Project Orion (19-20th Century Ancient History)
Project Orion was initiated in the 19-20th century of ancient humanity when a group of ancient humans came up with the idea to propel a spacecraft via blowing up explosives behind the spacecraft, while using what appears to be a pusher plate to shield the crew from the radiation and the sharp spikes in acceleration. While ion propulsion offers high efficiency but low thrust while chemical rockets offer high thrust and low efficiency, this would allow high thrust and efficiency, allowing quick access to the Sol system. This project never went forward, but our best historians think it might be to do with the fallout from thousands of nukes being ejected from a spacecraft travelling extremely fast around Earth posing a hazard to anyone within hundreds of kilometers of the orbit.
Modern Day Light Bomb Propulsion
In 75,858 the daredevil community would invent a spacecraft based off this ancient information. They created the L-ship. Instead of bombs, they'd use L-bombs, which blow up and emit lots of light instead of radioactive material. The pusher plate also uses a mirror instead of thermal resistant material. These ships can be seen for thousands of kilometers and usually can't be used around populated planets to avoid blinding their citizens.