There are many potential applications of scramjet technology.



James A. Jollyjamesajolly@Scramjets: From Supersonic to HypersonicAbstract: Scramjets represent an important next step in the future of air and space travel. But there are still many kinks that need to be worked out before we see scramjets in widespread use. This article will cover the basics of what a scramjet is, and the potential they have in air and space travel. Additionally, it will provide analysis on already existing scramjet designs and concepts like the X-43 and X-51; in both their strengths and weaknesses. Scramjets are a quickly developing and evolving field, and there are still many unknowns. Overall, however, it is clear there are numerous possibilities of scramjet applications for both military and commercial use and that there will undoubtedly be a great deal of future research and development in the. This article attempts to dive not only in to the past, present, and future of scramjets.Bio: James Jolly is a Bachelors Student at USC studying Astronautical Engineering and on track to graduate in the spring of 2015. He is interested space travel, specifically in regard to orbit insertion from Earth.IntroductionIn 1948, the X-1 became the first supersonic aircraft in the world, traveling faster than the speed of sound. A decade and a half later, in response to the U-2 spy plane being shot down during the Cold War, the SR-71 Blackbird was developed to fly higher and faster than any other aircraft before it, pushing the limit to over Mach 3, or 3 times the speed of sound [3]. In today’s times, aerospace companies and researchers are looking to push aircraft speeds even further, to what we call hypersonic speeds; or Mach 5 and above; and the application of such speeds goes far beyond just spy planes. Being able to fly faster has obvious applications in both commercial and military areas, including a lucrative market for faster commuter air travel, and increased lethality in air combat vehicles. But how do we make aircraft travel this fast? To achieve these hypersonic speeds, we must look to a specific type of engine, called a scramjet.What is a scramjet?All of the planes that you see flying around today use a form of a turbojet. A turbojet is a jet engine that uses a turbine to suck in and compress the needed oxygen for combustion in the engines. A scramjet is a specific type of more general type of engine called a ramjet, which operates slightly differently than a turbojet. To understand the difference between a turbine engine and a scramjet, it is important to take a step back and understand basically what any engine does. For simplicity we can think of aircraft engines in two categories; air breathing and non-air breathing. A non-air breathing engine would be a rocket, where everything needed for combustion to produce thrust is carried on the aircraft itself. For an air-breathing engine, oxygen is required from the surrounding air for combustion of the fuel in the engine to take place, meaning the aircraft does not have to carry the oxygen itself [1]. In this regard both turbojets and ramjets are air breathing engines, but beyond this they are very different.03924300Figure SEQ Figure \* ARABIC 1: Three types of engines; (a) Turbojet, (b) Ramjet, (c) Scramjet SEQ Figure \* ARABIC 1: Three types of engines; (a) Turbojet, (b) Ramjet, (c) Scramjet order to produce thrust, a jet engine must combust far more oxygen than would naturally flow through an engine at a standstill or at low speeds. In order to maintain this necessary flow of oxygen into the engine, a turbojet uses turbofans to suck in the outside air into the engine to be combusted. At higher velocities, these turbofans become unnecessary because the velocity of the aircraft itself is high enough that the surrounding air is automatically compressed into the engine [1]. This is the concept of a ramjet; where air is literally compressed by being “rammed” into the engine rather than from a compressor like a turbofan. This is where the term ramjet comes from.A scramjet is a specific type of ramjet known as a supersonic combusting ramjet. At high velocities, the air must be slowed down in order for it to be combusted and then reaccelerated out the back. A ramjet engine slows the incoming air to subsonic speeds before it is ignited with the fuel. A scramjet, however, is designed so that the flow of oxygen through the engine, while still being decelerated, never becomes subsonic [2]. This makes a scramjet capable of higher velocities than a ramjet.When talking about aircraft and aircraft engines it is far too simple to talk just about maximum speed capabilities. It is critical to talk about efficiency. While a scramjet is capable of greater speeds than both a turbojet and an ordinary ramjet, it is not superior in every way, and certainly not at