Cryogenic rocket engine and propellents

Rocket engine

A rocket engine, or simply "rocket", is a jet engine[1] that uses only propellant mass for forming

its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law. Since they need no external material to form their jet, rocket engines can be used for spacecraft propulsion as well as terrestrial uses, such as missiles. Most rocket engines are internal combustion engines, although non combusting forms also exist.

Rocket engines as a group have the highest exhaust velocities, are by far the lightest, but are the least propellant efficient of all types of jet engines.

Types of rocket engines

Rocket motor (or solid-propellant rocket motor) is a synonymous term with rocket engine that usually refers to solid rocket engines.

Liquid rockets (or liquid-propellant rocket engine) use one or more liquid propellants that are held in tanks prior to burning.

Hybrid rockets have a solid propellant in the combustion chamber and a second liquid or gas propellant is added to permit it to burn.

Thermal rockets are rockets where the propellant is inert, but is heated by a power source such as solar or nuclear power or beamed energy.

Monopropellant rockets are rockets where the propellant is one chemical, typically hi-test (85%+) hydrogen peroxide, which is decomposed by a catalyst producing steam and oxygen. There is no flame.

Liquid-propellant rocketA liquid-propellant rocket or a liquid rocket is a rocket engine that uses propellants in liquid form. Liquids are desirable because their reasonably high density allows the volume of the propellant tanks to be relatively low, and it is possible to use lightweight pumps to pump the propellant from the tanks into the engines, which means that the propellants can be kept under low pressure. This permits the use of low mass propellant tanks, permitting a high mass ratio for the rocket.

Liquid PropellantsIn a liquid propellant rocket, the fuel and oxidizer are stored in separate tanks, and are fed through a system of pipes, valves, and turbopumps 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.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 which 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.

Cryogenic rocket engineA cryogenic rocket engine is a rocket engine that uses a cryogenic fuel or oxidizer, that is, its fuel or oxidizer (or both) are gases liquefied and stored at very low temperatures.It is a type of liquid propellent rocket engine.

Cryogenic Propellants

In a cryogenic propellant the fuel and the oxidizer are in the form of very cold, liquefied gases. These liquefied gases are referred to as super cooled as they stay in liquid form even though they are at a temperature lower than the freezing point. Thus we can say that super cooled gases used as liquid fuels are called cryogenic fuels.

Cryogenic Engines are rocket motors designed for liquid fuels that have to be held at very low "cryogenic" temperatures to be liquid - they would otherwise be gas at normal temperatures. Typically Hydrogen and Oxygen are used which need to be held below 20°K (-423°F) and 90°K (-297°F) to remain liquid.

The Space Shuttle's main engines used for liftoff are cryogenic engines. The Shuttle's smaller thrusters for orbital manuvering use non-cyogenic hypergolic fuels, which are compact and are stored at warm temperatures. Currently, only the United States, Russia, China, France, Japan and India have mastered cryogenic rocket technology.The use of liquid fuel rocket engines was first considered by the German, American and Soviet engineers independently, and all discovered that rocket engines needed high mass flow rate for both liquid oxidizer and fuel, for generating the necessary thrust. Higher thrust levels were achieved when liquid oxygen (LOX) and liquid hydrocarbon were used as fuel.

At atmospheric conditions, liquid oxygen and low molecular weight hydrocarbons are in gaseous state and to get required mass flow rate, the only option is to feed them to the engine in liquid form. For that, these are stored in liquid form by cooling them down and hence the name cryogenic rocket engines. Various cryogenic fuels combination were tried and the liquid oxygen oxidizer and the liquid hydrogen (LH2) fuel combination caught special attention of engineers as: both components are easily and cheaply available, bio-friendly, non-corrosive and have the highest entropy release by combustion, among all non-toxic pairs. Liquefaction temperature of oxygen is 89 kelvins and at this temperature liquid oxygen achieves a density of 1,140 kg/m3 (1.14 g/cm3). And, for hydrogen it is 20 kelvins, a few kelvins above absolute zero, and gains a density of 70 kg/ m3 (70 mg/cm3). All cryogenic rocket engines work on expander cycle or gas-generator cycle or staged combustion cycle depending on thrust requirement, since the oxidizer and fuel are at sub-zero temperatures. LOX LH2 cryogenic rocket engines produce specific impulse up to 450 s (4.4 kN·s/kg).Construction

The major components of a cryogenic rocket engine are: The thrust chamber or combustion chamber, Gas turbine,

The combustion chamber in gas turbines and jet engines (including ramjets and scramjets) is called the combustor.

