Electrothermal Chemical Technology and Why it's Awesome

A_Mysterious_Stranger · June 30, 2018

Been delayed with stuff but I've wanted to post this. Actually I'm surprised I've never seen anything in detail about this before, because it's an interesting topic. (IF there IS a topic on this I apologize and it can be merged there.)

ETC tech is something you probably hear about if you hang out on tank, military or gun forums. Especially if Railguns or coilguns are mentioned. Or 'next step' in gun design like 140-152mm guns. There's lots of information out there if you look and you discover just how diverse it can be.

I'm sure most people are aware that Wikipedia has a article on ETC tech and as far as Wiki articles go it's decent. But the person who worked on it in the past also wrote an article on ETC for the Nationstates draft room. It's old but still good. So despite the origins it's still useful (the writer was also a member on Tanknet IIRC. Take that as you will.)

In Jane's Technology of Tanks, Ogorkiewicz also commented about ETC:

Quote

3.10 Electrothermal Guns

Work on electromagnetic guns has led since the mid-1980s to the development of another category of guns which combine some of their features with those of solid or liquid propellant guns. These are electrothermal guns, which resemble solid or liquid propellant guns in that gases are generated in them to push projectiles through the barrels. In this respect they differ radically from the electromagnetic guns. But they also differ from solid and liquid propellant guns on account of the method by which the propulsion gases are created in them, this being by the interaction of an electrically generated plasma with a working fluid.

Because their projectiles are propelled by gases, electrothermal guns are not subject to the erosion which occurs with electromagnetic guns and their barrels can be much the same as those of conventional guns. At the same time, their working fluids can be selected so that the gases generated from them have a lower mass and. therefore, absorb less energy than the gases of solid propellant guns. But, whatever the working fluid, some energy is still absorbed in accelerating the propulsion gases and to this extent electrothermal guns are less suitable than electromagnetic guns for achieving very high projectile velocities. In fact, they are generally considered most suitable for velocities of up to about 2500 m/s.

Like electromagnetic guns, electrothermal guns require a source of pulses of electrical power, but the amount of electrical energy they need to propel projectiles can be reduced by generating a part of the propulsion energy by the chemical reaction of the working fluid. In consequence, there are two types of electrothermal guns. One might be described as a 'pure' electrothermal gun and derives all of its propulsion energy from the electrical power source. The other type derives some, or even most, of the propulsion energy from the working
fluid.

The first type of electrothermal gun uses an inert working fluid such as water. This is heated endothermically by the plasma and vaporises to produce propulsion gases which have a lower molecular weight than those produced by the combustion of solid propellants. However, in this type of gun all the energy required to propel projectiles has to come from the electrical power source, with all the problems that this entails, as it does with electromagnetic guns.

The second type of electrothermal gun uses a reactive working fluid. This can be of a mildly exothermic nature, an example being a slurry consisting of aluminium, titanium hydride and water. When such a slurry is heated by the plasma, chemical reaction between its ingredients produces heat as well as low molecular weight products. As a result, an electrothermal gun using this kind of working fluid not only has the internal ballistic advantage of low molecular weight propellant gases but also requires considerably less electrical energy to propel projectiles than the 'pure" electrothermal guns. In fact, only 30 to 40 per cent of the energy required to propel projectiles may need to come from the electrical power source, the rest being generated by the chemical reaction of the working fluid.

The working fluid can also be of a highly exothermic nature, an example being a mixture of a fuel, such as octane or kerosene, and of hydrogen peroxide. In this case plasma energy is used to vaporise the fuel and mix it with the peroxide to produce a chemical reaction which results in moderate molecular weight products and generates most of the energy for projectile propulsion. The percentage of energy generated by the reaction of the fuel-oxidizer mixture may actually be as high as 80 per cent and this obviously reduces considerably the size of the electrical power plant required with a gun using this kind of working fluid. Guns using such a high energy working fluid have been developed in the United States since 1986 by FMC Corporation, which has called them 'combustion augmented plasma', or CAP, guns. However, because most of their propulsion energy comes from the fuel-oxidizer mixture, they can also be regarded as bulk-loaded liquid bipropellant guns in which the instabilities associated with bulk-loaded liquid propellants have been suppressed by the flow of the electrically generated plasma.

The plasma which introduces electrical energy into electrothermal guns is generated in a cartridge with a capillary tube containing a wire connected to a blocking electrode at one end and an annular, nozzle electrode at the other end. The wire explodes when a large surge of current is made to pass through it and this establishes the plasma which flows rapidly out of the capillary into the chamber containing the working fluid.

