Scramjets Pass Their Peak

From the outset, scramjets received attention for the propulsion of tactical mis­siles. In 1959 APL’s Gordon Dugger and Frederick Billig disclosed a concept that took the name SCRAM, Supersonic Combustion Ramjet Missile. Boosted by a solid-fuel rocket, SCRAM was to cruise at Mach 8.5 and an altitude of 100,000 feet, with range of more than 400 miles. This cruise speed resulted in a temperature of3,800°F at the nose, which was viewed as the limit attainable with coated materi­als.1

The APL researchers had a strong interest in fuels other than liquid hydrogen, which could not be stored. The standard fuel, a boron-rich blend, used ethyl deca – borane. It ignited easily and gave some 25 percent more energy per pound than gasoline. Other tests used blends of pentaborane with heavy hydrocarbons, with the pentaborane promoting their ignition. The APL group went on to construct and test a complete scramjet of 10-inch diameter.2

Paralleling this Navy-sponsored work, the Air Force strengthened its own efforts in scramjets. In 1963 Weldon Worth, chief scientist at the Aero Propulsion Labo­ratory, joined with Antonio Ferri and recommended scramjets as a topic meriting attention. Worth proceeded by funding new scramjet initiatives at General Electric and Pratt & Whitney. This was significant; these firms were the nations leading builders of turbojet and turbofan engines.

GE’s complete scramjet was axisymmetric, with a movable centerbody that included the nose spike. It was water-cooled and had a diameter of nine inches, with this size being suited to the company’s test facility. It burned hydrogen, which was quite energetic. Yet the engine failed to deliver net thrust, with this force being more than canceled out by drag.3

The Pratt & Whitney effort drew on management and facilities at nearby United Aircraft Research Laboratories. Its engine also was axisymmetric and used a long cowl that extended well to the rear, forming the outer wall of the nozzle duct. This entire cowl moved as a unit, thereby achieving variable geometry for all three major components: inlet, combustor, and nozzle. The effort culminated in fabrication of a complete water-cooled test unit of 18-inch diameter.4

A separate Aero Propulsion Lab initiative, the Incremental Flight Test Vehicle (IFTV), also went forward for a time. It indeed had the status of a flight vehicle, with Marquardt holding the prime contract and taking responsibility for the engine. Lockheed designed and built the vehicle and conducted wind-tunnel tests at its Rye Canyon facility, close to Marquardt’s plant in Van Nuys, California.

The concept called for this craft to ride atop a solid-fuel Castor rocket, which was the second stage of the Scout launch vehicle. Castor was to accelerate the IFTV to 5,400 feet per second, with this missile then separating and entering free flight. Burning hydrogen, its engines were to operate for at least five seconds, adding an “increment” of velocity of at least 600 feet per second. Following launch over the Pacific from Vandenberg AFB, it was to telemeter its data to the ground.

This was the first attempt to develop a scramjet as the centerpiece of a flight pro­gram, and much of what could go wrong did go wrong. The vehicle grew in weight during development. It also increased its drag and found itself plagued for a time with inlets that failed to start. The scramjets themselves gave genuine net thrust but still fell short in performance.

The flight vehicle mounted four scramjets. The target thrust was 597 pounds. The best value was 477 pounds. However, the engines needed several hundred pounds of thrust merely to overcome drag on the vehicle and accelerate, and this reduction in performance meant that the vehicle could attain not quite half of the desired velocity increase of 600 feet per second.5

Just then, around 1967, the troubles of the IFTV were mirrored by troubles in the overall scramjet program. Scramjets had held their promise for a time, with a NASA/Air Force Ad Hoc Working Group, in a May 1965 report, calling for an expanded program that was to culminate in a piloted hypersonic airplane. The SAB had offered its own favorable words, while General Bernard Schriever, head of the Air Force Systems Command—the ARDC, its name having changed in 1961— attempted to secure $50 million in new funding.6

He did not get it, and the most important reason was that conventional ramjets, their predecessors, had failed to win a secure role. The ramjet-powered programs of the 1950s, including Navaho and Bomarc, now appeared as mere sidelines within a grand transformation that took the Air Force in only 15 years from piston-pow­ered B-36 and B-50 bombers to the solid-fuel Minuteman ICBM and the powerful Titan III launch vehicle. The Air Force was happy with both and saw no reason for scramjet craft as alternatives. This was particularly true because Aerospaceplane had come up with nothing compelling.

The Aero Propulsion Laboratory had funded the IFTV and the GE and Pratt scramjets, but it had shown that it would support this engine only if it could be developed quickly and inexpensively. Neither had proved to be the case. The IFTV effort, for one, had escalated in cost from $3.5 million to $12 million, with its engine being short on power and its airframe having excessive drag and weight.7

After Schriever’s $50-million program failed to win support, Air Force scramjet efforts withered and died. More generally, between 1966 and 1968, three actions ended Air Force involvement in broad-based hypersonic research and brought an end to a succession of halcyon years. The Vietnam War gave an important reason for these actions, for the war placed great pressure on budgets and led to cancellation of many programs that lacked urgency.

