The following are some of the real pictures of scramjet, tested by the ISRO some months ago mounted on a Rohini Rocket. This is a brief description of scramjet based on these pictures. We are preparing a detailed article about different propulsive devices including scramjet, so followers will get more information about scramjet in the coming days.
What is a scramjet?
Scramjet, or Supersonic Combustion Ramjet is a kind of air breathing (It sucks the air from atmosphere) propulsion. In scramjet the combustion taking place at supersonic speeds while in other air breathing propulsive devices, combustion occurs at subsonic speeds. Scramjets are simple devices compared to jet engines which also are air breathing propulsion. Unlike jet engines scramjets doesn’t have any moving parts to compress the incoming air that makes scramjets less complex than the jet engines.
Scramjet typically works well in hypersonic speeds and is not suitable for supersonic and subsonic speeds. For supersonic speeds ramjets are the best option. In ramjets the intake decelerates the incoming air to subsonic speeds and then ignites the subsonic compressed air to supersonic speeds. But in scramjets the combustion take place at supersonic speeds so does the exhaust gases accelerate to hypersonic speeds.
Scramjets generally have a converging inlet (a duct that has a decreasing cross section in the direction of fluid flow is convergent, until a minimum area is reached) where incoming air is compressed and decelerated Unlike a typical jet engine, such as a turbojet or turbofan engine, a scramjet does not use rotating, fan-like components to compress the air; rather, the achievable speed of the vehicle moving through the atmosphere causes the Air intake to create shock waves these shockwaves compress the air, and we know scramjets works at hypersonic speeds air intake decelerates the air speed to supersonic speed. For Dual Mode Ramjet we use variable geometry inlets so that it can compress air at both supersonic and hypersonic speeds. A dual mode ramjet (DMRJ) is a type of jet engine where a ramjet transforms into scramjet over Mach 4-8 range, which means it can efficiently operate both in subsonic and supersonic combustor modes.
Cowl act as a door it usually be closed at subsonic & supersonic speeds. Cowl only open at hypersonic speed and allows the air to enter into the combustor. The opening of cowl is done through a pyro igniter & pyro actuator.
Pyro igniter & Pyro actuator
When the air vehicle reaches at hypersonic speeds sensors in the system bay detects and then sends signals to pyro igniter. Pyro igniter (it’s a kind of controlled blast through a chemical reaction) starts the pyro actuator working. Pyro actuator opens the cowl and when the cowl opens up fully a lock system permanently locks the cowl in position.
The combustor chamber is a duct where the combustion between free stream air and fuel takes place. In scramjet combustion is taking place at supersonic speeds. Supersonic combustion is very difficult to maintain. The main challenge of making a scramjet work is properly mixing the high-speed air with fuel while combusting and expanding that mixture before it exits the tail of the vehicle. The largest problem associated with combustion is the mixing between free stream air and fuel. If fuel cannot be properly injected and mixed into the air stream it will not ignite. This process typically occurs in less than 1 millisecond (0.001 seconds). Furthermore, the scramjet must burn enough fuel to generate an enormous amount of energy needed to overcome the tremendous drag forces experienced when flying at hypersonic speeds.
Hydrogen is commonly used as fuel in scramjets. In combustor fuel injector sprays hydrogen to the incoming air. Hydrogen has some particular advantages such as lower heating value; highly flammable, no hazardous emissions such as CO2, CO etc so hydrogen is a perfect choice for scramjets as a fuel.
Subsystem bay contains various sensors and telemetry systems to monitor the test and to send the stats and values to the ground station. And it also carries different sensors to monitor velocity, heat, pressure etc.
Energizer works like an afterburner of aircraft. It gives extra energy to the exhaust gas and thus improves the overall thrust.
Here Exhaust Nozzle is a diverging nozzle, where the heated air is accelerated to produce thrust. This nozzle is designed to produce maximum thrust.
Titanium alloys are the main contributor in scramjet. Because of the thrust-to-weight ratio of a scramjet being low compared to modern rockets, the scramjet needs more time to accelerate. Such a depressed trajectory implies that the vehicle stays a long time in the atmosphere at hypersonic speeds, causing atmospheric friction to become a problem. This is not only for space launch applications but also in missile or commercial transport applications. Heat addition produced by the combustion at these high velocities and temperatures is another significant factor to take into account. Therefore, the materials chosen for the structure must have good properties and be adequate in front of these phenomena. Titanium alloys are perfect for this
Other thing to be noted is that the scramjet engine must be integrated with the fuselage of the air vehicle, specially the air inlet and the nozzle. Part of the forebody of fuselage makes the function of air inlet compressing the free stream air, and similarly, the aftbody acts as a nozzle expanding the gases from the combustion. This design is called airframe – integrated scramjet.
Today, satellites are launched into orbit by multi-staged satellite launch vehicles that can be used only once (expendable). These launch vehicles carry oxidiser along with the fuel for combustion to produce thrust. Launch vehicles designed for one time use are expensive and their efficiency is low because they can carry only 2-4% of their lift-off mass to orbit. Thus, there is a worldwide effort to reduce the launch cost.
Nearly 70% of the propellant (fuel-oxidiser combination) carried by today’s launch vehicles consists of oxidiser. Therefore, the next generation launch vehicles must use a propulsion system which can utilise the atmospheric oxygen during their flight through the atmosphere which will considerably reduce the total propellant required to place a satellite in orbit.
Also, if those vehicles are made re-usable, the cost of launching satellites will further come down significantly. Thus, the future re-usable launch vehicle concept along with air-breathing propulsion is an exciting candidate offering routine access to space at far lower cost.
Scramjet can also use in ISRO’s reusable launch vehicle platform, called Avatar, is a concept that is capable of carrying out satellite launches – takes off vertically and lands back on a runway. The spacecraft is designed to use ramjets and scramjets for thrust. Each of these engines will be used in different stages of the flight – with ramjet used at lower speeds, scramjet at hypersonic speeds and cryogenic engines when the craft reaches the edge of the atmosphere.