Particle Drive

Thanks to the Particle Drive's very high efficiency and exceptional power, interplanetary travel takes days or weeks, not months or years. The particle drive is essentially a particle accelorator which works subatomic particles up to close to the speed of light--fast enough that their apparent mass begins to increase and approach infinity. This allows a tiny amount of fuel to accelerate a large mass with great ease, evading the rocket's curse of having to carry immense amounts of fuel.

Disadvantages
Particle drives have two downsides. First, they are immensely energy intensive, translating the concern in space travel from propulsive fuel to nuclear fuel. This limited possible uses for the particle drive until the fusion reactors made nuclear fuel cheap. The large energy footprint also requires any transport to have large Carnot ports.

The second downside is the radiation footprint. Particle drives are essentially cosmic ray guns, and too close to a planet's orbit they can irradiate satelites and stations. Often particle drives are equipped with dispersing magnets, which prevent the particles from coalescing over long range into a confined beam.

Affects on Transport
Before particle drives, the best designs used Induced Fusion, which was only moderately more efficient than chemical rockets or solar ion drives. Induced Fusion travel took months in a good situation, and you could be left stranded for years waiting for an orbital opportunity.

The particle drive's microscopic fuel footprint allows ships to constantly accelerate or decelerate between 2 and 5 Gs (depending on the ship and what the crew can put up with.) This allows ships to travel within the Solar system up to 1.5% of the speed of light.