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| ## PEGAS | ||
| *Powered Explicit Guidance Ascent System* is an autopilot for Kerbal Space Program made and ran in [kOS](http://forum.kerbalspaceprogram.com/index.php?/topic/61827-122-kos-scriptable-autopilot-system-v103-20161207/), designed to control launch vehicles under a modified version of the game running [Realism Overhaul](http://forum.kerbalspaceprogram.com/threads/99966). Its unique feature is an implementation of a real-word rocket guidance algorithm: previously [Powered Explicit Guidance](http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19660006073.pdf), more recently [Unified Powered Flight Guidance](https://ntrs.nasa.gov/search.jsp?R=19740004402) - as used in the **Space Shuttle** GN&C computer. | ||
| *Powered Explicit Guidance Ascent System*, from here referred to as *PEGAS*, is an ascent autopilot for Kerbal Space Program made and ran in [kOS](http://forum.kerbalspaceprogram.com/index.php?/topic/61827-122-kos-scriptable-autopilot-system-v103-20161207/), designed to control launch vehicles under a modified version of the game running [Realism Overhaul](http://forum.kerbalspaceprogram.com/index.php?/topic/155700-113-realism-overhaul). | ||
| Its unique feature is an implementation of a real-word rocket guidance algorithm: Unified Powered Flight Guidance, as used in the **Space Shuttle** GN&C computer for the standard ascent flight mode. | ||
| Short list of what PEGAS is capable of: | ||
| * estimation of a launch window, | ||
| * calculation of a launch azimuth, | ||
| * simple atmospheric ascent by pitching over and holding prograde with zero angle of attack, | ||
| * automatic guidance to orbits defined by: | ||
| * apoapse | ||
| * periapse | ||
| * inclination | ||
| * longitude of ascending node | ||
| * or, alternatively, selecting an existing target, | ||
| * executing of timed events (engine ignition, payload fairing jettison etc.), | ||
| * automatic staging with ullage handling. | ||
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| Right now PEGAS is undergoing a major rework: previously it implemented an old version of PEG, which (among its many limitations) only supported launching to circular orbits with no control over plane - constant azimuth was assumed throughout the mission. Now a full implementation of UPFG (in standard ascent mode) is being coded - this will allow it to target orbits not only of a given shape (apoapsis + periapsis), but also in a given plane (inclination + longitude of ascending node). | ||
| More info on my KSP [forum thread](http://forum.kerbalspaceprogram.com/index.php?/topic/142213-pegas-powered-explicit-guidance-ascent-system-devlog/), also see my [prototype repository](https://github.com/Noiredd/PEGAS-MATLAB). | ||
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| ### Recent advances | ||
| **IMPORTANT**: PEGAS repository has been split into two: [PEGAS-MATLAB](https://github.com/Noiredd/PEGAS-MATLAB) contains the prototype written in MATLAB, with all its bells, whistles and sophistication. This repository ([PEGAS](https://github.com/Noiredd/PEGAS)) *will contain* kOS code, ready-to-use for your launches. | ||
| ### How to use - see [tutorial](tutorial.md) and [reference](reference.md) | ||
| 1. Dowload files from this repository's [kOS folder](kOS) and place them in your `Script` folder. | ||
| 2. Define your vehicle and mission. | ||
| 3. Once on the launch pad, load the definitions from pt. 2. and type `run pegas.` in kOS terminal. | ||
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| PEGAS now uses a very general guidance algorithm, the Unified Powered Flight Guidance. Brief list of most important features includes: | ||
