4 ms·
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 molec
by Tuna-Fish 8y ago
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.
- deleted 8y ago[deleted]
- Gibbon1 8y ago> 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.
- avmich 8y agoIsp 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.
- ummonk 8y agoNote 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.