Insultingly Stupid Movie Physics

by Tom Rogers

  CIGARETTES AS LIGHTERS

  Call them cancer sticks, smokes, or whatever, but by any name, cigarettes are an amazing piece of technology—deliberately designed for reliable failure. Obviously combustible, they fail to burst into flame, even when burning at temperatures in excess of 1,000 degrees Fahrenheit (540°C)29. By design they smolder. Pull air through them, and they smolder really brightly and really quickly, but that’s all they do. They just don’t burst into flame.

  Light the wrong end (the filter), and it burns like a candle with a bright yellow-orange flame. But let a cigarette burn down to the filter and, remarkably, the tobacco smoldering at over 1,000 degrees Fahrenheit does nothing. It reliably fails to light the filter’s combustible cellulose acetate fibers.

  So what happens when a movie villain flicks his cigarette into a puddle of gasoline for some nefarious purpose? It bursts into flame. Try the same experiment in the real world, and the gasoline will snuff out the cigarette as soon as it lands in the liquid. Okay, it’s vapor, not liquid, that burns. The cigarette, however, has to pass through a combustible mixture of gasoline vapor and air before it lands in the puddle. Even if a smoker draws gasoline-vapor laced air through it, a cigarette will still not reliably ignite the vapor.

  It’s Not So Simple—The Fire Triangle

  Just about every grade school kid learns about the fire triangle and the three requirements for creating or putting out a fire. Although the triangle is a good starting point, actual combustion is not so simple. The three conditions needed for fire have to meet a number of conditions of their own. This is why it’s difficult (if not impossible) to ignite gasoline with either bullets (especially handgun bullets) or cigarettes.

  Combustion is a multistep process that first depends on the formation of free radicals. Free radicals are atoms or molecules with one or more unpaired electrons that provide a reaction site. After formation, the right free radicals have to bump into each other at the right moment in the right way—by a random process—so that they can bond and release energy. For example, an O2 molecule must absorb enough energy to break the double bond holding the two oxygen atoms together in order to form two free radicals— one of many types that must form. In turn, these O free radicals must bump into a carbon free radical to form CO or into a CO free radical to form CO2 or an H to form OH or any number of other free radicals to form other interim compounds. In the end, a hydrocarbon fuel (made of Cs and Hs) becomes CO2 and H2O—that is, if enough activation energy is present to start the process and enough excess oxygen to sustain it.

  When heat is liberated during combustion, it must provide the energy to continue activating the various steps in combustion, or the fire will go out. Simply diluting the reactants so that heat is transferred to nonreacting molecules instead of reacting ones can be enough to stop a combustion process. Interfering with the free radicals formed in interim steps can also end a combustion process. When heated, halogenated compounds such as Halon 1211 (CF2BrCl) form their own free radicals that effectively mop up the free radicals in combustion processes and extinguish fires. Generally, Halon is effective in concentrations of only 5 percent to 10 percent. Unfortunately, such compounds are also highly effective at destroying the ozone layer and have pretty much been banned. All in all, starting and maintaining a fire is not as simple as it looks.

  Unconvinced by testimony from several sources, a group of staff and volunteers at Intuitor.com tested the gasoline-igniting ability of cigarettes over three hundred times without a single success. They tried tossing lit cigarettes into a small puddle of gasoline, holding them (with really long tongs) in the vapor-air mixture at various distances above the puddle, placing up to forty smoldering cigarettes in and around the puddle, and using gasoline-soaked paper towels instead of a puddle, all with no effect. They devised a smoking machine and smoked numerous cigarettes directly over the puddle. Using the machine, they blew air backward through smoldering cigarettes, making sparks fly off the brightly glowing tip into the gasoline—but still no ignition. They lit an unfiltered cigarette and placed the unlit end in the gasoline. The flammable fluid wicked its way to the smoldering end and snuffed it out.

  In distress, the experimenters began thinking the gasoline was defective. They held a match above the puddle (at the end of a long pole) and—poof—the fuel burst into flame. They repeated this process several times, always with the same result. Clearly, cigarettes were just not reliable igniters even though matches were.