The first decision ended Air Force support for the X-15- In July 1966 the joint NASA-Air Force Aeronautics and Astronautics Coordinating Board determined that NASA was to accept all budgetary responsibility for the X-15 as of 1 January 1968. This meant that NASA was to pay for further flights—which it refused to do. This brought an end to the prospect of using this research airplane for flight testing of hypersonic engines.8

The second decision, in August 1967, terminated IFTV. Arthur Thomas, the Marquardt program manager, later stated that it had been a major error to embark on a flight program before ground test had established attainable performance levels. When asked why this systematic approach had not been pursued, Thomas pointed to the pressure of a fast-paced schedule that ruled out sequential development. He added that Marquardt would have been judged “nonresponsive” if its proposal had called for sequential development at the outset. In turn, this tight schedule reflected the basic attitude of the Aero Propulsion Lab: to develop a successful scramjet quickly and inexpensively, or not to develop one at all.9

Then in September 1968 the Navy elected to close its Ordnance Aerophysics Laboratory (OAL). This facility had stood out because it could accommodate test engines of realistic size. In turn, its demise brought a premature end to the P & W scramjet effort. That project succeeded in testing its engine at OAL at Mach 5, but only about 20 runs were conducted before OAL shut down, which was far too few for serious development. Nor could this engine readily find a new home; its 18-inch diameter had been sized to fit the capabilities of OAL. This project therefore died both from withdrawal of Air Force support and from loss of its principal test facil­ity.10

As dusk fell on the Air Force hypersonics program, Antonio Ferri was among the first to face up to the consequences. After 1966 he became aware that no major new contracts would be coming from the Aero Propulsion Lab, and he decided to leave GASL, where he had been president. New York University gave him strong encour­agement, offering him the endowed Astor Professorship. He took this appointment during the spring of 1967-11

He proceeded to build new research facilities in the Bronx, as New York Univer­sity bought a parcel of land for his new lab. A landmark was a vacuum sphere for his wind tunnel, which his friend Louis Nucci called “the hallmark of hypersonic flow” as it sucks high-pressure air from a stored supply. Ferri had left a trail of such spheres at his previous appointments: NACA-Langley, Brooklyn Polytechnic, GASL. But his new facilities were far less capable than those of GASL, and his opportunities were correspondingly reduced. He set up a consulting practice within an existing firm, Advanced Technology Labs, and conducted analytical studies. Still, Nucci recalls that “Ferris love was to do experiments. To have only [Advanced Technology Labs] was like having half a body.”

GASL took a significant blow in August 1967, as the Air Force canceled IFTV. The company had been giving strong support to the developmental testing of its

engine, and in Nucci’s words, “we had to use our know-how in flow and combus­tion.” Having taken over from Ferri as company president, he won a contract from the Department of Transportation to study the aerodynamics of high-speed trains running in tubes.

“We had to retread everybody,” Nucci adds. Boeing held a federal contract to develop a supersonic transport; GASL studied its sonic boom. GASL also investi­gated the “parasol wing,” a low-drag design that rode atop its fuselage at the end of a pylon. There also was work on pollution for the local utility, Long Island Light­ing Company, which hoped to reduce its smog-forming emissions. The company stayed alive, but its employment dropped from 80 people in 1967 to only 45 five years later.12

Marquardt made its own compromises. It now was building small rocket engines, including attitude-control thrusters for Apollo and later for the space shuttle. But it too left the field of hypersonics. Arthur Thomas had managed the company’s work on IFTV, and as he recalls, “I was chief engineer and assistant general manager. I got laid off. We laid off two-thirds of our people in one day.” He knew that there was no scramjet group he might join, but he hoped for the next-best thing; conventional ramjets, powering high-speed missiles. “I went all over the country,” he continues. “Everything in ramjet missiles had collapsed.” He had to settle for a job working with turbojets, at McDonnell Douglas in St. Louis.13

Did these people ever doubt the value of their work? “Never,” says Billig. Nucci, Ferris old friend, gives the same answer: “Never. He always had faith.” The problem they faced was not to allay any doubts of their own, but to overcome the misgivings of others and to find backers who would give them new funding. From time to time a small opportunity appeared. Then, as Billig recalls, “we were highly competitive. Who was going to get the last bits of money? As money got tighter, competition got stronger. I hope it was a friendly competition, but each of us thought he could do the job best.”14

Amid this dark night of hypersonic research, two candles still flickered. There was APL, where a small group continued to work on missiles powered by scramjets that were to burn conventional fuels. More significantly, there was the Hypersonic Propulsion Branch at NASA-Langley, which maintained itself as the one place where important work on hydrogen-fueled scramjets still could go forward. As scramjets died within the Air Force, the Langley group went ahead, first with its Hypersonic Research Engine (HRE) and then with more advanced airframe-integrated designs.