Stratolaunch has finally found a use for the world's largest airplane.

Twice in the last five months, the company launched a hypersonic vehicle over the Pacific Ocean, accelerated it to more than five times the speed of sound, and autonomously landed at Vandenberg Space Force Base in California. Stratolaunch used the same vehicle for both flights.

This is the first time anyone in the United States has flown a reusable hypersonic rocket plane since the last flight of the X-15, the iconic rocket-powered aircraft that pushed the envelope of high-altitude, high-speed flight 60 years ago. //

Zachary Krevor, president and CEO of Stratolaunch, spoke with Ars on Monday afternoon. He said the Talon test vehicle advances the capability lost with the retirement of the X-15 by flying autonomously. Like the Talon-A, the X-15 released from a carrier jet and ignited a rocket engine to soar into the uppermost layers of the atmosphere. But the X-15 had a pilot in command, while the Talon-A flies on autopilot.

"Why the autonomous flight matters is because hypersonic systems are now pushing the envelope in terms of maneuvering capability, maneuvering beyond what can be done by the human body," Krevor said. "Therefore, being able to perform flights with an autonomous, reusable, hypersonic testbed ensures that these flights are exploring the full envelope of capability that represents what's occurring in hypersonic system development today."

Stratolaunch's Talon-A is a little smaller than a school bus, or about half the size of the X-15. //

Engineers know less about the conditions of the hypersonic flight regime (in excess of Mach 5) than they do about lower-speed supersonic flight or spaceflight. The only vehicles that regularly fly at hypersonic speeds are missiles, rockets, and spacecraft reentering the atmosphere. They spend just a short time flying in the hypersonic environment as they transition to and from space.

There are two things you should know about hypersonic missiles. First, rockets have flown at hypersonic speeds since 1949, so when officials talk about hypersonic missiles, they are referring to vehicles that operate in the hypersonic flight environment, instead of just transiting through it.

Second, hypersonic vehicles come in a couple of variations. One is a glide vehicle, which is accelerated by a conventional rocket to hypersonic speed, then steers itself toward its destination or target using aerodynamic forces. The other is a cruiser that can sustain itself in hypersonic flight using exotic propulsion, such as scramjet engines. //

The Pentagon's emphasis on hypersonic weapons is relatively new. After the X-15's final flight in 1968, the government lacked any major hypersonic flight test programs for several decades. NASA flew the autonomous X-43 test vehicle to hypersonic speed two times in 2004, and the Air Force demonstrated an air-breathing scramjet engine at Mach 5.1 with the X-51 Waverider aircraft in 2013. While some of the X-43 and X-51 test flights failed, they provided early-stage data on hypersonic propulsion systems that could power high-speed aircraft and missiles.

But these were expensive government-led programs. Together, they cost nearly $1 billion in 2025 dollars, with only a handful of flight tests to show for it. The military now wants to lean heavier on commercial industry.

Since its founding 14 years ago, Stratolaunch has pivoted its mission from the airborne launch of satellites to hypersonic testing. //

Stratolaunch's founder, Microsoft billionaire Paul Allen, died in 2018, putting the company's future in doubt. Stratolaunch flew its huge carrier aircraft, named Roc, for the first time in April 2019 but ceased operations the following month. Cerberus Capital Management, a private equity firm, purchased Stratolaunch from Allen's heirs later that year and redirected the company's mission from space launch to hypersonic flight testing.

Through it all, Stratolaunch continued flying Roc, a twin-fuselage airplane with a wingspan of 385 feet (117 meters). For a time, it appeared Roc might share a fate with Howard Hughes' "Spruce Goose" flying boat, which held the record as the airplane with the widest wingspan, until Roc (officially designated the Scaled Composites Model 351) took off for the first time in 2019. The Spruce Goose flew just once after its business prospects faded in the aftermath of World War II.

Now, the Pentagon's hunger for hypersonic weapons seems likely to feed Stratolaunch's coffers for some time to come. //

David Mayer Wise, Aged Ars Veteran
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I'm uncertain how big a "game changer" endoatmospheric, maneuverable, hypersonic weapons will be. They should be relatively easy to detect, track, and target using existing systems, even if they have an incredibly low radar cross section, simply because they will be hot as all hell, appearing clearly to any infrared sensors.

Interception is another story, it all depends on how maneuverable the weapon is and if it can reliably detect interceptors. Available reaction time is going to be substantially lower than many other systems, just because of how fast the weapon is moving. Your interceptor doesn't need to match the speed, but it does need to deal with the maneuverability of the weapon. A 15° change in trajectory is a 7 kilometer difference in actual position vs expected position after 15 seconds. Lots of existing interceptors are going to have trouble coping with that.

If the weapon is regularly changing direction then the effective range of your interceptors is significantly reduced, meaning that any interception will have to be launched from closer to the defended area and will have to be launched later in the weapons flight. Launch too soon and the weapons maneuvering will exhaust the interceptors fuel.

If the weapon can detect the interceptor then it may chose an alternate target, heck, they might be maneuverable enough to come in for a second or even third approach.