 When the X-15 first rolled out of North American Aviation's Hangar in Los Angeles on October 15th of 1958, it wasn't quite the plane that was gonna set speed and altitude records. Today on Vintage Space, we're gonna talk about the X-15's big engine and specifically the first test that nearly killed a Scott Crossfield. The first X-15s to fly had two twin XLR 11 engines. These weren't giant engines. To take the X-15 to record-breaking speeds and altitudes, it would need one giant engine, the XLR 99, which had significantly more thrust and power than the XLR 11s. They had about the thrust of a redstone rocket, the one that launched Al Shepard and Gus Grissom on their suborbital Mercury flights. It also burned a different fuel, and idrosimmonia, and liquid oxygen as opposed to Efo alcohol and liquid oxygen. The first XLR 99 engine arrived in California in April of 1960, a year-and-a-half-pine schedule, but it didn't have to wait long to get into an airplane. North American and the NACA had actually kept an X-15 aside, the third X-15 built, specifically to put the XLR 99 engine in as soon as they got it, and that's exactly what they did, and almost immediately set it to ground testing. Testing the XLR 99 engine inside the X-15 demanded a pretty elaborate setup at Edwards Air Force Base in California. It involved an elaborate rig with steel clamps to hold the X-15 in place, underground observation bunkers, and all kinds of equipment to gather every piece of data during the test. The test day was June 8th, and Scott Crossfield was the pilot. But because Crossfield wouldn't be taking the X-15 in the air at all, he didn't need to put on a pressure suit. Instead, he climbed in his street clothes and took a breathing apparatus similar to what skin divers use. The test that day was designed to replicate an X-15 in flight. Crossfield would go through all the stages to ready the X-15 for a drop, fire the engine, and then throttle it back to simulate restarting in flight. And that's just what happened. Technicians assisting with the test that day took shelter in bunkers, and rescue personnel and fire trucks were parked about 600 feet away. As he moved through the engine start procedure, Crossfield called out every step over the radio. When he moved the main engine throttle to the engine start position, he opened the fuel and oxidizer lines. The engine roared to life, and the X-15 started vibrating. And, as planned, he throttled the engine back to about 15%. But this loss of thrust triggered a safety device in the engine, and it shut down automatically. It didn't really matter to Crossfield, though. He had no indications of an issue in the cockpit, and he started the restart procedure. He might as well have hit the plunger on a dynamite detonator. The X-15 exploded with enough force that the rear section was demolished instantly. Meanwhile, the front section, with the cockpit, with Crossfield inside, shot forward about 20 feet at an alarming speed. Inside the cockpit, Crossfield pulled about 50 Gs. It's a really good thing that he had his head resting on the headrest, otherwise he might have broken his neck. 900 gallons of ammonia and 60 gallons of hydrogen peroxide had ignited instantly. In the cockpit, Crossfield shut down the APU and switched to the airplane's own oxygen system. This was the only thing he could do to prevent another explosion. Then he braced his feet on the instrument panel, covered his face with his arms, and waited. Before long, the fire member there dousing the plane in water. And people were frantically trying to get him out of the cockpit, even though he knew that inside the cockpit of the X-15 was probably the safest place to be during a fire. Though the plane was completely destroyed, Crossfield walked away fine if shaken and a little bit wet. The airplane was eventually rebuilt and the XLR-99 engine was eventually installed and flown successfully on later flights. For more on the story of the X-15 and its evolution to high-speed, high-altitude flights, check out the latest article on vintage space over at Popular Science. And don't forget to follow me on Twitter for all kinds of vintage space updates all the time. And with new videos coming up every Tuesday and Friday, don't forget to subscribe right here so you never miss an episode.