• Currently in development, the X-60A will serve the hypersonic flight test and suborbital research communities with an air-launched single-stage liquid booster.  Original image by Generational Orbit. Edited by Chris D’Elia.
     Currently in development, the X-60A will serve the hypersonic flight test and suborbital research communities with an air-launched single-stage liquid booster. Original image by Generational Orbit. Edited by Chris D’Elia.

X-60A May Make Hypersonic Flight Routine

June 1, 2019
By George I. Seffers
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The test vehicle aims for initial flight next year.


Achieving and maintaining hypersonic flight—Mach 5 and above—remains a major challenge, but officials at U.S. Air Force Research Laboratory envision a day when hypersonic technologies are developed and deployed much more quickly and affordably than is currently possible.

The X-60A hypersonic flight test vehicle is central to that goal. The Generation Orbit system will be used to test technologies at hypersonic speeds. The idea is to increase the frequency of flight testing while lowering the cost of maturing hypersonic technologies in relevant flight conditions.

“The bumper sticker is affordable, routine and flexible access to hypersonic flight conditions,” says Barry Hellman, the X-60A program manager within the Aerospace Systems Directorate at the Air Force Research Lab. “We’re trying to develop a new flying facility to be able to test all the different hypersonic technologies.”

On its website, Generation Orbit describes the X-60A, previously known as the GOLAUNCHER1 or GO1, as a single-stage liquid rocket, launched from a Gulfstream III carrier aircraft. The rocket’s propulsion system uses liquid oxygen and kerosene propellants. The system is designed to provide affordable and regular access to high dynamic pressure flight conditions between Mach 5 and Mach 8 to a wide range of payloads for fundamental research, technology development and risk reduction.

Hypersonic flight conditions are both uncertain and brutal. “That’s where air begins to dissociate, and it’s characterized as the thermal barrier. You have extremely high heat on the vehicles,” explains Bob Mercier, chief engineer in the Air Force Research Lab’s High Speed Systems Division. He describes the temperatures as “a couple of thousands of degrees of toasty air on the outside of the vehicle.”

When William “Pete” Knight pushed the X-15 to Mach 6.7 back in 1967, the intense heat melted the pylons that held the scramjet engine in place, sending the engine crashing to the ground. Mercier points out that the Lockheed SR-71 Blackbird leaked fuel while on the ground, but at Mach 3, the panels expanded due to the thermal load and created a tight seal. “The thermal environment is extremely severe. Deformation can set in. You have to allow for thermal growth on the vehicle,” Mercier explains. “We have to be able to look at combined aerodynamic, thermodynamic and acoustic loads and how those influence the vehicle and change its shape during flight.”

Gas turbine engines, or other so-called air-breathing propulsion systems, present their own unique challenges at super speeds. “If you’re doing air-breathing propulsion, you have some serious technical challenges in terms of being able to establish a flame inside an engine when the air is still going at supersonic speed. We’re lighting a match in a blowing hurricane,” Mercier declares.

Every system or subsystem that generates heat must be carefully considered. “You have to look at using fuel as a coolant to keep the engine from melting. And then you have to have everything packaged inside the vehicle. Because of the thermal environment, you basically have to put it into a thermos bottle,” Mercier adds. “All the things that generate heat inside the vehicle—batteries, actuators, electronics—you have to be able to keep those things cool as well, so the overall vehicle thermal management becomes a significant issue.”

A limited number of adequate testing facilities compounds the challenge of developing hypersonic technologies, but that’s where the X-60A comes in. It is not unusual for research and development programs to reach a technology readiness level (TRL) of four or five in the nation’s laboratories but fail to advance beyond the labs. “People put a lot of money in hypersonic technologies, but to get over the technology Valley of Death where it can then go into an operational system has been very challenging in the past,” Hellman notes. “We’re hoping this vehicle can be that tool to get those technologies to TRL level six, which requires flight testing.”

Rapidly developing hypersonic technology has become a major priority for the Defense Department, in large part because China and Russia have gained an edge. “In the last year, China has tested more hypersonic weapons than we have in a decade. We’ve got to fix that,” Michael Griffin, undersecretary of defense for research and engineering, says in a written statement published in December on the U.S. Department of Defense’s website. Russia also is involved in hypersonics. Hypersonics is a game changer.”

