The North American X15

Suborbital winged spaceflight began in 1962 with the X-15 rocket research aircraft, which had already been flying below space altitude since 1959. Ballistic missiles had lobbed four men into space the previous year, and had done so with greater energy than the X-15 could muster. And yet the X-15 could be flown over and over, because it had wings. But it had something most aircraft did not have, and that was a rocket engine, enabling it to operate outside the sensible atmosphere.

The X-15 research aircraft routinely entered regions of the upper atmosphere where its wings were aerodynamically ineffective. It therefore had to be equipped with small thrusters to maintain control during the highest parts of the flight. These were located on the wingtips for roll control and in the nose for pitch and yaw control.

The US Air Force awarded astronaut wings to X-15 pilots who flew above 50 miles. NASA, on the other hand, required its pilots to fly above 62 miles -100 km - before considering them real astronauts. The X-15 program included pilots from prime contractor North American Aviation, the US Air Force, the US Navy,


Fig. 5.4 The X-15 just after landing on the dry lake at Edwards AFB, with its B-52 mothership flying over (courtesy NASA)

Fig. 5.4 The X-15 just after landing on the dry lake at Edwards AFB, with its B-52 mothership flying over (courtesy NASA)

Fig. 5.5 Space pilot Neil Armstrong with the X-15, about 1960 (courtesy NASA)

and NASA. As a result, some X-15 pilots earned astronaut wings, while others did not. One X-15 pilot who earned astronaut wings was Air Force Capt. Joe H. Engle, who later went on to pilot the Space Shuttle for NASA. One who did not was civilian pilot Neil Armstrong (Fig. 5.5), who only reached 39 miles. Ironically, he later became the first human to walk on the Moon, in July 1969.





NASA I> ■ (!1 .1 r.'t-l Research Center Pticilo Collection btip:/Avww,d frv. .•■<v I;: -XiIcry/PhoitVindei. Iicml NASA Photo: CTW-Û20S Dire: January 20, 1962 Phoroliy: NASA

Cutaway drawing of the North American X-15.

Fig. 5.6 Cutaway drawing of the X-15 showing internal structure and general arrangement of propellant tanks and engines, and landing gear. The rear landing gear was a set of retractable skids (courtesy NASA)

The X-15 was the ultimate rocket research aircraft of all time. Like its predecessors, it was launched from a large carrier aircraft (a modified B-52) and was flown by a single pilot. Its engine, the XLR-99, provided a thrust of 57,000 lb. It had very stubby wings, and tested the limits of flight at the edge of space. Three X-15s were built (Fig. 5.6) by North American Aviation, making a total of 199 flights between 1959 and 1968. During the research program, there were three crash landings, but only one fatality. One craft broke in half on landing, but its pilot, Scott Crossfield, survived. Another blew up on the test stand with the same pilot, but again he survived. A third craft suffered loss of control and made a high-speed landing in which the vehicle flipped over (see Fig. 5.7). The pilot, Jack McKay, was trapped inside the wreckage for 4 h after banging his helmet on the runway. Amazingly, both craft and pilot flew again, many times. The only pilot to lose his life was testing a procedure in which a computer was allowed to fly the plane. After successfully recovering from an extreme altitude 3,000 mph spin, a severe pitch oscillation ensued, knocking out USAF Captain Mike Adams because of excessive G forces, and the vehicle disintegrated before he woke up. He had just exceeded the 50-mile space altitude, as recognized by the US Air Force, and was posthumously awarded astronaut wings.

To really appreciate the success of the X-15, it is useful to compare some of its statistics with that of the Space Shuttle. By the time the Shuttle had made 115 flights, two vehicles had been lost, with their crews of seven each. The X-15 made 199 flights with the loss of one vehicle and one man. The shuttle has averaged six

Fig. 5.7 On November 9, 1962, X-15 pilot Jack McKay experienced multiple vehicle failures, forcing a crash landing. After the engine became stuck at 35%, the planned flight test was aborted. Landing flaps then failed to extend, resulting in a higher than normal-speed approach, and the left skid failed on touchdown, causing the vehicle to tumble. Yet because this vehicle had wings and direct human control, both plane and pilot flew again (courtesy NASA)

Fig. 5.7 On November 9, 1962, X-15 pilot Jack McKay experienced multiple vehicle failures, forcing a crash landing. After the engine became stuck at 35%, the planned flight test was aborted. Landing flaps then failed to extend, resulting in a higher than normal-speed approach, and the left skid failed on touchdown, causing the vehicle to tumble. Yet because this vehicle had wings and direct human control, both plane and pilot flew again (courtesy NASA)

flights per year, while X-15 averaged 22. The X-15 provided much of the data subsequently used in NASA's humans-in-space program.

The X-15's highest altitude flights had something in common with the first two manned American spaceflights. In both cases, flights were suborbital. How does the X-15 compare to these Mercury Redstone flights? Table 5.1 presents a few data points, which include NASA's first two manned spaceflights, all X-15 flights above 50 miles, and the five fastest flights of the X-15.

There are two matters of significance in these data. The first is that there were only two flights of a manned Redstone rocket - ever. The Redstone rocket had been developed by the US Army as a ballistic missile in the Redstone arsenal. It was modified for use by Project Mercury, because NASA knew that it could boost a small spacecraft into suborbital space. The Redstone had also been used to fly Ham, the space-chimp (Fig. 5.8) , on a flight similar to Commander Shepard's, on 31 January 1961. Including this flight, three separate launch vehicles and three separate spacecraft were used for a total of three suborbital flights. By contrast, the winged X-15 made 199 flights in 9 years, using just three vehicles, including 15 flights in 1961.