| * targetting orbits with periapsis/apoapsis and **plane** constraints (inclination + longitude of ascending node), | ||
| * support for multistage vehicles with constant-thrust and acceleration-limited modes, | ||
| * simple atmospheric stage guidance in a "pitch over and hold prograde" mode, | ||
| * automatically estimated launch window basing on launch site position and target orbit, | ||
| * no complicated pre-flight analysis needed - as long as the vehicle has enough power, missing the window slightly will not affect insertion precision. | ||
| ### Disclaimer | ||
| This is a first public release of PEGAS. | ||
| Due to sheer amount of work on prototyping and coding it, and the range of potential problems with launch vehicles, I have been unable to test it with many rockets. | ||
| Therefore, I cannot guarantee that it will handle *any* vehicle or that it is entirely bug-free. | ||
| Likely, it will take you several tries before you get your rocket flying - and maybe you will find yourself unable to do that at all. | ||
| I am willing to provide support, correct bugs and (to some extent) introduce new functionalities to PEGAS. | ||
| In case of problems: read the [how to submit issues](docs/issues.md) page and then visit the issue tracker. | ||
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| ### Preview | ||
| You can see the old version of PEGAS in my youtube video below. This will be soon replaced by a new thing, once it's ready. | ||
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| <a href="https://youtu.be/0LGAizO-6K4" target="_blank"><img src="http://img.youtube.com/vi/0LGAizO-6K4/0.jpg" width="240" height="180" border="10" /></a> | ||
| ### Demo | ||
| <a href="https://youtu.be/NEQD7AQoLXk" target="_blank"><img src="http://img.youtube.com/vi/NEQD7AQoLXk/0.jpg" width="240" height="180" border="10" /></a> |
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| // If no boot file was loaded, this check will immediately crash PEGAS, saving time that would otherwise be wasted on loading libraries. | ||
| IF NOT (DEFINED vehicle) OR NOT (DEFINED sequence) OR NOT (DEFINED controls) OR NOT (DEFINED mission) { | ||
| PRINT "". | ||
| PRINT "No boot file loaded! Crashing...". | ||
| PRINT "". | ||
| SET _ TO sequence. | ||
| SET _ TO controls. | ||
| SET _ TO vehicle. | ||
| SET _ TO mission. | ||
| } | ||
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| // The following is absolutely necessary to run UPFG fast enough. | ||
| SET CONFIG:IPU TO 500. | ||
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| // Set up constants. | ||
| GLOBAL g0 IS 9.8067. // PEGAS will launch from any planet or moon - "g0" is a standard constant for thrust computation and shall not be changed! | ||
| GLOBAL pitchOverTimeLimit IS 20. // In atmospheric part of ascent, when the vehicle pitches over, the wait for velocity vector to align will be forcibly broken after that many second. | ||
| GLOBAL upfgConvergenceDelay IS 5. // seconds before "upfgActivation" that we switch into loop 2 to give UPFG time to converge | ||
| GLOBAL upfgFinalizationTime IS 5. // When time-to-go gets below that, keep attitude stable and simply count down time to cutoff. | ||
| GLOBAL stagingKillRotTime IS 5. // That many seconds before staging, updating attitude commands will be forbidden, allowing clean separation. | ||
| GLOBAL upfgConvergenceCriterion IS 0.1. // Maximum difference between consecutive UPFG T-go predictions that allow accepting the solution. | ||
| GLOBAL upfgGoodSolutionCriterion IS 15. // Maximum angle between guidance vectors calculated by UPFG between stages that allow accepting the solution. | ||
