One Step Ahead, pt II

A Step Ahead in Propulsion

To understand why ii is so unique it is perhaps best to explain what a jet engine is and how it evolved. Though it does not look it at first glance, the jet engine is, in fact, simpler than the piston engine that first took man into the air. Its underlying principle can be explained by Newton’s third law of motion: For every action, there must be an equal and opposite reaction. Shaped like a cylinder in which both ends are open, air enters the jet through the front. That air is then mechanically compressed and fed into a combustion chamber, into which fuel is sprayed. The oxygen in the air burns intensely, and as its temperature and pressure rise, it expands and rushes with great force out the rear. As Newton demonstrated, that rearward exit must have an equal and opposite reaction, namely a forward thrust of exactly the same magnitude. It is that forward thrust that propels the aircraft. (Part of the energy released by the expanding gas is also used to spin the turbine that initially compresses the entering air. )

Using jets to propel aircraft was proposed as early as 1908, but it was not until 1930 that a patent was taken out by Frank Whittle, a 23-year-old officer in the Royal Air Force. After obtaining his patent, Whittle sought financial backing for his experiments from the British government. The Government’s firm but polite turndown must rank as one of history’s prime examples of official obtuseness. “Scientific investigation into the possibilities has given no indication that this method can be a serious competitor to the airscrew-engine combination,” proclaimed the Under Secretary of State for Air in 1934. Whittle found private help, and by 1939 the government had reversed itself and agreed to pay for future testing. With Whittle’s engine inside, the first British jet was flown on May 15, 1941. “My God, chaps!” one witness to the plane’s flight exclaimed in the officers’ mess that night, “I must be going round the bend. It hasn’t got a propeller!” That was not the first flight by a jet, however. The Germans, who had started later, had worked faster, using the ideas of one of their own scientists, Hans von Ohain. A jet-powered Heinkel He 178 was flight tested on August 27, 1939, just five days before Hitler invaded Poland. Under wartime pressure, progress was rapid, and by 1944 both the Germans and the British were flying their first jets in combat.

The British had sent one of their new engines, together with its drawings, to the United States not long after that first flight in 1941, and General Electric was chosen by Washington to build an American adaptation. Just a year later, the first American jet fighter, the Bell XP-59, took to the air. Lockheed was given responsibility for further work, and in January 1944, its XP-80 was also flown, reaching a speed of a little more than 500 miles an hour. The XP-80 nicknamed the Shooting Star, was put into service toward the end of World War 11, and in November 1950, it tangled with aMiG-15 over Korea. The Shooting Star came out on top in history’s first jet-to-jet dogfight.

Though it worked on the same principle, the engine finally employed by Germany was very different from von Ohain’s. Both Whittle and von Ohain favoured a centrifugal flow design, which increased compression by increasing diameter. The Junkers 004 engine designed by Dr. Anselm Franz was axial flow. Compression was increased by adding compressor stages, one behind the other. His engine was longer, but narrower, with a smaller frontal area offering resistance to the air. That meant less drag, particularly at high speeds. Franz’s concept eventually prevailed, and the axial flow design is the basis for most of today’s jet engines.

In either case, those early systems were pure turbojets, simple but wasteful of fuel. In the 1950s and 1960s engineers made a significant leap forward in thrust and efficiency with a modified engine called the turbofan. In the turbofan, an additional series of compressor-like blades called a fan is placed in front of the compressors which diverts part of the incoming air around the outside of the compressor. That air bypasses the core and rejoins the air that has gone through the core at the engine’s rear, thereby increasing thrust at minimal cost in fuel. For military use, both types of engines often have an afterburner attached where the hot air exits. A combustion chamber in the afterburner sprays more fuel into the mixture of gases and raises temperatures still further, to 3,000°F and above. This superheating increases exit velocity above what it would otherwise be and, again in accordance with Newton’s third law, gives the aircraft a correspondingly greater push forward. The afterburner of a turbojet can boost thrust by 55%; the afterburner of a turbofan can raise it by 70%. In either case, to reach those high temperatures the afterburner demands an enormous amount of fuel. Pilots usually rely on it therefore only for short dashes.

Just as airframe designers must consider the trade-offs between speed and maneuverability – high wing loading vs. low wing loading – so too must engine designers balance such conflicting factors as compression and weight. If air is more highly compressed, for example, it burns far more efficiently in the combustion chamber than it would at lower compression: Doubling compression reduces fuel consumption by one-quarter. But the trade-off is that increasing compression also means that more compression stages are necessary. These stages add weight to the engine and reduce the all-important thrust-to-weight ratio.

