Engines

Liquid Fueled Rockets

The opposite of solid is liquid.

Back before the 1900s, the only type of rocket fuel used was solid. Then, in 1926, Robert Goddard flew a very small rocket named Nell, after his daughter. This rocket had the distinction as being the first rocket to use liquid propellant. It reached the amazing height of about 100 meters. Not super high, but it was really a “Wright Brothers” moment. Nothing would be the same.

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Robert Goddard with his rocket Nell.

An aside on Robert Goddard: Goddard was basically the father of modern rocketry. When the press found out what he had done, they interviewed him and actually made fun of him, because they didn’t understand how rockets work. They thought that the rocket exhaust pushed on the atmosphere, enabling the rockets to go. They did not understand that you don’t need an atmosphere, or the ground, to lift something into space. In fact, the New York Times printed an editorial about Goddard that stated that he “does not know the relation of action and reaction, and of the need to have something better than a vacuum against which to react—to say that would be absurd. Of course he only seems to lack the knowledge ladled out daily in high schools.” Oops. That journalist didn’t read this blog, where I explain that rockets go because of Newton’s laws of motion. Goddard became very withdrawn and basically did his research in private. But, people still knew about him. In fact, German rocket scientists knew about him. They would call him up and ask him about different aspects of rockets. Being a scientist, he would share what he knew. Being an American, he told the government about the German’s interest in rocketry, which the government ignored. Robert Goddard died in August 1945. Just north of Washington DC, there is the NASA Goddard Research Facility, where scientists and engineers build many of the satellites that are in orbit today. The New York Times published an apology to Goddard the day after the launch of Apollo 11, which stated “Further investigation and experimentation have confirmed the findings of Isaac Newton in the 17th Century and it is now definitely established that a rocket can function in a vacuum as well as in an atmosphere. The Times regrets the error.”

An example of a liquid fuel for rocketry is hydrogen with oxygen used as an oxidizer. When these two molecules are mixed together, they release a very large amount of energy, and water results. I am sure that you all know that oxygen is a gas at room temperature, and I am sure that many of you know that hydrogen is also a gas at room temperature. Why, therefore, is this post about liquids and not gasses? Well, a gas takes up a huge amount of space compared to a liquid. Oh, sure, you could store gasses in pressure tanks, but have you ever tried to lift a gas tank? They are extremely heavy, since they have to hold the massive pressures of the gasses. So, the rockets would have to be very, very heavy in order to hold all of the gas. It is much easier to cool the gas down to extremely low temperatures and cause them to liquify. Then, you can store the liquids in relatively light weight tanks.

A liquid propellent engine typically uses two different liquids, a fuel and an oxidizer to create a large amount of energetic (hot) gas that is expelled at extremely fast speeds. The liquids need to be kept in separate tanks to keep them apart until they are pumped into the combustion chamber. The word “pumped” is important here, since, the flow rate of a pump can be controlled. This means that a liquid rocket engine can be throttled, such that the burn-rate of the propellent can be controlled, unlike a solid rocket booster. In fact, the liquid can be cut off completely, stopping all thrust. The flow can then be resumed at a later time, allowing the engine to restart. These are huge advantages over solid rocket boosters.

Liquid propellents are typically more efficient that solid propellents. This means that less fuel is needed, and since the amount of fuel exponentially increases as more is needed (see the rocket equation post!), any increase in fuel efficiency can dramatically decrease the size of the rocket.

Why would anyone ever use a solid propellent rocket, then? Well, there are a few disadvantages to liquid fueled rockets.

The first is that the liquids typically have to be stored at an extremely cold temperature. For example, oxygen turns into a liquid at -183 C (-297F). It is not super easy to store, nor is it super easy to fuel the rocket. Also, the rocket sits on the ground for a while before being launched into space, so there sometimes is insulation on the storage tank. For example, the large orange thing on the space shuttle is not rocket, but is a storage tank for the liquid oxygen and hydrogen. It is orange because that is the color of the foam insulation that wraps the entire tank. This insulation was the direct cause of the Columbia accident in 2003. I will talk about this in a separate post.

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The Space Shuttle Columbia during its final launch. The orange thing that the shuttle is attached to is the storage tank for the liquid hydrogen and oxygen.  It is orange because the tank is completely covered in several inches of (orange) foam insulation.

There is one advantage of having super-cooled liquids on the rocket, though. The combustion chamber and the rocket’s nozzle can get extremely hot, since the whole point of the rocket engine is to make extremely hot gasses and direct them out of the nozzle. The combustion chamber and nozzle can actually melt because of this. But, because there is very cold liquid around, the rocket’s plumbing goes around the nozzle and combustion chamber before leading into the chamber. This cools down the extremely hot metal, and warms up the liquids before they are combined together. All of this plumbing and pumping makes liquid engines much more complicated than solid engines.

