Okolo, Wendy A.

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NASA Technical Reports Server

CITY OF CUPERTINO SPEAKER SERIES
February 19th, 2020
Cupertino, CA
1
PTERODACTYL: DEVELOPMENT AND EVALUATION OF 
CONTROL DESIGNS FOR A MECHANICALLY DEPLOYED 
ENTRY VEHICLE 
WENDY A. OKOLO, PH.D.,
NASA AMES RESEARCH CENTER
https://ntrs.nasa.gov/search.jsp?R=20200001077 2020-03-28T19:02:26+00:00Z
TEAM
Ricky Howard
STMD ECI Program Executive
Michelle Munk
NASA STMD Mentor and
EDL Principal Technologist
Dr. Dave Kinney, Dr. Alan Cassell, and Ron Sostaric
NASA Mentors
2
Kenneth Hibbard, Jeffrey Barton, 
Dr. Gabriel Lopez, Jeremy John, 
and Larry Wolfarth
Dr. Stephen Robinson
Brandon Reddish
Dr. Sarah D’Souza (Principal Investigator)
Antonella Alunni (Lead Systems Engineer)
Breanna Johnson (Guidance and Trajectory Design Lead)
Dr. Wendy Okolo (Control System Design Lead)
Ben Nikaido (Aerodynamics and Aeroheating Lead)
Bryan Yount (Mechanical Design and Structures Lead)
Benjamin Margolis (Controls Engineer)
Zane Hays (TPS System Modeling)
NASA Core Team
Advisors Industry Partners
Pterodactyl Project, NASA STMD 
MOTIVATION, BACKGROUND, & VEHICLE 
OVERVIEW
Pterodactyl Project, NASA STMD 3
Pterodactyl Project, NASA STMD 4
MOTIVATION
Precision landing close to research facilities reduces operational
recovery costs and enables timely sample retrieval
BACKGROUND
Pterodactyl Project, NASA STMD 5
NASA’s Space Technology Mission Directorate is funding 
Pterodactyl through the Early Career Initiative (ECI) Award to 
address the need for deployable entry vehicles that can land 
small and large mass payloads precisely
Deployable Entry Vehicles (DEV)
WHY IS THIS A CHALLENGE?
Pterodactyl Project, NASA STMD 6
Heritage Entry Vehicle with 
Reaction Control System (RCS)
DEVs have no back shell
RESEARCH QUESTION
What control system can be integrated into the DEV 
structure and enable steering to a target location 
precisely?
Pterodactyl Project, NASA STMD 7
Pterodactyl Project, NASA STMD 8
V
V
V
BASELINE MISSION OVERVIEW
Lunar Return mission (Stress case for loads)
Focused on Entry phase
Target site: UTTR
Entry Interface 
hEI = 122 km
VEI = 11.0 km/s
Active Guidance
Descent System 
Activation
Ma = 2.0
EARTH
PTERODACTYL BASELINE VEHICLE 
(PBV)
9
asymmetric 
diameter = 1+ m
mass = 59.4 kg
L/Dtrim = 0.2
atrim = -12o
Leading Edge
Trailing Edge
LE
TE
V
Pterodactyl Project, NASA STMD 
CONTROLS GOALS
Pterodactyl Project, NASA STMD 10
Utilize aerodynamic data, mass, and inertia properties of LNA to: 
(i) understand vehicle, 
(ii) create representation of vehicle, 
(iii) command vehicle to follow feasible guidance trajectories within 
mechanical limits & employing unconventional control methods
(iv) compare control systems and down-select
CONTROL SYSTEM DESCRIPTIONS
Pterodactyl Project, NASA STMD 11
• Reaction Control System (RCS)
• Flap Control System (FCS)
• Moving Mass Control System (MMCS)
FLAP DESIGN
Pterodactyl Project, NASA STMD 12
Leading Edge
Trailing Edge
8 flaps to increase control authority, reduce of 
multi-axis coupling due to flap locations, & satisfy 
packaging/stowing constraints
Linear actuator
Connecting 
linkage
Linkage pin
Tab hinge pin
• Mass:  75.7 kg  (updated after TPS sizing)
• Deflection Range:  -45° to +20°
MASS MOVEMENT DESIGN
Pterodactyl Project, NASA STMD 13
Actuator motor
Actuator lead screw
Travel range
Moving mass with 
embedded nut
Mass size limited by stowed state Actuation similar to flaps
• Mass:  80.1 kg  (>ABC max allowable of 77 kg)
• Moving masses sizes:  2 kg each
RCS DESIGN
Pterodactyl Project, NASA STMD 14
• APL performed an RCS sizing study
èRecommended locating thrusters at a 
larger reaction radius
