Difference between Discovery and Discovery II capabilities

Question

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.

Answer 1

Discovery's fuel budget was always for a one-way trip.

For the last time, Discovery's main drive released its energies. For the last time, the incandescent fury of dying atoms blazed among the moons of Saturn. To David Bowman, the far-off whisper and rising thrust of the jets brought a sense of pride - and of sadness. The superb engines had done their duty with flawless efficiency. They had brought the ship from Earth to Jupiter to Saturn; now this was the very last time that they would ever operate. When Discovery had emptied her propellant tanks, she would be as helpless and inert as any comet or asteroid, a powerless prisoner of gravitation. Even when the rescue ship arrived a few years hence, it would not be an economical proposition to refuel her, so that she could fight her way back to Earth. She would be an eternally orbiting monument to the early days of planetary exploration.

_{^{2001: A Space Odyssey - Arthur C. Clarke}}

Discovery II's mission profile called for a return journey.

Yes. I’d be happy to get away sooner; but we don’t have the fuel for a higher-energy orbit…’ Tanya’s voice trailed away into uncharacteristic indecision. ‘I was going to announce this later, but now that the subject has come up…’ There was a simultaneous intake of breath, and an instant hush from the audience. ‘I’d like to delay our departure five days, to make our orbit closer to the ideal Hohmann one and give us a better fuel reserve.’

_{^{2010: Odyssey Two - Arthur C. Clarke}}

Presumably Discovery just had a much less favourable orbital profile but could afford to squander its fuel getting to the obelisk in a big hurry. Discovery II had a more favourable profile to begin with, and made its way to Jupiter more slowly. Additionally, Jupiter can be nearly twice as far from Earth at different points in their respective orbits so there's always that to take into consideration.