(A Raptor engine — SN-001 — on the test stand at McGregor, Texas. If you look carefully at the right side, you can see someone standing behind it for scale.)
（猛禽引擎 – SN-001 – 得克萨斯州麦格雷戈试验台上的规模，如果你在右侧仔细看，你可以看到有人站在它背后的规模）
Well, this gets a bit complex, since to understand what the sentence means you have to understand how a rocket engine works. I’m going to try to go from the basics up…
I mean, it’s not like it’s rocket science, right?
Just as a fair warning, this gets a little bit… long.
(For those who hate reading… why not try out Everday Astronaut’s video instead? Is SpaceX’s Raptor engine the king of rocket engines? – Everyday Astronaut)
(A pressure-fed rocket engine. Yes, it’s that small, including the tanks and valves. Photo: Nadir Bagaveyev)
The simplest (liquid-fueled) rocket engines work by having two pressurized tanks, one with fuel, one with oxidizer, with pipes that feed through a valve into a combustion chamber. Add an ignition source, open the valves, and you have a rocket engine.
However, there’s a lot of disadvantages to this “pressure-fed” rocket system. First, the tanks have to be at a higher pressure than the combustion chamber, or, obviously, the pressure will push the fuel and oxidizer back into the tank rather than the fuel and oxidizer pushing into the combustion chamber. That severely limits how powerful of an engine you can build, but this was the first real liquid rocket engines as built a little after the turn of the 20th century by guys like Robert H. Goddard.
然而，这种“压力输送”火箭系统有很多缺点。首先，燃料箱必须比燃烧室压力更高，或者否则压力将推动燃料和氧化剂流回燃料箱，而不是进入燃烧室。这严重限制了它。但这是第一款真正的液体燃料引擎，是在20世纪由 Robert H. Goddard这样的人建造起来的。
These kind of engines are still used today on things like the reaction control system rockets on most spacecraft, like the Draco engines on the SpaceX Dragon capsule.
这些类型发动机的至今仍在使用，如大多数航天器反应控制系统（Reaction control system，RCS），和SpaceX公司的龙飞船的Draco引擎。
But, like I said, these are limited by tank pressure, to make a more powerful engine, you need a higher-pressure tank, which needs to be built thicker and stronger, and you quickly run into diminishing returns.
If only there were a device to take a low pressure fluid and turn it into a high pressure fluid…. Oh wait, there is: a pump.
Even your car has pumps in it. One for taking the low pressure oil and pressuring it up to flow through all the places in the engine that need lubrication, a second for pumping the gas from the gas tank to the engine, and third, a water pump for pumping cooling water around the engine.
Let’s look at this last pump, the water pump. It works by simply spinning. You add low pressure water in the center, and as the pump spins, the water is thrown to the outside edge at high pressure, where it leaves the pump casing and flows through the system. This is known as a centrifugal pump, and it’s driven off a shaft or pulley directly from the spinning of the engine itself.
来看看最后一个泵，水泵。它工作时只是旋转而已。你把低压下的水加到中心，然后这个泵旋转，然后水以高压甩到叶轮周边的蜗壳通路，这叫 离心泵 ，它只驱动一个轴或者只旋转自身。
But, some cars have one more pump, used for pumping pressurized air into the engine. This pump looks a lot like the water pump, and is also connected to a shaft, but rather than the shaft going to the engine, it goes to a turbine.
A turbine is sort of an “anti-pump”. It take high pressure fluid, and uses it to turn a set of blades, and thus spin a shaft, resulting in low pressure exhaust. In a car, the hot exhaust of the car feeds into this turbine, and cooler, low-pressure exhaust feeds out of it, after spinning the turbine to high speed.
That turbine drives the pump that compresses air to feed into the engine. The combined turbine and pump is known as a turbopump, or, on most cars, a turbocharger.
(A car turbocharger — diesel in this case — used to compress air to get better performance from the engine. Photo: www.dieselnet.com)
（汽车涡轮增压器 – 柴油在这种情况下 – 用压缩空气从而得到从性能更好的发动机。照片www.dieselnet.com）
So, why did I just go through all those car parts? Because rockets use turbopumps to drive the fuel and oxidizer into the engine.
And that’s where we end up with all the complicated nomenclature.
