Sea level static firing tests of rocket engines and upper stages are standard industry practice and a matter of everyday aerospace observation, requiring no citation.
The claim asserts that sea level static tests of rocket upper stages are physically possible. This is a straightforward physical fact and a matter of standard aerospace engineering observation (rocket test stands operate at sea level worldwide). Per Step 0 instructions, trivial facts and direct everyday observations are classified as COMMON_KNOWLEDGE, requiring no citation, and indices are left empty.