# Is there a maximum ISp?

With ISp defined as ${v_e}\over{g_0}$, the naive approach would tell me, the maximum possible specific impulse, with propellant moving at speed of light relative to the craft, is $c\over{g_0}$ or 30,570,322s.

Of course, with relativistic physics naive approach is often wrong and I feel like I'm making some oversimplification here... so, if I am, could someone straighten it out? What happens with Specific Impulse when extreme energies are put into the propulsion, accelerating the propellant to relativistic speeds?

• Great question! Would the limiting case be a photon rocket? – Organic Marble Jul 10 '16 at 21:14