs, and laboratories for research on exhaust turbines, heat transfer, carburation, fuel injection, ignition, automatic controls, and 

materials. Research was officially initiated in May 1942, in the 

recently completed Engine Propeller Research Building, while 

construction continued on other Glenn buildings.

During World War II, as the country’s aircraft engine needs 

intensified, fundamental research assumed a lower priority 

than troubleshooting to solve the problems of engines in 

production. General Henry Harley “Hap” Arnold, Chief of the 

U.S. Army Air Forces, pushed the NACA to work on 

solutions, after concluding that the engine companies were 

responsible for the inferior status of the engine technology and 

that they lacked the ability to advance the technology to solve 

the problems. On October 14, 1942, Arnold issued an official 

directive for the NACA to concentrate on improving existing 

air-cooled piston engines to assist in winning the war.

Prior to Glenn’s construction, researchers at the NACA 

Langley Research Center had considered engine research to be 

inferior to aerodynamics research (conducted in wind tunnels), 

because they believed that advances in air transportation had 

more to do with aerodynamics than with power plants. 

However, combustion research by NACA researcher Cearcy 

Miller led to a method of predicting exactly when an engine 

would begin to knock—a problem common to all air-cooled 

radial engines. Miller’s success served as an example of how 

the NACA could carry out fundamental research to address a 

common problem and put this knowledge in the hands of the 

industry designers. The NACA also addressed several other 

fundamental problems, such as the use of fuel additives to 

increase the performance of combat aircraft, the erosion of 

engine cylinders because of sand ingestion, and the oil foaming responsible for oil draining during flight—demonstrating 

the value of fundamental precompetitive research by a 

Government laboratory.

Glenn research on the exhaust gas turbine, or turbosupercharger, was crucial to enhancing the horsepower of an 

aircraft engine. Adding turbosuperchargers to reciprocating 

engines enabled the high-speed, high-altitude flights of the 

B–17 Flying Fortress and the B–29 Superfortress (Fig. 2). The

NACA considered knowledge to be the end product of a 

research laboratory: research reports were emphasized, and 

Glenn had high editorial standards that mirrored those of the 

Langley Editorial Office. A great deal of attention was given 

to ensure that the reports were accurate and well organized. If

these guidelines had not been followed, the innovations 

proposed by the researchers after laborious testing might never 

have been reported.

Figure 2.—B–29 Superfortress.

Gas Turbine and Jet Propulsion Research

The gas turbine and jet propulsion concept for aircraft was 

pioneered in Great Britain and Germany simultaneously in the 

1920s and 1930s and led to the emergence during World 

War II of a military jet that revolutionized aerial warfare. Sir 

Frank Whittle, a British Royal Air Force Officer, obtained the 

first gas turbine/jet propulsion patent in 1930 and later built 

the first complete engine that ran on a test stand in 1937. In 

Germany, Hans von Ohain, a scientist from Gottingen 

University, patented an aeronautical gas turbine engine in

November 1935. Collaborating with aircraft manufacturer 

Heinkel Company, von Ohain built an engine and flew it on an 

He178 aircraft on August, 27, 1939. It was the world’s first 

gas-turbine-powered, jet-propelled airplane to fly (Kinney, 

2003). 

Official American involvement in the development of 

aeronautical gas turbine engines seemed to be reactionary 

rather than following the success of the Germans and the 

British in this area. Chief of the Air Corps, General H. H. 

Arnold asked the NACA to investigate the feasibility of jet 

propulsion in 1941, resulting in the creation of a Special 

Committee on Jet Propulsion headed by William R. Durand. 

The committee included members from the NACA, naval air, 

and other military organizations, the Bureau of Standards,

leading engineering universities, and industry—represented

only by manufacturers of industrial and marine turbines: 

Westinghouse, Allis-Chalmers, and General Electric (GE, 

Schenectady) (Kinney, 2003).

Three positive factors of flying a gas turbine compensated 

for the low efficiency of ground-based industrial turbines and 

compressors: (1) the low temperatures at high altitudes 

actually made the engine more efficient, (2) the forward speed 

of the aircraft created a ram effect that increased the efficiency 
NASA/TP—2013-216524 4 

of the compressor, and (3) only a portion of the energy 

released into the turbine was needed to drive the compressor—

the rest was used for propulsive thrust. Although the turbojet 

revolutionized aircraft propulsion, it was not a radical 

technology break from the reciprocating engine. The technology of the supercharger, which is essentially a compressor, 

was being used to augment the performance of the reciprocating engine. Because a compressor is an essential part of the 

turbojet engine, the supercharger technology provided continuity between the old technology and the new. Both Whittle 

and von Ohain chose centrifugal compressors for their turbojet 

engines because the centrifugal supercharger was in common 

use in reciprocating engines. However, it was believed that 

axial compressors would eventually be used for turbojet 

engines for better performance and higher power.

A Special Compressor-Turbine Panel was set up by the 

Durand committee to investigate the potential of the 

compressor-turbine combination. The panel considered an 

axial-flow compressor because of its smaller frontal area and 

higher potential pressure ratio. However, axial compressors 

that were lighter and more compact had complex aerodynamics across the blades of several stages that presented a 

challenge to designers and required better understanding of the 

underlying aerodynamics. In addition, fabrication of the complicated compressor proved to be very difficult, because the 

compressor blades needed to withstand vibrations to prevent 

them from breaking and flying off.

The Durand committee recommended that the Government

contract with three companies—Westinghouse, AllisChalmers, and the GE Turbine Group at Schenectady—to 

develop aircraft gas turbine engines. The three companies 

submitted designs for engines that resembled what would 

become the standard configuration for aeronautical gas 

turbines: a rotary compressor driven by a turbine wheel with 

axial rather than centrifugal compressors. Working with the 

Navy, Westinghouse submitted the design for a turbojet 

engine called the 19A, which became the only American 

engine (of the three mentioned) that flew during the war (in 

1942). Allis-Chalmers, also working with the Navy, designed 

a turbine-driven ducted fan, but the work suffered from a slow 

development program. The GE Turbine Group at Schenectady 

cooperated with the army to design a turboprop engine—the 

TG–100 (Kinney, 2003).

The most well-known U.S. aeronautical gas turbine program 

resulted when the British Royal Air Force helped the U.S.

Army import Whittle’s design. The British government provided the drawings of the improved Whittle engine, W2B, as 

well as an actual engine and the plans for the original model, 

to the GE factory in Lynn in October 1941. GE was chosen for 

its expertise in turbine and turbosupercharger technology to 

develop the U.S. military’s first jet engine, the I–A. A few 

other gas turbine development projects, in addition to the ones 

mentioned here, resulted in multiple engines. By late 1942, the 

American gas turbine program had a total of eight engines, 

Figure 3.—Altitude wind tunnel. 

which included the imported Whittle turbojet and seven U.S.-

designed engines (Kinney, 2003). The first turbojet engine, the 

GE 1–A, was brought to Glenn in late 1943. American 

researchers were armed with valuable pieces of British 

technology from the W2B and W1X plans as well as the actual 

W1X engine. In September 1943, construction of Glenn’s 

Static Test Laboratory was completed to test the GE 1–A 

turbojet engine.

As the work on jet propulsion continued, Glenn researchers 

built and tested the first afterburner in October 1943, in 

Glenn’s Propulsion Static Test Laboratory. They continued to 

gain hands-on research experience by testing the new GE and 

Westinghouse jet propulsion units in Glenn’s AWT (Fig. 3).

Glenn research staff built on this experience to become the 

Government’s experts in jet propulsion in the early postwar 

period.

Supersonics, Rockets, and Nuclear

Propulsion

The German V–2 rocket had flown at supersonic speeds in 

1944; therefore, supersonic aerodynamics could no longer be 

viewed as visionary. At the time, it was considered part of the 

NACA’s responsibility to advance the technology needed to 

make supersonic flight practical. Reports of European

advances in supersonics prompted NACA leadership to set up 

an interlaboratory high-speed panel to coordinate new research 

in this area. The U.S. Army Air Forces also had ambitious 

plans for a large-scale engineering development center and 

supersonic wind tunnels. Because postwar resources were too 

scarce to meet the demands of both the NACA and the U.S.

Army Air Forces, Congress enacted legislation to establish the 

National Unitary Wind Tunnels Plan (on Oct. 27, 1949). 

According to the plan, the NACA would build supersonic 

tunnels at each of its three centers, but the Air Force’s Arnold 

Engineering Center in Tullohoma, Tennessee, would still

receive a large portion of the appropriations. 

Under the Unitary Wind Tunnels Plan, the 10- by 10-Foot

Supersonic Wind Tunnel (10×10)

2 was built in 1955 at Glenn. 

 

2

3.05 by 3.05 m. 
NASA/TP—2013-216524 5 

The tunnel could operate at speeds between Mach 2.0 and 3.5 

(2124 to 3719 km/h; 1320 to 2311 mph) at altitudes of 50 000

to 150 000 ft (15 240 to 45 720 m). Another supersonic tunnel, 

the 8- by 6-Foot Supersonic Wind Tunnel (8×6),

3 could 

operate at speeds between Mach 0.55 and 2.1, with an altitude 

range from sea level to 40 000 ft (12 192 m). The 8×6 had 

been built in 1949 to study propulsion systems, including 

inlets, exit nozzles, combustion fuel injectors, flame holders, 

and controls on ramjet and turbojet engines. In the 1960s, a 

second (9- by 15-ft (2.7- by 4.6-m)) subsonic test section was 

added to the return leg of the 8×6 for testing scale models of 

propulsion systems for vertical and short takeoff and landing 

aircraft. Glenn research in supersonics had begun as early as 

the summer of 1945 in two wind tunnels, an 18- by 18-in.

