Hypersonic speeds by electromagnetic launching
On August 24, 1988, five blocks of aluminum powered by a Lorentz force rushed in quick succession with hypersonic speed between a rail construction of approximately 3 meters. For the first time in Europe, an electromagnetic launcher had successfully fired repetitively. The first important achievement of the Pulse Physics research group at the Prins Maurits Laboratory TNO (locations Rijswijk Plaspoelpolder and Delft Zuidpolder).
Accelerating macroscopic objects, such as projectiles and space vehicles, using chemical propellants has its limitations. The practical limit for projectiles with a mass of the order of one kilogram shot from a gun is in the vicinity of 2 km/s (7200 km/h).
The final speed of a rocket is determined, among other things, by the ratio of the mass of the full rocket before the start and that of the rocket when all the propellant has been used. In theory, that ratio can be made very large and with that the attainable final speed is large. In practice, a so-called payload ratio of 500 is pretty much the limit. End speeds in the order of 20 km/s required for a manned trip to the moon in an acceptable time are then just feasible with the most favorable chemical propellant: the combination of liquid hydrogen and oxygen. Further distances require higher final speeds. A chemical propellant then becomes inadequate.
We owe the idea of chemical propulsion to the Chinese who used it in artillery around the year 1200. More than one hundred years later, gunpowder was successfully introduced in battles in Europe. Since then, it forms the basis of our weapons and sky-rocket technology. Halfway through the 19th century, the idea arose that objects could also be accelerated by electromagnetic means. This idea was successfully introduced into experimental physics into the structure of atoms and atomic nuclei. The projectiles involved are very light particles such as electrons and protons. Electrons in particular are relatively easy to accelerate to speeds close to the speed of light. Nearing the speed of light, the supply of energy mainly results in an increase in the mass of the particles. The speed hardly increases anymore.
Electromagnetic (EM) acceleration of macroscopic objects requires a completely different technology. Pioneering work in this area was done by the Norwegian Kristian Olof Birkeland, who in the beginning of the last century succeeded in giving projectiles with a mass of 10 kg a speed of approximately 100 m/s. He used a so-called induction accelerator. Since then, a number of countries have continued to work on this idea without making any significant progress. It was Australian researcher Richard Marshall who accelerated research in this domain in the mid-1970s. He used a so-called rail accelerator (railgun), powered by a homopolar generator. He was able to accelerate plastic projectiles with a mass of 3 grams to a speed of 5.9 km/s (Mach 18). The technique he used, stood model for the TNO developments.
The basic mechanism of electromagnetic propulsion is based on the operation of the Lorentz force. An electric current, which is located in a magnetic field, experiences a force perpendicular to the plane of the field direction and the local current direction. The force is equal to the product of the current density J, the magnetic induction B, and the sine of the angle between the direction of both fields. The figure below shows the schematic design of a so-called rail accelerator. One rail provides the supply of power, the other the drain. The current circuit is closed by a connection that can move along the rails. This can be an arc discharge. However, it can also be a fixed conductor armature which is connected to the rails with special sliding contacts (brushes). The circuit itself generates a magnetic field (as happens by any current-carrying system), whose intensity is proportional to the current intensity, while the direction is perpendicular to the plane of the drawing. As long as the rails is rigidly supported, only mechanical stresses occur. However, the arc discharge or the armature can move along the rail and will be subjected to the Lorentz force. In this way a projectile can be propelled.
A variant is that an arc discharge, which consists of a plasma with high pressure and temperature, expands freely in a pipe in which the projectile is located. Using such electrothermal propulsion, the projectile is accelerated by the high pressure of the plasma. Another way to accelerate a projectile by electromagnetic means is with a so-called induction accelerator.
Its operation can be compared to energizing a relay, as a result of which a magnetic core is pulled into a coil. In an induction accelerator, the excitation coils are switched on one after the other. When the projectile passes a coil, it is switched off to prevent the projectile from being slowed down after passage. The excitation coils can also be designed as a long coil through which a current pulse passes. The current pulse generates a pulse-shaped moving magnetic field through which the projectile coil is sucked inwards.
The railgun accelerator
The propulsing force in the above-described railgun accelerator is L*I2 where L is the inductance of the parallel rails per unit length and I the current. Characteristic values of the force are 2*10-7 Newton per Ampère squared. Forces to become significant therefore require very large currents. The railgun accelerator only becomes interesting when one is able to generate current strengths of a few Mega Ampère (million Amps). At 1 MA, a force of 2*105 Newton is obtained.
