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.

 

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.

Space Vehicles Go Green – Reusable Launch Vehicle Concepts

In launch vehicles these days, it’s all about reusability. SpaceX recently tried to reignite its first stage booster, which is the initial step toward landing it vertically on the ground. If this first stage is landed successfully, it can be used again and again. DARPA is calling for proposals for a “reusable hypersonic vehicle with costs, operation and reliability similar to traditional aircraft.” Companies will propose their ideas on Monday October 7th.  Across the Atlantic, Reaction Engines is developing the SABRE engine, a radical engine for the Skylon spaceplane. Each design has the same intention, lowering the cost of access to space by reusing vehicles over and over again.

SpaceX has taken the most traditional approach to launch vehicles, using a vertical launch system and powerful rockets to get payloads into orbit. However, this system has not been reusable in the past. To add reusability to their system, SpaceX intends to have the massive first stage of the rocket land itself on the launch pad. The company is currently exploring the vertical landing technique through the Grasshopper research project, which completed a 325 meter leap test on June 14. While SpaceX’s system is not fully reusable, they are working toward that goal.

On the opposite extreme, Reaction Engines in the U.K. is developing a fully reusable spaceplane known as Skylon. Unlike SpaceX, which uses multiple stages of rockets to get into space (dropping used ones, such as the first stage, along the way), Reaction Engines wants their spaceplane to get into orbit using a single stage. To achieve this lofty goal, Reaction Engines is developing SABRE (see picture), a revolutionary rocket engine that uses atmospheric air as rocket oxidizer for a portion of the trip. Reaction Engines explains that SABRE sucks in atmospheric air through an inlet and then cools it to nearly liquid, basically generating its own oxidizer. However, once the Skylon exceeds Mach 5, the engine morphs into a typical rocket engine, using its own oxidizer stored onboard. This design allows Skylon to carry less liquid oxygen than a typical launch vehicle, saving space and weight for other essentials. With this radical engine, Skylon could be the start of a whole new family of launch vehicles.

In contrast, DARPA’s XS-1 spaceplane design concept seeks to be a melding of the traditional and radical. The radical part of the design is a suborbital spaceplane that can launch just like an aircraft, from the ground. This distinction is important because SpaceShipOne, a successful suborbital spaceplane, launched from a flying “mothership” airplane in 2004. The XS-1 concept, according to the proposer’s day announcement, is a reusable suborbital space plane with a (more traditional) secondary stage that would launch payloads into their required orbit. DARPA’s goal is ten flights in ten days, which would be a radical departure from current launch timeframes.

Although the goal of reusability is shared across these programs, the designs are completely different. They each also have their own timeframe. SpaceX is already conducting preliminary tests. The other two space planes are still in the concept phase. While parts of SABRE, such as the precooler system, have been tested, the Skylon system is years from launching. The XS-1 design is still flexible, so there’s no telling when it may be ready to fly. This flexibility shows how DARPA recognizes that they are many different ways to make reusable launch vehicles. The plethora of designs in development right now shows that reusable space vehicles are likely to be the next way to access space.

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.