Agreed, but to be pedantic, it applies to all momentum machines which carry its own "propellant", not "fuel".
For instance, in a NERVA rocket, the "fuel" is the uranium rods in the reactor, the "propellant" is the liquid hydrogen that is heated and shot out the exhaust bell.
Solar Sail, Laser Sail, Electric Sail, Magnetic Sail, Mini-magnetospheric plasma sail, MagBeam, Plasma Magnet Sail, Photon Drive, Bussard Ramjet, Ram-Augmented Interstellar Rocket.
Basically any spacecraft that does not carry its own propellant. Most of these have either pathetic thrust or are way beyond our current state-of-the-art.
I'm no expert but I think the answer is "No", due to other problems.
Distances are closer in Globular Clusters. Alas, those are "metal poor", so the only planets are gas giants like Jupiter.
Distances are closer in Open Galactic Clusters like the Pleiades. Unfortunately those cluster tend to disperse. By the time a space faring civilization has evolved, the stars are no longer close.
Distances are closer in the Galactic Core. Unfortunately that is a high radiation environment due to Sagitarius A* (the mega-black hole at the center of the galaxy_ and all the nebulae the hole is dragging in.
Short answer: places where the distance between stars is closer are unlikely to have space faring civilizations.
And about five minutes later, zillions of programmers would start writing similar apps, in hope of a big payday when Facebook shows up to buy them out.
Not much.
The main difference between a missile and a robot controlled spacecraft is that the latter has more propellant. The spacecraft presumably is intended to return while the missile is just has to arrive at the target. The spacecraft will have roughly 4x the propellant
[1] propellant to enter mothership to target trajectory
[2] propellant to slow to combat speed at target
[3] propellant to enter home to mothership trajectory
[4] propellant to slow to a stop at mothership.
A missile only needs [1].
Well, that turns out not to be the case. I'm the author of the Project Rho site, and I did not write the essay on the Spacebattles forum. But I agree with all the points the poster made, he just wrote the article more succinctly than I did in my website. ;)
I was under the impression that the extraction and purification of rare earth elements was such an ecological catastrophe that environmental protection laws in the US had made the existing mines in the US so unprofitable that they shut down.
Physicists choose Relativity since it has been tested to many decimal places and always comes through with flying colors. Relativity means that FTL travel is the exact same thing as a time machine. Time machines can cause temporal paradoxes like the Grandfather paradox. Temporal paradoxes render Causality impossible. And without Causality, the entire structure of science crumbles.
The only way to have all three is if there is some weird magic law that makes it impossible to use time machines to create temporal paradoxes.