Propellants, Explosives, Pyrotechnics 23, 17Ä…22 (1998) 17
Behaviour of a Working Fluid in an Electrothermal Launcher
Chamber
B. Baschung, M. Samirant, K. Zimmerman, C. Steinbach, and D. Mura
Institut Franco-Allemand de Recherches de Saint Louis (ISL), F-68301 Saint Louis Cedex (France)
A. Louati
Laboratoire Gestion des Risques et Environnement (GRE), F-68200 Mulhouse (France)
Das Verhalten einer ArbeitsŻussigkeit in der Kammer einer Comportement d'un Żuide moteur dans une chambre de lanceur
È
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elektrothermischen Kanone electrothermique
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Der elektrothermische Antrieb, dessen Konzept es gestattet, die Grenzen La propulsion electrothermique dont le concept permet de repousser
È Â
der Projektilgeschwindigkeit zu hoheren Werten zu verschieben, bietet les limites de la vitesse atteinte par le projectile, presente de nombreux
gegenuer einer konventionnellen Kanone viele Vorteile. Wie bei Pul- avantages par rapport a la propulsion conventionnelle. Comme pour un
È Á
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verkanonen, ist die Kenntnis der thermochemischen Daten und des lanceur a poudre, la connaissance des donnees thermochimiques et du
Verhaltens des Treibstoffs fur die Optimierung des Systems notwendig. comportement du propulseur sont necessaires a l'optimisation du
È Â Á
È È Á Â Á
Um die chemischen Phanomene zu verstehen, die wahrend des elek- systeme. Pour comprendre les phenomenes chimiques ayant lieu
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trothermischen Prozesses statt®nden, haben wir versucht, die Reak- pendant le processus electrothermique, nous entreprenons de car-
  Â
tionsprodukte von Methanol zu charakterisieren. Hierzu wurde ein acteriser les produits de reaction provenant de la decomposition du
manometrischer Kessel, der hohen Drucken standhalten kann, zusatzlich methanol. Dans cette perspective, une enceinte resistant aux hautes
È È Â Â
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mit einem Plasmagenerator ausgestattet. Wir identi®zieren und quanti- pressions est specialement modi®ee par adjonction d'un generateur de
®zieren die Verbrennungsprodukte von Methanol mittels chromato- plasma. Nous identi®ons et quanti®ons les produits de combustion du
  Â
gra®scher und spektometrischer Analyse. Die freigesetzte chemische methanol par analyse chromatographique et spectrometrique. L'ener-
   Á Â
Energie wird auf der Grundlage der Analysenergebnisse berechnet. Die gie chimique liberee est calculee a partir des resultats d'analyse. Les
   Á
Ergebnisse zeigen, daû es eine in das Plasma eingekoppelte Energie gibt, resultats montrent qu'il existe une energie injectee dans le plasma a
È Â Â Â Â
von der ab die gesamte Masse des Methanols zerfallt. Diese Energie wird partir de laquelle la totalite du methanol est detruite. Cette energie est
uber ein empirisches Verfahren abgeschatzt. Die theoretischen Tem- estimee par methode empirique. Les calculs theoriques de temperature
È È Â Â Â Â
È
 Á  Â
peraturberechnungen sind in guter Ubereinstimmung mit den beob- sont en bon accord avec les phenomenes observes. Il est montre que les
È Á Â Ã
achteten Phanomenen. Es wird gezeigt, daû die im Kessel benutzten matieres plastiques employees dans l'enceinte jouent un role important
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Kunststoffe eine bedeutende Rolle bei der Verbrennung spielen. dans la combustion. En outre, l'energie chimique liberee par la reac-
È Â Á
Auûerdem ist die wahrend der Reaktion freigesetzte chemische Energie tion n'est pas un facteur determinant quant a l'optimisation des per-
Â
kein entscheidender Parameter zur Optimierung der Leistung der Kanone formances du lanceur pendant le processus electrothermique. Une
È Â Â Â Â
wahrend des elektrothermischen Beschleunigungsprozesses. Es wird relation entre l'energie chimique liberee pendant la reaction et
È Â Â Â Â
eine mathematische Beziehung zwischen der wahrend der Reaktion l'energie injectee dans le plasma est etablie pour le methanol.
freigesetzten chemischen Energie und der in das Plasma eingekoppelten
elektrischen Energie fur Methanol vorgestellt.
