Interoffice Correspondence
Subject:
Chandra ISIM TV Test Temperatures and
Associated Data In Support of Decreasing
ACIS Focal Plane Temperature
To: Tan Trinh (Chandra FOT)
Date:
July 10, 2007
D255.2007-038
From:
Joe Vogrin
cc: Lee Harper; Pattie Geary
Rino Giordano
Perry Knollenberg
Neil Tice, LMA; Bill Till, MSFC
Paul Viens; Paul Plucinsky
Location/Phone
R4/1136
714-846-6713
References:
1) D255-2007-031, “Chandra ISIM Thermal Study to Lower ACIS Focal Plane Temperature”,
July 5, 2007, by Joe Vogrin
2) Email from Neil Tice, LMA: “ACIS Analysis Results”, June 26, 2007
3) Teleconference regarding ACIS Focal Plane Temperature on July 2, 2007.
4) Email from Paul Plucinsky, “Notes from Organizational Meeting for Turning off SIM FA6
and ACIS DH heaters”, July 5, 2007
5) Agreements following teleconference of Reference 3) between Tan Trinh and Joe
Vogrin on July 2, 2007.
6) AXAF.98.323.078, “ISIM Test Report for AXAF Observatory Level TV Test, 7/29/98,
by Joe Vogrin
7) AXAF.98.323.095, “ISIM Re-test Thermal Vacuum Test Report, 9/10/98, by Joe Vogrin
8) XKC98-0078CD, “Preliminary Report on AXAF Telescope Performance Measured During
Observatory-Level Thermal Test, 21 July 1998, by Keith Harvey and Nicolle Lynch
1.0 Introduction
The Chandra Flight Operations Team (FOT) and Science Instrument Teams are currently
investigating the possibility of de-activating some of the ISIM heaters in order to lower ACIS
focal plane temperature. This is because the focal plane temperature is beginning to warm
above it’s required temperature control range of -120 ± 1ºC . A study supporting this possibility is
documented in Reference 1). The selected heaters to be de-activated are the ACIS detector
housing operational heater and the SIM Focus Assembly FA6 heater. The Reference 1)
document indicates that if these heaters are de-activated:
- an improvement in ACIS focal plane temperature of about 3ºC can be attained
- the SIM Focus Assembly components and structures will be safely above their
minimum allowable temperatures
- the ACIS detector housing survival heaters will maintain the housing temperature
above the survival limits (concurred to by LMA thermal analysis in Reference 2).
In the teleconference of Reference 3), several concerns were identified that needed to be
investigated prior to implementing the de-activation of the ACIS detector housing and FA6
heaters. Action items were assigned in the follow-up email of Reference 4) and in the
agreements of Reference 5). The primary actions assigned to the author of this report are as
follows:
1. Review the Observatory TV test and summarize temperature exposures and
time of exposures for the following items:
- ACIS detector housing and Fid Lights during cold survival test conditions
- SIM Focus Assembly during cold conditions
- Telescope OBA at the interface with the ISIM during cold conditions
2. Review Observatory level testing and any previous ISIM testing at Ball Aerospace
to determine the SIM heater failure history. Specifically, did any failures occur on
any of the SIM operational heaters that the were being compensated for by the use
of the FA6 abort heater.
2.0 Summary and Conclusions:
A review of the Chandra Observatory level testing, as documented in Reference 6), indicated
that 2 failures occurred during this test. The 2 failures were:
1. Failure of the ACIS door to open
2. Failure of two SIM abort heaters: FA6 and TSC3, and one survival heater, FA3. No
operational (e.g. trim or gradient) heater failures occurred.
Subsequent to implementing design fixes, a re-test of the ISIM was performed, as documented
in Reference 7). The data presented herein covers both tests.
ACIS Detector Housing and Fid Light Temperatures: The ACIS detector housing, in which
the Fid Lights are located, experienced minimum temperatures of -73.1ºC when the detector
housing primary survival heater was operating, and -75.6ºC when the detector housing
redundant survival heater was operating. These temperatures were measured at the detector
housing flight telemetry temperature location: 1CBAT. These temperatures were experienced for
171.5 hours when the primary survival heater was active, and for 9.5 hours when the redundant
survival heater was active. Thus, the detector housing and Fid Lights have experienced
considerable test time at the same temperatures that will be experienced on-orbit if the detector
housing heater is de-activated.
