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So, suppose that you are not only engaged in building rockets, but also in constructing rocket engines or developing new alternative solid rocket fuels. Of course, you can use the rocket fuels offered to you, which have long since been discovered are also used, but it would be much more pleasant 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, but it could still be useful to you. This article is more intended for chemists who are engaged in the development of rocket fuels also for those who are engaged in the design of rocket engines. So see for yourself.

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

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

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

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

Simply put, fuel efficiency is determined by the fact that what kind of traction can be obtained by burning a certain amount of fuel into a piece of pores.

We know that the higher the pressure in the combustion chamber, the stronger the more intensely the gases escape from the nozzle, pushing the rocket in the opposite direction, plus, so the engine thrust increases. We also know that with increasing temperature the pressure of gases seems to increase, plus, therefore, the engine thrust also increases. Following from all this, it can be said that the fuel with the limit values ​​of the specific thrust should have the highest possible combustion temperature, and the molecular authority of its combustion products should be as low as possible.

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

69.9% (NH 4 ClO 4 ) + 12.04 (polyurethane) + 1.96 (hardener) + 16% (Al) + 0.07 ( Fe 2 O 3 )

- he is involved here as if a combustion catalyst, almost without affecting the specific thrust of the fuel. Aluminum powder is introduced in order to improve the energy characteristics of the fuel, but not everyone understands that the given characteristics increase.

So, ammonium perchlorate decomposes at a temperature, but at different temperatures in different ways. At high temperatures, it decomposes, giving up almost all of the oxygen that goes to oxidize the fuel (in this case, polyurethane). At the same time, products such as CO, CO 2 , N 2 , H 2 O, HCl , and small impurities of other products are formed preferentially. CO (carbon monoxide) density is lower than CO 2 (carbon dioxide), however, it means that under normal conditions it occupies 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, in which CO would be the main product of combustion. But this combination is not suitable for all fuels, 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, it is also one of the reasons why it is introduced into rocket fuels, but it is by no means the very first. You should also know that Al 2 O 3 and H 2 filled with its interaction with water at high temperatures will manifest Al 2 O 3 and H 2 - namely hydrogen, it is he who significantly improves the energy characteristics of the fuel, also don’t think that it plays the role of fuel . The fact is that under normal conditions it occupies a huge volume with a minimum weight, i.e. its density is very small. Of course, if we also consider that aluminum significantly increases the temperature of combustion of the 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 that is another question.

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

6C 6 H 14 O 6 + 26KNO 3 + 13S = 13K 2 S + 36CO 2 + 13N 2 + 42H 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 the purely theoretical reaction mechanism, because the reaction at the very early stage proceeds in that direction, and only then the products interact with each other giving the end products of combustion. I also want to note that at different pressures the products will be slightly different in composition. As I have already said, sulfur here displaces the oxygen atom, due to which the energy yield of the reaction increases, and a much larger volume of gaseous products is also formed. And unlike KOH , which is formed during the combustion of conventional caramel, K 2 S practically does not interact with CO 2 at all , keeping its volume in the chamber of the RD . All this contributes to a significant increase in the specific thrust of the fuel. This fuel is also very powerful compared to conventional caramel. Powerful caramel is very high-calorie fuel, i.e. It has a high combustion temperature, which acts more efficiently, but there are downsides: it can burn through the engine block if it is made of an insufficiently heat-resistant material. It is also very difficult to manufacture and not quite suitable for small engines, but ideally for large ones. From him, it is good to act fuel checkers from 100g . The reason why it cannot be successfully used in the manufacture of small engines is its low ductility. It is impossible to overheat it, because sulfur will just melt and the fuel will become inhomogeneous, this is also its main problem.

Let's now consider the usual caramel, the burning equation of which was proposed by Richard Nacka .

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. According to his equation, KOH is formed, but this association cannot form in any way, because it will simultaneously interact with CO 2 also turn into K 2 CO 3 , especially at such a high temperature. I want to seem to note that the CO 2 + H 2 O + CO + C reaction is not yet known in the cook course, the lesson is that at different temperatures and pressures it can flow in different directions: with the formation of such products as if CH 3 OH, CH 2 O, CH 3 COOH , H 2 is also another. In order for this reaction to proceed in the direction of bringing up elementary H 2 , a very high temperature is needed, which the fuel cannot give. So it would not at all be right to say that only H 2 is formed , and the thermal characteristics of an ordinary caramel are lower than powerful. There are still some reasons that act this fuel low-efficiency:

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

Secondly: The oxidation of sorbitol KNO 3 gives only a pair of oxygen atom, because of which fuel efficiency is significantly reduced.

Third: due to the relatively low temperature of combustion 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, due to the high content of sorbitol, it retains high plasticity for quite a long time, with sufficiently strong heating, it can be easily poured into the body without much difficulty. For those who are engaged only in the design of rockets, but not in any way engines, this fuel is also ideal to check the flight characteristics of their missiles.

In general, I want to say that although caramel is also considered to be chosen among the rocket-model occupation, the fuel, but still it is by no means effective enough if you undoubtedly engage in serious engine development. I also worked a lot with ordinary caramel before, but eventually I got down to the powerful one. Ordinary caramel is perfect for beginner rocket makers, but we would recommend professionals to try powerful ones.

After all these arguments, you will probably ask: "Why not introduce the same aluminum into the usual caramel, or why not to increase the essence of sorbitol so that CO is the main product of combustion?" 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 via a chain mechanism. And the more the energy yield of the reaction is also, the smaller its activation energy is, the faster and more fully the reaction takes. In this case, as if I said, the usual 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 } , and therefore the introduction of aluminum powder will only reduce the efficiency of the fuel. Well, well, once in the usual it is impossible, why not try a powerful one in any way? I will answer: because of the sulfur that will form the third-party product Al 2 S 3 with aluminum, which also reduces the fuel efficiency. But now I am still experimenting with this fuel with the addition of aluminum, because you still don’t quite know what happens in practice, then publish the test results.

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

In general, I want to say one thing: rocket fuels are a very delicate exercise than they seem to be, but there are a lot of modifications as well. And the selection of rocket fuels is limited not only by their composition and properties, but also by a number of other characteristics.

The main specific requirements imposed on solid RMS:

The uniform distribution of components plus, consequently, the constancy of the physicochemical and energy properties in the unit, stability is also a pattern of combustion in the RD chamber, but it looks like a complex of physicomechanical properties that ensure engine performance in conditions of overload, variable temperature, vibrations, it is also very proud to take into account when designing amateur rocket engines.

Also, the fuel must meet the basic environmental standards, but it is in the industry and, if possible, own low-toxic products that are safe for the environment as well as living beings, which cannot be said about perchlorate fuel.

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

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

From the creator: I set out in this article the very essence of the simple thing that you need to know about rocket fuels, mainly about solid fuels, and I also think that this information will be useful to you for your business. Whole books have been written in general about rocket fuels, but most of these books are intended only for professionals who develop LREs (liquid-propellant rocket engines), and also without good knowledge of mathematics and the laws of external and internal ballistics it will be very difficult to study them. 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: Oleg
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Publication date 02.02.2005