In the Voyager episode "Dark Frontier", Voyager got a transwarp coil from a Borg sphere. They used it all up and traveled 20,000 light years at once. It would have been very good for them to use about half of that or maybe a bit more and save a lot of it to use in combat situations when they need a quick escape, or when traveling through alien space that they are not allowed in.
Why did they use it continuously until it died, instead of saving some of it for emergency situations?
Borg technology loves to pack it in if its parent ship is in a bad spot; that's why Janeway went to go steal a transwarp coil from a healthy ship in the first place, because the one they originally recovered had committed suicide. Given the shenanigans that Voyager goes through on a weekly basis, it's likely no one wanted the coil to go kaput from the ship being smacked around (the very situation they'd most want to use it in).
It's also likely that they didn't know how much mileage the coil would provide before failing. The Federation had probably never used Borg transwarp coils before, so it was probably anyone's guess how well it would work when plugged into Voyager and left running for a while. Maybe it might have even ended up like a failing hard drive; if you leave it alone it will chug along for a while, but if you stop it, it gets much weaker or just dies prematurely. If you don't know how long you have, you can't plan around it, so you might as well go for broke.
I would have to rewatch the episode, but am not near my DVDs at the moment.
However there are two possibilities that I can see -
1) It was an all or nothing deal. Once you turn it on (due to incompatible tech), it stays on until it runs out. Kind of like the car race scenes where they press the nitro switch for a sudden boost.
2) Diminishing returns. Using it 1/2 the time gives you less than 1/2 the distance. Voyager was pretty good at getting out of trouble. Unless the plot required otherwise. So get the most out of the stollen tech in one hit.
Also, solves the 'why didn't they use the transwarp coil' questions you KNOW we would have asked. :)
Philip
they used it all up [...] at once
Who's to say they knew of any limitation on its usage? Maybe they hoped would work until they're home, maybe they thought it would last much longer, maybe they were lucky they got it working at all and wanted to use as much of it as possible before it would quit working.
Getting another functional coil was probably out of the question, and "coil" doesn't sound like a battery that simply discharges predictably, but more like an engine component that will just randomly break.
At the end of the episode, they just blew up the Borg Queen and her ship. Getting as far away from them as possible while they're still recovering from that would be a very good idea.
