Inspired by this question. For most orbital launch vehicles, do their engines have nozzle plugs installed and what happen to them at launch? Are they manually removed during the prelaunch/rollout or are they ejected during engine ignition?
In general, nozzle plugs like those depicted in the linked question, as well as other assorted engine protection setups like these nozzle FOD covers are almost always removed prior to flight, during the final assembly and preparation prior to launch. Many, such as turbopump inlet covers absolutely must be removed, as they otherwise interfere with integration of the engine.
These parts are used to prevent foreign object damage (FOD) or debris ingress, and in particular the above nozzle plug is almost certainly to help keep debris away from and out of the sensitive injector assembly. It could also be that the interior of the engine plumbing is "purged" with inert gas to prevent oxidation or corrosion prior to use or during shipping, but the procedures, if any, vary engine-to-engine.
It would generally be a bad idea to fire an engine with one of these plugs in place. Aside from the most flimsy of covers (paper, mylar, etc.) which will safely rupture, firing a large metal plug out of the nozzle is likely to damage it in the process, particularly if it tumbles and strikes the inner surface. Having the nozzle plugged at initial startup also runs the risk of an overpressure event the engine thrust chamber structure may not be designed to account for.
Certainly all the big shuttle engines: OMS, SSME and SRB covers and plugs were, ahem, "removed before flight".
However some of the shuttle Reaction Control System engines had covers applied to them that blew off at launch. This was intended to keep rain from collecting in the nozzles.
For details see
- Did any of the Space Shuttle RCS covers ever stay on?
- What are these membranes at the aft end of the Shuttle that get torn off during lift off?
Some solid rocket strapons used on expendable boosters have nozzle closures that are still affixed at liftoff (emphasis mine)
The air-start (altitude-ignited) GEM 40 motor configuration has a lengthened nozzle exit cone with higher expansion ratio, exit-plane-mounted nozzle closure system that is ejected at air-start motor ignition, and a different external insulation scheme.
Source: tweet by Tory Bruno