Even the very first rockets, the German A4 — known in the west as the V2 rocket, used a turbopump.
(The A4 rocket engine. Yes, it says Calcium-Permanganate, but Z-Stoff was Sodium Permanganate. I guess it was the fog of war? Photo: www.v2rocket.com)
To drive the turbine, it used super-heated steam, derived by adding sodium permanganate (German term: Z-Stoff) to highly pure hydrogen peroxide (German term: T-Stoff). This caused the peroxide to instantly decompose into super-heated steam.
The turbine, spun by the decomposing T-Stoff, would drive the shaft that connected the turbine to a pair of pumps. One pump was for liquid oxygen (A-Stoff) and the second was for a 75% ethanol, 25% water mix called B-Stoff.
These two pumps were manufactured at different sizes so that each would pump the proper amount of fuel and oxidizer to the engine.
In the engine was, basically, a firework that was ignited to provide a combustion source. Once the engine ignited, it was throttled up to 100% and then the rocket was released.
This system used two completely separate fuel cycles to power the engine, with gas generator driven by one fuel cycle, and dumping the waste steam overboard, and the two pumps were fed off one shaft, with no fuel or oxidizer mixing until they reached the combustion chamber.
To put that in perspective, that would be an “Open-cycle, dual fuel gas-generator rocket”. Open-cycle meaning that the exhaust from driving the pump is just wasted. It does nothing to aid the rocket in flight. The “gas-generator” portion means that there is an entirely separate system being used to generate the gas to push the turbine. The dual-fuel is sort of a weird outlier. Most modern rockets don’t carry a separate fuel to run a gas-generator cycle, they just use the same fuel that powers the rocket.
(The Rocket Lab Rutherford engine being held by their CEO, Peter Beck. The engine is actually 3-D printed, and the pumps are driven by an electric motor. It may look small, but it generates almost three tons (24 KN) of thrust. Photo: India Times)
（Rocket Lab的Rutherford引擎被他们的CEO Peter Beck拿着。这个引擎是3D打印出来的。泵被电动马达驱动。也许它看起来很小，但可以产生三吨（24KN）的推力。图片：India Times。）
One exception to that would be the Rocket Lab Electron rocket, which uses an electric motor instead of a turbine to power the pumps. Technically, this would be an open-cycle engine, since the chemical reaction of the lithium in the battery is driving the pumps, but the exhaust (which stays in the battery) is not aiding the thrust of the rocket. So, in that way, the Electron rocket has more in common with the 70 year old V2 rocket design than with the Raptor. Weird but true.
有一个例外，那将是 Rocket Lab 的电子火箭，其使用电动马达，而不是涡轮。从技术上讲，这将是一个开放式循环发动机，由于在电池中的锂的化学反应驱动了泵，替代了涡轮驱动泵，但废气依然不为火箭提供推力。所以，与猛禽相比，电火箭与70年历史的的V2火箭设计更接近。很奇怪但却是事实。
So, the first step is obviously to get rid of the separate fuel system. This means that you need to have a combustion chamber inside the engine to generate the gas to turn the turbine. In other words, inside your turbo-pump rocket, you have a pressure-fed rocket driving the turbine. This is called the gas-generator in a rocket engine, and it can actually be quite powerful. The gas-generator on the F-1 engines of the Saturn V generated about 55,000 horsepower each.
(The Open-Cycle Gas Generator engine. Photo: Wikipedia)
The exhaust from this gas generator is then fed to the turbines that turn the pumps. The exhaust is still dumped overboard — on the F-1 it was used to cool the exhaust nozzle, but that actually ever-so-slightly reduced the thrust, it didn’t help it.
然后从该燃气发生器产生的废气被供给涡轮机来转动泵。废气仍然被排放到外面 。在F-1中 它被用于冷却排气喷嘴，但实际上它几乎对推力降低没有影响。
In order to keep the turbine from melting, these gas-generators are usually run “fuel-rich” meaning that all the oxidizer gets burned, and the remaining cool fuel keeps the downstream turbine from melting. It’s eating rocket exhaust after all, right? So when you dump that exhaust overboard, it never goes to help push the rocket, and the fuel is lost.
So this is a single-fuel, open-cycle, gas-generator engine. A lot of the engines you actually see in use, like the Merlin engines, are actually this type.
