How Rockets Work

How a Solid Propellant Rocket Works

Solid Propellant Rocket - Taurus
Solid Propellant Rocket - Taurus. NASA

Solid propellant rockets includes all of the older firework rockets, however, there are now more advanced fuels, designs, and functions with solid propellants.

Solid propellant rockets were invented before liquid fueled rockets. The solid propellant type began with contributions by scientists Zasiadko, Constantinov, and Congreve. Now in an advanced state, solid propellant rockets remain in wide spread use today, including the Space Shuttle dual booster engines and the Delta series booster stages.

How a Solid Propellant Functions

A solid propellant is a monopropellant fuel, a single mixture of several chemicals i.e. the oxidizing agent and the reducing agent or fuel. This fuel is in its solid state and has a preformed or molded shape. The propellant grain, this interior shape of the core is an important factor in determining a rocket's performance. The variables determining grain-relative performance are core surface area and specific impulse.

Surface area is the amount of propellant exposed to interior combustion flames, existing in a direct relationship with thrust. An increase in surface area will increase thrust but will reduce burn-time since the propellant is being consumed at an accelerated rate. The optimal thrust is typically a constant one, which can be achieved by maintaining a constant surface area throughout the burn.

Examples of constant surface area grain designs include: end burning, internal-core and outer-core burning, and internal star core burning.

Various shapes are used for the optimization of grain-thrust relationships since some rockets may require an initially high thrust component for takeoff while a lower thrust will suffice its post-launch regressive thrust requirements. Complicated grain core patterns, in controlling the exposed surface area of the rocket's fuel, often have parts coated with a non-flammable plastic (such as cellulose acetate).

This coat prevents internal combustion flames from igniting that portion of fuel, ignited only later when the burn reaches the fuel directly.

Specific Impulse

Specific impulse is the thrust per unit propellant burned each second, it measures rocket performance and more specifically, internal thrust production a product of pressure and heat. Thrust in chemical rockets is a product of the hot and expanding gasses created in the combustion of an explosive fuel. The degree of the fuel's explosive power coupled with the rate of combustion is the specific impulse.

In designing the rocket's propellant grain specific impulse must be taken into account since it can be the difference failure (explosion), and a successfully optimized thrust producing rocket.

Modern Solid Fueled Rockets

The departure from the use of gunpowder to more powerful fuels (higher specific impulses) marks the development of modern solid fueled rockets. Once the chemistry behind rocket fuels (fuels provide their own "air" to burn) was discovered, scientists sought the evermore-powerful fuel, constantly approaching new limits.


Solid fueled rockets are relatively simple rockets. This is their chief advantage, but it also has its drawbacks.
  • Once a solid rocket is ignited it will consume the entirety of its fuel, without any option for shutoff or thrust adjustment. The Saturn V moon rocket used nearly 8 million pounds of thrust that would not have been feasible with the use of solid propellant, requiring a high specific impulse liquid propellant.
  • The danger involved in the premixed fuels of monopropellant rockets i.e. sometimes nitroglycerin is an ingredient.

One advantage, is the ease of storage of solid propellant rockets. Some of these rockets are small missiles such as Honest John and Nike Hercules; others are large ballistic missiles such as Polaris, Sergeant, and Vanguard. Liquid propellants may offer better performance, but the difficulties in propellant storage and handling of liquids near absolute zero (0 degrees Kelvin) has limited their use unable to meet the stringent demands the military requires of its firepower.

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Liquid fueled rockets were first theorized by Tsiolkozski in his "Investigation of Interplanetary Space by Means of Reactive Devices," published in 1896. His idea was realized 27 years later when Robert Goddard launched the first liquid fueled rocket.

Liquid fueled rockets propelled the Russians and Americans deep into the space age with the mighty Energiya SL-17 and Saturn V rockets. The high thrust capacities of these rockets enabled our first travels into space.

The "giant step for mankind" that took place on July 21, 1969, as Armstrong stepped onto the moon, was made possible by the 8 million pounds of thrust of the Saturn V rocket.

How a Liquid Propellant Functions

As with conventional solid fuels rockets, liquid fueled rockets burn a fuel and an oxidizer, however, both in a liquid state.

Two metal tanks hold the fuel and oxidizer respectively. Due to properties of these two liquids, they are typically loaded into their tanks just prior to launch. The separate tanks are necessary, for many liquid fuels burn upon contact. Upon a set launching sequence two valves open, allowing the liquid to flow down the pipe-work. If these valves simply opened allowing the liquid propellants to flow into the combustion chamber, a weak and unstable thrust rate would occur, so either a pressurized gas feed or a turbopump feed is used.

The simpler of the two, the pressurized gas feed, adds a tank of high pressure gas to the propulsion system.

The gas, an unreactive, inert, and light gas (such as helium), is held and regulated, under intense pressure, by a valve/regulator.

The second, and often preferred, solution to the fuel transfer problem is a turbopump. A turbopump is the same as regular pump in function and bypasses a gas-pressurized system by sucking out the propellants and accelerating them into the combustion chamber.

The oxidizer and fuel are mixed and ignited inside the combustion chamber and thrust is created.

Oxidizers & Fuels

Liquid Oxygen is the most common oxidizer used. Other oxidizers used in liquid propellant rockets includeing: hydrogen peroxide (95%, H2O2), nitric acid (HNO3), and liquid fluorine. Of these choices liquid fluorine, given a control fuel, produces the highest specific impulse (amount of thrust per unit propellant). But due to difficulties in handling this corrosive element, and due to the high temperatures it burns at, liquid fluorine is rarely used in modern liquid fueled rockets. The liquid fuels often used include: liquid hydrogen, liquid ammonia (NH3), hydrazine (N2H4), and kerosene (hydrocarbon).


