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The start of the "Union"

The start of the "Union"

The magnificent image of Ivan Timoshenko and Pavel Shvets captures the first seconds after the launch on February 72020 from the Baikonur Cosmodrome launch vehicles Soyuz-2.1B , which launched into orbit another portion of the project satellites. OneWeb . In the photo we see first stage three-stage rocket with working liquid rocket engines RD-107 (in the four side blocks) and RD-108 (in the central unit). Thirty two nozzles generate a bewitchingly beautiful picture of the expiration of jet jets. How are these jets arranged and why do they have such a complex shape?

In rocket engine fuel burned in combustion chamber , turns into a very hot compressed gas that flies out through the nozzle, creating reactive thrust . In liquid rocket engines, fuel and oxidizer (fuel vapor) are supplied under high pressure at 3-3-3351. nozzles located at the beginning of the combustion chamber. By mixing the components, the nozzles spray the fuel into the combustion chamber, where during the combustion process the 3–3–353 stored in the fuel is converted. chemical energy into compression energy and heat. The resulting hot gas rushes into the jet nozzle. The tapering subsonic part of the nozzle accelerates the flow, and in the narrowest part of the nozzle — the critical section — it acquires 3–3–355. speed of sound . Further, the flow is in the expanding part, becomes supersonic and continues to accelerate to the very nozzle exit. The outflow of this jet generates reactive force in the opposite direction: it forms the bulk of the engine's thrust force. The thrust of all engines is added to the thrust of the stage, accelerating the rocket. The RD-107 engines have four main combustion chambers and two small steering chambers, while the central RD-108 has four main and four steering chambers. The fuel for them is 3-3-3357. kerosene and an oxidizing agent is liquid oxygen .

The start of the "Union"

The scheme of the liquid rocket engine

So, from the nozzles of the rocket engine burst out hot gas jets. But what exactly do we see as tongues of a bright flame? It seems that they fly out from the inside of the nozzles, but this is not so: the flame arises only at the nozzle exit, and a little lower we will figure out how this happens. In general, such a bright flame is observed only on the Earth (more precisely, in the oxygen atmosphere). If one could look at the launch of a similar rocket from any other body of the Solar System, then only pale dim streams would be visible - and no blinding fire. It's all about burnout in the earth’s atmosphere, residues of kerosene and soot formed in the combustion chamber.

Most nozzles of the combustion chamber are two-component - they simultaneously receive kerosene and oxygen. They form nine tight concentric circles in order to burn as much fuel as possible per unit of time (and the greater the fuel consumption in a rocket engine, the 3–3–389. The higher its thrust is 3–3–3278.). But the nozzles of the outermost, tenth, circle are single-component, only kerosene is fed into them. By spraying it along the wall of the combustion chamber, the nozzles create a protective gas-liquid film that reduces the temperature and thereby protects the wall from burning. Kerosene sprayed by peripheral nozzles does not have enough oxygen, so it does not burn out completely, but partially evaporates or 3–3–391. thermally decomposes to pure carbon. These kerosene vapors and carbon black form a peripheral layer of the “exhaust” jet, which is enriched with combustible substances. Since the temperature of the jet at the exit of the nozzle is about 1700 ° C, when accessing atmospheric oxygen in this layer, combustion begins - we see it as bright yellow flames. In the inner part of the jet, kerosene, which burns with a sufficient amount of oxygen, ultimately decomposes into water vapor and carbon dioxide that are invisible in a hot state. It turns out that the exhaust jet of a rocket engine shines only with its surface.

But why is the surface of the jet not evenly lit? Bright stripes and thin fibers, separated by dark "gaps", are clearly visible on it. Atmospheric air drawn in by the movement of the jet is sucked to the nozzle exit by an irregular and smooth lateral flow. On the contrary, it rushes to the edge of the nozzle with such force that it is twisted into numerous separate vortices, which increase the flow of oxygen at the places of meeting with the edge of the nozzle. Combustion in these places becomes more intense and brighter, and the huge speed of the jet stretches the spots of enhanced combustion into almost even bright bands.