Whew. Hang in there, I’m about a third of the way done.
So, if you really want to squeeze everything you can out of an engine, the first thing you want to do is to use all that extra fuel you’re tossing overboard in an open-cycle engine. Well, if you do that, you get a closed-cycle engine.
Yes, that’s right, 1,000 words into the answer and I finally have the first term in your question.
(The Closed-Cycle engine. Now all of the fuel and oxidizer goes through the combustion chamber.)
The closed-cycle engine ensures that every drop of fuel and oxidizer run through the engine will go through the combustion chamber and aid in the act of producing thrust for the rocket.
To do this, we usually have to make changes to the turbopump design. The first thing is that the exhaust of the turbine has to feed back into the intakes of the pump. The exhaust was fuel rich if you’ll remember, which is why we have to send it to the fuel side pump.
The leftover fuel in the exhaust is re-pressurized by the pump and fed into the combustion chamber where it meets the oxidizer, and you get a closed-cycle engine.
One problem, however.
Most fuels in rocketry contain carbon, meaning if you mix hot exhaust with cryogenic fuel, the carbon drops out as soot, and you get “coking” of the pumps. That’s bad.
The turbopumps on rockets spin insanely fast. Turbochargers on most cars run at 80,000 RPM or more, but they’re 3–4 inches across. Turbines in rocket engines are typically 16–18 inches across and spinning at 36,000 RPM. By my math, that means the outside edges are moving at more than twice the speed of sound. Even the slightest imbalance in weight and they’ll tear themselves apart. Turbopump failures are the most common failures in rocket engines.
Thus, building up lots of sooty carbon on the pumps or turbines is bad, so, fuel-rich cycles are out on most engines; instead we see oxidizer rich closed-cycle engines.
The idea is the same, but instead of adding extra fuel, you add extra oxidizer, and the exhaust goes through the oxidizer pump.
Of course, now you have red-hot oxidizer inside a metal pump. Red hot pure oxygen tends to do things like set metal on fire. Have you ever seen an oxy-acetylene torch cut right through plate steel? You heat the steel with the acetylene, but the cutting is done by blasting hot oxygen into the hot steel — it actually burns the steel away.
So, the metallurgy had to catch up to create an alloy like Inconel that can handle super-hot oxidizer without burning. The Russians spent years perfecting this and built some amazingly powerful engines like the NK-33 and the RD-170. It was so hard to do this, the American space program said that the Soviets were just lying about their engine technology until after the Soviet Union fell and they actually managed to look at some of those engines in person.
所以，我们只好依靠冶金来创造出像 铬镍铁合金 这样让超热氧气烧不起来的合金。俄罗斯人花了很多年来完善，并建造出了像NK-33和RD-170这样强力的引擎。这样做太困难了。美国航天项目说苏联依靠他们的火箭引擎技术保持领先直到苏联解体，他们设法以个人名义去观察这些火箭引擎。
And if you do all this you finally have a closed-cycle engine.
Of course, the Americans cheated — they just put the turbines after the pump, and used the already pressurized fuel and oxidizer to run their gas-generator… which, since it was no longer generating gas was renamed the pre-burner. This meant they had to do things like install a pre-start helium injector to spin up the turbine before they ran the engine. It was a small issue compared to having to invent new forms of metal.
当然，美国人作弊了 – 他们只是把涡轮放在了泵后，并用已经加压的燃料和氧化剂来启动他们的燃气发生器……因为它已不再产生气体，所以更名为预燃器。这意味着他们还得做一些事情，如安装预喷注管将两种燃料均匀混合并且喷入燃烧室。【？？】 。与重新发明一种金属相比，这只是个小问题。
Pre-burning the fuel means that you are burning the fuel once in the pre-burner, and again in the combustion chamber. That’s two stages of burning…
Thus the term, staged-combustion. Yay! We have our second term.
However, these engines all still used a single-shaft from the turbine to the pumps. This causes it’s own problems.
You have to have a shaft, spinning at more than 30,000 RPM, yet that can still hold a seal against 300–500 atmospheres (Bar) of pressure. Because the last thing you want is fuel and oxidizer mixing inside the pumps.
