ACE Weekly 02/11/2015 - 02/18/2015 ACE spacecraft subsystems are performing as expected. See below for the discussion of maneuver performance. ======================================================================== 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. ======================================================================== OCRs: None ======================================================================== 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. ======================================================================== Anomalies: None