every speed. At lower speeds a turbojet is far more efficient in terms of fuel consumption, and even in terms of capability. This is because ramjets (and thus scramjets), by their design, cannot even operate at lower speeds. Also, at higher speeds however, the turbofan compressors get in the way and become a hindrance to higher flight speeds using a turbojet. Because of this, at around Mach 2 or 3 ramjets become far more effective and efficient. Because they slow the flow of oxygen to subsonic speeds, a ramjet will only remain effective up to around Mach 6. Anything above this speed requires a scramjet. The limits of scramjet speed is uncertain, but has been estimated as high as Mach 20 to 25 [6]! Modern Scramjets-114300241935000-117475110109000There has been a great deal of research and development into the field of scramjets on the part of aerospace companies and aerospace engineers throughout the United States and all over the world. But what kind of scramjets exist today, and how well do they work? To answer this question we will look at two particular and recent scramjet developments; the X-43 and X-51. These were not the first scramjets to be developed, nor do they encompass the entire field of scramjets research, but they are two good examples of scramjet capabilities and drawbacks.-24003001070610Figure SEQ Figure \* ARABIC 2: The X-43 shown by its self (bottom), and attached to Pegasus booster rocket and B-52 (top) SEQ Figure \* ARABIC 2: The X-43 shown by its self (bottom), and attached to Pegasus booster rocket and B-52 (top) X-43 was part of NASA’s Hyper X program, and was an attempt by engineers at NASA to develop a hypersonic aerial vehicle. The X-43 itself was attached to a rocket booster that was dropped from a B-52 as shown in Figure 2, and which accelerated the X-43 to high velocities before the scramjet engines engaged and the X-43 detached and flew on its own. In March of 2004 the X-43 flew at Mach 6.8, and in its last flight in November of 2004 it reached a speed of Mach 9.6. In each case the engines operated and recorded data for 10 seconds. In the Mach 6.8 flight the scramjet engines were able to accelerate the X-43 during their operation, and in the Mach 9.6 flight the engines were able to allow the X-43 to maintain its velocity [2]. The X-43 currently holds the speed record for an air breathing engine, and is an impressive display of the speed capabilities of a scramjet.35629852111375Figure SEQ Figure \* ARABIC 3: X-51 attached to a B-52 SEQ Figure \* ARABIC 3: X-51 attached to a B-52 X-51 was developed by Boeing and was aimed at more extended hypersonic flight. Like the X-43, the X-51 was accelerated by a rocket after being dropped from a B-52 as shown in Figure 3. In May of 2010, the X-51 was released at Mach 4.5, from which the X-53’s scramjet engines took over and accelerated it to Mach 5 for 143 seconds [7]. The X-51’s performance was a huge step forwards in displaying hypersonic capabilities for a more extended period of time. While both the X-42 and X-51 display critical advancements in Scramjet technology, there are still drawbacks and problems that need to be fixed before more relevant applications of Scramjet engines become available. The first problem is the limited duration for these engines. This will most likely be solved with further improvements to Scramjet vehicle design. The second critical drawback is the assisted launch requirement. Both the X-43 and X-51 required assisted takeoff from both a B-52 and a rocket. This has cruciual implications and concerns for Scramjet applications in regard to how they will actually take off and fly.Assisted Takeoff: ignore or solve?The question remains on what to do with assisted takeoff for Scramjets. It is clear that scramjets (or any form of ramjet) will not be able to operate at lower speeds. One possible choice is to simply use assisted takeoff for any future Scramjet applications, like the use of an external aircraft and a rocket to build up altitude and speed before the scramjet engine is engaged. It has also been proposed to use a rail gun to give a scramjet vehicle its necessary velocity boost required to start its scramjet engines. The concept is highly theoretical; but rather than using rockets it proposes using electricity and electromagnetic forces to accelerate a vehicle [5].