Similarly, an increase of the temperature in the combustion chamber, which raises the velocity of exiting gases, can be directly translated into greater thrust. Combustion at 2,500°F produces two-and-a-half times the thrust of combustion at 1,500°F. Again, however, there is a trade-off. This added thrust is valuable only if it is not offset by a too-large increase in the weight of the engine that might be made necessary by heavier, therefore more heat-resistant, parts.

In designing jet engines, engineers must strike a balance among all these conflicting factors and produce a strong, durable system at the same time. At very high heat, for instance, a metal temperature increase of as little as 50% can cut the life of turbine blades in half. Since the key to thrust is the intensely high heat that raises the velocity of gases exiting the engine, the challenge to engine designers is, even as it was in the days of pistons, a challenge to metallurgists as well. For it is the development of lightweight but heat-resistant metals that permits designers to increase the all-important turbine temperature.

Developing the F100

If the FX was to meet the standards of speed and maneuverability the Air Force has set for it, its engine would have to be advanced beyond any yet built, or perhaps even conceived. After establishing performance specifications, the Air Force issued initial engine development contracts to General Electric and Pratt & Whitney Aircraft in August 1968. Both companies prepared proposals and built demonstration engines. Both pushed the state of the art further than it had gone before, with significant refinements in all areas of design, including weight, thrust, and fuel consumption.

Less than two years after the development contracts went out, Pratt & Whitney was awarded the final contract for the engine, which was later designated the F100. Pratt & Whitney’s engine produces 23,820 pounds of thrust but weighs only 3,063 pounds, giving it a thrust-to-weight ratio of approximately 8:1. Only one previous engine, the J85, had achieved such a high thrust-to-weight ratio; but it provided only 5,000 pounds of thrust. And though the Pratt & Whitney TF30-P-100, used in the F-111, has a thrust equivalent to the F100’s, the F100 weighs 25% less a crucial difference. No other engine offers so much thrust from so little weight. The F100 links up three fan stages and ten compression stages, for an extremely high compression ratio of 23:1. To reduce weight it has used new technologies that create materials capable of sustaining the extraordinary temperatures that come from combustion. The principal among these advances is powder metallurgy: A metal is turned into powder, then heated and formed under high pressure to the shape of a part. This process yields parts more capable of tolerating high temperatures than those manufactured by earlier conventional processes.

Naval officials ordered an F100 variant, which they called the F401, for later models of the F-14. When the Defense Department later told the Navy to cut back its procurement plans for the F-14 from 550 to 390 planes, the plans for the F401 were dropped as well.

Pushing Forward – The Learning Curve

Since power is the key to performance, military pilots have always demanded engines that give more – and more after that. Engines that have been fully tested and proven in flight are not good enough simply because they have been tested and proven. What designers want – and what their country may need for survival -are engines that do not exist, engines that are a step ahead of all others. From the time of Charles Taylor and his motor for the Wright brothers, engine makers have tried to take that step, bringing together metal, fuel and their own imagination to produce machines that no one thought possible.

The best of them have succeeded, and their inventiveness can be read in the history of the fighters that are remembered: the Spitfire and P-51, the Me 262, and the F-86. But when engineers leap into unknown technologies, they also leap into unknown problems. Laboratory studies can tell them only so much about the system they are constructing. How a part will function after enduring thousands of hours of unprecedented pressures and temperatures is something that only experience can show. The result is that almost every truly advanced engine has required several years of actual flight before all of its major problems have been identified, let alone resolved.

Work on the Spitfire airframe started in January 1935, but the engine around which it was being built was far from ready. It overheated and its builders were still trying to cool it with a new ethylene-glycol mixture. The British Air Ministry, better aware of the Nazi menace than the politicians behind them, knew that they were in a race against time and pressed for speed. When the Merlin was finally ready, it was rushed to a waiting prototype, which flew in March 1936. Other problems were worked out during the following months, and when Luftwaffe bombers appeared in English skies in 1940, the Spitfire and the Merlin were there waiting. But if the Merlin had been perfect at the start – which is to say if it had been an old, tested engine – the Spitfire would not have been the Spitfire we remember and the Battle of Britain might have had a different conclusion.