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| // Load libraries. | ||
| RUN pegas_cser. | ||
| RUN pegas_upfg. | ||
| RUN pegas_util. | ||
| RUN pegas_misc. | ||
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| // Initialize global flags | ||
| GLOBAL upfgStage IS -1. // Seems wrong (we use "vehicle[upfgStage]") but first run of stageEventHandler increments this automatically | ||
| GLOBAL stageEventFlag IS FALSE. | ||
| GLOBAL systemEvents IS LIST(). | ||
| GLOBAL systemEventPointer IS -1. // Same deal as with "upfgStage" | ||
| GLOBAL systemEventFlag IS FALSE. | ||
| GLOBAL userEventPointer IS -1. // As above | ||
| GLOBAL userEventFlag IS FALSE. | ||
| GLOBAL throttleSetting IS 1. | ||
| GLOBAL steeringVector IS LOOKDIRUP(SHIP:FACING:FOREVECTOR, SHIP:FACING:TOPVECTOR). | ||
| GLOBAL upfgConverged IS FALSE. | ||
| GLOBAL stagingInProgress IS FALSE. | ||
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| // PREFLIGHT ACTIVITIES | ||
| // Set up UPFG target and silently upgrade mission target | ||
| SET upfgTarget TO targetSetup(). | ||
| // Calculate time to launch | ||
| SET currentTime TO TIME. | ||
| SET timeToOrbitIntercept TO orbitInterceptTime(). | ||
| GLOBAL liftoffTime IS currentTime + timeToOrbitIntercept - controls["launchTimeAdvance"]. | ||
| IF timeToOrbitIntercept < controls["launchTimeAdvance"] { SET liftoffTime TO liftoffTime + SHIP:BODY:ROTATIONPERIOD. } | ||
| // Calculate launch azimuth if not specified | ||
| IF NOT mission:HASKEY("launchAzimuth") { | ||
| mission:ADD("launchAzimuth", launchAzimuth()). | ||
| } | ||
| // Set up the system for flight | ||
| setSystemEvents(). // Set up countdown messages | ||
| setUserEvents(). // Initialize vehicle sequence | ||
| setVehicle(). // Complete vehicle definition (as given by user) | ||
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| // PEGAS TAKES CONTROL OF THE MISSION | ||
| createUI(). | ||
| // Prepare control for vertical ascent | ||
| LOCK THROTTLE TO throttleSetting. | ||
| LOCK STEERING TO steeringVector. | ||
| SET ascentFlag TO 0. // 0 = vertical, 1 = pitching over, 2 = notify about holding prograde, 3 = just hold prograde | ||
| // Main loop - wait on launch pad, lift-off and passive guidance | ||
| UNTIL ABORT { | ||
| // Sequence handling | ||
| IF systemEventFlag = TRUE { systemEventHandler(). } | ||
| IF userEventFlag = TRUE { userEventHandler(). } | ||
| // Control handling | ||
| IF ascentFlag = 0 { | ||
| // The vehicle is going straight up for given amount of time | ||
| IF TIME:SECONDS >= liftoffTime:SECONDS + controls["verticalAscentTime"] { | ||
| // Then it changes attitude for an initial pitchover "kick" | ||
| SET steeringVector TO LOOKDIRUP(HEADING(mission["launchAzimuth"],90-controls["pitchOverAngle"]):VECTOR, SHIP:FACING:TOPVECTOR). | ||
| SET ascentFlag TO 1. | ||
| pushUIMessage( "Pitching over by " + ROUND(controls["pitchOverAngle"],1) + " degrees." ). | ||
| } | ||
| } | ||
| ELSE IF ascentFlag = 1 { | ||
| // It keeps this attitude until velocity vector matches it closely | ||
| IF TIME:SECONDS < liftoffTime:SECONDS + controls["verticalAscentTime"] + 3 { | ||
| // Delay this check for the first few seconds to allow the vehicle to pitch away from current prograde | ||
| } ELSE { | ||
| // Attitude must be recalculated at every iteration though | ||
| SET velocityAngle TO VANG(SHIP:UP:VECTOR, SHIP:VELOCITY:SURFACE). | ||
| IF controls["pitchOverAngle"] - velocityAngle < 0.1 { | ||
| SET ascentFlag TO 2. | ||
| } | ||
| } | ||
| // As a safety check - do not stay deadlocked in this state for too long (might be unnecessary). | ||
| IF TIME:SECONDS >= liftoffTime:SECONDS + controls["verticalAscentTime"] + pitchOverTimeLimit { | ||
