By Donna Doleman, Aviation Aftermarket Defense Magazine
On November 1, 2013, Lockheed Martin’s Skunk Workssurprised the aerospace industry when they announced that an operational hypersonic aircraft, powered by an air-breathing jet engine and capable of speeds as high as Mach 6, could be flying by 2030.
For years, it had been thought that a thus far highly elusive, next generation, high-Mach turbine engine would be necessary for air-breathing hypersonic flight without rocket boosting. So the industry was abuzz when the engineers at Lockheed Martin indicated that they and their associates at corporate partner Aerojet Rocketdyne had solved the problem of reaching the transition to Mach 6 with a hybrid engine using an off-the-shelf turbine at relatively low speeds and a dual-mode ramjet to accelerate to higher speeds. It has been said that both the turbine and the ramjet will be fed through a single inlet nozzle.
Speeds in excess of Mach 6 have been achieved by other aircraft. For instance, the manned X-15 reached Mach 6.72, and the HTV-2 Falcon holds the record for the fastest unmanned aerial vehicle at Mach 20 — that is, 13,000 mph or approximately twenty times the speed of sound — at an altitude well beyond 100,000 feet. But those aircraft relied upon traditional rocket power to boost speed.
The excitement about this potential technology leap happens to coincide with this year’s fiftieth anniversary of the first flight of the Skunk Works’s SR-71 Blackbird, the aircraft that still holds the official world airspeed record (2,193 mph or Mach 2.88) for a manned, air-breathing jet aircraft. Anecdotal evidence shows that the plane was known to have cruised at Mach 3.3 (over 2,500 mph) at altitudes above 80,000 feet. If the SR-72 is to be capable of Mach 6, designers will face a number of the same design challenges as the SR-71 team —and then some.
Among the key problems will be dealing with the heat generated by friction and shock waves at high velocities, aerodynamics, materials, manufacturability, navigation, and a host of other critical details necessary for success. Looking back to understand how similar challenges were overcome in designing the SR-71 may offer some insight into how the Skunk Works team might be equally successful with its latest groundbreaking technology.
An Aerospace Revolution
On December 22, 1964, the revolutionary SR-71 made its first flight. Only 24 months before, Lockheed had signed a contract to produce the first six aircraft.
To put this feat in perspective, let’s turn back the clock to 1964, a time 5 years before the first transmission over the Advanced Research Projects Agency Network (ARPANET), the precursor to the Internet. Xerox had just introduced what most consider the first commercialized fax machine. BASIC (Beginner’s All Purpose Symbolic Instruction Code) computer language was brand new. The Intel 4004 Computer Microprocessor would not debut until 1971, and the first “consumer” computers, the Scelbi & Mark-8 Altair and the IBM 5100 would not become available until 1974.
The state-of-the-art in computer-assisted-design was “Sketchpad,” a limited program written in 1963 by Ivan Sutherland in pursuit of his PhD. The program ran only on the MIT-developed Lincoln TX-2 computer, which had all of 64K (36-bit) words of core memory (the random access memory of its day). Like engineers everywhere back then, the SR-71 designers still used slide rules.
Kent Burns was the chief engineer and deputy program manager during the SR-71 reactivation activities that occurred from 1995 to 1997. In addition to his own hands-on involvement with the aircraft, Burns was able to work with some of the people who had been instrumental in the original SR-71 program, and he heard many of the stories related to it.
“The SR-71 flew from 1964 to 1990 and then until 1999 for reactivated aircraft,” he says. “The first flight of the A-12 was in 1962. The A-12, the precursor to the SR-71, was the first of its kind design begun in the late 1950s. It was a very secret development program that added to what had been done on the U-2. There was a lot of interest in the program and a lot of resources were available, with significant presidential visibility under Eisenhower.”
In 1957, the head of U.S. Central Intelligence Agency (CIA) special projects, Richard Bissell, warned President Eisenhower of the growing risk of over-flights by the U-2, the world’s first real spy plane. (The U-2 shot down over the Soviet Union in 1960 proved Bissell right.) Edwin Land, of Polaroid camera fame, chaired a panel to consider the possibilities for a stealthier aircraft. Other members of the panel included Clarence “Kelly” Johnson of Lockheed’s Skunk Works, Vincent Dobson, President of General Dynamics, and the assistant secretaries of the U.S. Navy and the U.S. Air Force.
