Not necessarily. Usually you get to blast the escort ship, the demo is a bit weird like that. I also have no idea what the bottom type in that mission is, but it makes a huge difference in BL SSPs. If it's rock then maybe you really are screwed.
It's also abnormal for an escort ship to respond to a torpedo datum at 15 knots. They usually step on it, which means they are completely blind until they get where they're going and slow down to listen again. That gives you the window you need to either kill them or slip away. If you're going to slip away, it's crucial to start running when you first shoot the torpedo (don't wait until after your weapons have hit!) and to slow down the moment you see his speed drop.
It might be a little cheap, but it might be helpful for your torp salvo to include one weapon run at 40 knots or less aimed at the OHP, but fired from far enough out that you know the OHP won't get hit by it. But he'll sure be running for a long time, and he'll have longer to run back to find you after the torp shuts down.
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