In my last post, I detailed some of the very gory things that would happen to a human being in a high-gee centrifuge. Then I remembered that I have access to a high-gee centrifuge. Sort of. You see, I’ve got one of those fancy front-loading washing machines. It saves time on drying by spinning your clothes at a ridiculous speed at the end of the wash cycle. And when I say “ridiculous speed”, I’m talking 1,100 RPM (at least, according to the manufacturer). That’s 18 revolutions per second! I measured the drum’s diameter at 55 centimeters. If you do the math, it tells you that the acceleration on the inner surface of the drum, when the thing’s running full pelt, is 372 gees. Okay, so it’s not ultracentrifuge material, but that’s still a lot of acceleration.
And I thought, you know what? We’ve got some fruit in the refrigerator that would be just as tasty pulped as it would be whole. Let’s see what 372 gees does to it! (Sometimes, I worry about how close I came to growing up to be a serial killer…)
I’ve tried this experiment once before (for another blog, which is why this one might look a bit familiar). Let’s do it again, but this time, with our gory, scary centrifugal thought experiment in mind. Here are our astronauts:
That’s a plum and a lime. The plum was pretty soft. We had a firmer one, but it wouldn’t fit in the container, and, crazy as I am, I didn’t want to risk splattering the inside of my washing machine with plum pulp. The lime, on the other hand, was so hard it could probably cut glass. Either way, these are our volunteers (it was a pain in the bum getting them to sign the release forms, let me tell you…) Let’s seal them in their space capsule.
I must say, they look pretty brave, as far as produce goes. Note the extra precautions: each fruit sealed in an individual bag, and packing tape to seal up the container. I didn’t want it flipping over during spin-up and seeping stuff everywhere. But enough talk! Let’s get ’em in the centrifuge!
There they are, their capsule strapped into place. Can you tell how worried I was I’d end up painting the inside of my washing machine with fruit?
I could’ve sworn I heard a high-pitched shriek when the washer reached maximum spin. Then again, I’ve been hearing a high-pitched shriek ever since the Exploding Kikkoman Bottle Incident, so perhaps it’s just me.
This is what the picture above would have looked like if I’d remembered to turn the flash on. Believe it or not, the drum is actually spinning here. Sometimes, I’m impressed by what my cheap little point-and-shoot camera can do. And then I remember that it’s got no time-lapse or long-exposure settings, and I stop being impressed. Either way, in this picture, the top of the container is experiencing about 237 gees (2,322 m/s^2). The bottom is experiencing 372 gees (3,649 m/s^2). The difference is because the top of the container is significantly closer to the axis of rotation than the bottom, and the acceleration is the distance from the axis times the square of the angular velocity. I’m surprised how well the space capsule tolerated the gees. I shouldn’t be surprised. After all, lives are at stake here. The capsule was engineered to survive all conditions. Still, considering how many times that capsule has been through the dishwasher, I’m impressed that it didn’t collapse.
Other things, however, did collapse…
This is almost exactly how our juicy astronauts were when I pulled them out of the centrifuge. I moved them around for photographic purposes, but that’s it, and even I’m not clumsy enough to completely obliterate a plum just by touching it. At least not when I’ve had my coffee.
The lime did remarkably well. It was noticeably flattened on the bottom, but it was very much intact. Now, under 1 gee (earth surface gravity), my scale says that a similar lime (someone ate my surviving astronaut; the nerve!) weighed around 100 grams. Under 372 gees, that lime weighed the equivalent of 37 kilograms. That’s 82 pounds. That’s the size of the dumbbells those really gigantic scary guys with the tattoos are always curling at the gym. It’s heavier than a gold bar. But the lime had little trouble. It’s the toughest substance known to man. I feel sorry for whoever ate it.
The plum, as you can see, didn’t fare so well. Here’s a gory close-up:
(Just an aside: I wonder if there’s anybody who’s genuinely upset by the sight of squashed fruit. Not in a “that’s a waste of food” sense, but in a visceral sense, the way some people can’t stand the sight of blood. If that’s you, I apologize. And you might want to consider some counseling. I’d give you the number of my therapist, but she lives on Jupiter.)
