In 2001: A Space Odyssey, the Discovery mission is a one way trip. After the exploration phase, the astronauts will enter hibernation until rescued by Discovery II.
As the hibernation period will be longer than the return to Earth, the implication is that it is a lack of propellant that prevents Discovery returning under her own power.
Discovery II is described as still being unbuilt, and with transfer time looks like there will be three years for construction. This is quite a short time for a new ship design which, along with using the same name, has always led me to assume it was the same design.
For Discovery, it would be a one-way trip - yet her crew had no intention of committing suicide. If all went well, they would be back on Earth within seven years - five of which would pass like a flash in the dreamless sleep of hibernation, while they awaited rescue by the still unbuilt Discovery II.
Discovery used a slingshot around Jupiter to get to Saturn, so is already using gravity to achieve an efficient orbital transfer.
Is there any discussion to explain how Discovery was limited to a one way trip, but Discovery II would have the reserves to get to Saturn and return?
Happy for any answers that show my assumptions are incorrect, but a source from the novels or movies, or from Clarke, preferable to speculation.