In a previous post, I discussed some of the gory things that would happen if you put me into a centrifuge and spun it up until I was experiencing an acceleration of 1,000,000 gees.
Now, I’m a science-fiction buff, so I’m all about imagining wild new technologies, but frankly, if I tried to handwave a way to protect myself against 1,000,000 gees, I’d be going pretty far towards the fantasy end of the science fiction-fantasy scale. I’d be far to the right of Firefly, beyond Star Trek, past Star Wars. Hell, I’d probably be closer to Lord of the Rings than to Star Wars.
But I set myself a challenge: figure out a way that a human being could survive 100 gees. That’s 980.665 m/s^2. That’s the acceleration of the Sprint missile, the scary awesomeness of which I’ve talked about before. Here’s the video of it again, because if you haven’t seen it, you should:
I’m obligated to remind you that the tracking shot in that video is played at actual speed. This bastard could accelerate from liftoff to Mach 10 in 5 seconds.
So let’s pretend I’ve invented a handwavium-burning rocket engine that can accelerate my capsule at 100 gees for, say, an hour. In my previous post (and based on experimental data, mostly from race-car crashes), we decided that 100 gees applied over more than a second would be more than enough to kill me. The problem, as I said, is the fact that I have blood, and at 100 gees, even if I was supine (on my back, the ideal position for tolerating accelerations when traveling in the back-to-front direction), that blood would collect at the bottom of my body, rupturing blood vessels and starving the upper parts of oxygen. My heart would almost certainly stop within seconds, either from pure mechanical strain, from the effects of pressure differentials, or because my ribcage caved in and turned it into carne asada.
But I’ve devised some absurd ways to get around this. The first is to put my acceleration couch into a sealed steel coffin (let’s face it, I’m gonna end up in a coffin one way or another; might as well save everybody the cleanup). The coffin will be filled with saline that approximates, as close as possible, the density of my body. Let’s say the coffin is an elliptical cylinder long enough for my body, 1 meter across the short axis, and 2 meters across the long axis. And let’s say I’m positioned so that I’m as close to the top of the tank as possible. (The tank has to be filled right to the brim. If it isn’t, the undulations of the surface will probably be more than enough to kill me.) Let’s say no part of me is deeper than 50 centimeters. At 50 centimeters’ depth, the hydrostatic pressure from my blood would be fifteen times the blood pressure that qualifies as an instant medical emergency: over 3,000 mmHg. More than enough to burst every capillary in my back, and probably the rest of the bottom half of my body.
But when I’m floating in saline that’s very close to the density of my body, the problem all but disappears. In the previous post, my capillaries only burst because they were experiencing a blood-related hydrostatic pressure (sounds like a weather forecast in Hell) of 4.952 bars (just over 5 atmospheres), with only 1.000 bars to oppose it. Things flow from areas of high pressure to low pressure. In this case, that probably means my blood flowing from the high-pressure capillaries through the slightly-lower-pressure skin and out onto the low-pressure floor of the centrifuge.
Suspended in saline, the story is different. The saline exerts 4.952 bars of hydrostatic pressure, exactly (or very nearly, I hope) opposing the pressure exerted by the blood, therefore meaning my heart doesn’t have to work itself to death trying to get blood to my frontal organs.
Speaking of organs, though, my lungs are the next problem. While I’m suspended in saline, they’ll be filled with air. Normally, I like my lungs being filled with air. It keeps me from turning blue and making people cry and then bury me in a wooden box. But air, being so light, doesn’t produce nearly the hydrostatic pressure that saline does, and so there’s nothing to keep my lungs from collapsing. Here’s a brief picture of how the lungs work: your chest is a sealed cavity (if it’s not, you’d better be in the ER or on your way there). The diaphragm moves down when you inhale. This increases the volume of the chest cavity. The lungs are the only part that can expand in volume, since they’re full of gas. That lowers the pressure, which draws air in. Ordinary human lungs weigh something in the neighborhood of 0.5 kilograms. So we know the diaphragm can cope with the weight of 1 kilo. At 100 gees, though, that rises to 100 kilos. Not even Michael Phelps’s diaphragm could make the lungs expand against that much weight.
But fret not! There’s (kind of) a solution! It’s called liquid ventilation, and it’s one of those cool sci-fi things that’s a lot realer than you might think. Instead of breathing gas, you breathe liquid. Normally, that’s bad news (remember the blue and the wooden box and the sad from before?). But certain liquids (for example, perfluorodecalin, a slightly scary-looking fluorocarbon) happen to be very good at dissolving oxygen. Good enough that people and animals have been kept alive while getting part (or, in a few cases, all) of their oxygen from liquid.
There is, however, one snag. Perfluorodecalin is denser than water. Its density is 1.9 g/cc. If the frontmost parts of my lungs are 5 centimeters from the top of the tank, then the hydrostatic pressure from the saline is 0.450 bars. 20 centimeters deeper, at the back of my lungs, the hydrostatic pressure from the saline is 2.450 bars. Meanwhile, the pressure from that heavy column of perfluorodecalin (plus the pressure from the water on top of it) is 4.180 bars. That’s almost a two-fold pressure differential. More than enough to blow out a lung. You might be able to overcome this problem by mixing one part perfluorodecalin with three or four parts high-grade inert mineral oil, but not being a chemist, I can’t guarantee that it’ll end well.
