Thursday, June 22, 2017

Methods of Propulsion: Ramjet Engines

Ramjet engines are one of the more rare forms of propulsion employed in the world of aviation. They are a form of airbreathing jet engine which do not employ a fan to push air into the engine; instead, the speed of the aircraft is itself used to force, or 'ram' air into the engine, hence the name. Ramjets also go by other names such as flying stovepipes or athodyds (an abbreviation of aero thermodynamic duct).

Since ramjets use the speed of the aircraft itself to push air into the engine, they cannot start a plane up from a standstill. Indeed, they are extremely inefficient at low speeds, and attain maximum efficiency only at speeds exceeding Mach 3. Thus, another form of propulsion is required to get the craft to that speed before the ramjet can take over. The jets can work at speeds of up to Mach 6.


Image result for ramjet diagramA ramjet uses the high pressure generated in front of a craft moving through the air at very high speeds to force air into the engine. A spiked protrusion at the front of the engine compresses the air, heating it and slowing it down to subsonic speeds.

Once inside, some of that air is combined with fuel and burned, producing exhaust gases. These exhaust gases are channeled into a narrow stream by a exhaust nozzle, accelerating them to supersonic speeds, which pushes the ramjet forward, courtesy of Newton's Laws. (Above-Right): A diagram of a ramjet engine, with the speeds of airflow in different section given in Machs.

Parts of a conventional Ramjet

A ramjet engine consists of three main parts: the inlet/diffuser, the combustor, and the nozzle/s. 

The inlet/diffuser's job is to slow down incoming supersonic air to subsonic speeds so that it can be burned with fuel. The spiked tip of the ramjet (known as the innerbody) creates a conical shock wave in front of it - remember, it is always travelling faster than sound - and this forces more air into the space in between the innerbody and the tube, compressing it and slowing it down, although not to subsonic speeds. Once past the innerbody, the less-supersonic air again forms a shock wave inside the engine which finally slows it down to subsonic speeds. (Note that subsonic ramjets do not need such fancy equipment and have a simple hole as the inlet (without any innerbody), as the air is already subsonic.)

Once inside and slowed-down, the air is mixed with fuel, sprayed by pumps, and burned. A structure known as a flame holder prevents the flame from being blown out by the still-very-fast air. The structure of the flame holder may vary, and some designs use other methods to achieve flame stabilization.

Image result for convergent nozzle
The nozzle is the final part of a ramjet. Its purpose is to increase the velocity of the exhaust gases produced by the combustion of fuel. For subsonic ramjets, a simple convergent nozzle will do the trick, whereas for supersonic ones, a more complicated convergent-divergent nozzle is required. (Right): A diagram of a convergent and convergent-divergent nozzle.

Uses of Ramjets

Ramjets are used by a few aircraft and a good number of missiles. The SR-71 Blackbird is one of the more famous planes which employs turbojet-ramjet engines, which act like normal turbojets at subsonic speeds but then convert to ramjets at speeds exceeding Mach 1. You can learn more about the SR-71 here: 

(Below): A SR-71 Blackbird. One can clearly see the spiked innerbody of the turbojet-ramjet engines which propel this aircraft.

Image result for sr-71 blackbird

Sunday, June 5, 2016

Snippet: Rocket Engines (10 on n)

Mechanical issues

Rocket combustion chambers are normally operated at fairly high pressure, typically 10-200 bar (1 to 20 MPa, 150-3000 psi). 

When operated within significant atmospheric pressure, higher combustion chamber pressures give better performance by permitting a larger and more efficient nozzle to be fitted without it being grossly over expanded.

However, these high pressures cause the outermost part of the chamber to be under very large hoop stresses – rocket engines are pressure vessels.

Worse, due to the high temperatures created in rocket engines the materials used tend to have a significantly lowered working tensile strength.

In addition, significant temperature gradients are set up in the walls of the chamber and nozzle, these cause differential expansion of the inner liner that create internal stresses.

Acoustic issues

The extreme vibration and acoustic environment inside a rocket motor commonly result in peak stresses well above mean values, especially in the presence of organ pipe-like resonances and gas turbulence.

