Into space with sugar !: “Caramel” fuel
To space on the cheap
Nevertheless, the founders of the prize for cheap access to space (Cheap Access To Space, or abbreviated CATS Prize) were more decisive - to get the prize, you need to deliver a payload of 2 kg to a height of 200 km. The competition started in November 1997, and in order to get a prize of $ 250, 000, you had to manage to reach this height before November 8, 2000. More than 30 attempts were made, but nobody managed to climb above 25 km, and the prize remained untapped. No one was able to claim a "consolation" prize of $ 25, 000 for reaching an altitude of 125 km. Some teams continued to work and after the deadline - the impetus that the CATS Prize gave amateur rocket science cannot be overestimated. Some teams have become real business firms, only they no longer make rockets ...
Only one team - CSXT, led by former Hollywood stunt performer and master of special effects Kai Michelson - continued to work, trying to achieve the original goal. Michelson, known in narrow circles under the name The Rocketman for his commitment to jet propulsion, even having gone on vacation, continued to pursue his favorite pyrotechnics. After analyzing the failures of its predecessors, CSXT abandoned exotic launch schemes from a stratospheric balloon or an airplane.
Balloon launches date back to the 50s of the last century. Attempts to save on atmospheric losses were made even before the flight of the first satellite, but, both in the 1950s and in the 1990s, the result was unsatisfactory - a simple-looking scheme was fraught with many "rakes", which unsuccessful rocket builders 40 years came with the same enthusiasm.
Kai Michelson also had to abandon the two-stage design - its reliability in amateur performance left much to be desired, which he was convinced of during an unsuccessful attempt to reach the border of space in 1997. The second stage simply did not start. In addition, after unsuccessful starts of CATS Prize contestants, obtaining permissions to launch two-stage high-altitude rockets made it almost insurmountable for fans of slingshots.
Actually, the American laws governing amateur rocket science are the most liberal in the world. In addition to conventional rocket models launched around the world, the High Power Rockets classes are defined in the USA and, for those who lack it, Experimental Rockets. The classification is based on both the full engine impulse (the product of traction and the running time) and the starting mass and allows, with certain reservations, amateur missiles up to 16, 000 N • s in the High Power Rocketry class and up to 128, 000 N • s in the Experimental class Rocketry Compare this to a maximum of 80 N • s in rocket-modeling competitions! In Europe, there is nothing like this for lovers of large rockets, so the European record for flight altitude is still less than 10 km. Moreover, European amateurs are forced to carry their missiles to the United States, equip and launch them in Nevada!
But even in the desert, laws monitor security very carefully. Fans are forbidden to carry large charges from state to state - you need to equip a rocket right at the launch site. There are many other restrictions that seem far-fetched at first glance, but most of them were created during the analysis of some accident and are designed to eliminate such accidents in the future.
All Michelson could improve was rocket aerodynamics and fuel characteristics. CSXT has done a lot of research in order to achieve maximum performance. The volume of tests of engines of various calibers was unimaginable for most fans - more than a dozen do-it-yourself solid-propellant solid-propellant rocket engines with a caliber of 6 and 8 inches (15–20 cm) were burned during tests when trying to achieve reliable operation at the peak of capabilities. They say team costs exceeded $ 130, 000! But finally, in January 2002, a rocket capable of reaching space was ready. She received the name Primera, in honor of the sponsoring company, a CD maker. Only on June 1 was it possible to obtain permission to launch - however, it did not take place due to weather conditions. A new attempt at the end of September needed a new permit, which was received on August 27. But on September 21, 2002, this rocket, having managed to climb 720 m and gain a speed of 1700 km / h in just three seconds, collapsed in the air due to burnout of the engine housing near the nozzle and the rocket turned across the stream.
The refinement and manufacture of a new rocket, called GoFast, took a year and a half. The rocket was one and a half times heavier and weighed 328 kg at the start (of which 197.5 kg weighed fuel). The length of the rocket was 6.4 m, and the diameter of the hull was only 25.4 cm, that is, the rocket looked as thin as a nail! In professional rocket science, such proportions are almost never found, but it was necessary at all costs to reduce aerodynamic drag, which at hypersonic speed is achievable only by reducing the diameter. Yes, the rocket was supposed to gain hypersonic speed even in a dense atmosphere - at an altitude of about 8-10 km, where ordinary subsonic airliners fly. Therefore, her nose was a solid steel cone with a very small aperture angle and thin at the top - the turner was able to carve this part only on the third attempt.
