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It was early morning on August 11th, 2011 at Vandenberg Air Force Base in California with heavy fog clinging to the hills obscuring the launch complex where a Minotaur 4 light rocket sat waiting on the pad. Inside the control room engineers stared at their [music] screens scanning telemetry data that was about to come in faster than almost anything else in human history. The booster ignited punching through the marine layer and shaking the ground. As the rocket climbed staging its way out of the thick lower atmosphere, it began to arc over the Pacific. At the edge of space, the payload [music] fairing clamshelled open and kicked out a strange black arrowhead-shaped wedge. Inside the control room voices called out the numbers. The speed climbed past Mach 10, then Mach 15, finally settling around Mach 20, 20 times the speed of sound, 13,000 mph. And the data link was good, the GPS was locked, attitude was stable. And then, 9 minutes into the flight, the communications loop suddenly went quiet. Multiple sensors flatlined simultaneously, the telemetry screens froze because somewhere over the vast empty Pacific, the vehicle had gone silent. This was DARPA's Falcon Hypersonic Technology Vehicle 2, fortunately shortened to HTV-2. It was an experimental platform designed to answer a terrifyingly difficult question. Was it possible to build an aircraft that could fly anywhere in the world >> [music] >> in under 60 minutes? At those speeds, the plan was to be able to cross from New York to Los Angeles in under 12 minutes. >> [music] >> In some concepts, you could get from London to Sydney in less than an hour. But officially, the HTV-2 was just a glide vehicle. And considering that its two public flights both ended in what engineers politely call a rapid [music] unscheduled ocean interference, you might be tempted to say that, well, unofficially, it wasn't. But the failure of this specific machine hit a much bigger, much more significant story. You see, the Falcon program is at the heart of the American concept of prompt global strike. This was the desire to launch a conventional, non-nuclear weapon from the continental United States and hit a target, a terrorist training camp, a mobile missile launcher, or a rogue satellite facility anywhere on the planet without someone thinking that America had just pulled off a nuclear-tipped ICBM, which would be bad. It's a capability that has since become the new buzzword of a 21st century arms race. From China's DF-ZF to Russia's Avangard, covered that before in the channel if you want to check it out. The world's currently scrambling to master the technology that started right here with this small black wedge falling into the ocean. So, how do you design something that's supposed to survive skin temperatures of 3 1/2 thousand degrees Fahrenheit? That's hotter than molten steel, by the way, for 20 minutes at a time. And if both test vehicles crashed, how can DARPA still claim that this was a major step toward the new stealth? Well, let's find out, shall we? The dream, one hour to anywhere. The concept of a hypersonic glider was not exactly a new one. In fact, the dream of a vehicle that could launch like a rocket but glide like an aircraft had been haunting the dreams of US military planners since the very dawn of the space age. Back in the late 1950s, the Air Force was already pouring money into the [music] X-20 Dyna-Soar, a manned spaceplane designed to be boosted into space and then skip along the atmosphere's edge to drop bombs anywhere in the world. Also, my favorite name aircraft. It was a terrifyingly ambitious idea for the era. Effectively, it was a space bomber. But like so many high-speed dreams of the Cold War, and there were many, the X-20 was eventually killed off by a combination of spiraling costs, extreme technical challenges, and the [music] simple, albeit blunter, solution of ballistic missiles. The dream of global reach in under two hours was shelved, but it never really went away. It sat in the background gathering dust until the early 2000s when DARPA decided it was time to blow that dust off and try again. This time, they bundled the ambition under a new, appropriately aggressive acronym, Falcon, standing for Force Application and Launch from Continental United States. The Falcon project was designed as a sweeping umbrella program with two main components. First, [music] they wanted a small launch vehicle or SLV, which would be a cheap, effective rocket to throw hardware into the upper atmosphere. Second, and much more importantly, they wanted a family of Hypersonic