The combustor is fed high pressure air by the compression system, adds fuel and burns the mix and feeds the hot, high pressure exhaust into the turbine components of the engine or out the exhaust nozzle.

Fuel injector,

The injector implementation in liquid rockets determines the percentage of the theoretical performance of the nozzle that can be achieved. A poor injector performance causes unburnt propellant to leave the engine, giving extremely poor efficiency. The injector, as the name implies, injects the propellants into the combustion chamber in the right proportions and the right conditions to yield an efficient, stable combustion process. Placed at the forward, or upper, end of the combustor, the injector also performs the structural task of closing off the top of the combustion chamber against the high pressure and temperature it contains. The injector has been compared to the carburetor of an automobile engine, since it provides the fuel and oxidizer at the proper rates and in the correct proportions, this may be an appropriate comparison. However, the injector, located directly over the high-pressure combustion, performs many other functions related to the combustion and cooling processes and is much more important to the function of the rocket engine than the carburetor is for an automobile engine.

Additionally, injectors are also usually key in reducing thermal loads on the nozzle; by

increasing the proportion of fuel around the edge of the chamber, this gives much lower temperatures on the walls of the nozzle.

Fuel turbopumps,

A turbopump is a gas turbine that comprises basically two main components: a rotodynamic pump and a driving turbine, usually both mounted on the same shaft, or sometimes geared together. The purpose of a turbopump is to produce a high pressure fluid for feeding a combustion chamber or other use.

A turbopump can comprise one of two types of pumps: centrifugal pump, where the pumping is done by throwing fluid outward at high speed; or axial flow pump, where alternating rotating and static blades progressively raise the pressure of a fluid.

Pyrotechnic igniter, A pyrotechnic initiator (also initiator or igniter) is a device containing a pyrotechnic composition used primarily to ignite other, more difficult-to-ignite materials, e.g. thermites, gas generators, and solid-fuel rockets. The name is often used also for the compositions themselves.

Cryo valves, Regulators, The fuel tanks and

Rocket engine nozzle. A rocket engine nozzle is a propelling nozzle (usually of the de Laval type) used in

a rocket engine to expand and accelerate the combustion gases produced by burning propellants so that the exhaust gases exit the nozzle at hypersonic velocities.

The fuel flow can be differentiated into a main flow or a bypass flow configuration. In the main flow design, the entire fuel is fed through the gas turbines, which intern drive the cryopump for fuel and oxidizer, and then injected to the combustion chamber. In the bypass configuration, the fuel flow is split, the main part is goes to the combustion chamber to generate thrust, while a small amount of the fuel goes to the turbine, to drive the cryopumps for fuel and oxidizer and is subsequently injected to combustion chamber.

Rocket engine can generates 5,000 degree steam and 13,800 pounds of thrust form icicles at the rim of its nozzle. It's cryogenic. The Common Extensible Cryogenic Engine (CECE) has completed its third round of intensive testing. This technology development engine is fueled by a mixture of -297°F liquid oxygen and -423°F liquid hydrogen.

The engine components are super-cooled to similar low temperatures. As CECE burns its frigid fuels, gas composed of hot steam is produced and propelled out the nozzle creating thrust. The steam is cooled by the cold engine nozzle, condensing and eventually freezing at the nozzle exit to form icicles. Using liquid hydrogen and oxygen in rockets will provide major advantages for landing astronauts on the moon. Hydrogen is very light but enables about 40% greater performance (force on the rocket per pound of propellant) than other rocket fuels. Therefore, NASA can use this weight savings to bring a bigger spacecraft with a greater payload to the moon than with the same amount of conventional propellants. CECE is a step forward in NASA's efforts to develop reliable, robust technologies to return to the moon, and a winter wonder.