È
Summary 1. Introduction
Electrothermal propulsion allows higher limits of projectile Conventional launchers use the expansion of gases under
velocity. It provides many advantages over conventional
high pressure to accelerate a projectile. For a given initial
propulsion. As for a conventional gun, the knowledge of thermochemical
energy, the main parameter which controls the muzzle
data and propellant behaviour is necessary to optimize the system. The
velocity and the system ef®ciency is sound velocity in
reaction products issued from the decomposition of methanol during an
electrothermal process were characterized in order to understand the gases. This velocity determines the effective projectile base
chemical process. Therefore, a plasma generator is specially added to a
pressure during acceleration. Therefore, the gases must
closed vessel suitable for high pressures. The combustion products of
have a low molecular weight and a high temperature.
methanol were identi®ed and quanti®ed by the analysis of the gases with a
Electrothermal launchers use the expansion of a plasma,
chromatograph coupled with a mass spectrometer. The released chemical
energy is calculated from the analysis results. A de®ned electrical energy
generated by an electrical discharge in a chamber possibly
injected into the plasma is found by which the amount of methanol is
with the addition of the decomposition of a working Żuid
completely decomposed. This energy is estimated by an empirical
which allows to increase the volume of the gases and to
method. The theoretical calculations of temperature ®t well with the
maintain temperature within the limits consistent with the
observed phenomena. Results indicate that the plastic materials used in
the closed vessel have an important inŻuence on the combustion.
resistance of the metal of the launchers.
Moreover, the chemical energy released during reaction is not the deci-
As in the case of a conventional launcher, it is necessary
sive parameter for the optimization of the launcher's performances dur-
to know the thermochemical data and the propellant behav-
ing the electrothermal process. A relation between the chemical energy
iour (the composition of the combustion gases) to optimize
released during reaction and the plasma injection energy is established for
methanol.
the electrothermal system.
# WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998 0721-3115/98/0102Ä…0017 $17.50‡:50=0
18 Baschung, Samirant, Zimmermann, Steinbach, Mura, and Louati Propellants, Explosives, Pyrotechnics 23, 17Ä…22 (1998)
Numerous mixtures and pure substances have been stud- Expansion chamber
ied and identi®ed as working Å»uids(1,2). Several studies have
still been carried out, especially the thermochemical prop- It allows the expansion of the plasma generated by the
erties of working Å»uids(3), and the plasma=working Å»uid electrical discharge. The expansion chamber is ®xed at the
interaction in the electrothermal launcher chamber(4). downstream end of the plasma generator. It consists of three
Encouraged by the initial results, based on tests using pieces of metal, the interior walls of which are coated with a
methanol as working Żuid in the ISL 12 mm bore diameter thin polycarbonate sheath to protect them against erosion.
launcher(5), the reaction products were characterized in The chamber interior diameter is 20 mm, its length 130 mm.
order to understand the chemical process occurring during Before testing, the chamber is evacuated.
the electrical discharge. The aims of this report are (1) to
present the decomposition and reaction of methanol during
the electrothermal process, (2) to determine the chemical
Nozzle
energy released by the reaction, and (3) to establish a
relation between the energy injected into the plasma and the
The nozzle is a steel cylinder 20 mm in length with an
energy released by the decomposition of methanol.
internal bore of 1 mm in diameter. It is connected with the
expansion chamber. After the electrical discharge, the
pressure increases and the gases begin to stream down the
nozzle. Erosion of the nozzle leads to a very fast enlarge-
2. Experimental ment of its diameter and to a rapid reduction of pressure.
Then the combustion gases stream down from the closed
To perform this study, a vessel suitable for high pressures vessel into the combustion gas-holder.
was specially modi®ed by adding a plasma generator
(Figure 1).
Gas-holder
The combustion gases ejected from the nozzle are
Plasma generator
recovered in an evacuated steel gas-holder. The gas-holder
is connected to the closed vessel with a polyethylene tap
It allows the conversion of electrical energy into thermal
(Figure 2) and is ®tted with a gauge to measure the pressure
energy. The plasma generator consists of two electrodes, a
after the experience. All the volumes up to the polyethylene
rear electrode and an annular electrode, separated by an
membrane (expansion chamber, nozzle, piping and gas-
insulating tube of polyethylene. In this type of con®gura-
holder) are evacuated with an oil diffusion pump to a
tion, the rear electrode is conceived as a high voltage
pressure of about 10 3 Pa.
electrode. The second electrode is grounded. The distance
between the electrodes is 79 mm and the interior diameter of
the plasma generator is 10 mm. The arc ignition is obtained
by exploding a constantan wire (diameter 120 mm) between Electrical source
the electrodes.