(45.7- by 45.7-cm) tunnel and a 20-in.- (50.8-cm-) diameter 

round tunnel, capable of speeds up to Mach 2. Using these 

facilities and following the theoretical work by the British and 

Germans, Glenn’s researchers were able to make significant 

advancements, including the discovery of shock waves during 

wind tunnel experiments, which led to productive supersonics 

research accomplishments for many years to come.

In the 1930s, the Cleveland Rocket Society had a small 

group of amateur rocket enthusiasts, but membership declined 

drastically in the early 1940s. The flight of the V–2 across the 

English channel in the summer of 1944 renewed interest in 

rockets at Glenn. In 1944, after the U.S. Navy’s request that 

jet propulsion for aircraft and missiles be given priority to 

meet the military’s needs, Glenn researchers were eager to 

pursue rocket research. Around the same time, the NACA was 

authorized to build a complex of four rocket test cells at 

Glenn. In an effort to keep pace with the technology development following the V–2 flight, in June 1945, Glenn

submitted a proposal to conduct research on turbojets, ramjets, 

and rockets for guided missile powerplants. Initially, Glenn’s 

rocket program had to be proposed as high-pressure combustion research, because rocket research was believed to be 

outside of the NACA’s mandate of aircraft improvement. With 

minimal support for personnel, the group focused on highenergy liquid propellants, combustion, and rocket cooling to 

evaluate the performance of these propellants both theoretically and experimentally. In spite of underfunding and understaffing, the rocket group slowly built a technical competence 

that would lead to future achievements in liquid propulsion, 

and ultimately, to a major role for Glenn in the human 

exploration of space.

The initiative for nuclear propulsion also came from within 

Glenn—for a nuclear power plant for aircraft to enable longrange flights without refueling. Following the end of World 

War II, Glenn personnel who wanted to work with the 

laboratory at Los Alamos were told to wait until Congress had 

set up the Atomic Energy Commission. In May 1946, when 

the U.S. Army Air Forces initiated the Nuclear Energy 

 

3

2.44 by 1.84 m. 

Propulsion for Aircraft project, the NACA was placed on a 

board of consultants for the project along with nine companies 

and the Navy.

Glenn proposed a basic high-temperature research program 

in heat transfer and materials to support a gas-cooled nuclear 

reactor design. In recognition of the expertise of Glenn staff in 

analytical and experimental heat-transfer studies, the Atomic 

Energy Commission and the NACA made a formal agreement 

on a joint research program on July 15, 1948, and a number of 

Glenn personnel were sent to Oak Ridge National Laboratory 

for training. Glenn’s long commitment to nuclear propulsion 

continued until the termination of the nuclear effort in 1972.

Emphasis on Fundamental Research

The transition from the piston engine to jet propulsion at 

Glenn happened in a relatively short time, and this transition 

was liberating to leaders who had worked for years at Langley. 

Most of the work carried out at Glenn during World War II

was developmental in nature, and it came largely at the 

expense of fundamental research. It was hoped that after the 

war, the emphasis would return to the fundamental research 

that was characteristic of the NACA before World War II. In 

December 1945, Glenn sent a list of research topics, the 

Survey of Fundamental Problems Requiring Research, to the 

NACA’s Washington office (Headquarters), reflecting a desire 

to return to fundamental research. Glenn hoped to expand the 

scope of its research to further advance emerging jet 

propulsion technologies, such as turbojets, ramjets, rockets, 

and possibly aircraft nuclear propulsion. Table I shows the 

nine engine types in the December 1945 plan, along with the 

percentage of effort for each. The list was later revised to 

replace specific engine types with more general topics, such as 

compressor, turbine, combustion, fuels, materials, supersonics, 

and nuclear, because of criticism by the engine companies that 

the proposed research was not quite fundamental.

TABLE I.—RESEARCH IN DECEMBER 1945 PLAN

Engine type to be researched Planned effort, percent

Turbojet engines 20.0

Turbopropeller engines 20.0

Continuous ramjet engines 12.5

Intermittent ramjet engines 5.5

Rocket engines 4.0

Reciprocating engines 13.0

Compound enginesa 15.0

Icing 5.0

Engines for supersonic flight 5.0 a

Reciprocating engine plus turbosupercharger.

Even with a well-defined research portfolio, Glenn had to 

balance its efforts in the early postwar period between 

addressing the industry’s immediate development problems

and generating long-term scientific and engineering 

knowledge to serve national interests. Between the two world 
NASA/TP—2013-216524 6 

wars, the airframe industry had benefited from and relied on 

NACA innovations, such as the discovery, through testing in 

the NACA wind tunnels, that a change in the configuration of 

a wing, tail, or propeller could dramatically improve aircraft 

performance without affecting the overall design of the 

aircraft significantly. In an engine, however, even a small 

change in a component would usually affect the entire system 

because of the strong interrelationship among and complexity 

of the engine’s components, and it was not certain that Glenn

would ever enjoy the same favorable relationship with engine 

companies that the NACA had enjoyed with airframe 

designers. There was intense rivalry among the engine companies at the time, and Glenn had to ensure that its research 

would be available to the entire industry without benefiting 

any company more than another.

The turbojet was not considered to be a mature technology

around the end of World War II, and it was not clear that it 

would be suitable for the commercial market. The technology 

seemed suitable for fighter aircraft, where speed was more 

important than other issues, such as fuel consumption and life. 

However, it was not clear how it would benefit commercial 

applications during peacetime. Many design issues remained, 

such as axial versus centrifugal compressor, problems of high 

temperatures for combustor and turbine design, and whether 

new material development or turbine cooling technology 

advancement needed to be pursued. As a concept, the gas 

turbine seemed simpler than the piston engine, but the

complex physical processes involved in passing air through a 

compressor, combustor, and turbine required a much more 

indepth theoretical understanding of flow physics than had 

been necessary during World War II. 

In May 1945, the first American conference on gas turbine 

engineering, sponsored jointly by GE and the U.S. Army Air

Forces, was held in Swampscott, Massachusetts. Nearly 200 

members of the aeronautical community, including representatives from Great Britain, attended. Abe Silverstein, Chief of 

Glenn’s Wind Tunnels and Flight Division, presented a paper 

about the NACA’s contributions, including the test program in 

the AWT that helped GE to transform its prototype I–16 into the 

more reliable I–40. By the end of World War II, the I–40 had 

proved to be superior to the Rolls Royce engines based on 

Whittle’s design. Silverstein evaluated and compared the 

performance of five different gas turbine engines: two with 

centrifugal compressors, the GE I–16 and I–40, and three with 

axial compressors, including the Westinghouse 19–B and 

19–XB. 

Ben Pinkel, Chief of the Fuels and Thermodynamics 

Division, led Glenn’s work to evaluate the weight, altitude, 

range, and fuel consumption of different types of propulsion 

systems. Members of his division presented the results at the 

Second Annual Flight Propulsion Meeting of the Institute for 

Aeronautical Sciences held in Cleveland in March 1947. The 

group had evaluated six engines—the compound engine

(piston engine with turbosupercharger), the turbine-propeller 

engine, the turbojet engine, the turboramjet engine, the ramjet 

engine, and the rocket engine—by weighing different

parameters, such as thrust versus engine weight, frontal area, 

rate of fuel consumption, speed, and altitude to assess the 

advantages and disadvantages of each system. The analysis 

distinguished Glenn as the only U.S. institution that had a 

basic understanding of the entire aircraft propulsion picture, 

and it provided engine companies with the necessary basis for 

creating a realistic technology plan for developing different 

engine types.

Emphasis on fundamental research led to Glenn’s

reorganization in 1945, which was also consistent with the 

National Aeronautical Research policy that was approved in 

March 1946. The resulting management structure remained 

unchanged until the NACA became NASA in 1958. Much of 

the research scope for Glenn through the 1950s followed the 

framework set by NASA (in 1945). As the superiority of the 

turbojet over the turboprop and compound engine was 

demonstrated, further advancement of turbojet technology 

demanded the development of alloys, ceramics, and metal 

compounds that could withstand the high temperatures of 

gases. The resulting advancement of materials technology and 

early research in high-energy liquid propellants for rockets 

would lead to a prominent role for Glenn in rocket propulsion, 

including the success of the Centaur rocket. Research work on 

the supersonic ramjet led to stimulating basic research in 

aerodynamics and heat transfer, which was a key to the 

success of the Navaho, Bomarc, and T series of missiles for 

the Navy. Glenn’s nuclear propulsion initiative, though never 

considered to be a viable project, had inspired basic research 

in materials.

Although the NACA had a good reputation for technical

talent, it had few employees with advanced training in science 

and engineering. After World War II, the NACA’s relatively 

low civil servant salaries made it difficult to convince 

candidates with advanced degrees in science and engineering 

to choose Government employment over more lucrative offers 

from the industry, where their qualifications were in great 

demand. Between 1945 and 1958, the NACA and many 

national laboratories began to lose staff to universities and 

industry. Despite support from the Industry Consulting 

Committee for legislative change, it was not until in 1956 that 

Congress passed laws enabling the NACA to offer competitive 

compensation.

