Developing the Blueprint for a Scramjet

Although researchers are keen to develop a successful scramjet engine, designing and testing this type of engine has proved difficult. For example, the WaveRider scramjet test vehicle, built by the engineering powerhouses of DARPA, Boeing and Pratt and Whitney Rocketdyne, only had two successful flights of four. Scramspace also built and unsuccessfully tested a flight vehicle. Developing a successful design for a scramjet has not been a simple task, but, through patient research among experts on varying aspects of scramjet technology, a blueprint for a the engine is slowly taking shape.

Researchers are tackling varying design aspects of the scramjet engine, refining the physics and theorizing how components would work most effectively. In the Sep-Oct 2014 issue of the AIAA Journal of Propulsion and Power, at least three articles summarized research directly related to scramjet engine development. While each article addressed only a small aspect of component design, like the best width of cavities to increase fuel-air mixing (for more effective combustion), central design difficulties are being addressed. One of the most pressing issues with scramjets is having successful combustion at supersonic speeds. Two of the articles address this issue.

Korean researchers explored the issue of starting and sustaining combustion. The team created a two dimensional model for testing different lengths of combustor area. By varying the length of the combustor, the researchers could determine which configuration allowed ignition and sustained combustion. In four of seven tests, their “medium” length combustion area had supersonic combustion. The medium length allowed the fuel to atomize (small droplets) along the length of the combustion, so that the fuel was ignited successfully when it reached the flame. This short technical paper helped lay the foundation for designing a successful combustor length.

Cavities are another design idea intended to enable combustion at supersonic speeds. Previous work has established that cavities (like the semi-circular holes on golf balls) increase fuel-air mixing by making the air around them more turbulent. The turbulent air does not stream out of the engine as fast, allowing it to swirl around and mix with the fuel more thoroughly before reaching the flame (ignitor). A joint team of Korean and Indian scientists published their research on how the width of these cavities can increase or decrease the amount of turbulence just downstream of the cavity. Now future designers can arrange the width of cavities so that there is successful fuel-air mixing and therefore combustion.

While researchers are focused on determining how to have successful combustion at supersonic air speeds, other physics problems remain to be solved.  For example, how do they design an inlet that takes in air at hypersonic (above Mach 5) speeds? At these speeds, shocks play a vital role in aerodynamics – the assumptions of how air works at subsonic speeds do not apply. Shock waves and expansion fans, physical phenomena at high speeds, drastically alter the pressures and temperatures at the inlet. A Chinese team looked more carefully at the physical interaction of these phenomena, attempting to refine a theory developed in 1975, which they believed did not take into account the interference from expansion waves at the “shoulder” of the inlet. Further refining the community knowledge of the physical interactions at hypersonic inlets will eventually aid in the design of such inlets.

While scramjet testing in flight conditions (outside of the laboratory) can be expensive and has a historically low probability of success, theoretical refinements in component design are building a blueprint for scramjet designers to follow in designing the next generation vehicle.

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Sci-Fi Era Rocket Engines Work, but Questions Remain

Imagine this – in an orbit around Mars, spaceship engines ignite. Rather than the typical roaring core of flames, instead small explosions spin around the inner edge of the engine. They are rotating so fast that they blur into a blue ring of flame.

Researchers have built this type of engine in laboratories across the world (if in smaller scale), but do not understand this engine well enough to use it as spaceship propulsion  yet.

Called rotating detonation engines, or RDEs, these are not your typical rocket engine. Most rocket engines combust the propellant through deflagration – burning all the fuel at a continuous rate until it is gone. Detonation engines work more like bombs. They ignite the propellants all at once in one powerful explosion. The benefit is that the energy is released in a short time. Researchers believe that this trait can lead to powerful engines that are small in volume.

The key idea of the rotating detonation engine is that the detonation is a wave. In most detonations, waves go in all directions. The concussive waves of an explosion are a good example. However in a RDE, the waves go around an annular chamber (in a circle), re-igniting as they get fresh propellant. [To get a better idea of how they work, check out this Russian video, starting at 1:10 seconds. http://www.youtube.com/watch?v=EBcflqPNhCY – You can see how a single wave moves in a circle around the front of the engine. The later part of the video also shows the engine as if you are looking toward the exhaust, as imagine in the first paragraph.] The waves spin around the inner diameter of the engine, moving so fast that they look like a continuous flame.

The clip, though modeled, is based upon observations in a laboratory. In a recent survey published in the AIAA Journal of Power and Propulsion, Dr. Lu and Dr. Braun explored the rotating detonation engines developed and tested within the last decade. Through this survey, they revealed the challenges of building and testing these engines.

The authors noted that, while nine engines have been tested between 2006 and 2012, none has lasted for more than a few seconds. Three of the engines worked for an amount of time measured in milliseconds. If nine different engines had been constructed and successfully tested (if only for a short time), why hasn’t a more enduring engine been created? Simply, the physics behind RDEs is not well understood, so the typical engineering methods for design are replaced by a method approximating educated trial-and-error.

There is not yet any “standard” RDE hardware. Among the nine engines, six different ignition sources were constructed. These sources were intended to start a single wave speeding around the circular engine. The most reliable, with a reported repeatability of 95%, was also the most complex. It used three different pieces of hardware to achieve the reported rate. One system was not enough to guarantee success. The added complexity stems from uncertainty about how the waves begin and sustain themselves.

Even if successfully started, the waves have strange characteristics that make long-duration testing difficult. For example, researchers have observed the waves changing direction. They switch from running clockwise to running counterclockwise, or vice versa. The reason for this behavior is not well understood. Even if the waves do not unexpectedly about-face, maintaining them is difficult. Fuel injectors must give the waves enough fuel to sustain themselves, but at the right moment. The goal is not to become a deflagration engine, which simply burns the propellant in one continuous burn. Separate waves require specialized ignitors that support their travel around the engine. Each of the nine engines used a slightly different method to make this work.

It’s possible that one of these engine systems had the best mixture of ignition sources and ignitors required to sustain the waves of an a RDE. Yet, none of them ran for more than 2 seconds. It is likely that this is due, at least in part, to the extreme heat conditions in a rotating detonation engine. The authors experienced this difficulty first-hand. Their engine used a composite material that could withstand temperatures of 1800 degrees Celsius. It survived short duration tests of less than 0.5 seconds; further testing caused damage.

Why can’t engineers select the proper materials and hardware for testing? RDE models are not entirely accurate. The engines do not follow the typical thermodynamic cycle that deflagration engine designers use. The typical assumptions don’t apply. The Brayton thermodynamic cycle, used for analysis of deflagration engines, assumes constant pressure. Waves are inconstant. They move around the engine, causing rapid changes in pressure and velocity. Researchers have developed theories to estimate how many waves should be present and created new methods to describe the thermodynamic cycle. But the theoretical models do not yet seem developed enough to design a successful RDE using only models.

It may take some development time before we can use these engines to propel ourselves to distant worlds.

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.

 

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.