On April 12, 1981, the space shuttle Columbia achieved liftoff, successfully launching NASA’s eponymous Space Shuttle program with the power of three main engines and two solid rocket boosters (SRBs). The SRBs employed during STS-1 marked the first use of a solid fuel rocket as primary propulsion for a human spaceflight system. You might imagine it takes some powerful stuff to lift something as large as a space shuttle. But what is solid rocket fuel made of? The SRBs used throughout the Space Shuttle Program were massive; weighing about 2.6 million pounds, they provided 83 percent of the liftoff thrust burning 2.2 million pounds of propellant in just over 2 minutes. You may have noticed from those numbers that the propellant alone made up roughly 85% of the weight of the SRBs. Whether you are an aspiring rocket scientist or a model rocket enthusiast this guide aims to answer that question. While the specific materials used in solid rocket fuel vary depending on the application, the basic components remain the same. Solid rocket fuel consists of a fuel, an oxidizer, a binder and additional stabilizing agents. Let’s take a closer look at how these components work together to make rocket fuel.
The basic principle behind solid rockets holds its roots in 13th
century China where a variation of gunpowder was used to power rockets in battle. While the mixtures have grown more sophisticated since then, the basic idea behind rocket fuel remains the same: use a high energy substance that can burn quickly without exploding. Sometimes the difference between a fast burn and an explosion can be small; ancient Chinese took the explosive 75% nitrate, 15% carbon, and 10% sulfur mix of gunpowder and altered it to the fast burning 72% nitrate, 24% carbon, 4% mix of simple solid rocket fuel. In model rocketry, fine aluminum or magnesium powder is common fare, while the United States Military employs nitroglycerine in its rockets and guided missiles.
In the atmosphere, jet engines can use the oxygen in the air to help burn the fuel and provide thrust. However in solid fuel rockets, an oxidizer needs to be included in the propellant mix to help the fuel combust and generate a hotter more even burn. Nitrates or perchlorates are the most common forms of oxidizers. Nitrocellulose is the oxidizer of choice used in military applications while model rockets may use potassium nitrate or powdered sulfur.
In order to burn more efficiently, a binding agent is required to bind the fuel, oxidizer and other additives together into the grain or propellant charge. The structure and consistency of the grain allows the propellant to burn predictably; producing the steady stream of exhaust gases that provide thrust for the rocket. The most popular binders are polyurethane or polybutadienes. The type of binding agent is often used as a form of classifying composite fuels, with the two most common binders being polybutadiene acrylic acid acrylonitrile (PBAN) and hydroxy-terminator polybutadiene (HTPB). Since the binding agent is consumed as fuel along with everything else in the grain, it is important to note that the binder also affects the specific impulse, density, and burn rate of the grain. PBAN generally provides improved performance in these categories, at the cost of a more difficult mixing process and higher curing temperatures. As a finishing touch a small amount of epoxy curing agent is added to the fuel to stabilize the grain and prevent it from layering or dripping. The curing agent hardens the fuel making it easier store and transport.
Putting it all Together
When you mix a fuel, oxidizer, binding agent and additional additives like curing agents or catalysts, the end result is a propellant or rocket fuel. The variations in compositions of the components in rocket fuel are best illustrated using examples. The ballistite from our military example is a propellant composed of 43% nitroglycerine (fuel), 51.5% nitrocellulose (oxidizer), 1% plasticizer (binder) and 4.5% additives. Likewise the aluminum-based propellant used in model rockets is 16% aluminum powder (fuel), 65% ammonium perchlorate (oxidizer), 17% HTBP polybutadiene (binder) and 2% Iron (II) oxide stabilizer. As you can see there can be many possible formulations for solid rocket fuel. While the compositions and exact materials vary, the basic components remain the same. By understanding the different components that make up rocket fuel, it is possible to make informed decisions when purchasing propellant grain for your DIY space projects.