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table of contents foreword william h dana chapter 1 a new science x-planes what was achieved hypersonics the missile influence military support a convincing case the douglas model 671 chapter 2 a hypersonic research airplane developing a consensus engine options the competition the airframe evaluation second thoughts chapter 3 conflict and innovation changes the first industry conference 1956 mockup inspection structural fabrication high-altitude girdles escape system development stable platform development ball nose development flight control systems landing gear evolution the second industry conference 1958 chapter 4 the million-horsepower engine engine proposals the engine evaluation engine award the tr-139 the 1956 industry conference more problems rockets in the high desert continuing challenges retrospect post x-15 aerojet lr91 reaction motors xlr11 chapter 5 high range and dry lakes

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muroc to edwards the high-speed flight station the high range dry lakes chapter 6 preparations simulations carrier aircraft chase and support aircraft implications of sputnik more x-15s cost overruns chapter 7 the flight program rollout flight program overview flight description flight day the original contractor plan the original government plan revisions can it fly 1959 flight period 1960 flight period 1961 flight period 1962 flight period 1963 flight period 1964 flight period 1965 flight period 1966 flight period 1967 flight period 1968 flight period correcting an oversight chapter 8 the research program research instrumentation biomedical research mh-96 adaptive control system the follow-on program experiment accommodations early planning approved test-bed experiments other flown experiments recoverable booster system ramjets thin delta wings hires appendix a selected biographies michael j adams usaf neil a armstrong nasa

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john v becker nasa paul f bikle nasa a scott crossfield naa william h dana nasa hugh l dryden nasa joe h engle usaf charles h feltz naa iven c kincheloe jr usaf william j knight usaf john a manke nasa john b mckay nasa forrest s peterson usn robert a rushworth usaf hartley a soulÉ nasa harrison a storms jr naa milton o thompson nasa joseph a walker nasa alvin s white naa robert m white usaf walter c williams nasa appendix b x-15 flight log appendix c physical characteristics of the x-15 airplane 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 x15 was the only aircraft he had ever flown where he was glad when the engine quit.

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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-toback 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

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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 neil armstrong among others has called the x-15 the most successful research airplane in history that might be stretching a point but it was certainly the most successful of the highspeed x-planes given the major advances in materials and computer technology made in the 40

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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-theloop 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 1 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 2 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 soulé 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 3 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 it is interesting to note that although the x-15 is generally considered a mach 6 aircraft only two of the three airplanes ever flew that fast and then only four times on the other hand 108 other flights exceeded mach 5 accumulating

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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 [5 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 2 3 4 5 verify existing 1954 theory and wind-tunnel techniques study aircraft structures and stability and control under high 2,000 psf dynamic pressures study aircraft structures under high 1,200°f heating investigate stability and control problems associated with high-altitude boost and reentry 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 t]here 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

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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 dynasoar 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-152 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 [8 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.

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fifty years later the x-15 experience still provides the bulk of the available hypersonic data available to aircraft designers [9 perhaps we have not learned well enough dennis r jenkins cape canaveral florida acknowledgments robert s houston a historian at the air force wright air development center wrote the most frequently quoted x-15 history in 1959 this narrative unsurprisingly centered on the early air force involvement in the program and concentrated mostly as is normal for air force histories on the program management aspects rather than the technology dr richard p hallion later the chief historian for the u.s air force updated houston s history in 1987 as part of volume ii of the hypersonic revolution a collection of papers published by the aeronautical systems office at wright-patterson afb hallion added coverage of the last nine years of the program drawing mainly from his own on the frontier flight research at dryden 1946-1981 washington dc nasa 1984 and outline of the x-15 s contributions to aerospace technology written in 1977 by ronald g boston these historians did an excellent job but unfortunately their work received comparatively limited distribution i began this history by using these earlier works as a basis checking the sources expanding upon them as appropriate and adding a naca/nasa and navy perspective amazingly almost all of the original source documentation still existed in one archive or another allowing an evaluation of the tone and inflection of some of the earliest material although it is largely a new work anybody who is intimately familiar with the earlier histories will recognize some passages the original historians did a remarkably thorough job many people assisted in the preparation of this work and all gave generously and freely well beyond any reasonable expectation an author might have foremost were betty j love tony landis at dryden and dr roger d launius at the national air and space museum the surviving x-15 pilots neil a armstrong a scott crossfield william h dana brigadier general joe h engle usaf retired colonel william j pete knight usaf retired and major general robert m white usaf retired contributed immensely and several of them read the manuscript multiple times to ensure that nothing significant was missed or misrepresented john v becker and charles h feltz spent many hours explaining things i probably should have already known greatly improving the manuscript then there are the flight planners johnny g armstrong [10 richard e day and robert g hoey i would have missed many subtleties without the patient tutoring from these engineers all of whom read and commented on several versions of this manuscript and continued my education well past my two engineering degrees there was correspondence with many individuals who had been involved with the program william p albrecht colonel john e jack allavie usaf retired colonel clarence e bud anderson usaf retired bill arnold rmd/thiokol retired colonel charles c bock jr usaf retired jerry brandt richard j harer gerald m truszynski and alvin s white in addition jack bassick at the david clark company stephen j garber and colin a fries at the nasa history office michael j lombardi at the boeing company archives air force chief historian dr richard p hallion dr james h young and cheryl gumm at the afftc history office and john d jack weber at the afmc history office all provided excellent support friends and fellow authors gerald h balzer robert e bradley benjamin f guenther scott lowther mike machat michael