That plum is flattened. It looks like it was squashed under a very heavy weight. Which is exactly what happened. I don’t have a similarly-sized plum for comparison, but I’d say it’s reasonable to assume that, without all that weird white pithy stuff to decrease the density, the plum was at least twice as heavy as the lime, meaning, at maximum acceleration, it weighed almost 80 kilograms (176 pounds). That’s as much as my cousin. (I would invite her over for a comparison test, but even I recognize that “Will you come to my house and stand on a plum for me?” is a pretty weird request.)
But here you have an excellent practical demonstration of what I talked about in the last article. Under high acceleration, the weight of the plum exceeded its structural strength, and it split and oozed horribly all across the bottom of its bag. If the pit had been denser, it might very well have squelched down through the pulp and ended up on the bottom, but even my terrifying washing machine has its limits.
Oh, and before anybody complains that I’m wasting food on silly experiments… First of all, NYEH. Second of all, I didn’t waste it. I ate the plum. Somebody else ate the lime (for some reason). And you know what? That plum was one of the most delicious things I’ve ever eaten. I’m serious. It was all squishy and ripe. I used it because I thought it had gone over the edge already. But no. It was perfect. So not only did I get to centrifuge something, but I got some lovely fruit, too! I might have to try these practical experiments more often…
Or perhaps not. I must remember the Exploding Kikkoman Bottle Incident…
WARNING! Although it won’t contain any gory pictures, this post is going to contain some pretty gory details of what might happen to the human body under high acceleration. Children and people who don’t like reading about such things should probably skip this one. You have been warned.
In my last post, I talked a bit about gee forces. Gee forces are a handy way to measure acceleration. Right now, you and I and (almost) every other human are experiencing somewhere around 1 gee of head-to-foot acceleration due to the Earth’s gravity. Anyone who happens to be at the top of Mt. Everest is experiencing 0.999 gees. The overgrown amoebas at the bottom of the Challenger deep are experiencing 1.005 gees.
But human beings are exposed to greater gee forces than this all the time. For instance, the astronauts aboard Apollo 11 experienced up to 4 gees during launch. Here’s an awesome graph from NASA:
Fighter pilots have to put up with even higher gee forces when they make tight turns, thanks to the centrifugal acceleration required to turn in a circle at high speeds. From what I can gather, many pilots have to demonstrate they can handle 9 gees for 10 or 15 seconds without blacking out in order to qualify to fly planes like the F-16. Here’s an example of things not going right at 9 gees:
Yes, it’s the same video from the last post. I spent fifteen or twenty minutes searching YouTube for a better one, but I couldn’t find it. I did, however, discover this badass pilot in a gee-suit who handled 12 gees:
The reason we humans don’t tolerate gee forces very well is pretty simple: we have blood. Blood is a liquid. Like any liquid, its weight produces hydrostatic pressure. I’ll use myself as an example (for the record, I’m pretty sure I would die at 9 gees, but anybody who wants to let me in a centrifuge, I’d love to prove myself wrong). I’m 6 feet, 3 inches tall, or 191 centimeters, or 1.91 meters. Blood is almost the same density as water, so we can just run Pascal’s hydrostatic-pressure equation: (1 g/cc [density of blood]) * (9.80665 m/s^2 [acceleration due to gravity]) * (1.91 m [my height]). That comes out to a pressure of 187.3 millibars, or, to use the units we use for blood pressure in the United States, 140.5 millimeters of mercury.
It just so happens that I have one of those cheap drugstore blood-pressure cuffs handy. You wait right here. I’m gonna apply it to the fleshy part of my ankle and check my math.
It’s a good thing my heart isn’t in my ankle, because the blood pressure down there is 217/173. That’s the kind of blood pressure where, if you’ve got it throughout your body, the doctors get pale and start pumping you full of exciting chemicals. For the record, my resting blood pressure hovers around 120/70, rising to 140/80 if I drink too much coffee.
