14th Feb. 2001Page 1

LISA Mission Review

PresentationESTEC, 14th Feb.

2001

Prepared by the CDF* Team(*) ESTEC Concurrent Design Facility

LISA Mission Review

14th Feb. 2001Page 2

LISA Mission Review

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

14th Feb. 2001Page 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

14th Feb. 2001Page 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

231

2

3

1

2

3

SUN

EARTH 60º

ORBIT S/C 120º

ECLIPTIC

14th Feb. 2001Page 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 203mmhigh 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 >>>

14th Feb. 2001Page 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)

14th Feb. 2001Page 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

TelescopePrimary mirror: 30 cm diam, ULETM

Thermal shield

Mass: 99 Kg

14th Feb. 2001Page 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/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

14th Feb. 2001Page 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

14th Feb. 2001Page 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

14th Feb. 2001Page 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

14th Feb. 2001Page 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 / mech

anic

al interface

14th Feb. 2001Page 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

14th Feb. 2001Page 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 budget per S/C (Kg) Total (Kg)

Total wet mass according to Industrial Study (with 5% margin) 458.70 1376.10Corrected Total wet mass (with 5% margin) * 472.70 1418.10Total wet mass with CDF system margin (20%) 536.90 1610.70

Delta II 7925H mass performance 1380.00Delta III mass performance 2670.00Atlas IIA mass performance 2230.00

* Summing up the subsystem masses (inconsistency with the total budget)

Soyuz Fregat mass performance 1390.00

14th Feb. 2001Page 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)

14th Feb. 2001Page 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)

14th Feb. 2001Page 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

14th Feb. 2001Page 18

LISA Mission Review

System Design Issues (5/7)

Noise type Computedvalue

Requirement

Pathlength differencemeasurement noise

(m/Hz1/2)

38.264 e-12 40. e-12

Proof mass accelerationnoise (m/s2/Hz1/2)

2.77e-15 3.e-15

Noise Budget as presented in the Industrial Study

Very low margin considering all the uncertainties

14th Feb. 2001Page 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

25o

Side facing deep space

Tanks may run very cold

Thermal issue during Cruise - schematic

14th Feb. 2001Page 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)

14th Feb. 2001Page 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

14th Feb. 2001Page 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

14th Feb. 2001Page 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

14th Feb. 2001Page 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

14th Feb. 2001Page 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

14th Feb. 2001Page 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