How to Fix a Create Ultimate Selection 2 Server That Keeps Crashing
Create Ultimate Selection 2 bundles Create Aeronautics with the best Create addons on NeoForge 1.21.1, which means your server is doing engineering-scale mechanical simulation and flying-contraption tracking at the same time. Most crash reports for this pack trace back to one of a handful of causes: a Java version mismatch, airship/contraption tick cost piling up, an addon interaction conflict, or RAM provisioned below the pack's real floor. Work through these in order before assuming anything is broken.
See the Create Ultimate Selection 2 pack page for the full RAM and setup breakdown before you chase anything else.
1. Check you're actually running Java 21
NeoForge 1.21.1 requires Java 21 to boot at all. If the server was set up by copying a launch script from an older Minecraft version, or your host's default Java runtime wasn't updated, the server process will fail at startup rather than running badly — a stuck or instantly-dying server on first launch is almost always a Java version problem before it's anything Create-related.
- Confirm the JVM version your start script actually points to, not just what's installed system-wide
- A startup-only crash (never reaches world load) points here first, before you look at any mod logic
2. Airship and contraption tick cost adding up
Create Aeronautics adds real, ongoing tick cost for every flying contraption in the air — on top of the mechanical-network calculations Create already runs for cogs, shafts, belts and pumps. This is a well-documented pattern across Create-flight packs generally, not a bug specific to this one. If TPS drops noticeably whenever a large airship is airborne, or a crash consistently lines up with an airship taking off or flying over a long distance, the contraption load is the likely driver.
Start by checking server TPS/tick time during a flight versus on the ground, and see whether the problem scales with the size of the airship rather than the number of players.
3. Addon interaction conflicts
Create Ultimate Selection 2 runs multiple Create addons side by side, and combining several mods that each hook into Create's mechanical and contraption systems raises the odds of a specific addon-interaction conflict surfacing — even when each addon is stable on its own. If a crash only happens around a particular machine type or block combination rather than being random, that points toward an interaction between two specific addons rather than a general performance issue.
4. Desync from airships crossing unloaded chunks
Large airships crossing into unloaded or freshly-generated chunks is a known source of desync in Create-flight packs generally. The contraption's internal block state and the world's chunk-loading state can briefly fall out of sync at the boundary, which is more likely to produce a crash the farther and faster an airship travels from already-explored territory. If this matches your crash pattern, try keeping flight paths over pre-loaded terrain while you confirm it, rather than ruling out the mod entirely.
5. RAM under the pack's realistic floor
8 GB is the real floor for Create Ultimate Selection 2 on NeoForge 1.21.1. Running below that rarely causes an instant, clean crash — what you'll actually see is cascading lag and growing garbage-collection pauses over the course of a session as Create's mechanical networks and Aeronautics' contraption tracking compete for headroom, eventually tipping into a tick timeout kick or crash once the server can't recover between GC cycles.
FAQ
Why does my Create Ultimate Selection 2 server lag out specifically when a big airship is flying?+
Create Aeronautics tracks every block in a flying contraption and recalculates its position and collision each tick, on top of the mechanical-network calculations Create already does for cogs, shafts and belts. A large airship effectively stacks both costs at once, so the bigger the build, the more tick time it eats — this is a well-documented pattern in Create-flight packs, not unique to this one.
The server crashes only when players fly far from spawn — is that related to the mod, or just distance?+
It's usually related. Airships crossing chunk borders into unloaded or just-generated chunks are a known source of desync in Create-flight packs, because the contraption's blocks and the chunk's loaded state can briefly disagree. If crashes cluster around long-distance airship trips rather than base building, that's the first thing to isolate before assuming a general performance problem.
How much RAM does Create Ultimate Selection 2 actually need to run stable?+
8 GB is the real floor for this pack on NeoForge 1.21.1, even though a quick glance at the mod list might suggest less. Running below that doesn't usually produce a clean crash — it shows up first as rising GC pauses and tick lag that gets worse over a session, then eventually a tick timeout kick or crash once the garbage collector can't keep up.
Host Create Ultimate Selection 2 on hardware sized for its real floor
Create Ultimate Selection 2 needs a genuine 8 GB floor to handle Create's mechanical networks plus Aeronautics' contraption tracking without GC-driven lag spikes — and a backup taken before any big airship build or addon update means a bad crash costs you minutes, not the world.
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