为什么液体燃料火箭发动机如此复杂? 它其实不就是是个像火箭引擎一样工作的喷灯吗?Why are liquid fuel rocket engines so complicated? Wouldn’t it work to have basically a blowtorch act like a rocket engine?

【翻译自Quora】

@Makar Locke, Armchair Rocket Scientist 的回答

Sure. You can make a blowtorch.

当然了,制作个喷灯就行了。

Then you realize that the blowtorch breathes air so you’ll want to bring oxygen. When you want to bring oxygen, you’ll need a second tank.

然后你意识到喷灯需要消耗空气,所以你得让喷灯携带氧气,于是你要一个氧气箱。

Great! But now everything just got heavier—and the rocket isn’t going anywhere. And then you realize the blowtorch is producing heat but not thrust. You want thrust, so you learn that you can use a nozzle to convert heat into velocity of the gas, causing reactive thrust!

很好!但是它现在变重了——而且这个喷灯现在哪儿都去不了。然后你意识到这个喷灯只是产生热量,而没有产生任何推力。你需要推力,然后你知道了你可以用喷嘴来把热量变成气体的速度,由此产生了推力!

Awesome, but it’s barely any force. You want to go to space, don’t you? That’s fine—lets increase thrust. You learn that thrust is directly proportional to the pressure of your combustion. Oh man, but the blowtorch can’t handle high pressure. That’s ok, let’s redesign the blowtorch head and integrate the whole thing with the nozzle that converts heat to force.

好棒!但这推力太小了。你不是想进入太空吗?你想的没错——增加推力。您了解到推力与燃烧时的压力成正比。哎呀,这个喷灯没法承受那么大的压力。没关系,现在重新设计喷灯头,并且把能把热量转化为力的碰嘴与其他东西整合到一起。

Fantastic! You redesign the end of the blowtorch, that now eats oxygen and propane from your tanks and call that a combustion chamber. You design it to combust at a much higher pressure than the blow torch—that will do it! Let’s fire it up, three, two, one…boom!

精彩!你又重新设计了喷灯的尾部,现在它能从储存箱中吸收氧气和丙烷,你给它取名叫燃烧室。你设计的这个燃烧室比一般的喷灯能承受更多压力——这将会实现我们目标!点火,三,二,一…boom!

What the hell just happened? You walk over to the test stand and you see the tanks blew up. Oh no! You increased the pressure at the chamber but not at the tanks, which made the gas flow backwards. Ok, well now you know that pressure of the gases entering the chamber have to be higher than the gases combusting. Well now you have to make special tanks—pressure vessels.

到底发生了什么事? 您走到测试台上,看到储气箱炸没了。 不好! 您增加了燃烧室的压力,但没有增加储气罐的压力,这样气体就会倒流。 好了,现在你知道了进入燃烧室的气体的压力必须大于气体在燃烧室燃烧时的压力,于是你需要制作一个特殊的箱——压力管道。

That’s hard, and more expensive, but you can make it work. You revamp the tanks to hold more pressure but…how are you going to add pressure to the tanks? Let’s add a pressurized gas to the oxygen and propane so that it helps. Well that wouldn’t work, pressure would go down as the fuel gets used up and the gases would mix with the fuel and make it burn less hotly—so less heat for the nozzle to convert into thrust.

这很困难,而且成本更高,但是您最终让它正常工作了起来。 您需要对油箱进行改造以让保持更大的压力,但是……您将如何为油箱增加压力? 在氧气和丙烷中添加高压气体?嗯,那是行不通的,随着燃料的消耗,压力会下降,高压气体会与燃料混合,从而使其燃烧时产生的热量减少,从而使喷嘴转化为推力的热量减少。

Let’s switch to liquids so that we can push on them with a gas. We’ll use jet fuel now, and liquefied oxygen, and we will push on them with pressurized helium that we can let in dynamically from an external tank to keep the pressure steady. We have to revamp the tanks and all the lines to be compatible with the cryogenic oxygen, but that’s ok. The higher pressure in the tanks will let the fire burn much higher pressure, which will make more thrust. (We’ve arrived at the pressure-fed cycle.)