SIM Focus Assembly: During TV testing, the SIM Focus Assembly components and structure
experienced temperatures considerably colder at all telemetry locations than will be experienced
on-orbit if the FA6 heater is de-activated. Depending on the telemetry location, the minimum
temperatures experienced in the test were as much as 12 to 20ºC colder than are predicted to
occur on-orbit. The total accumulative time that the temperatures in the tests were colder than the
predicted on-orbit temperatures range from 206 to 403 hours depending on the telemetry location.
Thus, the SIM Focus Assembly components and structure have experienced considerable test
time at temperatures colder than will be experienced on-orbit if the FA6 heater is de-activated.
OBA Tempertures: The thermal verification tests that were conducted at the Observatory level
were intentionally very conservative to ensure a robust thermal design. For the cold operational
case, the environmental heating was assumed to be zero (e.g. no solar heating) and the FA6
heater was off. Two tests were conducted, one with ACIS in the viewing position, and one with
HRC in the viewing position. During these tests, the OBA met all thermal control requirements at
the OBA to ISIM interface as documented in Reference 8). As a result, it can be concluded that
the OBA has already demonstrated that it can meet operational temperature control
requirements with the FA6 heater off.
SIM Heater Failure History: A review of the observatory level TV test, the ISIM re-test, and the
previous ISIM testing at Ball Aerospace indicated that no failures to operational heaters (e.g. trim
or gradient) heaters occurred that resulted in the FA6 heater being used as compensation.
During the TV testing at Ball, however, the focus assembly ran colder than predicted and HRC
raised a concern that it would result in HRC detectors fal
ling below the minimum allowable survival temperature of -15ºC when HRC was off. There is no
problem for HRC when the instrument is operational or in standby because heaters on the
detector housing would maintain detector temperatures safely above the minimum temperature.
Rather than make a design change to add heaters, it was decided to use the FA6 abort heater,
which previously would not be active after Chandra left the space shuttle, to heat the focus
assembly. In order to guard against a condition when HRC would accidentally be turned off, it
was decided that the FA6 heater would always be active regardless if HRC was on or off. The
Chandra FOT has indicated that it is no longer possible to accidentally turn off HRC, and, thus,
there is no reason to have the FA6 heater activated to protect the HRC detectors.
During the thermal verification testing at the Observatory level, the cold operational test was run
with the FA6 heaters off and with no solar heating, a very conservative test. The minimum
measured temperatures for the HRC detectors during these tests was +3ºC, safely above the
-15ºC requirement. Thus, it has been conservatively demonstrated in test that the HRC
instrument will not have any thermal problems if the FA6 heater is turned off and the HRC
instrument is in the operational or standby mode.
As indicated earlier, there were failures to some SIM abort and survival heaters, including FA6,
at the Observatory level. Fixes were incorporated and were verified in the subsequent re-test of
the ISIM. A further description of this problem is discussed later in this report.
3.0 Discussion
A thorough review of test history at TRW (NGST) during Chandra Observatory testing and
during the subsequent re-test on the ISIM, yielded the information provided in the following
sections.
3.1 ACIS Detector Housing and Fid Lights TV Test Exposures at Survival Temperatures
The detector primary and redundant survival heaters were verified as to aliveness and “turn-on
temperature” during both of the initial Chandra Observatory TV test and the subsequent ISIM retest. The turn-on temperatures, as measured at the flight telemetry temperature location,
1CBAT, was -73.1ºC for the primary heater, and -75.6ºC for the redundant heater as
documented in Table E-5 (page 3) of References 6) and 7). These temperatures are slightly
higher than the specified nominal turn-on temperatures for the heaters which are -71.1ºC and
-73.6ºC. The higher than nominal can be attributed to the fact that the detector housing flight
telemetry, 1CBAT, has a 2.5ºC per count interval. The really important information, however, is
the fact that the detector housing and the Fid Lights underwent considerable time during the TV
test at survival temperatures, as is demonstrated in the following table. This data was obtained
by review of the test temperature/time plot history.