For each test, 2 g of methanol are located in the plasma The electrical source (Figure 3) is the same as the one
generator. The downstream end (level with the annular used for the ISL 12 mm launcher(6). It consists of a
electrode) is closed with a double polyethylene membrane. 35 kV=97 kJ capacitor bank. The bank consists of 31 par-
The wire between the electrodes is adjusted along the allel connected capacitors (5.13 mF). A spark gap initiates
plasma generator axis and is immersed in methanol. the discharge process. The natural inductance of the circuit
Figure 1. Representation of the closed vessel with the plasma generator.
Propellants, Explosives, Pyrotechnics 23, 17Ä…22 (1998) Behaviour of a Working Fluid in an Electrothermal Launcher Chamber 19
analysis is proceeded at 100 C. The sample injection is
obtained with a gas-tight syringe. The mass spectrometer
(HP-5972A) acts as a detector: it characterizes and quanti-
®es the gas mixture components.
3. Results
The current intensity in the plasma generator measured
with a Rogowski coil and the voltage in the plasma gen-
erator measured with a high-voltage gage, characterize the
Figure 2. Recovery system for the gases.
closed vessel process. The electrical energy injected into
the plasma is determined from the data for intensity and
voltage.
is 43 mH. An additional 40 mH induction coil allows
tailoring of the current. A crowbar consisting of a series of
diodes (72 kA maximum intensity and 2.2 kV maximum
4. Interpretation
voltage) avoids polarity reversal on the capacitor bank. The
plasma generator resistance varies between large limits
Figure 4 shows the percentage in volume of the major
during discharge.
constituents in the combustion gases for various plasma
By choosing the number of capacitors from 8 to 31, the
injection energies. The products contained in the gaseous
electrical energy on the capacitor bank can be varied from
mixture can be divided into two groups. The ®rst group is
25 to 97 kJ. Then, the energy injected into the plasma can be
formed by carbon monoxide, methane, carbon dioxide
increased from 13 to 70 kJ.
(which is always present in small quantities) as well as
methanol in some cases. They were obtained from the
methanol decomposition. In the second group hydrocarbons
Gas-container
are found, such as acetylene, ethylene and ethane. They are
probably degradation products of the plastic materials
A30 cm3 stainless steel gas-container is ®tted on the gas-
(polyethylene from the plasma generator and polycarbonate
holder. It allows to collect a sample of the combustion gases
from the expansion chamber). The quantities of hydro-
after each test.
carbons, except for acetylene, are very small. The results
are shown in Table 1.
For the lower energies, the increase of carbon monoxide
Gas analysis and methane percentages when increasing the plasma
injection energy can be explained by the progressive dis-
The combustion gases analysis is performed using the HP appearing of methanol. Methanol can be found in the
MS Chemstation system, a gas chromatograph coupled with combustion products when the plasma injection energy is
a mass spectrometer (GCMS). Light gases, hydrocarbons below 32Ä…33 kJ. Then methanol disappears by increasing
and alcohols can be separated by using a 25 m in length the energy to a larger level, being totally decomposed.
PoraPLOTQ capillary column, with an interior diameter of The amounts of carbon monoxide and methane slightly
0.32 mm. Helium is used as a carrier gas. The isothermal increase by increasing the energy between 35 and 50 kJ.
Figure 3. Electrical circuit.
20 Baschung, Samirant, Zimmermann, Steinbach, Mura, and Louati Propellants, Explosives, Pyrotechnics 23, 17Ä…22 (1998)
Figure 4. Percentages (in volume) of the gaseous mixture components, (carbon monoxide, methane and methanol) versus plasma injection
energy.