By today’s standards, Frank Whittle’s jet engine, with a thrust-to-weight ratio of about 1. 5:1, was almost as primitive as the piston engine Taylor made for the flight at Kitty Hawk. Though power and efficiency have vastly improved in the last 40 years, the same basic problems that bothered Whittle also confront the makers of modern jets. While it was being run at 13,000 r. p. m. , the turbine of Whittle’s engine failed because of “creep. ” The metal in the turbine blades began stretching under the high centrifugal force it was being subjected to, and the blades became unbalanced and separated from the turbine disc. Enough of the problems were eliminated to put the engine into the first British jet, the Squirt, but turbines remain the focus of trouble in all jets.

In the United States, the Whittle engine was renamed the General Electric I-A, after its first builder. It weighed only 1,000 pounds, but it also provided only 1,250 pounds of thrust, for a thrust-to-weight ratio of 1. 25:1. That was barely enough to propel the Bell P-59 to a maximum speed of 400 miles an hour, less than that of the best available piston-powered plane. There was clearly not sufficient thrust, and the Government asked General Electric for a more powerful engine, the 1-40, that would produce 4,000 pounds of thrust, more than triple that of the I-A. To free G. E. for the development of axial flow engines, the Government turned the 1-40 development and production over to the Allison Division of General Motors which was phasing out of piston engine production. The resulting Allison J33 was used in the Lockheed P-80. That engine weighed 1,830 pounds and provided 4,200 pounds of thrust. Yet the J33- an enormous achievement for those days – remained extremely troublesome for some time.

The pressures to produce better and more powerful engines did not let up with the surrender of Japan, and the J33 was not the only engine to encounter difficulties, both in the qualification tests and in the operational aeroplanes.

The Government has two major tests, the Preliminary Flight Rating Test (PFRT) and the Qualification Test (QT), and more than one otherwise good engine failed one or both. The first test is a benchmark that certifies that the engine is safe for flight testing. The second, the Qualification Test, is a durability test. Though the pass-fail requirements are individually negotiated between the contractor and the Government for each engine, the Preliminary Flight Rating Test usually requires that an engine be run for 50 problem-free hours before a test pilot can be asked to risk his life with it in the air. By the same token, the Qualification Test usually requires that the engine be run for 150 hours (not usually of continuous testing) before the Government will order the production go-ahead.

Try as they do to pass those tests, however, builders are often delayed by the unexpected and unforeseen. The Navy’s first jet plane, the FH-1 Phantom I, made its first flight early in 1945, for example, with only one if its two J30 engines (1,600 pounds of thrust). The other was not ready. Later, the first full-production aircraft were delayed because of a lack of engines. Late deliveries of J35 engines (3,700 pounds) also caused a delay in the flight of the first American jet bomber, the XB-43. The J35 also powered the F-84 and F-89, but in 1951, at the height of the Korean War, more than a third of the F-84s were grounded because there were not enough spare parts for their engines. This shortage, along with other problems of maintenance, was a serious blow to fighter commanders in Korea. The F-89 had different problems with the J35, and at one point the entire fleet was grounded for four months: Compressor blades had disintegrated when foreign objects were accidentally sucked through the engine inlet, a common occurrence in all jets. A more powerful version of the J35 caused F-89 “power dips,” particularly at altitudes above 30,000 feet. In all, it took seven years before the biggest bugs were eliminated from the engines of the F-89.

The J40 (9,200 pounds) performed so poorly that the Navy seriously considered cancelling the plane it powered, the F3H. Eleven crashes were attributed primarily to the J40, and it was eventually replaced by the J71.

The Air Force was eager to have the advanced J47 for its new F-86s and B-45 bombers, and it put the engine into production after only 60 hours of its 150-hour qualification test. The fighter made its first flight with the J47 six months later, in May 1948. So many problems remained, however, that the engine had to be overhauled after every seven-and-a-half hours of flight, causing enormous workloads for ground crews. Even after the J47 passed its qualification test in November, it was plagued with troubles, at one time causing the grounding of all planes carrying engines of a certain series.

A later version of the F-86, the F-86D, was set back in part because a newer model J47 dropped 18 months behind schedule. It did not pass its 150-hour test until late 1952. Even so, the F-86D engine was not fully reliable for another four years. Still, problem-plagued as it was, the J47, with all its changes, was a superior engine that helped to make the F-86 the best fighter of its time. Offered a choice between the J47 and its problems and a less powerful but more reliable engine, it is doubtful that any pilot or any commander would have chosen anything other than the J47. In the skies over Korea, the extra margin of power offered by the J47 meant the difference between living and dying.

