Kerbal Space Program Space Shuttle OPS1 Ascent Guidance

Updated June 2024 - please go through the README

My KSP ascent guidance script for the Space Shuttle, intended to be used in KSP Realism Overhaul. Uses Powered Explicit Guidance (also called UPFG) for vacuum guidance, adapted and modified to perform aborts. Allows for assisted manual flight.

References

• SPACE SHUTTLE ORBITER OPERATIONAL LEVEL C SOFTWARE REQUIREMENTS - GUIDANCE ASCENT/RTLS
• Flight Procedures Handbook - Ascent/Aborts (OI-30)

Installation

Required mods:

• A complete install of RSS/Realism Overhaul with Ferram Aerospace Resarch.
• kOS version 1.3 at least
• kOS Ferram, now available on CKAN
• My own fork of SpaceODY's Space Shuttle System.
  • if you use the latest version you will be required to also grab my Ferram Fork to use the custom aerodynamics module. Refer to the README
• My OPS3 Shuttle entry program required by RTLS and TAL aborts. Grab the latest version from its repo
  • no longer compatible with the older kOS-ShuttleEntrySim program

Known Incompatibilities

• Not compatible with SpaceODY's original fork or any other Shuttle mod
• RO Engines It should now be compatible, but not recommended

You will find one folder:

• Script

Put the contents of the Script folder inside Ship/Script so that kOS can see all the files. There are a few scripts you can run:

• ops1.ks to setup a launch of the Shuttle from the launchpad according to specified mission parameters (read on to learn about mission setup).
• ops1_3a.ks is an identical script with special parameters for Polar orbit launches from Vandenberg
• node.ks a little manoeuvre node executor that I use to execute OMS burns in orbit, it takes care of the offset OMS thrust position (so it even works with a single OMS burn) and is compatible with nodes created with Principia

Setup

The script needs to know accurately the mass of orbiter + payload + ET + propellants for closed-loop guidance, without the mass of SRB or launch clamps. The script will measure everything automatically provided that the part tree is set up correctly in the VAB.

Take care of the following things while building the Shuttle Stack in the VAB:

• The root part must be the Space Shuttle Orbiter part you find in my fork of Space Shuttle System
• The ET must be a child part of some orbiter part (for the Space Shuttle System mod it's attached to the cargo bay by default)
• Set fuel flow priority on the Orbiter lower than the External Tank, or else you won't have cryos for the fuel cells once you're in orbit
• The SRB decouplers must be attached to the External Tank, so that all SRB-related parts are children of the ET
• Any launch clamps/towers must be attached either to the ET or the SRBs, don't attach anything to the Orbiter
• Don't place clamps on the opposite side to where the Orbiter is because the angled SSMEs will push the stack in that direction and you may have a collision
• I advise you to disable the fairings on the RCS covers as once they are jettisoned they might collide with other parts and destroy them

Make sure the vessel staging is as follows (from the bottom stage upwards) :

• SSMEs and RCS staging toggles (required for aborts and MECO attiude control)
• SRBs and any launch clamps / platform
• SRB decouplers and separation motors (both nosecone and skirt)
• External tank separation and OMS engines
• Anything in the payload bay
• Tail parachute

• Don't forget to set the FAR control surface settings as required by the Entry script README.

• right-click on the SSMEs and then open up the Real Fuels GUI. Make sure you are selecting an appropriate version of SSME (refer to this Wikipedia table if you want to select the version accurately). Make sure you select the same version for all three SSMEs.
• if you do a launch from Vandenberg, switch the SRB type to 'Filament-wound casing' for extra performance

If you don't use SpaceShuttleSystem parts, or if you mismatch SSME versions, the script should detect this and break during vehicle initialisation. This is intended and meant to signal to you that something is wrong.

kOS configuration file

The mission parameters are specified in the main launch script ops1.ks or ops1_3a.ks. The scripts assume that the Shuttle stack has been assembled properly in the VAB, else it might fail to measure things accurately.

