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For main engine burns, nitrogen tetroxide is the oxidizer and hydrazine is the fuel; for the other reaction control system burns, those are monopropellant thrusters, where just the hydrazine will decompose across the catalyst beds to give you some thrust. Jeff Lewis Spacecraft Engineer, Operations Lead, Juno So very rarely do we burn things, only when we’re firing the main engine is there combustion going on. The rest of the thrusters use an exothermic monopropellant reaction. Hydrazine is effectively double ammonia N2H4 and it will break down energetically to create ammonia, nitrogen and hydrogen, and in doing so the exhaust products get hot. That sort of relates back to the rocket equation. The rocket equation likes high exhaust velocity, and exhaust velocity relates back to having a low molecular weight [exhaust gas]. You don’t want your exhaust product to be lead or depleted uranium. You want something with a low molecular weight that you can make go very fast, and that creates your efficiency, so having a propellant that breaks down into ammonia, nitrogen and hydrogen works great. You get it hot so that gives you more energy, and it all streams supersonically through the nozzle which gives you high velocity, and thus gives you a more efficient approach. Tim Martin Spacecraft Engineer, Propulsion Lead, Juno Chapter 8 – Propellants A model rocket engine resembles nothing more than a standard “safe and sane” firework– a hollow cardboard tube whose ends are covered by a clay-like material. Further investigation with a knife reveals the interior to contain a gray powder, which most people guess is the explosive propellant. Yet those people who have fortunate enough to witness a major launch, such as the Space Shuttle or other governmental rockets, and especially have heard the commentary, know that there are plenty of liquids involved – for instance, “LOX” which stands for liquid oxygen. How are there so many different kinds of propellants for rockets, and is there a best one? These are the questions to be answered in this chapter. There are many different ways to use the action/reaction phenomenon that Newton’s third law allows. That is, there are many different ways to throw material out the back end of a spacecraft; typically (but not always), the material that goes out the back is a gas or plasma (ionized gas). For convenience, we’ll break these down into the three principal types of generating the material that goes out the back: chemical reactions, plasma reactions and nuclear reactions. Chemical propellants are the most common method of moving a rocket PAGE 69 Since they are the most common, we’ll start with chemical propellants – this classification covers both the rockets that accelerate the model rocket and the Space Shuttle. Chemical propellants are the most used, tested and understood of all propulsion methods. Moreover, they produce the most reliable thrust and impulse. Chemical rocket motors are further divided by the state of the fuel: solid, liquid or hybrid. A hybrid engine uses both a solid and a liquid. Liquid chemical rocket motors are subdivided by the number of chemical compounds that must be used: monopropellant (one chemical) or bipropellant (two chemicals). Chemical propellants, whether liquid or solid, rely on a chemical reaction to occur. At least one fuel compound must react with at least one oxidizer compound; this reaction is called combustion (or burning). The fuel is oxidized by the oxidizer and the resulting products of the reaction, the exhaust gases, are ejected out of the back of the motor at a rate of about 2 to 5 km/s. Let’s start with the simplest motor: a solid chemical propellant rocket. The solid propellant chemical rocket has a long history. The earliest Chinese rockets from the thirteenth century were powered by black powder, which is a combination of charcoal, sulfur and potassium nitrate, which was lit by a flame. Black powder burns quickly because of the presence of oxygen in potassium nitrate. The exhaust gases are carbon dioxide, and sulfur and nitrogen oxides. Thus, in this case, the carbon and the sulfur are the fuel and the potassium nitrate is the oxidizer. Currently, the model rocket motors (classes A through E) use black powder. Clay is shaped to make the nozzle in the back end of the cardboard tube. The black powder in these motors is compressed to allow shaping and to make the burning of the substance more uniform. Once the amount of black powder becomes approximately E motor size, the compressed black powder becomes too brittle to form properly. For high-powered rocketry, a propellant made of ammonium perchlorate (AP), potassium nitrate and aluminum powder is often used. The ammonium perchlorate and the potassium nitrate are the oxidizers, and the aluminum is the fuel. The exhaust gases are aluminum oxide, hydrogen chloride and some nitrogen compounds. For even higher-powered rocketry, such as the Space Shuttle Solid Rocket boosters, an ammonium perchlorate composite propellant (APCP) is used. The formulation is slightly different than the AP propellant above; APCP includes an organic plastic binder, such as hydroxyl-terminated polybutadiene (HTPB), which acts as the fuel (in addition to the aluminum powder). The advantage of AP-based propellant is that they may be cast, rather than compressed like black powder propellants. This means they can be made much larger than the black powder propellants. In addition, AP-based propellants are more thermally stable than black powder, and behave more reliably when ignited. PAGE 70 The liquid propellants are classified according to the number of chemicals needed for the reaction. A monopropellant, such as hydrazine, is a single chemical that can decompose without another chemical to react with it. Hydrazine generates ammonia, hydrogen and nitrogen gases that go out the nozzle. Hydrazine itself is highly toxic and unstable. One advantage of hydrazine is the large quantity of gas it generates, especially at high temperatures, relative to the amount of fuel used. Liquid monopropellant engines have a long flight history, and do not require a lot of plumbing to get the propellant to the combustion chamber. They are also easy to turn off by shutting off the supply of propellant. The problem is that they don’t deliver a lot of thrust or specific impulse. A bipropellant is a system of two liquid chemicals that react to generate the gas or gases that go out the back. An example of this is unsymmetrical dimethyl hydrazine (UDMH) and dinitrogen tetroxide. The combination of these two chemicals is hypergolic, meaning that, when the two are combined, they