As Chief Engineer and later Vice President of Lockheed Advanced Development Projects, Kelly Johnson’s legendary achievements, among many other successes, included design leadership on the P-38 Lightning, the P-80 (America’s first operational jet fighter), and the Constellation (a propeller-driven, pre-World War II civilian airliner that could fly faster than a Japanese Zero fighter and also served as a military and civil transport). In 1955, Johnson and his team had launched the U-2.
The Skunk Works began internal development on a U-2 successor aircraft in late 1957. In July 1958, Johnson presented his concept to the committee; in August, he learned that his team had won the competition. “Everything had to be invented,” Johnson later said of the program. The design iterations were nicknamed “Archangel,” since the earlier U-2 designs had been known as “Angels.”
“They built the airplane around the amount of fuel it needed,” Burns says, “because they needed over 1 hour of flight at Mach 3. The finished aircraft could fly from San Francisco to New York in under an hour.”
Working around the fuel needs meant that designers had to solve the problem of the aircraft’s center of gravity. Since the SR-71 reportedly consumed upwards of a hefty 5,000 gallons of fuel per hour, the changing fuel load played a key role in the aircraft’s stability. SR-71 pilots constantly monitored the center of gravity and moved fuel around to balance the plane. (Modern computer systems likely will play a critical role in stabilizing the SR-72.)
“Archangel 1 through 11 [A-1 through A-11] designs were the study series,” Burns relates. “They concluded that A-11 offered the solution. The changes made from A-11 to A-12 were mainly about including stealth by reducing the radar signature. For 9 months to a year, it was all about “Will this or that work?”
The highly specialized black finish that earned the Blackbird its nickname not only enhanced stealth; this surface treatment also emitted more friction-generated heat at Mach 3 than it absorbed. Because some surface temperatures could rise as high as 1,200 degrees Fahrenheit (F), causing expansion of the surface material, the plane was engineered with a flexible skin made of about 85 percent titanium and 15 percent carbon composites. This combination allowed the Blackbird to withstand the heat while minimizing weight.
“Expansion joints enabled the plane to grow by 9 inches to a foot,” Burns says. “There were no bladders or tanks, which usually were used to hold the fuel. The skin of the aircraft formed the tank, and a special sealant was used. We had to reseal the fuel tanks of the aircraft we refurbished.”
Because of the high temperatures, a low volatility, thermally stable fuel was needed. “The special JP-7 fuel used was developed by Shell and refined by Ashland,” Burns continues. “Jimmy Doolittle was involved in it, too, as a friend of Kelly’s. There were many such close industry friendships in those days.
“The JP-7 fuel had a very high flashpoint,” he explains. “People have flicked cigarettes into that fuel with no effect. Triethylborane (TEB) was used as the igniter. They would spin the main rotor of the engine until it was hot and then shoot in the TEB to light the engine.”
Titanium was relatively unknown in those days. As one example of government support, the material was acquired from the world’s leading producer, the Soviet Union, secretly through the CIA. According to Burns, the fabrication technology [for building the aircraft] was developed by Kelly and his team: “They developed techniques for forging and working the titanium. They used the capabilities at Burbank (California) that had been developed during World War II, using that stamping, pressing, and other equipment. I understand they were able to cherry pick the right people for the team — those with the best experience from the U2 and A-12 programs, for example. The program was so secret that often the Burbank people did not know they were working on the SR-71.”
The team developed specialized techniques for working with titanium as they went along. “One thing they learned about titanium forming and manufacturing,” Burns continues, “was that the chromium plating (which incorporated cadmium) on normal tools was not compatible with titanium, so the tools had to be stripped of all the chrome plating. They also learned that the local water had elevated levels of chlorine in summer, which was not compatible with titanium either; the titanium cracked during sheet forming. So, they had to filter the chlorine out of the water.”
