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Most of that is actually because of the difference in fuel -- Raptor runs on Methane, which produces more H2O and less CO2 than RP-1, and H2O is a smaller molecule and therefore more efficient.

Yep. The actual physically relevant number is exhaust velocity, which is the mean velocity of the particles in the rocket exhaust.

Isp = Specific Impulse = exhaust velocity/(9.81ms^-2).

9.81ms^-2 there is not any actual acceleration, just an agreed conversion factor. The unit of Isp has no physical significance, the convention just arose because early rocketry was a collaboration of people who all used different units, and they wanted to be able to compare numbers. Since the unit they all shared was a second, and they all agreed (roughly) on the real value of g, they just divided exhaust velocity by their g to get a number that could be compared. The convention stuck.




Note that when using different fuel or oxidizer, specific impulse isn’t the only thing that matters. Density matters too due to its effect on non-payload mass ratio.


> 9.81ms^-2 there is not any actual acceleration, just an agreed conversion factor.

I'd look it up but my textbooks[1] are in boxes, but the units everyone uses are wrong. Actually inappropriately simplified.

[1] I have one and exactly one textbook that has the full units for gc.


Isp often defined as "thrust per unit of mass flow". That is, how many Newtons of thrust engine gets from each kilogram per second of propellant spent. Since N = kg * m / s^2 , and mass flow is in kg / s , Isp becomes (kg * m * s) / (s^2 * kg) = m/s , i.e. Isp has units of speed. In vacuum Isp is equal to the speed of gases flying from the engine.




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