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Comment by dmurray | Hacker News Reader

The part when they are falling is only at an acceleration of 1g (that's what falling is). If they experienced 8g acceleration, it was before the ballistic part of the descent.

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Atmospheric reentry is more than 1g

https://www.quora.com/What-is-the-acceleration-of-the-space-...

The reentry forces could definitely exceed 1g and usually do from orbital velocity. You're moving sideways however fast the rocket left you going, and you have fallen at 1g acceleration for some amount of time before hitting thicker parts of the atmosphere. Your velocity is now way higher than terminal velocity for that altitude and you experience acceleration (deceleration).

Whey you are falling in a vacuum, it's not 1g, but 0g. That's what they experience on orbit. When you are falling with atmosphere resistance, it can be anything. Consider yourself jumping into water. You feel weightless when you are falling, but what do you feel when you hit the water? That's what happens when you hit atmosphere at 8km/sec.

The difference is between acceleration and g-force. Falling towards earth in a vacuum (or orbiting earth in a vacuum) you are experiencing 1g of acceleration but zero-g of g-force because you and your spaceship are accelerating together and you aren't meeting any mechanical resistance to the acceleration.

Once you start hitting atmo the velocity you've built up from your 1g of freefall meeting the resistance of the atmosphere creates g-forces.

8g was probably from the parachutes deploying, although you do experience very slightly more than 1g acceleration while in freefall, as you encounter increasing air resistance (and significantly more in reentry).

I suspect the high G period is mostly when the escape rocket fires.

edit: Never mind. Chris Hadfield says it's hitting the atmosphere. https://twitter.com/Cmdr_Hadfield/status/1050385343625740289

The high g period was then they ran into the thicker part of the atmosphere at faster than terminal velocity (so the deceleration caused by the air is what led to the g forces).

Although that didn't happen this time - the escape rocket had already been jettisoned before the failure occurred.

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