The blood pressure at the level of my heart, meanwhile, should be (according to Pascal’s formula) (1 g/cc) * (9.80665 m/s^2) * (0.42 m [the distance from the top of my head to my heart]). That’s 41.2 millibars or 30.9 mmHg. I’m not going to put the blood-pressure cuff on my head. To butcher that Meat Loaf song “I would do any-thing for science, but I won’t dooooo that.” I can get a good idea of the pressure at head level, though, by putting the cuff around my bicep and raising it to the same height as my head. Back in a second.
Okay. So, apparently, there are things I won’t do for science, but there aren’t many of them. Among the things I will do for science is attempting to tape my hand to the wall so my muscle contractions don’t interfere with the blood-pressure reading. That didn’t work out. But the approximate reading, because I was starting to fear for my sanity and wanted to stop, is 99/56. 56 mmHg, which is the between-heartbeats pressure, is higher than 41.2, but it’s in the same ballpark. The differences are probably due to stuff like measurement inaccuracies and the fact that blood vessels contract to keep the blood pressure from varying too much throughout the body.
Man. That was a hell of a digression. But this is what it was leading to: when I’m standing up (and I’ve had coffee), my heart and blood vessels can exert a total pressure of about 131 mmHg. Ordinarily, it’s pumping against a head-to-heart pressure gradient of 41.2 mmHg. But what if I was standing up and exposed to five gees?
In that case, my heart would be pumping against a gradient of 154.5 mmHg. That means it’s going to be really easy for the blood to flow from my brain to my heart, but very hard for the blood to flow from my heart to my brain. And it’s for that reason that the handsome young dude in the first video passed out: his heart (most certainly in better shape than mine…) couldn’t produce enough pressure to keep the blood in his head, in spite of that weird breathing and those leg-and-abdomen-straining maneuvers he was doing to keep the blood up there. People exposed to high head-to-foot gees see their visual field shrink, and eventually lose consciousness altogether. Pilots call that G-LOC, which I really hope is the name of a rapper. (It stands for gee-induced loss of consciousness, in case you were wondering).
You’ll notice that, in almost all spacecraft, whether in movies or in real life, the astronauts are lying on their backs, relative to the ground, when they get in the capsule. That’s because human beings tolerate front-to-back gees better than head-to-foot gees, and in a rocket launch or a capsule reentry, the gees will (hopefully) always be front-to-back. There is at least one documented case (the case of madman test pilot John Stapp) of a human being surviving 46.2 front-to-back gees for over a second. There are a few documented cases of race drivers surviving crashes with peak accelerations of 100 gees.
But you should know by now that I don’t play around. I don’t care what happens if you’re exposed to 46.2 gees for a second. I want to know what happens if you’re exposed to it for twenty-four hours. Because, at heart, I’ve always been a mad scientist.
There’s a reason we don’t know much about the effects of extremely high accelerations. Actually, there are two reasons. For one, deliberately exposing a volunteer to gee forces that might pulp their organs sounds an awful lot like an experiment the Nazis would have done, and no matter your course in life, it’s always good if you don’t do Nazi-type things. For another, building a large centrifuge that could get up to, say, a million gees, would be hard as all hell.
But since I’m playing mad scientist, let’s pretend I’ve got myself a giant death fortress, and inside that death fortress is a centrifuge with a place for a human occupant. The arm of the centrifuge is 100 meters long (as long as a football field, which applies no matter which sport “football” is to you). To produce 1,000,000 gees, I’d have to make that arm spin 50 times a second. It would produce an audible hum. It would be spinning as fast as a CD in a disk drive. Just to support the centrifugal force from a 350-kilogram cockpit, I’d need almost two thousand one-inch-diameter Kevlar ropes. Doable, but ridiculous. That’s the way I like it.
But what would actually happen to our poor volunteer? This is where that gore warning from the beginning comes in. (If it makes you feel better, you can pretend the volunteer is a death-row inmate whose worst fears are, in order, injections, electrocution, and toxic gas, therefore making the centrifuge less cruel and unusual.)
