Imperceptible complexity of rocket technology. Part 2: Solid fuel engines

In the comments to the first article I was rightly pointed out that I did not at all tell about the solid-fuel engines used in astronautics. Indeed, in one article, even a simple educational program did not fit. Therefore, I invite those who wish to read the sequel.

Legends of antiquity deep

Black (smoky) powder was invented by the Chinese in the ninth century. And in the eleventh century, documentary evidence appeared on the creation of military rockets on black powder (Wujing Jingyao, 1044):

Pay attention to the design of the rocket in the center. This arrangement of combat rockets remained unchanged for more than eight hundred years, until the beginning of the twentieth century, and fireworks with it are still being made!
Human thought did not stand still. Already in 1409 in Korea, they came up with the multiple launch rocket system ( Khvach ):

There is also a legend about a Chinese official, Wang Hu , who in about the sixteenth century assembled an apparatus from an armchair, two serpents (not in all versions of the legend), and forty-seven rockets (obviously, from shells like Khvachi)

Alas, the inventors of that time were fearless from ignorance, they did not think about the methodology of flight design tests, and suffered from excessive optimism. Therefore, the first test was the last. When the roar of engines quieted down and the smoke cleared, neither Wang Hu nor his apparatus was found.
Rockets together with conquerors from the East (Mongols, Ottomans) came to Europe. The word "rocket" - from the Italian "little spindle." With varying intensity, rockets were used throughout Europe and Asia.
The next significant stage was the fourth Anglo-Mysore war (1798-1799). For the first time in the world, Mysore rockets had a steel shell, various purposes (incendiary, anti-personnel with cutting edges) and were massively used. The rocket ship Corps of Tipu Sultan consisted of five thousand people.

The impressed British, who also seized hundreds of missiles as trophies in Seringapatam, decided to reproduce the technology. This is how the Congrive missiles were born, which were widely used in the Napoleonic wars and subsequent conflicts, and even leaked to the US anthem.

Beginning in the mid-nineteenth century, rifled artillery began to outperform missiles both in range and accuracy, and quick-start use of the Khvachi type was forgotten. Therefore, combat missiles gradually disappeared from the scene, however, even in the First World War, they were still used. The photo shows the French Newpor-16 with Le Prieur missiles to fight airships and balloons. Despite the electric igniter and the installation on the plane, these are the good old powder rockets of the same layout as the Chinese of the eleventh century.

Traveled to the shore "Katyusha"

Rockets on black powder did not become more complicated and more powerful due to the limitations of the powder itself. It was impossible to make a powder test with stable parameters in the party, a large caliber, and burning at least a couple of seconds. For the development of solid-propellant missiles required new material. At the end of the nineteenth century, smokeless powder was invented. However, on an artillery smokeless powder rocket did not work. Began the search for smokeless rocket powders.
The gasdynamic laboratory of Tikhomirov and Artemyev in the USSR achieved the greatest success in this matter. They created a so-called. ballistic gunpowder, from which it was already possible to make fairly large pieces and put them in rockets. In addition, they remembered the idea of ​​volley fire in time. That is how Katyusha was born - RS-82 and RS-132 missiles for aviation, M-8 and M-13 for ground installations. In more detail about gunpowder, their types and production can be read here .

Advances in technology have led to the fact that during the Second World War, the USSR actively used combat rockets on solid fuel more actively than other countries. The weapon turned out to be very effective, was applied from air, land, ship carriers, new modifications of longer range or caliber were developed.

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Persistent Mixed Sergeant

Ballistic powder had its physical limitations. The maximum diameter of the checkers was measured in centimeters, and the burning time - in seconds. Even if von Braun wanted to, he could not have made the V-2 on ballistic gunpowder. We needed a new kind of solid fuel. It became so-called. composite fuel ("rubber fuel"). In 1942, John Parsons created the first engines of mixed-fuel engines using asphalt . And experiments with the components found that the most effective fuel is a mixture of ammonium perchlorate (oxidizer), aluminum and polyurethane (fuel) and polybutadiene to improve the parameters of combustion, molding and storage of the motor checkers. The first rocket with a mixed-fuel engine was the MGM-29 "Sergeant" (first flight - 1956), whose engine had a diameter of 0.7 meters and worked 34 seconds. It was a qualitative breakthrough - a rocket weighing 4.5 tons and a length of 10 meters could throw a warhead weighing 0.8 tons per 135 km, and did not require a convoy of vehicles with fuel components and tens of minutes to refuel.

After the medium-range missiles was developed ICBM "Minutemen" on mixed fuel. Its advantages can be seen by comparing with similar Soviet projects. The fact is that in the USSR Korolev attempted to create a ballistic missile on ballistic powder (RT-1) and on Soviet-recipe mixed fuel with worse characteristics (RT-2). Comparison of characteristics is very clear:

Please note that in the RT-1 rocket it was necessary to actually build an assembly of four separate engines due to limitations on the diameter of the ballistic gun checkers. At RT-2 and “Minuteman” there is one piece, a big one, but at the first stage there are 4 nozzles.