那就用液体吧,这样我们就可以用气体推动它们了。 现在,我们将喷射注料,以及液氧。并使用加压氦气将其推到燃烧室,我们可以从外部储罐引入氦气以保持压力稳定。 我们必须改造储罐和所有管线,使其与低温氧气兼容,但这问题也不大。于是燃料箱中更大的压力将使燃烧时的压力也更大,从而产生更大的推力。 (我们已经进入了压力补给周期。)

Now we are getting somewhere. We started with a blow torch, but as we wanted more thrust, we had to redesign it all over like that. That’s ok. We have a rocket now. Let’s fire it up! Three, two, one…boooooom! Rats, the combustion chamber burned through. Got too hot. We need to cool it. We can insulate it with something that would evaporate, but that would make it single use (ablative linear), we could spray fuel inside in a special way to make a cold layer at the chamber wall (film cooling) but it’s not enough. Alright this might sound crazy, but we learn that we can cool it with fuel passing through the chamber wall.

现在我们已经改造到了这种程度。 我们本来只有一个喷灯,但是由于我们想要更多的推力,我们不得不像这样重新设计。 没关系。 那我们现在有火箭。 让我们开始吧! 三,二,一……Boooooom! 老鼠,燃烧室被烧透了。 太热了。 我们需要冷却。 我们可以用会蒸发的东西绝热,但那只能使用一次( 线烧蚀 ),但我们可以通过特殊的方式向内部喷射燃料,从而在室壁上形成一层冷层(薄膜冷却),但这还不够。 好吧,这听起来可能很疯狂,但是我们了解到,可以通过在燃烧室壁的燃料来对其进行冷却。

That’s ok, we can make it work. We redesign the chamber to have a cooling jacket, with channels sandwiched between the walls so we can pass fuel through it before it goes in the chamber to absorb heat. (Regeneratively cooled pressure fed cycle.).

问题不大,我们还是可以使其正常工作。 我们对燃烧室进行了重新设计,让它有了一个冷却套,在各壁之间有通道,因此我们可以在燃料进入燃烧室之前通过它吸收热量。 (再生冷却的压力供料循环。)。

Let’s test it! Three two one…wow, it works! But it’s…kind of weak, actually. Man, that’s disappointing. We want to go to space, right? We need a lot of performance. Pressure-fed rocket engines kind of suck…well, what else could we do?

来试试吧!三,二,一…哇,成功了!但是它….太弱了。小伙子,有点失望吧。我们不是想去太空吗?我们还需要更好的性能,这个挤压循环引擎也有点烂….额,我们还能做什么呢?

Well, we could try using like a turbo to boost the gas as it goes in the chamber (gas generator cycle) but oh man, is that expensive, custom designing that turbo, and now we have exhaust that isn’t used for propulsion (open cycle). And then you realize it’s getting really hard with all the valves, timing the start sequence correctly so that nothing explodes, it’s hard to know what’s going on inside the tubing without sensors. All you have are math models and the only thing you can control is mass flow-rate.

好吧,我们可以尝试使用涡轮增压来提高气体在燃烧室中的流动速度(燃气发生器循环),但是,哦,但定制设计的涡轮增压很贵。然后我们就会生产出没法用来产生推力的废气了( 开式循环)。 并且您意识到正确地控制所有阀门,正确地设定启动顺序,以免发生爆炸很困难,很难知道没有传感器的油管内部发生了什么。 您所拥有的只是数学模型,唯一可以控制的就是质量流率。

And then you realize all your test hardware is still super heavy, and you have to lose a lot of weight to go to space. That’s hard and expensive because all the materials are now at their limit. Then you learn about the Rocket Equation. You need so much fuel, now you need structure, staging, controls and aerodynamics simulations.