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moore terry panopalis and mick roth also assisted others who contributed include lynn albaugh at ames jack beilman rodney k bogue at dfrc anita borger at ames john w boyd at ames russell castonguay at the jpl archives erik m conway at langley and nasm mark l evans at the naval historical center dr michael h gorn at the dfrc history office matt graham at dfrc fred w haise jr wesley b henry at the air force museum t.a heppenheimer james b hill at the john fitzgerald kennedy library dr j d dill hunley at the dfrc history office kenneth w iliff dfrc retired bob james dfrc retired jack kittrell dfrc retired christian ledet f robert van der linden at the national air and space museum marilyn meade at the university of wisconsin roger e moore claude s morse at the aedc karen moze at ames doug nelson at the afftc museum anne-laure perret at the fédération aéronautique internationale fai colonel bruce a peterson usmcr retired charles e rogers at the afftc mary f shafer dfrc retired bonita s smith at grc colonel donald m sorlie usaf retired and henry spencer it all would never have seen the light of day had it not been for tony springer of the aeronautics research mission directorate at nasa headquarters special thanks i owe a particular mention of jay miller author of the popular the x-planes x-1 to x-45 hinckley england midland publishing 2001 among many other works anybody interested in reading about the other x-planes should pick up a copy of this excellent book jay was responsible for the first photograph i ever had published and published my first book a short monograph on the space shuttle somehow i feel i have him to blame for the quagmire of aerospace history i find myself embroiled in i truly appreciate the help and friendship from jay and his lovely wife susan over the past 25 years or so thankfully my mother mrs mary e jenkins encouraged me to seize opportunities and taught me to write and type such necessary attributes for this endeavor as for so many things i owe her a great deal of gratitude along with my everlasting love and admiration after listening to my trials and tribulations about this project for a decade she passed away before publication i hope she has found the peace and rest she so richly deserves a note regarding terminology in the days before being politically correct became a prime influence on engineering and history engineers called piloted vehicles manned aircraft and the process of making them safe enough to fly was termed man-rating this work continues to use these terms since they are what were in use at the time [1 the armstrong quote is in the foreword to milton o thompson at the edge of space the x15 flight program washington dc smithsonian institution press 1992 p xii [2]john v becker the x-15 program in retrospect 3rd eugen sänger memorial lecture bonn germany 5 december 1968 pp 1-2 [3]harrison a storms x-15 hardware design challenges a paper in the proceedings of the x15 30th anniversary celebration dryden flight research facility edwards california 8 june 1989 nasa cp-3105 p 27 [4 in the 3rd eugen sänger memorial lecture in 1968 john becker stated that 109 flights exceeded mach 5 a reevaluation of the flight data shows that only 108 actually did see becker the x-15 program in retrospect p 3 for becker s original numbers.