Source: Wikipedia

Sunday, May 29, 2016

Snippet: Rocket Engines (9 of n)

For efficiency reasons, and because they physically can, rockets run with combustion temperatures that can reach ~3500 K (~3227 °C or ~5840 °F).
Temperatures in rockets are very often far higher than the melting point of the nozzle and combustion chamber materials. Two exceptions are graphite and tungsten (~1200 K for copper), although both are subject to oxidation if not protected.
Indeed many construction materials can make perfectly acceptable propellants in their own right. It is important that these materials be prevented from combusting, melting or vaporising to the point of failure.
Materials technology could potentially place an upper limit on the exhaust temperature of chemical rockets.
Alternatively, rockets may use more common construction materials such as aluminium, steel, nickel or copper alloys and employ cooling systems that prevent the construction material itself becoming too hot.
In rockets the coolant methods include:
  1. uncooled (used for short runs mainly during testing)
  2. ablative walls (walls are lined with a material that is continuously vaporised and carried away).
  3. radiative cooling (the chamber becomes almost white hot and radiates the heat away)
  4. dump cooling (a propellant, usually hydrogen, is passed around the chamber and dumped)
  5. regenerative cooling (liquid rockets use the fuel, or occasionally the oxidiser, to cool the chamber via a cooling jacket before being injected)
  6. curtain cooling (propellant injection is arranged so the temperature of the gases is cooler at the walls)
  7. film cooling (surfaces are wetted with liquid propellant, which cools as it evaporates)

Source: Wikipedia

Sunday, May 22, 2016

Snippet: Rocket Engines (8 of n)

Thrust-to-weight ratio
Rockets, of all the jet engines, indeed of essentially all engines, have the highest thrust to weight ratio. This is especially true for liquid rocket engines.
This high performance is due to the small volume of pressure vessels that make up the engine—the pumps, pipes and combustion chambers involved. The lack of inlet duct and the use of dense liquid propellant allows the pressurisation system to be small and lightweight, whereas duct engines have to deal with air which has a density about one thousand times lower.

Jet or Rocket engineMass
Thrust-to-weight ratio
RD-0410 nuclear rocket engine2,0004,40035.27,9001.8
J58 jet engine (SR-71 Blackbird)2,7226,00115034,0005.2
Rolls-Royce/Snecma Olympus 593
turbojet with reheat (Concorde)
Pratt & Whitney F1191,8003,9009120,5007.95
RD-0750 rocket engine, three-propellant mode4,62110,1881,413318,00031.2
RD-0146 rocket engine2605709822,00038.4
SSME rocket engine (Space Shuttle)3,1777,0042,278512,00073.1
RD-180 rocket engine5,39311,8904,152933,00078.5
RD-170 rocket engine9,75021,5007,8871,773,00082.5
F-1 (Saturn V first stage)8,39118,4997,740.51,740,10094.1
NK-33 rocket engine1,2222,6941,638368,000136.7
Merlin 1D rocket engine440970690160,000159.9

Of the liquid propellants used, density is worst for liquid hydrogen. Although this propellant is marvellous in many ways, it has a very low density, about one fourteenth that of water. This makes the turbo pumps and pipework larger and heavier, and this is reflected in the thrust-to-weight ratio of engines that use it (for example the SSME) compared to those that do not (NK-33).
Source: Wikipedia

Thursday, May 19, 2016

Snippet: Rocket Engines (7 of n)

Back pressure and optimal expansion

For optimal performance the pressure of the gas at the end of the nozzle should just equal the ambient pressure: if the exhaust's pressure is lower than the ambient pressure, then the vehicle will be slowed by the difference in pressure between the top of the engine and the exit; on the other hand, if the exhaust's pressure is higher, then exhaust pressure that could have been converted into thrust is not converted, and energy is wasted.

To maintain this ideal of equality between the exhaust's exit pressure and the ambient pressure, the diameter of the nozzle would need to increase with altitude, giving the pressure a longer nozzle to act on (and reducing the exit pressure and temperature). This increase is difficult to arrange in a lightweight fashion, although is routinely done with other forms of jet engines. 

In rocketry a lightweight compromise nozzle is generally used and some reduction in atmospheric performance occurs when used at other than the 'design altitude' or when throttled. To improve on this, various exotic nozzle designs such as the plug nozzle, stepped nozzles, the expanding nozzle and the aerospike have been proposed, each providing some way to adapt to changing ambient air pressure and each allowing the gas to expand further against the nozzle, giving extra thrust at higher altitudes.