Glenn’s contributions in basic research continued as it 

began to study solid and air-cooled turbine blades. Ernst 

Eckert, a well-known scientist who specialized in heat transfer 

and had worked in Germany during World War II, was 

brought to Glenn from Wright Field in 1949 and led the 

turbine cooling work. He understood the limitations of 

existing methods of internal air cooling and, on the basis of his 

experience in Germany, encouraged Glenn to consider other 

potentially more effective approaches like transpiration, film, 

and natural convection liquid cooling. Together with members 
NASA/TP—2013-216524 7 

of Glenn’s Turbine Cooling Branch, Eckert developed the 

theory of these cooling methods by analysis and advanced 

their development by experiments. Film cooling, now widely 

used in aircraft engine turbines, particularly attracted him, and 

he advocated further basic research for this method. While at 

Glenn, Eckert developed theories for transpiration cooling and 

natural convection liquid cooling, and he initiated experiments 

for applying these new theories.

Glenn provided a nurturing environment for outstanding 

young laboratory theoreticians. In the mid-1950s, researchers 

Simon Ostrach, Franklin Moore, Harold Mirels, and Steven 

Maslen were part of the Applied Mechanics Group, which 

worked on the fundamental research problems of threedimensional unsteady boundary layer theory (Moore and 

Ostrach) and acoustic screaming (Mirels and Maslen). By 

1987, they had all received the highest professional 

recognition—membership in the National Academy of 

Engineering. The theoretical skills of the Applied Mechanics 

Group were supported by the mechanical skills and creativity 

of Glenn’s support service personnel, who could build a small 

wind tunnel or a test rig, or modify a computer to validate 

theoretical models with experimental data. The NACA leadership strongly believed that basic research was the Nation’s 

technical capital. Although they were more comfortable with 

creativity expressed in tangible products, such as a new 

compressor, turbine, or afterburner, the principal part of 

Glenn’s mission still remained the full-scale testing of turbojet 

engines and components. 

During basic research advancements, Glenn staff collaborated with several universities, including Case Western 

Reserve University, Brown University, the University of 

Minnesota, the University of California at Berkeley, the 

University of Akron, the University of Pennsylvania, and the 

University of Rhode Island. Glenn’s basic research work with 

universities complemented very well its technology 

development collaborations with the aeropropulsion industry. 

These cooperative efforts enabled Glenn to pursue efforts 

ranging from fundamental research to focused technology 

development and transfer to the industry. Some of these 

technologies are being flown today in commercial as well as 

military applications. Glenn’s practice of working with 

academia, as well as industry, continues to the present day, 

and Glenn currently maintains a healthy balance between 

fundamental research and focused technology development by 

working with universities across the country and with all the 

aeropropulsion companies.

Icing Research—Aviation Safety

Operations research—dealing with the certification of an 

airplane—was considered to be at the applied end of the 

research scope. However, even in this area, the NACA made 

significant contributions to solving general problems that were 

of interest to the entire aircraft industry. 

During World War II, icing research dealt with providing a 

short-term solution to the icing problems of military aircraft. 

The development of anti-icing hardware was clearly 

considered to be development, but a fundamental understanding of the icing cloud was needed to determine how icing 

develops on aircraft flying under different conditions. In 1944, 

the NACA began a program to compile statistical data to 

define icing conditions. By the 1950s, Glenn’s guidelines for 

the design of ice-protection systems had become the basis for 

the Federal regulations used to certify these systems. As icing

research advanced, it became necessary to combine theoretical 

analyses with experimental studies. The type of ice accretion 

on an aircraft depends on the size of the supercooled droplets 

in the icing cloud that strike the aircraft surfaces. Small

droplets result in a porous white mass called rime ice. 

However, larger droplets can accumulate on a surface until 

enough heat is dissipated to freeze the water into a clear hard 

mass called glaze ice. 

In 1942, while the AWT was being built, Glenn proposed to

build an IRT to help researchers understand ice accretion 

phenomena. The IRT would take advantage of the extensive 

sophisticated refrigeration system needed for the AWT. The 

IRT was completed in the spring of 1944 (Fig. 4). As Glenn

staff tried to simulate the atmospheric conditions of an icing 

cloud, they realized that this was a very difficult engineering 

problem. They had no design experience for creating icing 

conditions in a wind tunnel, and therefore they used logic, 

theory, and speculation. It was not until the tunnel’s spray 

system was replaced in the early 1950s that the tunnel tests

Figure 4.—Glenn’s Icing Research Tunnel.
NASA/TP—2013-216524 8 

could yield icing data as accurate and reliable as flight 

research data. The research and technology advancement in 

the icing tunnel earned Glenn and the NASA-industry team 

the prestigious Collier Trophy in 1946 from the National 

Aeronautic Association for an outstanding contribution in the 

development of thermal ice-protection systems.

Glenn’s icing research contributed to better understanding 

of the icing phenomena through the development of 

instruments to determine the mean size and distribution of 

water droplets, instruments to determine liquid water content 

in the icing cloud, complex analysis of the supercooled water 

droplet trajectories, and analysis of the accretion mechanism 

by which droplets strike the aircraft surfaces and freeze. With 

their enhanced understanding of the natural icing cloud 

phenomena, the icing tunnel engineering staff set about the 

redesign of the spray system. In the early 1950s, the advent of 

high-speed, high-altitude turbojet aircraft brought new 

requirements for thermal anti-icing systems, which had 

required large amounts of hot air to be bled from the engine, 

severely impacting engine performance. Glenn researchers 

were able to attain a new level of sophistication in the heat 

transfer and aerodynamics required for the study of thermal

cyclic deicing systems. In 2009, Glenn received the 

prestigious Collier Trophy (as a member of the Commercial 

Aviation Safety Team) for technology advancements that had 

improved aviation safety, and icing research continues to be a 

strong technical competency at the Center. 

Compressor Research

Glenn’s Compressor and Turbine Division conducted 

research on both centrifugal and axial compressors. Early on, 

expertise in centrifugal compressors had been established by 

Stanitz (1948). However, the compressor researchers always 

had an interest in axial compressors. The axial compressor 

was considered a better choice for aircraft gas turbine engines 

because of its smaller frontal area, more compact dimensions, 

and lighter weight. However, the complex aerodynamics 

across the blades of several stages presented serious technical 

challenges to designers, who needed a better understanding of 

the underlying key flow physics to design axial-flow 

compressors with smooth flow across the blade rows with 

minimal losses. Research was initiated in 1944, with work on 

an eight-stage axial compressor designed by Eastman Jacobs

and Eugene Wasielewski forming the basis for compressor 

research at Glenn and leading to many accomplishments in 

this area. The isolated airfoil theory used in the design of the 

Jacobs-Wasielewski compressor was adopted as the accepted 

standard approach by American compressor designers until the 

mid-1950s (Nichelson, 1988). Later, Wu and Wolfenstein

(1949) developed a radial equilibrium theory for compressible 

flow across stages of an infinite number of blades to overcome 

the limitations of the isolated airfoil theory.

Figure 5.—Testing of transonic compressors.

In 1952, researchers in Glenn’s Compressor and Turbine

Division began to develop a transonic compressor. Robert 

Bullock and his team proposed a flow model developed to 

estimate losses caused by shocks in supersonic parts of the 

airflow through the transonic compressor. Seymour Lieblein 

developed a parameter called the diffusion factor D to 

represent the level of blade loading in a compressor. This 

factor is now universally accepted and used in the design 

guidelines. Ten years of research in single-stage and multistage compressors led to the development of an eight-stage 

axial transonic compressor test bed that was used extensively 

by in-house researchers (Lieblein, 1960). As the compressor 

research continued, Glenn staff designed and built a number of 

test rigs to test single-stage and multistage compressor 

configurations to provide essential data for industry engine 

designers, as well as to validate the analytical theories 

proposed by researchers (Fig. 5). NACA reports published

with these data were used extensively by the engine 

community and referred to as the Compressor Bible. The 

Compressor Division prepared a manual summarizing compressor research and published it as a three-volume NACA 

Confidential Research Memorandum in 1956 (edited by 

Johnsen and Bullock). A declassified edition was published 

later (Members of the Staff of Lewis Research Center, 1965). 

The unique engine test facilities developed by the NACA, 

and the associated research at Glenn, played a key role in 

securing U.S. dominance in aircraft engines. The American 

turbojet industry benefited and matured from this technology 

advancement, and market forces gradually narrowed the field 

to two companies: Pratt & Whitney and GE. During the Cold 

War, the United States was allowed continued access to 
NASA/TP—2013-216524 9 

British engine technology. In turn, Glenn shared its research 

experience in British engine technology with Pratt & Whitney 

and GE. As the United States started to strengthen its military 

power, American engine companies received large defense 

contracts. This enabled the military services and industry to 

develop new facilities that were comparable or superior to

those at Glenn, resulting in the marked decline of Glenn’s 

influence in the 1950s. It was time to reassess Glenn’s future 

role in the Nation’s propulsion research.

