Knight, J. Brent

NASA Technical Reports Server

Dynamic Analyses of the Proposed Habitable 
Exoplanet Astrophysics Facility
SPIE August 2019
J. Brent Knight
NASA/MSFC/ES63
https://ntrs.nasa.gov/search.jsp?R=20190030449 2019-09-26T18:56:30+00:00Z
HabEx
• HabEx is one of 4 large astrophysics facilities being 
considered by the 2020 decadal
• It is intended to, among other things, directly image 
planetary systems around sun-like stars
• Its main goal is to directly image earth like exoplanets 
and characterize their atmospheric content
• The current design is on the order of 17.2X5.25 m 
and the Primary Mirror (PM) is 4 m in diameter
• Performance requirements include an extremely 
stable system
NASA/MSFC/ES63/J. Brent Knight
Objective
NASA/MSFC/ES63/J. Brent Knight
• As part of the HabEx Pre-Phase A 
(feasibility) study, structural dynamic 
analyses have been performed to provide 
Systems Engineers (SE) with order of 
magnitude estimates of dynamic responses
• Results are consistent with feasibility studies 
and are based on first cut requirements, 
assumptions, and simplified inputs
• These results are rolled into system level 
performance predictions
• The objective of this presentation is to describe 
the models used, simulations performed, and 
results
Current
• Line Of Sight (LOS) jitter < 0.5 mas per axis
• Equates to low Nanolevel allowable linear and rotational jitter motion
• Dynamic analyses were performed to predict jitter due to 
Attitude Control System (ACS) dynamic inputs 
• Ring Down - Transient ring down associated with turning the system
• ACS thrusters will start turning the system then fire again to stop that 
motion
• How long does it take for transient jitter to subside?
• Jitter while pointing at a target and collecting data
• Micro-Thrusters (MT) will be used to maintain orientation 
• Thrust level is expected to change, ramp up/down, very slowly
• Considered to have no frequency content
• The MT have a continuous noise level up to 10 Hz and then it ramps down
• The MT noise is the only identified dynamic disturbance during science 
windows
NASA/MSFC/ES63/J. Brent Knight
PM
SM
Jitter
Model
• The analyses was performed via the Finite 
Element (FE) method
• MSC/NASTRAN was used as the solver and 
MSC/PATRAN was the pre/post processor
• The two primary systems are the Spacecraft (SC) 
and the telescope
• SC
• Modeled by JPL personnel
• Telescope 
• Modeled by MSFC personnel
NASA/MSFC/ES63/J. Brent Knight
SC FEM
Telescope FEM
• The SC and Telescope FEMs were integrated 
by JPL personnel
• FEM details
• FEM mass properties  
NASA/MSFC/ES63/J. Brent Knight
Model
Integrated
HabEx FEM
Approximately 636K DOF
Model
NASA/MSFC/ES63/J. Brent Knight
• Primary Mirror (PM) • Secondary Mirror (SM)
Mirror Models
PM FEM created using
The Arnold Mirror Modeler
PM has ≈ 14K DOF
SM has ≈ 10K DOF
NASA/MSFC/ES63/J. Brent Knight
Analysis Input Loads and Locations
• Ring Down
• JPL provided thrust loads associated with a 
simple maneuver
• Step function applied at 2 of the 4 ACS 
thruster locations – Y axis selected
• 8.8N for 20.5 s, drift 368 s, -8.8N for 20.5 s
• Jitter
• MT noise is the disturbance 
during science windows
• Noise input at all 8 MT locations
• Large MT pods
• F=0.8 µN, .1<f<10 Hz (spec)
• F=0.8 µN, .1<f<20 Hz (applied)
• Small MT pods
• F=0.4 µN, .1<f<10 Hz (spec)
• F=0.4 µN, .1<f<20 Hz (applied)
4 ACS thruster locations and 4 large MT pod locations 90°apart 4 small MT pod locations 90°apart
Analysis Output/Results
• NASTRAN Multi-Point Constrain (MPC) 
equations were incorporated to calculate 
Relative Motions (RM) within the solution 
sequence
• Per HabEx SE request,  MPCs were written 
to calculate RM’s between the PM/SM, the 
PM and Tertiary Mirror (TM), and the 
SM/TM
NASA/MSFC/ES63/J. Brent Knight
SM
PM
TM
Ring Down Analysis
NASA/MSFC/ES63/J. Brent Knight
• Physically, ring down is a transient event
• To assess the design to estimate the ring down time a NASTRAN transient (time domain) 
dynamic analysis (Solution 112) was performed
• Required damping of .05% was used
• No ring down time requirement has been determined in this feasibility study
• In the absence of a requirement, the simulation was run 5 minutes past the “stopping thrust”
• PM/SM RM for each DOF was output
• The max RM 5 minutes after the last thrust was reported
• PM to SM Relative Motion was the selected metric
• X Direction
NASA/MSFC/ES63/J. Brent Knight
Ring Down Results
Peak after 5 minutes = 1.2E-4 pm
• PM to SM Relative Motion
• Y Direction
NASA/MSFC/ES63/J. Brent Knight
Ring Down Results
Peak after 5 minutes = 4.1E-4 pm
• PM to SM Relative Motion
• Z Direction
NASA/MSFC/ES63/J. Brent Knight
Ring Down Results
Peak after 5 minutes = 1.0E-4 pm
• PM to SM Relative Motion
• Ɵx Direction
NASA/MSFC/ES63/J. Brent Knight
Ring Down Results
Peak after 5 minutes = 2.3E-5 pico-radians
• PM to SM Relative Motion
• Ɵy Direction
NASA/MSFC/ES63/J. Brent Knight
Ring Down Results
Peak after 5 minutes = 1.4E-6 pico-radians
• PM to SM Relative Motion
• Ɵz Direction
NASA/MSFC/ES63/J. Brent Knight
Ring Down Results
Peak after 5 minutes = 4.4E-7 pico-radians
Ring down Conclusion
• No required time to regain stability after thruster induced transients 
has been determined
• The maximum linear RM is on the order of 10-4 pm after 5 minutes of 
settling time
• The maximum rotational RM is on the order of 10-5 p-rad after 5 
minutes of settling time
• The LOS stability requirements are in the nm range
• Therefore, with small fractions of pm range motion predicted, 5 
minutes of settling time is a conservative number to use in this early 
study
NASA/MSFC/ES63/J. Brent Knight
Jitter Analysis
NASA/MSFC/ES63/J. Brent Knight
• Jitter is a continuous event that occurs as long as 
the source vibration is in operation
• It is well suited for a frequency response or 
harmonic dynamic analysis
• Damping was required to be .05%
• Results were predicted up to 350 Hz
Force, µN
Frequency, Hz
10 20
10 20
Force, µN
Frequency, Hz
.4
.8
Aft MT Pods
Forward MT Pods
Noise specified to 10 Hz
Conservatively applied out to 20 Hz
Jitter Results
NASA/MSFC/ES63/J. Brent Knight
• Results up to 300 Hz were provided to the HabEx SE team and were 
rolled into system level performance assessments
• Feasibility fidelity dynamic analyses have been performed for the 
HabEx Pre-Phase A engineering effort
• Based on the provided FEM and required inputs, no show stoppers 
were identified
NASA/MSFC/ES63/J. Brent Knight
Final Comments


Dynamic Analyses of the Proposed Habitable Exoplanet Astrophysics Facility

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