The start of the "Union"

The launch of the Soyuz-2.1A launch vehicle with the Progress MS-11 transport cargo vehicle as a payload on April 42019. Bright longitudinal stripes are clearly visible on the jet flowing out of the nozzles. It is also seen that the jet itself at the exit of the nozzle is transparent - almost everywhere you can easily see the far edge of the nozzle. Photo from the site roscosmos.ru

It is clearly seen that immediately after exiting the nozzle, the jets begin to narrow. This means that the jet exits overextended . Moving in the supersonic part of the jet nozzle, the gas flow expands and accelerates, but its temperature and pressure drop. The expansion is strong, 19 times (the degree of expansion is the ratio of the nozzle cut-off area to the critical section area). Because of this, the pressure at the nozzle exit is about 0.4 atm, and the surrounding air (at which the pressure is 1 atm) compresses the jet, narrowing it.

At an altitude of about ten kilometers, the pressure at the nozzle exit will be equal to atmospheric pressure and the jet will exit smoothly, strictly cylindrical. This is the calculated flow regime that is optimal from the point of view of 3-3-3125. gas dynamics , since there is neither a starting overexpansion (in which the atmosphere creates a pressure drop opposed to the flow at the nozzle exit that counteracts the outflow), nor does the altitude under-expansion. Underexpansion will begin on b about lower altitudes: there the atmospheric pressure is even lower, so the pressure of the jet at the nozzle exit will become greater than atmospheric. Because of this, it will continue to expand beyond the nozzle, but will no longer do useful work without contact with the nozzle wall.

Due to over-expansion, the jet after exiting the nozzle has the shape of an inverted truncated cone. At its narrowest point, a bright transverse ring is visible, after which the jet expands again. In the third photo you can count several such bright rings and narrowing-expansion cycles. These rings are Mach disks - represent shock wave seals in the effluent caused by interaction with atmospheric air. When narrowing, the supersonic jet is decelerated, 3–3–3135 appears in it. direct shock wave . It is important to emphasize that this braking is not associated with friction against the surrounding air: here a geometrical narrowing of the flow and purely gas-dynamic braking of the supersonic flow in the narrowing channel occur. Due to compression, the gas heats up, which enhances the combustion of fuel residues, and this leads to a local increase in the brightness of the jet. Areas of increased brightness have a circular shape due to a combination of the effects already described: the remains of kerosene and soot are still concentrated on the periphery of the “exhaust” jet, the majority of atmospheric oxygen is mixed in there, additional heating occurs due to 3-3-3137. shock wave .

The start of the "Union"

The launch of the Soyuz-FG launch vehicle with the Soyuz MS-13 transport manned spacecraft, which delivered to the ISS Alexander Skvortsov, Luka Parmitano and Andrew Morgan. Numerous Mach discs in each of the jets are clearly visible. Also, from this angle, it is seen that it is the peripheral annular layer that shines in the Mach disks. Photo from the site  roscosmos.ru

When the jet is compressed in a direct shock wave, the pressure increases and can slightly exceed atmospheric pressure. Then, behind the Mach disk, the jet expands slightly, while accelerating. The expansion goes into overexpansion, causing a narrowing of the flow and the formation of a new Mach disk. This cyclic process creates a chain of contractions. On each of them there is a slight loss of energy, and in general the stream gradually slows down. But due to the fact that at the exit from the nozzle the jet velocity is several times higher than the speed of sound, a whole series of Mach disks manages to form. They arise until the loss of speed in the seals and energy dissipation by the surface of the jet slows it down to subsonic flow and turbulent mixing with the surrounding air.

Thus, while inside the nozzle, the jet accelerates all the time, and after exiting it, it is inhibited by the atmosphere. At the nozzle exit, the jet velocity reaches 3 km /s. This corresponds to a value of 3-3-3167. Mach numbers 3-3-33278. about 3 - due to the high temperature, the speed of sound under these conditions is approximately 1 km /s. With a diameter of the main nozzles of 0.7 meters, the distance to the first narrowing of the jet is about a meter. The flow overcomes it in 00003 seconds.

