I'm looking for a slower-than-light (STL) drive for fast inter-system travel. It should make trips from one planet of a system to another (e.g. Earth, Mars, Jupiter) with sublight speed possible in a few days.
In many science fiction works we encounter starships that accelerate to high speeds, change direction and travel to nearby planets (without faster-than-light FTL) in a short time. With some glowing exhaust at the back end. It is often not explained at all or handwaved as "ion" or "photon" drive. In some scenarios, there are plenty of such ships in private hands of normal people.
My problem now is the following:
How could such a ship not be turned into a planet-crushing weapon by any terrorist or criminal? Why fight battles with laser or blaster if my ship is a much more powerful kinetic weapon? Why not use it (depending on how it works) to construct a perpetual motion machine?
I encountered this problem in SciFi (Role Playing Game) RPG sessions, where some player (OK, I admit it was me :-)) came up with such "abusive" ideas.
If the drive uses reaction mass, you have the problem of fuel. You want not to (and your favorable sci-fi story ship does not) run out of fuel after a few hours. And also you want not to sacrifice 99% of your ship's hull to fuel tanks (where are the fuel tanks of the Millenium Falcon?) So if you want to save reaction mass, you have to use high exit velocities. Using antimatter you could reach near light speed velocities. Using fusion (and only use the fusion products as reaction mass) the speed is much lower and you have to fuse a lot per second. Years ago (when I remembered enough from school to be able to do the calculations), I calculated the energies needed for a small craft to accelerate with one g. I ended up with energies that today's nuclear bombs pale beside. If you use this drive in an atmosphere or near a planet it will be like a nuclear strike to the surface. And you don't need any blaster cannons. Just point your drive to the enemy and - accelerate. He will be vaporized.
If you have a reactionless "impulse" drive that just consumes energy you have another "problem". Having a drive that constantly consumes a fixed amount of energy and produces a constant thrust and acceleration? Well, soon your kinetic energy will be greater than the energy you put into the drive. Magic. You have perpetual motion.
And as mentioned above, if you have a ship that can accelerate to 1 or 2 digit percents of light speed, you could aim it at a planet (with autopilot) and even if the defense forces of the planet manages to hit it, its debris will impact on the surface like nuclear bombs.
Giving ordinary people such ships would call for incredibly strong planetary shields (that need even higher energy levels) even the poorest outpost is equipped with. And the "weapons" of such ships must even be stronger than the "drive" to make any sense.
While most sci-fi works seem to ignore all this, I don't want to have this energy inflation and those powers in the hand of ordinary people. But I like to let them do fast STL travel. So what drive could they use?
I imagined a reactionless drive that somehow pushes against the background radiation frame. Because you have to spend more and more energy the faster you already move to gain constant acceleration (like a moving car pushing at the street) it solves the reaction mass and the perpetual motion problems, but not the weaponization problem. You also need high energies to go anywhere and you gain high speed to use as a kinetic weapon.
I also imagined a "virtual move drive". Similar to an STL warp drive. I think in one video game it was called "linear displacement drive". It gives you a virtual velocity. But as soon as you turn it off you stop instantly. The same happens, when you hit anything. So as a kinetic weapon only your "real" velocity counts. The problems here are, that the drive has to consume more energy if it "displaces" you up in a gravitational field to compensate for your increased potential energy. The energies you need will again be very high. Another problem is you need regular thrusters to align your speed to your target. Otherwise, after landing and turning of you drive you will crush into the ground if your real velocity is not zero.
Does anybody have any other ideas to circumvent this problem?
My Conclusion to all the good answers and ideas:
Hard SF => Heavy control of ships. No "Han Solo" smuggler with scrap ship who stealth lands near the heavily guarded base on a planet.
"Han Solo Style" => Handwaved linear displacement drive or similar and suspense of disbelieve (and hopefully bad knowledge of physics in the audience).
To all those who are asking why I think a reactionless drive would allow for perpetual motion look here: