When Paul Allen founded Stratolaunch in 2011, the hope was to make access to space cheaper and faster. The company’s massive carrier aircraft, the largest plane by wingspan ever to be built, would be able to carry rocket-powered vehicles up into the thin upper atmosphere on short notice under the power of its conventional jet engines. The smaller vehicle, free of the drag it would incur in the denser atmosphere closer to the ground, could then be released and continue its journey to space using smaller engines and less propellant than would have been required for a conventional launch.
But Allen, who died in October of 2018, never got to see his gigantic plane fly. It wasn’t until April 13th, 2019 that the prototype carrier aircraft, nicknamed Roc, finally got to stretch its 117 meter (385 feet) wings and soar over the Mojave Desert. By that time, the nature of spaceflight had changed completely. Commercial companies were putting payloads into orbit on their own rockets, and SpaceX was regularly recovering and reusing their first stage boosters. Facing a very different market, and without Allen at the helm, Stratolaunch ceased operations the following month. By June the company’s assets, including Roc, went on the market for $400 million.
Finally, after years of rumors that it was to be scrapped, Allen’s mega-plane has flown for the second time. With new ownership and a new mission, Stratolaunch is poised to reinvent itself as a major player in the emerging field of hypersonic flight.
The first scramjet, an airbreathing jet engine capable of pushing an aircraft beyond Mach 5, was successfully flown in the early 1990s. But while pretty much any other technology you could imagine has progressed by leaps and bounds in the nearly 30 years that have passed, the state-of-the-art in hypersonic scramjets hasn’t moved much. We still don’t have practical hypersonic aircraft, military or otherwise, and any missiles that travel at those sort of speeds are rocket powered.
This is somewhat surprising since, at least on paper, the operating principle of the scramjet is simplicity itself. Air rushing into the engine is compressed by the geometry of the inlet, fuel is added, the mixture is ignited, and the resulting flow of expanded gases leaves the engine faster than it entered. There aren’t even any moving parts inside of a scramjet, it’s little more than a carefully shaped tube with fuel injectors and ignitors in it.
Unfortunately, pulling it off in practice is quite a bit harder. Part of the problem is that a scramjet doesn’t actually start working until the air entering the engine’s inlet is moving at around Mach 4, which makes testing them difficult and expensive. It’s possible to do it in a specially designed wind tunnel, but practically speaking, it ends up being easier to mount the engine to the front of a conventional rocket and get it up to speed that way. The downside is that such flights are one-way tickets, and end with the test article crashing into the ocean once it runs out of fuel.
But the bigger problem is that the core concept is deceptively simple. It’s easy to say you’ll just squirt some jet fuel into the stream of compressed air and light it up, but when that air is moving at thousands of miles per hour, keeping it burning is no small feat. Because of this, the operation of a scramjet has often been likened to trying to light a match in a hurricane; the challenge isn’t in the task, but in the environment you’re trying to perform it in.
Now, both Aerojet Rocketdyne and Northrop Grumman think they may have found the solution: additive manufacturing. By 3D printing their scramjet engines, they can not only iterate through design revisions faster, but produce them far cheaper than they’ve been able to in the past. Even more importantly, it enables complex internal engine geometries that would have been more difficult to produce via traditional manufacturing.
When Charles “Chuck” Yeager reached a speed of Mach 1.06 while flying the Bell X-1 Glamorous Glennis in 1947, he became the first man to fly faster than the speed of sound in controlled level flight. Specifying that he reached supersonic speed “in controlled level flight” might seem superfluous, but it’s actually a very important distinction. There had been several unconfirmed claims that aircraft had hit or even exceeded Mach 1 during the Second World War, but it had always been during a steep dive and generally resulted in the loss of the aircraft and its pilot. Yeager’s accomplishment wasn’t just going faster than sound, but doing it in a controlled and sustained flight that ended with a safe landing.
