Hi,
Here is another update on Gravity Probe B. Gravity Proble B has been a little overshadow this week by the Cassini mission to Saturn.
#--------------------------------------------
=========================================
GRAVITY PROBE B MISSION UPDATE -- July 2, 2004
=========================================
Please Note: During the Initialization & Orbit Checkout (IOC) Phase
of the GP-B mission, we update our Web site and send out this email
update once a week (usually on Thursday or Friday) to keep you
apprised of our progress. From time to time, we may send out extra
updates, as warranted by mission events.
At a little over ten weeks into the mission, the spacecraft is in
excellent health, with all subsystems performing well. The
spacecraft's orbit remains stable, ready for the transition into the
Science Phase. All four gyros are digitally suspended and have
completed calibration testing at approximately 0.3 Hz (20 rpm) spin
rates. Two problematic micro thrusters, which were preventing the
spacecraft from sustaining a drag-free orbit, have been isolated, and
the thruster-control software was modified to optimize Attitude and
Translation Control system (ATC) functionality without them. Over the
past two weeks, the spacecraft's roll rate was increased from 0.3 rpm
to 0.9 rpm as part of the process of uniformly distributing and
balancing the mass of the spacecraft.
During this past week, we completed the mass trim procedure at 0.9
rpm, using movable weights on long screw shafts to alter the
spacecraft's center of mass from front to back and from side to side.
The mass trim operation is necessary to precisely align the
spacecraft's roll axis so that it passes through the centers of the
gyros and the telescope's line of sight. The mass trim procedure also
balances the mass of the spacecraft so that it rolls smoothly around
this axis.
Also, this past week, we decreased the spacecraft's roll rate
incrementally from 0.9 rpm back to 0.5 rpm. During the first
roll-down decrement to 0.7 rpm, we discovered that the distribution
of the liquid helium in the Dewar is less predictable during
roll-down than it is during roll-up. When the spacecraft's roll rate
is slowed too quickly, the liquid helium begins to slosh around. The
resulting displacement of the center of mass from the sloshing helium
affects the micro thrusters, resulting in a significant increase in
the time required to complete the roll-down.
Last weekend, in preparation for optimizing ATC performance with 14
instead of 16 micro thrusters, we uploaded revised drag-free
thruster-control software to the on-board computer. After completing
the mass trim maneuver and stabilizing the spacecraft at the 0.5 rpm
roll rate, we re-booted the on-board computer with the revised
software. The re-boot went very smoothly, and a checkout of the new
software confirms that the two problematic thrusters are isolated and
receiving no helium, while the remaining 14 thrusters are responding
to commands as expected. With the new software up and running, we
have begun successfully testing both primary and backup drag-free
modes around gyro #1.
Meanwhile, gyro #1 and gyro #3 are currently in the process of being
spun up to 3 Hz (300 rpm), and likewise, gyros #2 and #4 will be spun
up to 3 Hz next week. Over this weekend, the team will also re-lock
the science telescope on the guide star, IM Pegasi, with the
spacecraft rolling at 0.5 rpm and balanced along the telescope's axis
of sight. Before we begin calibration testing of the gyros at a spin
rate of 3 Hz next week, our goal is to have the spacecraft rolling
smoothly at 0.5 rpm, locked onto the guide star, and in a drag-free
orbit around one of the gyros.
This week, we are pleased to announce two welcome additions to our
GP-B Web site. First, we have added a Visual Tour of our spacecraft
and payload. You can access this tour by using the navigation menu
along the left edge of our Web pages. Choose the second item, The
Engineering Story, and from that sub-menu, choose Visual Tour, or
simply enter the following URL into your Web browser:
http://einstein.stanford.edu/content/vehicle_tour/index.htmlThe second addition to our Web site is a 1/20 scale, paper model of
the GP-B spacecraft that you can download as a PDF file, print out,
and assemble. There are two versions of the PDF file-a 12 MB
high-quality version and a 2 MB low-quality version. The only visible
difference between the versions is that the colors are not as bright
and saturated in the low-quality version, but it will download in
much less time for people with low-speed Internet connections. The
URL for these PDF files is:
http://einstein.stanford.edu/p_model.
Both versions include two pages of instructions and six pages of
images to cut out and assemble. You'll need scissors, an Exacto
knife, a straight edge, glue (glue sticks and hot glue guns work
well), Scotch tape, two 9.5" long, 1/8" diameter wooden dowels
(shish-kabob skewers work well) about 3-5 hours, and patience to
assemble the model. Kate Stephenson, a Stanford graduate in
Mechanical Engineering, created both the spacecraft visual tour and
the paper model. Over the past few years, Kate has worked on a number
of graphic design projects at GP-B, including our Web site.
The spacecraft is being controlled from the Gravity Probe B Mission
Operations Center, located here at Stanford University. The
Stanford-NASA/MSFC-Lockheed Martin operations team is continuing to
perform superbly.
Clarification of Centrifugal vs Centripetal Force from Last Week's Update
====================================================
Several subscribers emailed me to say that I should have used the
term centripetal force, rather than centrifugal force to describe the
bubble wrap process in last week's update--and they were correct. The
statement about bubble wrap should have read:
In the first procedure, called "bubble wrap," the spacecraft's roll
rate was increased in incremental steps, from 0.3 rpm to 0.9 rpm. The
increased roll rate begins to rotate the liquid helium, effectively
pushing it outwards as it tries to move in a straight line with its
inertia. The Dewar walls hold it in with a centripetal force. This
wraps the helium uniformly around the outer shell. Distributing the
liquid helium uniformly along the spacecraft's roll axis helps to
ensure that the science telescope can remain locked on the guide star
while the spacecraft is rolling.
The words centripetal and centrifugal are in fact antonyms defined as follows:
--centrifugal : tending to move away from a center.
--centripetal : tending to move toward a center.
#----------------------------------------------------
Regards,
LelandJ
