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

2. 14th Feb. 2001 Page 2 LISA Mission Review LISA Mission Status Agenda 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

3. 14th Feb. 2001 Page 3 LISA Mission Review 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

4. 14th Feb. 2001 Page 4 LISA Mission Review 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 1 2 3 1 2 3 1 2 3 SUN EARTH60º ORBIT S/C 1 20º ECLIPTIC

5. 14th Feb. 2001 Page 5 LISA Mission Review LISA Mission Overview (2/3) S/M >>> P/M >>> figure taken from LI-RP-DS-009 S/M >>> P/M >>> 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 >>>

6. 14th Feb. 2001 Page 6 LISA Mission Review 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)

7. 14th Feb. 2001 Page 7 LISA Mission Review Payload Description Y-shaped structure thermo-mechanically insulated from the S/C Two identical instruments Optical bench with Laser assembly Proof-mass (CAESAR design) Electronics Telescope Primary mirror: 30 cm diam, ULE TM Thermal shield Mass: 99 Kg

8. 14th Feb. 2001 Page 8 LISA Mission Review 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/Hz 1/2 at 1mHz –The temperature variation of the optical bench shall be  10 -6 K/Hz 1/2 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

9. 14th Feb. 2001 Page 9 LISA Mission Review 0.2 m LISA Composite S/C Design S/M solar array S/M 0.8 m P/M solar array S/M solar array 2.7 m EPS thrusters P/M thermal radiator

10. 14th Feb. 2001 Page 10 LISA Mission Review Science Module Design solar array (multi junction cells) shear walls ( isostatic interface between the module service unit and the payload ) tubes (load transfer during launch) thermal radiator Mass: 288 Kg (with 5% margin) Power: 284 W (average) FEEP’s HGA’s

11. 14th Feb. 2001 Page 11 LISA Mission Review Propulsion Module Design solar array (multi junction cells) EPS thrusters tubes (load transfer during launch) Mass: 172 Kg (with 5% margin) Power: 599 W (average) Hydrazine thrusters for AOCS

12. 14th Feb. 2001 Page 12 LISA Mission Review Interfaces launcher P/M support unit structure electrical / structural interface electrical / structural (isostatic) interface optical units proof mass AOCS incl. FEEP’s other subsystems Payload OBDH Basic building blocks S/M electrical / mechanical interface

13. 14th Feb. 2001 Page 13 LISA Mission Review LISA Study Review Summary The review team found: –Five major system design issues –Several minor design issues at subsystem design level

14. 14th Feb. 2001 Page 14 LISA Mission Review System Design Issues (1/7) Mass budget marginal Delta II allows only for 5% system margin Estimated unit/subsystem masses optimistic (especially Propulsion Module) Mass budgetper S/C (Kg)Total (Kg) Total wet mass according to Industrial Study (with 5% margin)458.701376.10 Corrected Total wet mass (with 5% margin) *472.701418.10 Total wet mass with CDF system margin (20%)536.901610.70 Delta II 7925H mass performance1380.00 Delta III mass performance2670.00 Atlas IIA mass performance2230.00 * Summing up the subsystem masses (inconsistency with the total budget) Soyuz Fregat mass performance 1390.00

15. 14th Feb. 2001 Page 15 LISA Mission Review System Design 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)

16. 14th Feb. 2001 Page 16 LISA Mission Review System Design 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)

17. 14th Feb. 2001 Page 17 LISA Mission Review System Design Issues (4/7) –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) Clear confirmation of the technical feasibility of the payload noise level control within the required limits still missing

18. 14th Feb. 2001 Page 18 LISA Mission Review System Design Issues (5/7) Noise Budget as presented in the Industrial Study Very low margin considering all the uncertainties

19. 14th Feb. 2001 Page 19 LISA Mission Review System Design 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 Sun direction 25 o Side facing deep space Tanks may run very cold Thermal issue during Cruise - schematic

20. 14th Feb. 2001 Page 20 LISA Mission Review System Design Issues (7/7) –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 Integration and Test Issues (AIT/AIV)

21. 14th Feb. 2001 Page 21 LISA Mission Review 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

22. 14th Feb. 2001 Page 22 LISA Mission Review 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

23. 14th Feb. 2001 Page 23 LISA Mission Review 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

24. 14th Feb. 2001 Page 24 LISA Mission Review 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

25. 14th Feb. 2001 Page 25 LISA Mission Review 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

26. 14th Feb. 2001 Page 26 LISA Mission Review 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