Yeah, yeah, yeah... We know grenades in movies aren't like the real thing. But that could make you wonder, "Why?"
Real grenades are puffs of smoke with a bit of high-moving metal. Why not give troops mobile fireballs that instill fear and awe in the hearts of all that see them? Why not arm our troops with something akin to Super Mario's fire flower?
First, we should take a look at what, exactly is going on with a real grenade versus a movie grenade.
The grenades you're probably thinking of when you hear the term "grenade" are likely fragmentation grenades, consisting of strong explosives wrapped up in a metal casing. When the explosives go off, either the case or a special wrapping is torn into lots of small bits of metal or ceramic. Those bits fly outwards at high speed, and the people they hit die.
The U.S. military uses the M67 Fragmentation Hand Grenade. 6.5 ounces of high explosive destroys a 2.5-inch diameter steel casing and sends the bits of steel out up to 230 meters. Deaths are commonly caused up to 5 meters away from the grenade.
U.S. Army soldiers throw live grenades during training in Alaska.
That's because grenades are made to maximize the efficiency of their components. See, explosive power is determined by a number of factors. Time, pressure, and temperature all play a role. Maximum boom comes from maximizing the temperature and pressure increase in as little time as possible.
That's actually a big part of why M67s have a steel casing. The user pulls the pin and throws the grenade, starting the chemical timer. When the explosion initiates, it's contained for a fraction of a second inside that steel casing. The strength of the steel allows more of the explosive to burn — and for the temperature and pressure to rise further — before it bursts through the steel.
As the pressure breaks out, it picks up all the little bits of steel from the casing that was containing it, and it carries those pieces into the flesh and bones of its enemies.
Movie grenades, meanwhile, are either created digitally from scratch, cobbled together digitally from a few different fires and explosions, or created in the physical world with pyrotechnics. If engineers wanted to create movie-like grenades, they would need to do it the third way, obviously, with real materials.
The explosion is easy enough. The 6.5 ounces in a typical M67 would work just fine. Enough for a little boom, not so much that it would kill the thrower.
But to get that movie-like fire, you need a new material. To get fire, you need unburnt explosives or fuel to be carried on the pressure wave, mixing with the air, picking up the heat from the initial explosion, and then burning in flight.
And that's where the problems lie for weapon designers. If they wanted to give infantrymen the chance to spit fire like a dragon, they would need to wrap something like the M67 in a new fuel that would burn after the initial explosion.
Makers of movie magic use liquid fuels, like gasoline, diesel, or oil, to get their effects (depending on what colors and amount of smoke they want). Alcohols, flammable gels, etc. all work great as well, but it takes quite a bit of fuel to get a relatively small fireball. The M1 flamethrower used half a gallon of fuel per second.
But liquid fuels are unwieldy, and even a quart of gasoline per grenade would add some serious weight to a soldier's load.
So, yeah, there's little chance of getting that sweet movie fireball onto a MOLLE vest. But there is another way. Instead of using liquids, you could use solid fuels, especially reactive metals and similar elements, such as aluminum, magnesium, or sodium.
The military went with phosphorous for incendiary weapons. It burns extremely hot and can melt its way through most metals. Still, the AN-M14 TH3 Incendiary Hand Grenade doesn't exactly create a fireball and doesn't even have a blast. Along with thermite, thermate, and similar munitions, it burns relatively slowly.
But if you combine the two grenades, the blast power of something like the M67 and the burning metals of something like the AN-M14 TH3, and you can create actual fireballs. That's how thermobaric weapons work.
U.S. Marines train with the SMAW, a weapon that can fire thermobaric warheads.
(U.S. Marine Corps Cpl. Brian J. Slaght)
In thermobaric weapons, an initial blast distributes a cloud of small pieces of highly reactive metal or fuel. Then, a moment later, a secondary charge ignites the cloud. The fire races out from the center, consuming the oxygen from the air and the fuel mixed in with it, creating a huge fireball.
If the weapon was sent into a cave, a building, or some other enclosed space, this turns the secondary fire into a large explosion of its own. In other words, shoot these things into a room on the first floor of a building, and that room itself becomes a bomb, leveling the larger building.
But throwing one of these things would be risky. Remember, creating the big fireball can turn an entire enclosed space into a massive bomb. And if you throw one in the open, you run the risk of the still-burning fuel landing on your skin. If that's something like phosphorous, magnesium, or aluminum, that metal has to be carved out of your flesh with a knife. It doesn't stop burning.
So, troops should leave the flashy grenades to the movies. It's better to get the quick, lethal pop of a fragmentation grenade than to carry the additional weight for a liquid-fueled fireball or a world-ending thermobaric weapon. Movie grenades aren't impossible, but they aren't worth the trouble.