Superspeed is a popular superpower but it comes with some challenges when rationalizing the physics behind it: friction at supersonic speeds would tear a human’s face off, accelerating and stopping would require impossible traction, a stray pebble in the air would rip through them like a bullet, staying on the ground when running on hilly terrain would be impossible… the list goes on.
Most comics waive those problems with a field that protects the hero from Newton’s laws. However, they still have to get dressed in the morning and their costumes, while possibly protected from obeying the laws of momentum or effects of friction, still have to cope with some other, less obvious perils. The following is an exploration of a couple of those dangers.
I wanted to know what kind of life expectancy a typical super-suit would have for a high-speed hero. To do that, first I needed to quantify some things. Fist, for comparison how fast does a normal human go?
The fastest human in the world, Ussain Bolt, tops out at 44.7 km/h or 12 m/s and has a stride length of around 2.5m. That puts humans, at their fastest, at 2.5 full cycles per second. That is five steps per second. That’s pretty fast, no?
Superspeed humans, though, regularly travel around at the speed of sound. At ground level, that’s around 1200 km/h or 333m/s. I am going to argue here that the superhero stride length can’t be much longer than a normal sprinter because they are rarely just cruising in a straight line, they tend to be accelerating or maneuvering which require footfalls. That being the case, as the speed of sound, a superhero would be laying down 133 steps a second or be running at 66Hz. For reference this is what 66hz sounds like
What does this mean, well it means that even without the effects of friction, the clothes might just wear out. There are a few mechanisms that can cause failure. I’m going to explore fatigue and hysteresis heating.
Fatigue refers to the eventual failure of material due to repetitive loading well below levels that would cause permanent deformation after a single load. I dug up a study on the fatigue life of polyester ropes to get a sense of the fatigue limit of polyester fibers. Why polyester, you ask? Because that’s what The Flash makes his costume out of. The study found that the fatigue life was on the order of 10^5 cycles. That means after a couple hundred thousand small, repetitive loads, the fibers in the material start to snap.
Given that failure mode alone, a human sprinter could expect their running clothes to last up to 100 hours of full-speed racing. The Flash or Quicksilver, though could expect their clothes to start falling apart after less than half an hour and would be lucky if they ever made it past four hours of casually cruising around at Mach 1 without some serious wardrobe malfunctions.
This is ignoring the effects of strain-rate. That is, materials react differently when they are deformed at different speeds. As an example, take some silly putty and stretch it out slowly, now do the same thing quickly. Note that it breaks when you pull hard and stretches when you move slowly. Similar things are happening when you bend fabric, albeit at a much smaller level. So small that normal use would never have to consider it, but the fact that the fabric is folding and unfolding at such a high rate might make strain-rate considerations very important.
Related to the speed of folding and unfolding, hysteresis heating may also be an issue. Whenever you bend a spring and then let it return to its original shape, a little bit of energy is converted to heat. This is a hysteresis loop. It’s a consequence of the second law of thermodynamics. This is a fundamental law of the universe and even though heroes can ignore the laws of physics, their trousers probably can’t.
This effect is tiny in normal use. You will notice that when you go jogging, your clothes do not catch fire from the hysteresis losses. However, when some areas of the costume are experiencing large deflections as they fully fold and unfold 66 times a second, these tiny effects add up. Even at this rate it is very unlikely that it will start to melt from the heat generated. But wait, the normal cooling process wouldn’t be active.
Normally we expect things passing through an air current to cool off pretty effectively. But we’ve already determined that there’s a field protecting the suits from air friction which means that there would be no air currents running over and through the fabric. As a consequence, there would be very little convection to draw the heat being generated away. It would just build and conduct slowly through the fabric and the skin. The end result would be that the suit would heat up significantly. It is still unlikely that there would be enough heat to melt the suit, but it would almost certainly become uncomfortably warm and would degrade even faster.
So there you go if you’re going to be an impossibly fast superhero, expect to tear through a lot of costumes. Alternately, you could just go naked. It’s a much more economic choice.