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MACHINE-GUN JETPACK
机枪飞行背包
Q. Is it possible to build a jetpack using downward-firing machine guns?
Q．用一挺向下射击的机枪能做出一个飞行背包吗？
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. -
A．我很惊讶地发现，答案是肯定的！不过要真的想把它做出来，你还得去向俄国人讨教一下。-罗布.B
The 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.
这东西的原理相当简单：如果你朝前方开枪，枪的后坐力会把你往后推。因而如果你向下开枪，那么后坐力就会把你往天上推。
The 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.
那么我们要弄清楚的第一个问题是：枪产生的后坐力能够抵消自己的重量吗？如果一把机枪重5千克，但只能产生折合4千克的后坐力，那么它就没办法把自己推离地面，更不用说再算上开枪的人了。
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. The Saturn V had a takeoff thrust-to-weight ratio of about 1.5.
在工程领域里，一个飞船的推力和自身重力的比值被称为推重比。如果这个比值小于1，那么飞船就飞不起来。"土星五号"在起飞时的推重比约为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.
虽然我在南方长大，但对于枪支却不怎么了解，因而为了回答这个问题，我找了一个在得克萨斯的熟人帮忙。
Note: Please, PLEASE do not try this at home.
警告：请你千万不要在家里尝试这么做！
As it turns out, the AK-47 has a thrust-to-weight ratio of around 2. This means if you stood it on end and somehow taped down the trigger, it would rise into the air while firing.
通过测试我发现AK-47步枪的推重比在2左右，换言之，如果你把枪口立在地上，然后拿胶带把扳机缠住，那么整支枪就能飞起来。
This isn't true of all machine guns. The M60, for example, probably can't produce enough recoil to lift itself off the ground.
但并不是所有机枪都有大于1的推重比，比如M60机枪的后坐力大概就不足以把它自己推离地面。
The 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. Thrust is the product of these two amounts:
火箭（或射击中的机枪）所能产生的推力取决于：（1）它能向后抛射出多少质量；（2）抛射出东西的速度有多快。推力等于这两个量的乘积。
If an AK-47 fires ten 8-gram bullets per second at 715 meters per second, its thrust is:
如果AK-47能在一秒内以715米/秒的速度射出10发8克的子弹，那么产生的推力为：
Since the AK-47 weighs only 10.5 pounds when loaded, it should be able to take off and accelerate upward.
推力＝抛射质量的效率×抛射物的速度
In practice, the actual thrust would turn out to be up to around 30 percent higher. The reason for this is that the gun isn't spitting out just bullets-it's also spitting out hot gas and explosive debris. The amount of extra force this adds varies by gun and cartridge.
在实际情况中，产生的推力还会比计算值大30％左右。原因在于枪口发射出来的不只是子弹，还有炽热的气体和炸药碎片。这些额外推力的大小取决于枪支和弹药的种类。
The 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. When they 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.
整体的推进效率还取决于弹壳是不是也随着子弹发射一起抛出去了。我问我那位得克萨斯的哥们儿能不能帮我称一下弹壳的重量，当我发现他家里似乎没有秤之后，我善意地提醒他，以他家里的军火库规模，去找一个家里有秤的人就好了。
We can try using multiple guns. If you fire two guns at the ground, it creates twice the thrust. If each gun can lift 5 pounds more than its own weight, two can lift 10.
10子弹/秒×8克/子弹×715米/秒＝57.2牛≈6千克重力
So what does all this mean for our jetpack?
那么对我们的飞行背包来说这意味着什么呢？
Well, the AK-47 could take off, but it doesn't have enough spare thrust to lift anything weighing much more than a squirrel.
而一把AK-47装满子弹的重量也不到5千克，因而它能够飞起来并向上加速。
At this point, it's clear where we're headed: You will not go to space today.
你今天去不了太空
If we add enough rifles, the weight of the passenger becomes irrelevant; it's spread over so many guns that each one barely notices. As the number of rifles increases, since the contraption is effectively many individual rifles flying in parallel, the craft's thrust-to-weight ratio approaches that of a single, unburdened rifle:
如果我们能把足够多的步枪绑在一起，那么搭载的那个人的重量就无关紧要了，因为他的体重已经分散到大量的枪身上去了。随着步枪数量的增加，由于这个装置本质上还是许多独立的步枪的并联，因而整体的推重比会渐渐趋近于一把不搭载负载的枪的推重比。
But there's a problem: ammunition.
但还有一个问题：弹药数量。
An AK-47 magazine holds 30 rounds. At 10 rounds per second, this would provide a measly three seconds of acceleration.
一把AK-47步枪只能装弹30发，以每秒10发的速度来算，这把枪只能提供3秒的加速度。
We can improve this with a larger magazine-but only up to a point. It turns out there's no advantage to carrying more than about 250 rounds of ammunition. The reason for this is a fundamental and central problem in rocket science: Fuel makes you heavier.Each bullet weighs 8 grams, and the cartridge (the "whole bullet") weighs over 16 grams. If we added more than about 250 rounds, the AK-47 would be too heavy to take off.
