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?

edit: oops. wrong forum. Could a moderator move?

[NecroBones]

The "jettison" button refers to the auto-shrouds when stacking them. It's nothing to worry about.

[Sharpy]

Wait, so it means I can't just dump the engine?

Eh, I've got a lander with a pair of pretty durable and heavy MK3 tanks, I guess getting the right speed on reverse will take care of the engines.

[annallia]

Just do a roughish landing on the engine, will give you a little bounce but they make surprisingly good shock absorbers.

[Sharpy]

Yeah, I know. I had a return/reentry vehicle that used the engine as a crumple zone to slow down the rest to a safe speed

[vixr]

Yeah, I know. I had a return/reentry vehicle that used the engine as a crumple zone to slow down the rest to a safe speed

I do this a lot... (not so much on purpose)

[X-SR71]

Who needs chutes! In my book, it's not really crashing if the command pod survives!

[moogoob]

You could use decouplers/separators with a fuel line. Just stage the engine(s) off when you no longer need them.

EDIT: Or even better, put the separator above the fuel tank, so you drop that dry mass too with the engine when it's spent.

EDIT AGAIN: Reading comprehension - the second idea wouldn't work for your fuel truck use, but would work OK for other landers.