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SOFIA Telescope Door:
A Monumental Effort
The world is full of magnificent doors
in the worlds of art, science and engineering, and it's about to
gain an extraordinary door of quite a different variety.
A first of its kind, SOFIA's upper
rigid door will cover an open port cavity nearly one-quarter of
the circumference of the SOFIA 747SP plane.
"No door this size has ever been put
on an airplane like this, especially that would open and close
in flight. It's a monumental effort," explains Raytheon Aircraft
Integration System's Don Gillespie, a structural integrity manager
who is coordinating SOFIA door design and fabrication.
"SOFIA has been called the aircraft
modification project of the century. It's destined to be one of
the marvels of the aerospace industry once it's fully operational," adds
Cavity Door Work Product Manager Bill Caldwell of the NASA Ames
Research Center, Moffett Field, California.
The 18-foot (arc length) by 13.5-foot-wide
curved upper door is part of a system intended to protect SOFIA's
40,000-pound infrared telescope at
the beginning and end of each night's mission, as well as on the
ground. During flight, it opens only when the plane has reached
its observing altitude of 39,000 to 45,000 feet, and on the ground
only when the telescope mirror needs to be removed for re-coating.
Most of the system design is taking place at Ames, while testing
and fabrication is centered at Raytheon's facilities in Waco, Texas.
Based on heritage from SOFIA's predecessor,
the Kuiper
Airborne Observatory, the system's on-the-ground goals include
keeping the telescope clean and allowing for pre-cooling of the
telescope down to -30 degrees Centigrade prior to each take-off.
In-the-air requirements include manipulating wind flow around the
open port cavity to minimize drag and keeping as much of the open
port cavity as possible closed off during science observations
in order to minimize turbulence.
Short animation of the SOFIA door
system in operation available in video formats, |
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Modules of the 4,500-lb. system include
the:
- Upper Rigid Door, which rotates over the top of the plane on
tracks to open and close over the stationary open port cavity.
Because of its sheer size, a simple, rigid aluminum door with
few interlocking or moving parts is preferable to a flexible
door.
- Lower Flexible Door, an 8-foot-high by 13.5-foot-wide aluminum
door fitted to the lower edge of the aperture (described below),
moving up and down with the aperture on tracks. The telescope
and aperture move over a 40-degree arc for science operations,
and this door ensures that as much of the open port cavity as
possible remains closed off and protected from buffeting winds.
Its lower portion doesn't arc in a perfect circle, in part because
the circumference of the 747SP's fuselage isn't a circle at all
but rather is an ellipse.
- Aperture System, a 9-foot-high by 13.5-foot-wide carbon fiber
reinforced plastic frame around part of the open port cavity
in the shape of a "D", exposing the telescope to the elements.
The remaining portions of the open port cavity are always covered
by the upper and lower doors. The tapered aft (rear) edge of
the aperture, a small ramp, catches air flow from the fairing
(described below) to minimize turbulence inside the cavity while
simultaneously minimizing the disruption of the airflow over
the aircraft fuselage. Raytheon has subcontracted fabrication
of this specialty part to Scaled
Technology Works of Montrose, Colorado.
- Fairing System, the aluminum fore (front) part of the open
port cavity, precisely angled to minimize the drag and aerodynamic
disturbances caused by the door and track system. When engineers
decided to put the upper rigid door on the outside of the plane,
raised up from the fuselage, instead of keeping it flush to the
exterior, a fairing was required to make the cavity as aerodynamically
benign as possible. It rests atop and independent of the doors
and aperture.
- Seal System, a silicon rubber inflatable seal (similar in principle
to an inflatable tube) between the inside surface of the upper
rigid door and the outer skin of the aircraft. Activated when
the door is closed, this system is important for pre-cooling
on the ground and for keeping the cavity sealed in flight during
ascent to or descent from observing altitudes.
Approximately 15 engineers are currently
working on cavity door systems design at Ames, while another 10
are on Raytheon's team in Waco and Greenville, Texas. In the coming
year, door components will continue entering the fabrication phase
as the plane is prepared for its scheduled flight to Munich, Germany,
for telescope installation. Upon the plane's return to the U.S.,
further work, particularly on software and controls, will continue
through the end of engineering test flights prior to the start
of science operations.
December 11, 2000 |