That’s hard to say. Unsurprisingly, there haven’t been many human or animal experiments over 10 gees. Probably because the kinds of people who like to see humans and lab animals crushed to a pulp are too busy murdering prostitutes to become scientists. But I’m determined to at least go through the thought experiment. And you know what, I’m starting to feel kinda weird talking about crushing another person to a pulp, so for this experiment, we’ll use my body measurements and pretend it’s me in the centrifuge. A sort of punishment, to keep me from getting too excited about doing horrible theoretical things to people.
The circumference of my head is 60 cm. If my head was circular, I could just divide that by pi to get my head’s diameter. For most people, that assumption doesn’t work. Luckily (for you, at least), my head is essentially a lumpy pink bowling ball with hair, so its diameter is about 60 cm / pi, or 19.1 cm.
Lying down under 1 gee, hydrostatic pressure means the blood vessels at the back of my head will be experiencing 14.050 mmHg more pressure than the ones at the front. From the fact that I don’t have brain hemorrhages every time I lie down, I know that my brain can handle at least that much.
But what about at 5 gees? I suspect I could probably handle that, although perhaps not indefinitely. The difference between the front of my head and the back of my head would be 70.250 mmHg. I might start to lose some of my vision as the blood struggled to reach my retinas, and I might start to see some very pretty colors as the back of my brain accumulated the excess, but I’d probably survive.
At 10 gees, I’m not so sure how long I could take it. That means a pressure differential of 140.500 mmHg, so at 10 gees, it would take most of my heart’s strength just to get blood to the front of my head and front of my brain. With all that additional pressure at the back of my brain, and without any muscles to resist it, I’m probably going to have to start worrying about brain hemorrhages at 10 gees.
As a matter of fact, the brain is probably going to be one of the first organs to go. Nature is pretty cool: she gave us brains, but brains are heavy. So she gave us cerebrospinal fluid, which is almost the same density as the brain, which, thanks to buyoancy, reduces the brain’s effective mass from 1,200 grams to about 22 grams. This is good because, as I mentioned, the brain doesn’t have muscles that it can squeeze to re-distribute its blood. So, if the brain’s effective weight were too high, it would do horrible things like sink to the bottom of the skull and start squeezing through the opening into the neck (this happens in people who have cerebrospinal fluid leaks; they experience horrifying headaches, dizziness, blurred vision, a metallic taste in the mouth, and problems with hearing and balance, because their leaking CSF is letting the brain sink downwards and compress the cranial nerves).
At 5 gees, my brain is going to feel like it weighs 108 grams. Not a lot, but perhaps enough to notice.
But what if I pulled a John Stapp, except without his common sense? That is: What if I exposed myself to 46.2 gees continuously.
Well, I would die. In many very unpleasant ways. For one thing, my brain would sink to the back of my head with an effective weight of 1002 grams. The buoyancy from my CSF wouldn’t matter anymore, and so my brain would start to squish against the back of my skull, giving me the mother of all concussions.
I probably wouldn’t notice, though. For one thing, the hydrostatic blood pressure at the back of my head would be 641.100 mmHg, which is three times the blood pressure that qualifies as a do-not-pass-Go-go-directly-to-the-ICU medical emergency. So all the blood vessels in the back of my brain would pop, while the ones at the front would collapse. Basically, only my brainstem would be getting oxygen, and even it would be feeling the strain from my suddenly-heavy cerebrum.
That’s okay, though. I’d be dead before I had time to worry about that. The average chest wall in a male human is somewhere around 4.5 cm thick. The average density of ribs, which make up most of the chest wall, is around 3.75 g/cc. I measured my chest at about 38 cm by 38 cm. So, lying down, at rest, my diaphragm and respiratory muscles have to work against a slab of chest with an equivalent mass of 24.4 kilograms. Accelerating at 46.2 gees means my chest would feel like it massed 1,100 kilograms more. That is, at 46.2 gees, my chest alone would make it feel like I had a metric ton sitting on my ribs. At 100 gees, I’d be feeling 2.4 metric tons.