Technology Vehicles or HTVs. The plan was to iterate quickly, starting with smaller demonstrators like HTV-1 and HTV-2, and eventually working up to a fully reusable hypersonic cruise vehicle that could take off from a runway, strike a target, and come back home. It sounded incredibly futuristic, but the strategic vision behind it was actually rather simple. It was known as Conventional Prompt Global Strike. The logic, the United States had this unrivaled nuclear arsenal that could hit anywhere on Earth in 30 minutes. But in a post-Cold War, post-9/11 world, nuclear weapons were essentially unusable. You cannot nuke a terrorist training camp or a rogue missile truck without maybe ending the world. It's definitely a bit of overkill, isn't it? So, the Pentagon was left with a time gap. If intelligence spotted a high-value target in a remote part of the world, it could take hours or even days to move an aircraft carrier or a bomber into position. But by the time they arrived, the target would be gone. Falcon was the answer to that time gap. The idea was to push a hypersonic glider off a booster, keep it within the atmosphere, and surf it to the target at speeds above Mach 5. It offered the speed of an ICBM, but with the precision of a sniper rifle, and crucially, you know, no radiation. But DARPA didn't just want Mach 5. [music] For the HTV-2, they targeted something much more extreme, Mach 20. Put that kind of speed into perspective. At Mach 20, you're covering about 3.6 miles or nearly 6 kilometers every [music] single second. If you were to fire the HTV-2 into the Channel Tunnel, well, it would go very, very wrong, but that's a 50-kilometer journey that takes the Eurostar about 20 minutes. Well, this glider would cover it in 9 seconds. At that speed, [music] if you can sustain the glide for just 30 minutes, almost any target on Earth is within reach. However, this capability came with a heavy political price tag. Developing a weapon that flies like a nuke but hits like a conventional bomb created a bit of a nightmare scenario for other nuclear powers. If Russia or China sees a fast-moving object launched from the US and streak toward their border at Mach 20, they only going to have minutes to decide if it was a conventional strike on a specific target or [music] the start of the end of the world. This warhead ambiguity became one of the defining controversies of [music] the program. DARPA wasn't just building a fast plane, they were building a system that threatened to destabilize the delicate balance of nuclear deterrence. But before they could worry about the geopolitics of the weapon, they first had to figure out if the thing could actually fly without completely melting. And as engineers were about to discover the gap between the dream of Mach 20 and the reality of Mach 20 was filled with a lot of very, very hot, very angry air. Inside the Falcon program and HTV-2's mission. Okay, so how exactly do you build a machine that is capable of surviving the journey that we just described? Well, in traditional aerospace engineering, the rule is pretty simple. You crawl, you walk, and then you run. You build a low-speed demonstrator, you test the basics, and then you slowly dial up the difficulty. That was the original plan for the Falcon program, too. The road map called for a vehicle named HTV-1. It was supposed to be the crawl phase, a lower-performance glider designed to prove the manufacturing techniques for the heat shield. But there was a bit of a problem. The design called for a curved, one-piece carbon-carbon aeroshell, and it turns out that building a curved, one-piece carbon-carbon aeroshell is quite difficult. It was essentially very difficult to make something that wouldn't shatter under that kind [music] of stress. The manufacturing issues with HTV-1 became so severe that DARPA made a bold, perhaps slightly reckless, decision. They canceled the crawl phase entirely. They decided they were just going to skip straight to running. They moved directly to the more advanced, more aggressive design, the HTV-2. HTV-2 was built by the legendary Skunk Works division at Lockheed Martin, the same people who gave us the SR-71 Blackbird and the F-117 [music] Nighthawk, two very, very badass planes. They adopted a design for manufacturing approach, simplifying the aeroshell's shape into a series of flat, wedge-like slabs that were easier to build but aerodynamically unforgiving. But make no mistake, this was not a weapon, at least not yet. In the official language of the program, HTV-2 was a data truck. Its primary mission wasn't to blow anything up, it was to scream through