Isn't there some way that a spaceship could just, well, go from planet to planet without this tedious waiting around? You'd need a better propulsion system, though.

NASA and the Defense Advanced Research Projects Agency (DARPA) announced Wednesday Lockheed Martin of Littleton, Colorado, as the prime contractor for the design, build, and testing of NASA and DARPA’s nuclear-powered rocket demonstration, in collaboration with other industry partners.

The Demonstration Rocket for Agile Cislunar Operations (DRACO) program will test a nuclear-powered rocket in space as soon as 2027.

"Working with DARPA and companies across the commercial space industry will enable us to accelerate the technology development we need to send humans to Mars," said NASA Deputy Administrator Pam Melroy. "This demonstration will be a crucial step in meeting our Moon to Mars objectives for crew transportation into deep space."

NASA and DARPA partnered on the DRACO program to advance development of nuclear thermal rocket technology, supporting both agencies' goals. For NASA, nuclear propulsion is one of the primary capabilities on the roadmap for crewed missions to Mars.

A nuclear-powered rocket would allow for a shorter, faster trip to the Red Planet, reducing the mission's complexity and risk for the crew. This type of rocket can be more than twice as efficient as conventional chemical rockets, meaning it requires significantly less propellant and could carry more equipment for scientific goals. A nuclear-powered rocket also could provide more power for instruments and communications systems.

That would be great! Heinlein contrasts the old-fashioned way with what he called the Lyle Drive in that same novel:

The Federation Ship Champion, manned by an all-male crew of eighteen experienced spacemen and carrying more than that number of male pioneers, made the crossing under Lyle Drive in only nineteen days. The Champion landed just south of Lacus Soli, as Captain van Tromp intended to search for the Envoy.

For those science fiction fans who are also scientifiction fans, an earlier description of this notion can be found in The First Martian (1932) by Eando Binder:

The inconceivable distance to be traversed, the enormous energy required to transport a heavy machine from planet to planet, and the all-powerful force of gravity, seemed insurmountable objects to even the most broadminded and optimistic thinkers. Feeble attempts were made even in 1931 to leave this speck of the universe and soar to other worlds, but constant failures dulled the ardor of those who wished to connect our world with the others which acknowledge the same central sun... Although the fatalities far exceeded these probable successes, the hope of interplanetary travel was still uppermost in our minds. We know now, that our failure can be attributed to the unfortunate lack of radioactive elements, and not to the lack of ingenuity or inventiveness. It was only too true. as Professor Billings stated in 1945, that the only type of engine which could possibly transport a heavy machine from our earth to some planet, would have to be the atomic-energy engine, which, with a minimum of fuel and the very necessary radioactive element, could develop an unlimited amount of energy.

(The First Martian by Eando Binder)

He even drew plans of the basic principles of such an engine, estimated the amount of radioactive material needed, and worked out the propulsion of the ship.