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WHAT YOU NEED TO KNOW ABOUT ROCKET FUELS

GREAT COLLECTION OF TECHNOLOGIES AND TECHNOLOGICAL RECIPES

Glossary of terms

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So, let's assume that you are not only engaged in the construction of missiles, but also with the design of rocket engines or the development of new alternative solid rocket fuels. Of course you can use the proposed rocket fuels that have long been open are also used, but it would be much nicer if you knew on what basis all the fuel data is created and as if they are working. I do not advise you to say this article to you, if you are not interested in it, but still it might be useful to you. This article is more intended for chemists who are developing rocket fuels for those who are engaged in the design of rocket engines. So look yourself.

So we know that the fuel efficiency is determined by its specific gravity - the indicator by which is judged on what total impulse can be obtained by burning 1 kg of fuel:

what total impulse can be obtained by burning 1 kg of fuel

The total impulse is the creation of thrust at the time of engine work.

The total impulse is the creation of thrust at the time of engine work.

Simply put, the fuel efficiency is determined by the fact that it is possible to obtain traction by burning a certain amount of fuel per pore.

We know that the more pressure in the combustion chamber, the stronger also the more intense the gases escape from the nozzle, pushing the rocket in the opposite direction, plus, so the thrust of the engine increases. We also know that when the temperature rises, the pressure of the gases seems to increase, plus, hence the thrust of the engine also increases. Following all this, we can say that fuel with the maximum values ​​of specific thrust should have the maximum combustion temperature as possible, while the molecular authority of its combustion products must be as small as possible.

For example, the fuel on which the solid fuel boosters of the shuttle operate consists of ammonium perchlorate ( NH 4 ClO 4 ), polyurethane, aluminum powder, also iron oxide ( Fe 2 O 3 ):

69.9% (NH 4 ClO 4 ) + 12.04 (polyurethane) + 1.96 (hardener) + 16% (Al) + 0.07 ( Fe2O3)

- it is involved here as if the catalyst of combustion, almost without affecting the specific fuel draft. Aluminum powder is injected in order to increase the energy characteristics of the fuel, but not everyone understands as if these characteristics increase.

So, ammonium perchlorate at a temperature decomposes, but at different temperatures in different ways. At high temperatures, it decomposes, giving away almost all the oxygen that goes into oxidizing the fuel (in this case polyurethane). In this case, products such as CO, CO 2 , N 2 , H 2 O, HCl , and small impurities of other products are preferably formed. The CO density (carbon monoxide) is below the CO 2 density (carbon dioxide), but it means under normal conditions, it takes a larger volume than CO 2 , but with increasing temperature, the volume of gases increases. So it can be said that it is advisable to use such a combination of fuel components, at which the main combustion product is CO . But not for all fuels, this combination is suitable, and after reading the entire article, you will understand why.

Now let's discuss aluminum - why is it needed ?. Surely from the chemistry course you know that aluminum has a high calorific value, and this is one of the reasons why it is introduced into rocket fuels, but it is not the first one. You should also know that, filled with its interaction with water at high temperatures, the final products will be Al 2 O 3 and H 2 - namely hydrogen, it greatly increases the energy characteristics of the fuel, also do not think please that it plays a role of fuel here . The fact is that under normal conditions it occupies a huge volume with a minimum weight, i.е. his density is very small. Of course, if you also take into account that aluminum significantly increases the temperature of combustion of fuel, then as a result, the pressure in the chamber is enormous, so this fuel is considered one of the best solid rocket fuels that can be used on an industrial scale at the moment. Of course this fuel is far from ideal, but this is another matter.

Let's now consider the fuel that I developed not so long ago. More precisely, we have finalized it, tk. Use sorbitol as a binder, I did not come up with anything. Surely you read about Powerful caramel - as if we called it, also observed that its composition is sulfur. There I briefly described why it is needed, but let's take a closer look. So, let us reproduce the combustion equation:

6C 6 H 14 O 6 + 26 KNO 3 + 13S = 13K 2 S + 36CO 2 + 13 N 2 + 42 H 2 O (theoretically) .

In this equation, we did not take into account the interaction of CO 2 with H 2 O or H 2 O with K 2 S , I prefer to first consider a purely theoretical mechanism of the reaction, because the reaction at the earliest stage proceeds precisely in this direction, interact with each other giving already end products of combustion. I also want to note that at different pressures, the products will be slightly different in composition. As I said, sulfur here displaces the oxygen atom, which increases the energy yield of the reaction, and a much larger volume of gaseous products is also formed. And unlike KOH , which is formed during the combustion of ordinary caramel, K 2 S practically does not interact with CO 2 , keeping its volume in the RD chamber. All this contributes to a significant increase in the specific thrust of fuel, and this fuel is very powerful, compared to conventional caramel. Powerful caramel is a very high-calorie fuel, i.e. It has a high combustion temperature, which acts more efficiently, but there are disadvantages: it can burn the motor housing if it is made of insufficiently heat-resistant material. It is also very difficult to manufacture and not entirely suitable for small engines, but ideal for large ones. It is good to operate fuel pellets from 100 gr . The reason why it can not be successfully used in the manufacture of small engines is its low plasticity. It can not be overheated, because sulfur just melts and fuel becomes non-homogeneous, it's also his main problem.

Let's now consider the usual caramel, the combustion equation which Richard Nakka proposed.

