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.

 

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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.

Technical Notes – Improvements in air-breathing propulsion pave the way to space

Aside

This section is geared those who have a background in aerospace engineering:

The concept of  multiple rocket exhaust areas was based upon the Strutjet concept, which used multiple rockets in an individual air duct. The mixing effects of multiple rockets entrained more air for combustion, leading to greater efficiency. The team wanted to examine having multiple rocket exhaust ares without multiple heavy thrust chambers, so they built a annular nozzle with three major circular arcs and small circular air entrainment tubes in between each arc. For comparison, the team also used a circular nozzle.

To enable accurate comparison, the mass flow and Mach number was kept constant across the two nozzles. The experiment was also set up have the maximum amount of entrained air at the duct exit. Pressure sensors were arranged around the nozzle and exit plane of the duct. To replicate high speed environments, ambient air was injected into the duct at high pressures, replicating up to Mach 2 speeds (after expansion of the flow). No fuel was injected.

Initial results showed the annular nozzle entrained more air than the circular nozzle at lower pressures. The pressures taken at the duct exit plane also showed that the air pressures were more uniform in the annular nozzle configuration, suggesting more mixing of the air had taken place. These results showed an average Mach number 58% higher in the annular configuration than the circular configuration.

Please see the article “Experimental Investigation of an Alternative Rocket Configuration for Rocket Based Combined Cycle Engines” in the July-August edition of the AIAA Journal of Propulsion and Power for more details.

XCOR Aerospace Borrows “Several Hundred Years of Experience” for its Piston Driven Rocket Pumps

On Monday, the partnership of XCOR Aerospace and the United Launch Alliance announced that they had adapted an old technology, the piston, into a high technology pump for liquid hydrogen rocket fuel. “…We have successfully operated our liquid hydrogen pump at design flow rate and pressure conditions,” said XCOR CEO Jeff Greason in a recent press release. The liquid hydrogen pump tested earlier this week is just one pump in a new family of piston driven fuel pumps. Flight Global reports that the company has already tested a rocket engine setup with their piston pumps, using the system to fire liquid oxygen and kerosene in March of this year.

So, why use an old automotive technology in the relatively new field of rocket development? Pistons provide a different way to drive fuel and oxidizer into the combustion chamber, where the chemical reaction that produces thrust occurs. Other current rocket technologies use a gas generator system (pump fed) or high pressure tanks (pressure fed) to drive the fuel and oxidizer into the combustion chamber (see below). The pump fed rocket relies upon reacting some of the fuel and oxidizer in a high temperature turbine, which generates the power for the pump system. Pressure-fed systems are simpler, using a high pressure gas (usually something nonreactive) to push the fuel and oxidizer into the combustion chamber. Both the pump fed and pressure-fed systems are fairly heavy, due to the gas generator and the large pressurization tanks. All three technologies have benefits and deficits, but XCOR argues that the piston pumped engines will cost less to manufacture and be easier to operate.

All three engine designs are competing to maximize thrust and reliability while keeping the mass of the system low, often leveraging other technologies to reach that goal. XCOR’s piston engine, the company explains, utilizes automotive technologies and a patented thermodynamic cycle to maintain a high specific impulse (a measure of thrust efficiency) and an easy start-stop feature. Like automobiles, XCOR’s pumps can run at a higher rpm than the original design, so the piston pump can be fitted to a larger rocket and pump more fuel if necessary. These pumps are a way toward an adaptable and reliable system through tweaking proven automobile technologies. Likewise, the pump fed rocket borrows high temperature, low mass materials from other industries, like aircraft engine manufacturing, to maximize the efficiency of the turbine and keep the mass low. Even the pressure-fed system draws from another industry, the materials industry, to create lighter tanks of new and exotic materials (such as carbon fibers).

Other non-aerospace technologies are also entering the aerospace sector. Designers are using enhanced video game graphics to simulate engineering tasks. Leveraging the developments of other industries is a great way for rocket propulsion and aerospace to progress with tight budgets. By utilizing automotive developments, XCOR flew through its first small piston rocket pump development, “taking fewer than four weeks from initial design to demonstration,” according to the site. Borrowing some concepts from other industries can help other companies do the same.

Rocket Hobbyists Show Off Unique Designs in Bonneville

Although the multi-million dollar rockets roaring off the launchpad often have similar shapes and designs, you’re bound to find some unique rocket designs at the Utah Rocket Club (UROC) Hellfire 18 event at the Bonneville Salt Flats. Unbound by needs of payload capacity and insertion points, model rocket designs probe the limits of design, often adding their own signature style.

Some rockets are designed simply to have a unique look. One was modeled after a chess piece. Another resembled a supersonic plane, built entirely from aluminum. One hobbyist showed off his glistening blue carbon fiber lined body tube, christened the Mockingjay after the iconic symbol of The Hunger Games. When pressed about the use of carbon fiber, he simply replied that he thought it looked the best.

However, not everyone cares about style; one hobbyist found practical reasons to modify his design. At a prior launch, one of his rockets plunged through the front window of a car due to a parachute deployment failure. Determined to make his rockets’ landings safer, this enterprising designer made two rockets of foam and balsa wood. Using a FUNNOODLE ® foam pool noodle as the body tube and nose cone and fins made of sturdy balsa, these rockets don’t need a successful parachute deployment anymore. When they smack the concrete-hard surface of the Bonneville Salt Flats, they simply bounce, ready for another launch as soon as the motor is replaced.

The diversity of designs is astonishing compared to large-scale rocket designs, even if the success rate isn’t as high. One can only hope that the commercial launch vehicles created by the likes of SpaceX, Blue Origin and Orbital Sciences will someday have as much diversity as one crate of rockets sitting in the blistering Utah sun. Who wouldn’t want to launch into orbit on a rocket shaped like a chess piece?

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