But at 100 gees, that’d be the least of my problems. At accelerations that high, pretty much everything around or attached to or touching my body would become deadly. A U.S. nickel (weighing 5 grams and worth 0.05 dollars) would behave like it weighed half a kilogram. But I wouldn’t notice that. I’d be too busy being dead. My back, my thighs, and my buttocks would be a horrible bruise-colored purple from all the blood that rushed to the back of me and burst my blood vessels. My chest and face would be horrible and pale, and stretched almost beyond recognition. My skin might tear. My ribcage might collapse.
Let’s crank it up. Let’s crank it up by a whole order of magnitude, and expose me to 1,000 continuous gees. This is where things get very, very messy and very, very horrible. If you’re not absolutely sure you can handle gore that would make Eli Roth and Paul Verhoven pee in their pants, please stop reading now.
At 1,000 gees, my eyeballs would either burst, or pop through their sockets and into my brain cavity. That cavity would likely be distressingly empty, since the pressure would probably have ruptured my meninges and made all the spinal fluid leak out. The brain itself would be roadkill in the back of my skull. Even if my ribs didn’t snap, my lungs would collapse under their own weight. The liver, which is a pretty fatty organ, would likely rise towards the top of my body while heavier stuff like muscle sank to the bottom. Basically, my guts would be moving around all over the place. And, at 1,000 gees, my head would feel like it weighed 5,000 kilograms. That’s five times as much as my car. My head would squish like a skittle under a boot.
At 10,000 gees, I would flatten. The bones in the front of my ribcage would weigh 50 metric tons. A nickel would weigh as much as a child or a small adult. My bones would be too heavy for my muscles to support them, and would start…migrating towards the bottom of my body. At this point, my tissues would begin behaving more and more like fluids. This would be more than enough to make my blood cells sink to the bottom and the watery plasma rise to the top. 10,000 gees is the kind of acceleration usually only experienced by bullets and in laboratory centrifuges.
By 100,000 gees, I’d be a horrible fluid, layered like a parfait from hell: a slurry of bone at the bottom topped with a gelatinous layer of muscle proteins and mitochondria, then a layer of hemoglobin, then a layer of collagen, then a layer of water, then a layer of purified fat.
And finally, at 1,000,000 gees, even weirder stuff would start to happen. For one thing, a nickel would weigh as much as a car. But let’s focus on me, or rather, what’s left of me. At 1,000,000 gees, individual molecules start to separate by density. The bottom of the me-puddle would be much richer in things like hemoglobin, calcium carbonate, iodine-bearing thyroid hormones, and large, stable proteins. Meanwhile, the top would consist of human tallow. Below that would be an oily layer of what was once stored oils in fat cells. Below that would come a slurry of the lighter cell organelles like the endoplasmic reticula and the mitochondria. The heavier organelles like the nucleus would be closer to the bottom. That’s right: at 1,000,000 gees, the difference in density between a cell’s nucleus and cytoplasm is enough to make the nucleus sink to the bottom.
I think we’ve gotten horrible enough. So let’s stop the centrifuge, hose what’s left of me out of it, and go ahead and call up a psychiatrist.
But before we do that, I want to make note of something amazing. In 2010, some very creative Japanese scientists decided to try a bizarre experiment. They placed different bacteria in test tubes full of nutrient broth, and put those test tubes in an ultracentrifuge. The ultracentrifuge exposed the bacteria to accelerations of around 400,000 gees. Normally, that’s the kind of acceleration you’d use to separate the proteins from the membranes. It’d kill just about anything. But it didn’t kill the bacteria. As a matter of fact, many of the bacteria kept right on growing. They kept on growing at an acceleration that would kill even a well-protected human instantly. Sure, their cells got a little weird-shaped, but so would yours, if you were exposed to 400,000 gees.
The universe is awesome. And scary as hell.
Actually, I think I’ll let Sam Neill (as Dr. Weir in Event Horizon) sum this one up: “Hell is only a word. The reality is much, much worse.”
As I’ve noted more than once, human beings like to make things go really fast. Part of me thinks that’s because we’re hunter-gatherers by nature, and somewhere deep in our limbic systems, we think that if we can make it to Mach 3, we’ll finally catch that damned antelope. The other part of me thinks we like it because it’s AWESOME!