This time, fate was more supportive of the team. On May 15, 2004, with a monstrous acceleration of 21.5 g (more than that of the catapult to save fighter pilots), a thin rocket rushed to the border of space. Attracted observers using radar tracked the speed and height of the rocket. After 13 seconds, the fuel in the engine completely burned out and the rocket flew by inertia at a speed 5.2 times the speed of sound. It became clear that the record would be held. After 2.5 minutes, the rocket reached space. Five minutes after the start, beacon signals were received - the payload module descended by parachute. Unfortunately - far from the estimated landing site. They managed to find him when the battery of the lighthouse was already exhausted. And the accelerator case had to be searched for more than two weeks - it fell 40 km from the launch site. These difficulties somewhat overshadowed the success, but an altitude of 115 km was taken, which, in addition to the radar, is now evidenced by the records of the onboard “black box”!
But back to sugar. The fuel used in the GoFast rocket was the closest amateur approach to the Shuttle launch booster (SRB) fuel. A typical mixed solid fuel consists of ammonium perchlorate, aluminum and synthetic rubber, initially liquid, hardening directly in the engine. But ammonium perchlorate and rubber are substances that are practically inaccessible to most "rocket enthusiasts." Their sale is under very serious control. Yes, and aluminum powder is needed not just any way - “silverfish”, for example, is not suitable, metal particles must have a spherical shape and a certain size.
As a result, engines using such fuel, even in the USA, are available only to units. The rest have to use something simpler. For example, the notorious sugar. Caramel rocket fuel is indeed an alloy of sugar with potassium nitrate. Its characteristics are modest, but still it is one and a half times better than the well-known smoke powder, on which rockets flew for almost a thousand years, before pyroxylin was invented. In addition, “caramel” is at least 10–20 times cheaper than fuel on ammonium perchlorate. It is difficult to establish who invented “caramel” fuel; it appeared in the middle of the 20th century. American sources claim that Bill Colburn first applied it in 1943 in California. Rare books on amateur rocket science did not reproduce his recipe, but it was put on a scientific basis only in the mid-1990s - amateurs began to study the properties of fuel, the dependence of its characteristics on composition variations, on the initial temperature, pressure in the chamber, etc. Of course, at the disposal of professionals there are energetically more profitable substances, but for the serious and safe use of the amateurs, all of this information was necessary for amateurs, and they could be obtained only experimentally.
It turned out that this fuel burns steadily in a wide range of pressures in the chamber, which made it possible to use both simple paper engines and rechargeable metal ones on it. Small deviations in the composition also do not interfere with its good work, therefore it is safer. However, this fuel also has disadvantages, first of all, it is fragility. For example, rubber-based fuels are very soft, professional rocket launchers
argue that from a piece of such fuel you can pinch off the crumbs with your hands, this allows you to tightly fasten the charge with the body. The charge also serves as thermal protection - until it burns out completely, the engine case will not heat up. You can’t do this with caramel - it can crack under working pressure reaching up to fifty atmospheres! Therefore, the caramel charge is a plug-in, between it and the body there should be a narrow gap for equalizing the pressure. But at the same time, the metal case must be protected from hot gases, because their temperature reaches almost 1400˚C, so that the metal will inevitably lose strength.
Another disadvantage of “caramel” is a large amount of “condensed phase”. So rockets call combustion products that are not gases. When burning caramel, potash, or potassium carbonate, is formed. In the chamber, it is liquid, and in the nozzle it becomes solid. The smallest particles of potassium carbonate create dense white smoke. This smoke is rather caustic, as potash has an alkaline reaction. Therefore, in no case should you burn "caramel fuel" indoors. But for a rocket engine, the condensed phase is harmful for another reason: solid or liquid particles cannot expand in the nozzle, like gases, and therefore do not create work; the heat from them to the gas is transmitted only by radiation, so the efficiency of the rocket engine is reduced. This means that the actual specific impulse of the “caramel” is noticeably lower than the theoretical one calculated from the heat of chemical reactions.
