Tutorial:Intermediate Rocket Design/zh-cn

物理学

在讨论火箭设计细节之前，先来了解一些必需的物理概念。亦即，重心、作用力点，以及他们的关系。

重心、作用力点，以及它们对于火箭对称性的要求

重心是火箭中各方向都能保持完全平衡的一个点。不考虑重力的影响，如果你在这个点上对火箭施加一个力，那么整个火箭都会按照这个力的矢量方向和大小进行相应的运动，因为不论上下、左右、前后，火箭在这个点都是对称等重，并保持平衡的。重心是绝对存在的，除非你把火箭设计得过于奇葩，这个重心都会位于箭体的内部。不走运的是，通常火箭的作用力点，也就是火箭发动机的推力作用点，都无法做到与重心重合。其实如果能做到这样，实在会非常的完美，因为我们可以随心所欲的让火箭按照需要运动（当然，要忽略空气阻力的影响）。

所以实际上我们只能退而求其次，把作用力点放在重心的“后”方，让推力矢量指向重心。简而言之，就是把发动机放在箭体重量的后方，然后向另一个方向推进。这种方法有几个明显的影响。首先，总推力矢量实际上是各个发动机推力的矢量和，那么就得吹毛求疵的要求其矢量方向与作用点和重心的连线重合。换句话说，你的火箭必须是对称形的才能保持稳定。

你自己也可以做个实验。拿起一把笤帚，把它竖起来，手托着一端，你应该可以保持它的平衡。你也会发现这两点：首先，只要你尽力，你能一直保持它的平衡，不过它往一边倒过去无疑更容易；其次，只要把一头顶住天花板，相比单手托着它，你可以更容易保持住平衡。

如果你能在手持的一段施加足够大的压力，你肯定能用笤帚把天花板捅破而不让它倒向一边（相信我吧）。这其实就是火箭工作的原理。现在，找点东西拴在笤帚的一侧，再来看看。如果你还想用刚才一样的方法来保持平衡，它肯定会倒向你栓东西的方向。除非你换个角度握住的话……如果它是火箭的话看起来就有点蠢了，不是吗？如果你给笤帚加一个向上的推力，它不会倒向那一边，实际上它会绕 X 轴旋转，然后做个“回环”……或者直接“坠毁”，这一点估计可能性更大，因为重力也会在这里起很大的影响。

所以，首先必须记住的是，请保持火箭的对称性，至少要保证重心位于作用力的矢量方向（如果你安装了多台发动机，那么你就得注意多个作用力点，并利用总作用力点的合推力矢量）。以物理的角度而言，我们必须保证作用力点与重心的连线与作用力矢量的重合。简而言之，火箭要保持平衡，总推力就必须通过重心，因此各个发动机的推力都指向重心，而喷射方向则指向反方向。

这也意味着“向内”的推力可以保持火箭的稳定，前提是各方向的推力能保持相等。这可以保证火箭按照目前的方向前进，但是有一部分燃料也被无谓的消耗了，因为你的各个发动机有一部分推力彼此“抵消”了。想象一下，这其实有点类似你把汽车车轮调整到一个内八字的状态下。

质量与重量

只要你身处 Kerbin，这两者就是可以互换的。你的质量直接决定重量。太空中就略有不同。重量是质量被重力，或运动加速后产生的。即便你在太空中失去了重量（也就是，重力加速度与你的反向脱离加速度相同），你的质量依旧存在。

把理论简单化就是，物体质量越大，改变其运动方向和速度所需要耗费的能量就越大。你的火箭越重，加速（或减速）所耗费的燃料就越多，假设你没有利用重力来协助你的话。通常，在游戏中，只要你超重了，重力就会跟你唱反调。

这也意味着，加速会让你超重。这并不是质量的增加（除非你接近光速，这不是我们讨论的范畴），而是施加在物体质量之上试图改变其速度的力。这叫做 G 力。位于 Kerbin ，你感受到的 G 力是 1g，即该行星表面的重力加速度。

这究竟能如何影响你的火箭呢？嗯，其实有两个方面。一方面，物体质量越大，把它送入轨道所需要的能量就越大。因此，一般而言并不是越大越好。我们以后再详细谈。另一方面，火箭的加速度越大，其组件承受的力就越大。有些组件能够承受，而有些则不行。大体上而言就是，设计缓慢爬升的火箭要比以 9g 加速入轨的火箭要更加容易，首先，我们的小绿人乘客肯定不喜欢体验胸前压着一辆大卡车的感觉（乘员承受的 G 力就目前的游戏版本而言倒不算是个问题），更重要的是，如果加速度过大，超过限度，火箭的结构组件会发生损坏，导致火箭解体。这就意味着你必须加入更多的结构件，它会带来额外的箭体重量，从而耗费更多的燃料。