If you take a closer look (it is best to look at the enlarged versions of 3-3-3171. The first 3-3-33278. And 3-3-3327. The second 3-3-33278. Photographs), you can see that the light streaks and fibers on the jet streams are not perfectly even: they have slight distortions, thickenings and irregularities. The distance estimates in the previous paragraph help to evaluate that the characteristic length of these curvatures is decimeters. This means that the time of their existence (i.e., the time it takes their length to flow) is of the order of 00001 seconds. They reappear all the time, so we can assume that this is a periodic process with a frequency of 10 kHz (10000 times per second). It occurs on the surface of supersonic flows of high power with a complicated shape - all this creates a complex resonant picture of high-frequency acoustic radiation and sound pressure. Not only can it be deafened from it - this sound is so powerful that even the massive trusses Starts are shaken by a dense frequent tremor. Well, we were lucky, and you don’t have to worry about the ears - the sound is not attached to the text, but in the uneven bends of the light lines on the jet streams, the manifestation of acoustic vibrations is directly visible.

The color of the rocket exhaust flame depends on the type of fuel. Below is the exhaust of a rocket. Proton-M . The fuel for its engines is unbalanced dimethylhydrazine . In its molecule, H 2 NN (CH 3-3-3191. 3 3-3-3192.) 3-3-3191. 2 there are only two carbon atoms, therefore the concentration of this element is much lower than in the more saturated with carbon (from C 3-3-3191. 8 3-3-3192. to C 3-3-3191. 15 3-3-3192.) kerosene components. When dimethylhydrazine is burned, carbon black is not formed - only transparent nitrogen, carbon dioxide and water vapor are in the exhaust.

The start of the "Union"

Above left: separation of "Proton-M" from the launch pad. In the lower part of the transparent blue jet streams, pointed whitish cones are visible behind the shock waves. Photo from the site roscosmos.ru . Bottom left: the exhaust jet of the soaring “Proton-M” in a more vertical perspective, whitish cones behind the shock waves are also visible. The red line on the exhaust from the near nozzle is the oxidizer stream, nitrogen tetraoxide having a reddish-brown color. It is vented to relieve excess pressure in the tank of the central block of the first stage of the rocket. Photo from the site roscosmos.ru . Right: general view of the torch of the blue jets of the first stage of the Proton-M2 operating on asymmetric dimethylhydrazine and nitrogen tetroxide. The yellowness of the lower part of the torch is due to the illumination of the water fog that appears for a short time with the projectors of the lighting mast, visible on the right. Photo from the site roscosmos.ru

With incomplete combustion, not free carbon is formed, but 3–3–3231. carbon monoxide (CO). Its reaction with atmospheric oxygen visually resembles the blue flame of a gas stove. Therefore, the dimethylhydrazine flame is always pale, transparent and similar to the flame. spirits , and the jets at the exit of the nozzle glow dimly. Burning down on the surface of CO jets in low concentrations gives a slight pale glow that does not obscure the inside of the jet. Due to this, whitish cones with their apex against the flow are clearly distinguishable - manifestations of supersonic shock waves in the stream. In jet jets of kerosene engines, they are hidden behind the bright burning of fuel residues.

Water vapor from the exhaust of oxygen-hydrogen engines is even more transparent - this is an almost invisible stream. The last photo on the left shows the working main engine of the Shuttles RS-25 . Impact seals in its stream are visible due to the high-temperature water fog having a dense milky white color that instantly falls behind them (in the region of a sharp pressure drop). So hot fog is nowhere else to be observed visually. American heavy rocket also flies on hydrogen. Delta-IV Heavy with engines RS-68 but the flame of her exhaust is painted in a rather bright yellow color. This vaporizes the protective ablative coating on the surface of the central part of the nozzle, the substance of which stains colorless water vapor by evaporating sodium ions.

The start of the "Union"

Left: Shuttle RS-25 main engine during bench tests. The flowing stream consists of pure water vapor and is therefore completely transparent. Inside the jet, below the invisible shock wave, a dense white condensate forms - a high-temperature water fog, which is visible at the very bottom of the picture. Photo from the site en.wikipedia.org . Right: Launch of the Delta-IV Heavy launcher with RS-68 oxygen-hydrogen engines. The transparent stream of water vapor is colored yellow by the combustion products of a protective ablation coating in the central part of the nozzle. Photo from forum.nasaspaceflight.com

Photo © Ivan Timoshenko, Pavel Shvets from roscosmos.ru .

Nikolai Tsygikalo

22 май 2020 /
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