-95252598420Figure SEQ Figure \* ARABIC 4: The SR-71 Blackbird in flight. SEQ Figure \* ARABIC 4: The SR-71 Blackbird in flight. possibility is the use of a hybrid vehicle, which makes use of both turbojets and scramjets. One of the best ways to look at this idea is to examine the work done by Lockheed Martin in the design and development of “hybrid” aircraft and concepts. The SR-71 Blackbird, developed by Lockheed, was able to fly at speeds over Mach 3. An interesting thing about the SR-71 was that it made use of both Turbojet and Ramjet technology. The Blackbird needed to be designed so that it could operate by itself all the way from takeoff to over Mach 3 [3]. It needed turbojets to operate at lower speeds, and ramjets in order to achieve higher speeds. To solve this problem Lockheed’s “skunk works” team developed an engine that could act as both a turbojet and a scramjet. Essentially, the way it worked was that at higher velocities inlets would open up that would bypass the turbofans and travel directly to the ignition chamber and would be ignited and reaccelerated to supersonic speeds and flow out the back. This would bypass the turbofans and prove the large amounts of thrust that the Blackbird would need to travel at such high speeds, and that the turbojets themselves would be incapable of providing. The Blackbird still holds all of the speed and altitude records for an aircraft that could take off and fly on its own, and is a great example of the possibility from combining turbojet and ramjet technology [3].34861504705350Figure SEQ Figure \* ARABIC 5: The SR-72 with artist rendering (top) and engine concept (bottom) SEQ Figure \* ARABIC 5: The SR-72 with artist rendering (top) and engine concept (bottom) main consideration and drawback when using both turbojets and scramjets, is that having both takes up more space. More space means more drag, and more drag means less efficiency and lower speed capabilities. The solution according to Lockheed has been to combine the two into a single operational engine. The SR-71 did this to some degree; but the goal is to push an aircraft to speeds beyond even that of the SR-71. For Lockheed, this concept has evolved into the SR-72. This plane would make use of a multi-operational engine that has a common inlet and nozzle, but two pathways for incoming flow to go through: a turbine engine and a dual-mode ramjet as shown in Figure 5. The turbine engine would take the plane up close to Mach 3, and then the dual-mode ramjet, which includes a scramjet, would accelerate to aircraft to hypersonic speeds of around Mach 6 [8]. The plane is still in early development, but nonetheless goes to show the potential of combining Turbojets with Scramjets for future aviation applications.Looking to the FutureThere are many potential applications of scramjet technology. One of the primary applications is military technology. Being able to travel at hypersonic speeds would provide a huge advantage on striking a target with little or no warning. This is one of the main objectives of planes like the SR-72, and other current projects like the DARPA (Defense Advanced Research Projects Agency) and United States Air Force Falcon Project, aimed at developing a supersonic military vehicle capable of striking anywhere in the world faster than any country would have time to react to. Travelling at such high velocities also has huge commercial possibilities, as people would be able to get anywhere in the world much faster than is currently possible. One of the biggest and most relevant potential applications of scramjets is cheaper space entry. A vehicle using scramjets could provide the speed and altitude conditions of near orbit, from which it would much easier to put a vehicle and any payload into orbit. A study done by DARPA has suggested that airborne assist can decreases space launch costs, and increase payload efficiency, or how much payload can be sent into space for a given launch and cost [4]. We are of course still far off from many of these applications; nevertheless, what we see is that scramjets have huge possibilities in carrying us into the future of air and space travel.Works Cited [1] B. Dunbar. (2004) What’s a Scramjet? [Online]. Available FTP: Directory: missions/research File: f_scramjets.html[2] B. Dunbar. (2014) NASA Armstrong Fact Sheet: Hyper-X Program [Online]. Available FTP: Directory: centers/dryden/news/FactSheets File: FS-040-DFRC.html#.Ux-ThPldUlI[3] Creating the Blackbird. Available FTP Directory: us/100years/stories File: blackbird.html[4] M. B. Clapp. (2012) Airborne Launch Assist Space Access (ALASA). Available FTP DARPA.mil Directory: WorkArea File: DownladAsset.aspc?id=2147485147[5] R. M. Pacella. (2010) NASA Engineers Propose Combining a Rail Gun and a Scramjet to Fire Spacecraft Into Orbit [Online]. Available FTP: Directory: technology/article/2010-11 File: nasa-engineers-propose-combining-rail-gun-and-scramjet-fire-spacecraft-orbit[6] Scramjets [Online]. Available FTP: Directory: Orbital%20Travel/Scramjets File: Scramjets.htm[7] (2012) X-51A WaveRider [Online]. Available FTP: Directory: assets/pdf/defense-space/military/waverider/docs File: X-51A_overview.pdf[8] (2013) Meet the SR-72 [Online]. Available FTP Directory: us/news/features/2013 File: sr-72.html ................
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