Date
Test Description
6/08/98
6/09/98
6/10/98
8/17/98
8/18/98
8/19/98
8/20/98
8/21/98
8/22/98
Observatory TV Test
Same as above
Same as above
ISIM TV Re-test
ISIM TV Re-test
ISIM TV Re-test
ISIM TV Re-test
ISIM TV Re-test
ISIM TV Re-test
Total Time
Time on Primary
Survival Heaters
at ~73.1ºC as
Measured by
1CBAT (Hours)
13
23.5
2
17
24
24
24
24
20
171.5
Time on Red.
Survival Heaters
at ~75.6ºC as
Measured by
1CBAT (Hours)
3
0.5
0
6
0
0
0
0
0
9.5
Total Time
at Survival
Temperatures
(Hours)
16
24
2
23
24
24
24
24
20
181
Thus, as demonstrated by the above data, the detector housing and Fid Lights have experienced
considerable test time at the same temperatures that will be experienced on-orbit if the detector
housing heater is de-activated.
3.2 SIM Focus Assembly Temperature Exposure During TV Testing
The minimum predicted temperatures that are expected to occur on orbit on the focus assembly
components and structure if the FA6 heaters are turned off are shown in the following table (see
Reference 1), Tables 5-2, 5-4, 5-6, 5-8).
SIM Focus Assembly Minimum Temperature Predictions for FA6
and ACIS Detector Housing Heaters Off
Location
MSID
Allowable Operating
Temperature Range
(ºC)
SIM Trans Drive Motor A
SIM Foc Assy Drive Mtr A
SIM Flexure A
SIM Flexure B
SIM Flexure C
SIM SEA A
SIM Foc Assy at Brg. B
SIM Foc Assy at Brg. C
SIM FA Struct, +X at -Y-Z
SIM FA Struct, +X at +Y+Z
SIM FA Struct at Rail, -Z
3TRMTRAT
3FAMTRAT
3FAFLAAT
3FAFLBAT
3FAFLCAT
3FASEAAT
3FABRAAT
3FABRCAT
3FAMYZAT
3FAPYZAT
3FARALAT
-50 to +50 (60) (A)
-50 to +50
-40 to +95
-40 to +95
-40 to +95
-20 to +40
-50 to +95
-50 to +95
-50 to +95
-50 to +95
-50 to +95
Minimum Predicted
Temperature for
all Pitch Angles
(ºC)
-28.4
-4.4
-8.9
-10.5
-4.1
+12.7
-9.7
-7
-9.2
-8.7
-10.7
Notes: (A) 60 is the current red line limit for the motor per “Chandra Thermal
Subsystem Status, Chandra FOT”, 10/2005, pages 37-39.
The minimum temperatures and the “times at temperature” experienced by the SIM Focus
Assembly Translation Drive Motor during testing is as follows:
Date
5/27/98
6/08/98
6/09/98
6/10/98
6/11/98
6/14/98
6/15/98
6/16/98
6/17/98
6/18/98
6/19/98
8/16/98
8/17/98
8/18/98
8/21/98
8/22/98
TV Test
Time 3TRMTRAT Time 3TRMTRAT Total Time 3TRMTRAT
Description
was between
was between
was below it’s Minimum
-40 to -50ºC
-30 to -40ºC
Predicted Temperature
(Hours)
(Hours)
of -28.4 ºC (Hours)
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Re-test
Re-test
Re-test
Re-test
Re-test
Total Time
4
6
24
5
5
0
0
0
0
3
9
0
12
20
0
0
88
1
4
0
2
6
19
18
21
24
21
8
11
12
2
24
22
195
5
10
24
7
11
19
18
21
24
24
17
11
24
22
24
22
283
The minimum temperatures and the “times at temperature” experienced by the SIM Focus
Assembly Focus Drive Motor, the 3 Flexures, and the SEA: are as follows:
Date
5/27/98
6/08/98
6/09/98
6/10/98
6/11/98
6/14/98
6/15/98
6/16/98
6/17/98
6/18/98
6/19/98
8/16/98
8/17/98
8/18/98
8/20/98
8/21/98
8/22/98
TV Test
Time Focus Motor Time Flexures
Time SEA
Description
was below it’s
were below their was below it’s
Minimum
Minimum
