The Feather

Mechanically, the most complicated system on SpaceShipOne was its feathering system. It was also the most important system on board for ensuring the safety of the pilot and the success of the mission. Rutan already had experience with movable-winged aircraft. His RAF analyzed the designs and loads for NASA’s scissor-wing AD-1 (refer to figure 4.13).

Before SpaceShipOne reentered the atmosphere, the aft section of both wings, including the tail booms, rose up as if the spacecraft were almost folding in half. With the feather extended, SpaceShipOne could reenter the atmosphere with very little pilot input required. This “carefree” reentry was one of the most important elements of Scaled Composites’ entire space program.

The feather is a separate structure from the forward wing sections and has its own spars and ribs. However, the aft wing sections and the tail booms do not move independently. Figure 4.14 shows the spar that runs through the fuselage from one end of the movable wing to the other, tying the feather all together.

Left and right pneumatic actuators, which are just cylinders with movable pistons, used air power to pivot the feather up or down along the hinge. The lower ends of each actuator are attached to the fuselage, and the upper ends are attached to the inner face of the aft wing section, as shown in figure 4.15.

There are just two positions of the feather, up or down. The angle the feather makes with the fuselage is preset to 65 degrees, so the pilot did not have to make any adjustments. It took about 13—14 seconds to raise or lower. The feather could be elevated on ascent once the airspeed was less then 10 knots equivalent airspeed. Figure 4.16 shows the 60-degree angle of attack for SpaceShipOne descending in the feather condition.

When the feather got to the fully extended position, there was no lock. It was just the force from the actuators that held the feather up. “As it turns out, it takes no load to put it up because you are weightless,” Rutan said. “When you are weightless, there are no aerodynamics. It takes no force. In fact, if you would unlock it in space, you could take a little cable and pull it right up. There is no load on it. Okay. But, you need to hold it up for reentry.” The reentry force on the flight test actually tried to push the feather down.

If the feather were to move or become disengaged at the wrong time, the results could be disastrous. To keep the feather in the retracted position and to keep it from moving during the other phases of flight, two I-shaped clasps from the locking system secure the trailing edges of the aft wing sections. To unlock the feather so it could be deployed, separate pneumatic actuators were pressurized to retract the clasps.

The design had built-in redundancy for the components that make up the feather system. The two clasps were coupled so they moved together. But they had separate pneumatic sources, lines, regulators, valves, and again, separate actuators. “So, I could have a fire. I could have a line come off. I could have a loss of pressure. I could have all of these things go wrong, and it doesn’t affect the other,” Rutan said.

The two different pressure sources could run either actuator. The redundancy of the locking system was identical to the redundancy of the elevating systems. They were two independent systems that, under normal conditions, acted in unison. However, either system could engage or disengage the clasps of the other system. Once SpaceShipOne was flying subsonically after reentry and the loading drops below 1.2 g, the pilot could retract the feather.