LISA Mission System Engineering

1
LISA Mission System Engineering

• LIST Meeting
• Roger Diehl
• Tupper Hyde
• Marcello Sallusti
• December 10, 2005

2
Agenda

• Error Budgets Technical Performance Measures
• Trade Studies Status
• Technical Baseline

3
MRD Requirements (Jan 05)
This allocation slightly misses Hard point at 5
mHz
4
Look at moving IMS low end up
This allocation also slightly misses Hard point
at 5 mHz
5
Astrium Performance Outlook
0.65 SRD 3.0 science requirements Makes hard
point at 5 mHz
6
IMS Error Budget Needs Work
7
CBEs for DRS
8
CBEs for IMS
9
Technical Performance Measures
10
Telemetry Budget
11
Trade Studies Completed

• Strap Down
• Old Baseline One proof mass to proof mass
  measurement
• New Baseline Three measurements (proof mass to
  optical bench in first sciencecraft, optical
  bench to optical bench between two sciencecraft,
  optical bench to proof mass in second
  sciencecraft)
• Rationale
• Decouples inertial sensor and inter-spacecraft
  interferometry
• Optimizes each functional assembly for its prime
  objective
• Strong heritage from LTP
• Impact Requires point-ahead-angle actuator on
  optical bench
• Optical Readout
• Old Baseline None
• New Baseline Minimum design- 3 axis
  (x,theta-y,theta-z) from back side bench
  interferometer
• Rationale Relax stiffness and crosstalk
  requirement, no extra hardware
• Impact Minor increase in interface complexity

12
Trade Studies Completed

• Photodiode Sampling
• Old Baseline None
• New Baseline 50 MHz
• Rationale Provides require 2.5 times factor to
  20 MHz max expected doppler frequency.
• Impact Implies orbit design requirement of 15
  m/s max arm length rate (with margin to 20 MHz).
  Implies frequency management scheme where doppler
  is only 1 times rate effect.
• Arm Locking
• Old Baseline None
• New Baseline Direct arm locking without
  reducing requirements on laser, cavity,
  phasemeter, clocks or TDI.
• Rationale Provides technology off-ramps (risk
  reduction) for reference cavity stability,
  clocks, and phasemeter dynamic range
• Impact No new hardware needed. Detailed
  simulations are underway to define hardware
  requirements implied by armlocking (method of
  cavity tuning).

13
Trade Studies Completed

• Science Data Rate
• Old Baseline 1500 bps to TDI master though
  inter SC comm (TDI ver I on board)
• New Baseline 400 bps per sciencecraft
  (allocation), TDI on ground
• Rationale Interpolation allows resampling for
  TDI processing on ground.
• Impact 400 bps includes margin on top of
  front, back and LO-LO phasemeters (each
  with two redundant quad output photodetectors).
• Telemetry Rate
• Old Baseline 0.45 kbps per SC continuous
• New Baseline 5 kbps per SC continuous
• Rationale Housekeeping reality and
  sciencekeeping first cut estimate, (first cut
  bottoms up telemetry estimate showed 4.1 kbps
  with margin)
• Impact relook at comm hardware (see next trade)

14
Trade Studies Completed

• End-to-End Data Architecture
• Old Baseline 30 cm HGA, 5 W X-band SSPA, DSN 8
  hours/2days, inter S/C comm over laser link
• New Baseline 30 cm HGA, 25W Ka TWTA (TBD), DSN
  8 hours/2days, inter S/C comm over laser link
  using spread spectrum on carrier
• Rationale Meet telemetry allocation. Inter SC
  comm required to support ranging and clock tone
  transfer. No extra hardware.
• Impact Software to support spread spectrum mod
  and demod.
• Dynamics Parameters
• Old Baseline Multiple sets of simulation
  parameters with no CM.
• New Baseline Database of parameters agreed to
  by lead dynamicists at both Astrium and NASA.
• Rationale All simulation should reference a
  common baseline.
• Impact Parameter database kept under CM by MSE
  (Tupper).

