Rocket Science – Fuel or Propellant?

2015-08-06

Propelling an object into a parabolic arc or an orbit is not an easy feat. The basic Delta-V requirement to achieve low-Earth orbit is around 7.8 kilometers per second (km/s). Atmospheric drag, and other factors often require 1.5-2.0 kilometers more Delta-V for the launch vehicle itself, placing the total requirement to reach orbit at around 9.4 km/s. These immense speeds cannot simply be reached by air-breathing engines, nor by the ancient rockets propelled by steam or gunpowder.

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A rocket functions by expelling mass backwards at extremely high speeds, utilizing Newton’s Third Law to push the payload in the opposite direction as the thrust. Specific Impulse, another important factor of rocket propellants, is the measure of a rocket or jet engine’s efficiency. This is the impulse delivered per unit of propellant that is consumed, and is dimensionally equivalent to the thrust generated per unit. If a unit of mass, such as a kilogram, is used as the unit of propellant, then the specific impulse has units of velocity. If a measure of weight, such as a pound is used instead, then specific impulse is measured in units of time, such as seconds.

The higher a rocket’s specific impulse, the lower the propellant flow rate required for a given thrust, and thus the less propellant needed to achieve a certain delta-v, as per the Tsiolkovsky Rocket Equation. This equation is commonly known as the “ideal rocket equation” and describes the motion of vehicles that follow the basic principles of rockets. These principles are defined as: A device that accelerates via expulsion of part of its mass with high speed, thereby moving due to conservation of momentum. The equation relates the Delta-V with the effective thrust velocity.

According to this equation, Delta-V is the Effective Exhaust Velocity, multiplied by the natural logarithm (ln) of the initial total mass including propellant, divided by the final total mass without propellant, or dry mass.

Equation for Ideal Rockets

Since rockets often are intended to travel into space or achieve extremely high velocities, typical air-fuel mixtures are nigh impossible. Thus, rocket fuel is known as propellant, the chemical mixture which is burned to produce gasses for propulsion. Propellant consists of a fuel and oxidizer, the fuel being a substance which burns when combined with oxygen, and an oxidizer being the agent that releases oxygen to combine with the fuel. The ratio of oxidizer to fuel is known as the mixture ratio. In modern rocketry, there are three classes of propellant, liquid, solid, and hybrid.

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Liquid Propellants

Liquid Propellant rockets are some of the most well-known rockets, examples such as the Saturn V Main Stage, the Space Shuttle, or NASA’s new Space Launch System come to mind. Liquid propellant engines are often very complex, but they do offer many advantages, such as flow control to the combustion chamber, engine throttle control, and shut-off control. Good liquid propellants have a high specific impulse, but also have drawbacks such as large fuel tanks which add mass to the launch vehicle, complicated storage temperatures, or propellant toxicity. Liquid Propellants can be broken down into three sub-categories; petroleum, cryogenic, and hypergolic.

Petroleum Propellants

Petroleum fuels are those that are derived from crude oil, and consist of a complex mixture of hydrocarbons. The petroleum used as rocket fuel is a highly refined version of Kerosene, called RP-1 in the United States. Kerosene generates a considerably lower specific impulse than some cryogenic fuels, but is generally preferred due to its stability and low toxicity. RP-1 was utilized in the first-stage boosters of Soyuz, Zenit, Delta I-III, Atlas, Falcon 9, and Tronador II. It also powered the main stages of the Energia, Titan I, Saturn I and IB, as well as the Saturn V. NASA’s SLS also intends to utilize a RP-1 and Liquid Oxygen (LOX) mixture.