spontaneously ignite. UDMH is carcinogenic but more stable than hydrazine. In addition, it can be stored for long periods of time in a fuel tank of a launch vehicle without losing efficacy. On the other hand, mixing two liquids in the right proportions at the right times poses a significant engineering challenge. Another example of a liquid bipropellant system is the Space Shuttle main engine: it uses liquid hydrogen (LH2) as the fuel and liquid oxygen (LOX) as the oxidizer. The exhaust gas is water vapor. There are over a hundred pumps and valves, as well as a control system, to deliver the fuel and oxidizer on time and in the proper amounts. Liquid bipropellant engines can deliver a higher thrust and specific impulse than monopropellant engines. They are also easy to shut off, and are more efficient than solid or hybrid engines. Hybrid systems use two chemicals: a solid and either a gas or liquid. For instance, a common hybrid system is using solid HTPB (the plastic binder) fuel and gaseous nitrous oxide as the oxidizer. The advantages of a hybrid system are simpler engineering than the liquid bipropellant system, more specific impulse and lower explosion risk. The disadvantages of a hybrid system are that they are more complex than a solid rocket, and they don’t have a long history; the first one was successfully fired in 1952. SpaceShipOne, the craft that won the Ansari X Prize in 2004, had a hybrid rocket system. Plasma reactions use magnetic fields to manipulate charged particle streams From previous chapters, you know that plasmas are charged particles in gaseous form. The solar wind, for instance, is a example of a plasma, and it flows outward, following the magnetic field lines of the Sun. A similar principle is involved on a much smaller scale with plasma-powered rockets. PAGE 71 There are three types of plasma-generating rockets; all use on-board electricity to generate the charged particles in the first place. Electrothermal systems use the electricity, as the name implies, to heat up a neutral gas such that it will lose electrons and therefore become charged. But rather than using a magnetic field to guide the plasma, the heat expanding the volume of the plasma is all that is required; that extra volume of the heated plasma can be directed out of the nozzle, and exhaust gases can be propelled at speeds much faster than chemical rockets. NASA’s Arc Jet project is an example of such a technology. In addition to the complexity of generating the plasma, these systems are not capable of generating high thrust, so their use at present is limited to station-keeping. Electrostatic systems, on the other hand, also generate a plasma using electricity, but do use a magnetic field to accelerate and direct the ions through the nozzle. These types of systems are quite efficient and generate high exhaust speeds. In a gridded ion thruster, such as that used by NASA’s Dawn spacecraft, the ions are accelerated by passing them through grids of alternating charges. In the Dawn rocket, the plasma is generated from xenon atoms; xenon is a gas that does not interact with most materials and thus is safe to store in a chamber for decades. The xenon atoms are also massive (compared to other atoms), so a comparatively low flow of these ions will produce as much thrust as a higher flow of lighter ions. The spacecraft, therefore, does not need to carry as much fuel. Dawn carried 425 kg of xenon for over five years that the rocket will be firing. The drawback is that the thrust of the rocket is miniscule – it would take Dawn four days to accelerate from 0 to 60 miles per hour. However, sustained thrust will result in the interplanetary speeds the spacecraft will need to reach its objective asteroids. Electromagnetic systems use an electrical current and a magnetic field to generate a Lorentz force. Current flowing through a conductor in a magnetic field causes a force on the conductor that is perpendicular to the direction of the current and the direction of the magnetic field. It is this force that will propel the rocket, and is successfully used on maglev trains on Earth. The pulsed plasma thruster is the oldest of these electromagnetic spacecraft propulsion systems. In this thruster, the electricity is used to heat a solid fuel (often the plastic Teflon) into a plasma, then use two charged plates to generate a magnetic field, with the plasma passing in between the plates. Because the plasma is charged, it is a current that will interact with the charged plates’ magnetic field and thus yield a Lorentz force that accelerates the plasma out of the nozzle. It has the advantage of being a fairly simple design, but it does not generate much thrust nor is it particularly efficient in its use of fuel, so its uses have been limited to pointing spacecraft and station-keeping. Nuclear propellants are the realm of science fiction PAGE 72 The idea of harnessing the tremendous power within the nucleus of an atom for space travel has been around since the development of the first nuclear bombs. In fact, the idea of using a nuclear bomb concussive force to move a spaceship was proposed in 1958 and became Project Orion. The idea was relatively simple; the spacecraft would hurl atomic bombs out the back and they would explode. There would be a large and strong concussion plate on the back of the spacecraft to absorb the shock of the blast wave, and, true to Newton’s Third Law, move the spacecraft forward. Since the bomb was to detonate about 60 meters behind the spacecraft, the thrust that each blast would provide was estimated at 1 megaNewton, so any humans on board would have to be cushioned sufficiently. It was also estimated that the craft could eventually achieve one-tenth the speed of light, far faster than any spacecraft today. Though the costs of such a craft would be enormous and the engineering quite difficult, the Nuclear Test Ban treaty of 1963 effectively killed off the project. The idea has resurfaced periodicially, most recently in the form of antimatter-catalyzed nuclear pulse propulsion which would involve the relatively small releases of nuclear energy initiated by matter-antimatter annihilation that would nevertheless “push” a rocket forward. The advantage of this type of nuclear propulsion is its relatively small thrust, which would not require special shock-absorbing protection for humans or their equipment. Currently, nuclear propulsion has been limited to nuclear thermal propulsion, in which a nuclear reactor heats up the gas to be expelled from the rocket, and nuclear electric propulsion, in which a nuclear reactor generates the electricity to power one of the electrical propulsion methods. PAGE 73