Another disadvantage of liquid propellents is the storage tanks. Imagine trying to ride a bike with a half-empty five-gallon bucket of water balanced on your handlebars. As the water slushes around, it moves the center of gravity around. If you are trying to go in a (relatively) straight line, and stay balanced, this can significantly complicate things. The same is true with a liquid propellent rocket. The tanks move from being full, to being empty, with every stage in between. All the while, the rocket has to stay on course. The tanks, therefore, need to have baffles and be specially designed so that the liquids won’t slosh. Yet another complication.

If you drove a car before about 1995, you probably remember having a carburetor. These were devices that combined the fuel with oxygen. Now, pretty much all cars have fuel injectors that automatically adjust the amount of fuel and oxygen mixture. So, imagine 50 years before the first automobile fuel injectors were built, building them for rockets. The fuel and the oxidizer had to be mixed just right in order for the burn to be as efficient as possible. On the Saturn-V, the fuel was being used at such a huge rate, that they couldn’t get the mixture to be very good, so they ended up having pockets of fuel and oxidizer, which would then cause explosions. The engineers didn’t have time to develop new fuel injectors, so they made the walls of the combustion chamber thicker in order to withstand the explosions. Engineering! Fuel injectors are complicated when you are using hundreds of gallons of fuel a second. Today, fuel injector technology is much better, but we don’t have many rockets that use fuel as quickly as a Saturn-V did.

In summary, liquid fueled rockets have some big advantages over solid rockets, namely the engines can be throttled and even turned off, allowing more precise orbital insertion; and the liquid fuels tend to be more efficient than solid fuels. On the other hand, rockets that use liquids as fuels tend to be more complicated, with lots of pumps, massive plumbing systems, and fuel injectors. Most rockets that launch satellites into space tend to have a main stack that is liquid with strap-on solid boosters that provide a bit of extra lift in the first minutes of the launch. That way the rockets compromise between the simplicity, but reduced performance of the solid rockets, while allowing the complex, but precise orbital insertion and raw power of the liquid fueled rockets.

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Engines

Back to the Basics: Solid Rocket Engines

As you can probably tell, I tend to get ahead of myself.  This blog has talked about all sorts of crazy different ways of getting into space, except for the two most common methods, which are solid rocket engines and liquid rocket engines. The difference may seem to be subtle, but they are actually quite big. I will talk about solid rocket engines first.

Most people have probably seen a solid rocket engine (maybe even held one in their hands), since they are what are used in model rockets. In fact, they have been used for about a thousand years. The first fire arrows were arrows with a little solid rocket engine tied on to them.  Basically, people used to take gun powder, shove it in a tube, tie it to an arrow, and light it.  The arrow would travel at much higher speeds than if an archer fired it, but they were also very hard to aim. Before World War II, the largest improvement in the technology came when William Hale, in about 1845, packed the gun powder into a cylinder and had the exhaust come out in such a way that the cylinder would spin, which provided stabilization to the rocket. It also eliminated the need for a big stick.

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Korean Fire Arrows. 

Did you know that we launched rockets against Mexico City?  How crazy is that?

When the Civil War started, we started to use modern artillery, like guns, which were much more accurate and could be used by a vast number of the solders. Rockets basically went out of favor during this time, although there were a few people working on liquid propellant rockets (like Goddard in the early 20th century), just to try to get the ideas right. I will talk about this when I talk about liquid rockets.

Anyways, during World War II, the Germans figured out how to really use rockets. They used liquid propellant engines in order to launch rockets against the allies. Now, the liquid propellant technology was a huge breakthrough, but they also came up with a bunch of other breakthroughs, namely a guidance system to actually steer the rockets on their 200 mile journey as well as a telemetry system to see if they actually were headed in the correct direction. This sounds like a simple thing to do, but this was before computers were invented, let alone GPS.  But, that is once again, for another post.

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A revolutionary German V-2 Rocket from WWII

The lesson here is that the Germans developed the underlying technologies that allowed the rocket to go from being a spinning cylinder that would sometimes work, to a supersonic vehicle that could hit a target accurately from 200 miles away.

Once World War II was over, the US and USSR militaries wanted to used rockets for a couple of uses, but one of the most important was for launching atomic bombs at other countries.  This was because an airplane could be shot down when trying to deliver an atomic bomb. Also, it could take an airplane hours to get to a target. With a rocket, almost any place on Earth could be hit in 45 minutes or less.

