B9 Part Switch Fatal Error | Fast Fixes For Modded KSP

Seeing a B9 Part Switch fatal error pop up while Kerbal Space Program loads can drain the fun out of a modded save. The message tells you to quit the game and fix your setup, yet the log spam looks like gibberish. The good news is that this type of crash usually traces back to a short list of issues you can track down with a calm, methodical approach now.

What Triggers This B9 Part Switch Error?

Big picture: B9PartSwitch is a plugin that lets parts swap fuel setups, meshes, or modules through config patches. When it throws a fatal error on load, it means a config somewhere asked B9PartSwitch to use a tank type or resource definition it cannot find, or the plugin itself does not match your game version.

Common log lines look similar across many reports. You might see lines about “No tank type named” a given label, or “no resource definition named” a resource such as MaterialKits, followed by a note that tank definitions have not loaded yet. Those clues point straight at a missing dependency or a broken configuration patch in another mod that relies on B9PartSwitch.

• Missing tank types in patches — A part config refers to a tank type name, yet still no matching tank definition exists anywhere in any loaded config file.
• Missing resource definitions — The log mentions a resource name like MaterialKits that does not exist in the game database, which often means a shared resource pack or a similar library mod is absent or out of date.
• Wrong mod version for your KSP build — B9PartSwitch or a dependent mod targets a different KSP version than the one you run, which can keep tank definitions from loading at the right time.
• Broken or half-installed mod folders — A mod that depends on B9PartSwitch ships its own patches, but some of its files were copied into the wrong folder or left behind from an older release, so B9PartSwitch parses patches that no longer match any tank definitions.

Once you know these patterns, the error box that says “B9PartSwitch has encountered a fatal error and KSP needs to close” starts to feel less like a dead end and more like a pointer toward the guilty file.

B9 Part Switch Fatal Error Fix Steps That Work

This part of the guide lays out a simple flow you can follow each time the game refuses to load due to the b9 part switch fatal error. Work through the steps in order and stop as soon as KSP loads cleanly; then make a backup of your working GameData so you have a safe restore point.

Step 1: Capture The Full Error And Log

First capture: When the pop-up appears, take a screenshot of the message and copy the log line that lists the missing tank type or resource, along with the part name if shown. Exit the game through the dialog so KSP writes a clean log file.

• Open KSP’s Player.log — On Windows this usually lives under your user folder in the Unity logs directory; on Steam installs you can also use the “Show Logs” option in mod managers.
• Search for “B9PartSwitch” — Scroll near the bottom of the file and search for the word B9PartSwitch; the final fatal error stack trace will sit close to the end of the log.
• Note missing names — Write down any missing tank type names, resource names, or part names mentioned in the trace, such as DS_H2O, MaterialKits, or a specific part ID.

Step 2: Check Dependencies With A Mod Manager

Dependency sweep: Many players hit this error when a dependency such as a shared resource pack, ModuleManager, or an NF library is missing or outdated. A mod manager makes that easy to spot.

• Launch CKAN or your chosen manager — Let it refresh its mod list so it knows about current releases that match your KSP version.
• Filter around B9PartSwitch — Select B9PartSwitch and inspect its listed dependencies; confirm that each one is installed and matches the version range for your game build.
• Update shared libraries — Bring CRP resource pack, NF libraries, and any fuel switch or resource packs up to date, then relaunch the game for a quick test run.

Step 3: Match The Error To A Mod Folder

Targeted search: The missing tank type or resource name almost always belongs to a specific mod. Matching that name to a folder cuts out guesswork and keeps you from deleting half your setup.

• Search GameData configs — Use a text search tool on the GameData folder for the missing name; many names appear inside B9_TANK_TYPE or RESOURCE_DEFINITION nodes.
• Link name to mod — Once you see which folder the definition should live in, you know which mod owns it; compare that mod’s files with a fresh download or CKAN install.
• Reinstall the owner mod — Delete that specific mod’s folder under GameData, then reinstall clean from its original archive to avoid stray old configs.

Log Hint	Likely Cause	Quick Fix
No tank type named ‘DS_H2O’	Water tank type missing from a life systems or food mod	Reinstall the mod that adds that tank type and confirm B9 configs are present
No tank type named ‘SEP supply tank type’	Starship-style expansion pack or a similar mod missing its tank config file	Update the SEP-style pack and ensure its B9 tank config file loads after dependencies
No resource definition named ‘MaterialKits’	CRP resource pack absent or too old for current mods	Install or update the CRP resource pack through CKAN

Checking Game Version And Core Dependencies

A stable base install matters a lot for B9PartSwitch. The plugin relies on a matching trio of versions: KSP, ModuleManager, and its own release. If one piece lags behind, the plugin can load at the wrong stage and throw fatal errors even when configs look fine.

