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Presentation ESTEC, 14th Feb. 2001

LISA Mission Review. Presentation ESTEC, 14th Feb. 2001. Prepared by the CDF* Team (*) ESTEC Concurrent Design Facility. LISA Mission Status Agenda. Objectives of review Mission overview Technical requirements Baseline design Simulation Results from study review (i.e. design issues)

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Presentation ESTEC, 14th Feb. 2001

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  1. LISA Mission Review Presentation ESTEC, 14th Feb. 2001 Prepared by the CDF* Team (*) ESTEC Concurrent Design Facility LISA Mission Review

  2. LISA Mission StatusAgenda • Objectives of review • Mission overview • Technical requirements • Baseline design • Simulation • Results from study review (i.e. design issues) • Recommendation for future activities • design/verification upgrade • detailed AIT/AIV approach LISA Mission Review

  3. LISA Study Review Objectives • Review of LISA industrial study • Ref.: Final Technical Report, astrium, LI-RP-DS-0009 • Review performed in ESTEC Concurrent Design Facility (CDF) using existing CDF models • Objectives • Review of proposed spacecraft and s/s design w.r.t. • consistency, completeness and maturity of the design • identification of critical issues • building of CDF model with data from industrial study • building of CATIA model • Bringing ESA technical staff up to date • Preparation of the plan for further activities LISA Mission Review

  4. EARTH 60º 3 ECLIPTIC 1 1 ORBIT S/C 1 20º 3 3 2 2 2 1 SUN LISA Mission Overview (1/3) • LISA requires 3 spacecraft (460kg each) positioned at the vertex of a quasi-equilateral triangle at distances of about 5 million km • Centre of the triangle in the ecliptic plane ~20 behind the Earth (50 Mkm) • Plane of triangle is at 60 with respect to the ecliptic • The orientation of the triangle rotates once a year • The angle between the line of sight from one S/C to the other 2 S/C oscillates around the nominal 60 with an amplitude < 0.6° • Inter-spacecraft distances oscillate with an amplitude < 30000 km • The rate of variation of these distances shall be < 15 m/s LISA Mission Review

  5. figure taken from LI-RP-DS-009 LISA Mission Overview(2/3) • The stack of 3 LISA spacecraft shall be launched by a single Delta II 7925H three stage launcher (3x800mm) • Each S/C (science module, S/M) is attached to a 203mm high Propulsion Module (P/M) with electrical propulsion (independent transfer to operational orbit) • Lifetime 2 years on station (ext.10 yrs) plus < 15 months transfer (difference of 1 - 2 months between S/C) • After cruise phase P/M is jettisoned • The LISA spacecraft will separate one by one, and perform autonomously any required attitude manoeuvre. • In science mode the S/C are controlled using the FEEPs to achieve drag-free mode. Only the gravitational forces of the Sun, planets, and other bodies determine the trajectory of each S/C S/M >>> P/M >>> S/M >>> P/M >>> S/M >>> P/M >>> LISA Mission Review

  6. LISA Mission Overview(3/3) • Nominal Orbit • Satisfies the scientific requirements • Provides very stable gravitational, and thermal environment • Only drag-free control will be applied in the operational phase • Present S/C design strongly depending on: • Payload configuration and dimensions • Mass performance of the launcher • Volume available in the fairing of the launcher • Payload stability requirement (instrument case concept) • Unique design; the spacecraft is actively involved in the measurement (high interaction S/C-P/L) LISA Mission Review

  7. Payload Description Telescope Primary mirror: 30 cm diam, ULETM Mass: 99 Kg Proof-mass (CAESAR design) Thermal shield Two identical instruments Optical bench with Laser assembly Y-shaped structure thermo-mechanically insulated from the S/C Electronics LISA Mission Review

  8. S/C Technical Requirements • To create a noise-free environment for the proof-mass by shielding from external disturbances • Acceleration by disturbing forces on the proof-mass shall be  3 10-15 m s-2/Hz 1/2 at 0.1mHz • To ensure high stability of the optical set-up • The temperature variation of the telescope shall be  10-5 K/Hz1/2 at 1mHz • The temperature variation of the optical bench shall be  10-6 K/Hz1/2 at 1mHz • To transfer the 3 S/C elements to the selected orbit and perform the insertion into the triangular formation and the acquisition and maintenance of the laser link • To act as service module for the payload LISA Mission Review

  9. LISA Composite S/C Design 2.7 m S/M solar array S/M solar array thermal radiator EPS thrusters S/M 0.8 m P/M 0.2 m P/M solar array LISA Mission Review

  10. Science Module Design Mass: 288 Kg (with 5% margin) Power: 284 W (average) HGA’s thermal radiator solar array (multi junction cells) FEEP’s tubes (load transfer during launch) shear walls (isostatic interface between the module service unit and the payload ) LISA Mission Review

  11. Propulsion Module Design Hydrazine thrusters for AOCS solar array (multi junction cells) tubes (load transfer during launch) EPS thrusters Mass: 172 Kg (with 5% margin) Power: 599 W (average) LISA Mission Review

  12. Interfaces Basic building blocks optical units proof mass Payload electrical / structural (isostatic) interface electrical / mechanical interface AOCS incl. FEEP’s P/M OBDH electrical / structural interface other subsystems launcher support unit structure S/M LISA Mission Review

  13. LISA Study ReviewSummary The review team found: • Five major system design issues • Several minor design issues at subsystem design level LISA Mission Review

