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 nozzle
Why Compressed Air Cans Get Cold
Anyone who’s used a can of compressed air to clean their computer or keyboard knows that the can quickly gets quite cold to the touch. This Minute Physics video explores some of the thermodynamics behind that process. Henry first identifies a few explanations that don’t quite line up with observations, before focusing in on the contents of the can: 1,1-difluoroethane. Inside the sealed can, this chemical sits in an equilibrium of part-liquid, part-vapor. But when pressure is released by opening the nozzle, the liquid boils, generating extra vapor and cooling whatever remains in the reservoir.
Although it’s not a good explanation for the compressed air can’s cooling, the cooling of an expanding gas is very important in applications like supersonic wind tunnels. That first equation you see at 0:36 in the video (for isentropic adiabatic expansion) is key to what happens in a nozzle with supersonic flow. As the flow accelerates to supersonic speeds, its temperature drops dramatically. When I was in graduate school, we actually had to preheat our hypersonic wind tunnel (in pretty much the same way you would preheat your oven at home) before we ran at Mach 6 because otherwise the temperature inside the test section would drop so low that the oxygen would liquefy out of the air! (Image and video credit: Minute Physics)
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)
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)
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)
Start Your Rocket Engine
When supersonic flow is achieved through a wind tunnel or rocket nozzle, the flow is said to have “started”. For this to happen, a shock wave must pass through, leaving supersonic flow in its wake. The series of images above show a shock wave passing through an ideal rocket nozzle contour. Flow is from the top to bottom. As the shock wave passes through the nozzle expansion, its interaction with the walls causes flow separation at the wall. This flow separation artificially narrows the rocket nozzle (see images on right), which hampers the acceleration of the air to its designed Mach number. It also causes turbulence and pressure fluctuations that can impact performance. (Image credit: B. Olson et al.)
SpaceShipTwo Lights It Up
Monday morning Virgin Galactic and their partners at Scaled Composites reached a new milestone in their commercial sub-orbital spaceflight program, firing SpaceShipTwo’s main engine for the first time and accelerating to supersonic speeds. The upper image shows hints of Mach diamonds, formed by a series of shock waves and expansions, in its exhaust. This is very common for rockets since most have a fixed geometry, and, by extension, a fixed Mach number and exhaust pressure. (Photo credits: Virgin Galactic and Mars Scientific)
Bottle Rocket Shock Waves
This high speed video shows schlieren photography of a bottle rocket’s exhaust. The supersonic CO2 leaving the nozzle is underexpanded, meaning its pressure is still higher than the ambient atmosphere. As a result, a series of diamond-shaped shock waves and expansion fans appear in the exhaust jet. Each shock and expansion changes the pressure of the exhaust until it ultimately reaches the same pressure as the ambient air. This distinctive pattern, also known as Mach diamonds or shock diamonds, often occurs in wake of rockets. (Video credit: P. Peterson and P. Taylor)
Unsteady Rocket Nozzle
This numerical simulation gives a glimpse of flow inside an unsteady rocket nozzle. The nozzle is over-expanded, meaning that the exhaust’s pressure is lower than that of the ambient atmosphere. A slightly over-expanded nozzle causes little more than a decrease in efficiency, but if the nozzle is grossly over-expanded, the boundary layer along the nozzle wall can separate and induce major instabilities, as seen here. In the first segment of the video, turbulent structures along the nozzle wall boundary layer are shown; note how the boundary layer becomes very thick and turbulent after the primary shock wave (shown in gray). This is due to the flow separating near the wall. The second half of the video shows the unsteadiness this can create. The primary shock wave splits into two near the wall, creating a lambda shock wave, named for the shape of the lower case Greek letter. This shock structure is indicative of strong interaction between the boundary layer and shock wave. (Video credit: B. Olson and S. Lele)
Homemade Hybrid Rocket Engine
In this video, Ben Krasnow details and demos a small hybrid rocket engine he built in his workshop. Hybrid rockets utilize propellants that are two different states of matter, in this case gaseous oxygen as the oxidizer and solid acrylic as the fuel. Krasnow’s verbal explanation of a convergent-divergent nozzle, used to accelerate flow to supersonic speeds is not quite right. In reality, a compressible fluid like air reaches the sonic point (i.e. Mach 1) at the narrowest point of the nozzle, also called the throat. The divergent portion of the nozzle causes the compressible fluid to expand in volume, which drops the temperature and pressure while the velocity increases beyond the speed of sound.
Krasnow says he did no calculations for his rocket, but I decided to have a little fun by doing some myself. Supersonic flow through the nozzle is only achieved if the flow is choked, meaning that the mass flow rate through the nozzle will not increase if the downstream pressure is decreased further relative to the upstream pressure. For Krasnow’s rocket, the downstream pressure is atmospheric pressure (14.7 psi) and the upstream pressure is provided by the oxygen canister, which he notes was at most 80 psi. Fortunately, the upstream pressure necessary to choke the nozzle is only 27.8 psi, so even with the ball valve partially closed, Krasnow’s rocket is definitely capable of supersonic speeds.
The Mach number achievable by any given supersonic nozzle is related to the ratio of the nozzle throat to its exit diameter (#). Krasnow gives the throat diameter as ¼-inch and the exit diameter as 5/8-inch. This means that the Mach number at the exit of the nozzle, assuming choked supersonic flow, is about Mach 3.4. (Video credit: Ben Krasnow; via Universe Today; submitted by jshoer)
