Ahmad, Naeem

Fill, Thomas

Hawkins, Matt

Von der Porten, Paul

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PEG Modifications & Enhancements for SLS Block-1 and 
Block-1B Vehicles
Paul Von der Porten, MSFC
Naeem Ahmad, MSFC
Matt Hawkins, MSFC/ESSCA/Jacobs Engineering
Thomas Fill, Charles Stark Draper Laboratory
.1
2018 AAS GNC Conference
https://ntrs.nasa.gov/search.jsp?R=20180002044 2019-08-30T12:49:22+00:00Z
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Outline
.2
• Introduction
• Block-1 Modifications to Powered Explicit Guidance (PEG) Since 
Shuttle
• Block-1B 1-target VS 2-target Ascent Guidance Problems
• Block-1-to-Block-1B Enhancements
• Conclusion
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Introduction
.3
• Marshall Space Flight Center (MSFC) Guidance, Navigation, & 
Control (GN&C) Team has an expanded responsibility going 
from Space Launch System (SLS) Block-1 to Block-1B vehicles
– Characteristics of Block-1 ascent burn allow for use of a modified 
version of Space Shuttle’s Powered Explicit Guidance (PEG) 
algorithm
– Long-arc burns and the need to carry out Lunar Vicinity and Earth 
Escape missions require enhancements to PEG for Block-1B
MSFC Block-1 Block-1B
GN&C Responsibility Ascent Only Ascent and In-space
Guided Burns
Core Stage (CS) Ascent 
Burn to Low Earth Orbit 
(LEO)
CS/EUS Ascent Burns to 
LEO,
Apogee Raise Burn (ARB),
Trans-Lunar Injection (TLI),
Earth Departure Burn (EDB),
Settling Motor Disposal Burn
Guidance Algorithm Modified Shuttle PEG Enhanced Shuttle PEG
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Block-1 Modifications to PEG Since Shuttle
.4
Modifications allow an outer loop to drive PEG for Block-1
• Block-1’s ascent burn arc (~13°) similar size as Space Shuttle
– Allows for straight adaption of Shuttle’s PEG with the ascent desired 
velocity mode
• Modifications:
– Multi-phase PEG and PEG Phase Manager
• Replaces Shuttle’s algorithmic approach to switch between 3 phases for a 
data-driven approach
• Moves calculation of most burn times and all mass-flowrate-to-initial-mass 
time constants out of time-to-go computation algorithm into an outer loop 
wrapper
– Lofting parameter for Launch Abort System Jettison
• Induces additional lofting by applying an altitude bias to the desired radius 
magnitude
– Engine-Out Logic
• Uses inertial velocity to decide if an alternate mission target is needed in 
response to a CS engine-out
– Thrust Factor
• Similar to Shuttle’s FT_FACTOR
• Provides updated propulsion knowledge to PEG
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Block-1B 1-target VS 2-target Ascent Guidance
.5
Challenging 1-target guidance problem led to several bullet-
proofing enhancements for PEG
• Block-1B Ascent Profile
– Boost stage:  Two Solid Rocket Boosters and CS engines burn to 
booster separation
– CS burn:  CS engines burn to intermediate point in ascent trajectory
– EUS Ascent:  EUS engines burn to LEO insertion
• Two flight techniques studied early in Block-1B design:
• 2-target baselined for Block-1B
• 1-target presented several convergence issues that led to 
several bullet-proof enhancements to PEG kept for Block-1B
1-target 2-target
CS Burn Approach Burn to completion Targeted burn with flight performance reserve
EUS Ascent approach Targeted burn with flight performance reserve
Targeted burn with flight 
performance reserve
PEG Modeling
Entire burn arc (~50°) 
from booster separation 
to LEO
Separate targeted burns for 
CS and EUS Ascent (i.e. PEG 
reset for EUS Ascent burn)
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Block-1-to-Block-1B Enhancements
.6
Safeguards developed for Turn Rate Vector to address stress 
ituations due to long-arc burns
• Safeguards for Constructing Turn Rate Vector
– Limiting Tangent of Thrust Angle
• Useful initial strategy to limit turn rate magnitude
• Bullet-proof enhancement from 1-target problem
– Elevation-limit
• Shuttle heritage
• Ensures thrust direction from PEG’s steering law does not have a 
component retrograde compared to cutoff radial direction
• Required to close engine-out 2-target scenarios
– Sign Reversal of Thrust Turning Rate Vector
• Constructs augmented coordinate frame to protect orthogonality 
constraint from yielding a thrust turning rate sign reversal
• Bullet-proof enhancement from 1-target problem
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Block-1-to-Block-1B Enhancements
.7
Additional enhancements bullet proof PEG and make PEG capable 
of carrying out Block-1B missions 
• Scaling Identity Jacobian in PEG Corrector
– Applies a contraction factor to PEG’s traditional Identity matrix 
Jacobian as a simplified scheme to improve PEG’s convergence for 
long-arc burns
– Bullet-proof enhancement from 1-target problem
• Plane Constraint Strategy
– Strategy to unify plane constraint for both ascent and in-space 
burns
– PLANE_OFF, RV_NULL, V_NULL, INTERCEPT
• New Desired Velocity Routines
– Linear Terminal Velocity Constraint
• Shuttle heritage
• Used for ARB and TLI burns
– Hyperbolic Target
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Conclusion
.8
• Space Shuttle PEG modified to accommodate initial 
evolution of SLS, Block-1
• Several enhancements to PEG required going from 
Block-1 to Block-1B to carry out demanding Block-1B 
missions
• Improvements make PEG capable for use on the SLS 
Block-1B vehicle as part of the GN&C System
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.9
Thank you!
Any questions?
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PEG Linear Tangent Guidance Geometry
.11


PEG Modifications & Enhancements for SLS Block-1 and Block-1B Vehicles

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