Space Propulsion

In July 1955, President Eisenhower announced the United 

States’ plans to launch the first Earth-orbiting satellite as 

America’s part in the International Geophysical Year. Glenn

would be involved in propulsion problems related to space 

exploration. Late in 1956, Glenn Research Chief, Silverstein, 

decided to shift the focus away from turbojet engine research 

to rocket propulsion and began to consider a major 

reorganization. In September 1957, Silverstein proposed that 

rocket research be expanded and that turbojet engine research 

be reduced proportionately. The launch of Sputnik by the 

Soviet Union in October 1957 led to President Eisenhower’s 

decision to form a space agency around the NACA in March 

1958. The official formation of NASA occurred on October 1,

1958. Glenn became involved in developing high-performance 

upper-stage rocket engines (including the Centaur upper stage) 

that used a combination of liquid hydrogen and liquid oxygen 

fuel. Work continued into the Apollo Program after President 

John F. Kennedy announced, in May 1961, that the United 

States planned to land an American on the Moon before the 

end of the decade.

Aircraft Engine Noise Reduction

The emphasis on rocket research had shifted Glenn’s focus 

away from air-breathing engines for almost a decade. In 1966, 

NASA and Glenn began to return their focus to aeronautical 

research. Several new aeronautics projects were initiated, but 

the majority of Glenn’s research efforts remained space 

related. Air travel had grown at a very rapid pace, and airports 

had become more congested. Environmental issues, such 

as noise and pollution-causing emissions, needed to be 

addressed. New aeronautics projects included the development 

of quieter engines and of aircraft that could take off and land 

on short runways to relieve airport congestion. Glenn also 

took part in planning the technology development for the 

supersonic transport (SST) airplane initiative proposed in 

response to British and French plans to develop a supersonic 

cruise aircraft: the Concorde. However, after 9 years without 

wind tunnel testing, Glenn’s facilities were no longer unique. 

The U.S. Air Force and aircraft engine industry facilities that 

had been developed in the postwar period were comparable or 

superior to those at Glenn. For example, the wind tunnels at 

the Air Force Arnold Engineering Development Center in 

Tullahoma, Tennessee, had capabilities similar to those of 

Glenn’s major wind tunnels, such as the 10×10.

Silverstein retired as Glenn’s director in 1969 after serving 

for 8 years, and Bruce Lundin became Glenn’s director. After 

the successful Apollo Program and Glenn’s successful participation in the Centaur and Agena upper-stage-rocket 

development programs, support for NASA’s human space 

programs started declining because of budget pressures from 

the Vietnam War and President Johnson’s War on Poverty. 

Because the prospect for healthy human space programs did 

not look very promising, Lundin directed more Glenn work 

toward aeronautics. The Quiet Engine Program effort that had 

begun under Silverstein had progressed to a full-scale engine 

demonstration in collaboration with the Federal Aviation 

Administration (FAA) and the Department of Transportation. 

The goal was to develop engine-noise-reduction technologies 

for a 22 000-lb (9979-kg) thrust engine that would demonstrate a noise reduction of 15 to 20 dB in comparison with the 

current commercial transport jets (Fig. 6). 

Glenn’s engine noise suppression research had begun in 

1957, but the early collaborations with industry had never 

reached the level of an engine demonstration. Air-breathing 

gas turbine engine technology had rapidly advanced after 

World War II, and by the late 1960s, the engine companies 

had introduced the turbofan engine, which was more efficient 

than the turbojet engine. In a turbofan engine, only a small 

portion of the air producing thrust passes through the 

compressor and the core. Most of the thrust comes from air 

that passes through the fan and bypasses the core. GE was 

awarded a $20 million Government contract to design and 

build three different fans and integrate them with an engine

Figure 6.—Engine noise reduction testing.
NASA/TP—2013-216524 10

core to demonstrate noise reduction. Glenn assumed 

leadership of the test program to measure fan noise through a 

contractual relationship with GE. In another effort to reduce 

engine noise, Glenn, Pratt & Whitney, Boeing, and Douglas 

Aircraft entered into a no-fee, cost-sharing contract in 1973 to 

refine an existing engine and test it at Glenn. Pratt & Whitney 

designed a single-stage fan to replace the existing two-stage 

fan in their JT8D engine, and Boeing and Douglas Aircraft 

developed acoustic liner materials to install in the nacelles that 

housed the engines. 

The renewed emphasis on air-breathing engine research also

included technology advancement in the small engines used 

for helicopters. This led to the establishment of a joint 

research program in low-speed rotary-wing transportation 

aviation with the U.S. Army Materiel Command. The program 

continued for four decades, with NASA and the Army 

leveraging their resources to advance technologies of mutual 

interest until 2010, when the Base Realignment and Closure 

mandated by Congress resulted in a major reorganization of 

the Army research and technology groups.

Emissions

In the early 1970s, heavy industry near downtown 

Cleveland produced significant air and water pollution. When 

industrial waste caught fire on the surface of the Cuyahoga 

River in 1969, the negative publicity from the national media 

increased the city’s commitment to clean up the air, the river, 

and nearby Lake Erie. In 1971, as support to address pollution 

strengthened, Glenn formed an Environmental Research 

Office to conduct research and advance sensor technology for 

accurately monitoring trace elements and compounds in 

Cleveland’s atmosphere. The researchers were able to develop 

neutron activation analysis, which can accurately detect 

minute amounts of mercury, arsenic, cadmium, and nickel in 

air samples. This success led to a new project funded by the 

Environmental Protection Agency (EPA) to study the effect of 

treated wastes on algae growth in Lake Erie.

In 1972, Glenn staff began a campaign that monitored air 

samples from eight meteorological stations in and around 

Cleveland. The data were used to estimate the impact of 

various factors on pollution levels and to adjust the mitigation 

strategy as needed. The research in the emissions area and the 

resulting expertise gained by Glenn staff would become a key 

competency in reducing emissions from aircraft engines during takeoff and landing to conform to emission standards 

proposed by the EPA to regulate local air quality in the 

vicinity of the airports. Later, this became part of the 

certification requirement. 

In 1970, Glenn turned its attention to pollution from 

automobiles and formed the Automotive Systems Office, also

funded through the EPA. It was proposed that the Automotive 

Systems Office develop a gas-turbine automobile engine 

because studies had shown that the Brayton cycle is more fuel 

efficient and cleaner than the piston engine. Glenn’s

advancements included a thermal reactor with very high 

combustion efficiency and advanced materials, such as 

ceramics, that could operate in a high-temperature, corrosive 

combustion environment. Technologies aimed at automotive 

engines continued to advance at Glenn, and in the mid-1970s, 

staff expanded the research scope by broadening the 

technology portfolio to include the Stirling engine and 

electric-powered cars.

Energy Crisis

In the early 1970s, environmental concerns and the 

country’s dependence on fossil fuels were top priorities for the 

U.S. public. Two senior managers at Glenn, Robert Graham 

and Robert English, urged Center Director Bruce Lundin to 

tackle the problems by employing Glenn’s unique expertise in 

thermodynamics, fluid mechanics, heat transfer, materials, 

chemistry, nuclear physics, plasma physics, and cryogenic 

physics, which were also relevant to energy conversion. 

Glenn’s expertise in Brayton cycle technology for gas turbine 

engines, as well as in closed Brayton-cycle technology for 

space power applications, was believed to be adaptable for 

ground power generation and mass transportation. Buses and 

trains running with cleaner and more energy-efficient engines 

showed great promise for the future. Glenn’s staff and 

management recognized that most of the aeropropulsion 

research and technologies that were being advanced by Glenn

would cause significant progress toward more efficient and 

cleaner ground power generation and mass public 

transportation.

Lundin, the U.S. Department of Transportation, and the 

EPA discussed ideas for pursuing research in clean coal 

burning based on Glenn’s experience in using the potassium 

Rankine system for space power generation. The Rankine 

system could be used as a topping cycle to extract more 

energy from coal for power generation. When the Arab oil 

embargo led to a U.S. energy crisis in 1973, President Nixon 

asked Dixie Lee Ray of the Atomic Energy Commission to 

form a task force to define the problems and offer solutions. 

Prominent Glenn staff members participated in 4 of the 15 

panels on the task force: the Energy Conversion Panel, Solar 

Energy Panel, Fusion Energy Panel, and the Advanced 

Transportation Systems Panel. 

The Solar Energy Panel included wind energy in its 

investigations because wind is driven by the Sun. Glenn

representatives on this panel translated their experience in gas 

turbine technology into advances in wind turbine technology. 

At the request of the Secretary of the Interior of Puerto Rico, 

Cruz Matos, Glenn designed a wind turbine to generate 

electricity on the island of Culebra. The successful design led 

to the National Science Foundation authorizing funds in 1973 

to construct and operate an experimental 100-kW wind turbine
NASA/TP—2013-216524 11

Figure 7.—Wind turbine at Plum Brook Station.

at Glenn’s Plum Brook Station campus, 50 mi (80.5 km) west 

of Glenn’s Lewis Field campus in Cleveland (Fig. 7). The

wind turbine generated about 180 000 kW/h/year for an 

average wind speed of 29 km/h (18 mph) and was upgraded to 

a capacity of 200 kW of power. In 1974, the National Science 

Foundation and the Energy Research and Development 

Administration provided $1.5 million for Glenn’s wind energy 

technology development. Subsequent funding by the Energy 

Research and Development Administration and its successor, 

the Department of Energy, resulted in 13 experimental wind 

turbines being put in operation between 1975 and 1979. The 

largest of these, a 3.2-MW Mod–5B wind turbine, is still 

operating on the island of Oahu in Hawaii.