Not to mention, you still end up with needing different sized pumps for the oxidizer and the fuel to maintain the correct ratio of injection for the most efficient combustion. And that ratio is “hard-coded” into the pumps themselves. If you have a pressure leak, you can’t adjust fuel pressure separate from oxidizer pressure.
Also, throttling gets tricky. Most engines can only be throttled through a small range of output because of this. The Merlin engines can only go between 70 and 100% thrust reliably. That’s because the combustion chamber always burns at the same temperature and the same heat, but when you spin-down the turbopump, the pressure drops, meaning that sometimes the pressure will drop below the pressure in the combustion chamber, and fuel/oxidizer will stop flowing until the combustion chamber pressure drops, which will allow more fuel/oxidizer in, it will ignite, chamber pressure will go up, and the cycle will repeat.
This is known as chugging, and most throttle-able rocket engines suffer from it. Even the amazing RL-10 engine with it’s 17.5:1 throttle (5%-104%) suffers from chugging at the low end.
So, how do you have separate control of fuel and oxidizer? Simple, you have two completely separate turbopump assemblies.
Yeah. You might be noticing this is getting complex…
Now you have a separate gas generator on each turbine, and you can separately control the throttle of each. This is, in fact, how the J-2 engine that powered the second and third stage of the Saturn V was built.
The difference with the J-2 is that, since it burns hydrogen, which has no chance of coking since it has no carbon, they run a fuel-rich cycle rather than an oxygen rich cycle.
So, the J-2 is, therefore, a fuel-rich, dual-shaft, closed-cycle staged-combustion engine.
(Note: I originally used the RL-10 as my example here, but as Duncan Oliver pointed out in the comments, I was lying by saying it was a gas-generator cycle engine. The RL-10 is actually a totally different kind of cycle called an expander cycle engine, but this was getting too long as it is. I suppose I could have picked the Russian made RD-180 used on the Atlas, but I was trying to stick to US made engines. Thus, I switched it to the J-2… which was the basis for the more complex Space Shuttle Main Engine, also known as the RS-25.)
(注意：我一开始用的是RL-10作为例子，但正如Duncan Oliver在评论区指出的那样，我说它是燃料循环引擎确实说错了。RL-10的循环与之前提到的完全不同，叫做膨胀循环引擎，但要解释起来又要花很长篇幅，于是我想讲讲 Atlas上的俄制RD-180，但我仍然想要讲美制引擎。因此，我选择了J-2….这是许多航天飞船主引擎的基础，叫做RS-25。)
(This starts to show how complex the real piping gets in an engine when you have to cover all the needs. This is a logical diagram of the space shuttle main engine, a dual-shaft, closed cycle fuel-rich engine.)
Yeah, still only two terms from your question in there.
Which gets us to the last term. Remember when I said mixing fuel and oxygen inside the pumps was bad?
Wouldn’t it be great to have a fuel-rich turbine on the fuel side, and an oxygen-rich turbine on the oxidizer side, meaning that the seals really wouldn’t matter that much, since anything that leaked through would just be the same stuff you were pumping anyway?
Well, that would mean you’d have to take a flow of oxygen from the oxidizer side and pump it to the pre-burner on both the oxidizer side and the fuel side, and you’d have to pump a flow of fuel from the fuel side to the pre-burner on the fuel side and the pre-burner on the oxidizer side.
This would mean you are flowing a full set of combustion goods to both sides of the engine pump system.
Hey, look, full-flow.
So, now you’re running a full-flow, closed cycle, dual shaft, staged-combustion engine.
(Finally, a full-flow, closed-cycle, dual-shaft, staged-combustion engine.)
And that’s what the Raptor is.
In case you’re wondering, it’s only the third one ever built — really, only the second full engine since the “full-flow power-head demonstrator” never actually had a combustion chamber attached. The other is the Russian RD-270 engine.
如果你还不知道这是什么概念，这其实是世界上第三个建造的——真的，甚至是第二个完整的引擎，那个“ full-flow power-head demonstrator ”还没有装好燃烧室。另一个是俄罗斯的RD-270引擎。
It’s also the first full-flow, closed-cycle, dual-shaft, staged-combustion rocket to ever fly. So far, it’s only 20 meters or so, but it did fly. (Update: 150 meters now…)
So there’s your answer.
Anyone still reading is now entitled to a cookie. You earned it.