In late 1954, just after the Korean War, the Soviet Union introduced the Bison, its first nuclear bomber with enough range to strike targets in the United States and return home again. Fortunately, the Air Force had a better strategic bomber of its own, the B-52. The B-52 was originally planned as a turboprop craft, but the Strategic Air Command demanded greater speed and an entirely new engine, the J57, was built to power it. With a thrust of 10,000 pounds, this versatile engine was also adapted to propel four fighters, the F-100, the F-101, the F-102, as well as the first U. S. commercial jet airliners. In lime, the J57 was replaced by the J75, which was used in the F-105 and F-106. Good as it was, the J75 nevertheless twice failed its 50-hour preliminary flight rating test.

The standard by which most other jet fighter engines have been measured is General Electric’s J79, the motive force behind the F-4 Phantom. The Turbine Engine Division of the Air Force’s Aero Propulsion Laboratory cites the J79’s program, indeed, as its “baseline for comparison” when it examines other programs. Yet the J79, like most other engines, also had severe problems along the way.

In the early 1950s, General Electric suggested a new engine to the Air Force, the now famous J79. A derivative of the older J53, the J79 was to have two versions, the J79-1 for the B-58 bomber and the J79-3 for the F-104 fighter. In the end, many versions of the J79 were used in four different aircraft, both for the U. S. Navy and the Air Force. However, the J79 went through typical growing pains in ground and flight testing, including severe compressor problems, and its 50-hour test was not completed until the end of 1955. When it finally reached its maturity, the J79, in all its versions, was generally considered an excellent powerplant.

One of the Pratt & Whitney F-100’s major problems, ironically, is that pilots, who have never had so much performance capability, have made use of far beyond expectation of its maximum potential. Original specifications called for a power plant that could go from idle to maximum power 1,765 times during every 2,000 hours of flight time. That transition from idle (below 73%) to maximum power (above 89%) is termed the transient cycle. Actually, Pratt & Whitney went far beyond those specifications: It built an engine that could withstand over 7,000 transient cycles during a 2,000-hour period – about four times as many as the Air Force had requested.

In the summer of 1979, American F-15 pilots in Europe were averaging more than 25,000 transient cycles for each 2,000 hours of engine time. Experience showed that everyone had vastly underestimated how many times pilots would actually take their planes through the cycle. This unanticipated and unforeseen use of the F10O’s power subjected it to equally unanticipated and unforeseen strains, particularly to its turbines, the sensitive part of the jet since Whittle tested its first machine in 1941. After 300,000 engine flight hours, the F100 has experienced 47 failures of its turbine blades. The problem was exacerbated by the intense combustion heat the F100 requires for its high thrust. By way of comparison, the turbine inlet temperature of the J79 is 1,775°F, and the TF30 reading is 2,150°F. For the F100 it is 2,500°F, or about a third again as much as it is for the J79.

The other major F100 problem was engine stalling. An engine ordinarily stalls when the angle at which the air strikes the compressor blades is greater than the stall angle of the blade airfoils, during a turn for example, and the compressor blades cannot accommodate, or digest, such a large influx. Occasional stalls are common to all power systems, especially in their early stages of development, and there is a certain low level that is considered acceptable. Turbofan engines are subject to a more severe type of stall in which the engine is not able to recover itself. In its early flight period, the F100 experienced an unacceptable level of as many as 16 non-recoverable stalls per 1,000 flight hours by the spring of 1979, it had been brought down to a much more tolerable level of 1. 8 per 1,000 hours. But after constant attention from the Air Force and Pratt & Whitney – and three major improvements – the level was quickly and systematically reduced. The goal was to cut non-recoverable stalls down still further, to 0. 15 per 1,000 hours.

The F-15

The only way an aircraft and its engine can, in the end, be measured is in combat, and that, fortunately, is hard to come by these days. In one such test for the F-15, however, the results speak for themselves. On June 27, 1979, the Israeli Air Force dispatched six F-15s and two Kfirs to engage eight to twelve Syrian MiG-21bis’ in the skies over Lebanon. In three minutes five MiGs were sent crashing to earth and several others were damaged.

American pilots, meantime, have logged thousands of hours in mock combat with the F15. Lt. Commander Randy Cunningham, the Navy’s only Vietnam pilot ace, describes what happened when he flew an F-4 against an F-15 in two such simulated dogfights.

“I climbed up to 40,000 feet, and went up to Mach 1. 9 and met the F-15 head-on, and he locked me up with his radar as soon as he turned the corner.