Cryogenic Propellants

Cryogenic propellants consist of liquefied gases stored at ultra-low temperatures. Most frequently, Liquid Hydrogen (LH2) is used, with Liquid Oxygen (LOX) as the oxidizer. LH2 must remain at a temperature of -253 degrees Celsius in order to keep its liquid form, and Oxygen at -183 degrees Celsius. Due to their low temperature, Cryogenic propellants are very difficult to store over extended periods, and are thus undesirable for missiles that must be kept launch-ready for months at a time. Liquid Hydrogen also has an extremely low density, and requires fuel storage that is considerably larger than other fuels. Despite these drawbacks, the efficiency of LOX/LH2 mixtures make these problems worth it when reaction time and storability are not critical. This mixture is used in the high-efficiency Space Shuttle’s Orbiter main stage, as well as the upper stages of the Saturn V, Centaur and Saturn 1B rockets.

Another, relatively untested fuel combination of Liquid Methane and LOX is higher-performing than other liquid fuels, but without the enormous volume of LH2, which results in a lower overall vehicle mass. LOX/Methane is also clean-burning and non-toxic. Future missions to Mars may use LOX/Methane, since it is believed that it can be produced on Mars. However, LOX/Methane has no flight history, and very few ground tests.

Liquid Fluorine, another cryogenic fuel that has been tested, is a super-oxidizer that violently reacts with anything other than nitrogen and the lighter noble gasses. Thus, fluorine produces impressive engine performance, and can be mixed with Liquid Oxygen to improve the performance of LOX-burning engines, resulting in a mixture known as FLOX. However, despite fluorine’s performance, it has been abandoned by nearly every space-faring nation simply due to the extreme toxicity of the chemical, though fluorine containing compounds, like Chlorine Pentafluoride have been considered for use as oxidizers in deep-space missions.

Hypergolic Propellants

Hypergolic fuels are propellants that ignite spontaneously when in contact with one another, and require no ignition source. This reactivity of hypergolic fuels give them an easy start and restart capability which make them idea for space-craft maneuvering thrusters. Hypergolic fuels also remain in liquid form at room temperature, so they do not pose the same storage problems as cryogenic fuels. However, hypergols are often extremely toxic and must be handled with extreme care.

Hypergolic fuels often include hydrazine, monomethyl hydrazine (MMH), and unsymmetrical dimethyl hydrazine (UDMH). Hydrazine is the most effective as a rocket fuel, but has a high freezing point and is unstable with use as a coolant. MMH is more stable, and gives the best performance when freezing point is an issue, such as deep-space propulsion applications. UDMH has the lowest freezing point, and highest thermal stability in large engines. Thus, UDMH is used in launch vehicle applications even though it is the least efficient of hydrazine variants. Mixed fuels, such as Aerozine 50, which consists of 50% UDMH and 50% Hydrazine, result in higher stabilities and performance than UDMH or Hydrazine alone.

Hypergols are often oxidized via Nitrogen Tetroxide (NTO) or Nitric Acid. In the United States, the nitric acid formula that is commonly used is Type III-A, known as Inhibited Red-Fuming Nitric Acid (IRFNA), which consists of HNO3 + 14% N2O4 + 1.5-2.5% H2O + 0.6% HF.  NTO is less corrosive than IRFNA, and provides better performance, but has a higher freezing point and is thus the oxidizer of choice when freezing isn’t an issue. In order to achieve the higher freezing point of IRFNA, and the effectiveness of NTO, the two are often mixed to create Mixed-Oxides of Nitrogen (MON). MON oxidizers commonly include a number, which dictates their percentage of nitric oxide by weight. For example, pure NTO has a freezing point of -9 degrees Celsius, while MON-25’s freezing point is -55 degrees Celsius.

Hydrazine is also used as a monopropellant in catalytic decomposition engines. In these engines, a liquid fuel is decomposed into hot gasses with the presentation of a catalyst. Decomposing hydrazine produces temperatures in excess of 1,100 degrees Celsius, and a specific impulse of 230, or 240 seconds. Hydrazine commonly decomposes into either mixtures of hydrogen and nitrogen, or ammonia and nitrogen.

