In-situ resource utilization to mine propellant from the Moon and near-Earth asteroids could bring a dramatic improvement in ∆v budget (and hence travel time),

In-situ resource utilization to mine propellant from the Moon and near-Earth asteroids could bring a dramatic improvement in ∆v budget (and hence travel time), using only conventional rocket propulsion. If you think about it, your car wouldn't get very far either without being able to refuel. If you wanted to travel across the country, the fuel tank would have to be dramatically larger, and then you'd have something very much like the rocket equation's exponential mass to ∆v relationship (except here it would be mass to distance traveled) as you have to carry more fuel just because the other fuel is weighing you down. As a result, eventually your fuel tank would be close to the entire mass of your car, and you might actually even require a two-stage vehicle, with your car detaching from a large tanker halfway across the country to finish the trip. And good luck getting back.