ACE Weekly 02/11/2015 - 02/18/2015

ACE spacecraft subsystems are performing as expected.  See below for the
discussion of maneuver performance.

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Orbit/Attitude:

	Type           Attitude       Attitude        SK-73
	Date          02/18/2015     02/18/2015     02/18/2015
	DOY            049 2015       049 2015       049 2015
	Thrusters      2R 4R+ 4R-     2R 4R+ 4R-      3A 4A
	Duration       14:51 min       7:39 min      63.406sec
	Start          20:30:26z      21:12:25z      21:57:07z
	Stop           20:45:17z      21:20:04z      21:58:10z
	ra,decStart   315.28,-19.48  321.73,-23.67  325.92,-24.81
	ra,decStop    321.73,-23.67  325.92,-24.81  326.77,-24.71 
	ra,decExpect  329.82,-24.68  329.82,-24.68
	Deviation     7.45deg(-49%)  3.54deg(-47%)   -40% cold
	HGAStart       -8.20 deg      -6.38 deg      +7.53 deg
	HGAStop        -6.34 deg      +7.45 deg      +7.29 deg
	SunStart       17.77 deg      15.53 deg      14.46 deg
	SunStop        15.50 deg      14.44 deg      14.02 deg
	SpinStart     5.0015 rpm     5.0036 rpm     5.0064 rpm
	SpinStop      5.0036 rpm     5.0064 rpm     4.9967 rpm
	Nutation        0.05 deg       0.04 deg       1.18 deg
	Firing         75 pulses      39 pulses      continuous
	FuelUsed        0.2250 lbm     0.1246 lbm     0.2104 lbm
	FuelRemain    100.8504 lbm   100.7260 lbm   100.5156 lbm
	FinalSCMass   1335.111 lbm   1334.986 lbm   1334.776 lbm

Every ~3 months (for about a month in duration) the maneuvers do not
reach their targets.

	05/13/2014 - 05/27/2014   -7% to -12%
	07/08/2014 - 08/26/2014   -7% to -16%
	10/28/2014 - 11/12/2014  -11% to -20%
	02/04/2014 - ongoing      -7% to -49%

We believe that the nitrogen from the empty B tanks fills the internal
lines over the ~2 months as the fuel in the external lines is slowly
used.  When the nitrogen reaches the thrusters, the nitrogen is used
instead of fuel resulting in underperformance.

With the release of nitrogen over a ~month, the pressure will eventually
be reduced and fuel from the A tanks backfills into the B internal
lines.  Then for the ~2 months, the maneuvers are closer to reaching
their expected target.

This week's maneuvers have been significantly short of their target.
This is the first instance of nitrogen being expelled since the B2 tank
completely emptied in December 2014 and is not completely unexpected.

Assuming that the original ratio of nitrogen distribution between the A
and B tanks is 40/60, then we estimate that we've expelled ~2% of the
nitrogen pressurant over the past year resulting in a ~40/58 ratio.
(Brief explanation: If the excess nitrogen in the B tanks was completely
expelled, the system pressure would drop ~20psi.  We are currently about
2psi below the expected pressure and therefore estimating 10% towards
having equal amounts of nitrogen between the A and B tanks.)

The plan is to allow the nitrogen still in the internal lines to be
purged over the next 1-3 maneuvers.  Once that nitrogen is purged, we
will see if the fuel backfills all of the way back to the B tanks which
would cause a change in the B tank temperature.  If the fuel does
backfill to the B tanks, some consideration will be given to allowing
the process to continue until the fuel is evenly distributed across the
4 fuel tanks.

If the fuel from the A tanks does not backfill to the B tanks (probably
the more likely outcome), then it is likely that we'll close the B latch
valves in order to return to consistently nominal maneuver performance. 

The next attitude maneuver will be Tuesday 2/24/2015.  FDF will plan
another station-keeping maneuver in the near future.

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OCRs:

None

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Activities:

Data Capture:  99.98%  DOY 039-046 2015
   DOY 044 17:22:07-17:24:13  127 second gap due to a partial slice

Last week had limited antenna time due to the DSCOVR launch on Wednesday
2/11/2015 (DOY 042).  There is a known issue with the ground system
redump calculator where a partially dumped slice is not redumped during
short passes.  This occurs rarely and the dump list had been fixed
manually in the past.  This week the dump list was not manually updated
and a 127 second gap resulted.

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Anomalies:

None