  The myth of cigarettes reliably igniting gasoline pervades popular culture so deeply it has worked its way into novels. In The Partner, John Grisham has a character splash gasoline around his vehicle (ironically, a Chevy Blazer), light a cigarette, and throw it in the gas, igniting an inferno.Tell a group of friends that the lit cigarette wouldn’t have ignited the gasoline, and invariably at least one will offer, “but they do it in movies.” (A few words of advice for aspiring novelists: don’t use movies for source material. The real-life experiences touted by writing teachers are actually much better.)

  So, how does a cigarette ignite a gasoline puddle in a movie? It doesn’t. At the right moment, a special-effects technician pushes a button that fires an electronic igniter located appropriately near the fuel. The camera is placed so the igniter can’t be seen. Sometimes, the fuel is lit by a trick cigarette containing combustible compounds other than tobacco. It is never, however, lit with anything as unreliable as an ordinary cigarette.

  The explosive or flammable range for gasoline is only about 1.4 percent to 7.6 percent gasoline vapor in air (21 percent oxygen). Concentrations below 1.4 percent don’t give off enough heat to sustain the chain reaction of combustion. Concentrations above 7.6 percent don’t have enough oxygen to sustain combustion. Outside these very narrow limits, gasoline cannot be ignited.

  Nevertheless, even the right mixture of gasoline vapor and air needs some activation energy for igniting, similar to the way some pressure is needed for tripping a mousetrap. At 536 degrees Fahrenheit (280°C)30 a combustible gasoline-air mixture already has enough internal energy to trigger combustion with no further energy input. Having a gasoline-air mixture contact an object at 536 degrees Fahrenheit (280°C) or higher, however, does not automatically heat the mixture to 536 degrees Fahrenheit (280°C). Heat must be transferred from the high-temperature object into the vapor-air mixture to elevate it to 536 degrees Fahrenheit before ignition occurs. Heat the gasoline-air mixture outside the cigarette and as soon as it gets warm, the vapor-air mixture will rise and mix with colder air, preventing it from reaching ignition temperature. A similar effect prevents firewalkers from burning their feet as they walk across glowing coals. If their feet contact the hot coals briefly, the heat transfer is too slow to cause burns, but stand on the coals too long and the feet get fried.

  Is there any other example of a combustion process that will not ignite a combustible fuel-air mixture? Yes, the Davy safety lamp. In the 1800s coal mines could only be illuminated with some sort of flame—better for igniting the mine’s methane gas than for illuminating the mine’s dark tunnels. The Davy lamp solved this problem by surrounding the flame with a fine wire screen. A flammable methane and air mixture could enter the lamp and make it burn brighter, but the flame would not propagate outward through the screen. Presumably, the screen disrupted heat transfer from the flame to the methane-air mixture and prevented it from reaching its ignition temperature.

  Can a cigarette ever light gasoline vapors? Yes, under special conditions, and it makes a convincing fire-safety demonstration in the process. A firefighter sets an object that looks like a skinny chrome-plated metal vase on the table. It has a square base with an 18-inch-long (45.5 cm) vertical tube, about 2 inches (5 cm) in diameter, welded to the base and open on the other end. He fills the tube with pure oxygen (not air), adds a few drops of gasoline, heats the outside slightly to vaporize the fuel, drops a lit cigarette in the open end, and—BOOM!—the gasoline explodes. So, yes, cigarettes can ignite gasoline vapors; yes, one shou
ld never risk smoking near gasoline; and yes, smoking is dangerous. But then there aren’t many gas stations, warehouses, or fuel dumps with pure oxygen environments.

  If anything, the exploding vapor demonstration shows that altering oxygen concentration makes a big difference in how gasoline vapors do or don’t ignite. Lower the oxygen content enough to the minimum oxygen concentration (MOC, about 12 percent for gasoline), and the gasoline vapor cannot ignite regardless of its concentration.

  If there’s a chance of an explosive mixture forming in the empty space of a storage tank, the chemical company that owns it will keep the empty space below the MOC of the tank’s contents, often by diluting the empty space’s air with nitrogen.