Liquid propellant rockets are the most powerful (in terms gross thrust) propulsion systems available. They are also among the most variable, that is to say, adjustable given a large array of valves and regulators to control and augment rocket performance.

Unfortunately the last point makes liquid propellant rockets intricate and complex. A real modern liquid bipropellant engine has thousands of piping connections carrying various cooling, fueling, or lubricating fluids.

Also the various sub-parts such as the turbopump or regulator consist of a separate vertigo of pipes, wires, control valves, temperature gauges and support struts. Given the many parts, the chance of one integral function failing is large.

As noted before, liquid oxygen is the most commonly used oxidizer, but it too has its drawbacks. To achieve the liquid state of this element, a temperature of -183 degrees Celsius must be obtained--conditions under which oxygen readily evaporates, losing a large sum of oxidizer just while loading. Nitric acid, another powerful oxidizer, contains 76% oxygen, is in its liquid state at STP, and has a high specific gravity--all great advantages. The latter point is a measurement similar to density and as it rises higher so to does the propellant's performance.

But, nitric acid is hazardous in handling (mixture with water produces a strong acid) and produces harmful by-products in combustion with a fuel, thus its use is limited.

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Developed in the second-century BC, by the ancient Chinese, fireworks are the oldest form of rockets and the most simplistic. Originally fireworks had religious purposes but were later adapted for military use during the middle ages in the form of "flaming arrows."

During the tenth and thirteenth centuries the Mongols and the Arabs brought the major component of these early rockets to the West: gunpowder.

Although the cannon, and gun became the major developments from the eastern introduction of gunpowder, rockets also resulted. These rockets were essentially enlarged fireworks which propelled, further than the long bow or cannon, packages of explosive gunpowder.

During the late eighteenth century imperialistic wars, Colonel Congreve, developed his famed rockets, which trave range distances of four miles. The "rockets' red glare" (American Anthem) records the usage of rocket warfare, in its early form of military strategy, during the inspirational battle of Fort McHenry.

How Fireworks Function

Gunpowder, a mixture composing of: 75% Potassium Nitrate (KNO3), 15% Charcoal (Carbon), and 10% Sulfur, provides the thrust of most fireworks. This fuel is tightly packed into the casing, a thick cardboard or paper rolled up tube, forming the propellant-core of the rocket in a typical length to width or diameter ratio of 7:1.

A fuse (cotton twine coated with gunpowder) is lit by a match or by a "punk" (a wooden stick with a coal-like red-glowing tip).

This fuse burns rapidly into the core of the rocket where it ignites the gunpowder walls of the interior core. As mentioned before one of the chemicals in gunpowder is potassium nitrate, the most important ingredient. The molecular structure of this chemical, KNO3, contains three atoms of oxygen (O3), one atom of nitrogen (N), and one atom of potassium (K).

The three oxygen atoms locked into this molecule provide the "air" that the fuse and the rocket use to burn the other two ingredients, carbon and sulfur. Thus potassium nitrate oxidizes the chemical reaction by easily releasing it oxygen. This reaction is not spontaneous though, and must be initiated by heat such as the match or "punk."


Thrust is produced once the burning fuse enters the core. The core is quickly filled with flames and thus, the necessary heat to ignite, continue, and spread the reaction. After the initial surface of the core has been exhausted a layer of gunpowder is exposed continuing, for the few seconds the rocket will burn, to produce thrust. The action reaction (propulsion ) effect explains the thrust as produced when the hot expanding gases (produced in the reaction burning of gunpowder) escape the rocket via the nozzle. Constructed of clay, the nozzle can withstand the intense heat of the flames that pass through.

Sky Rocket

The original sky rocket used a long wooden or bamboo stick to provide a low center of balance (by distributing the mass over a greater linear distance) and thus stability to the rocket through its flight. Fins usually three set at 120 degree angles of one another or four set at 90 degree angles of one another, had their developmental roots in arrow feather guides. The principles that governed the flight of an arrow were the same for early fireworks. But fins could be omitted altogether since a simple stick seemed to grant sufficient stability. With fins properly set (in creating a suitable center of balance) the extra mass of the drag (air resistance) creating guide-stick could be removed, increasing rocket performance.

What Makes the Pretty Colors?

The component of a rocket that produces these stars, reports ("bangs"), and colors is typically located just below the nosecone section of a rocket. After the rocket engine has consumed all of its fuel an internal fuse is lit that delays the release of the stars, or other effect. This delay allows for coasting time where the rocket continues its ascent. As gravity will eventually pull the firework back to earth, it slows and eventually reaches an apex (highest point: where velocity of the rocket is zero) and begins its descent. The delay usually lasts just before this apex, at an optimum velocity, where a small explosion shoots the firework's stars in desired directions and thus producing a brilliant effect. The colors, reports, flashes, and, stars are chemicals with special pyrotechnic properties added to bland gunpowder.


Gunpowder's relatively low specific impulse (amount of thrust per unit propellant) limits its capacity of thrust production on larger scales. Fireworks are the simplest of solid rockets and the weakest. Evolution from fireworks brought about more complex solid fueled rockets, which use more exotic and powerful fuels. The use of firework type rockets for purposes other than entertainment or education has virtually ceased since the late ninteenth century.
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