Aircraft Energy Efficiency

The information in this section and the next section is based 

on Bowles (2010). In 1975, the Aeronautical and Space 

Sciences Committee of the U.S. Senate directed NASA to 

initiate planning exercises to explore new fuel-saving technologies to address aircraft fuel conservation. Aircraft fuel 

prices had tripled between 1973 and 1975 as a result of the oil 

embargo. The efforts led to the Aircraft Energy Efficiency 

(ACEE) program (United States Senate, 1976), which was 

fully funded by 1976 as a 10-year, $500 million R&D program. The goal of the program was to reduce the amount of 

fuel used by the Nation’s commercial and military aircraft by 

50 percent. Of the six aeronautical projects that made up the 

ACEE program, Glenn was given responsibility for three 

dealing with propulsion: the Engine Component Improvement 

(ECI) project, which was to increase the efficiency of existing 

engines by making short-term, incremental changes to the 

components; the E3 project, which was to develop a new 

engine that would be significantly more efficient than the 

existing turbine-powered jet engines; and the ATP, which was 

an intensive effort to replace the turbojet with a much more 

efficient propeller.

The first of the three propulsion projects, ECI, focused on 

improving existing turbofan engines by redesigning the engine 

components that were most prone to wear. It was the least 

technically challenging of the three projects and aimed for a 

5-percent improvement in fuel efficiency. It was expected to 

return quick results, with new technologies being incorporated 

into engines within 5 years. Glenn worked with the engine 

companies, GE and Pratt & Whitney, through two major 

contracts. In addition, airframe companies and a number of 

airlines were subcontracted to participate in the project. The 

two main thrusts of the project were performance improvement and engine diagnostics. The performance improvement 

technologies included thermal barrier coatings and active

clearance control for turbines, improved aerodynamics for the 

fan and the high-pressure turbine, nacelle drag reduction, and 

compressor bleed reduction. The engine diagnostics work 

included evaluating existing data on performance deterioration, as well as developing special ground tests to simulate 

operating conditions. The ground tests were designed to 

determine the sources of performance deterioration and to 

identify components with failure rates that could be improved. 

The ECI project achieved the projected fuel-reduction 

technology and helped to maintain the competitive advantage 

of the U.S. commercial aircraft industry.

As part of the E3 project, Glenn had contracts with GE and 

Pratt & Whitney to develop new engines to improve fuel 

efficiency. Both companies were very much interested in 

working with Glenn—not only because of the financial and 

technical assistance but because continued innovation would 

help the companies continue to dominate the world engine 

market as competition from Japanese and European engine 

makers increased. NASA provided $90 million to each 

company, with the stipulation that each would share in the 

effort by spending $10 million of its own money as a cost 

share. The project’s four main goals were to (1) reduce fuel 

burn by 12 percent, (2) reduce operating costs by 5 percent (in 

comparison to existing turbine engines), (3) meet FAA noise 

regulations for areas around airports, and (4) conform to 

proposed EPA emission standards to regulate local air quality 

in the vicinity of the airports. It was expected that the new 

turbofan engine would be ready for commercial use by the late 

1980s or early 1990s. Because the propulsive efficiency is 

maximum when the jet velocity is close to the flight velocity 

of the vehicle, high-bypass turbofan engines with a large
NASA/TP—2013-216524 12

Figure 8.—Energy Efficient Engine (E3) technologies. LPT, lowpressure turbine; FADEC, full authority digital engine control; 

HPC, high-pressure compressor.

amount of air exiting at lower fan jet velocities would be more 

efficient than low-bypass turbofans or no-bypass turbojets.

The potential of the high-bypass turbofan engine was the 

main focus of the E3 project (Kerrebrock, 1982), and both GE 

and Pratt & Whitney were awarded contracts to design and 

build a new turbofan. The turbofans were to be used primarily 

as test beds, not as commercially ready engines. The new 

engines would need advanced component technologies—

compressor, fan, and turbine gas path improvements—as well 

as structural advances—improved blading and clearance 

control. Pratt & Whitney and GE used different strategies for 

the research, with Pratt & Whitney developing individual 

component technologies and GE integrating the advanced 

components into an engine. Because of budget reductions, in 

1982, NASA had to redirect the E3 project, and GE had its 

core engine demonstration test without the emission goal. 

Pratt & Whitney proceeded with its component technology 

focus. GE successfully met the program goal in 1983 by 

producing the “world’s most fuel efficient and best-performing 

turbofan” (Design News, 1983) (Fig. 8). GE incorporated its 

E3 technology in its engine designs, including the CF6–80E 

and the GE90 engine. Even though Pratt & Whitney’s 

component-level strategy technologies advanced at a slower 

pace, they helped lead to its energy-efficient Geared Turbofan 

engine in 2007. 

The ATP was one of the more radical and risky projects in 

the ACEE program. The ATP goals were to reduce fuel 

consumption by 20 to 30 percent in comparison with existing 

turbofan engines without any performance degradation or 

passenger discomfort at speeds to Mach 0.8 and altitudes to 

30 000 ft (9144 m). Glenn had been working with Hamilton 

Standard, a division of United Technologies, as early as 1973 

on the design for a propfan, a propeller powered by a gas

turbine. The potential for reducing fuel consumption had 

seemed attractive in spite of the disadvantages of turboprops 

in terms of slower speed, higher noise, and the complexity of 

gear boxes to drive the propeller. Daniel Mikkelson of Glenn

and Carl Rohrbach of Hamilton Standard came up with the 

technique of using propeller blade sweep to reduce noise and 

increase efficiency simultaneously. Initially, only Hamilton 

Standard was interested, but later other engine companies and 

airframe companies became interested in the turboprop, which 

had become more acceptable technically and socially, because 

it reduced fuel burn and noise. However, in spite of these 

demonstrated advantages, the airframe companies—Boeing,

McDonnell-Douglas, and Lockheed—still had concerns 

regarding propeller efficiency at cruise speeds, internal and 

external noise problems, installation aerodynamics, and 

maintenance costs (Hager and Vrabel, 1988). 

There were four technical stages in the ATP: concept 

development from 1976 to 1978; enabling technology from 

1978 to 1980; large-scale integration from 1981 to 1987; and 

flight research (demonstration) in 1987. During all of these 

stages, Glenn researchers worked with the aircraft engine 

industry to identify and solve technical problems in order for 

the ATP to meet its objectives. Even before the ATP was 

formalized, Glenn and Hamilton Standard collaborated to 

build and test an SR–1, a 0.61-m- (2-ft-) diameter singlerotation model propfan. High efficiency rates were demonstrated, providing data that helped lead to the formal award of 

the project in 1978. The enabling technology phase addressed 

the four areas that had concerned the airframe companies: 

modification of propeller aerodynamics for cruise efficiency, 

cabin and community noise, installation aerodynamics, and 

drive systems for reducing maintenance cost.

In the late 1970s, computational fluid dynamics (CFD) tools 

were beginning to be used in designing engine components, 

and for the first time, CFD tools were used for an aerodynamic 

propeller design, helping to improve aerodynamic efficiency 

and reduce noise. Numerous installation options—including 

the push or pull configuration and the most effective 

integration of the propeller, nacelle, and the wing—were 

investigated to determine the most advantageous configuration. Various drive-train problems, including in the gear 

boxes, were studied to determine the best option to achieve the 

project goals. The final stage of the project, flight testing, took 

place in 1987 with a successful flight of the SR–7A propfan 

(Fig. 9) on a modified Gulfstream II aircraft in Savannah, 

Georgia. The flight tests demonstrated that the project had 

achieved its goal of 20- to 30-percent fuel savings.

In 1981, GE started to explore engines that were more 

efficient in comparison to the existing ones, and in 1983, GE 

announced an innovative effort similar to NASA’s ATP, 

called the Unducted Fan (UDF). Like the ATP, GE’s design 

also had two rows of counterrotating blades, but the UDF 

propeller was driven directly by the low-pressure turbine, with 

each blade row connected to multiple turbine blade rows 

without any need for a gear box. In this novel low-speed, 

seven-stage free turbine, the turbine rotors drove one propeller 

in one direction, while the free turbine stators rotated the other

propeller in the opposite direction. In addition, the UDF would
NASA/TP—2013-216524 13

Figure 9.—Advanced Turboprop Project technology.

be installed on the airplane’s tail, not under the wings, to allow 

room for the propeller disk and to keep noise out of the cabin. 

GE teamed with Boeing to fly the full-scale demonstrator on 

the Boeing 727 test bed in 1986 (Sweetman, 2005).

The technology advancement under the ATP earned Glenn, 

NASA, and industry team the prestigious Collier Trophy for 

the outstanding contributions to aeronautics in 1987 from the 

National Aeronautic Association. By 1988, the energy crisis 

had passed, and the interest and need for the advanced 

turboprop vanished: the fuel savings would have been less 

than the higher initial cost (Facey, 1988). However, Glenn

staff involved in the ATP remained confident that future 

economic conditions would make the turboprop attractive

again. Apparently, their confidence was warranted. GE is 

currently exploring open rotor technology for next-generation 

aircraft, and Glenn and GE are collaborating to advance open 

rotor technology to meet the fuel efficiency and noise goals set 

by NASA’s Subsonic Fixed Wing project.