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[5 despite all that is written the program held very few official records mainly because it seldom invited the fai out to witness the flights in fact it appears that the 314,750-foot altitude record set by bob white is the only official record ever set by the program [6 ronald g boston outline of the x-15 s contributions to aerospace technology 21 november 1977 unpublished preliminary version of the typescript available in the nasa dryden history office for those interested in boston s original paper the easiest place to find a copy is in the hypersonic revolution republished by the air force history and museums program it constitutes the last section in the x-15 chapter letter william h dana chief flight crew branch dfrc to lee saegesser nasa history office transmitting a copy of the setp paper for the file a slightly rewritten more politically correct version of the paper was later published as the x-15 airplane lessons learned american institute of aeronautics and astronautics a paper prepared for the 31st aerospace sciences meeting reno nevada aiaa-93-0309 11-14 january 1993 boston listed 1,300°f as the maximum temperature but bill dana reported 1,350°f in his setp and aiaa papers boston also listed the max-q as 2,000 psf but in reality it was 2,202 psf on flight 1-66-111 [7 storms x-15 hardware design challenges pp 32-33 [8 becker the x-15 program in retrospect pp 1-2 [9 j d hunley the significance of the x-15 1999 unpublished typescript available at the dfrc history office [10 officially johnny armstrong who is now the chief engineer in the hypersonic flight test team maintains the afftc hypersonic flight test team project files and is fortunately something of a pack rat however to everybody at edwards and dryden this wonderful collection is simply the armstrong memorial library chapter 1 a new science the first 50 years of powered human flight were marked by a desire to always go faster and higher at first the daredevils be they racers or barnstormers drove this by the end of the 1930s however increases in speed and altitude were largely the province of government the cost of designing and building the ever-faster aircraft was becoming prohibitive for individuals as is usually the case war increased the tempo of development and two major conflicts within 30 years provided a tremendous impetus for advancements in aviation by the end of world war ii the next great challenge was in sight the sound barrier that stood between the pilots and supersonic flight contrary to general perception the speed of sound was not a discovery of the 20th century over 250 years before chuck yeager made his now-famous flight in the x-1 it was known that sound propagated through air at some constant velocity during the 17th century artillerymen determined that the speed of sound was approximately 1,140 feet per second fps by standing a known distance away from a cannon and using simple timing devices to measure the delay between the muzzle flash and the sound of the discharge their conclusion was remarkably accurate two centuries later the national advisory committee for aeronautics[1 naca defined the speed of sound as 1,117 fps on an iso standard day although this number is for engineering

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convenience and does not represent a real value 2 the first person to recognize an aerodynamic anomaly near the speed of sound was probably benjamin robins an 18th-century british scientist who invented a ballistic pendulum that measured the velocity of cannon projectiles as described by robins a large wooden block was suspended in front of a cannon and the projectile was fired into it the projectile transferred momentum to the block and the force could be determined by measuring the amplitude of the pendulum during these experiments robins observed that the drag on a projectile appeared to increase dramatically as it neared the speed of sound it was an interesting piece of data but there was no practical or theoretical basis for investigating it further 3 the concept of shock waves associated with the speed of sound also predated the 20th century as an object moves through the atmosphere the air molecules near the object are disturbed and move around the object if the object passes at low speed typically less than 200 mph the density of the air will remain relatively constant but at higher speeds some of the energy of the object will compress the air locally changing its density this compressibility effect alters the resulting force on the object and becomes more important as the speed increases near the speed of sound the compression waves merge into a strong shock wave that affects both the lift and drag of an object resulting in significant challenges for aircraft designers [4 austrian physicist ernst mach took the first photographs of supersonic shock waves using a technique called shadowgraphy in 1877 mach presented a paper to the academy of sciences in vienna where he showed a shadowgraph of a bullet moving at supersonic speeds the bow and trailing-edge shock waves were clearly visible mach was also the first to assign a numerical value to the ratio between the speed of a solid object passing through a gas and the speed of sound through the same gas in his honor the mach number is used as the engineering unit for supersonic velocities the concept of compressibility effects on objects moving at high speeds was established but little actual knowledge of the phenomena existed [5 none of these experiments had much impact on the airplanes of the early 20th century since their flight speeds were so low that compressibility effects were effectively nonexistent however within a few years things changed although the typical flight speeds during world war i were less than 125 mph the propeller tips because of their combined rotational and translational motion through the air sometimes approached the compressibility phenomenon [6 to better understand the nature of the problem in 1918 g h bryan began a theoretical analysis of subsonic and supersonic airflows for the british advisory committee for aeronautics at the royal aeronautical establishment his analysis was cumbersome and provided little data of immediate value at the same time frank w caldwell and elisha n fales from the army air service engineering division at mccook field in dayton ohio took a purely experimental approach to the problem [7 to investigate the problems associated with propellers in 1918 caldwell and fales designed the first high-speed wind tunnel built in the united states this tunnel had a 14-inch-diameter test section that could generate velocities up to 465 mph which was considered exceptional at the time this was the beginning of a dichotomy between american and british research over the next two decades the united states primarily the naca made most of the major experimental contributions to understanding compressibility effects while the major theoretical contributions were made in great britain this combination of american and british investigations of propellers constituted one of the first concerted efforts of the fledgling aeronautical community to investigate the sound barrier [8 within about five years practical solutions such as new thin-section propeller blades made

