When does "Jettison" appear on an engine?

[Sharpy]

I've sent two fuel trucks to the Mun with Poodles attached directly to them where the Klaw of the vehicle in front of them is supposed to go after they join the train-base, recalling I could get rid of Poodle by selecting "Jettison engine". Now as I'm in orbit I don't see that option, and I'm not sure why, and I'm worried I'll need to use some more destructive methods to get of the engines after landing. So, when does the option appear/disappear?

[Pecan]

It is meant for jettisoning the engine-shroud that normally goes with a previous decoupler anyway. Not used much, more just a left-over from past times.

[CalculusWarrior]

Yes, you need to attach a decoupler to split your ship; the engines will not do it on their own.

[mikegarrison]

What they should do is repurpose the idea and allow you to jettison your heat shields without a separate decoupler. So you could put a heat shield over an engine and do a shielded, parachute/airbrake entry into the atmosphere followed by a powered retrorocket landing without a clunky stage decoupler involved.

[Bev7787]

There's another post with more replies if you need it