So you know what? Screw the lungs! Let’s just fill ’em with saline! (I wonder how many frustrated respiratory therapists have screamed that in their offices…) Instead, I’m going to get my oxygen the SCIENCE way! That is, by extracorporeal membrane oxygenation. Now, while it might sound like something that would involve a seance and a lot of ectoplasm, ECMO is also a real technology. It’s a last-ditch life-saving measure for people whose hearts and/or lungs aren’t strong enough to keep them alive. ECMO is the ultimate in scary life-support. Two tubes as thick as your finger are inserted into the body through a big incision. One into a major artery, and one into a major vein. The one in the vein takes blood out of the body and passes it through a machine that diffuses oxygen into the blood through a membrane and removes carbon dioxide. The blood’s temperature and pressure are regulated, and usually blood thinners like heparin are added to stop the patient’s blood sludging up from all the foreign material it’s in contact with. Then a pump returns it to the body, well-enough oxygenated to keep that body alive.
There are several hundred problems with using ECMO for ventilation under high G forces. One of them is, of course, that I have to have a tube rammed up my aorta. Another other is that I’d probably have to be anesthetized the whole flight. And yet another is the fact that those tubes and fittings are likely to be significantly more or significantly less dense than saline, and might therefore have enough residual weight (after the effects of buoyancy) to either pierce through my abdomen and into my spine, or float up and pull all my guts out.
So breathing is a problem. But let’s do another hand-wave and say we’ve invented a special polymer that can hold enough oxygen to keep me alive and can dissolve in saline without adding too much density and doesn’t destroy my lungs in the process. There are still problems.
The balance between the hydrostatic pressure exerted by the weight of my body and the pressure exerted by the weight of the saline means there’s either no pressure gradient between body and tank, or the gradient is survivable. But, although most of pressure’s effects depend on pressure differences, some of them depend on absolute pressure. One of those effects is nitrogen narcosis. Air is mostly nitrogen (78% by volume). Nitrogen, being a fairly inert gas, isn’t too important in respiration. But when it’s under high enough pressures, more and more of it starts to dissolve into the bloodstream. If this happens, and then the pressure suddenly falls, it bubbles back out of the bloodstream and you get the horrifying affliction known only as the bends. (Actually, it has lots of different names. Damn. Spoiled my own drama again…) But even if you don’t have sudden pressure drops, when the pressure gets above 2 bars, all that extra dissolved nitrogen starts to interfere with brain function. Since the maximum pressure I’ll be experiencing is about 4.910 bars, Wikipedia’s handy table tells me I’ll probably be feeling a bit drunk and clumsy. When you’re accelerating at 100 gees on top of a magic super-rocket, you really don’t want to be drunk and clumsy.
And it turns out divers who have to work underwater where everything can kill you don’t want to be drunk and clumsy either. But they can’t solve the problem by just breathing pure oxygen. In a normal atmosphere, oxygen’s partial pressure is 0.210 bars. Breathing 100% oxygen at the surface means breathing 1.000 bars. While it’s probably not good long-term, when you’re flying on a super-rocket, “not good long-term” means you can worry about all the other things that are about to kill you.
However, at high pressures, pure oxygen becomes toxic. In a slightly worrying paper from the British Medical Journal, some scientists described how they exposed volunteers to pure oxygen at a pressure of 3.6 atmospheres (about 3.6 bars). Some experienced troubling symptoms like lip-twitching, nausea, vomiting, and fainting after as little as 6 minutes. Even their toughest subject only lasted 96 minutes before suffering “prolonged dazzle” and “severe spasmodic vomiting.” If I was breathing 100% oxygen (in magic-liquid form, of course), some parts of my body would be much higher than that, so I’d be in serious trouble.
But those clever divers have figured out a way around this, too. Sort of. Instead of breathing pure oxygen at depth, they breathe blends of gas containing oxygen, nitrogen, and an inert gas like helium. This means that, for a given pressure, the partial pressure of oxygen will be lower than it would be in an oxygen-nitrogen or pure-oxygen mixture. That saves the diver from oxygen toxicity.
Of course, when you go deep enough, the nitrogen becomes an issue. Very deep divers sometimes breathe a gas mixture called heliox, which is just oxygen and helium (I recognize heliox from reading Have Space-Suit–Will Travel as a pimply, lonely adolescent). Helium has a much smaller narcotic effect than nitrogen. Since I’m going to be experiencing as much as 4.91 bars (let’s call it 5, to be safe), I need to adjust the mixture so that the partial pressure of oxygen stays around 0.210 bars. That means I’ll be breathing a mixture of 96% helium and 4% oxygen.
Because I’m a geek, I know that my lungs can inflate comfortably to 3 liters (they max out at 4 liters). That means, at 4% oxygen, I’ll be getting 120 milliliters of oxygen per breath. When I’m exercising or under severe strain (say, for example, when I’m trapped in a metal coffin at the top of a rocket accelerating at 100 gees), I need 2.2 liters of oxygen per minute. To get that much oxygen when each inhale gets me 120 milliliters requires a respiratory rate of 36 breaths per minute. That’s awfully fast for an adult, and when you consider that I’m either breathing magic low-density fluorocarbons or magic oxygenated saline, that’s a lot of work for my lungs to do, and a lot of wear and tear.
So my conclusion is that, sadly, I won’t be able to strap myself to a speeding infinite-fuel Sprint missile for an hour. But all this math makes me think that it’s probably possible to protect the human body against milder accelerations (say 10 gees) for long periods, using the same techniques. Any fighter pilots who want to climb into a saline coffin and breathe Fluorinert, let me know how it turns out.