The aircraft was standing at the beginning of the runway, ready for takeoff. The engines were carefully brought to full power. Releasing the brakes, the pilot rolled his airplane forward and accelerated right to the end of the runway. Suddenly, the aircraft climbed almost vertically with unprecedented speed until it disappeared in the clouds. Franz later said he knew at that moment the jet age had begun.

The Junkers Jumo 004 was not the first turbojet invented, nor was the ME 262 the first aircraft to fly under jet power. But Franz's Jumo 004, an axial turbojet that produced 1,980 pounds of thrust, was the first jet engine to enter mass production. About 6,000 jet engines were built near the end of World War II in the face of acute shortages and damage to industry in Nazi Germany. The Jumo was brought from conceptual design to production in four years. The advanced ME 262 was not deployed expeditiously so it did not pose a serious threat to Allied air superiority.

Along with the introduction of low-wing monoplanes (circa 1935), jet propulsion allowed a sudden leap in fighter-aircraft speeds. The pioneers of the turbojet revolution were Frank Whittle in England and Hans von Ohain in Germany. Both of them envisioned flight speeds greater than 500 miles per hour at altitudes of 30,000 feet and above.

The first completely jet-powered flight, of the Heinkel He 178 powered by von Ohain's HeS3B turbojet, took place on Aug. 27, 1939, a few days before the start of World War II. Ernst Heinkel immediately informed German air-ministry officials of this momentous event, but he was met with indifference. Although ministry officials eventually showed some interest, in the end they did not use Heinkel's designs.

The cable pull-starter handle is located in the nose cone within the intake nose cowling of the Jumo 004

Two enterprising engineers working in the German air ministry, Helmut Schelp and Hans Mauch, had tried to interest the traditional aeroengine manufacturers in jet-engine development. In early 1939, Otto Mader, who was head of Junkers Engine Co. in Dessau, Germany, told Schelp that even if there was something in "this jet idea," he had no one to put in charge of such a complex project. Schelp suggested that Franz, who was then in charge of internal aerodynamics and turbo-supercharger development, be assigned to this project.

Born in Schladming, Austria, in 1900, Franz had studied mechanical engineering at the Technical University of Graz in his native Austria and earned a doctoral degree from the University of Berlin. Franz worked as a design engineer at a company in Berlin, where he developed hydraulic torque converters. In 1936, he joined Junkers, where he eventually became chief engineer. He was head of turbo-supercharger development when he was put in charge of designing the Jumo 004 turbojet. This engine was a success due to his leadership and the choices he made in terms of design compromise.

From the outset, Franz made a deliberate decision that his design would not aim for the maximum achievable performance but would instead focus on a conservative goal that had the greatest chance of success. He did not aim high, because he knew that failure to develop an engine quickly enough might cause Junkers or the air ministry to drop the entire program. His decision ensured that the Jumo 004 was the first jet engine to reach production.

At first Franz was given only a few people from his supercharger department, but his group grew steadily to about 500 people in 1944. According to Franz, there were never any constraints in terms of funding; also, his facilities were well-equipped with test rigs and stands, and they even had an altitude-chamber test cell.

With no time to design individual engine components, Franz designed an experimental engine, the 004A, that would be thermodynamically and aerodynamically similar to the final production engine. The goal in developing the 004A was to have an operating engine in the shortest time, without taking into account engine weight, manufacturing considerations, or the use of strategic materials. Even though Franz was familiar with centrifugal compressors from his supercharger work, he chose an axial compressor design because he was convinced that the frontal area was of fundamental importance and that gains could be achieved in efficiency with an axial design. The 004's compressor had a peak efficiency of 82 percent and an operating efficiency of 75 to 78 percent. The compressor used reaction blading and had a pressure ratio of 3.14:1 in eight compression stages. The engine airflow rate was 46.6 pounds per second. The turbine was based on the steam-turbine experience of AEG in Berlin and did not use a vortex design.

Franz recognized the superiority of an annular combustor design, but he opted for a six-can combustor because it would present less of a problem and permit bench testing with a single can. When Schelp reviewed the design of the Jumo 004, he was critical of its conservatism compared with the BMW 003, but he did not try to force Junkers to make any changes.