Working

cryogenic rocket engines (or, generally, all liquid-propellant engines) work in either an expander cycle, a gas-generator cycle, a staged combustion cycle, or the simplest pressure-fed cycle.

Pressure-fed cycle (rocket)

Pressure-fed rocket cycle. Propellant tanks are pressurized to supply fuel and oxidizer to the engine, eliminating

the need for turbopumps.

The pressure-fed cycle is a class of rocket engine designs. A separate gas supply, usually helium, pressurizes the propellant tanks to force fuel and oxidizer to the combustion chamber. To maintain adequate flow, the tank pressures must exceed the combustion chamber pressure.

Pressure fed engines have simple plumbing and lack complex and often unreliable turbopumps. A typical startup procedure begins with opening a valve, often a one-shot pyrotechnic device, to allow the pressurizing gas to flow through check valves into the propellant tanks. Then the propellant valves in the engine itself are opened. If the fuel and oxidizer are hypergolic, they burn on contact; non-hypergolic fuels require an igniter. Multiple burns can be conducted by merely opening and closing the propellant valves as needed. They can be operated electrically, or by gas pressure controlled by smaller electrically operated valves.

Care must be taken, especially during long burns, to avoid excessive cooling of the pressurizing gas due to adiabatic expansion. Cold helium won't liquify, but it could freeze a propellant, decrease tank pressures, or damage components not designed for low temperatures. The Apollo Lunar Module descent propulsion system was unusual in storing its helium in a supercritical but very cold state. It was warmed as it was withdrawn through a heat exchanger from the ambient temperature fuel.

Pressure-fed engines have practical limits on propellant pressure, which in turn limits combustion chamber pressure. High pressure propellant tanks require thicker walls and stronger alloys which make the vehicle tanks heavier. Thereby reducing performance and payload capacity. The lower stages of launch vehicles often use solid fuel and pump-fed liquid fuel engines instead, where high pressure ratio nozzles are considered desirable.[citation needed]

Expander cycle (rocket)From Wikipedia, the free encyclopedia

  (Redirected from Expander cycle)

Expander rocket cycle. Expander rocket engine (closed cycle). Heat from the nozzle and combustion chamber

powers the fuel and oxidizer pumps.

The expander cycle is a power cycle of a bipropellant rocket engine meant to improve the efficiency of fuel delivery.

In an expander cycle, the fuel is heated before it is combusted, usually with waste heat from the main combustion

chamber. As the liquid fuel passes through coolant passages in the walls of the combustion chamber, it undergoes

a phase change into a gaseous state. The fuel in the gaseous state expands through a turbine using the pressure

differential from the supply pressure to the ambient exhaust pressure to initiate turbopump rotation. This can provide a

bootstrap starting capability as is used on the Pratt & Whitney RL10 engine. This bootstrap power is used to

drive turbines that drive the fuel and oxidizer pumps increasing the propellant pressures and flows to the rocket

engine thrust chamber. After leaving the turbine(s), the fuel is then injected with the oxidizer into the combustion chamber

and burned to produce thrust for the vehicle.

Because of the necessary phase change, the expander cycle is thrust limited by the square-cube rule. As the size of a

bell-shaped nozzle increases with increasing thrust, the nozzle surface area (from which heat can be extracted to expand

the fuel) increases as the square of the radius. However, the volume of fuel that must be heated increases as the cube of

the radius. Thus there exists a maximum engine size of approximately 300 kN of thrust beyond which there is no longer

enough nozzle area to heat enough fuel to drive the turbines and hence the fuel pumps. Higher thrust levels can be

achieved using a bypass expander cycle where a portion of the fuel bypasses the turbine and or thrust chamber cooling

passages and goes directly to the main chamber injector. Aerospike engines do not suffer from the same limitations

because the linear shape of the engine is not subject to the square-cube law. As the width of the engine increases, both

the volume of fuel to be heated and the available thermal energy increase linearly, allowing arbitrarily wide engines to be

constructed. All expander cycle engines need to use a cryogenic fuelsuch as hydrogen, methane, or propane that easily

reach their boiling points.

