5/1/19 Seminar 11 Project Management & System Design Spacecraft Guidance Robert Stengel FRS 148, From the Earth to the Moon Princeton University Project Management & Spacecraft Design Fundamentals of Space Systems, Ch 1 NASA-SP-2016-6105 Spacecraft Guidance: Understanding Space Sec 12.3 Copyright 2019 by Robert Stengel. All rights reserved. For educational use only. 1 http://www.princeton.edu/~stengel/FRS.html Systems Engineering and Management § Introduction § Fundamentals of System Engineering § Concepts in Systems Engineering § Project Development Process § Management of the Development of Space Systems § Organization Pisacane, V., Fundamentals of Space System Design, Ch. 1 2 1 5/1/19 Systems Engineering 3 Pisacane, V., Fundamentals of Space System Design, Ch. 1 Product Development Pisacane, V., Fundamentals of Space System Design, Ch. 1 4 2 5/1/19 NASA-2016-6105 5 6 NASA-2016-6105 3 5/1/19 Spacecraft Mission Objectives and Requirements Fortescue 7 Requirements Definition • What must the system accomplish? • Why must it be done? • How do we achieve the design goal? • What are the alternatives? • What sub-systems perform what functions? • Are all functions technically feasible? • How can the system be tested to show that it satisfies requirements? Wertz and Larson 8 4 5/1/19 Program Management: Gantt Chart • Project schedule • Task breakdown and dependency • Start, interim, and finish elements • Time elapsed, time to go 9 Program Evaluation and Review Technique (PERT) Chart • Milestones • Path descriptors • Activities, precursors, and successors • Timing and coordination • Identification of critical path • Optimization and constraint 10 5 5/1/19 PERT Charts 11 Project Phases 12 6 5/1/19 Life Cycle Cost Impacts 13 Spacecraft Subsystems 14 7 5/1/19 Satellite Systems • Power and •Structure •Electronics Propulsion –Skin, frames, ribs, –Payload –Solar cells stringers, –Control bulkheads –Kick motor/ –Radio payload assist –Propellant tanks transmitters and module (PAM) –Heat/solar/ receivers –Attitude-control micrometeoroid –Radar shields, insulation –orbit-adjustment transponders –Articulation/ –Antennas –station-keeping deployment –Batteries, fuel mechanisms cells –Gravity-gradient –Pressure tanks tether –De-orbit systems –Re-entry system (e.g., sample return) 15 Functional Requirements of Spacecraft Subsystems 1. Payload must be pointed in the right direction 2. Payload must be operable 3. Data must be communicated to the ground 4. Desired orbit for the mission must be maintained 5. Payload must be held together and mounted on the spacecraft structure 6. Payload must operate reliably over some specified period 7. Adequate power must be provided 16 8 5/1/19 LADEE Typical Satellite Mass Breakdown • Satellite without on-orbit/de-orbit propulsion • Kick motor/ PAM can add significant mass 17 Communications Satellite Mass Breakdown 18 9 5/1/19 Recommended Mass Growth Margins Pisacane, V., Fundamentals of Space System Design, Ch. 1 19 Guidance, Navigation, and Control 20 10 5/1/19 Guidance, Navigation, and Control • Navigation: Where are we? • Guidance: How do we get to our destination? • Control: What do we tell our vehicle to do? 21 First Apollo Program Contract MIT Instrumentation Laboratory August 9, 1961 22 11 5/1/19 Components of Apollo CSM Primary Navigation Guidance and Control System (“PNGCS”) Mindell, D., Digital Apollo, Ch. 5 23 Landmark Tracking for Apollo Guidance Mindell, D., Digital Apollo, Ch. 5 24 12 5/1/19 IMU Alignment and State Update Mindell, D., Digital Apollo, Ch. 5 25 Apollo Guidance Computer • Parallel processor • 16-bit word length (hexadecimal) • Memory – 36,864 words (fixed) – 2,048 words (variable) • 1st operational solid-state computer • Identical computers in CSM and LM – Different software (with many identical subroutines) http://klabs.org/history/build_agc/ 26 13 5/1/19 Apollo Guidance Computer Magnetic Core Memory Ropes 1 core = 1 bit 1 Kiloword Memory Bank § No built-in redundancy § No redundant computers § No failures § Mean time between failures = ∞ 27 Apollo Lunar Module Radars • Landing radar • Rendezvous radar – 3-beam Doppler – continuous-wave radar altimeter tracking radar – LM descent stage – LM ascent stage 28 14 5/1/19 Apollo Guidance Computer Commands • Display/Keyboard (DSKY) • Sentence – Subject and predicate – Subject is implied • Astronaut, or • GNC system – Sentence describes action to be taken employing or involving an object • Predicate – Verb = Action – Noun = Variable or Program (i.e., the object) 29 Numerical Codes for Verbs and Nouns in Apollo Guidance Computer Programs Verb Code Description Remarks 01 Display 1st component of Octal display of data on REGISTER 1 02 Display 2nd component of Octal display of data on REGISTER 1 03 Display 3rd component of Octal