The X-15 had a propellant jettison capability if all engine light attempts failed. The pilot could jettison a full load of propellants in two minutes. This was extremely crucial since the aircraft would not survive a landing with any significant amount of propellants on-board.
The LR-11 engine was used in the X-15 on an interim basis while we waited for the LR-99 to complete its demonstration tests. The LR-11 engine had been previously used in the X-1 and the D-558-II rocket airplanes. It consisted of four separate barrels or chambers that developed 1,500 pounds of thrust in each barrel or a total of 6,000 pounds of thrust. Each barrel could be operated independently, and as a result the pilot had an incremental throttling capability. Two engines were installed in each X-15 to provide a total of 12,000 pounds of thrust. The engines burned a combination of water and alcohol using liquid oxygen as the oxidizer. The relatively low level of thrust developed by these engines severely limited the performance of the X-15, but use of these engines allowed the flight program to begin on schedule.
Three X-15 aircraft were constructed. Two were delivered to the government on completion of the contractor demonstration phase. The third aircraft blew up during a ground run prior to its first flight. It was subsequently rebuilt and delivered to the government almost a year later. The number two aircraft was severely damaged in November 1962 and was later rebuilt as the X-15A-2—a highly modified version of the X-15.
A. Scott Crossfield
Robert M. White
Joseph A. Walker
Forrest S. “Pete” Petersen
John B. “Jack” McKay
Robert A. Rushworth
Neil A. Armstrong
Joseph H. Engle
Milton O. Thompson
William J. “Pete” Knight
William H. Dana
Michael J. Adams
Left to right: Pete Knight, Bob Rushworth, Joe Engle, Milt Thompson, Bill Dana, and Jack McKay.
View from the X-15 during an altitude flight. Las Vegas is in the lower left. The Colorado River runs through the center of the photograph from left to right.
The LR-99 rocket engine.
Model of the proposed delta-wing X-15
Aerial view of the Dryden Flight Research Center, circa 1953. Short taxiway onto lakebed is at center top.
X-15 being hoisted up to pylon on B-52 wing. Hoists were located in X-15 servicing, fueling, and mating areas.
Preparing to start B-52 engines prior to taxi for takeoff.
X-15 servicing area during an attempt to fly two X-15s on the same day. Although several attempts were made, we never succeeded in launching two X-15s during one day. Both X-15s got airborne, but only one launched.
X-15 on B-52 in servicing area during preflight fueling operations. LOX vapor surrounds the aircraft.
Rear view of X-15 mated to the B-52 mothership during preflight fueling operations. Large hole in nozzle flange is engine turbopump exhaust duct.
Servicing crew loads hydrogen peroxide aboard the X-15.
Frost coats the exterior skin of the propellant tanks during fueling operations prior to flight.
X-15 being towed into position for mating to the B-52 pylon. Note rear-wheeled dolly used during ground towing operations. Nose boom was used prior to delivery of the ball nose that was used for all high-speed flights.
Dual LR-11 engines used prior to delivery of the LR-99 engine. Each rocket chamber produced 1,500 pounds of thrust.
LR-11 engine run in engine test stand.
X-15A-2 with ablative coating. Ablative charring can be seen on the canopy leading edge, the wing, horizontal and vertical tail leading edges, and the upper and lower speed brakes.
The number three X-15 on the Edwards lakebed. North Base hangars are visible above the horizontal stabilizer.
X-15 with F-104 landing chase aircraft approaching touchdown. Smoke from pyrotechnic flare provides pilot an indication of surface wind. North Base hangars are in background.
X-15A-2 engine light follows launch on maximum-speed flight. External tanks and dummy scramjet are visible. Vapor trail above X-15 fuselage is APU exhaust.
X-15A-2 with dummy scramjet installed accelerates and climbs after launch.
Chapter 2
The Operation
The only practical way to reach hypersonic speeds in a manned airplane in the late 1950s was to use rocket power. The largest jet engine available produced less than 30,000 pounds of thrust whereas rocket engines with over 200,000 pounds of thrust had been developed to power ballistic missiles. A rocket engine developing almost 60,000 pounds of thrust was designed for the X-15.
