nuclear pulse propulsion such as in the 1950s project Orion designs was estimated to be around 6000sec (theoretically up to 100k sec) https://en.wikipedia.org/

nuclear pulse propulsion such as in the 1950s project Orion designs was estimated to be around 6000sec (theoretically up to 100k sec) https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion https://en.wikipedia.org/wiki/Nuclear_pulse_propulsion