Confirm Your KSP Build

Version check: Open the main menu of KSP and read the version number in the corner. Compare that build with the versions listed on the B9PartSwitch download page or CKAN metadata. Running a much older or newer build than the mod expects raises your risk of load failures.

• Aim for listed ranges — If your game build falls outside the range listed for the latest B9PartSwitch release, pin the mod to an older release that matches your build.
• Avoid mixed installs — Do not copy GameData from another KSP install with a different version into your current folder; that mix leaves some mods targeting the wrong API level.

Update B9PartSwitch And ModuleManager

Core refresh: Out-of-date core plugins are a frequent source of B9 errors. Bringing them up to date clears a lot of silent incompatibilities before you spend time rebuilding your full mod list.

• Grab the latest releases — Through CKAN or manual download, install the current B9PartSwitch and ModuleManager versions that match your KSP build.
• Remove duplicates — In GameData, check that you do not have multiple ModuleManager DLL files or multiple B9PartSwitch folders left from older installs, as that can confuse load order.
• Test with a light mod set — Temporarily move large part packs out of GameData, then launch with only B9PartSwitch and its direct dependencies to confirm the core stack works.

Hunting Down Broken Tank Types And Resources

Once you know the core stack is healthy, remaining crashes usually come from specific part packs or patches that ask B9PartSwitch for resources or tank setups that no longer exist. A simple routine speeds up that hunt.

Use A Half-Split Method On Part Packs

Halving approach: When you run dozens of part packs, testing one by one can take ages. A half-split method reboots the game fewer times while still revealing which pack triggers the b9 part switch fatal error.

• Split part packs — Divide big part mods into two groups, move one group out of GameData, and launch the game.
• Watch for clean load — If the game loads without the fatal error, the problem sits in the group you removed; if it still fails, swap groups and repeat.
• Drill down by halves — Keep halving the suspect group until you reach a single mod that always triggers the error when present.

Repair Or Patch The Offending Mod

Local fix: After you find the mod that ties to the missing tank type or resource, you can repair it in several ways without losing your whole setup.

• Reinstall from a fresh archive — Delete that mod’s folder and reinstall it clean to wipe stray legacy configs that reference removed tank types.
• Check for compatibility notes — Many KSP forum threads mention small patches or extra config files that players apply when a pack update lags behind B9PartSwitch updates.
• Disable broken parts — As a last resort, move or comment out the specific part configs that mention the missing tank type, so the rest of the pack can load while you wait for an updated patch.

Cleaning Up GameData And Mod Installs Safely

After several rounds of installing and removing mods, a GameData folder can collect leftover configs, extra DLLs, and partial updates that confuse B9PartSwitch. A short cleanup session often removes hidden causes of fatal errors.

Take A Backup Before You Edit

Safety first: Copy your entire KSP folder, or at least GameData and saves, to another location. That snapshot lets you test changes without fear that a mistake will wipe a long-running career.

• Compress old installs — Zip older GameData layouts so you can roll back to a known working set if the new configuration misbehaves.
• Label backups with versions — Add the KSP build and date to backup names so you can match them to specific mod mixes later.

Clear Out Duplicates And Orphans

Folder sweep: With a backup safe, walk through GameData by hand and remove obvious problems that often sit behind fatal load errors.

• Delete stray ModuleManager files — Keep only the latest ModuleManager DLL; multiple versions can produce unpredictable patch order.
• Remove empty mod folders — Delete folders that contain only old version files or readme texts with no valid configs or DLLs.
• Check nested GameData folders — Some manual installs drop a GameData folder inside GameData, which leads to duplicated plugins and configs in the wrong path.

Final Checks Before You Relaunch Kerbal Space Program

Once you reach this stage, your setup should contain a matching KSP build, a current B9PartSwitch and ModuleManager pair, and part packs that all ship valid tank and resource definitions. A final round of checks helps lock in that stability.

• Run one clean launch — Start KSP, reach the main menu, then exit without loading a save so log files record a clean session.
• Scan logs for new warnings — Open Player.log and search for “B9PartSwitch”; confirm that no new warnings or fatal errors appear near the end of the file.
• Create a fresh test save — Build a simple craft that uses parts from the once-troublesome pack and confirm that fuel switch menus appear and behave as expected.
• Keep a “good state” backup — Copy the working GameData folder somewhere safe so you can roll back to this stable point if a later mod update triggers another fatal error.

Once you have a stable setup and a known-good backup, the b9 part switch fatal error loses its sting. If the message resurfaces after you add a new mod or change a part pack, you already have a clear, repeatable routine: capture the log, match the missing tank type or resource to its owner, repair the install, and relaunch. With that habit in place, you spend less time staring at loading screens and more time flying missions. That habit makes later fixes far quicker to handle again.