Flame Photography Discerns Peculiarity in Ramjet Ignition

Amid pictures of dazzling auroras and satellite passes, pictures of a flame may seem boring in comparison. However, a Chinese team’s recent photography of flames igniting in a high speed engine (Technical note, AIAA Journal of Propulsion and Power) captured an unexpected result.

Hiding their cameras behind a quartz window and taking pictures at a rate of 10,000 frames per second, the team photographed how a flame ignites in subsonic and supersonic conditions. Understanding combustion at different speeds is important to developing efficient ramjets and scramjets, which react atmospheric air with a fuel to accelerate the next generation of supersonic airplanes and space-planes. Despite our computing power, our knowledge of how air reacts in these high-speed, high temperature environments is limited. More insight into how flames ignite in this intense environment can lead to better ramjets and scramjets in the future.

Ramjets and scramjets use an inlet to swallow air at high speeds, which the engines mix with fuel and then ignite to provide thrust. The primary difference between a ramjet and scramjet is the speed at which the mix is ignited; ramjets combust at subsonic speeds and scramjets ignite at supersonic speeds. The fuel-to-air ratio influences whether combustion is subsonic or supersonic. In fact, the Chinese team was able to induce either subsonic or supersonic combustion simply by changing the fuel to air ratios. A lower fuel-to-air ratio produced supersonic combustion and a higher ratio allowed subsonic combustion. The speed of the heated air forced into the inlet never changed during the experiment.

By igniting a slow stream of oxygen and a kerosene fuel at different fuel-to-air ratios, the Chinese team was able to photograph how flames look in their infancy. The flame ignited at subsonic levels danced and transitioned through three distinct states before stabilizing at a steady glow. Conversely, the flame ignited at supersonic speeds (and a lower fuel-to-air ratio) stabilized more quickly. Through the photography, the Chinese team showed that the subsonic flame was affected by a counterflow, where the air moved toward the inlet instead of the exit.

Identifying the counterflow in the subsonic flame is an insight into how air moves and reacts after flame ignition. Better understanding of phenomena like this leads to accurate modeling of this extreme environment and development of more effective ignition sources. These pretty pictures may help in the design of the next space-plane.



Improvements in air-breathing propulsion pave the way to space

After reigning supreme for decades, traditional rockets may be supplanted by their more efficient air-breathing brethren as the preferred method of reaching space.

Air-breathing supersonic propulsion, such as a ramjet, may receive an efficiency boost from a new design recently proposed in the AIAA Journal of Propulsion and Power. Reaching space is accomplished at a massive energy cost, so any efficiency increase can have a huge impact on the number of satellites (or people) that a launch vehicle can take to space. Accessing space more efficiently reduces the cost required to propel people and satellites to space, enabling more new technology and adventures at a lower cost.

Although rockets are the only type of propulsion that works in space (since they have both N4NA_Ramjetpropellants on-board), air-breathing engines, which use atmospheric air as a propellant, provide greater efficiency when closer to Earth. Used together, rockets and air-breathing propulsion, such as ramjets, expend energy more effectively than rockets alone. In a rocket based combined cycle (RBCC), an air duct with a rocket inside can propel itself by rocket, ramjet or scramjet (See figure below). N4NA_ScramjetFirst, the internal rocket is used to accelerate the vehicle to supersonic (above Mach 1) speeds, using atmospheric air (1). Later (2), the air duct transitions to ramjet mode, where the air in the duct is ignited to provide thrust. After ramjet speeds are exceeded, the RBCC engine becomes a scramjet (3) and then eventually a rocket (4) once more. This set-up utilizes the most mass-efficient engine type at each supersonic stage*. RBCC_FunctionsEtele, Hasegawa, and Ueda proposed a modification to the internal rocket nozzle of a RBCC engine that boosted the combustion efficiency of the ramjet. The ramjet produces thrust more efficiently if there is more atmospheric air available. In order to increase the amount of captured air, or entrained air, in the ramjet area, the team proposed a deviation from the commonplace circular rocket nozzles and its replacement with an annular geometry. At the tested pressures, this new design entrained more air within the duct and more thoroughly mixed the air in the duct when compared with a circular nozzle. These conditions improve ramjet combustion, improving how fast the vehicle can propel itself.

Further efficiency increases in air-breathing engines, such as the annular rocket nozzle, could eventually improve launch vehicles, leading to more cost-effective access to space.

*Note: The turbine based combined cycle uses a turbine (like a jet) at low altitude and subsonic speed (below Mach 1), which is more effective at that altitude and speed, but has a huge mass cost. Turbines are heavy to lug around at higher altitudes when they are no longer effective.

A more technical synopsis of the experiment is available here.

Repurposing Spacecraft not a Novel Idea, but a Cost-Effective One

On August 15th, NASA officially issued a call for help, in the form of scientific white papers, in hopes of repurposing the crippled Kepler spacecraft. The telescope, which had led the charge in finding exoplanets, planets orbiting distant stars, has been unable to function due to issues with two of its four reaction wheels. These wheels, which, NASA explains, are used to point to distant galaxies, are vital to Kepler’s scientific mission, studying distant stars for the changes in light intensity that would signal a planet passing between Kepler and the star. Without the ability to point correctly, the spacecraft’s main mission is compromised. Rather than letting it drift in space, NASA is trying to repurpose the satellite to save money, a strategy that the space agency has used in years past.

NASA had success with repurposing two of the THEMIS spacecraft in 2010, according to an old press release. Originally intended to study the magnetosphere, the little understood magnetic field that surrounds the Earth, two THEMIS spacecraft were diverted toward the moon after finishing their original mission. Once in lunar orbit, the spacecraft began a new mission, ARTEMIS, studying the effect of solar wind on the moon’s surface. By simply sending the fueled spacecraft to another location, NASA scientists were able to get valuable new data, without spending much extra money.

“Using two repurposed satellites for the ARTEMIS mission highlights NASA’s efficient use of the nation’s space assets,” said Dick Fisher, director of the Heliophysics Division in NASA’s Science Mission Directorate at the agency’s headquarters in Washington. With its budget shrinking, NASA is doing all that it can to avoid the high cost of building and launching spacecraft.

NASA is not alone in trying to save money through repurposing satellites. John Keller reported in late January that the U.S. Defense Advanced Research Projects Agency, aka DARPA, is also looking to repurpose spacecarft, by recycling components from non-operational satellites. Defunct spacecraft often have still useful antennae and sensors. Building and launching these components is costly, in terms of both manufacturing and fuel. By recycling parts already up there, DARPA can lower the cost of a new satellite.

Using new ideas and clever innovations to repurpose or recycle spacecraft is both cost effective and a strong move toward sustainability. The junkyard of decommissioned satellites can instead become a useful tool shop for those with innovative ideas.