Hey Guys,
when I read the source of “UnrealBuildTool”, there is a “-rocket” used to build a “rocket” mode. I really don’t know what it is,…
It’s leftover from beta. Nothing to worry about.
Ya, I guess we can mention this now as the beta program is long over (dev’s can remove this if I’m mistaken), but the UE4 Beta was actually called ‘Rocket’ as a working name. That is likely a leftover argument from that.
Everybody can read about the rocket on answerhub -> there are many old rocket threads there
The good old rocket times 
I can still remember this picture:
Unreal Engine 4: Faster than a Cheetah on Jetpack Rollerskates.
These folks are correct, “Rocket” is a term leftover from the extended beta phase of Unreal Engine 4, however it still has an important meaning: During the Rocket beta test, only the “installed” versions of Unreal Engine were made available, there was no GitHub source code released yet.
The “installed” workflows are very important as they allow anyone to get up and running with a precompiled verison of Unreal Engine with minimum hassle. So if you see “-Rocket” parameters used, or just about anything else related to “Rocket” in the engine source code, it usually just means the “precompiled and installed” version of the Unreal Engine.
The version of the engine that you get from the Launcher and the version that you compile from GitHub are very similar. The main differences are that some workflows with project files are simplified (because you never have to compile the engine source code), and versioning of modules works a little differently. That’s the reason that the “installed” version of the engine runs Unreal Build Tool with the “-rocket” parameter – it is simply telling it to not worry about checking file dependencies for engine source code because it is not expected to have been changed. It makes iteration times faster.
Looking forward, we are actually working to reconcile the differences between the “installed” engine and the version of the engine that you would compile from GitHub, such that they will be very similar. It will help with stability (as there will be fewer combinations of things for our QA teams to test), as well as yield benefits to users – such as allowing third party binary plugins to work with Blueprint-only projects.
This is something we’re currently targeting for the 4.4 release and you can expect to hear more about it in a month or so.
–Mike
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