Burns knew the late Lou Schalk, the Skunk Works chief test pilot who was the first to fly the A-12 and the SR-71 and the first to surpass Mach 3. “Lou was a very humble person,” Burns recalls. “He told me once about the first flight of the SR-71. ‘I was scared to death about the first flight,’ he said. ‘One, we didn’t want to disappoint Kelly, and two, this was new technology, out of the realm of what had been done before.’”
“What they did first was a high-speed taxi test. That means it’s a flight test, but you can put the aircraft down really fast if it doesn’t fly,” Burns quips. “The stability of the SR-71 as it rotated off the ground left something to be desired, according to Schalk. But they tweaked a few things, and the first flight in front of the VIPs a few days later went flawlessly.”
“In 1990, the SR-71 aircraft was retired from Deal Air Force Base to museums.” Burns recalls. Then, Congress budgeted for a small reactivation program in the Department of Defense appropriations bill as part of the Fiscal Year 1995 budget. “Jay Murphy, a retired colonel, then at Lockheed, was tasked with being program manager,” he continues. “The program office was at Wright Patterson Air Force Base. They were given about $60 million to reactivate two [of the] aircraft, and they actually gave money back (about $20 million, I think) to the government when they were done! They cleaned up the aircraft and made them operational.
Restoring the SR-71s to operational status began in January 1995. Due to various disputes, the U.S. Air Force did not use them. But research pilots at the National Aeronautics and Space Administration (NASA) wanted to fly the two reactivated aircraft for high-speed and high-altitude tests, and the aircraft were loaned out to them.
It is said that the SR-71 could photograph a golf ball on the green from 80,000 feet. On reactivation in the mid-1990s, Lockheed Martin enabled the digital downlink of electronics, including photographs and radar information. This was a real coup for its time. “The A-12 had carried many sensors and cameras that also went onto the SR-71,” Burns explains. “They initially used film, of course, but the lag time after landing for developing the film was a problem. We pushed to get digital data links and radar downloads, technology we borrowed from the U-2 days. We had some capacity native to the U-2 and benefitted from that, but we also pushed the technology farther. In turn, the Global Hawk benefitted from it.”
“As one example of changes we made, the temperature of the SR-71 heated up to 800 to 900 degrees F in some areas. We invented a radome so we could transmit in that environment. A hard, fabric skin made of twenty-nine plies of AF6-700 protected the antenna, which stuck out from the aircraft. It was a challenge to avoid the drag, but we managed to do it.”
Advice for the Team
So how can the team benefit from the successes of the past? “I always wanted to work here,” Burns declares. “I have worked at other companies. There, your boss had to come up with the idea and sell it. The philosophy at Skunk Works has always been that if anyone — from engineers to shop workers — comes up with a good idea and can support it, then we can try it. It’s a far more collegial atmosphere than most. We’re all here to succeed. We have management and leadership who understand and are here to support us.”
“I personally believe that one of the more important things to our future and the future of aerospace is to foster an atmosphere pretty much as it was back in the 1930s and 1940s when they were inventing airplanes.” A lot of that still exists at Skunk Works.
Lockheed Martin says that the forthcoming design also will incorporate lessons learned from their work with the Defense Advanced Research Projects Agency (DARPA) on the HTV-2. That aircraft was said to survive surface temperatures of 3,500 degrees F as it transitioned to Mach 20 aerodynamic flight.
Materials science has come a long way since the SR-71 was conceived, so new nickel-based superalloys, ultra-high temperature ceramics, and carbon- and ceramic-matrix composites may play a role in the SR-72 design. Carbon-carbon composites might be borrowed from the space industry.
The SR-71 Astroinertial Navigation System, which used celestial observations, was surprisingly precise. But today, a global positioning system (GPS), which was not fully operational until 4 years before the last of the reactivated Blackbirds was finally retired in 1999, could well be used.
Naturally, Lockheed Martin is not revealing its proprietary solutions. But there will be plenty of industry speculation about how the engineers who are working on this truly futuristic aircraft may overcome the seemingly insurmountable challenges ahead. As the company points out, a game-changer in SR-72 design and operation certainly will be today’s advanced computer systems. Stay tuned for more on this and other aircraft of tomorrow.