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The Reality of Apollo / Re: Everyday Astronaut video about Apollo
« Last post by JayUtah on March 02, 2025, 01:02:52 PM »After sleeping I reran the video and I have a question concerning Tim's calculations concerning the rocket equation. In all of his calculations Tim used the Earth's gravitational constant of 9.8 m/sec^2. I wonder if the Moon's gravitational constant should be used for the CSM/LM while entering/leaving Lunar orbit and the LM's landing/takeoff. What's the verdict?
Short answer: no, you don't change g0 to be that for the Moon. This comes up all the time while teaching rocket science. A moment's thought reveals that specific impulse and exhaust velocity have nothing to do with the gravity of anything your rocket might be operating near. All that works in deep space too.
g0 appears in the rocket calculations to normalize the answer for differences in physical measurement units. "Specific" anything in science wants to be as dimensionless as possible. So something labeled "specific impulse" should be considered dimensionless even though it has units in seconds. To get there, you have to do undo all the physically-based measurements such as for mass, velocity, and force. Time in seconds is the only common thing in that relationship among all the measurement systems, so the physically-measured quantities are normalized to a "specific" quantity using something that exists as the same conceptual relationship in all systems. This is arbitrarily chosen to be Earth's gravity-based acceleration. It's a relationship that's defined in all systems and incorporates units of force, mass, time, and velocity—the quantities we care about when trying to measure rocket performance in terms of propellant behavior.
It's important to understand that this is arbitrary. It doesn't have anything to do with operating a rocket near Earth or anything to do with what the Earth's gravity is doing to the rocket. If you're working in SI, you undo the normalization for your units by using g0 in SI units. If working in EES, you undo the normalization for pounds-force, gallons, firkins, and cable-lengths by applying g0 in EES units to get exhaust velocity (for example) in feet per second instead of meters per second.
), and the production values are right up there with the best. He has come a long way from the funny, nerdy guy in an orange Russian flight suit. 