And another serious drawback - for classic sugar caramel, the temperature difference between the melting of sugar and the ignition of the finished mixture is too small. But this problem was successfully solved by replacing sugar with sorbitol. Sorbitol fuel burns more slowly than sugar, but working with it is much safer, because sorbitol melts already at 125˚C, and sucrose only at 185˚. All other useful properties of sugar fuel in sorbitol have been preserved.
After the triumph of GoFast, many rocket launchers made complaints against the CSXT team. They say that their rocket is “dishonest”, since it cannot be reproduced by almost any amateur, and also because of the large deviation of their rockets, high-altitude launches are now under much closer control: US officials decided that their legislation is too liberal. But on the other hand, once solved a problem a second time is much easier to solve. And the Canadian Richard Nakka, one of the main “caramel” enthusiasts, decided to achieve an “honest” result from the point of view of amateur rocket science, to reach the space border on sugar - or sorbitol - fuel. The project was named Sugar Shot to Space, in the free translation of “Sugar Into Space”.
But first, it was necessary to find out whether this problem can be solved in principle. If the atmosphere did not interfere, a speed of 1400 m / s would be enough to “jump” from the surface of the Earth to an altitude of 100 km. But at GoFast, the atmosphere “ate” about 300 m / s (more than 1000 km / h!). To reduce the amount of losses, it is necessary to accelerate in more rarefied air, at a higher height, and for this it is necessary to reduce the starting overload and increase the operating time of the engine. But for an unguided rocket, this is undesirable, since the area on which the stabilizers work poorly is increasing. It is necessary to increase either the height of the guide or the size of the stabilizers, which increases aerodynamic losses.
The analysis of aerodynamics was carried out very carefully, as a result the proportions of the rocket turned out to be even stranger than that of GoFast - the length was 30 times the diameter, three stabilizers instead of four, and they tried to optimize the shape of the bow. But all this did not bring closer to the desired result. I did not want to make the two-stage rocket, as this reduced reliability and increased the difficulty of obtaining permission to launch. Richard Nacca was familiar with these problems firsthand.
It was necessary to come up with such a traction profile (the dependence of traction on time) that could be implemented in a caramel engine and which would reduce aerodynamic losses and not increase gravitational losses too much. Anti-aircraft missiles use a fast launching stage with a thrust of a hundred tons and acceleration up to 50 g (in anti-ballistic systems) and a relatively "long-playing" marching - with much less traction. But the marching stage used to be done on the liquid propellant rocket engine, and now - on special solid fuels that provide a long operating time. For fans, this is not suitable - the volume of developmental tests is too large. For simple caramel engines, runtime is closely related to diameter.
But a solution was found - it became a two-stage engine. Such an engine consists of two chambers with two charges of fuel, in turn working on a common nozzle. Between the chambers there is a plug made of combustible material, which should not allow hot gases to reach the second charge during the operation of the first. After burning out the first stage, the rocket will fly up inertia for some time, gradually losing speed, but also getting out of the dense layers of the atmosphere, and only at the end of the ballistic pause the second half of the fuel supply will ignite. The maximum speed in this case will be noticeably lower than that of GoFast, and it will be possible to achieve it at a higher altitude - while aerodynamic losses will decrease.
However, with all the tricks, the starting mass and size of a rocket on sugar fuel should be larger than on perchlorate-rubber. Therefore, members of the SS2S group first built a two-stage engine model on a scale of 1: 4 (for linear dimensions; for fuel mass it is 1/64). Only with the fourth attempt did success come to them - the hardest thing was to ensure that the first stage camera did not burn out during the second operation, because it received a double dose of heat load.
However, having overcome all the difficulties, the rocket launchers realized that before building a full-sized rocket to storm the space, they would first have to work out technical solutions on something cheaper, and now they are building a rocket on a 1: 3 scale. Dolog is the way of lovers into space! But we hope that with time they will succeed, and we wish them perseverance and success.The article was published in the journal Popular Mechanics (No. 4, April 2008).