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practical by the use of metal instead of wood for their construction that minimized the effects of compressibility were in place however most of the solution was to avoid the problem the development of reliable reduction-gearing systems and variable-pitch constant-speed propellers eliminated the problem entirely for airplane speeds that were conceivable in 1925 because the propeller could be rotated at slower speeds at the time the best pursuit planes the forerunners of what are now called fighters could only achieve speeds of about 200 mph and a scan of literature from the mid-1920s shows only rare suggestions of significantly higher speeds in the foreseeable future accordingly most researchers moved on to other areas [9 the public belief in the sound barrier apparently had its beginning in 1935 when the british aerodynamicist w f hilton was explaining to a journalist about high-speed experiments he was conducting at the national physical laboratory pointing to a plot of airfoil drag hilton said see how the resistance of a wing shoots up like a barrier against higher speed as we approach the speed of sound the next morning the leading british newspapers were referring to the sound barrier and the notion that airplanes could never fly faster than the speed of sound became widespread among the public although most engineers refused to believe this the considerable uncertainty about how significantly drag would increase in the transonic regime made them wonder whether engines of sufficient power to fly faster than sound would ever be available [10 john stack head of the compressibility research division at naca langley was one of the driving forces behind the original set of experimental airplanes such as the bell x-1 and douglas d-558 series although he lent expertise and advice to the groups developing the x-15 he remained in the background and did not repeat the pivotal roles he had played on earlier projects nasa since the beginning of powered flight wind tunnels had proven to be useful tools but it appeared in the 1930s that simulation of the transonic regime was not possible due to the physical

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characteristics of the test sections however the beginning of the second world war increased the urgency of the research therefore on a spring morning in 1940 john v becker and john stack two researchers from the naca langley memorial aeronautical laboratory in hampton virginia 11 drove to a remote beach to observe a navy brewster xf2a-2 attempting to obtain supercritical aerodynamic data in free flight over chesapeake bay after it reached its terminal velocity in a steep dive about 575 mph the pilot made a pull-up that was near the design load factor of the airplane this flight did not encounter any undue difficulties and provided some data but the general feeling was that diving an operational-type airplane near its structural limits was probably not the best method of obtaining research information [12 x-planes as it happened john stack had already considered other alternatives the idea of a modern research airplane one designed strictly to probe unknown flight regimes came in a 1933 proposal by stack on his own initiative stack went through a preliminary analysis for a hypothetical airplane which however is not beyond the limits of possibility to fly well into the compressibility regime stack calculated that a small airplane using a 2,300-horsepower rollsroyce piston engine could obtain 566 mph in level flight far beyond that of any airplane flying at the time ultimately the naca did not pursue the suggestion and it would be another decade before the idea would come of age 13 ezra kotcher at the army air corps engineering school at wright field made the next proposal for a high-speed research airplane in 1939 kotcher pointed out the unknown aspects of the transonic flight regime and the problems associated with the effects of compressibility he further discussed the limitations of existing wind tunnels and advised that a full-scale flight research program would be an appropriate precaution by early 1941 john stack had confirmed that data from wind tunnels operating near mach 1 were essentially worthless because of a choking problem in the test section he again concluded that the only way to gather meaningful data near the speed of sound would be to build a vehicle that could fly in that regime again no action resulted from either kotcher s or stack s suggestions and determining the effects of compressibility on airplanes remained a largely theoretical pursuit [14 the real world intervened in november 1941 when lockheed test pilot ralph virden died trying to pull a p-38 lightning out of a high-speed dive that penetrated well into the compressibility regime by 1942 the diving speed of the new generation of fighters exceeded the choking speed of the wind tunnels then in use researchers increasingly supported the idea of an instrumented airplane operating at high subsonic speeds those involved do not remember that any one individual specifically championed this idea but john stack soon became the chief langley proponent [15 interestingly there was little interest within the naca in flying through the sound barrier it appeared that one of the early turbojet engines could push a small airplane to about mach 0.9 but the only near-term way to go faster was to use a rocket engine something that was considered too risky by the naca.

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