Here, a Soyuz rocket takes off in 2023, carrying three of the Expedition 70 crew to the International Space Station. This initial stage of the Soyuz launch vehicle uses four identical rocket boosters lashed around the second stage core. Each of the boosters has a rocket engine with four combustion chambers (and thus four exhaust nozzles) of its own. That creates the fiery flurry of engine plumes seen here. Most of the exhaust plumes are directed downward to provide the thrust needed to lift the rocket, but you can see a few angled slightly to either side to help stabilize the launch vehicle as it rises. (Image credit: NASA)
Tag: rocket engine
Test Firing a Rocket Engine
Watching a rocket engine start up in slow motion is always fun. This Slow Mo Guys video shows a test fire of one of Firefly’s engines, which is capable of 45,000 pounds of thrust. Gav walks us through the process of preparing to film the test as well as what his footage shows.
Green flames mark ignition of the initial fuel, and bursts of flame jerk back and forth as shock waves pass through the engine. That’s a necessary part of establishing supersonic flow through the bell-shaped diffuser at the end of the engine. Once the exhaust reaches supersonic speeds, expelling it creates a diamond-like pattern of standing shock waves and expansion fans that ultimately equalize the exhaust jet’s pressure to that of the surrounding atmosphere. (Video and image credit: The Slow Mo Guys)
Using Air to Break Up Jets
One method of breaking a liquid into droplets, or atomizing it, uses a slow liquid jet surrounded by an annulus of fast-moving gas. The gas along the outside of the liquid shears it, creating waves that the wind blowing past can amplify. This draws the liquid into thin ligaments that then break into droplets. This is a popular technique in rocket engines, where cryogenic liquid fuels often need to be atomized for efficient combustion. When things aren’t working exactly right, however, the liquid jet may start flapping instead of breaking up. In this case, the jet will swing back and forth, but only part of it will atomize. For a rocket engine, this would mean slower and less efficient combustion – never desirable outcomes! (Image credit: A. Delon et al.)
Burning a Rocket Underwater
In a recent video, Warped Perception filmed a model rocket engine firing underwater. Firstly, it’s no surprise that the engine would still operate underwater (after its wax waterproofing). The solid propellant inside the engine is a mixture of fuel and oxidizer, so it has all the oxygen it needs. Fluid dynamically speaking, though, this high-speed footage is just gorgeous.
Ignition starts at about 3:22 with some cavitation as the exhaust gases start flowing. Notice how that initial bubble dimples the surface when it rises (3:48). At the same time, the expanding exhaust on the right side of the tank is forcing the water level higher on that side, triggering an overflow starting at about 3:55. At this point, the splashes start to obscure the engine somewhat, but that’s okay. Watch that sheet of liquid; it develops a thicker rim edge and starts forming ligaments around 4:10. Thanks to surface tension and the Plateau-Rayleigh instability, those ligaments start breaking into droplets (4:20). A couple seconds later, holes form in the liquid sheet, triggering a larger breakdown. By 4:45, you can see smoke-filled bubbles getting swept along by the splash, and larger holes are nucleating in that sheet.
The second set of fireworks comes around 5:42, when the parachute ejection charge triggers. That second explosive triggers a big cavitation bubble and shock wave that utterly destroys the tank. If you look closely, you can see the cavitation bubble collapse and rebound as the pressure tries to adjust, but by that point, the tank is already falling. Really spectacular stuff! (Video and image credit: Warped Perception)
Watching a Model Rocket Burn
Rockets operate on a pretty simple principle: if you throw something out the back really fast, the rocket goes forward. Practically speaking, we accomplish this with a combination of chemistry and physics, by burning fuel and oxidizer together and accelerating the exhaust out a nozzle. Solid rocket propellant, like that found in the model rockets shown here, is a combination of fuel and oxidizer that don’t react until they’re ignited. You don’t want your rocket to just explode as soon as it’s lit, though, so solid rocket motors are carefully designed to burn in a particular way. By packing the propellant into different shapes – and even including patterns of propellants with different burn rates – engineers can create a rocket that burns with the thrust pattern they want.
