The funny thing is, GURPS has tauted the idea for a while. Specifically their fusion air-ram and fusion ram-rocket systems. Fusion Air Ram is, basically, a fusion version of the fission air ram concept. Use a fusion reactor instead of a fission reactor. Fusion Ram-Rockets are, essentially, a fusion between a fusion air-ram and a fusion rocket and switch modes when necessary with the added benefit of using water as the propellant as standard.
The problem? Those techs are considered TL10 techs, which is a good century or two down the line.