As of this writing, the world land speed record stands at a hair over 763 miles per hour (almost 1228 km/h, or 341 m/s). The record is held by Andy Green and his ThrustSSC. This is the first land speed record to break the sound barrier. I must also note that when I looked at that page, the entry at the top of the “Related Records” section was the world’s thinnest latex condom. I’m starting to wonder what exactly Andy Green gets up to when he’s not cruising through Nevada at Mach 1.002…
But that’s just the record for the fastest land vehicle with a person inside it. The ultimate land speed record, as far as I can tell, is held by a multi-stage rocket sled at Holloman Air Force Base, which deals with creepy secretive things. Their rocket sled reached Mach 8.47, or 6,453 mph (or 10,385 km/h, or 2,885 m/s; why are there always so many damn units…). They’re not saying why, exactly, they’re accelerating a rocket sled to railgun velocities, but they’ve done it.
There’s no theoretical reason a human being couldn’t go that fast. (There are lots of practical reasons, but I’ve never let that stop me.) In fact, there’s no theoretical reason a land vehicle couldn’t go much faster. Technically speaking, if we ignore aerodynamic effects (which we theoretical types always do, which is why there are engineers to explain to us that astronauts don’t like burning up in the atmosphere), the fastest a land vehicle could ever go is 7.91 kilometers per second. That’s orbital speed at sea level. It’s Mach 23. This is the speed at which the centrifugal acceleration from traveling around the circular earth exactly balances the acceleration due to gravity. To put it another way, this is the speed where your vehicle becomes weightless, and if you go any faster, you’re going to leave the ground.
7.91 km/s is fast. Here’s a good way to understand just how fast it is. Say you’ve got a really good reaction time (around 100 milliseconds; let’s say you’ve had a lot of coffee). If you were trying to time this ultimate land-speeder on a 1,000-kilometer track (about 10 football fields end-to-end) with a stopwatch, the speedy bugger would have traveled from the beginning to the end of the track by the time your brain noticed that it had entered the track, processed the fact, and sent the signal to your finger to press the button on the stopwatch. It wouldn’t matter, of course, because you’d be obliterated by a superheated shockwave a moment later.
But even 7.91 kilometers per second isn’t the ultimate limit on land speed. As a matter of fact, if you have a vehicle that can reach that speed anyway, it’s going to have to have some aerodynamic surfaces on it to keep it from lifting off the ground and turning into the world’s fastest plane. But, while we’re adding downward thrust (in the form of aerodynamic lift, or perhaps I should say anti-lift), why not go all the way? Why not put some rockets on this thing and make it stay on the ground?
The fastest a human being could reasonably expect to travel across flat ground and survive is 23.7 kilometers per second. Before I get into explaining just how horrifically fast that is, and why you can’t go faster than that without killing the pilot, I want to paint a picture of the vehicle we’re talking about.
In all likelihood, it looks more like a plane than a land vehicle. It’s got some sort of massive engine on the back that burns sand to glass behind it. It’s got enormous wings to keep it from bounding into the stratosphere. It’s got rocket motors mounted on the tops of those wings. And we’re not talking wimpy JATO motors. We’re talking ballistic-missile-grade motors. Motors powerful enough that, if you just strapped a human to them, the human would have a hard time staying conscious through the acceleration.
The cockpit’s weird, too. It’s a sort of pendulum, with a reclined seat aligned along the axis of rotation. Because of the pendulum arrangement, the seat rotates so that the occupant always feels the acceleration as vertical. You could be forgiven for thinking this is some kind of Edgar Allan Poe torture device.
I’ll explain all that in a minute. But for right now, I want to convey to you how fast 23.7 km/s is. It’s the speed of extinction-triggering asteroids. It’s Mach freakin’ 71. It’s twice as fast as the crew of Apollo 10 (the holders of the ultimate human speed record) were moving on their way back to Earth. It’s faster than both the Voyager probes and New Horizons. Matter of fact, there are only two human-constructed objects that have ever gone faster than this: the amazing Galileo atmospheric probe, which dropped into Jupiter’s atmosphere so fast that all the speeding bullets in the world momentarily blushed (47.8 km/s, for those who don’t like overwrought metaphors), and the equally amazing Helios 2 probe, which holds both the record for the fastest human-built object and the human object that’s gotten closest to the sun (at perihelion, it was moving at 70.2 km/s; hopefully, NASA won’t can Solar Probe Plus and we can break that record).