我们可以通过携带更多的弹药来克服这一缺点，但是这个方法有一个上限。你会发现当你携带超过250发子弹后，再继续增加子弹数将不会再给你带来任何好处，原因在于火箭科学中一个基础却重要的问题：燃料会增加你的重量。每枚弹丸质量8克，整颗子弹（弹丸、弹壳、火药、底火……加起来）质量超过16克。如果你携带超过250发的弹药，那么你的AK-47就会因过重而无法飞起来。
This suggests our optimal craft would comprise a large number of AK-47s (a minimum of 25 but ideally at least 300) carrying 250 rounds of ammunition each. The largest versions of this craft could accelerate upward to vertical speeds approaching 100 meters per second, climbing over half a kilometer into the air. So we've answered Rob's question. With enough machine guns, you could fly.
这意味着最佳配置是一大把（至少25把，最好超过300把）各携带250枚弹药的AK-47步枪，这个配置能让你的飞船加速至接近每秒100米的速度，你能飞上500多米的高空。所以至此，罗布的问题已经有了答案。只要你有足够多的机枪，你是能够飞起来的。
But our AK-47 rig is clearly not a practical jetpack. Can we do better?
但我们的AK-47步枪显然不是制作飞行背包的理想武器，能不能再给力一点儿？
My Texas friends suggested a series of machine guns, and I ran the numbers on each one. Some did pretty well; the MG-42, a heavier machine gun, had a marginally higher thrust-to-weight ratio than the AK-47.
我的朋友推荐了其他一些机枪，我也一一进行了计算。有些表现很好，比如重机枪MG-42，它的推重比要比AK-47更高一些。
Then we went bigger.
接下来就轮到大家伙了。
The GAU-8 Avenger fires up to 60 1-pound bullets a second. It produces almost 5 tons of recoil force, which is crazy considering that it's mounted in a type of plane (the A-10 "Warthog") whose two engines produce only 4 tons of thrust each. If you put two of them in one aircraft, and fired both guns forward while opening up the throttle, the guns would win and you'd accelerate backward.
GAU-8复仇者多管机炮能在1秒内射出60枚0.5千克的炮弹，它产生的后坐力将近5吨。这个后坐力是相当恐怖的，因为搭载它的A-10雷电攻击机的2个引擎每台只能产生4吨的推力。如果你在A-10上装上2台机炮，油门全开并同时向前开火，那么机炮会赢过引擎，飞机将会向后加速。
To put it another way: If I mounted a GAU-8 on my car, put the car in neutral, and started firing backward from a standstill, I would be breaking the interstate speed limit in less than three seconds.
换个角度，如果我把GAU-8装在我的车上，车子挂空挡停着，然后开始向后开火，那么我会在3秒之内超过美国州际公路的速度限制（约110千米/小时）。
"Actually, what I'm confused about is how."
"其实我更想知道你要怎么把我拦下来。"
As good as this gun would be as a rocket pack engine, the Russians built one that would work even better. The Gryazev-Shipunov GSh-6-30 weighs half as much as the GAU-8 and has an even higher fire rate. Its thrust-to-weight ratio approaches 40, which means if you pointed one at the ground and fired, not only would it take off in a rapidly expanding spray of deadly metal fragments, but you would experience 40 gees of acceleration.
虽然这炮拿来做阵列火箭引擎会很不错，但俄罗斯人还有更好的东西。格里亚泽夫－希普诺夫GSh-6-30加特林机炮的重量只有GAU-8的一半，但射速更高，它的推重比接近40。这意味着如果你朝地面开火的话，除了会有一股急速膨胀的致命金属碎片云，你还会以将近40倍重力加速度腾空而起。
This is way too much. In fact, even when it was firmly mounted in an aircraft, the acceleration was a problem:
GSh-6-30的后坐力过于恐怖，即使是牢固地安装在飞机上，后坐力产生的加速度仍然是个很大的问题。
The recoil . . . still had a tendency to inflict damage on the aircraft. The rate of fire was reduced to 4,000 rounds a minute but it didn't help much. Landing lights almost always broke after firing . . . Firing more than about 30 rounds in a burst was asking for trouble from overheating . . .
后坐力……仍然容易对飞机造成损伤，即使把射速降低到每分钟4000发也没有多大改观。在开火之后着陆灯几乎每次都被毁了……连续发射超过30发就会因过热而发生故障……
But if you somehow braced the human rider, made the craft strong enough to survive the acceleration, wrapped the GSh-6-30 in an aerodynamic shell, and made sure it was adequately cooled . . .
但是如果你设法把人类乘客安置好，建造出一个强度足够承受这么大加速度的飞船，把GSh-6-30包裹在气动外壳里，然后确保它能妥善地冷却……
. . . you could jump mountains.
那么你就能飞越重山啦。
1 Judging by the amount of ammunition they had lying around their house ready to measure and weigh for me, Texas has apparently become some kind of Mad Max-esque post-apocalyptic war zone.
1． 从他堆在家里用来给我测试的弹药数量来看，得克萨斯很明显已经变成某种程度上的"疯狂的麦克斯"式的后末日战争区了。--格雷格.戈贝尔，
2 Ideally someone with less ammo.
2． 最好是家里没这么多军火的。
