Rocket fuel

   


Rocket fuel is the propellant which is burned to produce thrust in rockets.

History

The earliest rockets were created hundreds of years ago by the Chinese. Used primarily for fireworks displays and or as armaments, they were relatively simple and were fueled by black powder. Black powder, a mixture of potassium nitrate, sulfur, and charcoal, was the only fuel used for centuries, so there was little improvement in rocket design or performance during that time. Since the 1920s, especially during World War II and the space race, rockets have improved a great deal. Much of the improvement in rocket performance relied on early improvements in rocket fuels.

Overview

Rockets make thrust by expelling mass backwards with velocity. Chemical rockets, the subject of this article, make thrust by reacting propellants into very hot gas, which is then expanded in a nozzle out the back. The thrust produced is the mass flow of the propellants multiplied by their exhaust velocity (relative to the rocket), as specified by Newton's third law of motion. It is the equal and opposite reaction that moves the rocket, and not any interaction of the exhaust stream with air around the rocket (but see base bleed). Equivalently, one can think of a rocket being accelerated upwards by the pressure of the combusting gases in the combustion chamber and nozzle. Rockets can move faster in outer space, because they do not need to overcome air resistance.

The velocity that a rocket can attain is primarily a function of its mass fraction and its exhaust velocity. This is known as the rocket equation: <math>V_f = V_e \ln(M_0/M_f)<math>. The mass fraction is just a way to express how much of the rocket is fuel when it starts accelerating. Typically, a single-stage rocket might be 80% fuel, which is a mass fraction of 5. The exhaust velocity is often reported as specific impulse.

The first stage will usually use high-density (low volume) propellants to reduce the amount of volume exposed to atmospheric drag. Thus, the Apollo-Saturn V first stage used kerosene-liquid oxygen rather than the liquid hydrogen-liquid oxygen used on the upper stages (hydrogen is highly energetic per kg, but not per m3). Similarly, the Space Shuttle uses high-thrust, high-density SRBs for its lift-off with the liquid hydrogen-liquid oxygen SSMEs used partly for lift-off but primarily for orbital insertion.


There are three main types of propellants:

Solid propellants

Solid fuels were the first type of propellant to be used in rockets. Gunpowder, obviously, was the original propellant to be used in rocketry, consisting of a mixture of charcoal, sulfur and potassium nitrate (saltpeter). Solid fuels (and really, all rocket fuels) consist of an oxidizer (substance providing oxygen) and a fuel. In the case of gunpowder, the fuel is charcoal and sulfur and the oxidizer is the potassium nitrate. More contemporary recipies employ such compounds as sodium or potassium chlorate and powdered aluminum. (This mixture is sometimes known as "white powder"; not only is it different in appearance than black powder, it has a considerably higher energy density.)

However, white powder has insufficient specific impulse for orbital or near-orbital boosters. During the 1950s and 60s researchers in the United States developed what is now the standard high-energy solid rocket fuel. The mixture is primarily ammonium perchlorate powder (an oxidizer), combined with fine aluminum powder (a fuel), held together in a base of PBAN or HTPB (rubber-like fuels). The mixture is formed as a liquid at elevated temperatures, poured into the rocket casing, and cools to form a single grain bonded to that casing.

Solid fueled rockets are much easier to store and handle than liquid fueled rockets, which makes them ideal for military applications. The LGM-30 Minuteman and LG-118A Peacekeeper (MX) missiles are four-stage rockets rockets capable of intercontinental suborbital flights. The first three stages are solid fuelled, and in each case the last stage is a precision maneuverable liquid-fuelled "bus" used to fine tune the trajectory of the reentry vehicle.

Their simplicity makes solid rockets a good choice whenever large amounts of thrust are needed and cost is an issue. The Space Shuttle and many other orbital launch vehicles use solid fuelled rockets in their first stages (solid rocket boosters) for this reason.

However, solid rockets have lower specific impulse than liquid fueled rockets. It is also difficult to build a large mass ratio solid rocket because almost the entire rocket is the combustion chamber, and must be built to withstand the high combustion pressures. If a solid rocket is used to go all the way to orbit, the payload fraction is very small. (The Orbital Sciences' Pegasus rocket is a three-stage solid rocket orbital booster.)

Solid rockets are difficult to throttle or shut down before they run out of fuel. Essentially, the burning grain must be vented to lower the chamber pressure. Venting generally involves destroying the rocket, and is usually only done by a range safety officer if the rocket goes awry. The third stages of the Minuteman and MX rockets have precision shutdown ports which, when opened, reduce the chamber pressure so abruptly that the interior flame is blown out. This allows a more precise trajectory which improves targetting accuracy.

Finally, casting very large single-grain rocket motors has proved to be a very tricky business. Defects in the grain can cause explosions during the burn, and these explosions can increase the burning propellant surface enough to cause a runaway pressure increase, until the case fails.

Liquid propellants

Main article: Liquid rocket propellants

Liquid fueled rockets have better specific impulse than solid rockets and are capable of being throttled, shut down, and restarted. Only the combustion chamber of a liquid fueled rocket needs to withstand combustion pressures and temperatures, and the fuel tanks can be built with less material, permitting a larger mass fraction. For these reasons, most orbital launch vehicles and all first- and second-generation ICBMs use liquid fuels for most of their velocity gain.

The primary performance advantage of liquid fuels is the oxidizer. Several practical liquid oxidizers (liquid oxygen, nitrogen tetraoxide) are available which have much better specific impulse than ammonium perchlorate when paired with comparable fuels.