  Cigarettes are designed to deliberately create an oxygen-poor, fuel (tobacco) rich environment in order to produce smoke. Draw a combustible mixture of gasoline and air into the smoldering tip, and the burning paper, which has a combustion temperature of 451 degrees Fahrenheit (233°C), along with the tobacco will greedily gobble up most of the available oxygen, lowering oxygen content below gasoline’s MOC of 12 percent before the gasoline vapors get to their ignition temperature of 536 degrees Fahrenheit (280°C). What happens to the oxygen? It mostly becomes CO2, the same stuff that’s in CO2 fire extinguishers. Diluting air’s oxygen content with CO2 is even more effective for fire suppression than diluting it with nitrogen.

  Past the glowing tip, fresh air will indeed seep in through the cigarette’s porous sides—boosting the oxygen content back up to around 13 percent coming out of the filter31. But in the process, it also lowers the temperature well below the gasoline vapor ignition point.

  Neither theory nor experiments can totally rule out the possibility of igniting a fire with an oddball cigarette combined with a special set of weather conditions and an unusual blend of gasoline. But there’s certainly enough theoretical and experimental information to show that cigarettes won’t reliably ignite gasoline—probably due to a combination of poor heat transfer and low oxygen content in the burning tip. What’s more, moviemakers know this. So, naturally, it makes perfect sense for them to continue filming clichéd scenes falsely depicting it.

  CARS AS BOMBS

  Movie bullets can be as effective as cinematic cigarettes for setting off gasoline. Shoot a car’s gas tank, and it explodes in a fireball. But how? It’s actually not as easy as it looks. First, automobile gas tanks aren’t put in exposed locations. They’re usually sandwiched between heavy frame members and surrounded by one or more layers of sheet metal in the car’s body. The tanks are hard to see, let alone to shoot. Second, if there’s any gas in the tank, gasoline is so volatile that enough of it would vaporize to ensure that the fuel-to-air ratio in the tank was too high for combustion.

  Stop a bullet suddenly, and it gets really hot because its kinetic energy changes into heat. In theory, a high-powered rifle bullet can reach high enough temperatures to set off a combustible mixture of gasoline vapor and air. But it’s just about impossible to stop a bullet abruptly enough when shooting a gas tank. The gas tank doesn’t offer enough resistance. The bullet will likely plow right through. If it is stopped inside, it’s because it has already lost most of its kinetic energy by penetrating the car’s body, frame, or gas-tank wall and will likely not have enough kinetic energy left to ignite anything.

  Sometimes, a bullet can cause a spark, especially if the bullet has steel parts or a static electric charge on it, but these are not the bright fiery flashes of light caused by overuse of pyrotechnic compounds in bullet impact special effects. They are modest sparks that are neither effective at starting fires nor clearly visible in normal daylight. For one thing, the sparks will likely occur outside the gas tank—where there are no combustible vapors—when the bullet first contacts the gas tank’s wall. For another, most bullets are made of lead or copper-jacketed lead, both materials that can be ground with high-speed grinders without producing any visible sparks, as opposed to grinding steel, which produces a shower of sparks.

  Sparks produced by grinding steel are generally orange-yellow in color, indicating a temperature of over 1,832 degrees Fahrenheit (1,000°C), way above the autoignition temperature of gasoline vapor (280°C). Will they ignite gasoline? Typically, no. Again, the doubters at Intuitor.com tested the gasoline-igniting ability of sparks from grinding steel by repeatedly grinding over a small pan of gasoline. The sparks showered into the pan for several minutes with no ignition. Again, to prove that a combustible mixture existed over the gasoline, a lighted match (on the end of a long pole) was held above it, and the mixture burst into flame. Although the sparks themselves were well above the autoignition temperature of a combustible gasoline vapor and air mixture, they did not contain the right amount of thermal energy to heat the combustible mixture around them to a high enough temperature for ignition.

  Once again it should be emphasized that neither theory nor experiments can totally rule out the possibility of igniting gasoline with sparks from a grinding operation, and so grinding near a puddle of gasoline has to be considered a very unsafe activity. However, sparks from a normal grinding operation are not a reliable source of ignition. This casts doubt on whether sparks from a steel bullet would be any more effective.