23.7 km/s is one of those speeds that just doesn’t fit very well into the human mind, unless it’s the kind of human mind that’s accustomed to particle accelerators or railguns, and frankly, those minds are a little scary. At this speed, our peculiar death-trap vehicle could circumnavigate the Earth in 28 minutes and 9 seconds. It could travel from New York to Los Angeles in 2 minutes and 47 seconds.
“But hell,” I can hear you saying, “we’ve already got a ridiculous impractical land-speed vehicle. Why not crank it up all the way? Why not go as fast as Helios 2? Or faster!” The problem is that I specified a vehicle being driven (or at least occupied) by a human being. Before I explain, here’s a video of a person making a very funny face.
That’s a pilot in training being subjected to 9 gees in a centrifuge. You’ll noticed that he briefly aged about 60 years and then passed out. But he was being trained for practical stuff (that is, not blacking out when making a high-speed turn in an airplane). That’s boring. And, more relevant to our speed record, he was almost certainly experiencing gees from head-to-foot. Humans don’t tolerate that very well. The problem is that human beings have blood. (Isn’t it always?) When gee forces get very high, it takes a lot of pressure to pump blood to levels above the heart. Unfortunately, when you’re dealing with vertical gees, the brain is well above the heart, and all the blood essentially falls out of the brain and into the legs. (There are some ways to compensate for that, like with the weird breathing technique the trainee was doing and the pressure-compensating suits most high-gee pilots wear, but there are limits).
But even if the gees were from back to front (that is, you’re accelerating in the direction of your nose), 9 gees would probably still be the upper limit. Because, even lying down, that acceleration is going to make the blood want to pool below the heart. It’s going to flood into places where you don’t really need it like your buttocks, your calves, and the back of your head. In fact, at 9 gees, you run a pretty good risk of rupturing blood vessels in the back of your brain from the pressure. But human beings can tolerate 9 forward gees for a few seconds, so we’ll pretend they can tolerate it for the 2 minutes and 47 seconds it takes to blaze from New York to LA.
And that’s why we’ve got the weird pendulum recliner in our hypothetical ultra-hypersonic land vehicle: at 23.7 kilometers per second, the vehicle’s going to have to accelerate towards the ground at 9 gees just to keep from flying off into space. The pilot’s seat will be upside-down, relative to the ground, with the pilot all smashed down and funny-looking for the duration of the flight. If we try to go any faster, our pilot isn’t going to be able to survive the acceleration for more than a few seconds at a time. According to this nifty graph
whose source material I unfortunately couldn’t verify, a human being can’t tolerate 10 gees for more than 10 seconds. A human can tolerate 20 gees for 1 second (this I know to be true, because lunatic rocket-sled pilot John Stapp did it; actually, he pulled 25 gees for a full second, and in spite of all his insane rocket-sled stunts, lived to be 89). And human beings have been known to survive 30 gees or more (up to about 100 gees) for very brief periods in car crashes.
But trust me, the weird French organization that certifies land speed records (and air speed records, and altitude records) probably isn’t going to be very impressed by your traveling 50 km/s for a tenth of a second. If you want to go that fast long enough to actually get anywhere, you’re limited to 8 or 9 gees, and even then, you’d damn well better make sure your life insurance is up to date.
So, unless you use weird technologies like liquid respiration (in which you breathe oxygenated liquid fluorocarbons instead of air, and which is a real thing that actually exists and is sometimes used for hospital patients with burned lungs) and those creepy full-body gee-tanks from Event Horizon, the 23.7 km/s land speed record can never be broken. Partly because of the gee-forces involved, but mainly because trying to go that fast on land is absolutely, certifiably insane.
Tune in next time, where I get all gory and try to imagine what would happen to a human body exerted to much larger gee forces.
Sublime Curiosity 
