ACE Weekly 11/19/2014 - 11/25/2014
ACE spacecraft subsystems are performing as expected.
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Orbit/Attitude:
Type Attitude
Date 11/25/2014
DOY 329 2014
Thrusters 2R 4R+ 4R-
Duration 7:56 min
Start 16:34:07z
Stop 16:42:03z
ra,decStart 247.22,-9.66
ra,decStop 254.61,-9.65
ra,decExpect 254.93,-9.78
Deviation 0.35 deg (-4% short)
HGAStart -9.16 deg
HGAStop +9.04 deg
SunStart 12.67 deg
SunStop 17.24 deg
SpinStart 5.0038 rpm
SpinStop 5.0043 rpm
Nutation 0.17 deg
Firing 40 pulses
FuelUsed 0.1274 lbm
FuelRemain 102.4844 lbm
FinalSCMass 1336.745 lbm
The next attitude maneuver and Station Keeping #72 are scheduled for
Tuesday 12/02/2014.
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OCRs:
None
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Activities:
Data Capture: 99.994% DOY 320-327 2014
DOY 322 16:20:41-16:20:44 4 second gap covered by ADC
DOY 327 08:54:52-08:55:21 30 second gap during SSR failover
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Anomalies:
Update for S13-0001 - B1/B2 Fuel Tank Temperature Anomaly
Others (APL, AETD, SSMO) have proposed that the Fuel Tank Temperature
Anomaly is the result of the "B" fuel tanks being empty. Given that the
B2 fuel tank temperature is now fluctuating (~2C) along with B1 (~4C),
the empty tank theory begins to seem more plausible than the detached
thermostat theory. Some of the issues with which to fit the theory are
provided below.
* The B1 fuel tank temperature did not increase continuously. There
were 3 distinct increases in the B1 temperature:
09/19/2013 18.4C to 19.5C spin increases 0.01%
11/05/2013 to 20.9C during maneuver
12/22/2013 to 21.2C fuel tank heaters start turning off
* For the spacecraft to spin up by 0.01% (9/19/2013), the moment of
inertia has to decrease by 0.01% (e.g. from 450kg*m^2 to 449.955kg*m^2).
Fuel emptying from one tank (B1) to a fuller tank (e.g. B2) would
decrease the moment of inertia by causing the fuel level in the fuller
tank to be closer to the center of rotation. With ~50kg of fuel in
Sept. 2013, each tank has ~12.5kg. The following scenarios (depending
on the tank volume at a particular fuel level) could decrease the moment
of inertia by 0.045kg*m^2:
1 kg at 0.8m moves inward by 2.9cm (to 0.771m)
5 kg at 0.8m moves inward by 0.6cm (to 0.794m)
10 kg at 0.8m moves inward by 0.3cm (to 0.797m)
Could differences in pressure and temperatures be enough to cause fuel
to empty from one tank into a fuller tank?
* There is ~0.075kg of fuel in each of four the internal lines (0.3kg
total). ACE's average fuel usage is 0.3kg/month. There were 8 to 5
months from the increase in the B1 fuel tank temperature (Sep-Dec 2013)
to maneuver underperformance (May 2014) believed to be due to bubbles in
the line.
* Could pressurant gas (nitrogen) flow from B1 fuel tank while the B2
tank still contained fuel?
Analysis of the fuel system continues. This is just reporting the
current state of thinking. The A fuel tank temperatures remain normal.
Correction on the Tank B2 temperature increase and -X External Fuel Line
temperature decrease during maneuvers on 11/5/2014 and 11/12/2014.
Investigation has found that the -X External Fuel Line has a history of
decreasing during maneuvers (this fuel line feeds the 2R thruster).
Therefore, the increase in the B2 temperature during the two maneuvers
is almost certainly not related to the -X External Fuel Line
temperature.