15
Work In-Progress

• Orbits
• Old Baseline NASA and ESA trajectories based on
  different launch dates with different delta-Vs
• Recommended New Baseline Select delta-V that
  envelopes all launch dates
• Rationale Provide flexibility to launch any day
  of the year
• Impact Operations orbits will either lead or
  trail Earth depending on launch date.
• Getting-To-Orbit
• Old Baseline Solar electric propulsion, Delta
  IV Med (4,2) (4045 kg capability)
• Recommended New Baseline Chemical (Dual-Mode)
  propulsion, Atlas V 531 (5185 kg capability)
• Rationale Chemical propulsion less risk due to
  less complexity and lower cost. Atlas family has
  more steps due to strap-on options.
• Impact Will stay on intermediate launch
  vehicle. Further LV options to be considered
  after further mass and delta-V reduction studies.

16
Work In-Progress

• PAA Actuator
• Old Baseline No PAA
• Rejected option rotating wedges (Risley
  prisms), periodic operation.
• Recommended New Baseline 1 DOF actuator, TNO
  Design (Ti-6Al-4V integral structure with piezo
  actuation and front face centered cross blade
  flexure). Continuous operation.
• Rationale the strap down concept do not allow
  to use the PM to steer the incoming laser beam on
  the science photodiode
• Impact Addition of actuator, delta mass on
  optical bench of 75g, additional noise due to
  actuator in the measurement path
• Pointing (TA) Mechanism
• Old Baseline 2-stage linear actuator with pivot
  bearings (FTR)
• Options 2 lever tiller with OTS linear
  actuators, rotary piezo stepper motors.
• Recommended New Baseline TBD
• Rationale TBD
• Impact TBD

17
Work In-Progress

• Wave Front Error Impact
• Old Baseline far-field effect budgeted to 8
  nrad/rtHz, 30 nrad offset, periodic calibration
  (TBD), flat spot centering.
• Rejected Options continuous dither, bright spot
  (not flat spot) centering, no on-ground
  correction.
• Recommended New Baseline periodic calibration
  (2 weeks), flat spot centering, near field
  pointing (phase center moment arm) budgeted, near
  and far field effects cancelled on ground
• Rationale flat spot centering results in only 2
  loss of power, pointing knowledge is 2-40 times
  better than pointing error
• Impact send down quadrant info from front
  science phasemeters, two tone dithering during
  calibrations to pick out near and farfield
  effect, PAA pointing noise directly degrades
  pointing knowledge
• Telescope
• Old Baseline Dall-Kirkham 40 cm (up from 30 cm
  in FTR), optical assy articulation
• Rejected options Off axis, Three mirror
  anistigmat, in-field pointing (/- 1.5 deg),
  telescope only pointing.
• Recommended New Baseline Cassegrain 40 cm,
  optical assy articulation
• Rationale Cassegrain is least complex but still
  allows for placement of pupils at PAA steering
  mirror and at quad science photodetector.
  Potential for larger FOV to support option of
  through telescope natural star tracking (TBD).
• Impact Additional reflective or transmissive
  optics required between secondary and first
  beamsplitter

18
Trade Studies In-Progress

• Redundancy Philosophy
• Old Baseline No credible single point failure
  shall degrade the mission below the minimum
  mission performance.
• Rejected Options A single failure cannot degrade
  the mission below the minimum mission success
  criteria (assuming two arm LISA is still a
  success).
• Recommended New Baseline No credible single
  point failure shall degrade the mission below the
  full science requirements. LISA system must be
  one failure tolerant (fail safe).
• Rationale Such a philosophy is appropriate for
  the scale of the LISA mission.
• Impact Design has to implement full redundancy.
  Failures must be detected before extant
  capability is damaged. Waivers are required for
  credible single point failures which are
  prohibitively expensive to make redundant and
  whose failure would degrade but not end the
  mission.
• The GRS may require such a waiver since there is
  no redundant GRS and its loss still allows two
  arm science operations).

