“Beware of the bull. He will carry you safely anywhere that you have to go and protect you from any enemies. He is awesome in battle. However, if you lose control of him or fall off, he will kill you as quickly as he would kill your enemy” 
from At the Edge of Space – Milton O. Thompson

On 8th June 1959, Scott Crossfield dropped away from beneath the wing of the B-52 carrier aircraft in X-15-1 and made a relatively uneventful glide back down to Rogers Dry Lake. During the brief flight, the sole planned glide flight of the entire programme, Crossfield allowed himself an aileron roll, all the better to check the new aircraft’s handling. Unfortunately his first landing was far less enjoyable with the X-15 bucking wildly as he struggled to put it down on the lake. The source of the control problems was soon isolated and fixed and the X-15 moved forward on its mission to unlock the secrets of high-speed, high-altitude flight.


As the North American Aviation (NAA) project pilot, Crossfield was the first to fly the X-15 and would carry out the initial demonstration flights to establish the craft’s suitability for its role. Once this was achieved to the satisfaction of all parties, the three X-15s would be in the hands of test pilots representing NASA, the Air Force and the Navy as the flight envelope would be pushed to the design limits and beyond. But before any records could be broken, Crossfield had a lot more work to do to get the X-15 flying with the interim XLR-11 engines before demonstrating the ‘big engine’ – the XLR-99.

As noted in Part 1, development of the XLR-99 was running massively over budget and behind schedule. Rather than delay the entire project while the engine edged to completion, the decision was taken to install two 4-chamber XLR-11 engines one above the other in X-15-1 and X-15-2. The third airframe would await the initial XLR-99. This meant initial flight tests could take place allowing the X-15’s low speed characteristics to be understood while pilot familiarisation flights for the initial group of pilots were carried out.

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An early shot of X-15-1 showing the dual 4-chamber XLR-11 engines [IMG: NASA]
The XLR-11 was a well understood engine out at Edwards AFB having been used on the X-1 series of research aircraft, but still it took some trial and error to get them working with the X-15 on the test stand. Once engine bugs were ironed out, captive-carry flights  demonstrated that the X-15’s tanks could be topped-off by the B-52 prior to powered flight and that the propellant jettison system worked effectively. Finally on September 17, Crossfield was able to make the first powered flight of the programme in X-15-2, reaching a tentative Mach 2.11 (surpassing his own previous best mark – the record breaking December 1953 Mach 2 flight in the Skyrocket). At last it looked like testing could move ahead after the long series of delays, but the X-15 had another surprise in store for Crossfield.

On his third flight, again in the second aircraft, Crossfield dropped away from the B-52 and began the ignition sequence at which point one of the XLR-11 chambers exploded. And so as well as his many other firsts on the programme, Crossfield received the dubious honour of making the first emergency landing. Unfortunately, due to the steep approach path, he had been unable to jettison sufficient propellant meaning as the X-15 landed, it came down hard. Crossfield noted that during the slide-out it seemed to slow down a lot quicker than usual – the reason for this was immediately apparent to observers as X-15-2’s fuselage had broken just behind the cockpit following the heavy impact and, as Crossfield later noted dryly “The broken fuselage dug into the lakebed, creating a very effective brake”.

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X-15-2, back broken after Crossfield’s heavy landing [IMG: NASA]
As X-15-2 headed back to NAA’s Downey plant for repairs, the first aircraft made its initial powered flight, again with Crossfield at the controls, on January 23 1960. Following this, X-15-1 was officially handed over to the Air Force (with whom the contract was signed) and readied for familiarisation flights by Air Force pilot Bob White and NASA’s Joe Walker. Walker flew the first government flight on 25th March followed by White on 13th April and soon they began to expand the XLR-11 powered X-15’s envelope with Walker exceeding Mach 3 and White passing 100,000ft during May that year. On August 12th 1960, Bob White took X-15-1 to 136,500ft – the highest altitude achieved with the XLR-11 engines. It was time for the XLR-99 to show what it could do.