Features of solid fuel engines

Ability to create a very large thrust engine

The most powerful rocket engine in history was the Space Shuttle solid-fuel booster. Its initial thrust is 1,250 tons, and its peak value reaches 1,400 tons, which is approximately 1.8 times the thrust of the most powerful rocket engines (F-1 and RD-170). The most powerful of the engines being operated is also solid propellant - these are the side accelerators Ariane-5, their thrust is 630 tons.

The thrust profile is specified during construction.

LRE can be throttled - to change the amount of thrust, sometimes in a very large range. The solid engine burns uncontrollably, and the amount of thrust can be adjusted only by means of an internal channel of a special profile. Different channel profiles allow you to have different profiles of thrust in time:

Impossibility of emergency shutdown

After the RTD has turned on, you cannot turn it off. On the side accelerators of the Space Shuttle they put charges of explosives so that in case of a catastrophic failure they would not fly in an arbitrary direction. All the flights of the shuttles took place with people, and the knowledge that a special person ( RSO ) was sitting in the bunker, who would blow up the boosters in case of an accident, added nervousness. The side accelerators of the Challenger in the 1986 crash were not damaged by the central tank explosion and were blown up a few seconds later.

Inability to restart

It follows from the previous paragraph. For each inclusion, you must have a separate stage with the engine. This is important for accelerating units, which should already be included in space several times.

Traction cutoff

If necessary, turn off prematurely working solid propellant rocket motors (for example, when accelerating to the required speed when shooting at an incomplete range), the only thing that can be done is so-called. cutoff thrust. Special charges open the upper part of the combustion chamber, zeroing the craving. The engine still works for some time, but the flames are pulled out from both sides, which, in fact, does not add speed.

Smaller specific impulse

The specific impulse (measure of fuel efficiency) of a solid propellant solid propellant is lower than that of a liquid rocket engine. This leads to the fact that in combat ICBMs usually one step more. Liquid UR-100 and R-36 have two stages, which is optimal in ballistics, and three stages have to be put on solid Topol. Therefore, mass perfection solid propellant worse.

Ease of manufacture and operation

After the fuel is poured into the combustion chamber, it becomes similar to rubber in consistency and does not require additional operations. In contrast to the booster blocks on the LRE, which need to be refilled and checked at the cosmodrome, the booster blocks with solid propellant rocket motors come ready from the manufacturer. Combat missiles with solid-propellant rocket motors also come ready from the manufacturer and have been on duty for decades, without requiring additional fuel operations from personnel. To be fair, it should be noted that combat ICBMs with LRE also come from the manufacturer “ampulized”, without requiring refueling in the mine.

The complexity of management mechanisms

In the LRE, you can select components after TNA and use them in hydraulic steering machines to deflect the nozzle. There is no such possibility in the solid-propellant solid propellant rocket motor, so you have to put powerful batteries or generators for steering gears. For example, on a solid-fuel accelerator, the Space Shuttle was equipped with special gas generators , which burned hydrazine from individual tanks and fed hydraulic steering gears, which deflected the nozzle for flight control. At the TTU PH "Titan-4" stood tanks with nitrogen tetraxide , which was asymmetrically injected into the nozzle through controlled nozzles, creating an asymmetry of thrust.
On overclocking units, it is necessary to install separate engines of orientation on liquid fuel, and for the time of engine operation to ensure stabilization by promotion.

Inability to regenerative cooling

The walls of the combustion chamber are not isolated yet burned fuel, this is an absolute plus solid propellant rocket motors, but the situation with the nozzle is the opposite. The matter is complicated by the fact that the combustion temperature of solid fuels is higher, and the combustion products have a much greater erosion effect than in the LRE. The nozzle is eroded by combustion products, which further degrades the engine parameters due to violation of the geometric parameters of the nozzle. Without the flow of components that can cool the nozzle, other methods have to be invented. There are two of them - cooling by radiation and evaporation (ablation). The critical section (the narrowest part of the nozzle, the greatest loads there) is made of very hard and heat-resistant materials (specially treated graphite), the less loaded parts are made of heat-resistant materials. More details can be read here .
But these solutions come at a price — the solid propellant nozzle is harder than the LRE. You can see it very well in the photos from this site :

Left LRE, right solid propellant rocket motors

Conclusion

In the modern solid propellant rocket technology found four main niches:

1. Military missiles. RDTTs provide high combat readiness, simplicity and reliability of intercontinental and other missile engines.
2. Starting accelerators. The ability to create a very powerful and cheap engine is used when it is necessary to tear a more efficient but less powerful LRE out of the ground.
3. Overclocking blocks. The prevalence, simplicity, reliability, mastery of industry, ease of storage led to the widespread use of solid propellant rocket motors as a booster in the United States. The specific impulse of the solid propellant rocket motor is only ~ 10% less than that of the heptyl / amyl pair (the mass of the RB IUS is even less than the “Breeze-M” because of the spaceport’s lower width), and one and a half times more efficient hydrogen / oxygen blocks were not used in the “Space Shuttle, which not so long ago took a large number of satellites.
4. Fireworks and rocket modeling. The simplicity of making a small solid-propellant solid propellant rocket motor generator has led to the fact that rockets are used in fireworks (there is almost certainly black powder) and in rocket-modeling. Simple home-made or purchased compositions (there are standard ones in stores) allow making small rockets for entertainment and training.

PS There will be a third part. About types of liquid fuel, the sizes of steps, starting constructions and money. Not very soon - in one article.