  14. Mass budget per S/C (Kg) Total (Kg) Total wet mass according to Industrial Study (with 5% margin) 458.70 1376.10 Corrected Total wet mass (with 5% margin) * 472.70 1418.10 Total wet mass with CDF system margin (20%) 536.90 1610.70 Delta II 7925H mass performance 1380.00 Delta III mass performance 2670.00 Atlas IIA mass performance 2230.00 * Summing up the subsystem masses (inconsistency with the total budget) SystemDesign Issues (1/7) Mass budget marginal • Delta II allows only for 5% system margin • Estimated unit/subsystem masses optimistic (especially Propulsion Module) Soyuz Fregat mass performance1390.00 LISA Mission Review

  15. SystemDesign Issues (2/7) Mass budget marginal (cont’d) • Option 1 (recommended): To increase the launch capabilities by switching to a more powerful launcher (Atlas IIA or Delta III) • This will also allow for more volume margin under the fairing • The drawback is the launch cost increase (about 50-60 %) • Option 2: Modification of the transfer scenario by launching to GTO or higher (Delta II capability up to 2000 Kg) and using electric propulsion all the way from there to the nominal orbit • This will significantly increase the cruise time (impact on cost of operations comparable to changing the launcher). • Long permanence time through the Van Allen belts (~ 9 months) • Mass saving not guaranteed a priori;it requires further analysis • Option 3: Radical re-design of the spacecraft aiming to mass reduction • This can only be achieved by payload redesign (very complex and time consuming) LISA Mission Review

  16. SystemDesign Issues (3/7) Spacecraft configuration extremely streamlined • Volume available under Delta II fairing very constraining (max height: 2.40 m, max diam. 2.75 m) for composite S/C COG position constraint • Propulsion Module only 0.2 m high • Accommodation of some equipment questionable (e.g. PCU, FEEP’s) Recommended solution: To go for a launcher with larger fairing volume The other possible options are: • Redesign of the spacecraft implying significant changes in the payload design • Re-examine the possibility of one single Propulsion Module for all 3 spacecraft (big impact on cruise complexity) LISA Mission Review

  17. SystemDesign Issues (4/7) Clear confirmation of the technical feasibility of the payload noise level control within the required limits still missing • Derivation of S/C system and subsystem requirements from the science requirements not clearly presented • Noise budget assessment not complete, e.g.: • Assessment of the effects of electronics power fluctuations on optical bench stability not conclusive • Assessment of the noise induced by the FEEP not conclusive • Proof-mass caging effect not fully discussed • Effect of antenna motion on proof-mass noise not computed (e.g. Self-gravity variations, noise induced by mechanisms) • Uncertainty on material properties and mounting not considered in the noise verification (e.g. Uniform CTE assumption) • Technology assumptions for the analysis not always justified/verified. Required developments not clearly identified • Numerical accuracy of the tools used for stability verification not discussed and verified (in the case of ESATAN for thermal analysis the tool accuracy is less than the computed stability 10-6 K vs 10-11 K) LISA Mission Review

  18. SystemDesign Issues (5/7) Noise Budget as presented in the Industrial Study Very low margin considering all the uncertainties LISA Mission Review

  19. Side facing deep space Tanks may run very cold Sun direction Thermal issue during Cruise - schematic 25o SystemDesign Issues (6/7) Propulsion Module design incomplete • Thermal design missing - Issues expected • SA design marginal even considering the highest available efficiency for the solar cells • Structures/configuration marginal Sun on the high electronic dissipating units and on the S/M radiator LISA Mission Review

  20. SystemDesign Issues (7/7) Integration and Test Issues (AIT/AIV) • International co-operation aspects not fully addressed by the contractor • Verification/Testing of the effects of the spacecraft on the payload performance not sufficiently addressed (special instrumentation and test methods not discussed, modelisation not described) • Integration issue not addressed in the configuration design LISA Mission Review

  21. Main Conclusions (1/2) • The contractor made a significant effort to fulfil the science-driven requirements within the very tight launcher mass and volume constraints • The nominal operational orbit and the constellation configuration selected satisfy mission requirements • The payload design has received much attention and is well advanced However LISA Mission Review

  22. Main Conclusions (2/2) • The Delta II capability is not adequate for the mission and it is strongly suggested to use a more powerful launcher • Feasibility of noise control methods is not fully convincing due to fragmented analysis ( i.e. elements addressed but total picture not presented) • The assessment of the noise induced by the spacecraft is incomplete and not thoroughly discussed • For a proper noise budget calculation there is a need to assess which kind of tools are needed and which numerical requirements must be fulfilled • With each noise source identified there should be a clear definition how it is tested or analytically verified • In same areas (e.g. P/M) the design is at low level of detail LISA Mission Review

  23. Example of Design Issues at Subsystem Level (1/2) AOCS design • approach seems sound but a comprehensive drag free control simulation is missing • Verification of the assumed hardware performance vs technology availability not fully convincing (clear requirements for technology development missing) Mechanisms • Design schematic, not all the required mechanisms clearly identified/selected Power • Potential contamination from the propulsion units on the SA of the PM not addressed • Power margin applied generally low • Electro-magnetic noise from power components not addressed LISA Mission Review

  24. Example of Design Issues at Subsystem Level (2/2) TT&C • Design schematic (link budgets not detailed, trade-offs not justified) Data Handling • Little attention paid to S/W development and integration with payload software LISA Mission Review

  25. Areas Requiring More Detailed Work (1/2) Mass budget marginal • Investigation of more powerful launchers:Atlas IIA or Delta III • Further mission trade-offs Propulsion Module design • Thermal, Power and Configuration issues to be addressed Noise budget • Re-assessment of the disturbance effects from the SM on the payload performance Thermal Design of SM and PM • Verification of the transfer phase & stability during the nominal operations LISA Mission Review

  26. Areas Requiring More Detailed Work (2/2) AOCS subsystem performance verification • Achievable accuracy performance of the hardware to be verified • More accurate dynamical model of the Control System to be built AIT/AIV approach to be addressed in detail LISA Mission Review

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