  If a vehicle’s gas tank is shot with a machine gun and gasoline leaks out, and it forms a combustible mixture, and a bullet just happens to hit in such a way that it makes a spark with just the right amount of energy inside the combustible mixture, then— poof! But that’s not likely. Even heavy .50-caliber machine guns in WWII airplanes were found to be unreliable for starting fires of this type.

  Incendiary bullets were invented to remedy the problem. They contain a pyrophoric material, which bursts into flame when it hits an object and burns like white phosphorus. Although the fire is very brief, it’s also very hot and highly effective at igniting flammable materials. Tracer bullets can also sometimes light fires but are less effective. The tracer compound is designed to produce a streak of light as the bullet flies toward its target so that gunners can see where their shots are going.The tracer compound does not burn as intensely as incendiary material, and it is often largely or entirely used up by the time the bullet reaches its target.

  Incendiary bullets are the only type capable of creating the large bright flashes of light on impact commonly portrayed in movies. Outside the military, virtually no one uses incendiary bullets in gunfights. The overdone bullet impact flashes in movies can be traced to Raiders of the Lost Ark [PGP-13] (1981). Bullet impact pyrotechnics were overused in this movie to create excitement. The bright flashes certainly give the impression that bullets would be effective fire starters. The movie was a hit and—as is Hollywood’s habit—was widely emulated. But that doesn’t change the fact that, on impact, real bullets don’t make such fiery flashes and very rarely start fires.

  Movie car crashes also routinely trigger infernos. Drive a car off a cliff, and it’s certain to burst into flame, sometimes even before it hits the ground. A character can flip, roll, and smash his car into the shape of an accordion, and yet walk away without so much as a broken nose. But if it’s time to kill him off, he’s going to get the fireball. Sometimes for dramatic effect he’s granted a few moments to ponder his fate while trapped upside down in the car as a puddle of gasoline ominously oozes beneath him. Nevertheless, a fiery end is assured—sometimes at the hands of a villain who sneers as he tosses a cigarette in the puddle.

  Yes, cars can catch on fire and sometimes even explode, but it’s rare. The narrow flammable limits of gasoline vapor in air make it unlikely. Not only does the gas tank have to rupture— hardly preordained—but a source of ignition has to occur in combination with an explosive mixture of gasoline and air.

  BURNING BUGS

  After crash landing on Mars, finding their habitat destroyed, and running out of air, the crew in Red Planet [RP] (2000) runs into a locust-like swarm of flesh-eating nematodes that can bore right through their space suits. But it’s not all bad news. The nematodes mad
e the oxygen atmosphere, which kept the crew alive when they removed their helmets as their air tanks ran dry. The voracious little guys are also highly flammable and, therefore, easily exterminated. Light one on fire and it bursts into flame, setting off a chain reaction with its nearby cousins in a veritable fireworks display.

  Apparently, viewers are supposed to believe that the nasty little critters not only make oxygen from Mars’s otherwise CO2 atmosphere, but also store it in their bodies under high pressure. According to Robert Zubrin, if Mars were terriformed so that it had a breathable atmosphere, the pressure would only be about five pounds per square inch (1/3 atm). Although it would be nearly pure oxygen—a condition favoring rapid combustion—the partial pressure of oxygen would be about the same as on Earth, a condition favoring normal combustion rates. Most of the oxygen needed for making the nematodes burn like fireworks would have to be stored inside them. The oxygen concentration outside the little guys would not be high enough for such spectacular combustion.

  It takes at least 3.45 grams of oxygen to completely burn 1 gram of paraffin wax (assuming a composition of C30H62). Oxygen free radicals have to “bump into” free radicals from fuel molecules for combustion to occur. As oxygen is consumed, these collisions become less and less likely. So, in reality, significantly more than 3.45 grams of oxygen must be available to insure complete combustion.

  Assuming that burning nematode carcasses consume as much oxygen as candle wax, the little guys would need to compress and store large quantities of oxygen at high pressure to be so flammable. The stored oxygen would weigh more than 3.45 times the critter’s deflated body weight. If the voracious little guys gave off oxygen as a waste product—enough to create an entire oxygen atmosphere—then why would they want to store and tote around such a heavy load of it?