well, a specific impulse of 7000 seconds is a bit deceiving. True, conventional fuel/oxidizer rocket engines have, at best, a specific impulse of around 450 seconds. But thrust = Isp*dm/dt*g0. For the ion engine, the mass flow, dm/dt, is very small. Despite a specific impulse much greater than conventional rocket engines, the thrust provided by the ion engine is miniscule. Moreover, because Prometheus is so massive, the corresponding acceleration is tiny.
The point is you cannot run a conventional rocket for months at a time. Also, the specific impulse of the nuclear powered ion engine is much larger than that of previously used ion engines (Deep Space One's ion engine had an Isp of about 1200 seconds I think).
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well, a specific impulse of 7000 seconds is a bit deceiving. True, conventional fuel/oxidizer rocket engines have, at best, a specific impulse of around 450 seconds. But thrust = Isp*dm/dt*g0. For the ion engine, the mass flow, dm/dt, is very small. Despite a specific impulse much greater than conventional rocket engines, the thrust provided by the ion engine is miniscule. Moreover, because Prometheus is so massive, the corresponding acceleration is tiny.
The point is you cannot run a conventional rocket for months at a time. Also, the specific impulse of the nuclear powered ion engine is much larger than that of previously used ion engines (Deep Space One's ion engine had an Isp of about 1200 seconds I think).