FOREWORD: WILLIAM H. DANA

The X-15 was an airplane of accelerations. When an X-15 pilot looks back on his X-15 flights. it is the accelerations he remembers. The first of these sensations was the acceleration due to B-52 lift. which held the X-15 at launch altitude and prevented it from falling to Earth. When the X-15 pilot hit the launch switch. the B-52 lift was no longer accessible to the X-15. The X-15 fell at the acceleration due to Earth’s gravity. which the pilot recognized as "free fall" or "zero g." Only when the pilot started the engine and put some "g" on the X-15 was this sensation of falling relieved.

The next impression encountered on the X-15 flight came as the engine lit. just a few seconds after launch. A 33.000-pound airplane was accelerated by a 57.000-lbf engine. resulting in a chest-to-back acceleration of almost 2 g. Then. as the propellant burned away and the atmosphere thinned with increasing altitude. the chest-to-back acceleration increased and the drag caused by the atmosphere lessened. For a standard altitude mission (250.000 feet). the weight and thrust were closer to 15.000 pounds and 60.000-lbf at shutdown. resulting in almost 4-g chest-to-back acceleration. The human body is not stressed for 4 g chest to back. and by shutdown the boost was starting to get a little painful. Milt Thompson once observed that the X – 15 was the only aircraft he had ever flown where he was glad when the engine quit.

X-15 ready for flight on the flight line. (NASA)

On a mission to high altitude (above 250,000 feet), the pilot did not regain any sensible air with which to execute a pullout until about 180,000 feet, and could not pull 1 g of lift until 130,000 feet. Flying a constant angle of attack on reentry, the pilot allowed g to build up to 5, and then maintained 5 g until the aircraft was level at about 80,000 feet. There was a deceleration from Mach 5 at 80,000 feet to about Mach 1 over the landing runway, and the pilot determined the magnitude of the deceleration by the use of speed brakes. This ended the high-g portion of the flight, except for one pilot who elected to start his traffic pattern at 50,000 feet and Mach 2, and flew a 360-degree overhead pattern from that starting point.

Flight to high altitude represented about two-thirds of the 199 X-15 flights. Flights to high speed or high dynamic pressure accounted for the other third, and those flights remained well within the atmosphere for the entire mission. The pilot of a high-speed flight got a small taste of chest-to – back acceleration during the boost (thrust was still greater than drag, but not by such a large margin as on the high-altitude flights). The deceleration after burnout was a new sensation. This condition was high drag and zero thrust, and it had the pilot hanging in his shoulder straps, with perspiration dripping off the tip of his nose onto the inside of his face plate.

Milt Thompson collected anecdotes about the X-15 that remain astonishing to this day. Milt noted that at Mach 5, a simple 20-degree heading change required 5 g of normal acceleration for 10 seconds. Milt also pointed out that on a speed flight, the (unmodified) X-15-1 accelerated from Mach 5 to Mach 6 in six seconds. These were eye-opening numbers at the time of the X-15 program.

Those of us in the program at flight 190 thought that the X-15 would continue indefinitely. Then, on flight 191, Major Michael J. Adams experienced electrical irregularities that made the inertial flight instruments unreliable and may have disoriented him. In any case, at peak altitude (266,000 feet), the X-15 began a yaw to the right. It reentered the atmosphere, yawed crosswise to the flight path, and went into a high-speed spin. It eventually came out of the spin but broke up

during the reentry, killing the pilot.

The loss of the airplane and pilot was the death knell for the entire program. Program management decided not to fly the X-15A-2 again, and to fly X-15-1 only for calendar year 1968. The X-15 flew its last flight on 24 October of that year, and then faded into aeronautical history.

William H. Dana

Test Pilot, Dryden Flight Research Center Pilot, last X-15 flight

Bill Dana greets his family after the last flight of the X-15 program on 24 October 1968. (NASA)

PREFACE: ROCKETS OVER THE HIGH DESERT

years since the flight program ended, it is unlikely that many of the actual hardware lessons are still applicable. Having said that, the lessons learned from hypersonic modeling and pilot-in-the – loop simulation, and the insight gained by being able to evaluate actual X-15 flight test results against wind-tunnel and theoretical predictions greatly expanded the confidence of researchers during the 1970s and 1980s.m

It would not have surprised anybody involved that the actual X-15 technology did not find further application. Researchers such as John Becker and Norris Dow, and engineers like Harrison Storms and Charlie Feltz never intended the design to represent anything other than a convenient platform to acquire aero-thermo data. Becker once opined that proceeding with a general research configuration rather than a prototype of a vehicle designed to achieve a specific mission was critical to the ultimate success of the X-15. Had the prototype route been taken, Becker believed, "we would have picked the wrong mission, the wrong structure, the wrong aerodynamic shapes, and the wrong propulsion." They are good words of advice.-12

