From the Everett and Seattle Astronomical

From the Everett and Seattle Astronomical
June 9, 2004 - 040609.doc
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From Celestial North, this is IT'S OVER YOUR HEAD for the week of June
9, 2004, a look at what's up in the sky over Puget Sound.
The Hypervelocity Impact Technology Facility, or HITF, located at the
Johnson Space Center, Houston, Texas, is used to test many materials and
spacebound items for their susceptibility to micrometeoroid impact. The nearearth space environment - where most satellites, the Shuttle, and the
International Space Station orbit the earth - is cluttered with man-made debris
and naturally occurring meteoroids. Hypervelocity impacts between spacecraft
and man-made debris or meteriods can lead to catastrophic failure. Because of
the potential of hypervelocity inpacts, there is an entire science devoted to
investigating this problem. The HITF facility is also used to develop passive
shielding techniques for spacecraft.
Hypervelocity impact testing has some extreme requirements such as
launching projectiles at speeds more than seven times faster than the fastest
bullet, measuring the speed of the projectile at impact and photographing the
impact that lasts only a few microseconds or millionths of a second. Two stage
light gas guns are used to accelerate projectiles to hypervelocity speeds on
Earth.
The first stage is a large diameter cylinder filled with compressed hydrogen
gas at 50 psi. The breech contains a powder charge, while the other end of the
cylinder, called the pump tube, is tapered. Inside the pump tube is a nylon piston;
when the powder charge is ignited, the piston is propelled down the pump tube to
the end of the first stage, compressing the hydrogen gas to extremely high
pressures.
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June 9, 2004 - 040609.doc
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The second stage of the light gas gun consists of the barrel, the flight
range, and the target chamber. The second stage is separated form the first
stage by a scored rupture disc. The projectile is located at the front of the barrel,
just downrange of the rupture disc. The pressure in the second stage, like space,
is near vacuum. When the pressure in the first stage becomes sufficiently high,
the rupture disc fails, releasing the hydrogen gas at hundreds of thousands of
pounds of pressure into the vacuum of the second stage. The rapidly expanding
hydrogen propels the projectile down the barrel to the target. With the high
pressure behind, and the vacuum in front, the projectile will reach velocities in the
neighborhood of 7 km/s before striking the target.
Lasers are used for measuring the velocity of the impacting projectile. Two
laser curtains are positioned a short distance uprange of the target. Electronics
attached to the laser system can detect disruptions in the uprange and
downrange beams as the projectile passes through them. Because the distance
between the laser curtains is known, the speed of the projectile can be calculated
from the elasped time between the disruption of the two laser beams.
Light detectors can also be used to measure projectile velocity because
light is generally emitted during a hypervelocity impact. Light is emitted when the
projectile impacts the target, and light is also emitted when the sabot pieces
impact the sabot stripper uprange. A sabot is a protective sheath that holds the
projectile. The projectile speed can be calculated from the distance and the
elasped time between the two light emissions.
© Celestial North, Inc. All rights reserved.
June 9, 2004 - 040609.doc
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The high speed cameras that are used at the HITF are Cordin High Speed
Shadowgraph Cameras. These cameras are capable of taking images at a rate
of 2.5 million per second. The film is fixed around around a circular housing. At
the center of the circle is a rotating mirror powered by a compressed gas turbine.
There is no shutter. Instead, the light source is a pulsed laser, timed to strike the
rotating mirror in such a way that it exposes one frame of film per pulse. Since
the film is stationary, each test is limited to only 80 frames of film. If you are
operating the camera at 1 million frames per second, that's 80 microseconds of
filming. Fortunately, that's plenty of time, since impacts last only a few
microseconds.
One of the main functions of the Hypervelocity Impact Technology Facility
is the development of advanced shielding concepts to protect spacecraft in orbit.
The goal is always to develop a shield that is effective and lightweight.
Spacecraft shield designers must work carefully to produce shielding solutions
which are within the allocated mass, volume, and cost budgets of the spacecraft.
There is no single method used to design and develop a spacecraft shield. Most
shield innovations are developed using a "let's see how well this works"
approach, coupled with some intensive HVI testing and analysis.
Hydrocodes are sophisticated computer programs used to run simulations
of hypervelocity impact events. In orbit, velocities well in excess of what can be
produced in the lab may be present. Hydrocodes are used to investigate impacts
beyond what is testable. Hydrocodes are also used as a visualization tool to help
understand the physics that occur during an impact.
© Celestial North, Inc. All rights reserved.
June 9, 2004 - 040609.doc
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We're on the web at celestialnorth.org. Until next time, this is
____________ and ____________, with a reminder that the night is very large
and full of wonders!
© Celestial North, Inc. All rights reserved.
June 9, 2004 - 040609.doc
REFERENCES:
http://spaceflight.nasa.gov/shuttle/support/engineering/
http://hitf.jsc.nasa.gov/hitfpub/main/index.html
© Celestial North, Inc. All rights reserved.
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