Bye Bye Blackbird

However gracefully the SR-71 Blackbird has aged, aviation’s Belle of the Ball will likely have its successor. In a bid to overcome some of the most daunting hypersonic propulsion technology issues known, the legendary Lockheed Martin Skunk Works has finally managed to pull off what can only be termed a seriously problematic marriage in powered flight. Known as the “turbine-based combined cycle (TBCC) propulsion system”, it dovetails an off-the-shelf fighter jet engine with a ramjet/scramjet thrust configuration. This is the engineering equivalent of successfully conjoining a Formula One car engine with a Saturn booster. Although details are scarce or vague, it is clear that some very basic design fundamentals have been driven out to the “bleeding edge” of technology.

The article mentions how, “Aerodynamically, the forebody appears to be shaped for inlet compression at high speed, but without the characteristic stepped “wave-rider” configuration of the X-51A.” For a quick overview, below is a brief video of how the X-51A, successor to the original X-15 multiple-Mach rocket plane, uses ramjet technology. Note carefully how at video time point 00:32 the booster stage separates from this experimental launch vehicle. This probably was the deal-breaker that drove innovation of the TBCC. Haphazardly tossing off chunks of astronomically expensive propulsion gear doesn’t exactly fit the flight profile of a mission-ready aeronautical platform.

Known as a HSSW (Hypersonic Strike Weapon), the XR-72 is designed to gather intelligence or deliver weapons anywhere around the world in a very short time. With the same range as the SR-71, but twice the speed, it has a reach of 2,900 nmi (5,400 km)—which converts to over 3,300 statute miles—meaning that a coast-to-coast flight over the continental USA wouldn’t take much more than half an hour.

Skunk Works Reveals SR-71 Successor Plane
(Images courtesy of Lockheed Martin)

From the Aviation Week article by Guy Norris:

Just as importantly, the Skunk Works design team developed a methodology for integrating a working, practical turbine-based combined cycle (TBCC) propulsion system. “Before that, it was all cartoons,” Leland says. “We actually developed a way of transforming it from a turbojet to a ramjet and back. We did a lot of tests to prove it out, including the first mode-transition demonstration.” The Skunk Works conducted subscale ground tests of the TBCC under the Facet program, which combined a small high-Mach turbojet with a dual-mode ramjet/scramjet, and the two sharing an axisymmetric inlet and nozzle.

Below is a glimpse of the piggyback dual-path engine configuration.

What the image does not reveal is the methods used to cool incoming air. Intake air arrives at an elevated temperature due to speed-related compression and contact with the aircraft’s fuselage. During regular flights, the old SR-71 Blackbird would land with a canopy temperature of over 300 °C (572 °F). Starting at video time point 01:31, the above clip gives a guided tour of the ramjet in action. At time point 01:50 its point-of-view shifts to the X-51A’s engine intake. The concentric gray rays are fuel lines that are used to both cool the intake structure but also, of equal importance, reduce the temperature of incoming air. A useful comparison is automotive intercooler technology that removes heat from the turbocompressor’s gas-air mixture in order that more combustible fuel can be drawn into engine cylinders. Reversing this expansion cycle is critical for getting a ramjet to function at its lower end speeds where insufficient oxygen is arriving at the intake.

In fact, the article hints at exactly this sort of solution:

Lockheed will not disclose its chosen method of bridging the thrust chasm. The company funded research and development, and “our approach is proprietary,” says Leland, adding that he cannot go into details. Several concepts are known, however, to be ripe for larger-scale testing, including various pre-cooler methods that mass-inject cooler flow into the compressor to boost performance. Other concepts that augment the engine power include the “hyperburner,” an augmentor that starts as an afterburner and transitions to a ramjet as Mach number increases. Aerojet, which acquired Rocketdyne earlier this year, has also floated the option of a rocket-augmented ejector ramjet as another means of providing seamless propulsion to Mach 6. [emphasis added]

