Between Spaceflights

Preparing SpaceShipOne for its next spaceflight was a relatively simple task that required only minimal maintenance. The spent compo­nents of the rocket engine were replaced with fully fueled components, and the oxidizer tank was refilled. The air bottles used to activate the feather and run the reaction-control system and other systems had to be recharged. The ablative coating for the thermal protec­tion system was restored. And since every flight was an envelope expansion, theTONU was updated after a thorough review of the flight data.

X-15 Comparison

The North American X-15 was the first of three winged vehicles ever to have reached space, the Space Shuttle and SpaceShipOne being the other two. The basic mission profile was similar for these vehicles in that they all used two stages to reach space and glided back to Earth for an unpowered landing. However, the X-15 and SpaceShipOne shared much more in common compared to the Space Shuttle, which was a fully operational spacecraft designed to transport large payloads back and forth from orbit, whereas the other two were research and proof-of-concept vehicles that only reached suborbital altitudes.

Originally conceived in 1954, the X-15 first flew in 1959 and flew the last time in 1968. Its two primary goals were to fly at Mach 6— hypersonic speeds begin at Mach 5—and reach an altitude of 250,000 feet (76,200 meters). The X-15’s 199 powered flights directly influ­enced the Mercury, Gemini, Apollo, and Space Shuttle space programs as well as the U-2 and SR-71 reconnaissance aircraft.

Table 3.2 shows a comparison between the X-15 and SpaceShipOne.

To conserve fuel, the X-15 was dropped from the wing of a NASA B-52 carrier aircraft at an altitude of 45,000 feet (13,720 meters), as shown in figure 3.20. For its high-altitude mission, the rocket engine burned for up to 2 minutes, and then the X-15 returned from space and glided in for a landing.

This trajectory flown by the X-15 was, however, quite a bit differ­ent from that flown by SpaceShipOne. Figure 3.21 shows the trajecto­ry flown by SpaceShipOne and compares it with both the high-speed and high-altitude trajectories of the X-15.

The most apparent difference was in the profile width of the X-15 high-altitude mission and the SpaceShipOne mission. The X-15 had to cover 331 miles (533 kilometers) in order to reach an apogee above 62.1 miles (100 kilometers), whereas SpaceShipOne only needed 40 miles (64 kilometers) to accomplish the same feat.

Both had the same amount of weightless time and view, but the X-15 traveled much faster to achieve this. The higher speeds meant greater aerodynamic loads and thermal protection require­ments. But the most important difference was that the X-15 had to expend much more energy to reach the same altitude. The greater the energy needed to get from point A to point B, the more expensive it is to fly.

Primarily constructed of lightweight, high-strength titanium, it had skin of Inconel X, a chrome-nickel alloy that would withstand temperature as high as 1,200 degrees Fahrenheit. The black coating helped dissipate heat, and it was necessary to design gaps into the fuselage to allow for temperature expansion, which was a feature carried over to the SR-71.

A liquid oxygen oxidizer and an anhydrous ammonia fuel powered the liquid rocket engines, providing a thrust of 28,000—57,000 pounds-force (125,000—254,000 newtons). A reaction-control system that used hydrogen peroxide thrusters on the nose and wings allowed the X-15 to maneuver outside the atmosphere.

Another similarity was the landing gear. To reduce weight and simplify the design, the X-15 used two landing skids at the rear of the vehicle, compared to the single skid at the nose used by SpaceShipOne.

On August 22, 1963, the X-15 set an altitude record of 354,200 feet (108,000 meters). Four years later, on October 3, 1967, a high­ly modified version renamed the X-15A-2 set a speed record of Mach 6.70, or 4,520 miles per hour (7,270 kilometers per hour). The entire aircraft had to be covered in a white ablative coating to increase the thermal protection of the skin up to 2,000 degrees Fahrenheit. This was a peak speed, and the X-15 could only run its engine for about two minutes. However, if this speed could be maintained, it would be possible to travel the distance from New York City to Fos Angeles in just over a half an hour.

Although it was a tremendously successful program, four major accidents occurred. One of them claimed the life of test pilot Michael Adams due to a control-system failure during reentry. This accident and the Space Shuttle Columbia accident, also occurring during reentry, were key influences that drove Rutan to develop the “carefree” feather reentry.

Although White Knight began flying about a year before SpaceShipOne, construction of both vehicles began at about the same time. High strength, lightweight composites of carbon fiber/epoxy were used to build the primary structure of both vehicles. Tyson V. Rininger