A rocket propulsion system has the advantages of being powerful and relatively simple. It does not require air inlets, compressors, turbines, or other airflow and pressure control components. It provides its own oxidizer for combustion of the fuel. It makes a nice, neat propulsion system. It has its disadvantages, however. It burns horrendous amounts of fuel and oxidizer. Thus, you have to use this fuel in the most effective manner possible. The operational concept of air launch and unpowered landings was developed to optimize the use of the available rocket fuel. In this concept, the rocket powered airplane is carried up to altitude by a mothership and then launched to make its own flight on rocket power. All of the rocket fuel can then be used to accelerate to the desired speed or to climb to much higher altitudes. No fuel needs to be retained for landing, because the airplane is designed for a power off or deadstick landing.
This operating concept had been used very successfully for each of the early rocket airplanes. B-29s or B-50s were used as motherships to carry these early rocket aircraft to a launch altitude of 35,000 feet. These early rocket aircraft would generally be launched within 30 to 40 miles of the Edwards lakebed, heading toward the lakebed. After launch, the pilot would light the rocket engine and then accelerate to the desired speed or climb to the planned altitude. The flights were generally planned so that fuel exhaustion occurred as the aircraft passed over the Edwards lakebed.
After engine burnout, the pilot would decelerate and descend while turning back to land. Using this procedure, the aircraft were never beyond gliding distance of the Edwards or Rosamond lakebeds. It was essential that the aircraft remain within gliding distance at all times during free flight to ensure that it could be recovered at any time if the rocket engine failed to light or if it quit prematurely. This same operating procedure was used for the first fourteen X-15 flights and for all of the pilot checkout flights using the LR-11 engine. A B-52 was used as a mothership for the X-15. Two early production B-52s (AF serial numbers 52-003 and 52-008) were modified to carry the X-15s.
As flights to higher speed and altitude were planned, it became obvious that more distance was required to allow for acceleration, deceleration, climb, and descent. Circling flights around Edwards were impractical to acquire stabilized flight conditions. At maximum speed, for example, the X-15 would have to be turning continuously at 6 g to stay within gliding distance of Edwards. This was totally impractical. The solution was to move the launch point farther away from Edwards to allow more straight line distance to conduct the higher-speed and higher-altitude flights.
A constraint on moving the launch point was the requirement to be within gliding distance of a landing site at launch, in case the rocket engine failed to light. The solution to this problem was to locate some lakes at various distances from Edwards that could be used as launch sites. These lakes would then be available for emergency landings if the engine failed to light after launch. Initially only four lakebeds were used as launch lakes: Silver, Hidden Hills, Delamar, and Mud. Two others, Smith Ranch and Railroad Valley, were used during the latter portion of the program for very high altitude flights. Wendover was originally designated as a launch lake, but was never used.
Silver Lake was 105 miles from Edwards, Hidden Hills was 130 miles, Mud Lake was 200 miles, Delamar was 220 miles, Smith Ranch was 280 miles, and Railroad Valley was 230 miles. This combination of launch lakes gave us a gr
eat deal of flexibility in planning flights to various speeds and altitudes, because the ground distances to Edwards varied from 100 miles to almost 300 miles among these six lakebeds.
The X-15 would be launched at these remote launch lakes on a heading back to Edwards. We always planned our flights to land at Edwards in order to minimize recovery and turnaround operations. Edwards was the only landing site that had all the facilities to conduct an X-15 operation. If we landed at any other lakebed, we had to bring the airplane back on a flatbed truck since there were no facilities to load it on the B-52 to fly it out. We could have installed facilities for X-15 operations at some other lakebeds, but it made more sense to plan to recover at Edwards on every flight. The only problem using Edwards as the recovery site was that there were no lakebeds to the south or west of it. Thus, if you overshot Edwards coming from the north or northeast where most of the launch lakes were, you had a problem.
