1. ## Maximum G

I was just watching a TV programme about Racing Drivers Crashes and it discused safety advances in a big way. The old cars of 10 years ago were much worse at safety and a 70G+ crash would almost certainly kill. Whereas today a Formula One Car Crashes at over 200G and a driver can survive.

It got me to thinking about Maximum G for sustained space travel, I know there was a post a while back asking a similar question and I put in that if you immersed a pilot in a Liquid with similar to the weight of the pilot he would be able to sustain much greater G's. But how much greater?

David Blaines latest cunning stunt has him in sphere of water with much the same setup as I envisenged for the G-pod.

Anyway 3 questions:
1. at what point do normal blackouts occur? (G wise)
2. pilots wear special G suits, what G's can be sustained with them on?
3. what G could you sustain in a ball of liquid?

2. Order of Kilopi
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1. at what point do normal blackouts occur? (G wise)
2. pilots wear special G suits, what G's can be sustained with them on?
3. what G could you sustain in a ball of liquid?

1. It depends on the direction of the G forces. For example, positive Gs are essentially parallel to the spine in a head to foot direction. When you pull positive Gs, the blood tends to drain out of your head causing a grayout followed by a blackout. Without any protection, most people start to black out at perhaps 6-7Gs, although competition aerobatic pilots and airshow pilots can sometimes pull up to 12 Gs for very brief periods. Negative Gs are in the opposite direction as positive Gs (blood flows towards the head). These can be very uncomfortable and there isn't any G suit that can protect against them. High negative Gs can even cause blood vessels in the eyes to rupture. Top level aerobatic pilots can sometimes pull upwards of 10 negative Gs but only for very brief periods.

There's another type of G forces, sometimes called transverse Gs. These G forces are mostly perpendicular to the spine and are the forces that you get in the case of a car crash. Blood doesn't flow towards or away from the brain, so a person can sustain transverse Gs for a long time (although it can be uncomfortable and difficult to work). As you pointed out, properly restrained, a person can pull very high transverse Gs for very brief periods. Astronauts experience transverse Gs during liftoff. During the Mercury and Gemini missions, they'd pull upwards of 8-10Gs for sustained periods. Likewise, when they rode out reentry, they'd pull a lot of transverse Gs. The Apollo missions were much gentler - some astronauts called the Saturn V an "old man's rocket" because they didn't pull much more than 3 Gs. G loads for the Shuttle are interesting. The crew is sitting facing forward like on an airplane. During liftoff and the trip to orbit, they're pulling transverse Gs (but only 3 Gs or so). However, during reentry, they're sitting upright while the Shuttle is in a nose high position, meaning they're pulling mostly positive Gs. However, the G load is pretty low. The Soyuz cosmonauts pull transverse Gs during liftoff and reentry but I don't know off hand how many.

2. US fighter pilots can pull 9 positive Gs with a combination of G suits and a grunting maneuver that helps force blood to their heads. The G suits offer no protection against negative Gs. They can sustain 9 Gs for short periods (how long can you grunt?). I've read that the G suits developed for the Eurofighter allow pilots to sustain 9 or more positive Gs for longer periods but I don't have any details.

3. I don't know. I suspect that the person could sustain more G forces if his/her lungs were filled with a breathable liquid like shown in the movie "The Abyss", but I really don't know for sure. There was a thread discussing this topic several weeks ago. You might want to search for it.

3. Other things modern fighter pilots do is develop really big leg muscles through weight training. The Grunting described above is basically a flexing of the leg muscles. This combined with the constriction of the G-suit helps to keep the blood from pooling in the legs by basically giving it nowhere to go.

The F-16 also uses a steeply reclined chair for the pilot so that a high G (positive) turn sends the blood more to the lower back than the legs and gives them more of a buffer before blacking out.

In the water ball thing, what would keep the pilot from sloshing about in the liquid?

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Originally Posted by Tog_
In the water ball thing, what would keep the pilot from sloshing about in the liquid?
I think it's the lack of an air pocket if the bubble is completely filled with the liquid... Kinda like those "snowy balls" where you shake it about and tiny pieces of white material get suspended in the liquid and then slowly settle down due to gravity once the shaking stops. Those toys/souvenirs always have a little bit of air inside. I think otherwise, they wouldn't work...

I think it's the air that allows the liquid to actually behave like a liquid. If the liquid took up all of the space inside, I think it wouldn't act as a liquid but as 1 'solid' body of mass (I mean, the liquid would not be able to 'slosh' about). But I'm not sure of that...

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Come to think of it, this reminds me of a trick I once read in one of those "Do it yourself at home science is fun" books (don't ask which one):

Take an empty bottle and put in something small and light that would float on water (like a little plastic ball). Now fill the bottle completely with water, right to the top. Cork the bottle, making sure no air gets in. The little ball will be 'floating' underneath the cork. If you then apply extra pressure on the cork, the ball will start to drop towards the bottom. You can even keep it suspended by maintaining the same amount of pressure during its descend.

Once you've maintained a constant pressure keeping the ball in the middle of the bottle, I'm pretty sure you can even start shaking the bottle, without having the ball move position no matter how hard you shake it (provided you don't loose grip of the cork)...

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The technique that fighter pilots use is called a Valsalva maneuver.

http://en.wikipedia.org/wiki/Valsalva_maneuver

Essentially the maneuver raises the pressure in the chest cavity and as such constricts the blood vessels there, helping to impede flow away from the brain. Its also useful for forcing things out of the abdominal space, as constriction of the abdominal muscles helps in the efficiency of this maneuver.
You can also perform several of these in quick succession, so you can perform more than one 'grunt'.

Training the muscles in the legs helps to return blood to the core of the body. By constricting the leg muscles, the blood vessels are constricted, thus moving blood back toward the core, or preventing it from 'draining' to the legs. This is the same principle that helps our blood to avoid 'pooling' in our legs and feet as we stand upright (along with valves in our veins) and walk, and is commonly called the 'muscle pump'. Infantry soldiers, or anyone who has had to stand at attention for long periods of time will generally flex their leg and calf muscles to accomplish this as well.

And, the liquid in the movie Abyss is a compound called a Perfluorcarbon. The US Navy experimented with it a lot in the 70s. It can carry quite a lot of oxygen for a liquid, and has some very interesting applications, some testing for medical applications has already been done. One problem with it though is that long exposures have some toxic side-effects, so not optimal yet but I'm sure there are chemists working to overcome these problems.

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