Where would the LLM get the training data? It's not as if supersonic jet designers commonly post easy-to-ingest design data online.
Besides, most countries couldn't build a modern jet engine even if they had the exact engineering drawings of an existing one available. From fashioning single-crystal turbine blades to establishing a supply chain capable of the quality controls needed, the amount of hours spent is unreal.
As a corollary to AI taking up easy jobs like CRUD apps and spreadsheets, will human work be pushed to making things like silicon chips, nuclear reactors, jet engines and space elevators?