“I was already doing 1. 9. When I had done a high-speed profile for about fifty miles, I knew he was below me and I remember thinking ‘There’s no way he’s going to come up above me. ‘ Well, he did wait, and when we saw each other, I put the nose of the F-4 in about a 45° climb at 1. 8 at 40,000 feet. Figuring:

“Okay, come on up after me and when you drop off, I’ll show you you’re not going to get above me. He came up with me. I got the F-4 up to about 53,000 feet. But I started overspeeding an engine and had to shut it down, then relight it. So I put the nose back down -and on the way down he shot me. He did come up with the power advantage. So my plan didn’t work.

“The second time I met him, I said: Okay, I just want to see what the aeroplanes will do, one on one. I met him head-on and just tried to match a turn. While I’ve got my speed up, I’ve got the same g available as he does. I did a nose-low turn and kept my energy up. He stayed with me. But as I increased my g, I decreased my airspeed, and then he started really making a tight turn. When I made a hard move into him, he just pulled back, levelled his wings and went right up over me. His brute power was just more than I could handle.

“I went into a nose-low turn from about 20,000 feet, and held about 6-1/2 gs all the way, trying to maintain my speed. We were staying pretty neutral until he squared the corner on me. Then when I had to match his turn rate, my airspeed fell off. To maintain with him. (But) he just pulled up and went over the top of me.

“I was out of airspeed and ideas. With his extra g available at slower airspeeds, he was able to maneuver behind me there also. It seemed no matter what I did against him he countered every move.”

Lt. Cunningham says of the F-15’s competitiveness:

“If I’d been in the F-15 in Vietnam, I’d have had a Jot more time to devote to a couple of engagements I got into. I spent a lot of time worrying about my energy. I had to run to get energy and then come back and fight. Plus, I have a gun in the F-15, which I didn’t have in the F-4.

“You get down to 350-400 knots in the F-4 and you try to take on a MiG -he’s going to clean your clock. You want to build up to about 450 knots to sustain your best turn rate. But if the MiG, or any other airplane, makes a big move on you, all you have to do in an F-15 is level your wings and go pure vertical, and he’s bled 200 knots. You’ve got a 200-knot energy advantage even if you start off equal. In the F-15 you’ve got an energy advantage to start with. “

The F-16

The F-16, smaller than the F-15 and with only one engine, has also outperformed the F-4. The F-4, for example, must use its afterburner to maintain an airspeed of 450 knots when it is executing a loop below 20,000 feet. The F-16 can do the same loop at 275 knots – without its afterburner. To perform a Zimmermann (an altitude-gaining maneuver that reverses direction) the F-4D requires 7,000 feet of airspace and a speed of 300 knots. The F-16 can accomplish the same maneuver at the same speed in exactly half that space, 3,500 feet.

A test pilot had this to say about the power the F100 engine provides the F-16:

“During a dogfight with an F-4, I found that I was using power-management in the F-16 rather than setting power at 100% and leaving it there. At one point, at low altitude, I was indicating 550 knots and pulling eight gs without using the afterburner.”

As a demonstration of the F-16’s capabilities, an F-16 pilot will give an F-4 pilot a significant combat advantage, allowing the Phantom to fly about a mile-and-a-half behind him, at the same altitude (10,000 to 15,000 feet) and at the same airspeed (325 to 350 knots). The F-4 pilot is thus in an ideal attacking position, but within a few seconds, the F-16 pilot takes away his advantage – turning 180° and meeting him head-on.

The range of the F-16 is as impressive as its maneuverability, the result of the airframe’s high fuel capacity and the low fuel consumption of the F100 engine. In another exercise, two F-16s flew against two F-4Ds. Although the F-4Ds each carried two external fuel tanks, the F-16s soon ran the F-4Ds out of fuel. While one of the F-16s returned home, the other engaged another F-4D in a dogfight. That Phantom, too, ran dry and was forced to disengage. The second F-16 still had enough fuel left to fly almost 100 miles back to base and make four touch-and-go practice landings before finishing for the day.

The F-16’s range stems in part from its ability to fight and win in air-to-air combat without using the fuel-consuming afterburner. The F100 engine is so powerful that the pilot usually does not even need an afterburner. In actual battle, says one F-16 pilot, he would probably use his afterburner in no more than five out of 100 dogfights. Even then, he says, he would turn it on for no more than a few seconds to gain airspeed.