Most liquid fuels are also cheaper than solid fuels. The cost savings do not, and historically have not mattered; the cost of fuel is a very small portion of the overall cost of a rocket, even in the case of solid fuel.

The main difficulties with liquid fuels are also with the oxidizers. These are generally difficult to store and handle, either due to extreme toxicity (nitric acids), extreme cold (liquid oxygen), or both (liquid fluorine is a perennial favorite of wild-eyed enthusiasts). Several exotic oxidizers have been proposed: liquid ozone (O3), ClF3, and ClF5, all of which are unstable, energetic, and toxic.

Liquid fuelled rockets also require troublesome and highly stressed pressurization systems, plumbing and combustion chambers, which greatly increase the cost of the rocket. Many employ turbopumps which raise the cost still more.

Though all the early rocket theorists proposed liquid hydrogen and liquid oxygen as propellants, the first liquid-fuelled rocket, launched by Robert Goddard on March 16, 1926, used gasoline and liquid oxygen. Liquid hydrogen was first used by the Lockheed CL-400 Suntan reconnaissance aircraft in the mid-1950s. In the mid-1960s, the Centaur and Saturn upper stages were both using liquid hydrogen and liquid oxygen.

The highest specific impulse chemistry ever test fired in a rocket engine was lithium, fluorine, and hydrogen (a tripropellant), which was measured at 542 seconds specific impulse with a high-expansion nozzle in a vacuum. The combination is completely impractical: the hydrogen must be kept below -252 C (just 21 K), the lithium must be kept above 180 C, both lithium and fluorine are extremely corrosive, lithium ignites on contact with air, fluorine ignites on contact with almost everything else, fluorine is very toxic, both lithium and fluorine are expensive, and the rocket exhaust is both ionized (interfering with radio communication with the rocket), and toxic, which leads to questions about the effect on the environment.

The common liquid fuel combinations in use today are:

  • LOX and kerosene (RP-1). Used for the lower stages of most Russian and Chinese boosters, and the first stage of the Saturn 5. Very similar to Robert Goddard's first rocket.
  • LOX and liquid hydrogen, used in the Space Shuttle and the Centaur upper stages.
  • Nitrogen tetroxide (N2O4) and hydrazine (N2H4). Used in military, orbital and deep space rockets, because both liquids are storable for long periods at reasonable temperatures and pressures. Hydrazine decomposes energetically to nitrogen and hydrogen, making it a fairly good monopropellant all by itself.

Hybrid propellants

The observation that most of the trouble with solid fuelled rockets is with the oxidizer has led to the development of hybrid motors, which generally consist of a liquid oxidizer and a solid fuel.

There has been much less development of hybrid motors than solids or liquids.

The primary difficulty with hybrids is with mixing the propellants before burning. In solid propellants, the oxidizer and fuel are mixed in a factory in carefully controlled conditions (and even then it is tricky). Liquid propellants are generally mixed by the injector at the top of the combustion chamber, which directs many small fast-moving streams of fuel and oxidizer into one another. Injector design is a black art. In a hybrid motor, the mixing happens at the surface of the melting or evaporating surface of the fuel. The mixing is not a well controlled process and generally quite a lot of propellant is left unburned, which limits the efficiency and thus the exhaust velocity of the motor.

Another difficulty with hybrids is that they have liquid or pressurized gaseous oxidizers, and it is generally the oxidizer that gives so much trouble when handling all-liquid propellant combinations.

Mixture ratio

The theoretical exhaust velocity of a given propellant chemistry is a function of the energy released per unit of propellant mass (specific energy). Unburned fuel or oxidizer drags down the specific energy. Surprisingly, most rockets run fuel-rich.

The usual explanation for fuel-rich mixtures is that fuel-rich mixtures have lower molecular weight exhaust, which by reducing <math>M<math> increases the ratio <math>\frac{\sqrt(T_c)}{M}<math>, which is approximately equal to the theoretical exhaust velocity. This explanation, though found in some textbooks, is wrong. Fuel-rich mixtures actually have lower theoretical exhaust velocities.

The nozzle of the rocket converts the thermal energy of the propellants into directed kinetic energy. For this to happen, energy must transfer very quickly from the rotational and vibrational states of the exhaust molecules into translation. Molecules with fewer atoms (like CO and H2) store less energy in vibration and rotation than molecules with more atoms (like CO2 and H2O). This resulting difference improves nozzle efficiency so much that real rocket engines actually improve their actual exhaust velocity by running rich mixtures with somewhat lower theoretical exhaust velocities.

The effect of exhaust molecular weight on nozzle efficiency is most important for nozzles operating near sea level. High expansion rockets operating in a vacuum see a much smaller effect, and so are run less rich. The Saturn-II stage (a LOX/LH2 rocket) varied its mixture ratio during flight to optimize performance.

LOX/hydrocarbon rockets are run only somewhat rich (O/F mass ratio of 3 rather than stochiometric of 3.4 to 4), because the energy release per unit mass drops off quickly as the mixture ratio deviates from stochiometric. LOX/LH2 rockets are run very rich (O/F mass ratio of 4 rather than stochiometic 8) because hydrogen is so light that the energy release per unit mass of propellant drops very slowly with extra hydrogen. In fact, LOX/LH2 rockets are generally limited in how rich they run by the performance penalty of the mass of the extra hydrogen tankage, rather than the mass of the hydrogen itself.

Another reason for running rich is that off-stochiometric mixtures burn cooler than stochiometric mixtures, which makes engine cooling easier. And as most engines are made of metal or carbon, hot oxidizer-rich exhaust is extremely corrosive, where fuel-rich exhaust is less so.

See also

External links


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