The Big Engine arrives at last

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A shot showing the ‘Million-Horsepower engine’: the XLR-99 [IMG: NASA]
The first XLR-99, dubbed the ‘Million-Horsepower engine’ had actually arrived at Edwards in late 1959 and testing without an airframe began soon after. X-15-3 had never received the XLR-11 engines and was fitted with an XLR-99 and ready for the test-stand by mid-1960. These tests required a pilot to be in the cockpit as the engine couldn’t be operated remotely and so on June 2nd 1960 Scott Crossfield climbed into aircraft 3 and tested the definitive propulsion system for the first time. Initial tests went well until June 6th when Crossfield attempted to demonstrate throttling and the restart capability. Unfortunately after restarting, a malfunction caused by a frozen regulator resulted in an explosion which sent the forward section of the aircraft 30ft further along the test area and left the smouldering engine section secured to the stand. Crossfield was pulled dazed but otherwise unhurt from the cockpit and once again an X-15 was loaded up and sent back to Downey for repairs.

An enquiry into the test-stand explosion found that there was no fundamental design problem with the XLR99 installation and so the now-repaired X-15-2 was also fitted with the ‘big engine’ while X-15-1 continued to fly with its 2 XLR-11s. Between September and December 1960, 4 new pilots- Forest Petersen (the sole Navy pilot to fly during the programme), Jack McKay (NASA), Bob Rushworth (Air Force) and Neil Armstrong (NASA) – got their first flights. While these familiarisation flights continued, Crossfield was back in the air in X-15-2 to test the XLR-99 in flight. After a number of aborted attempts, he was finally able to launch on 15th November taking the X-15 to Mach 2.97 on 50% power. He made two further demonstration flights with the XLR-99 testing the full range of throttle settings and the restart capability. On the 6th of December 1960, Crossfield made his 14th and final X-15 flight as the contractor demonstration phase of the programme ended. NAA handed the XLR-99 equipped X-15 over to the government – it was now time to see if it could live up to expectations.

Down to business
The research phase of the X-15 programme broadly aimed to: Investigate hypersonic aerodynamics and heating, understand effects on aircraft structure of high heating and flight loads, gain experience of flight dynamics during atmospheric exit and re-entry and finally gain a better understanding of human factors and pilot performance under the stresses of high-speed or high-altitude flight.

Two main flight profiles were developed – speed and altitude. For speed flights, the X-15 would stay within the atmosphere with the pilot pushing over after an initial climb an accelerating at a level altitude. For altitude flights, the pilot would pull back after launch and maintain a high angle of climb until they had exited the atmosphere. The X-15 would then follow a ballistic arc before reentering the atmosphere at a high angle of attack followed by a pull-out to level flight and approach to Edwards.

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B-52 008 prepares to drop the X-15 with a T-38 chase plane in attendance [IMG: NASA]
The final approach pattern used for the X-15 was unusual with the aircraft reaching an initial ‘high-key’ point around 2 miles from the runway while still at an altitude of  at least 20,000ft and a speed in excess of 300 knots. A 180° turn was now executed to bring the X-15 to the ‘low-key’ point at around 12,000ft and 250 knots. The pilot would then make a final 180° turn to line up for landing. During this turn he would jettison the aircraft’s lower ventral fin (if carried) and flare for landing, dropping the gear late to avoid generating additional drag. This spiral approach pattern allowed the pilot a high degree of flexibility regarding energy management – if he was high on energy a wider turn could be flown from high to low-key, similarly if he was low on energy a tighter pattern could be flown.

In general each X-15 flight took many months to reach fruition. A flight planner would work with the chosen pilot, developing a test plan to capture the required data. For every minute in the air, hours would be spent in the simulator practicing all aspects of the flight and a multitude of contingencies, and this level of planning was vital as the average X-15 flight only lasted around 10 minutes from launch to slide out. Workload was extremely heavy for the pilots with the immediate priority being to get the engine started. Should the engine fail to ignite a rapid restart could be attempted and, if successful, the pilot could generally get back on profile. If this second attempt also resulted in failure, the pilot needed to start jettisoning propellants and positioning for an emergency landing at the launch lake. As well as the B-52, the X-15 was accompanied throughout the launch and landing phases by a fleet of chase planes, able to check out the research plane’s condition and offer advice. Many attempts were often needed to launch a flight – aborts could occur for a multitude of technical reasons or due to poor weather at any of the planned landing lakes, meaning all of the X-15 pilots experienced countless hours crammed into the cockpit in their full pressure suit often making a frustrating slow trip back to Edwards.

