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HST MOSES
TITLE

MISSION OPERATIONS, SYSTEMS ENGINEERING, & SOFTWARE PROGRAM

ENGINEERING MEMORANDUM
EM: MOSES-1268 DATE: December 2, 2005

Light Shield Baffle Temperature Predictions for One Gyro Science Operations Mode AUTHORS: /s/ Andrew M. Wong PHONE: 301-901-6172 ORGN: 46-12

REVISED: APPROVALS: J. Abel /s/ L. Dunham /s/ T. Cruz /s/

SUMMARY Thermal analysis was conducted to assess forward Sun Angles (SA) and their effect on HST light shield (LS) baffle temperatures. The analysis results were used to provide attitude constraints for One Gyro Science (OGS) operations mode. Pointing Control Subsystem (PCS) studies indicate OGS can result in a maximum Large Attitude Uncertainty (LAU) of 15°. The LS baffle maximum allowable temperature is limited by the bake-out temperature. Given these goals and requirements, the recommendation is that commanded attitude shall be constrained to 65° SA and 5° Off Nominal Roll (ONR), with occasional allowance for 61.5° SA when Science Mission Scheduling (SMS) flexibility is required. BACKGROUND, ASSESSMENT CRITERIA AND ANALYSIS ASSUMPTIONS The Venus Observations analysis model was used to predict the temperature results. Reference 1 describes the details of the thermal model and the earlier model sensitivity assessment. Table 1 lists changes made to the model for the present analysis. The low earth orbit (LEO) thermal environmental constants were changed from nominal values to 3 values per Reference 2 (more conservative). LS and baffle solid thermal conduction was assumed for the present model. Note that the original model only considered conduction for the sensitivity assessment, but excluded that assumption from the final results. The present model removes that conservatism.

Table 1. Thermal Model Changes from Reference 1.
Analysis Factor HST Environment Original Model Solar [W/m2] 1397 LS Conduction Baffle Conduction Albedo 0.38 None None Earth [W/m2] 241 Present Model Solar [W/m2] 1419 Albedo 0.35 Earth [W/m2] 265 -6 to -16 °C
1

Approximate Impact [°C] +1 to +2 °C

0.813 mm thick Mg (no longitudinal stiffeners) 0.381 mm thick Al

1. The temperature reduction is dependent upon the amount of area being impacted by solar heating. For angles resulting in a larger heated area, the temperature reduction gained by including conduction is decreased.

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EM: MOSES-1268 December 2, 2005

The criteria for assessing the OGS operations mode are similar to those for the Two Gyro Science (TGS) operations mode documented in Reference 3. Maintaining the black painted baffle temperature below the outgassing concern limit of 148.9 °C (300 °F) was the major thermal criteria [Reference 4]. This temperature is based on protecting the HST primary and secondary mirrors in the forward shell (immediately aft of the LS) from excessive outgassing due to high paint temperatures. With consideration of thermal modeling uncertainties and standard temperature margin practices, as well as consistency with TGS analysis, the maximum allowable transient temperature was selected to be 142.2 °C (288 °F) [Reference 3]. The PCS LAU of 15° represents the resultant error from a commanded attitude. Representing HST attitude as SA and ONR, the combined resultant uncertainty or error () is specified by the following vector equation:



LAU

=(



SA

2

+



ONR

2

)

Ѕ

(1)

Per Reference 3, the commanded ONR was limited to ±5° for the forward SA under consideration (less than 90°). RESULTS Figure 1 shows the maximum temperature predictions as a function of SA and ONR with LAU when the pointing is commanded to 61.5 SA and 0 to 5 ONR (0 to -5 ONR was not considered due to symmetry). The shaded rectangular area shows the allowable commanded pointing attitude region in terms of SA and ONR; although the region extends vertically upward in the figure to 90° SA, the higher SAs do not pose a threat because no solar flux enters the HST boresight. The resultant attitude curve represents the worst-case pointing attitude with the addition of the maximum LAU per Equation 1. Transient temperature predictions were performed at each resultant attitude. The temperature prediction curve of Figure 1 shows the maximum baffle temperatures. Figure 2 shows the location of the baffle hot spot (142 °C or 288 °F) just before eclipse entry for the worst resultant attitude of 46.6° SA and 7° ONR. Figure 2 also shows the temperature-time history of that location (model node 11100) for the last 3 orbits from the 8orbit simulation that was analyzed for each case. Table 2 lists the data for the curves in Figure 1. The boldfaced SA and ONR in Table 2 represent one (positive spacecraft roll) extreme forward corner of the pointing attitude corridor. RECOMMENDATIONS Based on these results and for consistency with TGS operations mode, CHAMP Thermal Engineering recommends that Reference 4 be updated to include a commanded forward SA limit of 65° for OGS operations mode. On occasions when SMS flexibility is required or desired, an additional clause may be added to Reference 4 to note that the OGS commanded forward SA must not be allowed to fall below 61.5°.

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EM: MOSES-1268 December 2, 2005

Commanded SA & ONR Extreme
65 63 61 59 29 5.0 29 0.0 28 5.0 28 0.0 27 5.0 27 0.0 26 5.0 26 0.0 25 5.0 25 0.0 24 5.0 0 5 10 15 20

57 55 53 51 49 47 45

ONR
R es ul ta nt Attitud e SA & ONR Te m pe rature Pre di cti on

Figure 1. Commanded and Resultant Attitude, with Corresponding Maximum Temperatures.

Node 11100

Figure 2. Worst Hot Case, Baffle Hot Spot Location with Temperature versus Time History.

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Peak Nodal Temperature [F]

Sun Angle


EM: MOSES-1268 December 2, 2005

Table 2. List of Resultant Pointing Attitudes and Predicted Maximum Temperatures
Resultant Pointing Attitude SA ONR Thermal Analysis Peak Nodal T [°F] 247 256 264 270 275 278 281 286 288 285 279

61.5
58.5 55.5 52.5 50.32 48.81 47.75 47.04 46.63 46.5 46.5

20 19.70 18.75 17 15 13 11 9 7

5
0

REFERENCES Reference 1 ­ "Light Shield Thermal Analysis for Venus Observations," EM MOSES-1067, P.T. Gruver, 9 December 1994. Reference 2 ­ "Thermal Environment Fluxes," Memo from 442/HST Thermal Branch to HST Thermal Team Members, 20 March 1995, Goddard Space Flight Center, Greenbelt, MD. Reference 3 ­ Presentation "TGS Operations Readiness Review," pp 122-130, Analysis by C.E. Cottingham, Presented by A.M. Wong, 4/12/2005, intranet link
http://edocs1.hst.nasa.gov/hstsysman/TGSOWG/TGS%20ORR%20Presentation%2004122005.pdf

Reference 4 ­ "Hubble Space Telescope Constraints and Restrictions Document (CARD)," SM-1020 latest revision and change, HST Library #TM-030520, Goddard Space Flight Center, Greenbelt, MD. Distribution for EM: MOSES-1268 D. Smith/LMTO/441 M. Reinhart/STScI J. Pilkington/LMTO/441 J. Pepe/GSFC/441 D. Nguyen/GSFC/441 O. Lupie/GSFC/441 P. Finneran/GSFC/441 L. Dunham/MOSES/441 J. Abel/LMTO/441 Data Center

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