Here's the exciting conclusion. I've left out, for now, the reference list.
I've also, once again, left out the pictorial data, but will summarize the
graph of penetration rates in Figure 9.
________________________________
5. RESULTS OF CRYOBOT PERFORMANCE TESTING
As stated in Section 4. the focus of the FY 2000 research and development
effort was to derive and validate an accurate model of the fluid dynamic and
heat transfer processes. Once validated, the model would be used to optimize
the vehicle design. Indeed. this was the first major accomplishment of the
research task. Figure 9 shows the projected penetration rates for a 1 kW
thermal probe in different temperature ice, and also displays the corrected
value for likely penetration rates in Europan ice based on empirical test
data (i.e., note that the 0.5 meters/hr rate was corrected to 0.3
meters/hr). Most importantly, the vehicle dynamic modeling accurately
predicted melt performance for both passive and active phase-change
processes.
Using a prototype probe of known geometry (particularly frontal area
equivalent to the model; i.e., 12 cm), ice with known properties (both ice
structure and temperature; i.e., -10 deg C), and known energy input (i.e.,
0.6 to 0.8 kW), and known water jet temperatures/flow rates (i.e., 25 deg C
at the jet outlet and 1 liter/minute respectively), the team was able to
test and validate the predictions of the fluid and heat transfer models and
obtain melt rates of 0.5 to 1 meter/hour.
The model and lab tests were used to establish the probe power requirements,
vehicle size (diameter and length), and functionality. The prototype probe
was tested with a split-nose, two-heater configuration. Similar to the
four-quadrant nose design, the split nose used a ceramic fin to split the
two hemispheres and prevent heat transfer between hemispheres during heater
switching and steering. The water jet nozzle was inserted in the ceramic web
along the axial centerline of the nose to accommodate the active melting
subsystem. It should be stated up front that the custom heaters designed to
provide 250 W thermal had not been delivered yet by the vendor. Therefore.
off-the-shelf standard heaters were employed and run at lower power to
prevent burnout. The actual probe that will accommodate the full suite of
acoustic sensors, hightemperature custom passive heaters, pump/reservoir
motor, instruments, electronics, and tether is shown in Figure 11 [not
shown].
The primary test of the complete system was to melt through a 5-meter ice
column. Figure 12 shows the ice tower and probe melting through the ice
column. The test results are summarized in Table 2.
______________________________________
Summary of Figure 9 -- Ice penetration as a function of temperature and
pressure
(for 1 kilowatt thermal input):
Place Temperature Drill speed
(deg C) (meters/hour)
Earth glacier - 15 1.3
Vostok,
Antarctica - 55 .9
Mars - 120 .5
Europa - 170 .3
[Note from Moomaw: The very low listed temperature of Europan ice applies
only to the upper few km. As you descend, the ice gets steadily warmer.
Below the "conductive" shell of very cold, rigid, non-convecting ice --
which is only a few km thick in all models -- the very slow convective
churning of the "ductile" ice that makes up most of Europa's ice crust may
make the temperature pretty close to 0 deg C all the rest of the way down.]
______________________________________
Table 2 -- Summary Results of Cryobot
Performance Tests:
Test Parameter Results/Observations
Total melt distance 5 meters
(plus 1 later short melt of 3
meters)
Total elapsed melt time 11.2 hrs
Average power 418 W (range: 240-536 W)
Average descent rate 43.4 cm/hour
(range: 34-57 cm/hour)
Passive melt rate 34.5 cm/hour
Active melt rate 60 cm/hour
(at water-jet temp of 30 deg C
and 1 liter/minute flow)
_____________________________
Vehicle attitude control Made 2 planned attitude corrections
Correction #1: Corrected 3 degrees off-vertical,
moving ~10 deg in opposite
direction
Correction #2: Corrected 7degree over-shoot
back to vertical
Vertical travel
during correction:
Correction #1 28 cm
Correction #2 8 cm
_________________________________
Variable ice conditions:
Firn ice (low density
passive rate): 60 cm/hour
Firn Sediment ice
(1-10 micron size
particles,
5% volume)
passive rate: 60 cm/hour
(NOTE: sediment remained suspended and did not appear to adhere to nose of
probe and cause heat transfer barrier)
Clear ice (high density)
passive & active rates: 60 cm/hour
(ice at 0 deg C)
Observed correction dynamics:
Heater element actuated was on side of adverse tilt. This allowed the melt
cavity on that side to advance and exceed the diameter of the nose. At that
time, the melt regime was able to initiate heat convection along the leading
edge of the shell and allow the melt regime to increase in volume and slowly
creep up along the shell. This convection process broke down the ice
barrier on the side of adverse tilt, and so the vehicle was able to follow
its gravity vector and slowly move back towards vertical.
[Note from Moomaw: observe the remarkably small downward movement needed for
the probe to veer onto a significantly new course. This thing has more
steering ability than I thought.]
6. CONCLUSIONS
In conclusion, the FY 2000 research not only developed and validated the
fluid dynamic and heat transfer models, but the team was successful in
designing, building, and testing the first prototype cryobot system. The
test results were of particular importance in that inroads were made into
understanding the dynamics of steering, as well as the importance of water
jet vortices in the transfer of heat to the melt front and debris removal.
The prototype probe will also be tested over a range of short melts in
Antarctica to obtain melt rate data in actual pack-ice (testing to be done
by Dr. H. Englehardt, California Institute of Technology, Department of
Geophysics). Current plans for FY '01 include continued modeling and testing
of sediment-laden ice. The team will also complete the assembly of the full
probe system (i.e., the active melt subsystem. full suite of vehicle
state/control sensors. state assessment/sequencer micro-controller). The
acoustic imaging research will be initiated in partnership with an industry
partner, as will the tether design. FY '02 research and development will
include the final integration/ test of the acoustic imaging system, tether,
and a suite of two science instruments, followed by performance of a deep
descent (100 to 300 meters) in actual pack (dirty) ice.
[Note from Moomaw: These tests were actually carried out in a Scandinavian
glacier in 2001 -- an event briefly described in an AGU meeting abstract.
I'll print it shortly, and see what else I can dig up on the test results.]
==
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