推重比

基本上，推重比就是推力除以重量得到的值，推力单位牛顿，重量是质量乘以加速度，kg*m/s²，所以单位也是牛顿。推力是把你送上天空的那份原力，而重量则是把你热情挽留于地面的力量。如果推力 > 重力，也就是推重比大于 1，你就向上飞。如果推力 < 重力，那么无论你的发动机如何热情的嚎叫，你肯定一步都动不了。历史上，土星 5的第一级火箭发动机的推重比是 94.1 。换句话说，它可以推动自重 94 倍的物体。我嘞个去！

这对我们的航天器意味着什么呢？基本上就是说，无论我们用什么样的火箭推进飞船，它都必须克服整个飞行器的总重量。另外，如果你采用的是多级火箭，除了第一级之外，其他各级在出发时都是“死”重。没错，你没听错，即便那儿竖立着的推力巨大的火箭，它们的绝大多数部分在开始时都属于一无是处的玩艺。推力只包括某个时刻真正给箭体施加的作用力，而不是最终的全部作用力。

要注意的是，就其字面含义而言，每一台火箭发动机的推重比都是大于 1 的。推重比小于 1 的发动机只有同时借助空气动力学才能产生足够的升力来起飞。不过，关键问题是，我们必须把所谓的死重加入计算公式，再得到的推重比仍旧大于 1 才可以。土星 5 的 F1 发动机 94.1 的推重比并不是说我们只需要 100g 的箭体就可以飞入太空。它恰恰意味着这些发动机需要推动其上那个巨大的家伙升空，而且把这个的大块头完整的送上轨道也基本属于不可能完成的任务。

我的猜测是 Kerbin 的重力加速度大约等于 10m/s² （和地球差不多），这意味着 最大推力 200 的火箭发动机（如non-gimballed stock engine）可以推动 20 质量单位的载荷（或者说 8 个液体燃料储存单元）。假设一枚火箭包括 1 个指令舱，7 个燃料储存单元和1 台火箭发动机（总质量为 20.5，7*2.5+2+1），它是肯定无法起飞的；把燃料单元减为 6 个才能飞起来，对，就是这样设计火箭的。

所以，在建造火箭时，一定要把所有组件的质量加在一起，再乘以 10，然后除火箭发动机的推力，不过必须是实际工作的发动机。这个数字越大，你的火箭就爬升的更快。考虑到发动机在最大推力下工作的过热问题，你的第一级火箭推重比应该至少为 1.7。我的火箭第一级推重比大概是 2.2，足够同时又不会产生过大的 G 力。

另外，要记住的是你的飞行需要消耗燃料。火箭发动机的工作会逐渐淘空燃料储存单元，火箭也就越来越轻，爬升需要克服的重力就越来越小。而且，随着距离的增大，重力也会减弱，所以 Kerbin 引力也会在你升高的过程中逐渐降低。不过不会降低的很明显。

Staging, and when to do it

Staging usually means tossing dead weight. You jettison spent rocket parts to make your craft lighter. Less mass means less energy required to move the rest of the mass. The obvious choice would now be to stage as much as possible, to carry around as little dead weight as possible. This is not the best strategy, though.

Staging also means that you have to carry around the weight for the staging equipment and, in case of a liquid fuel set, another liquid engine. A spent stock booster weighs 0.36. The equipment to jettison it weighs 0.4. A spent liquid tank weighs 0.3. The additional engine and the staging equipment to toss it weighs 2.8.

A compromise has to be found. There is no hard limit to tell when to stage and when not to, what matters is how long you'd have to haul around the dead weight (if it's just a few seconds between the booster's end of life and until the other engine of this stage burns out, just keep the booster attached, it's not worth the extra weight for another set of staging couplers. If it's for the rest of the flight, tossing it pays off easily), whether the spent stage prevents you from firing the next (a lower stage burned up covering an upper stage has to be jettisoned, of course) and what the stage is used for (an upper stage is usually in use longer than a stage to reach orbit that is burning at max power constantly, i.e. a fuel tank in upper stages lasts much, much longer). I find the sweet spot for liquid tanks to be around 4-5 for lower stages and about 2 for upper stages.

As much thrust as possible to the bottom

Also easy to see, the more thrust you apply right from the start, the less dead weight you carry around. It's usually quite pointless to have a lot of thrust further up if you cannot get off the launch pad. On the other hand, as mentioned above, the more thrust you put behind your crate, the more g-force it has to endure and the more you stress your parts. Not to mention the air resistance which is of course worst lower in the atmosphere.

draaaaaaag

While we're at it, drag. I hope I got that one right, it's kinda hard to tell how that part really works. Basically, every part you add has air resistance. Doesn't matter once you're in orbit (and hence satellites rarely look streamlined), but it's a big issue until you hit that magical 70,000 meters. I still have very limited data on how drag really works and what affects what, so far all I can say is that it's there and that you should probably take it into account, i.e. creating insanely wide rockets to cram in a lot of boosters to fire at the same time might be a drag. Literally. Especially if you try to fly such a rocket at high speeds.