display of data on REGISTER 1 Noun Code Description Scale/Units 01 Specify machine address XXXXX 02 Specify machine address XXXXX 03 (Spare) 04 (Spare) 05 Angular error XXX.XX degrees 06 Pitch angle XXX.XX degrees Heads up-down +/- 00001 07 Change of program or major mode 11 Engine ON enable 30 15 5/1/19 Verbs and Nouns in Apollo Guidance Computer Programs • Verbs (Actions) • Selected Nouns • Selected Programs – Display (Variables) (CM) – Enter – Checklist – AGC Idling – Monitor – Self-test ON/OFF – Gyro Compassing – Write – Star number – LET Abort – Terminate – Failure register – Landmark Tracking – Start code – Ground Track – Change – Event time Determination – Align – Inertial velocity – Return to Earth – Lock – Altitude – SPS Minimum – Set – Latitude Impulse – CSM/IMU Align – Return – Miss distance – Test – Delta time of burn – Final Phase – First Abort Burn – Calculate – Velocity to be gained – Update 31 A Little AGC Digital Autopilot Code 32 16 5/1/19 Apollo GNC Software Testing and Verification • Major areas of testing – Computational accuracy – Proper logical sequences • Testing program – Comprehensive test plans – Specific initial conditions and operating sequences – Performance of tests – Comparison with prior simulations, evaluation, and re-testing • Levels of testing – 1: Specifications coded in higher-order language for non-flight hardware (e.g., mainframe then, PCs now) – 2: Digital simulation of flight code – 3: Verification of complete programs or routines on laboratory flight hardware – 4: Verification of program compatibility in mission scenarios – 5: Repeat 3 and 4 with flight hardware to be used for actual mission – 6: Prediction of mission performance using non-flight computers and laboratory flight hardware 33 Lunar Module Navigation, Guidance, and Control Configuration 34 17 5/1/19 Lunar Descent Guidance 35 Lunar Module Transfer Ellipse to Powered Descent Initiation 36 18 5/1/19 Lunar Module Powered Descent 37 Lunar Module Descent Events 38 19 5/1/19 Lunar Module Descent Targeting Sequence • Braking Phase (P63) • Approach Phase (P64) 39 • Terminal Descent Phase (P66) Characterize Braking Phase By Five Points 40 20 5/1/19 Lunar Module Descent Guidance Logic (Klumpp, Automatica, 1974) • Reference (nominal) trajectory, rr(t), from target position back to starting point (Braking Phase example) – Calculated before mission – Three 4th-degree polynomials in time – 5 points needed to specify each polynomial ⎡ x(t)⎤ t 2 t 3 t 4 ⎢ ⎥ r (t) = r + v t + a + j + s r(t) = y(t) r t t t 2 t 6 t 24 ⎢ ⎥ ⎣⎢ z(t)⎦⎥ 41 Coefficients of the Polynomials t 2 t 3 t 4 r (t) = r + v t + a + j + s r t t t 2 t 6 t 24 • r = position vector • v = velocity vector ⎡ ⎤ • a = acceleration vector ⎡ x⎤ ⎡ x! ⎤ vx ⎢ ⎥ dr ⎢ ⎥ • j = jerk vector (time v ⎢ y ⎥ v r = y = = ⎢ ! ⎥ = ⎢ y ⎥ derivative of acceleration) ⎢ ⎥ dt ⎢ z⎥ ⎢ z! ⎥ ⎢ ⎥ ⎣ ⎦ ⎣ ⎦ vz • s = snap vector (time ⎣⎢ ⎦⎥ derivative of jerk) ⎡ ⎤ ⎡ ⎤ ⎡ ⎤ ax jx sx dv ⎢ ⎥ da ⎢ ⎥ dj ⎢ ⎥ a = = a j = = j s = = s dt ⎢ y ⎥ dt ⎢ y ⎥ dt ⎢ y ⎥ ⎢ a ⎥ ⎢ j ⎥ ⎢ s ⎥ ⎣⎢ z ⎦⎥ ⎣⎢ z ⎦⎥ ⎣⎢ z ⎦⎥ 42 21 5/1/19 Corresponding Reference Velocity and Acceleration Vectors t 2 t 3 v (t) = v + a t + j + s r t t t 2 t 6 t 2 a (t) = a + j t + s r t t t 2 • ar(t) is the reference control vector – Descent engine thrust / mass = total acceleration – Vector components controlled by orienting yaw and pitch angles of the Lunar Module 43 Guidance Logic Defines Desired Acceleration Vector • If initial conditions, dynamic model, and thrust control were perfect, ar(t) would produce rr(t) t 2 a (t) = a + j t + s ⇒ r t t t 2 t 2 t 3 t 4 r (t) = r + v t + a + j + s r t t t 2 t 6 t 24 • ... but they are not • Therefore, feedback control is required to follow the reference trajectory 44 22 5/1/19 Guidance Law for the Lunar Module Descent Linear feedback guidance law (real time acommand (t) = ar (t) + KV [v measured (t) − vr (t)]+ KR [rmeasured (t) − rr (t)] KV :velocity error gain KR :position error gain • Nominal acceleration profile is corrected for measured differences between actual and reference flight paths • Considerable modifications made in actual LM implementation (see Klumpps paper on Blackboard) 45 LM Manual Control Response During Simulated Landing 46 23 5/1/19 Simulated LM Manual Control Response To Rate Command 47 Next Time: The Future of Space Flight Telemetry, Communications & Tracking 48 24 5/1/19 Supplemental Material 49 Apollo GNC Software Specification Control • Guidance System Operations Plan (GSOP) – NASA-approved specifications document for mission software – Changes must be approved by NASA Software Control Board • Change control procedures – Program Change Request (NASA) or Notice (MIT) – Anomaly reports – Program and operational notes • Software control meetings – Biweekly internal meetings – Joint