A while back, I asked if FTL drives mandated time travel, and I was referenced this previous question:
Where the top-rated answer provides what is apparently a very-well crafted explanation for why any given FTL system can be exploited to create time travel. Unfortunately, I don't fully understand it. Looking at the 'Time Travel Happens' section, though, where the scenario is illustrated with time cones and world lines, a thought occurred to me: Could we exploit this system by going backwards? That is, can be exploit the same mechanics that turn FTL travel into time travel in order to create FTL travel without time travel? I'll try to explain what I mean.
Here's the 'time cone' illustration from the question I'm referencing.
When traveling within the 90 degree cone 'above' A or B, you are traveling towards/away from either destination with the bounds of the speed of light, and all is well. A horizontal line between A or B represents instantaneous travel, and a line between the horizontal and 45 degrees above the horizontal is faster-than-light, while the 45 degree line above the horizontal is the speed of light. A line BELOW the x-axis is trouble, as that means you're going back in time.
IF I understand it, Derrel is traveling at a relativistic velocity in such a direction when he engages his FTL drive to go from A to B, the horizontal line representing instantaneous travel 'tilts' downwards, below the x-axis, creating time travel, which we don't want. This can happen with any speed between infinite speed and the speed of light, because depending on your direction and velocity, your 'travel line' can be tilted up to 45 degrees down. I'm not sure if I'm talking gibberish or not, but if you had a ship with the ability to reach, say, 90+% of the speed of light using conventional engines (somehow), AND had an Alcubierre drive that could take you up to exactly (or slightly below) the speed of light, couldn't you use relativity to 'tilt' your line of travel 40 degrees down, so that when you engage your speed-of-light Alcubierre drive, it's only 5% above the horizontal, and will therefore get you to your destination faster than light, without actually being faster than light?
I have no clue what I'm talking about, don't go easy on me.