>>11432024I'm gonna imagine a 1000 ton (wet) vehicle. It weighs 200 tons dry, 150 of that is ship parts (structure, power, radiators) and 50 is payload including people and their food. 80% wet-dry mass fraction, achieved by liquid fuel boosters right now (the best stages are ~90% propellant by mass).
80% wet-dry ratio at 10,000 Isp gets you a delta V budget of slightly under 158 km/s. That's certainly enough to get you to a low orbit around pretty much every object in the solar system and back to Earth, except for the Sun itself. As for how *long* it would take, that's a different story.
Given a total budget of 157,800 m/s of delta V, and setting aside 17,800 m/s of that as exploration/reserve delta V (for actually scooting around the Neptune system), we have 140,000 m/s to plan our transfer around. We can't do a single burn of 140 km/s out, because then we can't stop at Neptune. We also can't do 70 km/s out, because we'd like to come back from Neptune in about the same time it took to get there. So, our outbound delta V will be 35,000 m/s from Earth to Neptune and later from Neptune to Earth, and there will be an equal magnitude braking burn at either end (not really, because the Sun's gravity will slow us down/speed us up depending on which direction we're going, but it about evens out and we're moving so fast that it doesn't make much difference anyway).
In an ideal world those delta V's will be applied instantaneously. Earth is ~149,600,000 km from the Sun, Neptune is ~4,494,000,000 km from the Sun, which means at closest approach Neptune is about 4,344,400,000 km from Earth. At 35 km/s, it takes the ship ~1436.6 days of coasting to reach Neptune. Note that in reality, unless the ship could pull decently close to 1 g of acceleration, it'd probably take months or years to reach top speed.