The speed that could theoretically be achieved is ‘currently’ (1988) under discussion. If the strength of the electric field E which generates the current is greater than the strength of the magnetic induction B generated by the current, the theoretical speed limit would be the speed of light. If E<B, the limit becomes a fraction E/B of the speed of light.
In practice, the speed limit is determined by the mechanical strength of the accelerator construction which is exposed to very high mechanical stresses. With the available materials this means that projectiles with masses in the order of one kilogram can in principle be accelerated to many tens of km/s. The military interest in this technology is primarily related to the high firepower that can be achieved. A firepower that is basically adjustable to one’s need. In conventional guns, the propulsion of a projectile takes place explosively, as a result of which large peak stresses have to be absorbed by artillery equipment. In an EM gun, the propulsion can be spread over time. Another important advantage is the lack of chemical propellants. The required energy can be generated on the spot, repeatedly and in a controlled manner, making such a weapon eminently suitable for use by robots.
As explained, very high currents are required for an EM launch. For weapon applications, these currents must be more or less strongly pulsed. The generation system consists of three stages. The first stage generates electrical energy in a conventional manner, for example with a gas turbine. In the next stage, the energy is stored in such a way that it can be released in a very short time. In the last stage, the energy is withdrawn from the storage medium and supplied to the accelerator in the form of a current pulse. An energy converter that is ideally suited for the production of very high currents is the homopolar generator.
The effect of a homopolar generator is based on the fact that in a metal disk rotating in a magnetic field perpendicular to the disk a potential difference arises between the centre and the edge of the disk. The voltage difference thus generated is relatively low (several tens of volts). However, because the solid rotor has a very low internal resistance it is an almost ideal power source with which very large currents can be generated. If such a source is used, the kinetic energy of the rotating disc is momentarily converted into electrical energy in the load. The disc immediately comes to a stop. However, the voltage of a homopolar generator is too low to allow the current in the railgun to rise sufficiently fast. In order to give the current pulse the desired shape, the generator is therefore connected to a special storage coil (capacitors are less useful here because they release their energy too quickly). At the moment the very solid coil is charged to the desired current, the charging circuit is interrupted by a special switching mechanism in a programmable manner, so that a rapidly increasing voltage difference appears at the beginning of the rails.
Below is a sketch of the setup as it is operational at TNO. The system is capable of producing energy pulses of 6.7 MJoule.
The TNO research, which actually started in 1983 by seconding an employee to the US, aimed at the following main areas: generation of very high current pulses, electromagnetic and electrothermal launching, and material research. The research topics include pulse forming networks with rapidly discharging batteries, inductors and a pulsed homopolar generator as energy source, in which the electromechanical and dynamic properties are also studied. Another important area of attention is the special electrical switching technology that is necessary for these activities. As far as launching is concerned, the focus will be on the behaviour and development of ultrafast sliding contacts and on the properties of high-pressure plasmas. Little is known about the processes that occur at very high speeds and currents. The development of a reliable system requires further material research. This applies in particular to the accelerator, the switches and the projectiles.
Other applications envisioned
The idea of electromagnetic propulsion also exerts a great attraction to designers of future spacecraft. This assumes a plasma (gas mass, in which a substantial part of the atoms has lost an electron) which is produced by generating electric discharges in the propellant. These discharges could be generated by means of a small nuclear reactor or solar batteries. As with the railgun, the plasma will be blown away by its own magnetic field if a sufficiently high current can be generated in the discharge. This is the principle of the magnetoplasmic dynamic arcjet.
The technique to generate very strong current pulses is also interesting for welding applications. It has been found that welds of high quality can be achieved with a homopolar pulse generator, for example in high-strength pipelines. The welding process is very fast, requires no filling material or a special shield. Relatively little metal evaporates and the distortion by the heat is minimal. The pressure generated by a magnetic field on a current-conducting system can also be used to form metal objects.
The generation of extremely strong magnetic fields is also interesting from a fundamental point of view. Little is known about the properties of matter under these exotic conditions. A fairly curious application of the EM launch technique is the investigation into the processes that occur during the impact of meteorites. The speed regime, which is necessary for this, was until recently not available.
Gerard van de Schootbrugge for the article in TNO Magazine in 1988 on which this web page is based.
Status in 1993
A later article (Roering jg 30 nr 2, p 41-48) gives an indication of the performance of the installation: max 1 MA current pulses with a capacity of 3.7 MJ, a capacitor bank alllowing for 1 ms pulses of 600 kA, and a fast bi-polar battery/pulse transformer combination that delivers 3 ms pulses of 250 kA. Projectiles of 500 g are fired with a muzzle velocity of 3 km/s.
At the end of the 90’s, Defence spending cuts caused the termination of the Dutch research efforts in this area.