I always thought "Speed Drive" was shorthand for "drive causing the 'Space is a Drag' quirk". Vessels with Speed Drives will start rapidly decelerating, as though in contact with air, water, or a surface, if they lose power or deliberately cut the engines. In fact, they are always subject to that drag, which effectively puts an upper limit on their speed.
Or at least, that's how I've always thought of it. Space Friction is a central trope of Space Opera, after all.
Or at least, that's how I've always thought of it. Space Friction is a central trope of Space Opera, after all.