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The end of another flight, back on Rogers dry lake ‘Where God intended rocket planes to land’ as Milt Thompson put it [IMG: NASA]
With the XLR-99 now up and running the flights began to move away from the relative safety of Rogers dry lake and further up the high range giving them the distance needed to achieve the desired speed or altitude profiles. Although White had already set a new altitude record with his August 12th 1960 flight, the ‘big engine’ would now allow the X-15 to chase its design goals. On March 7th 1961, Bob White became the first person to exceed Mach 4, a mark that only stood until the 23rd of June when he broke Mach 5. He would go on to pass Mach 6 in November the same year, but not before setting a new altitude record of 217,000ft that October. The X-15’s capabilities were truly staggering compared to the research aircraft that had preceded it, but in exploring these new and extreme flight regimes it was extremely demanding to fly – as Neil Armstrong was about to find out.

On April 20th 1962, Armstrong dropped away from the B-52 on a flight planned to reach a velocity of Mach 5 and an altitude of 205,000ft. Initially, the flight proceeded well with Armstrong flying an accurate profile, but during re-entry the X-15’s altitude began to balloon – Armstrong had inadvertently ‘bounced’ off the upper atmosphere and rather than entering denser air where speed and altitude would rapidly decrease, he now found himself sailing past Edwards and heading for Pasadena. Eventually he was able to bring the situation under control and get the aircraft heading back towards the safety of Rogers dry lake, but the question now was would he have sufficient energy to make it home? Using a high Lift/Drag configuration he was able to make up the 45 mile overshoot, landing at the southern edge of the lake to complete the longest X-15 flight of the programme – one which became jokingly referred to as ‘Neil’s cross-country flight’!

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A happy looking Neil Armstrong post flight [IMG: NASA]
Things went more smoothly for Bob White on July 17th when he took the X-15 to a new record altitude of 314,750ft. At peak altitude White could clearly see the curvature of the Earth with a view stretching from the Gulf of California all the way to San Francisco bay. Asked later about the view, he commented with a real sense of understatement “You could see just as far as you looked”. The next day White, representing the USAF, Crossfield, Walker (NASA) and Petersen (Navy) travelled to the White House to be presented with the Collier Trophy by JFK in recognition of the X-15’s contribution to the field of aviation, but White had one more important appointment to keep. Under Air Force rules any pilot exceeding an altitude of 50 miles would qualify as an astronaut, so on 19th July he was presented with his astronaut wings by Gen. Curtis LeMay becoming the first non-NASA astronaut. Bob White moved on from the X-15 programme following his 16th flight in December 1962.

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JFK presents the Collier Trophy to (L to R) Scott Crossfield, Bob White, Joe Walker and Forrest Petersen [IMG: Abbie Rowe. White House Photographs. John F. Kennedy Presidential Library and Museum, Boston]

A Bad day at Mud Lake
The dangers of flying the X-15 were never far from the minds of those involved with the programme. As noted in the opening section of this article, Milt Thompson would liken the experience to old stories of riding a bull into conflict and on November 9th 1962, NASA pilot Jack McKay would experience the first serious crash of the programme. After dropping from the B-52 in X-15-2 over Mud Lake, Nevada, McKay advanced the XLR-99’s throttle but found it unresponsive beyond 30% power. Unsure he could make it back to Edwards, McKay elected to make an emergency landing at the launch lake and although he jettisoned some fuel, the X-15 was still heavy as he lined up for landing. Unfortunately for McKay his problems were far from over as the flaps failed to deploy meaning he came in faster and heavier than normal. Following touchdown one of the aircraft’s main gear struts failed causing the wing tip to dig into the lakebed and send the aircraft rolling. McKay had jettisoned the canopy knowing he may need to get out quickly, but as the aircraft rolled to a stop inverted his head impacted the lakebed causing serious injuries and complicating attempts to get him out of the stricken vehicle.

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X-15-2 following Jack McKay’s crash at Mud Lake [IMG: NASA]
Fortunately for McKay, rescue crews were located at launch lakes and help was quickly at hand. As crews struggled to release McKay, the pilot of the rescue helicopter was able to use his rotor wash to blow away ammonia fumes from the residual propellant. McKay was badly injured and although he was later able to return, flying the X-15 another 22 times, his injuries contributed to problems in later life and his early death at the age of 52.