As in the case of electromagnetic guns, the pulses of electrical power required to generate the plasma can come from a number of sources but they involve only two basically different methods of intermediate energy storage. In one, energy supplied by an engine is stored in the form of the rotational kinetic energy of a homopolar generator or of a compulsator. In the other, electrical energy supplied by an engine-driven alternator is stored in a bank of capacitors, either by charging them directly, or indirectly, through an intermediate battery storage subsystem. With either form of energy storage the pulses of power which are drawn from it have to be suitably shaped in a pulse forming network.

Ogorkiewicz also discusses the concept in Tanks: 100 years of evolution:

Quote

In contrast, the prospects of arming tanks with another type of electric gun, the electro thermal-chemical or ETC gun, were brighter from the start because only part of the energy it used to launch projectiles was electrical, the rest coming from the chemical reaction of a solid or liquid propellant. In consequence, the ETC guns did not require electrical equipment as large and as heavy as the EM guns.

Development of ETC guns was pioneered by GT Devices, a small US company that started firing 20mm ETC guns in 1985 and was subsequently taken over by General Dynamics Land Systems (GDLS). In 1985 FMC Corporation also started work on what it called Combustion Augmented Plasma Guns, in which originally most of the projectile propulsion energy was expected to be electrical but which were in effect ETC guns. The early work on ETC guns was so promising that by the end of 1989 a competitive trial was arranged between 120mm tank guns converted by GDLS and FMC into ETC guns, which were intended to demonstrate that an ETC gun could arm the next version of the US M1 tank. The trial was clearly premature, and proved so disappointing that it led to an equally rash view that ETC guns were less promising than EM guns. This view was reached, among others, by the US Army Science Board, which recommended in 1990 that development funds be diverted from ETC to EM guns. 23 Similar views were held in Britain, where RARDE had already shown little enthusiasm for ETC guns.

However, the US Army continued to support research into ETC guns and ordered a 9 MJ 120mm ETC laboratory gun from FMC, which was installed in 1991 and from which projectiles were fired at up to 2,500m/s. Work on ETC guns was also pursued in Germany, where it started in 1987, and resulted in the construction by Rheinmetall of a 105mm ETC gun thatby 1995 fired projectiles at up to 2,400m/s. This was followed by the design of a 120mm ETC gun that began to be used for firing trials in 1999, and by collaboration with France, where another 120mm ETC gun was built by GIAT and started firing trials in 2003.

Since 1986, work on ETC guns has also been pursued in Israel at the Soreq Nuclear Research Centre, which pioneered the use of solid propellants as the source of the chemical part of the projectile propulsion energy instead of the liquid or slurry propellants used originally by FMC and GDLS. Soreq’s lead was followed by others, and since the early 1990s the development of ETC guns has concentrated on the solid propellant form of them, becoming focused during the 1990s on guns of 120mm calibre.

The object of the development of the solid propellant 120mm ETC guns that was pursued in the United States, Germany and elsewhere became that of making them a potential alternative to 140mm solid propellant guns that were being developed for the defeat of future enemy tanks. In the course of this development, the use of a 120mm ETC gun was considered in the early stages of the US Future Combat Systems programme and in 2004 United Defense LP (originally FMC and now BAE Systems) successfully fired a 120mm ETC gun from a light tank developed from a much modified M8 Armored Gun System. An ETC gun was also included in the plans for a new family of armoured vehicles that were drawn up in Germany in the late 1990s, and by 2002 Rheinmetall demonstrated
a 120mm ETC capable of generating 30 per cent more muzzle energy than the 120mm solid propellant gun on which it was based. 24

However, even though 120mm ETC guns were considered capable of firing projectiles with a muzzle energy of 15 MJ, their performance still fell short of that of 140mm solid
propellant guns, which could fire projectiles with an energy of 18 to 23 MJ and at the same time enjoyed the advantages of being based on well proven technology.

One realization from this is ET/ETC technology is quite diverse and can be confusing. One of the better sources covering that concerned Rheinmetall research into a German 120-140mm (courtesy of Wayback because the original source fell to link rot):

Link to image of Rheinmetall ETC classifcations

On the amateur experime which discusses ET/ETC stuff in detail too. If you prefer the more 'hype' side of things, ETC was also tied to the Future Combat Systems - a link some people may recognize:

Quote

Encouraging results have been obtained with   Electro-Thermal-Chemical   (ETC) experimental guns.   In principle, an ETC gun utilizes a chemically energetic (reactive) working liquid instead of conventional solid propellant. It requires considerably less electrical energy to achieve adequate projectile propulsion than its predecessor, the Electro-Thermal experimental   (ET)   gun.   It   needs   relatively smaller and lighter auxiliary equipment to   produce   and   store   electricity.   This equipment could ultimately be reduced to a suitable size to warrant its installation   in   an   armored   vehicle.   Energetic working liquid is naturally prone to be problematic in operation, handling, storage, and supply, such that its utilization will pose a potential safety concern and a logistic burden, much similar to LP guns. As in LP, ETC implementation requires new industrial and military infrastructures   for   production,   deployment, and logistics.