In the case of this model rocket motor, what we observe is not really how it is intended to burn; you can see how some of the combustion products are working their way out of cracks that wouldn’t normally exist. But the video and animation do show how the burn front moves gradually through the engine, allowing it to produce a relatively steady amount of thrust for a longer period before reaching the darker burning propellant on the left, which would normally launch the model rocket’s parachute. (Image and video credit: Warped Perception; via Gizmodo)
Bottle Rocket Shock Diamonds
Mach diamonds or shock diamonds can often be seen in the exhaust of rocket engines. Here they’re shown in high-speed video of a bottle rocket’s launch. The rocket’s exhaust exits at a pressure that is higher than the surrounding atmosphere, which causes the exhaust to bulge outward and forms two expansion fans, seen in pink, to lower the pressure. The pressure actually drops too low, however, causing shock waves, seen in turquoise, to form in order to raise the exhaust’s pressure. This back-and-forth between shock waves and expansion fans continues, forming the diamond shapes we see. Each subsequent set gets weaker as the exhaust closes in on the right pressure, and ultimately the series of diamonds fades into turbulence. (Image credit: P. Peterson and P. Taylor, source)
Capturing SLS
NASA’s recent full-scale ground test of their Space Launch System (SLS) rocket was notable for more than just the engine. It was an opportunity to use a new high dynamic range, high speed camera prototype,
HiDyRS-X, to capture the rocket’s exhaust in detail never seen before. Usually the extreme brightness of the rocket exhaust makes it impossible to see any structure in the flow without completely obscuring the ground equipment. With this camera, however, engineers can see how the engine, exhaust, and surroundings all interact. Be sure to check out the full video. I particularly like watching how the rocket’s exhaust entrains dust and sand from the ground nearby. (Image credit: NASA, source; submitted by Chris P. and Matt S.)
Shock Diamonds
Rocket engine exhaust often contains a distinctive pattern known as shock diamonds or Mach diamonds. These are a series of shock waves and expansion fans that increase and decrease, respectively, the supersonic exhaust gases’ pressure until it equalizes with atmospheric pressure. The bright glowing spots visible to the naked eye are caused by excess fuel in the exhaust igniting. As awesome as shock diamonds look, they’re actually an indication of inefficiencies in the rocket: first, because the exhaust is over- or underexpanded, and second, because combustion inside the engine is incomplete. Both factors reduce a rocket engine’s efficiency (and both are, to some extent, inescapable). (Photo credit: XCOR)
Inside a Rocket
Rockets often utilize liquid propellants for their combustion. To maximize the efficiency during burning, the liquid fuel and oxidizer must mix quickly and break up into an easily vaporized spray. One method to achieve this is to inject the fuel and oxidizer as liquid jets that collide with one another. For high enough flow rates, this creates a highly unstable liquid sheet that quickly atomizes into a spray of droplets. The animation above shows an example of two impinging jets, but rockets using this method would typically have more than just two injection points. Other rockets use co-axial or centrifugal injectors to mix and atomize the fuel and oxidizer prior to combustion. (Image credit: C. Inoue; full-scale GIF)
Mach Diamonds
Rocket engines often feature a distinctive pattern of diamonds in their exhaust. These shock diamonds, also known as Mach diamonds, are formed as result of a pressure imbalance between the exhaust and the surrounding air. Because the exhaust gases are moving at supersonic speeds, changing their pressure requires a shock wave (to increase pressure) or an expansion fan (to decrease the pressure). The diamonds are a series of both shock waves and expansion fans that gradually change the exhaust’s pressure until it matches that of the surrounding air. This effect is not always visible to the naked eye, though. We see the glowing diamonds as a result of ignition of excess fuel in the exhaust. As neat as they are to see, visible shock diamonds are actually an indication of inefficiencies in the rocket: first because the exhaust is over- or under-pressurized, and, second, because combustion inside the engine is incomplete. (Photo credit: Swiss Propulsion Laboratory)