When exhausting into a sufficiently low ambient pressure (vacuum) several issues arise. One is the sheer weight of the nozzle—beyond a certain point, for a particular vehicle, the extra weight of the nozzle outweighs any performance gained. Secondly, as the exhaust gases expand within the nozzle they cool, and eventually some of the chemicals can freeze, producing 'snow' within the jet. This causes instabilities in the jet and must be avoided.

Source: Wikipedia

Tuesday, May 17, 2016

Snippet: Rocket Engines (6 of n)

Propellant efficiency
For a rocket engine to be propellant efficient, it is important that the maximum pressures possible be created on the walls of the chamber and nozzle by a specific amount of propellant; as this is the source of the thrust. This can be achieved by all of:
  • heating the propellant to as high a temperature as possible (using a high energy fuel, containing hydrogen and carbon and sometimes metals such as aluminium, or even using nuclear energy)
  • using a low specific density gas (as hydrogen rich as possible)
  • using propellants which are, or decompose to, simple molecules to maximise translational velocity
Since all of these things minimise the mass of the propellant used, and since pressure is proportional to the mass of propellant present to be accelerated as it pushes on the engine, and since from Newton's third law the pressure that acts on the engine also reciprocally acts on the propellant, it turns out that for any given engine the speed that the propellant leaves the chamber is unaffected by the chamber pressure (although the thrust is proportional).
However, speed is significantly affected by all three of the above factors and the exhaust speed is an excellent measure of the engine propellant efficiency. This is termed exhaust velocity, and after allowance is made for factors that can reduce it, the effective exhaust velocity is one of the most important parameters of a rocket engine (although weight, cost, ease of manufacture etc. are usually also very important).
For aerodynamic reasons the flow goes sonic at the narrowest part of the nozzle, the 'throat'. Since the speed of sound in gases increases with the square root of temperature, the use of hot exhaust gas greatly improves performance. By comparison, at room temperature the speed of sound in air is about 340 m/s while the speed of sound in the hot gas of a rocket engine can be over 1700 m/s; much of this performance is due to the higher temperature, but additionally rocket propellants are chosen to be of low molecular mass, and this also gives a higher velocity compared to air.
Expansion in the rocket nozzle then further multiplies the speed, typically between 1.5 and 2 times, giving a highly hypersonic exhaust jet. The speed increase of a rocket nozzle is mostly determined by its area expansion ratio—the ratio of the area of the throat to the area at the exit, but detailed properties of the gas are also important. Larger ratio nozzles are more massive but are able to extract more heat from the combustion gases, increasing the exhaust velocity.
Source: Wikipedia

Sunday, May 15, 2016

Snippet: Rocket Engines (5 of n)

Combustion Chamber
For chemical rockets the combustion chamber is typically just a cylinder. The dimensions of the cylinder are such that the propellant is able to combust thoroughly; different propellants require different combustion chamber sizes for this to occur.
The combination of temperatures and pressures typically reached in a combustion chamber is usually extreme by any standards. Unlike in air-breathing jet engines, no atmospheric nitrogen is present to dilute and cool the combustion, and the temperature can reach really high values. This, in combination with the high pressures, means that the rate of heat conduction through the walls is very high.
Rocket nozzles
The large bell or cone shaped expansion nozzle gives a rocket engine its characteristic shape. In rockets the hot gas produced in the combustion chamber is permitted to escape from the combustion chamber through an opening (the "throat").
When sufficient pressure is provided to the nozzle (about 2.5-3x above ambient pressure) the nozzle chokes and a supersonic jet is formed, dramatically accelerating the gas, converting most of the thermal energy into kinetic energy.
The exhaust speeds vary, depending on the expansion ratio the nozzle is designed to give, but exhaust speeds as high as ten times the speed of sound at sea level air are not uncommon.
About half of the rocket engine's thrust comes from the unbalanced pressures inside the combustion chamber and the rest comes from the pressures acting against the inside of the nozzle. As the gas expands (adiabatically) the pressure against the nozzle's walls forces the rocket engine in one direction while accelerating the gas in the other.
Source: Wikipedia