Current developments are aimed at a medium caliber (60-80mm), antitank gun with a firing rate of 10-15 rounds/min. At this caliber range, various types of rounds could be comprised of KE projectiles and CE rounds, as well as future ‘smart’ sensor-fuzed munitions. The ultimate objective is aimed at an ETC automatic gun with a muzzle energy of 20+ MJ (corresponding to 2500-3000 m/sec for medium calibers) which is comparable to that of the conventional, solid propellant 140mm gun. Much like LP guns, ETC technology allows better control of the pressure (propulsion) generated, so that it is maintained relatively close to its maximum while the projectile is moving down the barrel, resulting in more energy conveyed to the projectile.

This is quite contrary to conventional SP    technology,    where    the    pressure quickly diminishes as the projectile departs   from   the   combustion   chamber. ETC technology is recognized by many to show promise of “infinite” or multistage   variable   lethality   and   improved propulsion controllability. It also requires significantly   less   electrical   energy   in comparison   to   Electro-Magnetic   (EM) guns that use only electricity for projectile propulsion. Nevertheless, ETC technology, as promising as it may seem, requires further fundamental research beyond the laboratory stage. Much detailed research and testing has yet to be accomplished in the field and at weapon system level. It must achieve maturation to warrant its applicability as a stand-alone solution, or in conjunction with other mature   technologies,   or   with   existing 120/140mm guns.

As an additional practical alternative, ETC technology could be combined with existing conventional SP 120mm and/or future   140mm   guns   and   ammunition, though   a   new   cartridge   and   modified gun chamber are required. It represents a near-term upgrade application of already leveraged and proven technology. The size of the electrical equipment is much smaller than that of current EM research guns and present ETC as a viable upgrade proposition. Research has shown that specially designed ammunition and ETC gun technology could be combined with existing conventional SP guns to further enhance the performance of the latter up to 30% and beyond. Augmenting the energy of solid propellant is possible by implementing a plasma regenerative injector and combustion control to the conventional pressure chamber. In the event that ETC technology will become   practical,   existing   conventional 120mm and future 140mm guns could be economically converted into ETC/SP guns as one more step in the evolution of SP guns. There are still various predominating   problems   to   be   addressed and resolved before ETC guns can become a practical proposition in conjunction with conventional solid propulsion. The combination of controllable, repeatable inner ballistics with a compatible solid propellant, and the significant increase in performance (e.g. muzzle velocity) in large caliber guns, has yet to be demonstrated.

Regardless of whether ETC technology will become a viable proposition, the use of large consumable ammunition in addition to ‘energetic’ liquid propellant is contradictory to the requirement of reduced    dependency    on    logistics    and weight.   The   combined   implementation of SP with ETC, will probably not justify the enormous investment in design, development and deployment associated with the fielding of an entirely new tank fleet. Though new and promising technology, it will not change the nature of armored warfare.

As you can see, ETC is evolutionary not revolutionary like EM guns. It takes existing technology and builds on it: You can settle for improving propellant ignition (minimizing electrical cost) or add electricity to boost performance (up to the 'pure' ETC idea) You can also utilize the technology on Liquid propellant and possibly even Light Gas guns - it stacks quite nicely with other ideas. You can even use it with a bigger caliber. This is part of the ETC charm.

Further information on ETC stuff can be found here:

AN END-TO-END MODEL OF AN ELECTROTHERMAL CHEMICAL GUN

Electro-Thermal Chemical Gun Technology Study

Both of these are articles I like, but there's more stuff:

Electrothermal-Chemical (ETC) Technology Weaponization Issues

Electrothermal-Chemical Gun Systems Utilizing Novel Electric Solid Propellants

And of course DTIC is a wealth of ETC stuff:

(direct pdf links):

Overview on the German R&D Programs on ETC Gun Technologies for Main Battletank Weaponization

ELECTROTHERMAL-CHEMICAL PROPULSION AND PERFORMANCE LIMITS FOR THE 120-MM, M256 CANNON

And some dtic links to ETC stuff that requires download:

Electrothermal-Chemical (ETC) Propulsion with High Loading Density Charges.