In the spring of 1940, the 004A made its first test run; by January 1941, the engine was brought to its full speed of 9,000 rpm and a thrust of 946 pounds. At this juncture, the engine's compressor was plagued by vibration failures in the sheet-metal stator vanes, which were originally cantilevered from the outside. Max Bentele, a renowned blade-vibration specialist, was asked to help solve the problem. The stator's design was changed, and by August 1941 a thrust of 1,320 pounds was attained. In December 1941, a 10-hour run at a thrust of 2,200 pounds was demonstrated. The engine was flown in an ME 110 test bed on March 15, 1942, and on July 18 the first flight of the ME 262 powered by two Jumo 004 jets took place. The flight lasted 12 minutes.

Based on the excellent flight results, the air ministry issued a contract for 80 engines. These 004B engines, rated at a thrust of 1,850 pounds, were used for further development and airframe testing. The 004A could not be mass-produced because of its considerable weight and its high use of nickel, cobalt, and molybdenum, but the 004B was designed to use a minimum of strategic materials. All the hot metal parts, including the combustion chamber, were changed to mild steel (SAE 1010) and were protected against oxidation by aluminum coating.

Extensive air cooling was used throughout the engine. A later version of the 004B engine had hollow air-cooled stator vanes. Compressor discharge air was used to cool the blades. With hollow blades of Cromadur sheet metal, the complete 004B engine contained less than 5 pounds of chromium. The first production model of the 004B weighed 220 pounds less than the 004A. Additional modifications were made to the first compressor stages. A series of 100-hour tests were completed on several engines, and time between overhaul of 50 hours was achieved.

During the summer of 1943, a sixth-order excitation caused several turbine-blade failures. Franz resorted to asking a professional musician to stroke the blades with a violin bow and use his trained musical ear to determine the ringing natural frequency. The air ministry, however, was getting increasingly impatient and scheduled a conference in December 1943. Bentele attended the conference and listened to the numerous arguments pertaining to material defects, grain size, and manufacturing tolerances.

When his turn came, Bentele told the assembled group that the culprits were the six combustor cans and the three struts of the jet nozzle housing after the turbine. These induced forced excitation on the turbine rotor blades where a sixth-order resonance occurred with the blade-bending frequency in the upper speed range. The predominance of the sixth-order excitation was due to the six combustor cans (undisturbed by the 36 nozzles) and the second harmonic of the three struts downstream from the rotor. In the 004A engine, this resonance was above the operating speed range, but in the 004B it had slipped because of the slightly higher turbine speed and the higher turbine temperatures. The problem was solved by raising the blade's natural frequency—increasing blade taper, shortening the blades by 1 millimeter, and reducing the operating speed of the engine from 9,000 to 8,700 rpm.

In 1936, when the first work on turbojets began, a high-temperature Krupp steel known as P-193 was available. This material, which contained nickel, chromium, and titanium, could be given good high-temperature strength by means of solution treating and precipitation hardening. Franz used an improved version of P-193 known as Tinidur.

The first turbine blades of the 004A version were solid. Early tests showed that even supposedly identical blades would have a large scatter life. By 1944, Junkers had solved the problem and obtained uniform quality of the blade by close control of manufacturing, especially of the critical forging process. Attempts to produce hollow blades by folding flat sheets of Tinidur and welding down the trailing edge resulted in failure, as Tinidur could not be welded. Eventually, a deep drawing process was used, in which the stock for the blade was a flat circular blank. Hollow blades could be manufactured faster than solid blades by this process.

To do away with Tinidur's 30-percent nickel content, Krupp developed the alloy called Cromadur, which proved easy to weld. The process of folding the blade flat and welding it turned out to be superior to deep drawing, so the Cromadur blades proved more reliable than the Tinidur blading despite Cromadur's lower creep strength.

In the Jumo 004's inlet section, the diameter of the intake was 20 inches. The circular nose cowling contained two annular gas tanks. The 0.75-gallon upper gas tank contained fuel for the two-cylinder, two-stroke, horizontally opposed gasoline starter engine made by Riedel. This engine, which produced 10 horsepower at 6,000 rpm, had its own electric starter motor, but for emergencies it also had a cable pull starter in the nose cone. The 3.75-gallon lower tank fed starting fuel to the combustion chambers.