| SET ascentFlag TO 2. | ||
| pushUIMessage( "Pitchover time limit exceeded!", 5, PRIORITY_HIGH ). | ||
| } | ||
| } | ||
| ELSE IF ascentFlag = 2 { | ||
| // We cannot blindly hold prograde though, because this will provide no azimuth control | ||
| // Much better option is to read current velocity angle and aim for that, but correct for azimuth | ||
| SET velocityAngle TO 90-VANG(SHIP:UP:VECTOR, SHIP:VELOCITY:SURFACE). | ||
| SET steeringVector TO LOOKDIRUP(HEADING(mission["launchAzimuth"],velocityAngle):VECTOR, SHIP:FACING:TOPVECTOR). | ||
| // There are two almost identical cases, in the first we set the initial message, in the next we just keep attitude. | ||
| pushUIMessage( "Holding prograde at " + ROUND(mission["launchAzimuth"],1) + " deg azimuth." ). | ||
| SET ascentFlag TO 3. | ||
| } | ||
| ELSE { | ||
| SET velocityAngle TO 90-VANG(SHIP:UP:VECTOR, SHIP:VELOCITY:SURFACE). | ||
| SET steeringVector TO LOOKDIRUP(HEADING(mission["launchAzimuth"],velocityAngle):VECTOR, SHIP:FACING:TOPVECTOR). | ||
| } | ||
| // The passive guidance loop ends a few seconds before actual ignition of the first UPFG-controlled stage. | ||
| // This is to give UPFG time to converge. Actual ignition occurs via stagingEvents. | ||
| IF TIME:SECONDS >= liftoffTime:SECONDS + controls["upfgActivation"] - upfgConvergenceDelay { | ||
| pushUIMessage( "Initiating UPFG!" ). | ||
| BREAK. | ||
| } | ||
| // UI - recalculate UPFG target solely for printing relative angle | ||
| SET upfgTarget["normal"] TO targetNormal(mission["inclination"], mission["LAN"]). | ||
| refreshUI(). | ||
| WAIT 0. | ||
| }. | ||
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| // ACTIVE GUIDANCE | ||
| createUI(). | ||
| // Initialize UPFG | ||
| initializeVehicle(). | ||
| SET upfgState TO acquireState(). | ||
| SET upfgInternal TO setupUPFG(). | ||
| // Main loop - iterate UPFG (respective function controls attitude directly) | ||
| UNTIL ABORT { | ||
| // Sequence handling | ||
| IF systemEventFlag = TRUE { systemEventHandler(). } | ||
| IF userEventFlag = TRUE { userEventHandler(). } | ||
| IF stageEventFlag = TRUE { stageEventHandler(). } | ||
| // Update UPFG target and vehicle state | ||
| SET upfgTarget["normal"] TO targetNormal(mission["inclination"], mission["LAN"]). | ||
| SET upfgState TO acquireState(). | ||
| // Iterate UPFG and preserve its state | ||
| SET upfgInternal TO upfgSteeringControl(vehicle, upfgStage, upfgTarget, upfgState, upfgInternal). | ||
| // Manage throttle, with the exception of initial portion of guided flight (where we're technically still flying the first stage). | ||
| IF upfgStage >= 0 { throttleControl(). } | ||
| // For the final seconds of the flight, just hold attitude and wait. | ||
| IF upfgConverged AND upfgInternal["tgo"] < upfgFinalizationTime { BREAK. } | ||
| // UI | ||
| refreshUI(). | ||
| WAIT 0. | ||
| }. | ||
| // Final orbital insertion loop | ||
| pushUIMessage( "Holding attitude for burn finalization!" ). | ||
| SET previousTime TO TIME:SECONDS. | ||
| UNTIL ABORT { | ||
| LOCAL finalizeDT IS TIME:SECONDS - previousTime. | ||
| SET previousTime TO TIME:SECONDS. | ||
| SET upfgInternal["tgo"] TO upfgInternal["tgo"] - finalizeDT. | ||
| IF upfgInternal["tgo"] < finalizeDT { BREAK. } // Exit loop before entering the next refresh cycle | ||
| // We could have done "tgo < 0" but this would mean that the previous loop tgo was 0.01 yet we still didn't break | ||
| refreshUI(). | ||
| WAIT 0. | ||
| }. | ||
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| // EXIT | ||
| UNLOCK STEERING. | ||
| UNLOCK THROTTLE. | ||
| SET SHIP:CONTROL:PILOTMAINTHROTTLE TO 0. | ||
| WAIT 0. | ||
| missionValidation(). | ||
| refreshUI(). |