A research paper shows that Stratolaunch, the company that owns the world's largest airplane, the Roc, was exploring using it as a platform to launch hypersonic research vehicles, in addition to space launch systems, more than a year ago. Last month, the firm, under new ownership following the death of founder Paul Allen, also co-founder of Microsoft, reportedly shifted toward primarily offering hypersonic test support rather than space launch services, which makes this work especially relevant now. Stephen Corda, a Senior Technical Fellow-Hypersonics at Stratolaunch, Curtis Longo, a Principal Engineer-Hypersonics at the company, and Zachary Krevor, the firm's Vice President of Engineering, co-presented the paper on Roc's potential for use in hypersonic testing at the American Institute of Aeronautics and Astronautics's International Space Planes and Hypersonics Systems and Technologies Conference in September 2018. They said that, at that time, at least two hypersonic vehicles were in development, the Hyper-A and the Hyper-Z.

"Stratolaunch is investigating a variety of space launch vehicle configurations, all of which must operate in the hypersonic flight regime," Corda, Long, and Krevor wrote. "These space launch vehicles share many technologies with vehicles that are designed to accelerate or cruise at hypersonic speeds within the atmosphere." Hyper-A, the smaller of the two designs they described in the paper, was just over 28 feet long, had a wingspan of just under 11 and a half feet, and a maximum launch weight of around 6,000 pounds, according to the paper. Stratolaunch's engineers described it as having a top speed of greater than Mach 6. Hypersonic speed is technically defined as being above Mach 5.

Stratolaunch An artist's conception of the Hyper-A hypersonic research vehicle, showing the internal layout of its rocket motor and fuel tanks, as well as its projected specifications compared to those of NASA's X-43A and the X-15, which the US Air Force and NASA tested in the 1960s.

The larger Hyper-Z, with a projected top speed of more than Mach 10, was significantly larger, at 83 and a half feet long and with a wingspan of 32 and a half feet. Its maximum launch weight would be around 65,000 pounds, nearly twice that of the North American X-15 experimental hypersonic rocket plane that the U.S. Air Force and NASA used for research purposes during the 1960s.

Stratolaunch An internal schematic of the Hyper-Z hypersonic research vehicle.

Stratolaunch A three-view drawing of the Hyper-Z hypersonic research vehicle, as well as its projected specifications.

Both designs were unmanned and powered by liquid fuel rocket motors that Stratolaunch was also developing at the time, according to the paper. Hyper-A and Hyper-Z would be recoverable and reusable, flying their routes autonomously and then landing like conventional aircraft at a designated point. The Stratolaunch paper says that Hyper-A could potentially be launched straight from the ground, without the need to be carried aloft by the Roc, but doesn't explain how this would be feasible. Beyond the discussions about Hyper-A and Hyper-Z, it also has interesting details about the Roc mothership's own performance and capabilities. The aircraft, which has a maximum gross takeoff weight of 1.3 million pounds, can carry a single payload on its main pylon, suspended under the center wing box that connects its two separate fuselages, weighing up to a whopping 500,000 pounds.

Stratolaunch A graphic depicting the massive Roc against various other objects, such as a blue whale and the SR-71 Blackbird.

Stratolaunch A graphic showing the large "box" available for payloads, which can weigh up to 500,000 pounds, under the Roc's center wing box.

Even at its maximum takeoff weight, the Roc has a range of 1,000 nautical miles. A typical flight profile would involve traveling to a designated launch location, then ascending to the desired altitude and reaching the required launch speed, before conducting a pull-up maneuver to release the payload. "Typical release conditions [for a space launch] are a Mach number of 0.63 at an altitude of 30,000 feet," the paper explained. For space launch, purposes, the vehicle would then boost itself into orbit and carrying out whatever its mission might be, such as releasing a small satellite. This whole process is commonly known as two-stage-to-orbit, which you can read about in greater detail in this past War Zone feature. The procedure for deploying the Hyper-A or Hyper-Z hypersonic test vehicles would be largely the same, but they would remain in the atmosphere the entire time.

Stratolaunch A graphic showing a typical launch sequence using the Roc.

Stratolaunch A graph showing the Hyper-Z's launch against the envelopes that Stratolaunch would employ for launching a space launch vehicle or a system using an air-breathing hypersonic propulsion system.

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A hypersonic test vehicle could be used in a variety of applications. It could be used as a platform to experiment with new high-speed airbreathing engines or carry shapes presenting smaller vehicles or hypersonic weapons to see how they respond to flight at speeds above Mach 5. The strain of flying so fast, which also generates extreme heat, means that it is valuable to have reusable platforms for exploring new materials or subsystems, as well. Additional payloads could include telemetry recording equipment and other sensors to collect valuable information during a test and datalinks to transmit it to personnel on the ground in near-real-time. The Stratolaunch paper describes the potential for loading Hyper-A and Hyper-Z with "hyper spot" canisters, which could contain "research electronics, systems, etc."

Stratolaunch A table describing the possible testing purposes where a hypersonic research vehicle would be applicable, as well as how many of those would apply equally for research and development of space launch vehicles.

The U.S. Air Force is already developing the X-60A hypersonic research vehicle, which is smaller than the Hyper-A, for similar purposes. Other countries, such as Russia, also have hypersonic test vehicles for the same reasons. China recently revealed that it is working on a two-stage-to-orbit system that it could also use for hypersonic research and development purposes. It's unclear if Stratolaunch is presently working on the Hyper-A or Hyper-Z. In January 2019, just three months after the company's engineers presented their paper on its hypersonic test potential, the firm announced it was halting work on its entire family of proprietary space launch vehicles, including the Black Ice reusable spaceplane. It's not clear if this also impacted work on the two hypersonic research vehicles.

Stratolaunch An undated photo Hyper-Z wind tunnel model undergoing testing at the US Naval Academy.

Stratolaunch A graphic showing the entire planned family of Stratolaunch vehicles, including two views of the Black Ice reusable space plane and one of the hypersonic test vehicles at right.

We have reached out to Stratolaunch for additional information, but if the Hyper-A and Hyper-Z projects were canceled, it certainly possible that the company has revived them or might be thinking about doing so now. Even if there are no plans yet to build either of those test vehicles, the experience gained from those efforts would be very valuable in the firm's apparent new focus on offering hypersonic test services, evidence of which first emerged in December 2019. Two months earlier, the late Paul Allen's Vulcan, Inc. holding company had sold Stratolaunch Cerberus Capital Management. You can read more about the sale and the implications of the shift away from space launch services in this previous War Zone piece. Whatever happens, it's clear that Stratolaunch was interested in entering the hypersonic research and development space, which has expanded dramatically in recent years due to interest from the U.S. military, in particular, even before Paul Allen's death and the subsequent sale of the company and its announced movement away from the space launch sector. Special thanks to Twitter user @ToughSf for alerting us to this paper. Contact the author: joe@thedrive.com