Winged spacecraft are reusable because of their wings. Extensive testing of any flight vehicle requires that vehicle to have wings so that it can be reused and tested over and over again. This is the fundamental difference between ballistic capsules and spaceplanes. And this is why spaceplanes will be inherently safer than missiles and modules.

Fig. 5.8 Ham, the astro-chimp, was the first chimpanzee to enter space, blazing the trail for NASA's human astronauts. Meanwhile, the Russians were sending dogs, like Laika, into space (courtesy NASA)
Table 5.1 Comparison of Mercury Redstone and X-15 suborbital flights1,2

Max. altitude

Max. speed






05 May 1961




Alan Shepard

21 July 1961




Gus Grissom

09 Nov. 1961




Robert White

27 June 1962




Joseph Walker

17 July 1962




Robert White

17 Jan. 1963




Joseph Walker

27 June 1963




Robert Rushworth

19 July 1963




Joseph Walker

22 Aug. 1963




Joseph Walker

05 Dec. 1963




Robert Rushworth

29 June 1965




Joe Engle

10 Aug. 1965




Joe Engle

28 Sep. 1965




Jack McKay

14 Oct. 1965




Joe Engle

01 Nov. 1966




William Dana

18 Nov. 1966




Pete Knight

03 Oct. 1967




Pete Knight

17 Oct. 1967




Pete Knight

15 Nov. 1967




Mike Adams

21 Aug. 1968




William Dana


Although the Redstone lifted off from sea level, it easily - and admittedly - outperformed the X-15 in both speed and altitude. Ballistic missiles are inherently good at lobbing fast projectiles. While the X-15 and SpaceShipOne each had zero forward speed at the apogees of their respective suborbital trajectories, Mercury had a significant suborbital velocity. This is the reason it landed 300 miles down-range from the launch point, and a large part of the reason Shepard subsequently experienced such high G forces.

The fastest X-15 flight was number 188 on 3 October 1967, which reached 4,520 mph. This was only 660 mph slower than Alan Shepard's suborbital lob 6 years earlier. This X-15 flight carried extra propellant tanks and had the longest burn time of its rocket engine, more than 140 s. The fastest X-15 flights did not reach the highest altitudes, however. Pete Knight's unofficial X-15 speed record of 4,520 mph never took him or his craft above 20 miles, compared with Alan Shepard's apogee of 1161/2 miles. Again, ballistic missiles outperform aircraft in both speed and altitude, but not in the vital areas of reusability and reliability.

Although the X-15's performance cannot match that of even the relatively underpowered Redstone rocket, the fact that it had wings meant that it could be flown time and time again. And this is the true value in all winged space vehicles. This point will be stressed repeatedly throughout this book. Reusability in flight requires wings. The reason spaceplanes have wings is so they can fly, yes, but more important, it is so they can fly again. This, in turn, leads to much greater reliability and safety because of the additional flight experience gained. In the case of the X-15, the engineering data gleaned from 9 years' worth of flight experience proved invaluable in the design of the Space Shuttle in the following decade. This research program could not have been carried out without a reusable, reliable vehicle such as the X-15.

What steps need to happen to bridge the gap between vehicles such as the X-15 and future orbital spaceplanes? There is more than one answer to this question, and it depends on our vision of the future. The process of gradual improvement in spaceplanes is already underway. As spaceplanes are enlarged, they will have the capability of transporting more passengers to greater speeds and altitudes. Yet, the road to space is long and arduous. The energy required to reach orbit is 30 times greater than what is required to reach space height, because orbital energy is mainly in the form of speed.

For a spacecraft in a circular orbit 200 miles, or 1 million ft, above Earth, the specific energy due to altitude is gravity times height, gh, or roughly 32,000,000 ft2/s2. When an actual orbital velocity of 25,000 ft/s is added into the equation, the specific energy (gh + %v2) becomes roughly 344,000,000 ft2/s2 . Fully 90% of this is in the kinetic or "velocity" term - that little /v2 . SpaceShipOne reached an altitude of 112 km on 4 October 2004, but by the time it reached this altitude, it had lost all of its speed, just like the X-15. Its specific energy at burnout can easily be found by calculating its specific potential energy at apogee, because this is where the speed and the kinetic energy went to zero. This is simply gh or (32 ft/s2) x [(112 km)/(0.0003048 km/ft)] = 11,700,000 ft2/s2. Dividing the first result by the second, we have

344,000,000/11,700,000, which is nearly 30 times as much energy to reach space height and orbital speed as required to reach space height only. This means spaceplanes need to be improved by a factor of 30 before they will reach low-Earth orbit. At present, spaceplanes are little more than manned sounding rockets.

How long will this process take? Let us take a look at the altitudes and speeds associated with the Wright Flyer. The original Wright Flyers had airspeeds in the 10-30-mph range. The X-1 broke the sound barrier - about 700 mph - in October 1947. This pivotal event required speeds roughly 30 times what the Wright brothers could achieve. So it took just under 44 years. An increase in speed is tantamount to an increase in energy, because as we have seen, for a spacecraft in a 200-mile orbit, 90% of a spacecraft's energy is tied up in speed. If the development of the spaceplane takes place at the same rate as that of the airplane, then the first orbital spaceplane should be flying by the year 2047.

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