eli5: How do rockets not go over 3g’s while accelerating to 16958 mph?


I saw a really stupid post on Facebook talking about “how rockets don’t work.” Flat earther conspiracy nonsense. I started reading on all the points the post made and the only one I didn’t understand is how at 22 x the speed of sound a rocket does not really go over 3g’s of force.

My assumption is that as the rocket travels further from the earth the gravity influence also influences g’s on an accelerating object?

Please help me with this one.

Thank you

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6 Answers

Anonymous 0 Comments

G-force is the perception of weight due to **acceleration**. We feel acceleration, not speed. When you are sitting in your seat in a car going 60 mph you aren’t being pressed back into your seat, you only feel that when you are accelerating to that speed. Once something is moving at a steady speed it won’t experience any g-force other than the acceleration of gravity.

The gravity of Earth extends out into space more than many people realize; at the altitude of the international space station there is about 90% of the gravity experienced at the surface. However since both the ISS structure and everything in it are falling around Earth at the same speed they experience “micro-gravity” where they can float around in relation to each other.

So for the rockets they only accelerate at a maximum of 3 G’s of force. How fast they end up going just depends on how long they hold this acceleration, so they can end up going 22 times the speed of sound while only experiencing 3 G’s during the acceleration to that point. Afterwards once they stop thrusting they don’t experience any G-forces while going that speed.

Anonymous 0 Comments

**G’s are a unit of** ***acceleration***, not speed. The more gradually you speed up the longer it takes, but you can still eventually get to any speed you want.

1G = speeding up by 9.8 m/s, per second. It’s written as 9.8 m/s^(2).

So if you start from a standstill and accelerate at 1G, then after 1 second you’ll be going 9.8 meters/second speed. After 2 seconds you’re going 19.6 m/s. After 3 seconds, 29.4 m/s, etc.

Hopefully from that you can see that you can get to any speed you want without ever exceeding even 1 G of acceleration. You could get to 16958 mph without ever going over even 0.01 G if you want! It would just take long.

Example: You want to get to 16958 mph. That’s about 7581 m/s.

* If you accelerate at 1G, that will take (7581/9.8) = 773 seconds, or a little under 13 minutes.
* What if you accelerate at 3G? Well 3G = 3×9.8 = 29.4 m/s per second. So getting to 7581 m/s at 3G’s of acceleration would take 258 seconds, just over 4 mins.

Anonymous 0 Comments

The Space Shuttle’s three liquid fueled engines could be throttled, to some degree.

During Max-Q, the point with the most stresses on the vehicle, the Shuttle would throttle -down its engines to reduce stresses. Afterwards, they were:

*”Go for throttle-up.”*

As more fuel was burned, the vehicle became lighter & lighter. This meant increasing acceleration, unless the main engines were throttled down to keep G forces below 3.

On the Saturn V moon rockets, they would intentionally cut the middle engine, as neither the F-1 engines on the first stage, nor the J-2 engines on the second stage were throttleable.

Anonymous 0 Comments

Most boosters throttle down their engines to reduce the amount of thrust as they get lighter due to burning off fuel.

Anonymous 0 Comments

In this context ‘g’ (lowercase) refers to the acceleration due to gravity at the surface of the Earth: 9.81 m/s^2 . It’s a change in the velocity of an object over a period of time. We often talk about acceleration in terms of Earth’s gravity because it’s relatively constant everywhere on Earth and because you can think about the relationship between acceleration and weight more easily. Like if you’re on the surface of the Earth and you start accelerating upwards at one g you would feel as though you were twice as heavy (because you already get one g from gravity). In application it doesn’t necessarily have anything to do with the force of gravity acting on an object.

If a rocket is accelerating at 3g’s that’s equivalent to 29.43 m/s^2 . To relate acceleration, velocity, and time you can use the equation V = 1/2 at^2 where V is the final velocity at the end of whatever period of acceleration you’re interested in. Rearranging gives t = sqrt(2V/a), and solving with a=29.43 and V=7581m/s gives t=22.69 seconds of constant acceleration, which is not unreasonable for a rocket burn.

Anonymous 0 Comments

Thrust is cumulative. Orbital velocity is about 7500 meters per second, and 3g is about 30 meters per second per second. 7500/30 is 250 seconds of acceleration to reach orbit. Because the rocket is not at 3g for its entire burn, and losses from aerodynamic drag and gravity it actually takes about 500 seconds to reach orbit.

Why 3g? Humans. A human can only comfortably sustain 3g for long durations. Any more and they would be fighting to not pass out.