It contains variable definitions for:

• a "target_orbit" structure describing the shape of the desired orbit. Periapsis and cutoff altitude are standard for a Shuttle launch and shouldn't be changed. Only change the apoapsis and inclination to suit your needs.
• a disabled variable engine_failure_time which you can uncomment to trigger an automatic engine failure at the specified time. More on aborts later on.
• a variable to enable telemetry logging in the UPFG_OPS1/LOGS/ folder if you want to plot it with your own means

The main Ascent script

Upon running ops1.ks you will be greeted with a GUI with buttons and a data display to interface with the program:

• The X button will interrupt the program at any point
• DAP selects the digital autopilot modes. By default it's AUTO but it can be switched to CSS for manual flight. More on autopilot modes later on.
• The red ABORT button triggers a manual abort and the selector menu to its left selects the modes currently available. More on aborts later on.
• the TAL site selector is specifically to choose an available landing site for a manual TAL abort
• the red button with the ! symbol opens the Engine Failure setup panel, more on this in the aborts section
• the Display will show different things depending on the flight phase. There will be plenty more about this later on.
• the Message Window at the bottom displays printouts of the program state. Useful to monitor the abort state

The abort buttons and selectors, except for engine failure, are frozen at program start and unfreeze after SRB separation

The ASCENT TRAJ displays

These two displays show useful data and trajectory printouts:

• TRAJ 1 displays first stage (SRB) flight until right before SRB sep
• TRAJ 2 takes over from there all the way to MECO
• At the top you have the display title and the running mission elapsed time
  • the display title is ASCENT in the nominal case, but in an abort it will change to RTLS, TAL, ATO or CONT
• On the left side you have:
  • current vertical speed Ḣ
  • the errors with respect to guidance commands: roll R, pitch P, yaw Y, throttle T
  • R,P,Y errors are in angles with respect to the commanded thrust direction in the vehicle-fixed frame of reference, T is a percentage.
  • When the errors are too large, the numbers will turn yellow
• CONT ABORT shows the current active contingency abort regions for 2- and 3-engine-out situations. More about this in the aborts section
• The TRAJ 1 central plot shows altitude vs. surface velocity
  • The ticks show the pitch angle that the Shuttle should have at that moment
  • The solid line is the typical nominal trajectory
  • The dashed line is the trajectory in case of one engine out at liftoff
  • The triangle is the current state, the circle is the predicted state 15 seconds into the future
• The TRAJ 2 plot is altitude vs. inertial (orbital) velocity
  • the solid line shows the early lofting and droop which is typical of the Shuttle
  • the ticks show roughly the intact abort boundaries: negative return (RTLS), press to ATO (ATO), last TAL opportunity (TAL)
  • The triangle is the current state, the circle is the predicted state 30 seconds into the future
• On the right side you have:
  • a slider with the current acceleration in G-units, the indicator will turn yellow if it exceeds 3G
  • THROT is the current throttle level in units of Rated Power Level
  • PROP is the percentage of remaining propellant in the External Tank
• On the TRAJ 2 display, below the title, there is the MECO velocity line indicator, close to MECO it shows how far off from the targeted cutoff velocity (CO) you are
• On the TRAJ2 display, in the top right corner, are some UPFG-specific values:
  • TGO is the guidance-calculated time to MECO
  • VGO is the guidance-calculated delta-V to gain to MECO
  • the numbers will be yellow when the algorithm is unconverged, green upon convergence

Meaning of Rated Power Level and the THROT indicator

The Space Shuttle Main Engine had several performance improvements in its operational life, and you have all the variants at your disposal in KSP RO.
The caveat is that "full power" is an ambiguous term in this situation. For this reason we define the Rated Power Level (100% RPL) as 2090 kN, the max thrust of the original variant. This is significant because for the later part of the Shuttle program, when they used the RS-25D variant, the nominal throttle setting was 104% RPL and boosted to the full 109% RPL in case of an abort. This was done to reduce wear and risk of failure on the engines, it's all realistic and simulated by the script.

How to fly manually with CSS

CSS means control-stick-steering, a fly-by-wire mode that allows you to use keyboard or joystick to command changes in pitch, roll or yaw letting kOS worry about keeping stability.
At any time you can switch between the two. A manual ascent from liftoff to orbit in CSS is possible but challenging. Here are some tips and remarks on how to do it:

• switch the KSP camera to Locked or switch to IVA mode
• the errors are defined relative to the body axes, not the Earth's horizon
• Positive Roll and Yaw errors means you need to correct to the right. Positive pitch error means you need to pitch up. Positive throttle error means you need to throttle up
• You will need to constantly correct, especialy in pitch
• Make small corrections and see the result in the Display before correcting further
• The initial roll manoeuvre is by far the hardest:
  • it's a combined roll, pitch and yaw manoeuvre that puts you on the right azimuth for ascent
  • It's useful not to rely solely on the numbers and also have a mental image of where the Shuttle is and how it needs to move to ascend heads-down
  • use continuous roll inputs and correct pitch and yaw as required by the error numbers
• During TRAJ 1 you can use the trajectory plot as a guide to correct your pitch, you shouldn't go too far off the line
• During TRAJ 2 I don't suggest you try to follow the line closely, the Guidance commands already account for any error you accumulated