Minimum
Pred. Temp of
Pred. Temp of
Pred. Temp of
-4.4ºC (Hours)
-10.5ºC (Hours) +12.7ºC (Hours)
Initial Test
8
8
7
Initial Test
9
14
16
Initial Test
24
24
24
Initial Test
6
2
5
Initial Test
10
8
0
Initial Test
21
0
9
Initial Test
17
0
0
Initial Test
23
0
21
Initial Test
24
0
10
Initial Test
21
10
24
Initial Test
19
10
1
Re-test
12
12
12
Re-test
24
24
24
Re-test
24
24
24
Re-test
24
24
24
Re-test
24
24
24
Re-test
22
22
22
Total Time
312
206
247
The minimum temperatures and the “times at temperature” experienced by the SIM Focus
Assembly Bearings, Rails and Structure: are as follows:
Date
TV Test
Description
5/26/98
5/27/98
6/08/98
6/09/98
6/10/98
6/11/98
6/12/98
6/14/98
6/15/98
6/16/98
6/17/98
6/18/98
6/19/98
8/16/98
8/17/98
8/18/98
8/20/98
8/21/98
8/22/98
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Initial Test
Re-test
Re-test
Re-test
Re-test
Re-test
Re-test
Total Time
Time
3FARALAT
was below it’s
Minimum
Pred. Temp of
-10.7ºC (Hours)
22
24
19
24
3
19
24
24
24
24
24
23
19
17
24
22
0
24
20
380
Time
3FABRCAT
was below it’s
Minimum
Pred. Temp of
-7ºC (Hours)
19
24
19
24
2
18
24
24
24
24
24
23
19
17
24
22
24
24
24
403
Time
3FAMYZAT
was below it’s
Minimum
Pred. Temp of
-9.2ºC (Hours)
19
23
17
24
1
18
24
24
24
24
24
23
7
17
24
22
0
0
0
315
Time
3FAPYZAT
was below it’s
Minimum
Pred. Temp of
-8.7ºC (Hours)
24
20
18
24
2
18
24
24
24
24
24
23
7
17
24
22
0
0
0
319
Time
3FABRAAT
was below it’s
Minimum
Pred. Temp of
-9.7ºC (Hours)
17
21
16
23
1
18
24
24
24
24
24
23
14
17
24
22
24
24
24
388
The above results for all focus assembly telemetry locations are summarized in the table below:
Location
MSID
Allowable
Operating
Temperature
Range (ºC)
Minimum
Predicted
Temperature
For all Pitch
Angles for
FA6 Heaters
Off (ºC)
SIM Trans Drive Motor A
SIM Foc Assy Drive Mtr A
SIM Flexure A
SIM Flexure B
SIM Flexure C
SIM SEA A
SIM Foc Assy at Brg. B
SIM Foc Assy at Brg. C
SIM FA Struct, +X at -Y-Z
SIM FA Struct, +X at +Y+Z
SIM FA Struct at Rail, -Z
3TRMTRAT
3FAMTRAT
3FAFLAAT
3FAFLBAT
3FAFLCAT
3FASEAAT
3FABRAAT
3FABRCAT
3FAMYZAT
3FAPYZAT
3FARALAT
-50 to +50 (60) (A)
-50 to +50
-40 to +95
-40 to +95
-40 to +95
-20 to +40
-50 to +95
-50 to +95
-50 to +95
-50 to +95
-50 to +95
-28.4
-4.4
-8.9
-10.5
-4.1
+12.7
-9.7
-7
-9.2
-8.7
-10.7
Time this
Telemetry
Location was
below it’s
Minimum
Predicted
Temperature
During TV
Testing
(Hours)
283
247
206
206
206
247
388
403
315
319
380
Notes: (A) 60 is the current red line limit for the motor per “Chandra Thermal
Subsystem Status, Chandra FOT”, 10/2005, pages 37-39.
Minimum
Temperature
Experienced
by this
Telemetry
Location
During TV
Testing (ºC)
-48
-21
-22
-23
-26
-13
-22
-37
-33
-22
-44
Based on the above summary, it is seen that that the focus assembly experienced temperatures
considerably colder (as much as 12 to 20ºC) during testing than will occur on-orbit with the FA6
heater commanded off. In addition, during the testing, the focus assembly experienced
temperatures below that which will occur on-orbit for a considerable period of time (≥206 hours).
