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. – 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.


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.

ScramSpace Grapples with Scramjet Testing Challenges

Yesterday, an Atlas V and an Antares rocket both roared off of the United States eastern coast and into space. However, in a remote area of Norway, another rocket launch didn’t go as planned. ScramSpace, a scramjet developed by researchers at the University of Queensland, disappeared over the Atlantic Ocean without sending back the hoped-for data. Scientists were left to clean up the two rocket sections that launched ScramSpace skyward and head home.

What’s the difference? How can complex orbital missions like the Atlas V and Antares be successful when a scramjet flight so easily goes awry? Part of the difference is the physics, which makes scramjets harder to test. While both rockets and scramjets are means of achieving high speeds, they differ in design. Scramjets squeeze high speed (greater than Mach 5) air into a small tube, compressing and heating it. Then a fuel is injected into the airstream. They only work above Mach 5. In contrast, rockets supply their own fuel and oxidizer (air is an example of an oxidizer, oxygen is another), mixing them to cause the explosive reaction that gets them moving. The physics of rockets is fairly well understood after years of launch. Scramjets have not had the luxury of so many tests, mainly because testing scramjets is difficult.

First of all, the testing setup for scramjets is tricky. Unlike rockets, which can be ignited at any speed, scramjets only work when they are already traveling at Mach 5 or greater. Since no research planes currently achieve these speeds, rockets are required. ScramSpace, the University of Queensland explains, uses a two-stage rocket to send the scramjet into space. Then the test is completed as the scramjet accelerates toward Earth (see graphic at top). Other projects, like the X-43 and X-51 projects in the US, launch the scramjet from a B-52 bomber, accelerate it with a rocket and then perform the experiment, according to NASA. These tests are not simple or easy to do.

Plus, once you get to Mach 5, the conditions are nasty. At high speed, aerodynamic heating causes the scramjet to experience extreme temperatures. This means that scramjets require high temperature alloys to simply maintain their structural integrity. The physics also get complicated at hypersonic (above Mach 5) speeds. Shockwaves bounce around inside the scramjet tube and must be “swallowed” correctly in order to achieve maximum thrust. In an ABC Catalyst special, a ScramSpace engineer explains that the air must also rush into the inlet at the right angle. Extensive ground tests help scientists determine these conditions, but they don’t depict how the vehicle will react in actual flight.

The test setup also leaves very little room for error. In the same ABC short video, a ScramSpace engineer explains how vital proper controls are to the experiment, demonstrating how reaction thrusters are used to properly align the craft in space. If improperly aligned, the ScramSpace vehicle would not work correctly. U.S. projects have also had difficulties controlling scramjets at high speeds. An X-51 WaveRider scramjet failed to ignite when one tiny fin (of four) became loose before the ignition, according to Zach Rosenburg. Controlling the vehicles is difficult and the conditions at Mach 5 and above leave little room for error.

Despite all these difficulties, scramjets have accomplished some amazing test flights. The third test of the X-43 still holds the world record for an air breathing engine (as opposed to a rocket engine, which carries its own “air” or oxidizer). According to NASA, it achieved a speed of Mach 9.6, and, theoretically, scramjets can go even faster, reaching Mach 14 or 15. That’s almost fast enough to reach space! The last test of the X-51 Waverider flew successfully for over 6 minutes, according to Space Ref. This was huge accomplishment for a test period that is typically 5-15 seconds.

Overcoming the difficulties of scramjet flight is difficult, not impossible. Perfecting scramjets is challenge worth taking on. Since scramjets use atmospheric air as the oxidizer, instead of carrying big oxygen tanks as rockets often do, they offer great weight savings over rockets. Yet, funding for scramjets is often limited. The ScramSpace team is heading back the University of Queensland, according to a recent report, not to begin building again, but to be disbanded. Once perfected, scramjets can become the transportation of the future, streaking through the skies at speeds above Mach 10, but they face many physical and budgetary challenges in the near future.

NASA Has a History of Crashing Helicopters for Science

In a video reminiscent of Discovery Channel’s Mythbusters, NASA dropped a 45 ft long Marine CH-46E helicopter fuselage into the dusty ground at NASA’s Langley Research Center. NASA reports that the helicopter, having plummeted from 30 feet in the air, smashed into the ground at 30 mph. This was a simulation of a survivable crash, but the 15 crash-test dummies locked inside did not have a smooth ride. At the conclusion of the video posted by NASA, one almost expects to hear Mythbuster Adam Savage giggle.

Perhaps smashing things does make NASA engineers giggle; they have a history of crashing helicopters and other experimental aerial vehicles. In 2009, the U.S. Army donated an MD-500 helicopter to NASA, according to a NASA press release. What did NASA do with it? They crashed it (“for science”). The initial test had a “deployable energy absorber,” a Kevlar honeycomb design originally intended to cushion space capsules. Miraculously, the helicopter survived the first test relatively intact, thanks to the new technology.

Four months later, NASA explained that it had dropped the helicopter a second time. This time, however, the MD-500 had no honeycomb cushion. As a result, the helicopter was too damaged for further testing. Engineers had recorded over three times the “g” forces compared to the previous test with the cushion. The picture below shows the windscreen shattered and the skids bent. It was destructive testing, indeed.

However, unlike Mythbusters, NASA conducts the tests for more than the wanton destruction and cool video footage. In the 2009 and 2010 tests, NASA was able to demonstrate that the honeycomb cushion designed for space capsules could also increase the survivability of a helicopter crash. It worked so well that they were able to simulate another crash for comparison.

Last week’s crash test of the much larger CH-46E helicopter also provided useful crash data. Unlike the 2009-10 tests, this time NASA crashed the unmodified helicopter first. Although the test already had scientific use as a basis for which to compare a future test of a composite airframe, additional experiments abounded. The Navy, Army and FAA all contributed different crash test dummies. CONAX Florida Corporation DBA Cobham Life Support tested a restraint system in the cockpit. Unlike Mythbusters, NASA now has 350 different instrumentation points to analyze, as well as high speed camera data.

Destructive testing, like NASA’s recent helicopter crash, is not only fun and entertaining, but extremely useful. New technologies, like the honeycomb cushion, can be tested and vetted for future use. The cost, however, is high. Not everyone has the money available to conduct these necessary tests. With NASA facing flat budgets for the next few years, we wonder ‘who will crash our helicopters for us in the future?’