Why Isro's scramjet flight demonstration is significant

It will position the organisation as a major space fairing agency with the ability to offer services for low-cost access to space

The flight demonstration of the hypersonic air-breathing dual mode ramjet engine, which uses atmospheric oxygen in a portion of its journey, is a major step for Indian Space Research Organisation (Isro) in its pursuit for a future space transportation system.

An experimental mission to demonstrate supersonic combustion using atmospheric oxygen was conducted on Sunday. It was the maiden experiment of Isro's Dual Mode Ramjet (DMRJ) engine, which uses hydrogen as fuel and air from the atmosphere as the oxidiser, at hypersonic conditions.

The programme, along with another ongoing programme for developing a re-usable launch vehicle, would position the organisation as a major space fairing agency with technological competence to offer services for low-cost access to space in the long term.

 

The programme will help the organisation to reduce the number of stages in the current multi-staged expendable launch vehicles to two stages. Most of the current launch vehicles are designed for one time use and are expensive. Further, their efficiency in terms of payload to lift-off mass is low, of the order of 2-4 per cent. Current rocket technology offers little opportunity in this regard given that nearly 85 per cent of the lift-off mass is propellant.

Technologies to reduce weight and cost of space craft

There are ongoing worldwide efforts to reduce launch cost. In order to achieve low-cost access to space, the strategy has been to try and reduce propellant mass, reduce the rocket size to have a higher payload to lift-off mass ratio, and make the rocket re-usable.

When it comes to reducing propellant mass, nearly 70 per cent of the propellant mass is oxidiser. The next generation of launch vehicles will have to use propulsion systems which can utilise atmospheric oxygen during the atmospheric flight regime. This will reduce the total propellant required to place a satellite in orbit for lower stages.

While the ultimate goal is the development of a single-stage-to-orbit (SSTO) launch vehicle configuration, to finally make access to space a routine affair at comparatively lower cost than conventional expendable rockets, a fully rocket-powered SSTO is not feasible with existing material and propulsion technology. However, air breathing propulsion can largely improve the feasibility of a SSTO if the air breathing mode can be efficiently used on a very large flight Mach number range, for example from Mach 1.5 to Mach 12.

There are uncertainties related to air breathing mode performance and the existing material technology puts a question mark on the real feasibility of such a SSTO launcher in the near future. It could be relatively easy to develop a two-stages-to-orbit (TSTO) launcher with an air breathing first stage. The key technology to enable the operation of two stage and finally the SSTO launch vehicle is air breathing propulsion which can operate over a wide Mach number regime. Besides, if the first stage of the TSTO is made re-usable, the cost of launching satellites will come down by a magnitude two. The future combination of re-usable launch vehicles along with air-breathing propulsion, thus, would lead towards routine space missions at far lower cost.

Concepts of air breathing technology

One of the concepts for air breathing technology being studied worldwide is a ramjet. A ramjet is a form of air breathing jet engine that uses the vehicle's forward motion to compress incoming air for combustion without an axial compressor. Fuel is injected in the combustion chamber where it mixes with the hot compressed air and ignites. Ramjets cannot produce thrust at zero airspeed; they cannot move an aircraft from a standstill. A ramjet-powered vehicle, therefore, requires an assisted take-off like a rocket assist to accelerate it to a speed where it begins to produce thrust. The ramjet works best at supersonic speeds and as the speed enters the hypersonic range, its efficiency starts to drop.

Another concept is the scramjet. A scramjet engine is an improvement over the ramjet engine as it operates at hypersonic speeds and allows supersonic combustion, which gives it its name — supersonic combustion ramjet, or scramjet. The exhaust gases are then accelerated to hypersonic speeds using a divergent nozzle.

The scramjet is composed of three basic components — a converging inlet where incoming air is compressed, a combustor where gaseous fuel is burned with atmospheric oxygen to produce heat, and a diverging nozzle where the heated air is accelerated to produce thrust. Unlike a typical jet engine, a scramjet does not use rotating, fan-like components to compress the air. Instead, the speed at which the vehicle moves through the atmosphere causes the air to compress within the inlet. As such, no moving parts are needed in a scramjet, which reduces the weight and the number of failure points in the engine.

While scramjets are conceptually simple, actual implementation is limited by extreme technical challenges, including that when the vehicle is within the atmosphere, it will be at a higher temperature than the surrounding air and it would be similar to lighting a candle in a hurricane, as the fuel must be injected, mixed, ignited, and burned within milliseconds. Worldwide efforts are on to achieve powered flight using scramjet engines.

The third concept is a mix of ramjet and scramjet, which is called DMRJ. There is a need for an engine which can operate at both supersonic and hypersonic speeds. A DMRJ is an engine design where a ramjet transforms into a scramjet over Mach 4-8 range, which means, it can operate in both the subsonic and supersonic combustor mode.

Most of the research on air breathing engines is classified. Computational fluid dynamics (CFD) tools also have recently reached maturity levels to make reasonable computations in solving scramjet operation problems possible. However, it is yet to be perfected and as such, flight testing is the only tool available to master all the key technologies of scramjet propulsion and have an assessment of the integrated engine performance.

Isro's mission

The mission objectives of the ATV-D02/DMRJ mission are to demonstrate supersonic ignition, sustain the flame for combustion, and evaluate integrated DMRJ engine performance at hypersonic flight conditions. The flying test bed for Isro's DMRJ engine at supersonic conditions is provided by a sounding rocket platform — the ATV D02.

Dual mode engine systems and subsystems has undergone an extensive qualification and testing programme, which includes wind tunnel testing, CFD runs, structural tests, material characterisation, static test of solid motor, supersonic ignition and combustion experiments.

The space organisation says that the technological challenges for the mission include hypersonic air intake geometry with high pressure recovery and better shock wave-boundary layer management; design of supersonic combustor, which includes fuel injection and flame holding device; performance and operability across wide range of flight speeds, which is the transition from ramjet to scramjet; thermal management of engines; computational tools to simulate hypersonic flow and arrive at optimum Mach number-dynamic pressure window; thermo-structural tests of scramjet engines and ground test at higher Mach number conditions; among others.

For Isro, the technology demonstration of hypersonic air breathing dual ramjet engines would lead to the design and development capability of advanced air breathing engines, including engines with variable geometry air intake for its future space transportation system.