What rocket scientists learned is that using liquid propellants on Intercontinental Ballistic Missiles is not really smart.  This is because liquid propellants can’t actually be stored in the rocket – they have to be stored in coolant tanks that keep the liquids very cold.  For example, liquid oxygen needs to be stored at about -300° F (-183° C), which is a wee bit cold. So, when someone wants to fire a liquid propellant rocket, they have to fuel it first, which can take a long time.

On the other hand, those old rockets from the 1840s were packed with gun powder and were stable for a many years, and could be fired immediately when needed.  A perfect solution for global thermonuclear war! Therefore, scientists developed solid propellant engines. These are pretty simple.  You take a solid fuel and a solid oxidizer and mix them together with something that will bind them.  You put it in a tube and shape it, and ta-da, you have a solid rocket engine. There is obviously more to it than that, but that is the basic premise. There are a lot of resources out there that will tell you the chemical makeup of solid rocket fuels.

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A Minuteman-III ICBM

When you pack the fuel (called grain) into the cylinder, where are a wide variety of shapes that it can be.  The simplest is packing the outer walls and leave a hole in the center (see wonderful drawing below).  The point here is that only the part of the grain that is exposed will burn, so when you ignite this, then a very small portion of the grain starts to burn. Then, as the grain burns away, more surface area is exposed, so more grain can burn. If you are looking up the rocket from the bottom, you will see a little circle of clear area at first, then as the grain burns, the circle gets larger and larger, with more and more grain burning. Since more grain is burning, the thrust actually increases as a function of time, until the outer wall is reached, and the thrust stops, since the fuel is completely depleted. This is called a progressive burn. Model rockets have this grain pattern.

progressive_burn
A progressive burn grain pattern.  This is viewing up the rocket from the bottom.

 

If you pack the grain in exactly the opposite way, with a big circle in the middle, and a thin shell of clear area at the outer shell, then as the grain burns, the circle of grain gets smaller and smaller, which creates a regressive burn.

regressive_burn
A regressive burn grain pattern, viewed from the bottom of the rocket.

Interestingly, you can do some strange shapes that will allow the burn to be neutral, where the exposed surface area of the grain remains mostly constant as a function of time.  (If you think about it, the area always has to roughly be the area of outer shell, since this is the final area.) This is what the space shuttle’s solid rocket booster engines used – an 11-point star configuration for a roughly constant burn for the entire 104 seconds that they thrusted.  More on the space shuttle’s solid booster in another blog post!

One huge disadvantage of solid rocket boosters is that once you fire them, you can’t stop them – they burn until all of the grain is used up. With a liquid propellant rocket engine, you can throttle the engine and even turn it off and back on. There is none of that control in a solid rocket engine. Engineers design the shape of the grain to give them the type of burn that they want, and when it is lit, it goes.  This can be somewhat mitigated by making the top of the rocket so that it can pop off, exposing the grain at both the top and bottom, allowing the rocket to thrust both down and UP at the same time. This creates a balance in the (upward and downward) forces, allowing the rocket to go nowhere. That is pretty insane, and, I would imagine, quite dangerous, but it is something.

Another disadvantage is that, because the rocket exhausts all of the fuel in one go, the rocket basically can’t do any final adjustments. Therefore, the uncertainty on where it will end up can be rather large. For nuclear weapons, being off by a few (10s of) miles is not a big deal, since there are a lot of warheads on each one and they all go sort of cluster-bomb at the end anyways. If you want to get something to orbit, being off by a few (10s of) miles can be somewhat bad.

Ok, let’s summarize. Some advantages of solid rocket engines:

  1. Solid rocket engines are dead simple and are therefore easy to design and cheap.
  2. You can store a fully fueled solid rocket engine for a long time and fire it whenever.
  3. Solid rocket engines can be strapped on to liquid propellant rockets to give them an initial boost. Hence, solid rockets are sometimes called boosters.
  4. Did I mention that they are so simple that you can buy them at Walmart?

Some disadvantages:

  1. Once you ignite the solid rocket engine, it will fire until used up.
  2. Because they can’t be relit, they are not super accurate.
  3. They are almost always not as powerful as liquid propellant engines. I didn’t talk about this, but it is true!

Solid rocket engines end up being used as strap-on boosters and intercontinental ballistic missiles, and are not used very often to put stuff into orbit on their own.  That is, unless you don’t have a lot of money and need to get to space for cheap. You can pick up a bottom of the line Pegasus-XL for about $28,000,000 from Orbital-ATK (and build a paper one for free!). And yes, they accept bitcoin.

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