the upper atmosphere, collect terabytes of telemetry, and verify the physics models that engineers have been arguing over the decades. The objectives were a checklist of aerodynamic nightmares. [music] First, prove that this arrowhead shape actually generated enough lift to glide thousands of miles. Second, validate the carbon-carbon thermal protection system, ensuring the hot structures didn't melt when the air turned into plasma, which is something I'm just learning that air does. Third, demonstrate that you could actually steer this thing, providing precise guidance, navigation, and control at speeds where a tiny twitch of a flap could send you tumbling into space or disintegrating into [music] the ocean. And finally, there was one more critical system to test, the autonomous flight safety system. This was a piece of onboard software with a really specific job. If the vehicle ever sensed that it was losing control or flying outside a safety corridor, it was programmed to immediately command a termination maneuver, forcing the glider into a suicide roll straight into the ocean. As we'll see, the system worked perfectly. Twice. The flight plan for this data truck was practically identical for both missions. The launch site was Vandenberg Air Force Base in California. The ride was an Orbital Sciences Minotaur 4 Lite, a modified Peacekeeper missile stack that used three solid fuel rocket stages to punch the glider out of the atmosphere. The profile called for the booster to lift the HTV-2 to the edge of space, reaching an altitude of roughly 100 mi. Then, the clamshell payload fairing would open and the glider would separate. At this point, gravity would pull it back down into the atmosphere, where it would perform a pull-up maneuver to level out at around Mach 20. From there, the plan was to surf the atmosphere across the Pacific Ocean for about 30 minutes, covering over 4,000 nautical miles, about 7,700 km. The target was a patch of ocean just north of the US Army's Kwajalein Atoll test range in the Marshall Islands. Now, in theory, the vehicle would end its mission with a controlled dive into the water. In practice, that didn't happen. The flights never made it to Kwajalein. Both vehicles went silent around the 9-minute mark. But despite the short flights, the program leaders insisted that this was worth it. Major Chris Schulz, the US Air Force program manager, later emphasized that wind tunnels on the ground simply lie to you at these speeds. You can't simulate the chemical changes in the air at Mach 20. As Schulz [music] put it, they needed actual flight data to understand what was really happening to the skin of the aircraft. DARPA director Regina Dugan later framed the stakes even more [music] clearly. She referred to hypersonics as the new stealth. The logic was pretty straightforward. As stealth technology spreads to other nations, the US advantage of being invisible is kind of eroding. If you can't be invisible, well, then you have to be fast. Fast enough that even if they do see you coming, they can't do about it. The HTV-2 was the first stumbling, expensive step toward that reality. How do you surf Mach 20 air? Okay, so what does it actually take to fly at Mach 20? Well, to understand the madness of the HTV-2, we first got to agree on some definitions because fast kind of a relative term. Now, in the world of aerodynamics, we usually talk about regimes. Subsonic is anything slower than the speed of sound. That's small commercial airliners, Cessnas, birds, those kind of things. A supersonic bird would be kind of awesome. Supersonic covers everything from Mach 1 to Mach 5. This is the domain of fighter jets, the Concorde, and rifle bullets. But once you cross Mach 5, you enter the hypersonic regime. And the HTV-2 wasn't just hypersonic, it was designated to sit in the high hypersonic regime, cruising at Mach 20. At these speeds, the air doesn't really behave like air anymore. When a normal plane flies, it pushes air molecules out of the way. But at Mach 20, the vehicle is moving so fast that the air molecules can't get out of the way in time. Instead, they get smashed against the front of the vehicle with such violence that they literally break apart. The kinetic energy is so intense that the gas chemically decomposes and ionizes, creating a sheath of superheated plasma around the vehicle. You aren't flying through a breeze anymore, you're flying through a blowtorch, which is definitely trying to melt you, and it's also sandblasting you and chemically dissolving you. To survive this incredibly harsh environment, you can't just build a regular-ass plane. The