Well, the Ramjet collects protons in-situ while flying through the interstellar medium for use as fusion fuel.

Putting aside that fusion rocketry is very different from combustion rocketry, even the solar wind isn't dense enough for that, which is the medium in question for interplanetary travel.

At 1AU, solar wind has ~1 proton per cubic centimeter, moving at about 500 km/s, per current readings at time of writing. That's 1e6 protons per cubic meter, and a flux of 5e11 protons per second passing through each square meter.

Now let's move in to Mercury at ~0.38 AU (+/-eccentricity). Mercury receives on average ~10x the flux of particles assuming 1/r^2. Suppose we make a ship with a collector for solar wind the size of Mercury, with a radius of ~2.5e6 m. Our ship then collects 2e13 square meters of flux, or ~1e26 protons per second. A proton is ~1e-27 kg, so that's almost 10% of a kg per second to power our now dwarf-planet-sized collector infrastructure.

Oh, you wanted an engine? Good luck having anything leftover. You can't collect the solar wind and use it in real time. But wait an Earth year, and now your collector has collected a million kg, which is about enough to fuel a single one of these. Looks to me like all stages and boosters here burn H2, from what I'm reading. This seems like the appropriate place to say TIL about Saturn V stage 1. Thanks.

Collection at background solar wind levels is not possible. The time to go proton fishing is during a CME. A single CME ejects billions of tons of protons, per NASA's space weather prediction center's page explaining them. A billion tons is a trillion kg, and enough to launch a million of those Delta IV Heavies from a single flare event.

I understand that "catching a solar flare" may be easier said than done.