After launch at one of these remote launch lakes, the X-15 would climb and accelerate to build up enough speed and altitude to allow it to glide back to Edwards following engine shutdown or burnout. The X-15 could glide over 400 miles once it achieved its maximum speed, although we never attempted to fly that far. There was a potential problem, however, if the X-15 did not build up enough energy to get back to Edwards. This might be the case if, for example, the engine quit prematurely. If that happened, the X-15 would need an intermediate landing site. This requirement had to be considered in the selection of suitable launch lakes. Each launch lake had to have one to three usable lakebeds located between it and Edwards to accommodate these potential emergency situations. Luckily, the southern California desert region is liberally sprinkled with dry lakebeds. Suitable combinations of launch and intermediate lakebeds were found to accommodate all of our needs.
In all, we examined 40 or 50 lakebeds in the southern California and western Nevada deserts and selected fifteen or so that were suitable for X-15 emergency use. We preferred lakes that were at least 3 miles long, but reluctantly accepted some as small as 2 miles, hoping that we would never have to use them.
These launch lakes and intermediate lakes had to be carefully inspected for adequate hardness and smoothness. They were marked with tar strips to define the best landing area or areas if the lake was large enough for more than one runway. The runways that were marked off were standardized at 300 feet wide. The tar strips outlining the edge of the runway were also standardized at 8 feet wide. The width of the strips was quite critical because it was our only good reference measurement to judge height. Many of the lakebeds were very smooth with no surface texture. Looking down on a lakebed of that type was like looking down on smooth water. We could not judge height, thus the need for a known reference dimension such as the tar strips.
The Edwards Flight Test Center devoted a lot of effort to marking out the runways on each of the lakebeds. These runways had to be re-marked at least once a year, and sometimes more frequently if we had heavy rain or snow. The rain and melting snow tended to cover the tar strips with mud which made them almost invisible. Over the years, the thickness of the tar strips increased with each new marking until they exceeded 3 or 4 inches in height above the lakebed surface.
The rain and snow caused other problems on these lakes. It softened the lakebed surface and, in the case of heavy rain or snow, it created a gooey mud or even a shallow lake full of water. I have seen some of the lakebeds covered with as much as a foot of water. If that happened, we could not use the lake for 3 or 4 months. It took that long for it to dry up and regain its hardness.
NASA took on the responsibility of checking the lakebeds for hardness throughout the year. We used our Gooney Bird to fly up to each of the lakebeds with a small team of people to survey the marked runways for soft spots and sinkholes. We had a 6-inch-diameter lead ball that we dropped from a height of 5 feet on the lakebed surface. We would measure the diameter of the depression and then compare that to some reference measurements on a hard useable lakebed to determine whether the runway would support the weight of the X-15. It was a crude but effective means of verifying the usability of the lakebeds.
Prior to landing on the lakebed to inspect it, we would make a low pass or two to visually look for wet spots or standing water. If the lakebed appeared to be damp, we would make another pass and roll the Gooney Bird wheels on the lakebed and then come around to check the depth of the tracks. That could be a tricky maneuver if the lakebed was softer than it appeared to be, because we could be sucked down into the mud. That happened on a couple of occasions. In his book, Chuck Yeager described one such occasion during an inspection of a lakebed using a T-33. Neil Armstrong was flying the T-33 and he and Yeager ended up stuck in the mud and stranded on the lakebed. It took a lot of effort to salvage that airplane.
We carried a motorcycle on the Gooney Bird to ride up and down the runways to inspect them. Walter Whiteside, a retired air force maintenance officer and veteran dirt bike rider, was among other things, our primary lakebed inspector. He really loved to get out on those big lakebeds with that motorcycle. Whenever I went out in an F-104 to practice X-15 landings, I would look for him if he were uprange inspecting a lakebed. I would try to catch him out on the lakebed on his motorcycle and sneak up behind him at 600 knots so he could not hear me coming and then try to blow him off the motorcycle by buzzing him at 10 to 15 feet above his head. To make it even more dramatic, I would light the burner as I passed over him. He admitted that he nearly lost control a few times during these buzz jobs. It really is a terrifying experience to be buzzed like that if you cannot see the airplane coming. You cannot hear it coming, at that speed, until it is just a fraction of a second away. Then the noise hits you like a bomb blast. You can really get someone’s attention like that.