As for X-15-2, the remains were trucked back to NAA while a decision was made on the aircraft’s future.

The first American in Space…twice
Although the X-15 had already proven it could comfortably exceed its maximum design altitude of 250,000ft, Joe Walker took on the challenge of performing a series of flights to what were felt to be the safe limits for the vehicle. With the lower ventral fin removed to improve reentry stability and using the MH-96 adaptive control system the programme engineers calculated that the X-15 could reach an altitude in excess of 400,000ft but there was less certainty that it could be safely recovered from these altitudes. A decision was made to aim for a maximum altitude of 360,000ft – a 40,000ft overshoot was far from impossible in the X-15!

On July 19th 1963 Walker conducted a final build up flight towards this ultimate target altitude. On this flight he aimed to take X-15-3 to an altitude of 315,000ft but a combination of factors including higher than expected thrust from the XLR-99 and a slightly steeper than planned climb led to Walker topping-out at 347,800ft. Having exceeded the 100km Kármán line Walker had become the first NASA astronaut to fly himself to space and back.

On the follow-up flight which took place on August 22nd Walker would reach 354,200ft – an altitude of over 67 miles – and in doing so become the first American to enter what was internationally recognised as space twice. Going out on a literal high, this was Walker’s final X-15 flight.

New Faces, new challenges and a nearly-new aircraft…

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(L to R) Joe Engle (USAF), Bob Rushworth (USAF), Jack McKay (NASA), Pete Knight (USAF), Milt Thompson (NASA) and Bill Dana (NASA) [IMG: NASA]
With many of the original pilots having moved on from the programme, October 1963 saw Joe Engle (USAF) and Milt Thompson become X-15 pilots. With the X-15 having reached its original design goals, a follow-on programme was initiated with the X-15 to act as a flying testbed taking experiments into desired flight regimes. Apart from initial pilot checkout flights, the majority of the remaining sorties using X-15-1 and X-15-3 fell into this category. New hardware such as tailboxes and wing-tip pods allowed experiments to be carried and a ‘skylight hatch’ was placed in the equipment bay behind the cockpit of X-15-1 to allow large experiments to be extended up out of the airframe once the vehicle was beyond the atmosphere.

Following the severe damage sustained during Jack McKay’s crash at Mud Lake in November 1962, X-15-2 had been rebuilt and extensively modified to allow for new higher speed research. The fuselage was extended to allow more fuel to be carried and provision was made for jettisonable external tanks. A mounting point on the lower ventral stub would allow experimental scramjet engines to be carried and small propellant tanks for these were also included. The new configuration was designated the X-15A-2 and was flown for the first time by Bob Rushworth in June 1964. It was hoped that the X-15A-2 would be capable of reaching speeds of Mach 7 and throughout 1965-66 build-up flights took place to test out the drop tanks and other modifications.

A maximum speed attempt would mean exposing the aircraft to far higher heat loads than it had previously experienced, so to cope with this X-15A-2 was covered in a spray-on ablative heat shield. It was hoped that this could be partially removed and re-applied after every high-speed flight, but the process of application proved incredibly time consuming. The ablative coating also caused a problem for the pilot as residue would collect on the canopy windshield and obscure his vision. To combat this the left-hand pane was fitted with a mechanical ‘eyelid’ which would remain closed until the speed run had been completed at which point it would open offering the pilot at least one clear window to use for approach and landing. The eyelid caused its own problems though as, when open, it generated a small canard effect causing the aircraft to roll slightly. Coupled with the asymmetrical weights of the external drop-tanks it was clear that flying X-15A-2 was going to be a challenging assignment.

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the X-15A-2 in its pristine protective coat [IMG: NASA]
Between September 1965 and October 1966, three new pilots joined the programme – Pete Knight (USAF), Bill Dana (NASA) and Mike Adams (USAF) and they would conduct the majority of flights for the remainder of the programme.