Interest in High-Speed Flight

The price of oil and the Nation’s interest in fuel efficiency 

have risen and fallen together like synchronized swimmers. 

The increased cost of oil in the 1970s led to the ACEE 

program. During the late 1980s and 1990s, with the perception 

of oil abundance and the resulting oil price decrease, the 

national emphasis on fuel efficiency was abandoned. 

Consequently, in 1990, NASA’s interest in high-speed flight 

returned in the form of the High Speed Research (HSR)

program. The desire for addressing technology barriers for 

high-speed flight had followed the need to address fuel 

efficiency. One example is America’s interest in the development of an SST, evidenced by three Government attempts 

since the 1950s to advance technologies to enable the 

commercial viability of a supersonic aircraft (Bowles, 2010). 

The first attempt, the national SST program, began in 1961

and was terminated in 1971. The second attempt, the

Figure 10.—Artist’s concept of High Speed Research aircraft.

Supersonic Cruise Aircraft Research program, began in 1972 

to advance technology for flight in the 1980s. It was 

terminated in 1981 (Bowles, 2010). The third attempt, the 

HSR program, began in 1990 and was terminated in 1998 

(Conway, 2005). 

Glenn supported the development of propulsion technology

in all three high-speed programs, utilizing its unique wind

tunnels, such as the 10×10 and the 8×6, to study supersonic 

propulsion systems, including inlets, exit nozzles, combustion 

fuel injectors, flameholders, controls on ramjets, and turbojet

engines. The 10×10 could operate between Mach 2.0 and 3.5 

at altitudes from 50 000 to 150 000 ft (15.24 to 45.72 km). 

The 8×6 could operate between Mach 0.55 and 2.1 with an 

altitude range from sea level to 40 000 ft (12.19 km). 

Glenn also took part in planning all three SST airplane

technology development programs. From several studies, it 

was believed that supersonic aircraft, if environmentally 

acceptable and economically viable, could successfully compete in the 21st century marketplace (Fig. 10). However, 

several barriers needed to be resolved to make these aircraft a 

reality. The HSR program was established to address these 

barriers through collaboration between NASA and industry. 

The Critical Propulsion Components (CPC) element of HSR 

was created to develop the propulsion component technologies

needed to (1) reduce cruise emissions by a factor of 10 and (2) 

meet ever-increasing airport noise restrictions (Federal

Aviation Regulation Part 36 Stage III) with an economically 

viable propulsion system. The emission goal was an Emission 

Index of less than 5 for nitrogen oxide (NOx) at cruise. The 

noise goals were a reduction of 4 to 6 in effective perceived 

noise in decibels (EPNdB) at the sideline, 8 to 10 EPNdB at 

cutback, and 5 to 6 EPNdB during approach.

The CPC identified the critical components as ultra-lowemission combustors; low-noise/high-performance exhaust

nozzles; low-noise fans; and stable, high-performance inlets. 

Propulsion cycle studies, coordinated with Langley-sponsored 

airplane studies, were conducted throughout the CPC program 
NASA/TP—2013-216524 14

to help evaluate candidate components and select the best 

concepts for the more complex and larger-scale research 

efforts. The propulsion cycle and components ultimately 

selected were a mixed-flow turbofan engine employing a lean, 

premixed, prevaporized combustor coupled to a twodimensional mixed compression inlet and a two-dimensional

mixer/ejector nozzle. The CPC program began in 1994 and 

was planned for completion in 2002. Unfortunately, in 1999, 

NASA chose to end the HSR program prematurely. 

Nevertheless, the HSR program demonstrated that an economically viable and environmentally acceptable supersonic 

aircraft and propulsion system were achievable.

Air-Breathing Hypersonic Propulsion

For air-breathing propulsion in the hypersonic speed 

regime, access to space seems to be the only area relevant to 

civilian applications in the foreseeable future. In comparison 

with conventional rocket propulsion, air-breathing propulsion 

offers a significant advantage of higher effective specific 

impulse for space access to low Earth orbit. The combinedcycle propulsion research includes turbine-based and rocketbased cycles, as well as concepts such as pulsed-detonation 

rocket engines. In addition to a higher specific impulse, 

combined-cycle engines with air-breathing propulsion offer 

the following advantages over conventional rocket propulsion:

(1) The higher propulsion performance allows for more 

robust structural mass fraction and reusability, especially 

for turbine-based systems, resulting in lower life-cycle 

cost and higher payload fraction. 

(2) Combined-cycle systems are amenable to improved 

operations, translated from the efficiency requirement in 

civil aircraft operations, enabling efficient abort scenarios and ground operations.

However, combined-cycle systems also have several disadvantages in comparison to traditional rocket propulsion:

(1) Because of their complex nature, combined-cycle 

systems have higher development and manufacturing 

costs and are susceptible to more failure modes. 

(2) Because these systems follow higher dynamic pressure 

trajectories to benefit from longer air-breathing propulsion durations, they are subject to higher aerodynamic 

heating and structural loads and complicated staging 

issues.

(3) Combined-cycle propulsion technology is considered to 

be relatively immature in comparison to rocket propulsion and must have significant further development 

before it can be applied for space access as readily as 

rocket propulsion.

Hypersonic air-breathing propulsion technologies—especially 

combined-cycle and scramjet engine technologies—are of very 

high interest to the Department of Defense for cruise missiles; 

intelligence, surveillance, and reconnaissance; and long-rangestrike aircraft applications. Consequently, NASA has worked in 

hypersonic air-breathing propulsion and airframe technologies 

over the last several decades, even though there was not a 

significant technology pull from space technology missions. 

Although hypersonic air-breathing propulsion has been studied 

for 60 years in the United States, through numerous scramjet 

tests in several ground facilities (Guy et al., 1996; Andrews and 

Mackley, 1994; Anderson et al., 1987; Guy and Mackley, 1979; 

Stalker et al., 1994), there have been only four attempts to 

develop a flight vehicle and, of those, only two resulted in flight. 

The first effort, the Hypersonic Research Engine project of the 

1960s, focused on flying an axisymmetric scramjet mounted on 

the X–15 rocket-powered aircraft. The X–15 project was 

canceled before an operating Hypersonic Research Engine

could fly (Freeman et al., 1997). 

The second effort was the National Aerospace Plane (NASP)

funded by NASA and the U.S. Department of Defense (funding 

was divided approximately equally between NASA, the 

Defense Advanced Research Projects Agency, the U.S. Air 

Force, the Strategic Defense Initiative Office, and the U.S.

Navy). In 1986, the NASP began as an advanced technology 

demonstrator project to create a single-stage-to-orbit (SSTO) 

spacecraft and passenger spaceliner. Research suggested a 

maximum speed of Mach 8 for scramjet-based aircraft, because 

the heat generated by atmospheric friction would cost considerable energy. The NASP project showed that much of this 

energy could be recovered by passing hydrogen over the aircraft 

skin and carrying the heat into the combustion chamber. 

Mach 20 seemed possible. McDonnell Douglas, Rockwell

International, and General Dynamics competed to develop 

technology for a hypersonic air-breathing SSTO vehicle, while 

Rocketdyne and Pratt & Whitney competed to develop engines. 

In 1990, the companies joined under the leadership of Rockwell 

International to overcome technical and budgetary obstacles.

Development on the X–30 (Fig. 11), as it was then designated,

continued until 1993, when it was terminated amid budget cuts 

and technical concerns. 

As the NASP project came to an end, the Hypersonic 

Systems Technology Program (HySTP) was initiated to 

continue joint NASA and U.S. Air Force cataloguing of 

hypersonic technology. In January 1995, the Air Force ended 

its participation in HySTP, marking the true end of NASP. 

Glenn researchers were fully involved in NASP from its initial 

planning stages, through its implementation, its termination, 

and even beyond in the generic hypersonic research that 

followed NASP on a smaller scale. Glenn staff made 

significant contributions to air-breathing hypersonic propulsion under NASP, including advancing fundamental technologies, such as the fuel-air mixing mechanism for effective 

supersonic combustion and technologies for components, 

including inlets, isolators, and nozzles. A number of 

computational and experimental studies were carried out to
NASA/TP—2013-216524 15

Figure 11.—Artist’s concept of X–30 single-stage-to-orbit 

vehicle.

Figure 12.—X–43 scramjet test aircraft.

Figure 13.—X–51 scramjet test vehicle.

provide much needed understanding of the flow physics 

involved in the performance of key propulsion system 

components—the inlet, combustor, isolator, and nozzle—

which were highly integrated with the airframe of the 

hypersonic vehicle. The performance of these key components, as the vehicle transitions from lower speeds to 

hypersonic speeds, is crucial to the stability and overall 

performance of the vehicle. Glenn played a key role in 

enhancing the fundamental understanding of and advancing 

the technology for stable and efficient operation of the highly 

integrated propulsion system for the entire range of the vehicle 

operating conditions for the mission.

After NASP and HySTP ended, NASA management issued 

small contracts to examine scaled-down, but realistic, vehicles 

(size was a major cost driver) that could be constructed and 

flown to demonstrate the NASP technology. NASA initiated a 

third effort to develop a flight vehicle, the Hyper-X program, to 

demonstrate that scramjet engines could be designed, 

constructed, and flown at the high specific impulses necessary 

for access-to-space vehicles. Looking much like a scaled-down 

X–30, the Hyper-X (known as X–43) was a small, unpiloted 

vehicle intended to test an integrated scramjet engine from 

Mach 7 to 10. The X–43 was designed to be mounted on the 

nose of a Pegasus rocket carried aloft and released by a B–52. 