HTV-2 used a waverider concept, a flattened arrowhead-shaped wedge with a razor-sharp leading edge. The entire vehicle was designed to ride on top of the shockwave it created, trapping the high pressure underneath it to generate lift. This gave the HTV-2 a lift-to-drag ratio, or LD, of about 2.6. Now, compared to a modern airliner, which has a lift-to-drag ratio of around 18, that sounds, well, pretty It has the aerodynamic efficiency of a very sleek brick. But for something traveling at 13,000 mph while being engulfed in plasma, a ratio of 2.6 is [music] actually basically a miracle of engineering. It means that for every kilometer the vehicle descends, it can skip forward 2.6 km. >> [music] >> It allows the vehicle to skim along the top of the atmosphere like a stone skipping across a pond, extending its range by thousands of miles before gravity finally wins. But staying in the air is only half the battle. The bigger problem is keeping the air from turning your vehicle into a streak of molten slag. The thermal environment for the HTV-2 [music] is difficult to even comprehend. Engineers expected the surface temperature of the vehicle to reach roughly 3,500° F or 1,930° C. To give you some context, the melting point of steel is around 2,500° F. So, if you made this aircraft out of standard aerospace materials, it wouldn't just fail, it would liquefy and vaporize before it was even halfway to the target. To handle this, Lockheed Martin built the aeroshell out of a specialized carbon-carbon composite, the tricky-to-make one we mentioned earlier. Now, this material is incredibly light, incredibly strong, and it stays rigid even when it is glowing white-hot. But manufacturing it is a real nightmare. You've got to put sheets of carbon fabric together, impregnating them with resin, and then baking them all at extreme temperatures for weeks to turn the resin into pure carbon graphite. Imagine trying to build a paper airplane that needs to stay perfectly rigid while someone holds it inside a blast furnace and then throws it through a hurricane. That is effectively the design for the HTV-2. Then there's the problem of steering. DARPA identified three main technical hurdles for this program: aerodynamics, aerothermal effects, and guidance, navigation, and control, or GNC. GNC is the art of telling the vehicle where to go. But at Mach 20, everything is a lot harder. You see, the vehicle is moving at 3.6 mi/s. That means the onboard sensors and GPS receivers have to lock onto satellite signals while moving faster than almost any other man-made object in history. In fact, maintaining a GPS lock at that speed was one of the few firsts DARPA proudly bragged about after the first flight. The control system has to be absolutely superhuman. The vehicle used aerodynamic flaps to steer, but in the thin upper atmosphere, the flaps don't have much air to bite into. So, the HTV-2 also carried a reaction control system, or RCS, tiny set of rocket thrusters capable of puffing gas to shove the nose left, right, up, or down. The computer has to mix these two systems perfectly. If the nose drifts just a fraction of a degree off course, at Mach 20, that error translates to missing the target by hundreds of miles, or worse, tumbling out of control. And because the vehicle is surrounded by a sheath of ionized plasma, radio signals can also get blocked, meaning the computer is often flying entirely on its own, making survival decisions in milliseconds. And this brings us to the autonomous flight safety system. Now, testing experimental weapons near populated areas makes governments rather nervous. If the HTV-2 decided to take an unexpected turn toward San Francisco or Tokyo, it would be a bit of a disaster. So, the engineers gave the vehicle a suicide pill. The autonomous flight safety system monitored the vehicle's position and health. If the glider ever sensed that it was losing control or drifting outside of its safe flight corridor, it was programmed to instantly command a termination maneuver. It would deliberately force the vehicle into a violent roll and pitch straight down, diving into the ocean to ensure it didn't hurt anyone. Now, ultimately, this disposable nature was kind of the whole point. DARPA explicitly referred to the HTV-2 not as a prototype, but as that data truck system we mentioned earlier. It was packed with sensors [music] designed to record temperatures, pressures, and aerodynamic forces that no wind tunnel on Earth could simulate. The goal wasn't to build a fleet of these specific gliders. The goal was to gather data that would feed into the computer models for future