The “hyperburner” is reminiscent of technology used on the earlier SR-71 Blackbird. Bringing it to maximum speed involved using aft-mounted “ring injectors” that would dump unburned fuel directly into the engine’s outtake zone. This combustion would consume so much air that, in combination with special engine pod airflow control systems, sufficient suction was created to, literally, drag the plane forward. This was a critical issue in that attempting to conventionally feed the engines any more fuel would elevate internal operating temperatures to the point where the Pratt & Whitney J58’s turbine blades would begin to melt. Operating at 10,000 RPM the rotating turbine blades experienced centripetal forces of the order of 20,000g. Using a process known as “directional solidification”, turbine airfoil grain boundaries were eliminated, essentially rendering each compressor blade into a single crystal.

By manipulating airflow in this manner, some estimates were that, at top speeds, up to an astonishing 90% of the forward motion was due to this suction. Obviously, this same method could prove exceptionally useful as an “augmentor” to increase the draw of oxygen into the ramjet during the crucial transition speed at around Mach 2.0 when regular turbine jet engines begin to develop problems with ingesting shock waves. The SR-71 crews referred to such engine failures as “unstarts”.

From another Aviation Week article about the SR-72:

Described as a “supercharged ramjet” by the late Ben Rich, the former Skunk Works director and member of the SR-71 and its propulsion design team, the inlet and exhaust ejector generated up to 90% of the aircraft’s thrust at cruise speed. Rich told Aviation Week that at Mach 3 plus, almost 60% of the thrust came from the inlets and 30% from the ejectors. The engine only produced 10% of thrust at cruise.

A Wiki page for the Pratt & Whitney J58 cites the following thrust statistics:

At Mach 2.2 inlet 13% – engine 73% – ejector 14%

At Mach 3.0+ inlet 54% – engine 17.6% – ejector 28.4%

At the following link is a chart showing how engine nacelle configuration and airflow changes during the SR-71’s transition to Mach 3.0 from regular sub-sonic flight. As can be seen, at speeds over Mach 3.0, the ejector (outtake) overtook engine performance by a considerable margin. This same technology will, undoubtedly, play a role in the turbine portion of the SR-72’s propulsion package.

Although details of the proposed thrust-augmentation concept remain under wraps, Leland says a large part of a successfully integrated mode-transition design is the inlet. “That’s because you have to keep two compressor systems [ramjet and turbine] working stably. Both will run in parallel,” he adds.

This is much like saying that “we will harness together a Chihuahua and a Greyhound and still win races.”

The SR-72 is being designed with strike capability in mind. “We would envision a role with over-flight ISR, as well as missiles,” Leland says. Being launched from a Mach 6 platform, the weapons would not require a booster, significantly reducing weight. The higher speed of the SR-72 would also give it the ability to detect and strike more agile targets. “Even with the -SR-71, at Mach 3, there was still time to notify that the plane was coming, but at Mach 6, there is no reaction time to hide a mobile target. [emphasis added]

The outline plan for the operational vehicle, the SR-72, is a twin-engine unmanned aircraft over 100 ft. long (see artist’s concept on page 20). “It will be about the size of the SR-71 and have the same range, but have twice the speed,” he adds. The FRV would start in 2018 and fly in 2023. “We would be ready to launch the SR-72 shortly after and could be in service by 2030,” Leland says.

According to Al Romig, Skunk Works engineering and advanced systems vice president, “speed is the new stealth.”

This is an unchanging factor in the design of transonic and hypersonic aircraft. The SR-71 Blackbird was never shot down because by the time it had been detected, it was already out of missile range. A physical “lock-on” by a launch-on-notice missile simply was not an option.

Through international basing, the SR-72 will allow for response times across most areas of the world in less than an hour and, more often, in just one or two dozen minutes. With advanced stealth technology and a larger payload of un-boosted missiles, this new craft promises to project American air power in ways that no other super power can dream of.

While it may not have the SR-71 Blackbird’s superbly elegant lines or the mind-bending cachet of having been designed only with slide rules, this new Blackbird will control the skies with the same ease and agility that its predecessor once did. Clearly, the message remains the same, “Anywhere, anytime, your @ss is ours.”