然后,您意识到所有测试硬件仍然非常重,您必须减轻很多重量才能进入太空。 这很困难也很贵,因为所有材料现在都已达到极限。 然后,您将学习火箭方程。 您需要大量的燃料,现在需要结构,分级,控制和空气动力学模拟。

Then you learn that the engine nozzles aren’t efficient at sea level, you need to make a new engine for vacuum. Then it gets more and more and more expensive, oh boy, you start learning so much wacky stuff about supersonic fluid dynamics that happens in your engine, and dumb failure modes creep in like acoustic instabilities (screeching), and then fatigue and stress on your materials gets bad and…..

然后,您会发现引擎喷嘴在海平面上效率不高,您还需要制造一台用于真空的新引擎。 然后它变得越来越贵,哦,小伙子,您开始学到很多关于引擎中发生的超音速流体动力学的古怪知识,而愚蠢的故障模式在不稳定性声学中蔓延开来,然后是金属疲劳和承受压力的材料坏了……

Well. You get the picture. You can most certainly make a blow torch, but as you search for more thrust, more efficiency, more robustness, you will be driven to complexity, cost, higher pressure, lower tolerances, more expensive materials, harder to model gas flows, power cycles, toxic chemicals, and more.

好,你大概知道了会是怎么样。。 当然,您可以只制造一个喷灯,但是当您需要更大的推力,更高的效率,更高的耐用性时,您将被迫接受复杂性,成本,更高的压力,更低的容错率,更昂贵的材料,更难于模拟的气体流动,动力循环 ,有毒化学物质等。

I love rocket engines. But…I don’t wish they were as easy as blowtorches. Where would be the fun in being a rocket scientist?

我喜欢火箭发动机。 但是……我不觉得它们像喷灯一样容易。 成为火箭科学家会带来什么乐趣?

edit: thank you to all the reads, upvotes, comments, and encouragement thus far. I thoroughly enjoy sharing the wonderfully bizarre discipline of rocket engine design, and I’m glad you enjoyed reading it. To be perfectly honest I wrote this up in about 40 minutes lying on my floor avoiding studying for exams, and I want to improve it since it’s doing so well.

编辑:到目前为止,感谢您的所有阅读,支持,评论和鼓励。 我非常喜欢分享火箭发动机设计奇妙的学科,也很高兴您喜欢阅读它。 老实说,我在大约40分钟的时间里躺在地板上把答案写了出来,以避免为了考试而学习,并且我希望对此加以改进。

Comment below: do you think an expanded version of this would work well in a video with my sarcastic voice and images of the rockets working or not working? Let me know.

请您在下面评论:您认为这个答案的扩展版本,即有我略沙哑的声音和图片的视频来介绍火箭好不好?让我知道你的看法。

【评论区】

@ David A Kelly I can read and I listen. I even paid attention to your post.

As a kid, I used play “rocket engine” by punching holes in the bottom and lid of empty paint cans, and pour some gasoline or turpentine into the can lying on its side. I would put the hose from my Mother’s vacuum cleaner on the discharge end instead of suction, ignite the vapours, and then direct the pressurized air into one end of the can. It would make a great flame four or five feet long with a satisfying roaring sound.

小时候,我玩“火箭发动机”的方法是在空的油漆罐的底部和盖子上打孔,然后将汽油或松节油倒入侧面的罐子中。 我会把妈妈的真空吸尘器上的软管放到排放端,并非用来吸尘,点燃蒸汽,然后将压缩空气引到罐的一端。 它将喷出四到五英尺长的巨大火焰,并发出令人满意的轰鸣声。

That is, until Mom discovered my activity!

就是这样,直到被我妈妈发现!

@ Kelly Kinkade IT geek, former law student, many other things

This is why I say that rocket science is easy (computing trajectories, thrust requirements, etc. is all fairly straightforward, if tedious, math) but rocket engineering is incredibly complicated.

这就是为什么我说火箭科学很容易(计算轨迹,推力要求等都相当简单,即使是乏味的数学也是如此),但是火箭工程却异常复杂。

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