Introduction
The development of travel
Throughout history mankind has found the need to travel greater distances and with this science has developed new means of travelling these distances at greater rates, with relative ease. From the uses of the horse and cart, to cars and planes being able to circumnavigate the earth has become less and less of a daunting task.
The development of aviation is one such example. It was no long ago when travelling across the Atlantic took weeks and holidays were limited to national locations compared to simply hopping down to Greece for the week. Propulsion systems have made huge advancements from simple propeller planes creating low and high pressure around the blades to create lift. Achieving speeds of only a few meters per second. To internal combustion air breathing jet engines which would allow mankind to travel faster than the speed of sound known as Mach one (approximately 320ms-1). While the core concept has remained the same, high and low pressure in the air is created by the wings to create lift and keep the plane off the ground.
As the popular phrase states space the final frontier… [1] Space has become the next vast expanse for humanity to traverse. We are all too familiar with science fiction where huge ships make travelling space seem like a trip to the local post office. But where are we at the moment. How far through space can we go and what problems are we currently facing. In this essay I intend to look at our current methods of navigating the depths of space and the future technologies we are currently developing. Talking about the similarities and difference, advantages and limitations of modern day space travel and the ideas that science fiction have made us come to expect as the future of space exploration.
Where are we at the moment?
Virtually all modern day propulsion systems use newton’s third law: if object A exerts a force on object B, then object B also exerts an equal and opposite force on object A. This principle is the groundwork for any rocket engine. Simply put if you throw something in one direction it will push you in the other direction. That’s what rocketry does, you throw large amount of fuel one way and it pushes you in the other direction. However it is much more complicated than that.
Currently the most popular methods of getting rockets into space and moving them when they are up there include: chemical rockets, electronic propulsion systems
Modern day propulsion systems
Chemical rockets
Anyone who sits on top of the largest hydrogen-oxygen fuelled system in the world; knowing they’re going to light the bottom—and doesn’t get a little worried—does not fully understand the situation[2]. Chemical rockets, the primary source for transporting space vessels from Earth to space. These are most often made up of three propellants: liquid, solid and hybrid.
Liquid propellant
In a liquid propellant rocket, the fuel and oxidizer are stored in separate tanks, and are fed through a system of pipes, valves, and turbo pumps to a combustion chamber where they are combined and burned to produce thrust. Liquid propellant engines are more complex than their solid propellant counterparts, however, they offer several advantages. By controlling the flow of propellant to the combustion chamber, the engine can be throttled, stopped, or restarted.
The thrust provided by the rocket is dependent on three things the mass flow rate, exit velocity of exhaust and the pressure at the nozzle exit. These variables are determined by the design of the nozzle. The nozzle has three parts, the combustion chamber, the throat and the diverging section (or nozzle cone). The fuel and oxidiser are combined in the combustion chamber this hot exhaust then converges into the throat of the nozzle, the smallest part. The throat is used to choke the hot exhaust which sets the mass flow rate. The exhaust travels through the throat at Mach one. Once passed the throat the nozzle widens out again this leads to the isentropic expansion (called so due to a consideration of the second law of thermodynamic as the flow maintains a constant value of entropy and so is a combination of the Greek word iso- same and entropy) of the flow leading to an increase in the Mach number to supersonic (faster than sound). This expansion of the area and thus gas causes static pressure and temperature to decrease. The exit temperature or energy determines the velocity of the exhaust exiting the nozzle.[3]
A good liquid propellant is one with a high specific impulse or, stated another way, one with a high speed of exhaust gas ejection. This implies a high combustion temperature and exhaust gases with small molecular weights. However, there is another important factor that must be taken into consideration: the density of the propellant. Using low-density propellants means that larger storage tanks will be required, thus increasing the mass of the launch vehicle. Storage temperature is also important. A propellant with a low storage temperature, i.e. a cryogenic, will require thermal insulation, thus further increasing the mass of the launcher. The toxicity of the propellant is likewise important. Safety hazards exist when handling, transporting, and storing highly toxic compounds. Also, some propellants are very corrosive; however, materials that are resistant to certain propellants have been identified for use in rocket construction.
Liquid propellants used in rocketry can be classified into three types: petroleum, cryogens, and hypergols.
Petroleum fuels are those refined from crude oil and are a mixture of complex hydrocarbons, i.e. organic compounds containing only carbon and hydrogen. The petroleum used as rocket fuel is a type of highly refined kerosene, called RP-1 in the United States. Petroleum fuels are usually used in combination with liquid oxygen as the oxidizer. Kerosene delivers a specific impulse considerably less than cryogenic fuels, but it is generally better than hypergolic propellants.
Specifications for RP-1 where first issued in the United States in the late 1950’s when the need for a clean burning petroleum rocket fuel was recognized.[4] Prior experimentation with jet fuels produced tarry residue in the engine cooling passages and excessive soot, coke and other deposits in the gas generator. Even with the new specifications, kerosene-burning engines still produce enough residues that their operational lifetimes are limited.
Liquid oxygen and RP-1 are used as the propellant in the first-stage boosters of the Atlas and Delta II launch vehicles. It also powered the first-stages of the Saturn 1B and Saturn V rockets.
