Use of Bussard ramjets is a distinct historical phase in the numerous stories set in Niven's known space [0] universe - they are how mankind reaches and colonis

Use of Bussard ramjets is a distinct historical phase in the numerous stories set in Niven's known space [0] universe - they are how mankind reaches and colonises the nearby stars before being given access to FTL hyperdrive by the Outsiders during the first Man-Kzin War. [0] https://en.wikipedia.org/wiki/Known_Space https://en.wikipedia.org/wiki/Known_Space