èRecommended Hydrazine or Green 
Prop to provide adequate Impulse
4U RCS Tank / 
Controller 
Module
4X rib mounted thrusters
(1N nominal thrust ea)
• Mass:  69.1 kg  
• Capability:  
• 1N thrusters on lateral ribs
• Thrust reaction radius: 0.39m
GUIDANCE COMMANDS
Pterodactyl Project, NASA STMD 15
Heritage:
bank (s) commands
Alternative:
angle of attack (a) and side slip (b) 
commands
CONTROL SYSTEM DESCRIPTIONS
Pterodactyl Project, NASA STMD 16
Configuration Capability Control Outputs Requirements 
Flaps Flap Deflection 
Angle
Map flap deflections 
(in/out of the flow)
Moving masses Mass movement 
distance 
Map mass 
displacement as 
fraction of maximum 
distance
RCS Moments/torques 
required
Map thruster 
selection & duration
a/b
s
Decoupled
down/cross 
range control
OR
Coupled
down/cross 
range control
SIMULATION DEVELOPMENT & 
CONTROL DESIGN
Pterodactyl Project, NASA STMD 17
Pterodactyl Project, NASA STMD 18
Credit: https://www.grc.nasa.gov/www/k-12/airplane/forces.html
Pterodactyl Project, NASA STMD 19
Credit: https://www.grc.nasa.gov/www/k-12/airplane/forces.html
Pterodactyl Project, NASA STMD 20
Credit: https://www.grc.nasa.gov/www/k-12/airplane/forces.html
KEY ELEMENTS
Pterodactyl Project, NASA STMD 21
• Represent non-linear vehicle EOMs in state space form 
and derive linear controller
• Consider non-linear EOMs
Drag, Lift, & Side Forces
Roll, Pitch, & Yaw Moments
NEED AERODYNAMIC FORCES AND MOMENTS
AERODYNAMIC DATA
Pterodactyl Project, NASA STMD 22
• Three force & moment coefficients:
• 𝑪𝑫 , 𝑪𝑺 , 𝑪𝑳 , and 𝑪𝓛 , 𝑪𝓜 , 𝑪𝓝
• Dependent variables:
• Mach 𝑴, Dynamic pressure 𝒒, angle of attack 𝛂, 
and sideslip 𝛃
• Tab deflection angles 𝛅𝑖 for flap configuration only𝑪𝑭/𝑴 = 𝑓(𝑴, 𝒒, 𝛂, 𝛃, 𝛅𝑖)
USE AERO FORCES & MOMENTS TO CHARACTERIZE VEHICLE & DESIGN CONTROLLER
AERODYNAMIC DATA
• Aerodynamic forces and moment
• Aerodynamic coefficients
Pterodactyl Project, NASA STMD 23
0 for RCS and MMCSPBV Coefficients
LQR CONTROL DESIGN OVERVIEW
Pterodactyl Project, NASA STMD
24
• Consider linear system ?̇? = 𝐴𝑥 + 𝐵𝑢
• Require feedback gain matrix 𝐾 such that𝑢 = −𝐾(𝑥 − 𝑥=>?)
• Plant dynamics becomes?̇? = (𝐴 − 𝐵𝐾)𝑥 + 𝐵𝐾𝑥=>?
• Goal: Solve for K such that
• System is stabilized
• 𝑢 drives 𝑥 to 𝑥=>? while balancing objectives – small 𝑢 and 𝑥
goes to 𝑥=>? “fast enough”
• Linear Quadratic Regulator (LQR) theory to solve
Desired states/control inputs
Closed loop “A matrix” Closed loop “B matrix”
PTERODACTYL LQR DESIGN
Pterodactyl Project, NASA STMD 25
• Require linear dynamics of the form
• Linearize system about a state, xi*and input, ui*
• By introducing
where
• This splits the linearization and provides two key benefits:
• Aero database updates do not require a full linearization of the EOMs
• Only need partial derivatives of the aero forces & moments w.r.t states & 
control input
• Utilizing forces/moments as inputs allows us independently map control 
design to different control architectures with similar EOMs
Ref: Evaluating & Augmenting Fuel-
saving Benefits in Aircraft 
Formation Flight, Okolo 2015
PTERODACTYL LQR DESIGN
Using linear system, we can design a feedback control system
that minimizes the performance index:
Pterodactyl Project, NASA STMD 26
where Qi and Ri are weighting matrices that 
determine state performance requirements 
and control surface use respectively
Linearization can be done about these points to 
obtain A & B matrices for LQR control design
Local min/max of dynamic pressure along 
trajectory for controller evaluation
where K𝒊 is the matrix of control gains
Active 
guidance Max q
CAN UTILIZE CONTROL 
DESIGN FOR ALL THREE 
CONFIGURATIONS… 
BUT FIRST…
Pterodactyl Project, NASA STMD 27
WHAT ARE THE LIMITS OF 
CONTROLLING THIS VEHICLE?