After the Pegasus powered the test craft to about 100 000 ft 

(30 480 m), the X–43 would separate and its scramjet engine 

would be ignited. The Hyper-X would fly for only a few 

seconds before falling into the ocean, but the data collected 

from these test flights were to be used to develop practical 

hypersonic scramjet engines for future vehicles. Three unpiloted 

X–43A (Fig. 12) research aircraft were built. Each of the 12-ftlong, 5-ft-wide (3.7-m-long, 1.5-m-wide) lifting-body vehicles 

was designed to fly once and not be recovered. The first and 

second vehicles were designed to fly at Mach 7 and the third at 

Mach 10. The first flight attempt in June 2001 failed when the 

booster rocket went out of control, but the second and third 

attempts resulted in highly successful, record-breaking flights. 

Mach 6.8 was reached in March 2004, and Mach 9.6 was 

reached in the final flight in November 2004.

The most recent hypersonic vehicle effort is the X–51 

program, a collaborative effort of the U.S. Air Force Research

Laboratory, the Defense Advanced Research Projects Agency, 

Pratt & Whitney Rocketdyne, and Boeing. The X–51A 

(Fig. 13) was designed to demonstrate the flight of a scalable, 

robust endothermic hydrocarbon-fueled scramjet propulsion 

system in the Mach 4.5 to 6.5 range. During its first flight test 

in May 2010, after being released from a B–52, the solid 

rocket U.S. Army Tactical Missile booster ignited and took the 

X–51A Waverider to approximately Mach 4.5; then the 

scramjet engine took over and accelerated the vehicle to 

approximately Mach 5.0 for about 140 s. The scramjet engine 

decreased thrust and acceleration for another 30 s before the 

test was terminated. The test was the longest of its kind, 

beating the previous record of 10 s set by the X–43. The data 

are being analyzed before the next flight is scheduled. Glenn

support of the computational investigation of the scramjet flow 

field in ground test and flight configurations helped significantly in validating the CFD tools and in meeting the need for 

physics-based models to accurately simulate the effect of 

key flow phenomena, and therefore performance could be 

predicted with reasonable accuracy.
NASA/TP—2013-216524 16

Return to Subsonic Transportation, 

Energy, and Environment: 1990s–2000s

A subsonic transportation technology project was established in the 1990s, a few years after HSR commenced. The 

Advanced Subsonic Technology (AST) program was an 

intellectual offspring of the ACEE program of the 1980s and 

was one of the major NASA aeronautics programs that began 

in 1993. It involved research at Glenn in combustor emissions, 

fuel efficiency, composites technology, and noise reduction. It 

included NASA, GE Aircraft Engines, Pratt & Whitney, 

Allison Engines, and AlliedSignal Engines. The high-level 

objectives of the propulsion portion of AST included a 

demonstration of a 1500 °F (816 °C) compressor, a demonstration of a low-emissions combustor at 3000 °F (1649 °C),

an integrated component technology demonstration in a highpressure ratio core, a large-scale demonstration of a highefficiency turbine, and a demonstration of technology to 

reduce, in just 6 years, the uninstalled engine noise levels by 

3 to 4 dB, relative to the state of the art. Although AST was an 

important technology program, its funding support was shortlived, and AST was terminated in 2000. 

AST was replaced in 2000 with the Ultra-Efficient Engine

Technology (UEET) program, which had fewer resources and 

less industry involvement in comparison with AST (National 

Research Council, 2002). UEET’s mission was to develop and 

transfer revolutionary turbine engine propulsion technologies to 

industry. The technology development program was designed to 

address fuel efficiency and emissions reduction to decrease 

ozone depletion and the role of airplanes in global warming 

(Shaw, 2001). The goal for emissions reduction was 70 percent

below 1996 International Civil Aviation Organization standards 

for NOx emissions at landing and takeoff. In addition, UEET 

was to address potential ozone depletion concerns by demonstrating combustor technologies that would prevent any 

discernible aircraft impact on the ozone layer during cruise 

operation (up to a 90-percent reduction). The goal for fuel 

efficiency was an overall fuel savings of about 15 percent for 

large subsonic transport and up to 8 percent for supersonic or 

small aircraft. Glenn managed the program, but other NASA 

centers, engine companies (GE, Pratt & Whitney, Honeywell, 

Allison/Rolls-Royce, and Williams International), and airplane 

manufacturers (Boeing and Lockheed Martin) also participated. 

In addition, the team collaborated with other Government agencies, such as the Department of Defense, Department of Energy, 

EPA, and FAA.

Glenn’s organizational structure has adapted over the last 

70 years depending on the focus of the Agency’s mission and 

the work assigned to the Center. During the early years of 

World War II, the focus was on developing gas turbine engine 

technology equal to that in Europe, but in the postwar period, 

the focus was on fundamental research in various technical 

disciplines. When Glenn became part of NASA, space 

propulsion received more emphasis; and when the energy 

crisis occurred, energy efficiency received more attention. 

Subsequently, environmental issues, such as noise and 

emissions, were added to energy efficiency, and the current 

aeronautics projects reflect these priorities. Glenn’s current 

organizational structure is based on assigned work for current 

aeronautics and space projects that are based on the Center’s 

core competencies.

Current Aeronautics Research Programs

NASA’s aeronautics research programs were restructured in 

2005 when Lisa Porter was appointed NASA’s Associate 

Administrator for the Aeronautics Mission Directorate. 

Aeronautics research was reorganized into four programs: the 

Fundamental Aeronautics Program (FAP) to conduct longterm, cutting-edge research in the core competencies of aeronautics in all flight regimes, the Aviation Safety Program 

(AvSP) to develop unique safety-related research capabilities 

to improve the safety of new and legacy vehicles as well as to 

overcome safety technology barriers, the Airspace Systems 

Program to address air traffic management R&D needs, and 

the Aeronautics Test Program to protect and maintain key 

research and test facilities. The FAP produced knowledge,

data, capabilities, and design tools in the old NACA style.

Industry partnerships were to be transformed from near-term, 

evolutionary procurements to long-term, intellectual partnerships to provide long-term, stable investment in capabilities 

that would benefit all of the aviation industry. The FAP was 

organized into four projects: Subsonic Fixed Wing, Subsonic 

Rotary Wing, Supersonic, and Hypersonic. In 2010, a fifth 

program, the Integrated Systems Research Program was added 

to mature technologies that had already proven their merit at 

the fundamental research level and to transition them more 

quickly to the aviation community. The emphasis is on integrated system-level research of interest and importance to the 

aviation stakeholder community. AvSP consists of three projects: the System-wide Safety Assurance Technologies project, 

Vehicle System Safety Technologies, and Atmospheric 

Environmental Safety Technologies (AEST). Glenn is advancing air-breathing propulsion technology at the fundamental, 

component, and system level in all the programs within the 

Aeronautics Mission Directorate.

The National Aeronautics Research and Development 

Policy was established by an Executive Order for the first time 

in December 2006 to help guide U.S. aeronautics R&D 

programs through 2020 (National Science and Technology 

Council, 2006). NASA’s Aeronautics Research Mission 

Directorate’s plan, developed in conjunction with the National 

Aeronautics Research and Development Policy, established 

aeronautics R&D challenges, prioritized goals, and timephased, long-term objectives for the Nation’s research to 

benefit the public in civil aviation (Porter, 2007; Alonso, 
NASA/TP—2013-216524 17

2008). The plan calls for collaborative partnerships with other 

Government agencies, academia, and industry and ensures 

the availability of world-class personnel, facilities, knowledge, and expertise. The Aeronautics Research Mission 

Directorate’s FAP has identified near-term, midterm, and 

long-term technology development goals in subsonic and 

supersonic air transport, as well as in the hypersonic technology advancement needed to enable affordable space access 

for low-Earth-orbit applications. 

In the subsonic regime, goals for the near and long term 

(extending to 2025) are in the areas of (1) emissions—NOx, 

particulate, and green-house gases, (2) noise, and (3) fuel 

efficiency. The quantitative targets for near-term, midterm, 

and long-term technology development, and the baseline 

subsonic and supersonic reference vehicles were presented at 

the second annual meeting of the FAP (Alonso, 2008). The 

goals and the timeframes were based on consultations with 

industry and on studies of system-level impacts that the 

various technology advancements are likely to have in the 

corresponding timeframes. The goals will be reviewed periodically and adjusted as necessary to reflect the revised outlook 

of the technology advancement path. The noise, emission, and 

performance goals for subsonic aircraft are consistent with 

those of Europe, as outlined in their aviation and aeronautics 

research plans. One example is the integrated approach of 

focused propulsion research along with cross-cutting applications under the Clear Sky Technology project sponsored by 

the Advisory Council for Aeronautics Research in Europe in

2001: Vision 2020. Glenn’s research is aimed at advancing 

propulsion technology and meeting the propulsion system 

targets (which were based on systems analysis) in all three 

areas. To meet the stringent, long-term goals (2025), we 

anticipate that unconventional architecture may be needed for 

the airframe as well as the propulsion systems (Fig. 14). 

