The scramjet–supersonic combustion ramjet–engine has been a holy grail of aerospace engineering for 50 years. It is an air-breathing engine with no moving parts capable of propelling crafts at hypersonic speeds beyond Mach 5. As indicated in the name, combustion in the scramjet occurs at supersonic speeds, where the heating due to air compression is sufficient to ignite fuel when injected into the engine. At present the record for the highest speed attained in scramjet flight is held by the NASA X-43A, which reached Mach 9.8 in 2004 after about 10 seconds of scramjet free-flight. The longest scramjet flight belongs to the Boeing X-51 Waverider with 140 seconds of burn time in a 2010 test flight. Few tests of these experimental hypersonic crafts have been completely successful; they represent the frontier of aerospace technology.
Tag: combustion
Microgravity Combustion
This collage of three combustion images reveals the beautiful symmetry of flames in microgravity. In the absence of gravity, flames are spherical, and, in the confines of a spacecraft, any combustion is extremely dangerous. Thus, most microgravity combustion experiments occur in drop towers. From NASA:
Each image is of flame spread over cellulose paper in a spacecraft ventilation flow in microgravity. The different colors represent different chemical reactions within the flame. The blue areas are caused by chemiluminescence (light produced by a chemical reaction.) The white, yellow and orange regions are due to glowing soot within the flame zone. #
Atomization
Atomization–breaking a flowing liquid into a fine spray–is important for fuel injection in a variety of engines, including automobiles, jet engines, ramjets, scramjets, and rockets. The more effectively a liquid fuel can be dispersed as a spray in an engine, the more efficient and stable the combustion will be. The apparatus in this high-speed video injects an annular water sheet with concentric jets of air on either side of the water. The video series shows the effects of increasing the outer and inner air velocities relative to the water on the breakup of the liquid. What to the naked eye looks like a deluge, high-speed video reveals as a complex undulating structure.
Carboy Combustion
Lighting a thin layer of ethyl alcohol in a jug produces some beautiful pulse jets and a moving wall of flame that shifts and flows according to the changing pressures inside the jug. Like the video’s author, we do NOT recommend trying this combustion demo yourself.
As for the video’s questions, firstly, blowing into the jar helps the flame because humans do not exhale pure CO2. With regard to the second question, the interior of the jug is initially thinly coated in ethyl alcohol vapor. Combustion starts at the top of the jug and the sheet of flame moves downward as the fuel at the top is spent. As that flame moves downward, however, it’s heating the air inside the jug, which expands and is forced out the opening. When the flame goes out in the upper part of the jug, that does not mean all of the fuel has combusted, simply that the ratio of air/fuel is insufficient for continued combustion. I suspect the flame persists at this opening because the air/fuel mixture is concentrated at that point. Any residual ethyl alcohol in the container is forced out through that narrow opening, and the resulting concentration of fuel there may be high enough to keep the flame burning there. (idea submitted by davidbenque #)
X-51A Scramjet Test Flight
The X-51A Waverider hypersonic aircraft had its second test flight earlier this week. Unfortunately, its supersonic combustion ramjet (scramjet) engine failed to transition from its start-up fuel to its primary fuel. According to the US Air Force Research Laboratory:
A US Air Force B-52H Stratofortress released the experimental vehicle from an altitude of approximately 50,000 feet. After release the X-51A was initially accelerated by a solid rocket booster to a speed just over Mach 5. The experimental aircraft’s air breathing scramjet engine lit on ethylene and attempted to transition to JP7 fuel operation when the vehicle experienced an inlet un-start. The hypersonic vehicle attempted to restart and oriented itself to optimize engine start conditions, but was unsuccessful. The vehicle continued in a controlled flight orientation until it flew into the ocean within the test range. #
Un-starting is the term used when supersonic flow is lost in an engine or wind tunnel. If the pressure or temperature in the engine deviates too far from the ideal conditions, the upstream mass flow through the engine will be greater than the downstream mass flow and the engine will choke (video). A shock wave forms and travels upstream, leaving subsonic flow in its wake. Loss of supersonic flow inside the engine would likely also result in losing ignition of the fuel/air mixture, resulting in flameout. #
If you haven’t guessed already, engineers like to make up words.
Microgravity Combustion
Combustion in microgravity is markedly different than that on earth, due to a lack of buoyant convection. The combustion of a droplet of heptane is shown here as a composite image. The bright yellow structure shows the path of the droplet, which gets smaller as it burns. The green structures show the initial development of soot, which eventually streams outward as long streaks. # (submitted by jshoer)
Combustion
Fluid dynamics are vital to combustion. Like here, many practical flames–such as those responsible for internal combustion in automobiles, jet engines, and rockets–are turbulent. The turbulence aids mixing of the fuel and oxidizer, resulting in more complete combustion and greater efficiency. #
Air Force Gears Up For Hypersonic Missile Test
Air Force Gears Up For Hypersonic Missile Test
The U.S. Air Force has announced another test of the X-51 Waverider coming up on March 22nd. This will be the latest in only a handful of tests of a new supersonic combustion ramjet engine, also known as a scramjet. The test should involve flying at Mach 6 for about four minutes. Hopefully we’ll have see some exciting results from that test flight in a week or so.
Hungarian Fire Tornado
This fire tornado formed over a burning plastic-processing plant in Hungary a week ago. Fire tornadoes aren’t rare, but footage of them is because they typically occur amidst wild conflagrations. Take a look at our explanation of how they form. #
Godspeed, Discovery!
The space shuttle, despite three decades of service, remains a triumph of engineering. Although it is nominally a space vehicle, fluid dynamics are vital throughout its operation. From the combustion in the engine to the overexpansion of the exhaust gases; from the turbulent plume of the shuttle’s wake to the life support and waste management systems on orbit, fluid mechanics cannot be escaped. Countless simulations and experiments have helped determine the forces, temperatures, and flight profiles for the vehicle during ascent and re-entry. Experiments have flown as payloads and hundreds of astronauts have “performed experiments in fluid mechanics” in microgravity. Since STS-114, flow transition experiments have even been mounted on the orbiter wing. The effort and love put into making these machines fly is staggering, but all things end. Godspeed to Discovery and her crew on this, her final mission!