Table 1. Overview
Eplasma Gaseous products obtained (percentages in volume)
DHreaction(a) %Echemical(b)
(kJ) CO CH4 CO2 C2H2 C2H4 C2H6 H2O CH3OH (kJ=2 g MetOH)
13.05 44.30 14.60 0.40 2.40 0.40 0.40 0.50 36.90 78.06 38.16
15.00 47.80 24.00 0.60 0.60 0.40 0.50 1.20 24.80 77.64 33.75
30.02 57.30 21.50 0.90 9.80 1.90 1.50 1.50 5.50 74.58 13.24
30.18 51.30 21.50 0.80 10.50 2.60 1.30 1.10 10.80 74.61 13.26
31.06 53.90 29.30 1.10 9.20 2.50 1.20 1.90 1.00 74.31 12.17
33.80 52.50 30.40 1.00 10.20 3.00 1.20 1.80 0.00 73.94 10.43
34.69 53.30 32.00 0.60 9.20 2.90 1.10 0.70 0.30 73.98 10.29
48.22 54.00 32.60 0.60 8.00 2.90 1.50 0.40 0.00 74.16 7.95
50.31 54.50 33.20 0.50 8.00 2.60 1.10 0.10 0.00 74.15 7.61
61.06 62.00 27.10 0.50 4.60 2.40 1.50 0.00 0.00 74.71 7.17
64.39 61.20 29.90 0.30 4.40 2.30 1.10 0.00 0.00 74.83 6.98
64.95 55.80 33.70 0.50 5.20 2.70 1.50 0.30 0.00 74.74 6.80
66.88 56.90 33.70 0.20 5.20 2.60 1.10 0.10 0.00 74.64 6.49
69.48 63.40 28.30 0.20 4.80 2.30 0.90 0.00 0.00 74.90 6.58
70.43 61.30 29.20 0.30 4.80 2.60 1.00 0.00 0.00 74.75 6.32
(a)
Chemical energy is the variation of enthalpy between ®nal products (determined from the chromatographic and spectrometric analysis) and the
initial product, methanol.
(b)
Chemical energy percentage that is [DHreaction=DHreaction ‡ EplasmaÄ…].
Whereas above 50 kJ a progressive increase of carbon energy injected into the plasma. The calculations performed
Â
monoxide can be observed, the quantity of methane does by K. Daree for the ISL 12 mm electrothermal launcher
not change signi®cantly above 50 kJ. show that the temperature gradients are very important in
Experiences using an energy of about 65 kJ showed that the launcher. The achieved temperature is 1500 K during
the amount of methane is larger whereas the amount of 0.2 ms for 69 kJ at a distance of 15 cm from the rear elec-
carbon monoxide is smaller: hydrogenation is preferred to trode (in the expansion chamber centre) using 2 g methanol
oxygenation but the actual reasons for this phenomenon are in the plasma generator. In the arc core, the temperature
not well understood yet. High temperature may favour reaches 30 000 K. Accordingly, when the plasma injection
hydrogenation against oxygenation. Moreover, a catalytic energy is larger than 35 kJ, the high temperature zone is
hydrogenation may be enhanced by the presence of metal suf®ciently large to totally decompose the methanol in the
from electrodes erosion. expansion chamber. On the other hand, when the plasma
The presence of methanol in the ®nal gas using a plasma injection energy is below 33 kJ, the high temperature zone
injection energy below 33 kJ can be explained as follows. in the expansion chamber is too localized to decompose all
At the beginning of the electrical discharge, methanol is methanol: only a part will react before the opening of the
ejected from the plasma generator into the expansion nozzle.
chamber, since the membrane of the plasma generator Figure 5 shows the chemical energy (DHreaction) liberated
opens. Once in the expansion chamber, it reacts. The as a function of plasma injection energies. The total
reaction rate depends essentially on the temperature reached decomposition of methanol is indicated by a bending point.
in the chamber, on the localization of the methanol in the Up to 33Ä…34 kJ, the chemical energy released by the reac-
chamber (at the entry, near the annular electrode, or near the tion diminishes gradually until methanol completely dis-
nozzle) and on the time the nozzle opens. The temperature appears. Then, the chemical energy increases slightly to an
reached in the expansion chamber depends on the electrical amount of 4.8 kJ as 65Ä…70 kJ were injected into the plasma.
Propellants, Explosives, Pyrotechnics 23, 17Ä…22 (1998) Behaviour of a Working Fluid in an Electrothermal Launcher Chamber 21
Figure 5. Chemical energy (reaction enthalpy) as a function of the plasma injection energy.
Figure 6. Percentage of the chemical energy as a function of the plasma injection energy.
The presence of methanol in the ®nal gaseous phase intervals given before: below 33 kJ and above 35 kJ). Thus,
explains the large amount of chemical energy obtained by a relation between the energies above-mentioned is estab-
injecting small energies into the plasma (7.64 kJ chemical lished with an exponential function:
energy released for 15 kJ plasma injection energy).
eb ‡ a Eplasma
In Figure 6 the percentage of the released chemical
DHreaction ˆ Eplasma
energy is plotted against the various plasma injection
100 eb ‡ a Eplasma
energies and shows a bending point for the same abscissa as
on the diagram in Figure 5. The percentage of the released
where a and b are, respectively, the slope and the ordinate
chemical energy is very large for small plasma injection
from the point of origin of the straight lines.