Some expander cycle engine may use a gas generator of some kind to start the turbine and run the engine until the heat

input from the thrust chamber and nozzle skirt increases as the chamber pressure builds up.

In an open cycle, or "bleed" expander cycle, only some of the fuel is heated to drive the turbines, which is then vented to

atmosphere to increase turbine efficiency. While this increases power output, the dumped fuel leads to a decrease in

propellant efficiency (lower engine specific impulse). A closed cycle expander engine sends the turbine exhaust to the

combustion chamber (see image at right.)

Advantages

The expander cycle has a number of advantages over other designs:[citation needed]

Low temperature. The advantage is that after they have turned gaseous, the fuels are usually near room temperature, and do very little or no damage to the turbine, allowing the engine to be reusable. In contrast Gas-generator or Staged combustion engines operate their turbines at high temperature.

Tolerance. During the development of the RL10 engineers were worried that insulation foam mounted on the inside of the tank might break off and damage the engine. They tested this by putting loose foam in a fuel tank and running it through the engine. The RL10 chewed it up without problems or noticeable degradation in performance. Conventional gas-generators are in

practice miniature rocket engines, with all the complexity that implies. Blocking even a small part of a gas generator can lead to a hot spot, which can cause violent loss of the engine. Using the engine bell as a 'gas generator' also makes it very tolerant of fuel contamination because of the wider fuel flow channels used.

Inherent safety. Because a bell-type expander-cycle engine is thrust limited, it can easily be designed to withstand its maximum thrust conditions. In other engine types, a stuck fuel valve or similar problem can lead to engine thrust spiraling out of control due to unintended feedback systems. Other engine types require complex mechanical or electronic controllers to ensure this does not happen. Expander cycles are by design incapable of malfunctioning that way.

Gas-generator cycle (rocket)From Wikipedia, the free encyclopedia

Gas generator rocket cycle. Some of the fuel and oxidizer is burned separately to power the pumps and then discarded.

Most Gas-generator engines use the fuel for nozzle cooling.

The gas generator cycle is a power cycle of a bipropellant rocket engine. Some of the propellant is burned in a gas-generator and the

resulting hot gas is used to power the engine's pumps. The gas is then exhausted. Because something is "thrown away" this type of

engine is also known as open cycle.

There are several advantages to the gas generator cycle over its counterpart, the staged combustion cycle. The gas generator turbine

does not need to deal with the counter pressure of injecting the exhaust into the combustion chamber. This simplifies plumbing and

turbine design, and results in a less expensive and lighter engine.

The main disadvantage is lost efficiency due to discarded propellant. Gas generator cycles tend to have lower specific impulse than

staged combustion cycles.[citation needed]

As in most cryogenic rocket engines, some of the fuel in a gas-generator cycle is used to cool the nozzle and combustion chamber.

Current construction materials cannot stand extreme temperatures of rocket combustion processes by themselves. Cooling permits the

use of rocket engines for relatively longer periods of time with today’s material technology. Without rocket combustion chamber and

nozzle cooling, the engine would fail catastrophically.[1]

Staged combustion cycle (rocket)

Staged combustion rocket cycle. Usually, all of the fuel and a portion of the oxidizer are fed through the pre-burner

(fuel rich) to power the pumps. An oxygen rich circuit is possible also, but less common because of the metallurgy

required.

The staged combustion cycle, also called topping cycle or pre-burner cycle,[1] is a thermodynamic cycle

of bipropellant rocket engines. Some of the propellant is burned in a pre-burner and the resulting hot gas is used to power

the engine's turbines and pumps. The exhausted gas is then injected into the main combustion chamber, along with the

rest of the propellant, and combustion is completed.

The advantage of the staged combustion cycle is that all of the engine cycles' gases and heat go through the combustion

chamber, and overall efficiency essentially suffers no pumping losses at all. Thus this combustion cycle is often called

closed cycle since the cycle is closed as all propellant products go through the chamber; as opposed to open cycle which

dumps the turbopump driving gases, representing a few percent of loss.