We had three primary launch lakes, Hidden Hills, Mud, and Delamar. Each of these launch lakes had a separate set of intermediate lakes on a line leading back to Edwards in case we could not make it all the way home. We also used Smith Ranch and Railroad Valley as launch lakes for the very high energy flights.
Perhaps surprisingly, altitude flights required the greatest distance from launch to landing. We covered a lot of ground while we were outside the atmosphere, and we could not kill off any energy while we were in space. Speed brakes do not provide any drag in a vacuum. The maximum altitude flight of 354,200 feet made by Joe Walker, required a ground distance of 305 miles to climb out of the atmosphere coast to peak altitude, descend, make the pullout, and then slow down before reaching Edwards. The maximum speed flight of 4,520 MPH required a ground distance of only 225 miles since on a speed flight, we stayed within the atmosphere and we could slow down relatively quickly.
One of the major reasons for utilizing longer distances on each flight was to allow time to accomplish test maneuvers or to conduct various experiments. During the early X-15 test flights, for example, we performed maneuvers to determine the stability of the aircraft at various speeds, the control effectiveness at various speeds, the aerodynamic performance or lift/drag ratio, the loads and the stress imposed on various parts of the aircraft.
All of these maneuvers required a finite amount of time to set up a quasi-steady state airspeed and altitude condition and then perform the maneuver. This is the kind of thing a research pilot is required to do to earn his money—accomplishing good maneuvers for data purposes. Flying the airplane is just something the pilot does to get the desired test maneuver. He can be the greatest stick and rudder pilot in the world, but if he cannot do the required data maneuvers, he is worthless as a research pilot.
Where the X-15 would land during any given flight was precomputed on the basis of planned versus actual engine burn time. If the engine did not light in two attempts after launch, we made an emergency landing at the launch lake. If the engine did light, but only burned for a short period of time, we would end up making a landing at the launch lake since we could still turn around and make it back. If the engine burned over half of the available burn time, which wa
s approximately 82 seconds, but less than 80 percent, for example, we would very likely end up at one of the intermediate lakebeds since we could not make it to Edwards nor could we get back to the launch lake. If the engine burned longer than 80 percent of the planned burn time, we could make it home to Edwards. These percentages quoted are representative and vary with each launch lake and type of flight. Some representative decision times for a typical flight out of Delamar Lake are: 0 to 40 seconds—land at Delamar, 40 to 46 seconds—land at an unnamed lakebed in a highly classified restricted area, 46 to 64 seconds—land at Hidden Hills, 64 to 68.5 seconds—land at Cuddeback, and 68.5 seconds or longer—land at Edwards.
Figure 3. Map of X-15 operating area and all X-15 lakebeds.
Hidden Hills was used as a launch lake on many flights, but it also served as an intermediate lakebed for flights from Delamar Lake since these flights passed almost directly over Hidden Hills. Mud Lake, another launch lake, also served as an intermediate lakebed for flights out of Smith Ranch.
The precomputation of landing sites was part of a process called energy management. In an unpowered aircraft, the distance you fly depends on how much initial energy you have. Speed and altitude are the primary components of energy. With plenty of speed and altitude, it is possible to glide a long distance. With a minimal amount of speed and altitude, you can only glide a short distance. With the optimum combination of speed and altitude, in the X-15, we could glide over 400 miles. This was not a slow speed glide either. We normally averaged over 2,000 MPH during gliding flight after engine burnout. If we needed to maximize our gliding distance we flew at the airspeed that gave us the maximum lift-drag ratio. Using a combination of these techniques, we could vary our gliding distance from over 400 miles to less than 50 miles, starting from Mach 6 at 100,000 feet altitude.
At the Edge of Space Page 7