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Solid Propellants

Solid Propellant motors are the simplest of all rocket designs, and hearken back to the days of gunpowder rockets in China’s Three Kingdoms Period (220-280 AD). These rockets consist of a casing, usually steel or some other high-strength metal, filled with a mixture of solid propellant components (fuel and oxidizer) that burn at a rapid rate, expelling gasses through a thrust nozzle. When ignited, solid propellants burn from the center, outwards to the sides of the casing. The shape of the center channel determines the rate and speed of the burn, thus providing a simple means to control the thrust. However, unlike a liquid propellant engines, solid propellants motors cannot be shut down, and will burn until all their propellant is exhausted. Some avid Kerbal Space Program players may understand this concept and have found themselves at the business end of a runaway solid booster rocket one too many times. There are two families of solid propellants, and these are known as homogenous, and composite. Both homogenous and composite propellants are dense, stable at room temperature, and very easily stored.

Homogenous Propellants

Homogenous propellants consist of a simple base, or double base. Simple base propellants use a single compound, typically something along the lines of Nitrocellulose, which has an oxidation capacity and reduction capacity. Double-base propellants commonly consist of Nitrocellulose and Nitroglycerine. Homogenous propellants have very poor specific impulses, usually no greater than 210 seconds under normal conditions, but their main advantage is that they do not produce traceable fumes and are therefore commonly used in tactical ballistic weapons. They are also often used to perform basic functions such as jettisoning spent parts, or separating stages from one another.

Composite Propellants

Composite propellants are heterogeneous powders that use a crystallized or finely ground mineral salt as an oxidizer, such as ammonium perchlorate. This mineral salt often constitutes around 60 to 90 percent of the mass of the propellant. The fuel itself is typically aluminum, and is held together via polymeric binders such as polyurethane or polybutadiene, which is also consumed as fuel. Additional compounds are sometimes added, such as catalysts to increase the burning rate, or chemicals to make the powder easier to manufacture. The final product is a rubber-like substance, with the texture and consistency of a common pink eraser.

Composite propellants are typically identified by the time of polymeric binder used, such as Polybutadiene Acrylic Acid Acrylonitrile (PBAN) and Hydroxyl-Terminator Polybutadiene. (HTPB). PBAN formulas give a slightly better specific impulse, density, and burn rate than formulas utilizing HTPB, but PBAN propellants are considerably harder to produce and requires an elevated curing temperature. HTPB is also stronger and more flexible than PBAN, but both formulas result in propellants that deliver excellent performance, and are extremely reliable in both mechanical and burn properties. These types of fuels have been used on the Titan, Delta, and Space Shuttle launch vehicles as strap-on propellant rockets in order to provide extra thrust at lift-off. The Space Shuttle had the largest solid rocket boosters ever built, each one containing 500,000 kilograms of propellant, and producing 3.3 million pounds of thrust.

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Hybrid Propellants

Last, and sort-of least, we have Hybrid Propellants. These engines represent a middling group that exists between both solid and liquid propellant engines. One component is solid, typically the fuel, while the other, typically the oxidizer, is liquid. The liquid is injected into the solid component, whose fuel reservoir also serves as the primary combustion chamber. The advantage of these engines is their high performance, similar to solid propellants, though with the moderation and restart capabilities of liquid fuel engines. However, it is difficult to make this process work with extremely large thrust capabilities, and thus the hybrid propellant engine is rarely built. Most recently, a hybrid engine burning Nitrous Oxide, and HTPB rubber powered SpaceShipOne, which won the Ansari X-Prize.

Rocketry is a complex science, but all-the-more interesting to study if you’ve got the spare time, or the interest. For the reader’s enjoyment, I’ve included a few graphs comparing every fuel that the article has spoken about with one another, as well as popular rockets.

PROPERTIES OF ROCKET PROPELLANTS

ROCKET PROPELLANT PERFORMANCE

POPULAR ROCKETS AND THEIR PROPELLANTS

This article originally appeared on TheMittani.com, written by Kristoff Merkas.

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