Another very significant advantage that staged combustion gives is an abundance of power which permits very high

chamber pressures. Very high chamber pressures mean high expansion ratio nozzles can be used, whilst still giving

ambient pressures at takeoff. These nozzles give far better efficiencies at low altitude.

The disadvantages of this cycle are harsh turbine conditions, that more exotic plumbing is required to carry the hot gases,

and that a very complicated feedback and control design is necessary. In particular, running the full oxidizer stream

through both a pre-combustor and main-combustor chamber (oxidizer-rich staged combustion) produces extremely

corrosive gases. Thus most staged-combustion engines are fuel-rich, as in the schematic.

Staged combustion engines are the most difficult types of rocket engines to design. A simplified version is called the gas-

generator cycle.

Power CyclesLiquid bipropellant rocket engines can be categorized according to their power cycles, that is, how power is derived to feed propellants to the main combustion chamber. Described below are some of the more common types.Gas-generator cycle: The gas-generator cycle, also called open cycle, taps off a small amount of fuel and oxidizer from the main flow (typically 3 to 7 percent) to feed a burner called a gas generator. The hot gas from this generator passes through a turbine to generate power for the pumps that send propellants to the combustion chamber. The hot gas is then either dumped overboard or sent into the main nozzle downstream. Increasing the flow of propellants into the gas generator increases the speed of the turbine, which increases the flow of propellants into the main combustion chamber, and hence, the amount of thrust produced. The gas generator must burn propellants at a less-than-optimal mixture ratio to keep the temperature low for the turbine blades. Thus, the cycle is appropriate for moderate power requirements but

not high-power systems, which would have to divert a large portion of the main flow to the less efficient gas-generator flow.As in most rocket engines, some of the propellant in a gas generator cycle is used to cool the nozzle and combustion chamber, increasing efficiency and allowing higher engine temperature.

Staged combustion cycle: In a staged combustion cycle, also called closed cycle, the propellants are burned in stages. Like the gas-generator cycle, this cycle also has a burner, called a preburner, to generate gas for a turbine. The preburner taps off and burns a small amount of one propellant and a large amount of the other, producing an oxidizer-rich or fuel-rich hot gas mixture that is mostly unburned vaporized propellant. This hot gas is then passed through the turbine, injected into the main chamber, and burned again with the remaining propellants. The advantage over the gas-generator cycle is that all of the propellants are burned at the optimal mixture ratio in the main chamber and no flow is dumped overboard. The staged combustion cycle is often used for high-power applications. The higher the chamber pressure, the smaller and lighter the engine can be to produce the same thrust. Development cost for this cycle is higher because the high pressures complicate the development process. Further disadvantages are harsh turbine conditions, high temperature piping required to carry hot gases, and a very complicated feedback and control design.Staged combustion was invented by Soviet engineers and first appeared in 1960. In the West, the first laboratory staged combustion test engine was built in Germany in 1963.Expander cycle: The expander cycle is similar to the staged combustion cycle but has no preburner. Heat in the cooling jacket of the main combustion chamber serves to vaporize the fuel. The fuel vapor is then passed through the turbine and injected into the main chamber to burn with the oxidizer. This cycle works with fuels such as

hydrogen or methane, which have a low boiling point and can be vaporized easily. As with the staged combustion cycle, all of the propellants are burned at the optimal mixture ratio in the main chamber, and typically no flow is dumped overboard; however, the heat transfer to the fuel limits the power available to the turbine, making this cycle appropriate for small to midsize engines. A variation of the system is the open, or bleed, expander cycle, which uses only a portion of the fuel to drive the turbine. In this variation, the turbine exhaust is dumped overboard to ambient pressure to increase the turbine pressure ratio and power output. This can achieve higher chamber pressures than the closed expander cycle although at lower efficiency because of the overboard flow.

Pressure-fed cycle: The simplest system, the pressure-fed cycle, does not have pumps or turbines but instead relies on tank pressure to feed the propellants into the main chamber. In practice, the cycle is limited to relatively low chamber pressures because higher pressures make the vehicle tanks too heavy. The cycle can be reliable, given its reduced part count and complexity compared with other systems.

