Needless to say, ships having a maximum velocity in space makes no sense; but Forstchen did his best to retcon for Origin and, in his novel Action Stations, he came up with the idea of electromagnetic scoops that extend forward of ships for miles, and collect hydrogen from space to replenish the fuel tanks. Sort of like Bussard Intakes, but invisible; and he explains that the drag generated by hydrogen collection is what limits the speed of ships, and allows them to maneuver in space as if they were in an atmosphere. The idea doesn't stand to much scrutiny at all, but it's better than nothing under the circumstances. Additionally, he gave ships the ability to switch off the scoops, which allows them to accelerate to virtually unlimited speeds, but with the undesirable consequence of fuel stores rapidly decreasing. The TCS Tarawa, in End Run, at one point flies towards Kilrah with scoops closed and does a tractor beam assisted sling shot around the planet.