programs, things with names like Tactical Boost Glide and Hypersonic Air Breathing Weapon Concept. The engineers knew the vehicle was going to die. They just hoped that it would live long enough to tell them something interesting before it did. As it turned out, on the first flight, it had a very short story to tell. Flight one. On April the 22nd, 2010, the first major test of the Falcon program took place. On the pad that day stood a Minotaur 4 Lite, a Frankenstein's monster of a rocket derived from decommissioned Peacekeeper missiles. Perched on top, hidden inside the fairing, was the first HTV-2 vehicle. The plan was a straightforward one. The booster would launch and perform a series of complex energy management maneuvers, basically S turns during the ascent to burn off excess energy so it didn't overshoot the target speed. At the edge of space, the fairing would open, the glider would separate, and the vehicle would begin its long, lonely glide across the Pacific toward Kwajalein Atoll. At first, everything looked brilliant. The launch was clean, the separation was clean. For the first few minutes of the glide, the control room was buzzing with what looked like a historic success. DARPA had assembled the largest network of data collection assets ever deployed for a hypersonic test. Ships, planes, satellites, ground stations were all watching for this one black wedge. And the vehicle was talking back. It established two-way communication with the ground. The reaction control system, RCS, was firing correctly to keep it stable. Most impressively, the onboard computer maintained a GPS lock while traveling at Mach 20. But then, the data stopped. At roughly 9 minutes into the flight, eight separate telemetry assets lost the signal simultaneously. The screens went blank. The vehicle was gone. So, what happens? It took months for an independent engineering review board to piece together the autopsy. The culprit wasn't a melted wing or a computer crash. It was a phenomenon known as yaw roll coupling. Think of the HTV-2 like an ice skater spinning in a really tight circle. If the skater leans just a tiny bit too far to the side, the spin turns into a tumble. At Mach 20, that tumble happens in milliseconds. The review board found that the vehicle was flying at an attack angle that produced a higher than predicted yaw. Basically, the nose drifted sideways more than models said it would. This yaw forced the vehicle into a roll. The onboard computer saw this happening and fought back firing its thrusters, moving its flaps to correct the spin. But, the aerodynamic forces always stronger than the control surfaces. The vehicle's control authority, its physical ability to push back against the air, was exceeded. And this is where the HTV-2 realized something. It was going to lose the fight. The autonomous flight safety system, the suicide pill, detected that the vehicle was about to violate its safe flight [music] parameters. Rather than risk tumbling uncontrollably and landing somewhere unpredictable, the computer executed its final command forcing the vehicle into a controlled roll and pitched its nose down commanding a rapid unscheduled ocean interference deep in the Pacific. In the immediate aftermath, the mood was pretty grim. The headlines were not kind. The fastest plane ever built had barely lasted 9 minutes. Critics began to wonder if this was going to be a repeat of the spaceplane failures of the '80s and '90s. Was the HTV-2 destined to become just another hangar queen too expensive to fly and too fragile to succeed? DARPA was left with a choice. They'd gathered some good data, but the primary mission had failed. They had one vehicle left in the inventory. They could pack it up and go home, or they could try to fix it, load the last remaining glider onto a rocket, and roll the dice one more time. Autopsy. For the second flight, they decided to adjust the flight plan. First, they adjusted the center of gravity by shifting the mass inside the vehicle. This could make it naturally more stable, kind of like putting a paperclip on the nose of a paper airplane. Second, they changed the flight profile. They programmed the autopilot to fly at a shallower angle of attack. By keeping the nose lower, they kept the vehicle in a safer aerodynamic envelope where the control surfaces had more margin to work with. And third, they unleashed the thrusters. For the second flight, the software was updated to use the reaction control system to augment the aerodynamic flaps much more aggressively, especially early in the glide when the air was thinnest. If the flaps couldn't handle the turn, the