Advantages

The cryogenic engines uses liquid nitrogen as the fuel and the exhaust is also nitrogen. Since nitrogen is present nearly 78% in our atmosphere, the engine is non pollutant. whereas in the case of normal IC engine, the exhaust is carbon monoxide, CO2 and other harmful gases. Efficiency is higher in the case of cryogenic engine than that of petrol engines. High Energy per unit massPropellants like oxygen and hydrogenin liquid form give very high amounts of energy per unit mass due to which the amount of fuel to be carried aboard the rockets decreases.    

Clean FuelsHydrogen and oxygen are extremely clean fuels. When they combine, they give out only water. This water is thrown out of the nozzle in form of very hot vapor. Thus the rocket is nothing but a high burning steam engine.  

EconomicalUse of oxygen and hydrogen as fuels is very economical, as liquid oxygen costs less than gasoline.  Cryogenic propellants suffer from certain drawbacks. Let us see what these drawbacks are and how they can be overcomed.

Drawbacks of Cryogenic PropellantsBoil off RateSince these propellants are in extremely low temperature conditions they are very hard to handle. They must be protected from heat so as to prevent boiling of gases. When liquid propellants are stored at temperatures above their boiling point they vaporize. If these vapors are contained in a tank, then the pressure increases with temperature.Since the tank weight increases with design pressure, a pressure relief valve is generally provided to prevent the tank from over pressurizing and exploding. When the relief valvereleases the pressure, some of the propellant escapes from the tank. This lost propellant is referred to as boil-off loss. and proves of no use for propulsion.For liquid hydrogen the boil off rate per day is 0.127% while that of liquid oxygen is 0.016%. Thus the boil of rates of liquid hydrogen per month is 3.81% while that of liquid oxygen is 0.49%. Boil off rate is governed by heat leakage and by the amount of propellant in the tanks. With partly filled tanks, the percentage loss is higher. Heat leakage depends on surface area, while the original mass of propellant in the tanks depends on volume. So by square/cube law smaller the tank, the faster the liquids will boil off.To lower the boil off rates, we must protect the propellant tank from the heat of the sun. Tanks at more distance from the sun have lower boil off rates. Lower boil off rates can also be ensured if tanks are maintained at an angle by which their cross sectional area exposed to sun is minimum or tanks are protected by a sunshade.

Highly reactive gasesCryogens are highly concentrated gases and have a very high reactivity. Liquid oxygen, which is used as an oxidizer, combines with most of the

organic materials to form explosive compounds. So lots of care must be taken to ensure safety.

LeakageOne of the most major concerns is leakage. At cryogenic temperatures, which are roughly below 150 degrees Kelvin or equivalently (-190oF), the seals of the container used for storing the propellants lose the ability to maintain a seal properly. Hydrogen, being the smallest element, has a tendency to leak past seals or materials.Hydrogen can burst into flames whenever its concentration is approximately 4% to 96%. It is hence necessary to ensure that hydrogen leak rate is minimal and does not present a hazard. Also there must be some way of determining the rates of leakage and checking whether a fire hazard exists or not. The compartments where hydrogen gas may exist in case of a leak must be made safe, so that the hydrogen buildup does not cause a hazardous condition.

Hydrogen EmbrittlementDue to cryogenic propellants, various significant thermal stresses are introduced into the launch vehicle. These stresses can damage the structural integrity of the vehicle. Hydrogenreacts with certain materials to alter its grain structure causing it to become brittle, in a process known as hydrogen embrittlement

Zero Gravity conditionsIt is difficult to store cryogenic propellants in zero gravity conditions. Generally, the propellants are cooled continuously, allowing excess heat to be carried away as the gasses boil off. The vapors of the gas are vented away. But in zero gravity conditions, the liquid may flash into vapor, and go away from the nozzle that dumps the gas overboard. This pushes the propellant in liquid form, out of the nozzle resulting in wastage of large amounts of fuel into space.The cryogenic propellants certainly have their own disadvantages. But their advantages outweigh the disadvantages by far. Thus they are preferred for use in rockets.The next liquid propellant is hypergolic propellant.