energies (35% for an energy injected in the plasma of
The straight lines coef®cients are the following:
15 kJ). The best results for the launcher are obtained by
injecting 65Ä…70 kJ energy into the plasma since the 8Eplasma; such as 10 < Eplasma < 33 kJ;
launcher's performances increase as the plasma injection
a ˆ 6:26 10 2 and b ˆ 4:459;
energy increases. It seems that the released chemical energy
8Eplasma; such as 35 < Eplasma < 70 kJ
of the electrothermal process is not completely used to
optimize the performances of the launcher. a ˆ 1:29 10 2 and b ˆ 2:746:
However, a relation between the released chemical
energy of the reaction and the plasma injection energy is The value for which the chemical energy is the smallest is
established. Figure 7 shows the Naperian logarithm of the determined. It is the value of the chemical energy for the
chemical energy for various plasma injection energies. It abscissa of the intersection point: Eplasma ˆ 34.5 kJ.
displays two straight lines. The abscissa of their intersection Further investigations will be carried out using different
point agrees with the abscissa of the inÅ»ection point. Those working Å»uids in order to con®rm this relation for other
two straight lines de®ne two energy intervals (the two substances.
22 Baschung, Samirant, Zimmermann, Steinbach, Mura, and Louati Propellants, Explosives, Pyrotechnics 23, 17Ä…22 (1998)
Figure 7. Naperian logarithm of the chemical energy percentage as a function of the plasma injection energy.
5. Conclusion particular water, are now performed in order to ®nd out the
characteristic parameters for high performances and to
The reaction products issued from the decomposition of con®rm the existence of the same type of relation between
methanol during an electrothermal process were character- the different energies involved in the process.
ized. The experiences performed in a closed vessel were
based on the analysis of the combustion gases with a gas
chromatograph coupled with a mass spectrometer (GCMS).
6. References
A de®ned plasma injection energy was found by which the
amount of methanol is completely decomposed. The theo-
(1) W. Oberle, A. Juhasz, D. Downs, T. Doran, J. Copley and L. T. C.
retical calculations of temperature ®t well with the observed
S., Kee, ``An Overview and Analysis of US Efforts in Electro-
phenomena. As the membrane opens, methanol is ejected
thermal-Chemical Gun Technology'', Proceedings of Ballistics
from the plasma generator towards the expansion chamber.
'93, Vol. 1, 14th International Symposium, 26Ä…29 September 1993,
Quebec City, Canada.
There it decomposes partly for a plasma injection energy up
(2) D. Bruce and D. Van Deusen, ``The Selection of Propellants for
to 34.5 kJ. For higher energies, decomposition is complete.
Electrothermal Chemical Guns'', Proceedings of the 5th Interna-
Carbon monoxide and methane are the components pre-
tional Gun Propellant & Propulsion Symposium, November 19Ä…
sent in largest amounts in the gaseous mixture. The products
21, 1991, Picatinny Arsenal, New Jersey.
(3) William F. Oberle and Eli Freedman, ``Thermochemical Evalu-
from the decomposition of plastic materials are present in
ation of Proposed Electrothermal-Chemical Propellants'', Tech-
the mixture (especially acetylene). The plastic materials
nical Report ARL-TR-12, November 1992.
have an important inŻuence on the combustion and con-
(4) A. E. Wildegger-Gaissmaier and G. P. Wren, ``InŻuence of Plasma
sequently on the electrothermal process. Duration on the Performance of Solid Propellant Electrothermal
Guns'', Proceedings of Ballistics '95, Vol. 3, 15th International
Chemical energy released by the reaction is not the
Symposium on Ballistics, Jerusalem, Israel, May 21Ä…24, 1995.
deciding parameter for optimization of the launcher's per-
(5) K. Zimmermann, I. Raupp, D. Mura, and C. Steinbach, ``Etude
formances. It appeared that when the largest amount of
 Â
parametrique sur un lanceur electrothermique de calibre 12 mm'',
chemical energy is released, the ISL 12 mm launcher has its ISL Technical Report RT 514/92.
(6) K. Zimmermann, I. Raupp, D. Mura, and C. Steinbach,
lower performances.
``Elektrothermischer Antrieb. Anlage und erste Ergebnisse'', ISL
These experiments in the closed vessel are not suf®cient
Report CO 205=91.
to con®rm the good experimental results of the launcher
(7) Results were given at the ISL Internal Conference CCRE.
using methanol as working Żuid (2100 m s 1 muzzle
velocity for a 5 g projectile and 70 kJ plasma injection
energy)(7). Therefore, tests using different working Żuids, in (Received September 2, 1996; Ms 62=96)