rockets would muscle through it. But, notice what they didn't change. They didn't change the arrowhead shape, and crucially, they didn't change the thermal protection system. The engineers were confident that the aeroshell, that high-tech carbon skin, was fine. The review board concluded that the failure of flight one was purely a steering issue. They assumed that if they could just keep the vehicle pointing straight, the materials would hold up against the heat. As we're about to see, that assumption was uh let's say optimistic. The team was running out of time. By 2011, the political clock was ticking. The Prompt Global Strike program was under scrutiny and other hypersonic efforts like the Air Force's X-51 Waverider were also suffering high-profile failures. Patience in Washington was wearing thin and the budget was drying up. DARPA had one vehicle left. There was no backup. It was flight two or bust. Flight two, controlled hell. >> [music] >> On August the 11th, 2011, that fog had returned. The launch had already been scrubbed once the previous day due to bad weather adding another 24 hours of agonizing delay to a program that was already living on borrowed time. On the pad stood the second and final HTV-2 vehicle. There were no backup gliders in the hangar. There was no budget for a flight three. The engineers tweaked the center of gravity, adjusted the flight path, and rewritten the control software. Now, they just had to hope that it would all work. At 7:45 a.m. Pacific time, the Minotaur 4 light roared off and carried the last hope of the program toward the edge of space. The booster did its job, the fairing opened, and the glider separated cleanly. It pitched over, reentered the atmosphere, and leveled out at Mach 20. And for the first few minutes, once again, it was glorious. The telemetry boards were lighting up with green. The engineering fixes had worked. The vehicle achieved stable, aerodynamically controlled flight at speeds up to Mach 20 for nearly 3 minutes. It was flying straight. It was responding to commands. [music] It was doing exactly what a hypersonic glider is supposed to do. But, while the guidance computer was keeping the nose straight, the air outside was tearing the vehicle apart. Later analysis revealed that during the stable glide, the vehicle was experiencing shockwave disturbances more than 100 times stronger than what it was designed to handle. The glider was fighting for its life, correcting itself over and over again until it couldn't do it anymore. The independent engineering review board later concluded that the primary cause of the failure was unexpected aeroshell degradation. In plain English, its skin started to peel off. The models had predicted that the carbon-carbon composite would ablate, basically wear away slowly like a bar of soap. But, in reality, larger than expected chunks of the vehicle's skin were being ripped away by the airflow. When a chunk of skin flies off at Mach 20, it doesn't really just leave a dent. It creates a jagged gap that generates a massive shockwave. These shockwaves slammed into the side of the vehicle like a hammer causing violent rolls. Hypersonic flight is just brutal. We're used to seeing spacecraft reenter the atmosphere enduring thousands of degrees of heat, but a capsule like Apollo or Soyuz only dips through that heat for a few minutes while it slows down. The HTV-2 was trying to stay in that heat. Imagine the heat shield on a spacecraft, but instead of dipping in for a few minutes and slowing down, you're trying to stay at that reentry heat for half an hour. Even tiny imperfections or microcracks can magnify into catastrophic failure. The vehicle put up a hell of a fight. The telemetry showed that it endured several of these shockwave-induced rolls and actually managed to right itself using its thrusters and flaps to wrestle the nose back to center. But, eventually, the skin degradation became too severe, the shockwaves too violent. The disturbances exceeded the ability of the control system to compensate. And so, for the second time in 2 years, the autonomous flight safety system looked at the data, realized the vehicle was doomed, and triggered the termination sequence. It used its remaining energy to guide the vehicle into a controlled splashdown in the Pacific Ocean. To the outside world, this looked like an embarrassing disaster. The headlines were pretty brutal again. "Fastest plane ever built crashes," screamed ABC News. The Guardian noted that contact was lost after just 9 minutes marking the second major hypersonic setback of 2011 following the failure of the X-51 Waverider earlier