What If? by Randall Munroe Read Free Book Online Page B

radiation, and it’s seen in the distinctive blue glow of nuclear reactor cores.
    Some of our radioactive waste products, such as cesium-137, are meltedand mixed with glass, then cooled into a solid block that can be wrapped in more shielding so they can be safely transported and stored.
    In the dark, these glass blocks glow blue.
    Cesium-137 has a half-life of thirty years, which means that two centuries later, they’ll still be glowing with 1 percent of their original radioactivity. Since the color of the light depends only on the decayenergy, and not the amount of radiation, it will fade in brightness over time but keep the same blue color.
    And thus, we arrive at our answer: Centuries from now, deep in concrete vaults, the light from our most toxic waste will still be shining.

    1 When Enrico Fermi built the first nuclear reactor, he suspended the control rods from a rope tied to a balcony railing. In case something went wrong, next to the railing was stationed a distinguished physicist with an axe. Th is led to the probably apocryphal story that SCRAM stands for “Safety Control Rod Axe Man.”
    2 Th e purposeof the crash was to safely incinerate the probe so it wouldn’t accidentally contaminate the nearby moons, such as the watery Europa, with Earth bacteria.
    3 Th e USSR built some lighthouses powered by radioactive decay, but none are still in operation.

Machine-Gun Jetpack
    Q. Is it possible to build a jetpack using downward-firing machine guns?
—Rob B
    A. I was sort of surprised to find that the answer was yes! But to really do it right, you’ll want to talk to the Russians.
    Th e principle here is pretty simple. If you fire a bullet forward, the recoil pushes you back. So if you fire downward, the recoil should push you up.
    Th e first question we have to answer is “can a gun even lift its own weight?” If a machine gun weighs 10 pounds but produces only 8 pounds of recoil when firing, it won’t be able to lift itself off the ground, let alone lift itself plus a person.
    In the engineering world, the ratio between a craft’s thrust and the weight is called, appropriately, thrust-to-weight ratio. If it’s less than 1,the vehicle can’t lift off. Th e Saturn V had a takeoff thrust-to-weight ratio of about 1.5.
    Despite growing up in the South, I’m not really a firearms expert, so to help answer this question, I got in touch with an acquaintance in Texas. 1
    Note : Please, PLEASE do not try this at home.
    As it turns out, the AK-47 has a thrust-to-weight ratio of around 2. Th is means if you stood it onend and somehow taped down the trigger, it would rise into the air while firing.
    Th is isn’t true of all machine guns. Th e M60 , for example, probably can’t produce enough recoil to lift itself off the ground.

    Th e amount of thrust created by a rocket (or firing machine gun) depends on (1) how much mass it’s throwing out behind it, and (2) how fast it’s throwing it. Th rust is the product of these two amounts:

    If an AK-47 fires ten 8-gram bullets per second at 715 meters per second,its thrust is:

    Since the AK-47 weighs only 10.5 pounds when loaded, it should be able to take off and accelerate upward.
    In practice, the actual thrust would turn out to be up to around 30 percent higher. Th ereason for this is that the gun isn’t spitting out just bullets — it’s also spitting out hot gas and explosive debris. Th e amount of extra force this adds varies by gun and cartridge.
    Th e overall efficiency also depends on whether you eject the shell casings out of the vehicle or carry them with you. I asked my Texan acquaintances if they could weigh some shell casings for my calculations. Whenthey had trouble finding a scale, I helpfully suggested that given the size of their arsenal, really they just need to find someone else who owned a scale. 2
    So what does all this mean for our jetpack?
    Well, the AK-47 could take off, but it doesn’t have enough spare thrust to lift anything weighing