It is, therefore, concluded that, with respect to the focus assembly, it is acceptable to command
off the FA6 heater on-orbit.
Further protection of the focus assembly temperatures, is provided by the fact that the primary
and redundant FA3 survival heaters are always enabled (see SIM-C-004).
3.3 Effect on Telescope OBA Temperature Control if FA6 Heater is Turned Off
The thermal design verification testing that was performed during the Observatory level TV test
was intentionally very conservative to ensure a robust thermal design. In order to avoid running
multiple tests, the environmental simulation used in the testing was conservative in that the hot
case environments assumed the sun was located at 90º pitch angle for the telescope and 180º
pitch angle for the ISIM. For the cold case, zero solar heating was assumed for both the
telescope and the ISIM. In addition, during cold case testing it was assumed that the FA6 heater
was off. Two separate cold case tests were run, once with ACIS in the viewing position and one
with HRC in the viewing position. During these tests, the OBA met all thermal control
requirements at the OBA to ISIM interface as documented in Reference 8). As a result, it can be
concluded that the OBA has already demonstrated that it can meet operational temperature
control requirements with the FA6 heater off.
3.4 SIM Heater Failure History
During TV testing, as documented in the test report of Reference 6), heater anomalies were
noted for 3 ISIM heater circuits. The affected heaters were as follows:
- FA3 survival heater: One heater element was mis-wired into the FA6 heater circuit
- FA6 prime and redundant abort heaters: Burn-out or discoloration of 3 of the 7 heater
patches in each circuit occurred due to high watt densities at high voltage (~32.5
volts). Both circuits were enabled simultaneously. Primary and redundant heater
circuitry are contained in each heater patch, thus doubling the watt density in each
patch.
-TSC3 abort heater: Burn-out of one heater patch occurred due to high watt density.
Only the primary heater was enabled.
Fixes were made to all of the failed heater circuits, as appropriate. All heater circuits were reverified in the ISIM re-test of Reference 7). Mission constraints were established to prohibit the
primary and redundant FA6 and TSC3 heater circuits from being enabled at the same time (see
SIM-C-004).
It should be noted that there were no failures of any of the “operational” (e.g. other than abort)
heaters during the TV testing at the Observatory level or during earlier TV testing of the SIM at
Ball Aerospace. The SIM did run colder than anticipated during the Ball testing and the solution
was to activate the FA6 abort heater during on-orbit operational conditions to preclude the HRC
detectors, which have a survival limit of -15ºC, from falling below this temperature when HRC is
in the “off” condition. According to Chandra FOT (Tan Trinh), it is not possible for HRC to be
accidentally turned off, thus the requirement for the FA6 heater to be enabled to protect HRC is
no longer required.
It should also be noted that during the thermal design verification testing of Reference 6), the
FA6 heater was disabled for the cold verification test. The minimum temperature for the HRC
detectors was +3ºC, an 18ºC margin relative to the detector cold limit of -15ºC. The test was
conducted for both HRC and ACIS in the viewing position. Thus, it can safely be said that it has
been demonstrated that HRC minimum temperatures can be safely maintained on-orbit if the
FA6 heater is turned off provided HRC is either in the operational or standby modes.
It should further be noted that the constraint SIM-L-006 allows for turning off of the FA6 heater if
the temperature at the ACIS collimator attachment to the translation table (1DACTAT,
1DACTTB) exceeds 0ºC provided that HRC is operating.
In addition to Reference 6), the SIM heater failure history was obtained by reviewing the
following additional references:
9) AXAF.97-323.059, “Test Plan for ISIM TV Test”, July 9, 1997, by Joe Vogrin
10) AXAF 97.323.069, “ ISIM Heater Verification Methodology and Heater Enable/Disable
Status During TV Testing at TRW”, August 11, 1997, by Joe Vogrin
11) AXAF 97.323.94, “ISIM Focus Assembly Heater Power”, December 12, 1997, by Joe
Vogrin
12) AXAF 97.323.097A, “ISIM Heater Power Usage, Revision A”, December 22, 1997, by
Joe Vogrin
13) AXAF 98.323.007, “Summary Test Report for ISIM Cold Thermal Balance Re-test”,
January 30, 1998, by Joe Vogrin