For supersonic transportation—in addition to emission and 

noise reduction goals during takeoff and landing—sonic 

boom, high-altitude emissions, and cruise efficiency have been 

identified as areas of research. Quantifiable goals must be met 

to enable economically viable supersonic air transport while 

minimizing the impact on the environment. Sonic boom reduction, cruise-efficient propulsion, aeroservoelasticity, highaltitude emissions, and the development of lightweight and 

high-temperature materials are the areas of active research at 

Glenn. Near-term, midterm, and long-term system-level 

targets in fuel consumption, sonic boom, airport noise, and 

emission reductions have been established. For the near term, 

technology development is planned for business-jet-sized aircraft flying at Mach 1.6 to 1.8. For the midterm, the goal is a 

commercial jet with 70 to 80 passengers flying at Mach 1.6 to 

1.8. For the long term, the aircraft will approach the size of a 

large commercial jet with about 200 passengers flying at Mach

1.8 to 2.0 (Fig. 15). 

For hypersonic speeds, a balanced portfolio of combinedcycle air-breathing propulsion and entry, descent, and landing 

Figure 14.—Subsonic advanced architecture concept to meet 

long-term project goals (Kim et al. 2008).

Figure 15.—Notional N+3 (long-term) supersonic aircraft 

(Alonso 2008).

(EDL) on planets with an atmosphere have been identified as 

the major technology areas for supporting affordable access to 

space. However, all the R&D work related to EDL was recently 

moved to the purview of the Office of the Chief Technologist, 

which funds advancements in space technology. Glenn and 

Langley are carrying out the remaining hypersonics work in 

combined-cycle and scramjet air-breathing propulsion work. 

Starting in fiscal year 2013, because of budget constraints, 

NASA terminated all hypersonic air-breathing propulsion 

research. Glenn’s hypersonic air-breathing propulsion work 

included turbine-based combined-cycle propulsion research and 

the advancement of fundamental technologies, such as the fuelair mixing mechanism for effective supersonic combustion; 

technologies for components, such as inlets, isolators, and 

nozzles; and technologies for integrating the components using 

computational and experimental techniques. Mode transition 

from low-speed (Mach 3 to 4) to high-speed dual-mode 

scramjet operation is considered to be a critical technology 

challenge to make combined-cycle propulsion viable for 

hypersonic air-breathing propulsion. To address this challenge, 

Glenn staff have designed and built, and tested in Glenn’s

10×10, the combined-cycle-engine large-scale inlet mode transition experimental configuration (Fig. 16).

Under the AEST project of the AvSP, Glenn is building on 

its strong competency and is continuing to make significant 

contributions to advancements in icing research. Glenn 
NASA/TP—2013-216524 18

Figure 16.—Combined-cycle engine—large-scale inlet mode 

transition model in the 10- by 10-Foot Supersonic Wind 

Tunnel.

researchers are working to reduce flight test cost and to improve 

safety by developing methods to simulate, experimentally and 

computationally, the process of ice growth on aircraft surfaces. 

The aircraft industry and the Government use these methods for 

design, analysis, and certification efforts. Ice growth needs to be 

modeled on swept wings and on future-generation aircraft 

configurations (e.g., the blended-wing body). In addition, 

supercooled-large-droplet ice growth needs to be modeled and 

validated with a comprehensive database, which is lacking at 

present, to understand the ice accretion phenomena and to 

develop intelligent controls that can respond to an icing 

encounter. After icing in engines recently caused several engines 

to lose power when the aircraft flew in convective cloud environment with high-ice-water-content ice crystals, the FAA and the 

aircraft industry asked Glenn to lead engine icing research to 

improve understanding of the ice growth process within engine 

flow paths and how it affects engine performance. In response, 

Glenn, along with domestic and international partners, initiated a 

High Ice Water Content Flight Campaign under AEST to 

provide insight into cloud properties that cause engine power 

loss. In addition, Glenn has added icing capability in its 

Propulsion System Laboratory to enable full-scale engine testing 

in icing environments. For this effort, methods will be developed 

to simulate, experimentally and computationally, the degradation 

in aircraft engine performance during in-flight icing conditions. 

These methods will then be used for design, analysis, and 

certification efforts by the aircraft industry and Government, and 

to add input to controls-based remediation efforts.

Future Technology Trends

On the basis of the global energy demand forecast, there will 

be extraordinary pressure on the transportation industry in 

general and on aviation in particular to advance technologies to 

improve fuel efficiency. Aircraft engine technologies that will 

increase the overall efficiency of engines will be the focus of the 

aviation propulsion research community for the foreseeable 

future. For the long-term (2025 to 2030), a number of advanced

propulsion concepts are being considered because of the 

possibility of some emerging technologies. For example, the 

maturing of high-temperature superconductor technology during 

this timeframe will enable some alternative propulsion concepts, 

such as the electric drive and hybrid propulsion devices. Even in 

conventional gas turbines, concepts such as recuperative cycles, 

including interstage cooling of the compressor for very high 

operating pressure ratios (100 and above), are being considered. 

Advances in materials technology are anticipated to lead to 

lightweight materials that can enable the very lightweight heat

exchangers needed for these aggressive cycles. Other concepts, 

such as constant-volume combustion to increase thermal efficiency, integrated energy optimization for propulsion and powerconsuming devices, such as auxiliary power units, and the 

utilization of energy dissipated during landing also are being 

considered for long-term propulsion (Reddy and Blankson, 

2010). 

Alternative fuels with low carbon content, such as liquefied

natural gas combined with optimal operations, such as flying 

at lower cruise speeds and formational flying might help to 

reduce the carbon footprint of aircraft, which are likely to 

receive increasing emphasis over the next 30 years. 

Alternative power plant concepts, such as fuel cells, need to 

have power densities (power per unit weight) comparable to

those of modern gas turbine engines to be considered for 

aircraft propulsion. Even though research in these concepts is 

being actively pursued for terrestrial applications (e.g., 

automotive and electric power), based on the current rate of 

technology advancement, the power densities of fuel cells are 

not going to be comparable with those of gas turbines within 

the next 15 years. However, with technology advances in 

high-energy-density batteries and lightweight electric motors, 

the aircraft engine industry is exploring the possibility of 

developing some form of hybrid propulsion technology (a 

combination of battery/fuel cell and gas turbine engine) or allelectric propulsion within 25 years.

For the very distant future (∼50 years), concepts such as 

low-energy nuclear reaction might hold some promise 

depending on how the feasibility of the technology progresses 
NASA/TP—2013-216524 19

for aeropropulsion applications. The ongoing and anticipated 

advances in high-temperature superconducting electric technology are expected to yield the needed breakthroughs to 

make electric turbine power transmission an efficient mechanism to meet the efficiency and noise goals for the near term 

to midterm. Lightweight generators and motors can be 

exploited (Kim et al., 2008) to distribute the fans over a wide 

area and to exploit the very large bypass ratio and boundary 

layer ingestion configuration advantages, yielding very high 

overall engine efficiency. Control and aerodynamic benefits 

are possible if the power-generating gas turbines are distanced 

from the fans in an optimum location (Fig. 14).

Commercial viability will be the dominant factor in 

technology development roadmaps for supersonic propulsion. 

Sonic boom mitigation for overland supersonic flight, cruise 

efficiency, and cruise emissions, in addition to airport noise 

and emission limits, will continue to be enabling technologies 

for commercial applications. Future technologies might 

include near-stoichiometric combustion; staged combustion; 

multifunctional components, such as continuous-wave 

combustion, and adaptive structural and material systems; 

multiaxis turbine engines; multifan cores; off-axis topping 

cycle cores for distributed thrust acoustics at takeoff and 

improved cruise efficiency; and highly integrated and 

continuously variable airframe and propulsion systems.

In the hypersonic speed regime, if the technology development satisfactorily addresses all the technical challenges 

and reaches a technology readiness level that can fully exploit 

the higher effective specific impulse of the air-breathing 

system, air-breathing propulsion will become more attractive 

than rocket propulsion for access to space. Some of the 

challenges include the higher aerodynamic heating and structural loads caused by the higher dynamic pressure trajectories 

needed to derive benefit from longer air-breathing propulsion 

durations and the associated complicated staging issues. 

Significant advancements in materials and better understanding of the various underlying key flow phenomena and 

their complex interactions will be needed to address these 

challenges. In addition, computational tools will need to be 

advanced significantly at the component and system levels to 

reduce uncertainty and to be used effectively in trade studies 

so that different propulsion systems can be assessed and 

compared accurately.

Concluding Remarks

This report briefly outlined the research and technology 

development programs and accomplishments in air-breathing 

propulsion at the NASA Glenn Research Center since its 

inception in 1941. Through the initial stages of growing pains 

and various budget priority cycles, Glenn management and 

staff have always been able to maintain a healthy balance 

between conducting fundamental research and advancing 

technology to higher levels to enable effective technology 

transfer to industry. Partnerships with academia, the aeropropulsion industry, and other Government agencies, 

wherever appropriate, have played a key role in enabling 

Glenn to maintain its leadership in the development and 

transfer of aeropropulsion technology. As we face the current 

austere budget environment, knowledge of how the Center has 

been able to adapt to the changing needs and political climate 

of our Nation while maintaining its focus on core NASA 

values could be a very valuable guide as we plan the future.

Glenn Research Center

National Aeronautics and Space Administration

Cleveland, Ohio, September 9, 2013 

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