The Air Force Research Lab is partnered with the Defense Advanced Research Projects Agency on hypersonic research. The Hypersonic Air-breathing Weapon Concept (HAWC), for instance, is developing critical technologies to enable an effective and affordable air-launched hypersonic cruise missile. The program intends to emphasize efficient, rapid and affordable flight tests to validate key technologies. It is pursuing flight demonstrations in three critical technology challenge areas: air vehicle feasibility, effectiveness and affordability. Technologies of interest include advanced air vehicle configurations capable of efficient hypersonic flight; hydrocarbon scramjet-powered propulsion to enable sustained hypersonic cruise; approaches to managing the thermal stresses of high-temperature cruise; and affordable system designs and manufacturing approaches.

Meanwhile, the Tactical Boost Glide program aims to enable future air-launched, tactical-range hypersonic boost glide systems. In a boost glide system, a rocket accelerates its payload to high speeds. The payload then separates from the rocket and glides unpowered to its destination. Like the HAWC program, the focus is on vehicle feasibility, effectiveness and affordability.

The development schedules for the programs are classified, but the X-60A may be built rapidly enough to contribute to ongoing programs. “I can’t get into program specifics, but we think this is being developed quickly enough. We are looking at plans for many years going into the next decade for a variety of different hypersonic programs going on. This vehicle is already being considered for how it can influence and help support those programs,” Hellman states.

The X-60A can be developed relatively rapidly in part because it will be made up almost entirely of readily available, commercial off-the-shelf technologies. Some components, such as antennas or thermal protection systems, need to be custom-built, but most are ready to go. “There’s not a lot of new technology in the vehicle itself. The hardest part is just finding the right components out there in industry and integrating that,” Hellman explains.

Officials from the Air Force Research Lab announced in March that the test vehicle had completed its critical design review, a major milestone. Hellman indicates the team has a few “close-out actions” to address following the review, but the system formally progresses to the fabrication phase with the initial flight expected about a year from now. The next steps include a second “hot fire test at the end of the summer,” he adds.

The first so-called hot fire test was performed last summer. It demonstrated integration of the engine with propellant tanks, valves, pressurization system and flight controls. Further, the first hot fire test demonstrated the throttling capabilities of the system necessary to meet the thrust levels for Mach 6 cruise at 80,000 to 90,000 feet, according to the contractor.

Some lessons learned from the first hot fire should allow an even more successful second test. “There were some propellant tanks and feedlines and all of that kind of integration and how to get propellant to the engine,” Hellman reveals. “Sometimes the thing that weighs the least, the software on these vehicles, is the hardest to get working.”

The flight test will be from Cecil Spaceport, Jacksonville, Florida. Using a 30-degree flight path angle, the Gulfstream III will lift the X-60A to 35,000 feet before releasing it to reach a speed of Mach 7. “We’ll go through a Federal Aviation Administration-approved corridor over land to minimize population overflight to get out over the water, and then we’re going to use some of the restricted airspace over the coast of Florida to do some of our flight testing,” Hellman reports. “We are planning two flight tests next year.”

Assuming those initial flights are successful, additional payloads will be included for later flights. “For the first couple of flights, the primary payload is the vehicle,” Hellman notes. “We have a few secondary payloads that will go on the vehicle as ride-alongs, and if those flights are successful, we’ll be able to switch into more primary payloads in late 2020 and into 2021.”

The researchers are looking at other ways to streamline the development and fielding of hypersonic systems, but it is too early to reveal details, Mercier indicates. “We’re in the process of looking at how we can become more risk tolerant and how we can use things, such as the X-60, to get systems ready to transition much sooner. We’re restructuring our investment portfolio to help get things out the door sooner,” Mercier offers. “Basically, we’re looking at the nearer-term applications that might support weapons or hypersonic cruising platforms and what we can do to allow those to transition quicker. Part of it is more acceptance of risk and changing the way we do things.”

Because of the renewed attention and technological advances, Mercier concludes that it is an interesting time to be developing new hypersonic systems. “We’re expanding our knowledge base. We’re writing history. It doesn’t get any better than that.”

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