Jet propulsion has long attracted the attention of scientists and designers around the world. However, the first production vehicles with jet engines of various types appeared only in the forties of the last century. Until that time, all equipment with rocket or air-jet engines was created only for experimental purposes. So, at the end of the twenties, the German company Opel began to implement the Opel RAK project. The purpose of this work was to create several types of technology with rocket engines. It was proposed to test new machines, determining the prospects for such technology.
The inspiration behind the Opel RAK project was one of the company's leaders, Fritz Adam Hermann von Opel. Interestingly, after the first tests of the new technology, the nickname "Rocket Fritz" was assigned to him. Leading experts in the field of rocketry were involved in the implementation of the project. The development of rocket engines was taken up by Max Valier and Friedrich Wilhelm Sander, who had extensive experience in this matter. Opel specialists were responsible for the creation of "platforms" for rocket engines.
In the spring of 1928, work on the Opel RAK project led to the construction of the first experimental vehicle, designated RAK.1. According to available data, other experimental devices of various types later received this name. The reasons for this are unknown. Probably, German engineers planned to use separate numbering for experimental equipment of various classes. So, starting from one, rocket cars, railroad cars and rocket aircraft were supposed to be numbered. However, errors in records and historical documents cannot be ruled out.
The RAK.1 rocket car was built on the basis of one of the Opel race cars of that time. This car had a classic "racing" layout with a front engine, closed with a characteristic long hood, and a single cab at the rear. The body of the car had smooth contours designed to reduce air resistance. The four-wheel undercarriage had steerable front wheels and drive to the rear axle. For use in the experimental project, the racing car was significantly modified. The native gasoline engine and transmission units were removed from it, as well as all other components necessary for the old power plant. At the same time, eight solid-propellant rocket engines were installed in the rear of the body.
The Opel RAK.1 was equipped with engines developed by M. Valier and F. V. Zander based on special gunpowder. Each such unit had a cylindrical body 80 cm long and 12.7 cm in diameter, in which a charge of gunpowder was placed. Valier and Zander developed two engine options that differed from each other in thrust. The engine charge of the first version burned out in 3 seconds, providing a thrust of 180 kgf, and the second burned for 30 seconds and gave 20 kgf of thrust. It was assumed that more powerful engines will be used to accelerate the car, and the remaining ones will turn on after them and will be able to maintain speed while driving.
Testing of the RAK.1 began in the spring of 1928. The first run on the test track ended in failure. The car only accelerated to 5 km / h and drove about 150 m, spewing out a large amount of smoke. After some modifications, the rocket car was again able to enter the track and show higher performance. However, the RAK.1 had a relatively low power-to-weight ratio. Due to the insufficient total thrust of the engines and the large mass of the structure, the car could not reach a speed of more than 75 km / h. This record was set on March 15, 1928.
Due to the lack of other rocket engines with higher characteristics, German engineers were forced to take the path of increasing the number of engines on one machine. This is how the Opel RAK.2 rocket car appeared. Like the first car, it had a streamlined body with a rear cockpit. An important feature of the RAK.2 is the rear wing. Two half-planes were placed in the middle of the body. It was assumed that due to aerodynamic forces, these units will improve the grip of the wheels with the track and thereby improve a number of characteristics. In the rear of the car there was a package of 24 powder engines with different thrust.
It didn't take long to assemble the Opel RAK.2. Tests of this machine began in mid-May 28th. On May 23, the jet car, in the cockpit of which Fritz von Opel was, was able to reach a speed of 230 km / h. This test run used the entire set of 24 rocket engines. It was after this that von Opel got his nickname Rocket Fritz.
In parallel with the development of ground vehicles with rocket engines, Opel, Valle, Sander and other German specialists worked on other options for using jet thrust. So, at the beginning of June 1928, the construction of a glider equipped with rocket engines was completed. Various sources refer to this aircraft as Opel RAK.1 and Opel RAK.3. In addition, it is sometimes referred to simply as a rocket glider, without specifying a special designation. The Ente glider ("Duck") designed by Alexander Lippish, built according to the "duck" scheme, was taken as the basis for the experimental apparatus. A starting engine with a thrust of 360 kgf and an operating time of 3 s was installed on it, as well as two main engines with a thrust of 20 kgf and an operating time of 30 s.
On June 11, the RAK.1 rocket glider took to the air for the first time with the pilot Friedrich Stamer in the cockpit. A special rail was used to launch the aircraft. In this case, takeoff was to be carried out only with the help of the existing powder engine. Outside assistance from a towing aircraft or ground crew was not required. During the first test, the pilot successfully lifted the glider into the air. Already in flight, F. Stamer switched on two propulsion engines in sequence. In 70 seconds, the RAK.1 apparatus flew about 1500 m.
