Apogee to Atmosphere

Burt Rutan described the idea of the feather maneuver as the pivotal piece of the puzzle needed for the design of SpaceShipOne. Reentry into Earth’s atmosphere was the most critical point of every spaceflight. By deploying the feather mechanism, referred to as “carefree” and “hands – off,” the aerodynamic drag increased substantially, resulting in very low thermal loads. This was because the spaceship slowed down so quickly in the upper atmosphere that when it reached the thick atmosphere, it was traveling with much less energy. Because there was less energy, there was less heat being generated, and SpaceShipOne didn’t get as hot. On ascent after the boost, as SpaceShipOne continued to slow and close in on apogee, the test pilot put the feather up. “We put the feather up because we want to have as much time as possible to troubleshoot if it doesn’t
go up,” Melvill said. “It goes up with two different pneumatic actuators, either one of which can do the job. They are fed out of two separate high-pressure bottles. And you can put both bottles to one or the other. We had redundancy.”

Going up to space had its challenges, but coming back down was where a space program was truly tested. And the feather was SpaceShipOne’s ticket back home. Figure 3.15 shows SpaceShipOne with its feature in the extended position.

“So, we would put it up as soon as we were out of the atmosphere because if we put it up in the atmosphere, we would start doing loops.”

The rear half of each wing folds upward about a hinge line, looking like a jack-knife. It took less than 20 seconds for the feather to deploy to an angle of 65 degrees, and the pilot watched the instrument panel to make sure it went all the way up.

“As soon as you moved the handle, that unlocked it, and as soon as it was unlocked, you’d hear it,” Melvill said. “Without the motor running, it was very quiet. And you would hear it go konk in the back as it unlocked. As soon as we heard that, you would put the feather up with the handle. There were two handles right next to each other on the left side. The feather took a long time, and it made a noise going up. You could hear the air hissing into the large-diameter actuators.”

After SpaceShipOne reached apogee, going over the top, it began to pick up speed and continued to follow a ballistic arc downward much

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Fig. 3.16. The feather extends and retracts by air-powered actuators, or pistons, that are attached on either side of the fuselage and connected to each side of the wing near their trailing edges. Mojave Aerospace Ventures LLC, video capture provided courtesy of Discovery Channel and Vulcan Productions, Inc.

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like the parabola traced by a ball thrown into the air and as it drops back down to the ground.

The spacecraft had a lift-to-drag ratio of about 0.7 in the feathered configuration, so the descent was nearly vertical, with an angle of attack of 60 degrees. While in space, a video camera mounted in SpaceShipOne’s tail boom captured the image of the feather shown in figure 3.16.

Now as SpaceShipOne transitioned from space to reentry, the atmosphere began to get thicker and thicker. Mach 3.25 was the fastest reached by SpaceShipOne. This corresponded to an airspeed below 160 knots equivalent airspeed.

“You come back a little faster than you go up,” Melvill said. “You get a tremendous amount of g-forces on your body when you are coming back. We were looking at 5.5 g’s on reentry.”

The pilot didn’t wear a g-suit. But he did experience a decelera­tion force above 5 g’s for around 10 seconds. Since the pilot sat upright the entire flight and SpaceShipOne reentered the atmosphere belly first, it was critical for the pilot to train beforehand in order to build up his g-tolerance.

“You see because of the rods and cones in your eyes,” Melvill explained. “They need oxygen-enriched blood to feed them. If the blood gets drained down to where there is not much oxygen – enriched blood behind your eyes, you go blind. You just black out. And you can still hear and think and move the stick. But you can’t see. And that would be hard to fly if you couldn’t see.”

In the feather configuration, SpaceShipOne acted like the conical­shaped shuttlecock, or birdie, used in the sport of badminton. Originally made of feathers and now more commonly made of plastic, the shuttle­cock’s skirt has such high drag compared to its base that after a hit from a racket, the shuttlecock automatically orients itself base-first in the direction of flight. The Scaled Composites team took advantage of the two important aspects of this concept when considering the reentry of SpaceShipOne. The high drag caused rapid deceleration, and the self-aligning tendency ensured the proper orientation.