Pterodactyl Project, NASA STMD 28
Leading Edge
Trailing Edge
LE
TE
V
Pterodactyl Project, NASA STMD 29
Zero yaw at 18° sideslip in full yaw left 
configuration
Non-zero roll at 18° sideslip in full yaw 
left configuration
Yaw induces roll moment that vehicle needs to trim 
YAW AND ROLL MOMENT VS β
𝒀𝒂𝒘𝒎𝒐𝒎𝒆𝒏𝒕 𝒗𝒔 𝒔𝒊𝒅𝒆𝒔𝒍𝒊𝒑 𝑹𝒐𝒍𝒍 𝒎𝒐𝒎𝒆𝒏𝒕 𝒗𝒔 𝒔𝒊𝒅𝒆𝒔𝒍𝒊𝒑
VEHICLE CHARACTERIZATION - TRIMMABILITY
Pterodactyl Project, NASA STMD 30
Side slip commands require constant reaction roll moments to trim induced roll.
Bank commands need roll moments only when roll is changing
à roll moments not required at steady state roll. 
TRIMMABILITY CHARACTERIZATION
Pterodactyl Project, NASA STMD 31
Stability & Control Characterization for Complex 
Control Systems
Provides control envelope for vehicle/control 
system configurations that are then provided to 
guidance for targeting performance assessment
MAJOR FINDING a and b have coupled operable 
ranges because the side slip commands require 
constant reaction roll moments to trim induced roll
Determined that a/b is not possible for this 
vehicle/control configuration and mission
Pterodactyl Project, NASA STMD 32
OPTIONS:
(I) RE-DESIGN CONTROL HARDWARE 
CONFIGURATION TO TRIM OUT ROLL OR, 
(II) UTILIZE INDUCED ROLL FOR BANK ANGLE 
GUIDANCE TRACKING – YAW TO BANK
MODIFIED CONTROL SYSTEM DESCRIPTIONS
Pterodactyl Project, NASA STMD 33
Flaps
Moving Masses
RCS
Control Variables
s
Control Effector 
Command
Flap 
Deflection 
Angle
Mass 
Movement 
Distance
Jets on/off 
and duration
Challenge
Coupled
down/cross 
range control
Adequate fidelity to capture the 
aerodynamic flap increments for 
active control
Capture adequate aeroheating for 
TPS sizing
Identifying achievable shift in the 
center of gravity
Develop control algorithm that 
can identify feasible distance 
commands
Identify thruster position off of 
the payload 
Feasibility to integrate thrusters, 
fuel lines, and fuel payload 
YAW-TO-BANK ARCHITECTURE
Pterodactyl Project, NASA STMD 34
Bank angle/roll gains à drives 
bank angle using sideslip
Sideslip computed from 
bank angle commands
Nonlinear plant
Block diagram for yaw to bank cascading control architecture 
CONTROL COMPARISON METRIC
• Metric for control performance à bank angle acceleration
• Evaluate metric at two different points in flight envelope
Pterodactyl Project, NASA STMD 35
G&C activation
Max O𝒒
CONTROL CAPABILITY COMPARISON
Pterodactyl Project, NASA STMD 36
Bank acceleration minimum for good performance: 5 deg/s2
RCS – TRACKING
Pterodactyl Project, NASA STMD 37
• Generalized jet selection algorithm takes inputs along with the control 
algorithm's desired torque to output ON/OFF commands
• Before flight, the jet selection tool is run to derive all torque combinations  and 
pre-load into tables
Bank angle (deg)
Truth
Command
FLAPS – TRACKING
Pterodactyl Project, NASA STMD 38
Tracking 
bank angle 
using sideslip 
& holding 
trim angle of 
attack 
𝜷, 𝜶, 𝒂𝒏𝒅 𝝈
Sideslip changes only to drive 
bank and returns to 0
CONTROL SYSTEM SELECTION
Downselection: FCS
• Exceptional control system 
capability
• Notable available payload 
volume
• Added trim augmentation, 
increased maneuverability
Pterodactyl Project, NASA STMD 39
SUMMARY
Pterodactyl Project, NASA STMD 40
• Control architecture can handle alpha-beta or bank 
angle commands 
• MIMO-LQR controller allows for over-actuated vehicle 
control
• Control design adaptable to other vehicles and actuator 
configurations
• Roll-yaw coupling renders alpha-beta control infeasible 
for good guidance with this vehicle, control designs, & 
mechanical limits
• Yaw to bank à attractive solution
CONTROL CAPABILITY ESTIMATE FOR YAW TO BANK à
ACHIEVABLE BANK ACCELERATION WITH SIDESLIP ANGLE
DEPENDENCY
CONCLUSION
• Yaw-to-bank leverages roll-yaw coupling to achieve 
increased maneuverability compared to heritage RCS
• FCS has the highest bank maneuverability of the three 
control architectures
• Control system recommendation à FCS