The second test flight did not take place due to the accident. During takeoff, the starting rocket engine exploded and set fire to the wooden structure of the airframe. F. Stamer managed to get out of the aircraft, which soon completely burned down. It was decided not to build a new rocket glider and not to continue testing.
The next two experiments were carried out using railway platforms. In the summer of 1928, Opel built two missile railcars, during the tests of which some success was achieved.
On June 23, two test runs of the Opel RAK.3 missile railcar took place on the Hanover-Celle railway line. This device was a light four-wheeled platform, in the rear of which there was a driver's cabin and a set of rocket engines. The car was not equipped with a steering mechanism, and the cab had the smallest possible size, limited only by the convenience of the driver's seat. In addition, the rocket railcar received lightweight wheels.
The trials of the vehicle were announced in advance, which caused a large number of spectators to gather along the tracks. For the first pass, the rocket railcar was equipped with ten engines. Under the control of the tester, the car developed a high speed: figures from 254 to 290 km / h are mentioned in various sources. Despite this difference in data, it is safe to assume that the Opel RAK.3 rocket railcar was one of the fastest vehicles in the world.
Immediately after the first race, it was decided to hold the second one. This time, the project leaders ordered the installation of 24 rocket engines on the railcar. We must pay tribute to von Opel and his colleagues: they understood the risk, so the car had to go on the second run without a driver. This precaution was fully justified. The thrust of 24 engines turned out to be too great for a light car, which is why it quickly gained high speed and flew off the tracks. The first version of the missile trolley was completely destroyed and could not be restored.
In the summer of 1928, another rocket railcar was built, designated RAK.4. By its design, this machine differed little from its predecessor. Not only the design turned out to be similar, but also the fate of the two machines. The railcar, equipped with a set of rocket engines, was unable to complete even one test drive. During the first tests, one of the engines exploded and provoked the explosion of the rest. The trolley was thrown from its place, it drove a little along the rails and flew off to the side. The car was destroyed. After this incident, the leadership of the German railways banned the testing of such equipment on existing lines. Due to the lack of its own tracks, Opel was forced to discontinue the rail section of the RAK project.
Until the early autumn of 1929, German specialists were engaged in various projects, including promising jet technology. However, no tests were carried out on the finished samples. In September 29th F. von Opel, A. Lippisch, M. Valier, F. V. Zander and their colleagues have completed the rocket-powered airframe, designated the Opel RAK.1. It should be noted that there is a certain confusion with the names of jet gliders due to the lack of reliable information about the designation of the first spacecraft that flew in 1928.
The new glider designed by A. Lippisha received 16 rocket engines with a thrust of 23 kgf each. A special 20-meter structure was intended for take-off. On September 30, 1929, the first and last flight of the RAK.1 glider took place, which was flown by Rocket Fritz himself. Takeoff and flight were successful. The power of the sequentially switched on engines was enough for acceleration, ascent into the air and the subsequent flight lasting several minutes. However, the landing ended in an accident. The weight of the structure with the pilot exceeded 270 kg, and the recommended landing speed was 160 km / h. Fritz von Opel lost control and the glider was seriously damaged.
Shortly after the emergency landing of the Opel RAK.1 glider, a special letter arrived from the United States to Germany. The main shareholder of Opel at that time was the American company General Motors, whose management was concerned about several unsuccessful tests of experimental rocket technology. Not wanting to put personnel at risk, GM executives have banned German specialists from engaging in rocketry. An additional prerequisite to this ban was the economic crisis, which did not allow spending money on dubious experimental projects.
After this order M. Valier, F. V. Sander and other specialists continued their research, and F. von Opel soon left his company. In 1930, he moved to Switzerland, and after the outbreak of World War II he left for the United States. Despite his nickname, Rocket Fritz was no longer involved with the jet-powered vehicles theme.
The Opel RAK project is of great technical and historical interest. He clearly showed that already at the end of the twenties, the development of technology made it possible to build equipment with unusual engines. Nevertheless, all the cars built were nothing more than demonstrators of technology. It is not hard to guess that the rocket car and rocket railcar can hardly find their place on highways and railways. Much more viable was the rocket-powered aircraft. In the second half of the thirties, A. Lippisch began the development of the aircraft, which later received the name Me-163 Komet. This machine with a liquid propellant rocket engine was the first mass-produced rocket plane, and was also limitedly used in the Luftwaffe. Nevertheless, planes with rocket engines also did not become widespread, most of these developments remained purely experimental equipment that did not find application in practice.