Nominal mission scenario

Running the ops1.ks or ops1_3a.ks script is the only action required for a nominal launch (aborts are different). Simply running the script will program the launch about 10 seconds after running the script, meaning that the LAN of the target orbit depends on the time of day you choose to launch.
You can launch to rendezvous with a vessel in orbit by selecting it as a target in the map view BEFORE running the script. This will override the launch inclination to match and warp to the right time to launch so the LAN is correct.

These are the main events durign ascent:

• Fuel cells are automatically activated at liftoff
• Dynamic pressure (Q) is monitored throughout and SSME throttle will be reduced to 75% RPL during the Max-Q period. If you fly CSS you have to be alert and adjust throttle
• SRB separation is thrust-dependent and usually happens around the T+02:00 mark
• 5 seconds after SRB sep the program transitions to closed-loop guidance using the UPFG algorithm
• It should take around 10 iterations for the algorithm to converge and in the nominal case it should stay converged throughout
• On a nominal mission a roll to heads-up attitude is performed at T+05:50
• About a minute before MECO, Guidance will start throttling down the engines to maintain a maximum acceleration of 3G. Again, if you're flying CSS you need to be alert for this
• 5 seconds before the targeted MECO the program transitions to terminal guidance at minimum throttle
  • alternatively it can happen 5 seconds before the Shuttle runs out of propellant
• After MECO the program will automatically:
  • disable SSME gimballing
  • trigger ET sep
  • command an RCS vertical translation manoeuvre
  • close the umbilical doors
• The program then prints a message in the message window, displaying the results of an orbital analysis calculating the erros with respect to the targeted orbit
• 5 seconds after this, the program will quit itself
• Do not forget that the nominal ascent puts the shuttle on a trajectory that dips back into the atmosphere for ET disposal. You must perform manually an OMS bun to circularise.

Abort scenarios

General remarks

Abort modes are usually required whenever one or more engines fail. You can either shutdown an engine manually or setup an automatic failure using the Engine Failure panel:

• The failure mode by default is OFF
• the RAND setting will run a probability-based algorithm to decide whether to trigger a single, double or even triple engine failure at any moment
• the TIME setting allow you to schedule up to three failures at any MET you like. Click the + button to add a failure

There are two kinds of abort modes:

• Intact aborts where there always is a procedure that will take the Shuttle to a safe trajectory, either to space or to a landing runway
• Contingency aborts where the Shuttle cannot achieve a nominal safe trajectory and will not reach space in any way. There is no guarantee to reach a runway or even survive the reentry into the lower atmosphere

Intact aborts

Intact aborts mean that there always is a procedure that will take the Shuttle to a landing runway with sufficient energy. The Shuttle has continuous intact abort coverage from liftoff to orbit in case of a single engine failure.
In some regions, even a double engine failure will allow an intact abort. This is valid both for a sudden double-engine failure or a second engine failure during an ongoing abort

• Intact aborts can be triggered either manually or automatically
• Remmebr that the abort selectors and buttons are inactive until after SRB separation
• Manually, you can select an available mode from the GUI menu and then press the red ABORT button to activate it
• In case of an engine failure, you have 10 seconds to select a manual abort, after which the program will automatically activate an abort mode
  • The automatic intact abort logic is: ATO-TAL-RTLS in this order of preference, based on availability
• You can manually activate an abort even without an engine failure, guidance will work just the same

The above chart shows the intact abort modes and their boundaries:

• Return to launch site (RTLS) is available from liftoff to about 2400 m/s surface-relative (the actual boundary depends on inclination). The boundary is called NEGATIVE RETURN
• Transoceanic Abort Landing (TAL) is available from just before Negative Return until late in the ascent (until Single-engine Press to ATO is available)
  • late in the ascent, Single-engine TAL should become available, this also disappears when Single-engine Press to ATO is available
• Press to ATO is available some time after Negative Return until Press to MECO
  • later in the ascent, Single-engine press to ATO is available
• Press to MECO is available roughly a minute after press to ATO, this is not really an abort mode
  • later in the ascent, Single-engine press to ATO is available

Contingency aborts

Coming soon, not implemented yet