In that way, the current status of hypersonic flight is not entirely unlike that of supersonic flight prior to 1947. We have missiles which travel at or above Mach 5, the start of the hypersonic regime, and spacecraft returning from orbit such as the Space Shuttle can attain speeds as high as Mach 25 while diving through the atmosphere. But neither example meets that same requirement of “controlled level flight” that Yeager achieved 72 years ago. Until a vehicle can accelerate up to Mach 5, sustain that speed for a useful period of time, and then land intact (with or without a human occupant), we can’t say that we’ve truly mastered hypersonic flight.
So why, nearly a century after we broke the sound barrier, are we still without practical hypersonic aircraft? One of the biggest issues historically has been the material the vehicle is made out of. The Lockheed SR-71 “Blackbird” struggled with the intense heat generated by flying at Mach 3, which ultimately required it to be constructed from an expensive and temperamental combination of titanium and polymer composites. A craft which flies at Mach 5 or beyond is subjected to even harsher conditions, and it has taken decades for material science to rise to the challenge.
With modern composites and the benefit of advanced computer simulations, we’re closing in on solving the physical aspects of surviving sustained hypersonic flight. With the recent announcement that Russia has put their Avangard hypersonic glider into production, small scale vehicles traveling at high Mach numbers for extended periods of time are now a reality. Saying it’s a solved problem isn’t quite accurate; the American hypersonic glider program has been plagued with issues related to the vehicle coming apart under the stress of Mach 20 flight, which heats the craft’s surface to temperatures in excess of 1,900 C (~3,500 F). But we’re getting closer, and it’s no longer the insurmountable problem it seemed a few decades ago.
Today, the biggest remaining challenge is propelling a hypersonic vehicle in level flight for a useful period of time. The most promising solution is the scramjet, an engine that relies on the speed of the vehicle itself to compress incoming air for combustion. They’re mechanically very simple, and the physics behind it have been known since about the time Yeager was climbing into the cockpit of the X-1. Unfortunately the road towards constructing, much less testing, a full scale hypersonic scramjet aircraft has been a long and hard one.
With a highly publicized test firing and pledge by President Vladimir Putin that it will soon be deployed to frontline units, Russia’s Avangard hypersonic weapon has officially gone from a secretive development program to an inevitability. The first weapon of its type to enter into active service, it’s capable of delivering a payload to any spot on the planet at speeds up to Mach 27 while remaining effectively unstoppable by conventional missile defense systems because of its incredible speed and enhanced maneuverability compared to traditional intercontinental ballistic missiles (ICBMs).
In a statement made after the successful test of Avangard, which saw it hit a target approximately 6,000 kilometers (3,700 miles) from the launch site, President Putin made it clear that the evasive nature of the weapon was not to be underestimated: “The Avangard is invulnerable to intercept by any existing and prospective missile defense means of the potential adversary.” The former Soviet KGB agent turned head of state has never been one to shy away from boastful claims, but in this case it’s not just an exaggeration. While the United States and China have been working on their own hypersonic weapons which should be able to meet the capabilities of Avangard when they eventually come online, there’s still no clear deterrent for this type of weapon.
Earlier in the year, commander of U.S. Strategic Command General John Hyten testified to the Senate Armed Services Committee that the threat of retaliation was the best and perhaps only method of keeping the risk of hypersonic weapons in check: “We don’t have any defense that could deny the employment of such a weapon against us, so our response would be our deterrent force.” Essentially, the threat of hypersonic weapons may usher in a new era of “mutually assured destruction” (MAD), the Cold War era doctrine that kept either side from firing the first shot knowing they would sustain the same or greater damage from their adversary.
With President Putin claiming Avangard has already entered into serial production and will be deployed as soon as early 2019, the race is on for the United States and China to close the hypersonic gap. But exactly how far away is the rest of the world from developing an operational hypersonic weapon? Perhaps more to the point, what does “hypersonic weapon” really mean?