19
Trade Study (Dec 05- Feb 06)

• The 2 million km arm Study
• ESA and NASA quick-look to find significant
  mass/complexity/risk/cost reductions
• Goal to maintain full science performance
• Evaluate cost payoff of slight relaxation at 5
  mHz requirement point
• Reduce Arm Length to 2 million km
• Cruise deltaV reduced from 1000 m/s to under
  700 m/s, 500 kg savings, opens monoprop option
  again
• Use PM forcing on sensitive axis (1x10-9 m/s2)
  to buck out earth induced perturbation
• Max doppler reduced from 15 to 1 MHz
• Triangle breathing angle reduced from 1.5 deg to
  0.15 deg
• Eliminate optical assy articulation
• Removes large structural stage, large mechanism
  and launch locks
• Replaced with in-field pointing (perhaps
  periodically)
• Phasemeter bandwidth reduced from 50 Mhz to 3 Mhz
• Laser power reduced from 1 W to just meet
  requirements (300 mW?)
• Reduce electronics box count (combine functions)
• Payload mass, power, and size savings
• SC diameter reduction from 2.7m to 2.0 m
• Average thruster force reduced from 9 to 5
  micronewtons
• Bus mass savings due to size reduction and
  smaller payload

20
Short Arms (full science)
21
Short Arms (missing 2 points)
22
Future Trade Studies

• Trade Studies due Feb 06
• Spacecraft Mass Reduction
• Acquisition
• No Vacuum
• Optical Bench Layout
• Arm Locking Details
• Trade Studies due April 06
• Thermal Interfaces
• Grounding/Power Distribution
• Frequency Plan
• Avionics Arch
• Flight SW Arch
• Magnetic Zones/PM mag reqs
• Ranging/Clock
• DRS Cal
• Critical Alignments

23
Timeline for Chemical Transfer
24
Optimal Chemical Transfer
25
Structural Baseline

• Sciencecraft mass estimate down to 550 kg
  (including 30 percent margin)
• Analyzed several spacecraft/prop module
  configurations
• Electric (SEP) and Chemical Prop Module Versions
• Chem Prop Module looks promising with spaceframe
  construction
• Outer cylinder of prop module is the main load up
  the launch path
• Spacecraft not required to carry full launch
  stack
• Spacecraft designed for 40cm telescope
• Y-tube opening housed inside of tuna can,
  providing thermal protection from sunlight and
  contamination shielding during launch and cruise
  phase.
• Develop spacecraft to prop module and PM to PM
  separation systems
• Only six separation systems are required (3 PM to
  PM and 3 S/C to PM)
• Eliminate release mechanisms on top deck

26
LISA Spacecraft Configuration

• Volume and mass are design drivers!!
• Coordinated electronics boxes in solid model with
  agreed to mass/power budget
• Currently 35 electronic boxes are housed inside
  of the Sciencecraft
• Redundancy philosophy needs to be worked (combine
  functions)
• Placement of electronics boxes need to address
  self gravity, thermal, magnetics (adopt zone
  philosophy)
• Consider requirements carefully!

VIEW Top of structure with Solar Array and Top
Panel removed
27
Top Hat/Redundant Thruster Clusters

• Current design puts redundancy in cluster (there
  are only 3 clusters, not 6 as shown)

28
Welcome to crowded LISAapolis
29
Telescope to Bench Options
Astrium MAR (Oct 05) design
Astrium lightweight (Dec 05) design
NASA Large FOV (Nov 05) design
30
Laser Subsystem Configuration

• Old Baseline
• 2 2 laser subsystems (everything out to fiber
  to bench)
• Switching to backup laser was by TBD fiber
  positioner mechanism
• Under evaluation
• 3 laser units modulators to provide redundancy
• A single box serving the three laser heads and
  amplifiers
• A single box serving the three modulators
  electronics
• A single box for switching the lasers
  (switchyard)
• A single box for the electronics needed for the
  cavity and power stabilization of each laser
• Total mass of system unchanged at 29 kg

31
LIMAS Functions Overview
GRS
3 phase msrmts Laser freq. GRS tilt GRS
position Clock msrmnt Range msrmt Comm
message (all to ground)
Phase/range/clock/comm/GRS
In from distant Sc/C
Phase Measurement
InterSc/C comm demod signal
Laser phase lock signal (inc. armlocking)
S/C Bus
Frequency distribution
LIMAS Avionics
Inter-Sc/C Comm message Ground
commands Acquisition commands
Laser Freq. offset Clock tone Ranging
tone InterSc/C comm signal
laser
Out to distant Sc/C