The Jumo 004's inlet section contains the Riedel starter motor, the auxiliary drive, and the front compressor bearing; the starter motor's fuel tank is within the nose cowling

The starter engine was bolted to the six struts in the casing that contained bevel gears to drive the accessories. Two drive shafts were used, one of which extended down to the main oil pumps inside the lower part of the casing. The rear part of the casing housed the front compressor bearing mounted in steel liners set in a light hemispheric housing. The housing was kept in contact with the female portion of the intake housing by the pressure of 10 springs held in place by a plate bolted to the intake casting. The outer bearing races were mounted in separate sleeves that fit on the compressor shaft. This design allowed preloading of the bearings to ensure an even distribution of thrust. The other advantage was that the bearing assembly could be left intact during disassembly by withdrawing the compressor shaft from the inner sleeve.

The Jumo 004 compressor was an eight-stage unit with an airflow rate of 46.6 pounds per second and an outer casing of uniform diameter. The compressor rotor was made of eight aluminum disks held together by 12 bolts and located by spigots. The entire assembly was pulled together by a 38.75-inch-long tie rod (0.75 inch in diameter), estimated to have a stress of 40,000 pounds per square inch with a pull force on the assembly of 16,000 pounds.

The first two rows of the compressor each had 27 stamped aluminum blades and the rest of the stages had 28. All had machined roots that fit into the pyramidal slots in the rotor disk. The stagger of the blades increased and the chord decreased in successive stages. The rotor turned on two steel shafts attached to the outside faces of the first and last discs. The compressor's front bearing was made up of three ball races, each capable of taking end thrust. The rear bearing consisted of a single roller race.

Cooling airflow was derived from between the fourth and fifth compressor stages, and led to the double skin around the combustion-chamber assembly. Most air passed down one exhaust cone strut to circulate inside the cone and through small holes to cool the downstream face of the turbine disk. Air was also taken in through three tunnels in two of the casting ribs and into the space between the two plate diaphragms in front of the turbine disk. Most of this air passed through the hollow turbine nozzle guide vanes, emerging through slits in the trailing edges.

The Jumo 004 had six combustor cans arranged around the central casting. This casting carried the rear compressor bearing and the turbine shaft bearing. Three cans carried spark plugs. The engine was designed to run on diesel fuel. The approach to combustor design was to have a flame chamber region in the combustor for primary combustion at a close-to-stoichiometric ratio. To obtain good mixing and a short flame length, the primary combustion air was introduced in this chamber with swirl, and fuel was injected with a swirl against the airflow. The combustion chambers were made of aluminized sheet steel.

The turbine, designed in collaboration with AEG, had a degree of reaction of 20 percent, which represented a compromise between AEG, which wanted less, and Junkers, which wanted more (from afterburner considerations). The single-stage turbine had 61 blades fixed to the turbine disk by a formed root and kept in position by rivets. The production version had air-cooled hollow blades. The absolute discharge velocity was 663 feet per second. The enthalpy drop across the turbine was approximately 64 Btus per pound. A movable 'bullet' was mounted in the tailpipe and controlled by a servomotor to vary the nozzle area.

Bringing the Jumo from conceptual design to production in a span of four years at the dawn of the jet age was a pioneering achievement for Franz. After the war, Franz came to the United States, where he worked for the U.S. Air Force; in 1951, he joined Avco Lycoming and soon moved to Stratford, Conn. He established the gas-turbine department there and was responsible for several successful engine-development programs, including the T53 (which powers the U.S. military's AH-1S Cobra, Grumman OV-1 Mohawk, and Bell UH-1 helicopters) and T55 series of turboshaft engines, as well as the T55 high-bypass turbofan (named the ALF502). In the 1960s, Franz led a team to design the three-spool, 1,500-shaft-horsepower AGT-1500 gas turbine, the power plant for the U.S. M1 Abrams main battle tank. He retired as vice president of Avco Lycoming in 1968.

Franz, who passed away in 1994 at the age of 94, was a Fellow of ASME and received numerous awards, including the U.S. Army Outstanding Civilian Service Medal, the R. Tom Sawyer Award from ASME, and the Grand Decoration of Honor from Austria.

This article was adapted from an ASME paper presented at the 1996 International Gas Turbine and Aeroengine Congress and Exhibition in Birmingham, England.

Cyrus B. Meher-Homji is an engineering specialist at Bechtel Corp. in Houston