“You don’t even need to do anything coming back down. It is a ‘carefree’ reentry,” Melvill said. “You could put your hands behind your head and take your feet off the rudder pedals and just wait. And you could reenter in any attitude. You could be tumbling when you reenter. You could be upside-down when you reenter. You could be knife-edge and the feather will turn you around and straighten you out. It happens real slowly.”

A terminal velocity of 60 knots equivalent airspeed is reached in this high-drag configuration. This corresponds to a ballistic coefficient,
calculated by using the weight, drag, and cross-section of SpaceShipOne, of 12 pounds per square foot (psf), compared to that of 60 psf for the early Mercury capsules. A low value of the ballistic coefficient means that the spacecraft will begin to slow down quickly in the thin atmosphere. So, SpaceShipOne experiences only low overall structural and thermal loading. It goes from supersonic to subsonic in about a minute and a half.

“The temperature when we reenter around the airplane is very high,” Melvill said. “It is about 1,200 degrees, but that’s the air temperature against the skin. Because that happens at 100,000 feet [30,480 meters] or more, there is so little air to conduct the heat into the structure. The molecules of air are so far apart because it is only 1 percent, or less, atmosphere up there. So, it takes time for that heat to be conducted into the structure, and we’re through that heating period before it has time to get into our airplane. Burt designed it that way, and that was very clever. He made sure that we wouldn’t spend very much time under the conditions where we could melt the airplane.”

During a presentation at the Experimental Aircraft Association’s 2006 AirVenture at Oshkosh, Wisconsin, Rutan described the differ­ence between the thermal protection system of SpaceShipOne and the Space Shuttle: “You don’t have the problems going Mach 4 that you do going Mach 25.

“On the boost, SpaceShipOne sees temperatures that are too hot for the skin at the nose and on the leading edges. That’s all. To be conser­vative, we protected some of the areas that got relatively hot on reentry. That’s why you see that stuff down under the nose and up underneath the wing. However, our measurements there showed that none of that was required. SpaceShipOne doesn’t need any thermal protection at all for reentry. It only needs several pounds of material for boost.”

In cooperation with the U. S. Air Force, SpaceShipOne reentered into restricted airspace controlled by Edwards Air Force Base. But within the restricted airspace, the Office of Commercial Space Transportation (AST) had designated a location, a box roughly 2.5 square miles (6.5 square kilometers) in size, for SpaceShipOne to come down through during reentry.

This was certainly a piloting challenge because in order to reenter through the box, the pilot already had to be in position on the way up. So not only did the pilot have to make sure the wings were level, the nose was pointed up, asymmetries were compensated for, and the occasional wind shear was counteracted, he had to try to position SpaceShipOne so that once the engine shut down and the atmosphere was gone, it would coast more than 100,000 feet (30,480 meters)

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Fig. 3.17. SpaceShipOne returns from space as a glider and makes a horizontal landing similar to way the Space Shuttle does it. Although not the most efficient glider, SpaceShipOne had a glide range of around 60 miles (97 kilometers). Mojave Aerospace Ventures LLC, photograph by Scaled Composites

V_________________ ) feet to apogee, free fall back down, and then drop through a relatively small-sized box.

“Our priorities were we wanted to get altitude, and we wanted to leave the atmosphere without a lot of body rates or gyrations,” Binnie said.

The third goal was to come back inside the box. “But controlling the body rates and maneuvering the vehicle to find that box were kind of at odds with each other.”

SpaceShipOne continued to descend with its feather up. This configuration was so stable that in the atmosphere at the higher altitudes, it was easier for the pilot to just leave the feather up, even though SpaceShipOne had performed a safe reentry. However, there wasn’t much control. The pilot couldn’t pitch the nose and roll the wings, but he was able to change the direction that the nose pointed.

“This is something we didn’t feel necessary to test, but it is likely that you could survive a feather-up landing in SpaceShipOne,” Rutan said. “We did not plan to ride it down if the feather didn’t come down. We planned to jump out.”

At an altitude below 70,000 feet (21,340 meters), the feather was retracted and locked. SpaceShipOne, flying subsonically, transformed into a glider.