C 6 H 14 O 6 + 3.345 KNO 3 -> 1.870 CO 2 + 2.490 CO + 4.828 H 2 O + 2.145 H 2 + 1.672 N 2 + 1.644 K 2 CO 3 + 0.057 KOH (practically)

I do not like one thing in his reaction. By its equation, KOH is formed, but this combination can not be formed in any way, because it simultaneously interacts with CO 2 and turns into K 2 CO 3 , especially at such a high temperature. I want to notice that the CO 2 + H 2 O + CO + C reaction still does not know in the COO course, the occupation is that at different temperatures and pressures it can flow in different directions: with the formation of such products as CH 3 OH, CH 2 O, CH 3 COOH , H 2 , etc. To ensure that this reaction proceeds towards the education of elementary H 2 , a very high temperature is needed, which the fuel can not give. So it would not be absolutely correct to say that only H 2 is formed , and the thermal characteristics of a regular caramel are lower than that of a powerful caramel. There are some other reasons that this fuel is not effective:

First: KOH is formed in the products, which absorbs an impressive amount of CO 2 , resulting in a decrease in the number of gaseous products released.

Secondly: For the oxidation of sorbitol KNO 3 gives only a pair of oxygen atoms, which significantly reduces the efficiency of the fuel.

Thirdly: due to the relatively low combustion temperature of the fuel, the reaction proceeds in the course of reducing the volume of gaseous products.

Well, this fuel has impressive advantages: it is easy to manufacture, however, because of the high content of sorbitol, it retains high plasticity for quite a long time, with sufficiently strong heating it can easily be poured into the hull without much difficulty. For those who are only engaged in the design of rockets, but not engines, this fuel is also ideal for testing the flight characteristics of its rocket.

Generally I want to say that although caramel is also considered to be chosen among rocket-making classes, fuel, but still it is not effective enough, if undoubtedly engage in serious engine development. I also used to work quite a lot with the usual caramel, but eventually it came down to a powerful caramel. Usual caramel is perfect for beginner rocket-modelists, but professionals in this district, we would still recommend trying a powerful one.

After all this reasoning, you will probably ask the task: "Why not introduce the same aluminum into the ordinary caramel, or why not increase the essence of sorbitol so that the main product of combustion is CO ?" In chemistry there is such a thing, as if the activation energy is the minimum energy that must be spent on the reaction. Moreover, if the energy yield of the reaction is greater than its activation energy, then such a reaction proceeds along the chain mechanism. And the more the energy yield of the reaction is, the less its activation energy, the faster and more fully this reaction proceeds. In this case, as if I said, ordinary caramel is a very low-calorie fuel, and the energy yield of the reaction does not allow to fully activate the complex {Al + H 2 O + CO + CO 2 } , so the introduction of aluminum powder will only reduce fuel efficiency. Well, once in the ordinary can not, why not try in a powerful one - I will answer: because of sulfur, which will form an Al 2 S 3 product with aluminum, which will also reduce the fuel efficiency. But now I'm still experimenting with this fuel with the addition of aluminum, because still do not quite know what happens in practice, the results of the tests will then be published.

I also want to make a note about the combustion catalysts. In case of a powerful caramel, their use is inappropriate. In the case of a traditional caramel, copper salts can be used that do not form crystals at all, but also in the case of designing particularly impressive engines. Various compounds of transition metals, for example Fe 2 O 3 , are used for chlorates and perchlorates, which significantly increases the burning rate of the fuel, with a content of only 0.5% in the initial mixture. But for the preparation of fuel for engines on the model of rockets, it is not used in any way, because Perchlorate fuel in itself has a solid combustion rate also very high energy characteristics.

In general, I want to say one thing: rocket fuels - the occupation is very subtle, than it seems to the main eye, but there are a lot of modifications of their varieties. And the choice of rocket fuels is limited not only by their composition and properties, but also by a number of other characteristics.

The main specific requirements for solid RT:

Uniformity in the distribution of components plus, consequently, the persistence of physicochemical and energy properties in the block, the stability of the combustion in the RD chamber is also stable, but a complex of physical and mechanical properties that ensure the engine's performance under conditions of overload, variable temperature, vibrations is also very when designing amateur rocket engines.

Also, fuel must meet the basic environmental standards, but it is in industry and, if possible, to own low-toxic products, which are also safe for the environment, also living things, which can not be said about perchlorate fuel.

Personally, we think that in the future there will undoubtedly be new types of fuels that will significantly exceed by their characteristics, those that are available at that moment.

But for us this is not particularly important, because we work with very small quantities in comparison with industry. In the future, everything is possible. Already engines are being developed using alternative energy sources, for example, a plasma engine has already been practically developed and its tests are being conducted, it seems that the elementary components of the ion engine are being tested. The specific thrust of such engines in thousands is also tens of thousands of times higher than the specific thrust of engines using chemical energy. But even these engines, which now seem to us fantastic, will also lose their significance in the distant future, and they will also give way to others, which now exist only theoretically.

From the Creator: I have stated in this article the very essence is also simple, what you need to know about rocket fuels, mainly solid ones, also think that this information will be useful to you, for your work. The whole of books on rocket fuels has been written, but most of these books are intended only for professionals who develop liquid-propellant rocket engines, and without studying the mathematics of the laws of outer and inner ballistics, studying them will become very problematic. I want to say that I wrote this article personally from myself, based on my own theoretical knowledge, I also think that everything is clear to you.

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Author: Олег
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Date of publication 22.02.2005гг