Cryogenic propellants are liquefied gases stored at very low temperatures, most frequently liquid hydrogen (LH2) as the fuel and liquid oxygen (LO2 or LOX) as the oxidizer. Hydrogen remains liquid at temperatures of -253 oC and oxygen remains in a liquid state at temperatures of -183 oC.[5]
Because of the low temperatures of cryogenic propellants, they are difficult to store over long periods of time. For this reason, they are less desirable for use in military rockets that must be kept launch ready for months at a time. Furthermore, liquid hydrogen has a very low density (0.071 g/ml) and, therefore, requires a storage volume many times greater than other fuels. Despite these drawbacks, the high efficiency of liquid oxygen/liquid hydrogen makes these problems worth coping with when reaction time and storability are not too critical. Liquid hydrogen delivers a specific impulse about 30%-40% higher than most other rocket fuels.
Liquid oxygen and liquid hydrogen are used as the propellant in the high efficiency main engines of the Space Shuttle. LOX/LH2 also powered the upper stages of the Saturn V and Saturn 1B rockets, as well as the Centaur upper stage, the United States’ first LOX/LH2 rocket (1962).
Another cryogenic fuel with desirable properties for space propulsion systems is liquid methane (-162 oC). When burned with liquid oxygen, methane is higher performing than state-of-the-art storable propellants but without the volume increase common with LOX/LH2 systems, which results in an overall lower vehicle mass as compared to common hypergolic propellants. LOX/methane is also clean burning and non-toxic. Future missions to Mars will likely use methane fuel because it can be manufactured partly from Martian in-situ resources. LOX/methane has no flight history and very limited ground-test history.
Liquid fluorine (-188 oC) burning engines have also been developed and fired successfully. Fluorine is not only extremely toxic; it is a super-oxidizer that reacts, usually violently, with almost everything except nitrogen, the lighter noble gases, and substances that have already been fluorinated. Despite these drawbacks, fluorine produces very impressive engine performance. It can also be mixed with liquid oxygen to improve the performance of LOX-burning engines; the resulting mixture is called FLOX. Because of fluorine’s high toxicity, it has been largely abandoned by most space-faring nations.
Some fluorine containing compounds, such as chlorine pentafluoride, have also been considered for use as an ‘oxidizer’ in deep-space applications.
Hypergolic propellants are fuels and oxidizers that ignite spontaneously on contact with each other and require no ignition source. The easy start and restart capability of hypergols make them ideal for spacecraft manoeuvring systems. Also, since hypergols remain liquid at normal temperatures, they do not pose the storage problems of cryogenic propellants. Hypergols are highly toxic and must be handled with extreme care.
Hypergolic fuels commonly include hydrazine, monomethyl hydrazine (MMH) and unsymmetrical dimethyl hydrazine (UDMH). Hydrazine gives the best performance as a rocket fuel, but it has a high freezing point and is too unstable for use as a coolant. MMH is more stable and gives the best performance when freezing point is an issue, such as spacecraft propulsion applications. UDMH has the lowest freezing point and has enough thermal stability to be used in large regeneratively cooled engines. Consequently, UDMH is often used in launch vehicle applications even though it is the least efficient of the hydrazine derivatives. Also commonly used are blended fuels, such as Aerozine 50 (or “50-50”), which is a mixture of 50% UDMH and 50% hydrazine. Aerozine 50 is almost as stable as UDMH and provides better performance.[6]
The oxidizer is usually nitrogen tetroxide (NTO) or nitric acid. In the United States, the nitric acid formulation most commonly used is type III-A, called inhibited red-fuming nitric acid (IRFNA), which consists of HNO3 + 14% N2O4 + 1.5-2.5% H2O + 0.6% HF (added as a corrosion inhibitor). Nitrogen tetroxide is less corrosive than nitric acid and provides better performance, but it has a higher freezing point. Consequently, nitrogen tetroxide is usually the oxidizer of choice when freezing point is not an issue, however, the freezing point can be lowered with the introduction nitric oxide. The resulting oxidizer is called mixed oxides of nitrogen (MON). The number included in the description, e.g. MON-3 or MON-25, indicates the percentage of nitric oxide by weight. While pure nitrogen tetroxide has a freezing point of about -9 oC, the freezing point of MON-3 is -15 oC and that of MON-25 is -55 0C.
The Titan family of launch vehicles and the second stage of the Delta II rocket use NTO/Aerozine 50 propellant. NTO/MMH is used in the orbital manoeuvring system (OMS) and reaction control system (RCS) of the Space Shuttle orbiter. IRFNA/UDMH is often used in tactical missiles such as the US Army’s Lance (1972-91).
Hydrazine is also frequently used as a monopropellant in catalytic decomposition engines. In these engines, a liquid fuel decomposes into hot gas in the presence of a catalyst. The decomposition of hydrazine produces temperatures up to about 1,100 oC and a specific impulse of about 230 or 240 seconds. Hydrazine decomposes to either hydrogen and nitrogen, or ammonia and nitrogen.
Solid Propellants
Of all the rocket designs solid propellant motors are the simplest. A typical rocket consisting of a casing, most commonly steel, filled with a mixture of fuel and oxidiser that react (burn) at a rapid rate. This ejects hot gases from a nozzle to produce thrust. A major difference between solid and liquid fuel rockets is that once ignited, a solid propellant motor cannot be shut down. Once the oxidiser and fuel have been ignited they will burn until all the propellant is exhausted. The fuel burns from the centre towards the sides of the casing. The shape of the centre channel can be used to control the thrust of the burn as it determines the rate and pattern once ignited. Similar to liquid propellants there are more than one groups of possible fuels : homogeneous and composite. Both types are stable and dense which makes them easily storable.
Homogeneous propellants consist of a simple base, or double base. Simple base propellants use a single compound, typically chemicals similar to that of nitrocellulose due to its capacity to be oxidised and reduced. Double based propellants also commonly consist of nitrocellulose but also contain nitroglycerine.