Pterodactyl Project, NASA STMD 41
KEY TECHNICAL 
CONTRIBUTIONS
First of its kind study that closes a DEV control system design 
with hardware feasibility and guidance & control performance 
(July 2019)
Pterodactyl Project, NASA STMD 42
IMPACT Enables further application of 
inflatable/mechanical DEVs for science missions and high mass 
to Mars
KEY TECHNICAL 
CONTRIBUTIONS
First of its kind study that closes a DEV control system design 
with hardware feasibility and guidance & control performance
Toolset that can be applied to any vehicle/control system 
configuration (July 2019)
Pterodactyl Project, NASA STMD 43
IMPACT Pterodactyl can retire risk by addressing 
integrated assessment hardware and G&C at an early 
conceptual phase in both conventional and non-traditional
entry system technologies
KEY TECHNICAL 
CONTRIBUTIONS
First of its kind study that closes a DEV control system design 
with hardware feasibility and guidance & control performance
Toolset that can be applied to any vehicle/control system 
configuration 
Closed a NOVEL control solution that makes asymmetric vehicle 
configurations controllable (formerly not controllable, July 2019)
Pterodactyl Project, NASA STMD 44
IMPACT Demonstrates feasibility for vehicles not previously 
considered .... for non-traditional entry vehicles that better 
serve mission needs versus traditional symmetric designs
KEY TECHNICAL CONTRIBUTIONS
Pterodactyl Project, NASA STMD 45
Identified a 100% non-propulsive entry control system (June 2020)
IMPACT Enables significant mass, volume and operational 
cost savings
KEY TECHNICAL CONTRIBUTIONS
Pterodactyl Project, NASA STMD 46
Identified a 100% non-propulsive entry control system (June 2020)
Built 6DOF Simulation Testbed with core Flight Software capability 
and developed Technology Maturation plan (June 2020)
IMPACT Provides foundation to understand challenges, risks, 
costs for future flight projects
NEXT STEPS
Pterodactyl Project, NASA STMD 47
• Investigate option 1: Redesign flap control architecture 
to enable alpha-beta tracking
• Obtain follow-on funding for hardware prototyping, 
demonstrator, and sub-orbital test flight
REFERENCES
48
Yount, B. C., Cassell, A. M., and D’Souza, S. N., “Pterodactyl: Mechanical Designs for Integrated Control Design of a 
Mechanically Deployed Entry Vehicle (DEV),” AIAA SciTech 2020 Forum, AIAA, Orlando, FL, 2020.
Nikaido, B. E., D’Souza, S. N., Hays, Z. B., and Reddish, B. J., “Pterodactyl: Aerodynamic and Aeroheating Database 
Development for Integrated Control Design of a Mechanically Deployed Entry Vehicle,” AIAA SciTech 2020 Forum, 
AIAA, Orlando, FL, 2020.
Johnson, B. J., Rocca-Bejar, D., Lu, P., Nikaido, B. E., Yount, B. C., D’Souza, S. N., and Hays, Z. B., “Pterodactyl: 
Development and Performance of Guidance Algorithms for a Mechanically Deployed Entry Vehicle,” AIAA SciTech 
2020 Forum, AIAA, Orlando, FL, 2020.
Okolo, W. A., Margolis, B. W., D’Souza, S. N., and Barton, J. D., “Pterodactyl: Development and Comparison of 
Control Architectures for a Mechanically Deployed Entry Vehicle,” AIAA SciTech 2020 Forum, AIAA, Orlando, FL, 
2020.
Hays, Z. B., Yount, B. C., Nikaido, B. E., Tran, J., D’Souza, S. N., Kinney, D. J., and McGuire, M. K., “Pterodactyl: 
Thermal Protection System for Integrated Control Design of a Mechanically Deployed Entry Vehicle,” AIAA SciTech 
2020 Forum, AIAA, Orlando, FL, 2020.
Alunni, A.I., D’Souza, S.N., Yount, B.C., Okolo, W.A., Nikaido, B.E., Margolis, B.W., Johnson, B.J., Barton, J.D., Lopez, 
G., Wolfarth, L. S., and Hays, Z. B., “Pterodactyl: Trade Study for an Integrated Control System Design of a 
Mechanically Deployable Entry Vehicle,” AIAA SciTech 2020 Forum, AIAA, Orlando, FL, 2020.
Pterodactyl Project, NASA STMD 
QUESTIONS
Pterodactyl Project, NASA STMD 49
“wendy_okolo”


Pterodactyl: Development and Evaluation of Control Designs for a Mechanically Deployed Entry Vehicle

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