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Scheduling Challenges for JWST JIM Feb. 19, 2004

Scheduling Challenges for JWST JIM Feb. 19, 2004. Peter Stockman. Major planning constraints. Sun avoidance: well known Field of Regard Earth-Moon scattered light: Will constrain some orientations (still not completely understood)

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Scheduling Challenges for JWST JIM Feb. 19, 2004

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  1. Scheduling Challenges for JWSTJIM Feb. 19, 2004 Peter Stockman

  2. Major planning constraints • Sun avoidance: well known Field of Regard • Earth-Moon scattered light: Will constrain some orientations (still not completely understood) • Orbit maintenance (11 days between angular momentum dumps for FDF): would constrain roll choice/orientation for long observations • Fuel conservation (22 days between angular momentum dumps): would constrain the roll constraints of all observations and potentially the mix of observations in a 22 day period. Red = New and Exciting JIM

  3. 1.) Sun Avoidance Simple model based upon 2001 TRW sunshield design by Dennis Skelton • The sunshade provides: • -5° to 45° pitch from the ecliptic poles • ~± 5° of operational roll • Required for 10 day fixed-roll NIRSpec observations. • 5° safety band in both pitch and roll Sunshield shadows Primary & Secondary Mirrors Primary shadows Secondary Mirror Stayout Zone JIM 56°

  4. 2.) Simple L2, Earth, Moon Geometry in X-Y plane shows how Earth and Moon light can strike OTE ±10° roll shadow band, ±5°in MRD Sun projection • The Earth and/or Moon can illuminate the optical surfaces, particularly at L2 orbit (Y and Z) extremes • Could be improved by tighter L2 orbit or larger sunshade. Earth Moon 37° 27° L2 JWST JIM

  5. Earthshine typical example OTE components overhanging sunshield coverage will be illuminated by Earth crescent • Northern hemisphere • Pitch = 0 • Sunshield Roll = 0 • Yaw = 45 JIM

  6. Scattered Earthshine can exceed the zodiacal background at l> 3 mm. Earthshine Zodi • Worst case assumes: • 100% of 1 mirror (SM or PM) • 1% dust • Nominal BRDF • Moonlight is less important (1-3% Zodi) From Larry Petro JIM

  7. Beckman analyzed one DRM for Earth/Moon Impacts • For analysis, he used: • Skelton’s stay out zones from 2001 TRW Phase 1 design • 15 yr ephemeris and DRM v3.6b • Periods exist when either the Earthlight or Moonlight would strike the primary or secondary mirror 15 yr JWST orbit seen from the Sun JIM

  8. Results • 70% of observations were “dark” • Earth and Moon each affected 25% of observations: • Earth intruded as much as 22° into keep-out zone • Moon intruded as much as 30° into keep-out zone Earth seen in L2 XY plane • Very little correlation with time, but both Moon and Earth most easily seen at X-Y-Z extremes of the orbit. 1.5Mkm Earth JIM

  9. The new sunshield (June 2003) is 43% smaller than previous design to reduce angular momentum buildup and mass Design in proposal New design • 67% area (based on inner layer) • 57% area (based on outer layer) The smaller sunshade will increase the impact of scattered light from the Earth and Moon. JIM

  10. A rough idea of the constraint and how it changes per year NEP NEP Increased scattered light regions • Pattern repeats • 90 L2/2 • 180 L2 • ~1 year • Can create shorter observing seasons and impact 180 day repeats FOR in JWST frame Z JWST 3 weeks Later L2 JWST L2 Y JIM

  11. 3.& 4.) Angular Momentum and Orbit Maintenance: • To determine orbit, FDF is allowing at most 2 momentum dumps per 22 day period (e-folding time for orbit errors). • Limited propellant mass for orbit maintenance and momentum dumps has led to concept of 1 dump/22 days (24 hrs before orbit burn) • Flexibility for scheduling depends on wheel momentum storage capability • 6 wheels = 40 n-m-s • 4 wheels = 22 n-m-s JIM

  12. Schematic Maneuver Sequence Possible Additional Momentum Unload 21-day Tracking Arc Momentum Unloads (~ 1 day prior to SK maneuver) Station-Keeping Maneuvers (8 per rev, ~ 22 days apart) JIM

  13. Momentum accumulation is dominated by roll offsets in current design 1/5th of 22 day total accumulated in one day! dJ/dt Pitch Roll JIM

  14. Comparison of momentum accumulation for both new sunshields designs Current design (negative dihedral) Note significant angular momentum due to pitch alone Positive dihedral alternative JIM

  15. Possible ways to manage angular momentum in the scheduling system • Baseline today: Monitor: Check long range plan to see if there is a potential for exceeding the momentum between 22 day dumps. Feasible if problems are rare • Restrict average momentum buildup per observation to less than 2 n-m-s average during development of LRP. • Constrain roll orientation and start-dates • Significantly decreases scheduling flexibility. • Failed observations will necessitate replan since all observations would be shifted • Actively Manage momentum by balancing angular momentum build-up over each 22 day period (and potentially beyond) in the LRP. • Increases science return, but may create a a very brittle schedule. JIM

  16. Monitor Study: DRM shows 30-40% of dump intervals less than 22 days Monitor method will not work. Fails in 30-40% of cases even with all reaction wheels working. JIM

  17. Restricting to an average momentum : 10 day observations need special planning to avoid excessive momentum build-up At high ecliptic latitudes, the visits must be centered within one day: either fixed start times or intervention needed if started early by failure of previous observation JIM

  18. Restricting to an average momentum : One day visits are also constrained Note drop in available Start-time at high ecliptic latitudes even for a 1-day early visit Restricting average momentum method will be very constraining…worth going to total momentum management JIM

  19. Restricting average momentum : Loss of reaction wheel leads to drastic constraints Note loss of all flexibility above 45° even for 1 day observations. Restricting average momentum method is not viable. Must go to total momentum management or change vehicle JIM

  20. A typical 22 day managed schedule • Possible 22 day rules • Only one 8-10 day obs • Only one 4-7 day obs • Fill in with 1 day obs JIM

  21. Summary • Overall scheduling of JWST has become more complicated and may significantly impact JWST science: • Long observations are almost time critical • Full roll (± 5°) is not routinely available • Even observations with varying roll but in the same part of the sky will be limited to ≤ 10 day stretches. • Thermal radiation from the Earth can produce significant scattered light and preferred observing seasons (potentially impacting NGP & SGP depending on launch date) • Angular momentum issue could be mitigated with positive dihedral design, increased momentum wheel capability or added fuel (~ 70 kg). • Scattered light issue needs to be confirmed by Ball, STScI, and GSFC (Beckman/Skelton). Larger sunshield makes angular momentum problem worse. JIM

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