that year. But, inside the Pentagon, the mood was different. Yes, the vehicle was gone, but before it died, it had beamed back more than 9 minutes of high-fidelity data including that crucial window of stable aerodynamic flight. For the first time, engineers were not guessing what happens to carbon skin at Mach 20. They had watched it happen. The data truck had crashed, but it had delivered the cargo first. Legacy in the new stealth. So, after two flights, two rockets, and two premature splashdowns, what did the engineers actually learn? The second flight was actually a vindication of the vehicle's aerodynamic design. [music] When the skin was peeling off, the HTV-2 flew beautifully. It proved that the arrowhead shape was stable and controllable at Mach 20 validating the fixes made after the first failure. The problem, as it turned out, was entirely thermal. The existing computer models and ground tests had severely underpredicted just how quickly and violently the carbon [music] skin would degrade under real flight conditions. Major Chris Schulz, the program manager, summed it up perfectly. He noted that flight one corrected their aerodynamic models teaching them how to steer. Flight two corrected their understanding of the materials teaching them how to survive. It was a classic engineering moment. We thought we knew, we realized we were wrong, and now, thanks to 18 minutes of flight time, we actually do know. Fantastic. And what they now know is fueling a concept DARPA calls the new stealth. For the last 30 years, American air dominance has relied on being invisible to radar. But, as stealth technology proliferates and anti-stealth radars improve, that advantage is starting to erode. So, the Pentagon is looking for a new edge, extreme [music] speed, reach, and range. Logic's simple. Stealth hides you from the enemy's radar, hypersonics compresses [music] the enemy's time to react. Even if they see you coming, a weapon moving at Mach 20 crosses entire countries in minutes. It shrinks the defender's decision loop from hours to seconds [music] forcing them to panic or freeze. The HTV-2 never became an operational weapon, but its DNA is everywhere. DARPA explicitly states that the data collected from these two failed flights is now feeding directly into policy, acquisition, and operational decisions for the next generation of conventional Prompt Global Strike [music] programs. The lessons learned specifically about how carbon composites fail at high Mach numbers have been baked into follow-on programs with names like Integrated Hypersonics, [music] Tactical Boost Glide, and the Hypersonic Air-Breathing Weapon Concept, or Hawk. While the specifics of modern [music] systems like the Army's Common Hypersonic Glide Body or the Air Force's Arrow missile often classified, sensibly so, they all live in an engineering ecosystem that was defined by the failures of the HTV-2. [music] By 2013, DARPA decided that the HTV-2 had done its job. They decided to end the program choosing not to build a third. They'd answered enough of the crazy Mach 20 questions to realize that perhaps they should refocus their research on more tactical survivable systems, things that don't try to fly halfway around the world in one go, but rather offer shorter, sharper strikes. But, while the US paused to regroup, the rest of the world kept pushing. The physics that the HTV2 explored have since been weaponized by America's rivals. Reports suggest that China's DFZF Hypersonic Glide Vehicle and Russia's Avangard system have both reached operational status. These weapons rely on the same basic boost-glide physics that DARPA was testing back in 2011. The HTV2 wasn't a weapon itself, it was the instruction manual on what a Mach 20 weapon would need to survive. And it seems everyone else was reading that manual, too. And in the end, we have to look back at the human cost of a project like this, not in lives, thankfully, but in effort. Hundreds of engineers spent years designing, building, and coding these machines. They spent countless [music] nights in simulators and control rooms. And the result of all of that work was two flights, 9 minutes each, both ending in the ocean. It takes a special kind of resilience to work in hypersonics. Engineers had to write reports explaining why the vehicle's skin peeled off faster than they thought, and then convince Congress that this was actually progress. For most of us, success means building something that still exists at the end of the project. For the people behind HTV2, success was 9 minutes of data and a splashdown that they planned all along. Thank you for watching.