G-Force, Jerk, and Passing Out In A Centrifuge

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- This is the Royal Air Force training centrifuge at Farnborough.

And the team here are going to push me

as far as they're allowed to push a civilian.

- The centrifuge has been here since 1955.

The device was originally installed for research purposes,

although these days it's used for training as much as it is for research.

What it does is recreate the forces that you feel in an aircraft.

For our routine pilot training, the first time pilots go on the centrifuge,

we expect them to get up to 5g without a g-suit,

and then up to 7g with an anti-g-suit.

- Now I'm not taking a significant risk here.

I'm healthy, I've pulled a few gs before.

And the human body can take this.

And the reason we know that is because in the 1950s,

the US Air Force used rocket sleds to push volunteers to incredible speeds.

But that rocket-powered acceleration wasn't the dangerous,

or even the really high-g part of the test.

See, high-g acceleration takes a lot of incredibly expensive rockets

or a big ol' centrifuge like this.

But high-g deceleration?

All you need for that is a wall.

Or for something less destructive, like the rocket sleds,

you use scoops dropped down into a water trough.

USAF flight surgeon John Stapp,

aboard the rocket sled Sonic Wind Number One,

holds the record for the highest sustained g-force

anyone has ever voluntarily endured,

25g for 1.1 seconds, with a brief peak over 46g.

And he was badly injured, but he survived

and he recovered and he lived to the age of 89.

The human body is an incredible thing

particularly because we didn't evolve for this.

- G-tolerance is something that's innate in all of us.

Some of us have high G-tolerance. Some of us have low G-tolerance.

Over the years, people's g-tolerance doesn't really adapt.

Any shortfall in g-tolerance has really got to be made up

by physical exertion and the g-straining manoeuvre.

- What this centrifuge doesn't have is much jerk.

And jerk is a technical term.

In the same way that acceleration is the rate of change of speed,

jerk is the rate of change of acceleration.

And because it takes time to spin up and spin down...

Oh, here we go! Even though the acceleration is high,

the jerk here is relatively low, about 1g per second.

Jerk is the difference between a rocket to space

which might take a couple of minutes to reach peak acceleration

and a fighter jet, where manoeuvres might change the G force acting on you

in a fraction of a second.

And it can go further than that.

You can measure the rate of change of jerk which is called snap or jounce.

The two derivatives after that are called crackle and pop but

they're not all that useful in the real world.

- As we increase the G that Tom is exposed to,

the blood's going to be pushed down into his feet,

and he's going to have to work really hard to push that blood back up

to feed oxygen to his brain to keep him conscious.

And in real life, we would be expecting that person to be flying an aircraft whilst doing that.

- I'm getting a little bit of grey-out.

I can't quite see. Agh.

- We teach the anti-g straining manoeuvre which composes of two separate parts.

First of all, muscle tensing,

so both the buttocks and legs squeezing the blood vessels

to try and get the blood back up into the chest and the head.

But also the second part is a breathing manoeuvre

which increases the strain in the chest

directly increasing the blood pressure to the great vessels in the chest

and keeping him conscious.

And when you lose blood pressure to your head,

you could even lose consciousness.

And we term that g-induced loss of consciousness or G-LOC.


- Blimey!

I lost everything there.


- G-LOC in itself is not dangerous.

But the real point is when you G-LOC you're flying an aircraft.

So if you're not able to fly that aircraft,

I'm sure you can appreciate that that is a real problem.

- Because of John Stapp and all the volunteers like him

that rode the rocket sleds,

there is a lot of research

into acceleration on the human body.

How many gs can be withstood for minutes at a time?

How many gs can be withstood for brief moments?

And how many can be withstood with training that I clearly don't have.

Rocket scientists and roller coaster designers use that data.

But there's not much research into jerk

because how do you test it without also testing acceleration?

Over on the Starrship channel, I am not passing out

pulling gs with the Blades aerobatic team(!)

And as for this video, thank you so much

to all the team at the RAF Centre of Aviation Medicine,

to the team at Qinetiq, and to the team at Starrship.