In fact, the decision to pursue a pure research shape was somewhat controversial at the beginning. Kelly Johnson, for one, believed the vehicle should be adaptable as a strategic reconnaissance aircraft. Indeed, several of the proposals for the X-15 sought to design a vehicle with some future application. Nevertheless, the original Langley concept of a vehicle optimized to collect the desired data as safely as possible ultimately won. As Harley Soule told Harrison Storms, "You have a little airplane and a big engine with a large thrust margin. We want to go to 250,000 feet altitude and Mach 6. We want to study aerodynamic heating. We do not want to worry about aerodynamic stability and control, or the airplane breaking up. So, if you make any errors, make them on the strong side. You should have enough thrust to do the job." North American succeeded brilliantly.-13

It had taken 44 years to go from Kitty Hawk to Chuck Yeager’s first supersonic flight in the X-1. Six more years were required before Scott Crossfield got to Mach 2 in the D-558-2 Skyrocket. A remarkably short three years had passed when Mel Apt coaxed the X-2 above Mach 3, before tumbling out of control to his death. There progress stalled, awaiting the arrival of the three small black airplanes that would more than double the speed and altitude milestones.

The X-15 flight program began slowly, mostly because the XLR99 was not ready. This undoubtedly worked in the program’s favor since it forced the engineers and pilots to gain experience with the airplane and its systems prior to pushing the envelope too far. The first 20 months took the X-15 from Crossfield’s glide flight to essentially duplicating the performance of the X-2: Mach 3.5 and 136,500 feet. Then the XLR99s arrived and things got serious. Six days after the last flight with the interim XLR11s, Bob White took X-15-2 past Mach 4, the first time a piloted aircraft had flown that fast. Mach 5 fell, also to Bob White, four months later. Mach 6, again to White, took six more months. Once the X-15 began flying with the ultimate engine, it took only 15 flights to double the maximum Mach number achieved by the X-2.

Altitude was a similar story. Iven Kincheloe was the first person to fly above 100,000 feet, in the X-2 on 7 September 1956. Thirteen flights with the big engine allowed Bob White to fly above 200,000 feet for the first time. Three months later, he broke 300,000 feet. Once it began flying with the ultimate engine, the X-15 took only 19 months to double the maximum altitude achieved by the X-2. These were stunning achievements.

1 hour, 25 minutes, and 33 seconds of hypersonic flight. At the other end of the spectrum, just two flights were not supersonic (one of these was the first glide flight), and only 14 others did not exceed Mach 2. It was a fast airplane. Similarly, there were only four flights above 300,000 feet (all by X-15-3), but only the initial glide flight was below 40,000 feet.[4]

Despite appearances, however, the program was not about setting records.-151 The actual speed and altitude achieved by the program was not the ultimate test, and the fact that the basic airplane never achieved its advertised 6,600 feet per second velocity was of little consequence. What interested the researchers was the environment in which the airplane flew. They wanted to study dynamic pressures, heating rates, and total temperatures. More specifically, the goals were to:

1. Verify existing (1954) theory and wind-tunnel techniques

2. Study aircraft structures and stability and control under high (2,000 psf) dynamic pressures

3. Study aircraft structures under high (1,200°F) heating

4. Investigate stability and control problems associated with high-altitude boost and reentry

5. Investigate the biomedical effects of both weightless and high-g flight

The X-15 achieved all of these design goals, although Project Mercury and other manned space efforts quickly eclipsed the airplane’s contribution to weightless research. The program ultimately achieved a velocity of 6,629 fps (with X-15A-2), 354,200 feet altitude, 1,350°F, and dynamic pressures over 2,200 psf.[6]

With 40 years of hindsight, it is apparent that the most important lessons to be learned from the X-15 concern not the hardware, but the culture. The world was different during the 1950s, certainly within the government-contracting environment. The military and NACA initiated and funded the X-15 program without congressional approval or oversight, although this was not an effort to hide the program or circumvent the appropriations process. The military services had contingency funds available to use as they saw fit. They ultimately needed to explain to Congress and the White House how they spent the funds, but there was little second-guessing from the politicians. This allowed the program to ramp up quickly and absorb the significant cost overruns that would come. Following its likely origin in February 1954, the Air Force awarded the X-15 development contract in September 1955 and North American rolled out the first airplane in October 1958. The maiden glide flight was in June 1959, just over five years from a gleam in John Becker’s eye to Scott Crossfield soaring over the high desert. It could not happen today.