Where do you need the most power?

That's a simple one again: From ground to orbit. You will NEVER in your flight have to spend as much energy as in that part of your flight. Getting from orbit to the Mun, landing on the Mun, getting back off the moon, flying back to Kerbin and landing there? Easily done with about 1/6th of the fuel spent to get into orbit. I am NOT kidding or exaggerating here. Remember that Saturn V rocket that sent Apollo to the moon? Remember how friggin' huge that thing was? And what a tiny little bit of it actually went to the moon, with the rest being tossed somewhere along the way? And how that little service module that got them basically from orbit to moon also got them back? It's the same here. You will spend a good 80% of fuel and dump about as much of your rocket before you reach the Mun.

Long or wide?

Preferably neither. Making your rocket longer is about as bad as making it wider. For various reasons. Wide rockets tend to be bottom-heavy (because, usually, they are wide at the bottom, to maximize thrust at liftoff), making them harder to control because they sway easily, and they are prone to out of control rolling if the thrusters on the outer edges are not PERFECTLY aligned (which they are, well, never), due to leverage. Also, I'd expect them to be very susceptible to drag, meaning a lot of power is lost due to air resistance. Wide rockets usually need quite a bit of SAS to keep from spinning out of control. And they are prone to "flipping", i.e. uncontrollably going upside down because they are easy to tilt and bank. Think of the broom example at the beginning.

Long rockets are very hard to tilt and bank, making them hard to steer and very sluggish. They also usually suffer from top-heaviness, especially after a good deal of their lower stage fuel is spent, which can result in rockets that are very hard to control and to keep from going "keel-up", i.e. nose-down without a lot of RCS thrust. Long rockets usually need quite a few wings to keep them manageable and responsive. And even then they are very slow to react and need foresightful piloting. They usually keep their direction pretty well as long as they are balanced and there's a lot of thrust applied, but once you bank and tilt them, they can very easily oversteer, especially in horizontal flight with a center of mass that's very close to the top (as it is usually just before your ascent stage is burned up, with a lot of empty and near empty fuel tanks hanging on your tail). Still, I prefer long over wide rockets.

So, with all that, what IS now the best design?

From these tidbits we can puzzle together a few cornerstones that give us a good idea what a GOOD design would be, and what would be a BAD one.

It's a GOOD idea to put every engine that CAN actually thrust at launch to work right at launch. Else it's dead weight we first have to haul upwards. If that gives you too much thrust and your rocket starts to fall apart due to excessive G forces, slap on another can of gas for that liquid fuel rocket, or take off some boosters (yeah, right...). It MAY be a good idea to not run that liquid engine at full power if you get so fast that your friction is killing most of the power you put behind it. Actually, I usually take off with full throttle, only to ease off a little as I climb to keep the speed from going overboard and being burned in friction. But if you have a big rocket, it CAN be a very good idea to make the first stage(s) only of solid boosters, they're very light for their push and even with a coupler on them they have a better TWR than liquid engines. Their main drawback, the inability to control their thrust output, doesn't matter for the first 20,000 Meters since you actually just want to get the hell up there. Do not expect too much from that, a full complement of two solid-only stages underneath every single engine of my actual first stage only got me about half a fuel tank. Yes, half a stock fuel tank is all you get for slapping two rows of solid stages under your rocket. The diminishing returns are stunning!

With bigger rockets, you'll run into the need to add SAS to keep them manageable. Only one ASAS module gives you any benefits, so put only one of them into your rocket. The key difference between SAS and ASAS is, as the description says, that ASAS is more like an autopilot, SAS is more like a gyroscope. In other words, ASAS only works as well as YOU could, or, in other terms, if the rocket is uncontrollable, ASAS cannot control it either. If you have wings (and, IMO, you should have some at least as long as you're hauling a big ass rocket about), you might even be able to forgo the normal SAS modules. My Mun rocket only has one ASAS module and no SAS modules. Your rocket should get thinner as it progresses upwards. From afar, it should look like a very steep pyramide. At least IMO. Top-heavy rockets are usually very hard to control, since their center of gravity is far from the point of action. The further away, the bigger the lever, the more wings and other control tidbits you need to keep it upright.

You need most of your fuel on your way up. Once you're in orbit, even the trans-lunar shot is peanuts compared to the expense to get into an orbit. It's quite ok to create an unwieldy, but powerful lower stage and create a very manageable and precisely controlable stage for upper orbit that has rather little fuel compared to it. Try different designs here, it's all right to have zero control (aside of "keep it upright by ASAS") over the rocket for the first 20,000 or even 40,000 Meters of its trip.