Tuesday, September 21, 2010
We Fly The Space Shuttle (Simulator, That Is)
What’s it like to reenter the atmosphere from Earth’s orbit?
If you guessed the Shuttle makes a terrible glider, you guessed right. Most pilots had better hope they never come close to experiencing the glide characteristics of a Space Shuttle. My Mooney and most other general aviation aircraft come downhill power-off at about a 10 to one L/D, 10 feet forward for every foot of descent. When I earned my glider rating 30 years ago, I flew a Czechoslovakian Blanik L13 trainer that offered a glide ratio of roughly 28 to one. The most efficient high-performance sailplanes record L/Ds as high as 70 to one.
In contrast, the Shuttle’s glide at high altitude is barely distinguishable from that of a set of car keys. The spacecraft experiences a variable glide ratio, because it operates in variable levels of air density. It’s a spacecraft in space and a conventional aircraft in air (duh!), so its glide characteristics change as the air becomes thicker. At hypersonic velocities in the upper atmosphere, above about Mach 5 and 200,000 feet, the Shuttle glides at a 1:1 ratio, one foot of forward travel for each foot of descent, about the same as a Steinway. That increases to 2:1 at supersonic velocities and improves to about 4.5:1 when you’re on final approach.
In contrast to the extreme nose-up attitude of reentry, the world’s largest glider must assume a nose-down pitch for the last 50,000 feet of descent. During this phase of the Shuttle’s approach, it’s dropping out of the sky at 10,000 fpm. Pilot error makes an undershoot possible, but the computer flies the airplane down to about 50,000 feet before the commander or pilot assumes control. In theory, the pilot takes over just before the shuttle reaches the HAC (heading alignment cone) that’s located about 50,000 feet above the Kennedy Space Center. In other words, the computer doesn’t guide the Shuttle directly to the threshold of Kennedy’s three-mile-long runway but to a cone of airspace high above the Cape. From there, the pilot flies the aircraft around an arcing pattern to final approach.
Flying the approach is simplified somewhat by the HUD (head-up display) that overlays the windshield. The HUD allows you to look through the display at the runway and still receive visual cues of the approach. The HUD presents the pilot with a round, green flight-path marker representing the Shuttle, and a guidance diamond. Theoretically, if you fly the flight-path marker on top of the guidance diamond at all times, the STS will slide right down final to a squeaker landing. It says here.
Follow the HUD precisely, and it will arc you around to final approach at 12,000 feet, six miles out, about a minute from touchdown. A 20-degree glideslope is seven times as steep as a normal ILS, so if you try to fly the approach visually without the help of the HUD, you’ll be challenged, to say the least. In the Shuttle, you’ll be dropping out of the sky at 1,000 feet every six seconds, a pretty ferocious descent rate. In his capacity as unofficial Shuttle sim test pilot, Dye has flown many pure VFR approaches to Edwards AFB where the dry lake precedes the actual runway, and Dye has learned to do it reasonably well. For the rest of us, don’t try this at home.
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