There is a story in the main text about a meeting Harrison Storms attended at Edwards, and some important words of wisdom: "[TJhere is a very fine line between stopping progress and being reckless. That the necessary ingredient in this situation of solving a sticky problem is attitude and approach. The answer, in my opinion, is what I refer to as ‘thoughtful courage.’ If you don’t have that, you will very easily fall into the habit of ‘fearful safety’ and end up with a very long and tedious-type solution at the hands of some committee. This can very well end up giving a test program a disease commonly referred to as ‘cancelitis,’ which results in little or no progress."[7]

Storms must have had a crystal ball. In today’s environment, the system will not allow programs to have problems. If the Air Force and NASA were trying to develop the X-15 today, Congress would cancel it long before the first flight. A series of configuration changes and production problems added weight and lowered the expected performance before the airplane flew. The XLR99 engine was tremendously behind schedule, so much so that the program selected interim engines just to allow the airplane to begin flying. Ultimately, however, the airplane and the engine were hugely successful. Compare this to how the X-33 program reacted to issues with its composite propellant tanks.

When Crossfield finally released from the carrier aircraft on the initial glide flight in X-15-1, his landing was less than ideal. In today’s world, the program would have stood down to work out this issue and assess the risk. In 1959 North American made some adjustments and launched Crossfield again three months later. It was a short-lived reprieve. Less than 60 days later, Crossfield broke the back of X-15-2 during a hard landing that followed an in-flight abort. Instead of canceling the program, the X-15 went back to the factory for repair. Three months later Crossfield was flying again.

During the initial ground-testing of the ultimate XLR99 engine in X-15-3 at Edwards, an explosion destroyed the airplane. Nobody was seriously hurt and North American subsequently rebuilt the airplane with an advanced flight control system intended for the stillborn X-20 Dyna – Soar. The program was flying two months later using X-15-1 and the rebuilt X-15-3 went on to become the high-altitude workhorse.

It was the same across the board. When Jack McKay made his emergency landing at Mud Lake that essentially destroyed X-15-2, the Air Force did not cancel the program. Five weeks later Bob White made a Mach 5.65 flight in X-15-3; McKay was his NASA-1. North American rebuilt X-15- 2 and the airplane began flying again 18 months later. Jack McKay went on to fly 22 more X-15 flights, although the lingering effects of his injuries shortened his lifetime considerably.

In each case the program quickly analyzed the cause of the failure, instituted appropriate changes, and moved on. Always cautious, never reckless. No prolonged down times. No thought of cancellation. It would not happen that way today. One of the risks when extending any frontier is that you do not understand all the risks.

Paul Bikle, the director of the Flight Research Center, had long warned that the flight program should end when it achieved the design speed and altitude. However, the X-15s provided an ideal platform for follow-on experiments that had little or nothing to do with the design aero-thermo research mission. The temptation was too great, and NASA extended the flight program several years. Bikle knew that eventually the odds would catch up with the program. The day they did, Mike Adams was at the controls of X-15-3, and the consequences were as bad as anything Bikle could have imagined. The crash killed Mike Adams and destroyed X-15-3. Even so, the program made sure it learned from the accident and was flying again less than four months later. This time, however, it would not be for long. Eight more flights were conducted before the program ended when funding expired at the end of 1968.

John Becker, arguably the father of the X-15, once stated that the project came along at "the most propitious of all possible times for its promotion and approval." At the time, it was not considered necessary to have a defined operational program in order to conduct basic research. There were no "glamorous and expensive" manned space projects to compete for funding, and the general feeling within the nation was one of trying to go faster, higher, or further. In today’s environment, as in 1968 when Becker made his comment, it is highly unlikely that a program such as the X-15 could gain approval.-18

Dill Hunley, a former DFRC historian, once opined that "This situation should give pause to those who fund aerospace projects solely on the basis of their presumably predictable outcomes and their expected cost effectiveness. Without the X-15’s pioneering work, it is quite possible that the manned space program would have been slowed, conceivably with disastrous consequences for national prestige." It is certain that the development of the Space Shuttle would have carried a far greater risk if not for the lessons learned from the development and flight-testing of the X-15.

Fifty years later, the X-15 experience still provides the bulk of the available hypersonic data available to aircraft designers.-19

Perhaps we have not learned well enough.

Dennis R. Jenkins Cape Canaveral, Florida