Knight would make one of the programme’s more troubled flights  on June 29th 1967 when, during climb out in X-15-1, he suffered a complete loss of power due to the failure of both auxiliary power units. Helpless to do much except try and retain some control of the aircraft as it passed up through 173,000ft, Knight recalled looking out and seeing Mono Lake (to the east of Yosemite NP) and deciding to enjoy the view as it might be his last. Fortunately he was able to maintain some control of the uncooperative X-15 before getting emergency battery power back during reentry and restarting one of the troublesome APUs. Knight made an emergency landing at Mud Lake and was able to extract himself from the aircraft before help arrived.

Maximum Speed
Knight was also the pilot for the maximum speed attempt in the X-15A-2 and on October 3rd 1967 he dropped away from the B-52 in the gleaming white research plane and lit the XLR-99 heading for a maximum mark of Mach 6.7. The X-15 had fallen just short of the hoped for Mach 7 but unbeknownst to Knight or the ground controllers, shockwaves from the dummy scramjet X-15A-2 was carrying caused extremely high local temperatures around the lower ventral fin which actually melted through the ablator-protected Inconel-X skin causing severe structural damage. As he made his initial turn into the landing pattern, the scramjet tore free and only following a successful landing did it become apparent how close to disaster the flight has come. The heavily damaged X-15A-2 would never fly again but Knight was safe. Sadly in the following month, another X-15 pilot would be far less lucky.

A final tragedy
Mike Adams had joined the X-15 programme having previously been an Astronaut on the Air Force Manned Orbiting Laboratory (MOL) project. As the military space station ran into funding troubles and schedules lengthened, Adams transferred across to Edwards figuring it was better to be involved in a programme that was actually flying than to wait for a ride into space that MOL may never provide. On November 15th 1967 he was making his 7th X-15 flight to a planned altitude of 250,000ft in aircraft 3. Initially it looked as though the flight was proceeding well with the climb executed on profile, but as the X-15 reached space a number of factors began to conspire to make the piloting task more difficult.

A short-circuit in one of the experiments caused interference with flight controls and while battling against this problem Adams inadvertently began to use one of the cockpit displays in the incorrect mode, leading to the aircraft diverging from its correct alignment to a point where it began to reenter at 90° to the flight path. Adams became disoriented and seemed unaware of the error until the X-15 entered a hypersonic spin. Although he was able to recover from this, the loads on the airframe became too great and X-15-3 broke up high above the Mojave.

Mike Adams was posthumously awarded Astronaut wings for the flight as he had exceeded the USAF 50-mile mark reaching a maximum altitude of 266,000 ft.

The end of the programme
Following Adams’s fatal crash and the loss of X-15-3, only one serviceable aircraft remained – X-15-1. By now the X-15 had been flying for nearly a decade and funding for any sort of additional development was proving difficult to obtain. Now, in the wake of a fatal crash and with only one plane remaining the decision was made to terminate the programme at the end of 1968. Between them Pete Knight and Bill Dana managed to make eight more flights with Dana flying the last of these – the 199th flight of the programme – on October 24th 1968. Attempts were made for a 200th flight before the end of year deadline, but for a variety of reasons these were never able to get off the ground.

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A classic shot of the X-15 poised for flight – “Beware the Bull”  [IMG: NASA]
On December 20th one final attempt had to be abandoned because of snow at Edwards AFB. The X-15 had made its last flight of an amazing career. The very next day Apollo 8 took off from Cape Kennedy taking astronauts Borman, Lovell and Anders to the Moon. The X-15 could never go as high, as fast or as far as the big rockets and capsules launched from Florida, but it provided the first winged flights beyond the atmosphere and would have a huge influence on projects that followed Apollo.

Part 3 – The Final Steps and Legacy can be found here

Part 1Toward the Unknown can be found here


Sources

At the Edge of Space – Milton O Thompson

Illustrated History of Space Shuttle: US Winged Spacecraft X-15 to Orbiter – Melvyn Smith

The X-Planes: X-1 to X-45 – Jay Miller

Hypersonic: The story of the North American X-15 – Dennis R. Jenkins & Tony R. Landis

The X-15 Rocket Plane: Flying the First Wings into Space – Michelle Evans

The Right Stuff – Tom Wolfe

X-15: Extending the Frontiers of Flight – Dennis R. Jenkins

Hypersonic Before the Shuttle: A Concise History of the X-15 Research Airplane – Dennis R. Jenkins

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