The possibility of using an electric arc under the influence of a magnetic field in ambient air to transform the heat energy of the working fluid arc into the kinetic energy of the jet was investigated. A convergent-divergent type nozzle was used. Variation of specific thrust and chamber pressure are discussed. Nitrogen was the propellant used

Oggero, M.

Robotti, A. C.

English

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'-	 NASA TECHNICAL MEMORANDUM	 NASA TM-77474
EXPERIMENTAL RESEARCH ON ELECTRIC PROPULSION
NOT' ' V: EXPERIMENTAL STUDY OF A MAGNETIC FIELD STABILIZED ARC-JET
A. C. Robotti and M. Oggero
i
O
I
(NASA-TM-77474) EXPERIMENTAL RESEARCH ON
	 N84-34266
ELECTRIC.' PRtUPULSICN. NOTE 5: EXPERIMENTAL
STUDY OF A MAGNETIC FIELD STABILIZED ARC-JET
(National Aeronautics dnd Space
	 Unclas
Administration) 13 P RC A02/MF A01 CSCL 201 G3/75 22664
Translation of "Ricerche sperimentali sulla propulsions elettrica -
Nota V: Studio sperimentale, di un plasmagetto ad arco stabilizzato
con campo magnetico", La Ricerca Scientifica, Year 33, Rendiconti A
Vol. 3, No. 1, 1963 - Series 2, nn ?A
r
,I/'	
r/(}P e y
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
WAHINGTON, D.C. 20546
	
JUNE 1984
1.	 Re p ort No.
NASA TM-77474
2,	 Covornment Accession No. Y.	 Reclpient'i Cotoioo no.
A.	 Tllle end Subtille 3. RspaN Dele
Experimental Research on Electric June 1984
Propulsion.
	
- Note V:	 Exper imental Study 6. Petfanolno Otgonizorer Code
of a Magnetic Field Stabilized Arc,-Jet
7.	 Auttrot(e1
Aurelio	 C. Robotti and Mario Oggero
L Petfaminp Organization Report No,
School of Aerospace Engineering, Turin 10.	 Work Unit No,
Polytechnic
9,	 Perlormtng Organisation Nonye and Address
Leo Kanner Associates
11.	 Contract of Grant No.
NASW-3541
Redwood City, California 94063 13.
	 (you of Report end Period Corerert
Translation
12,	 rSponr g rin; Agency Name and Address
National Aeronautics and Space Adminis-
tration,	 Washington,
	
D.C.	 20546 Tf,	 Sponser)ng Agency Code
15,	 Sv;;ler..tntcry Notes
Translation of	 "Ricerche	 sperimentali sulla propulsione
elettrica	 - Nota V:	 Studio sperimentale di un plasmagetto
ad arco stabilizzato con cameo magnetico", La Ricerca Scienti-
Pica,	 year 33,	 Rendiconti A Vol. 3, No. 1, 1963 - Series 2, pp 28-35.
10.	 L.117UC1
Phis paper describes a number of experiments carried out on a
magnetic field stabilized arc-jet, 	 using nitrogen as a pro-
pellant:
17,	 Key Words (ScIcCtcO by Auihor(t)) 18.	 Dlstribviion Statement
Unclassified-Unlimited
19.	 Secvnly Closell, (of this report) 2220,	 Sacvrtly Claeelf. (of this pope) 21• No. of Paget 22.
Unclassified Unclassified 13
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STANDARD TITLE alr-n
- 2 -
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Flo. ^'T -^' M^ f	 5'.^3' A .n s t .•	 x	 ',	 z	 ^' J.
atr^
EXPERIMENTAL RESEARCH ON ELECTRIC PROPULSION
NOTE V: EXPERIMENTAL STUDY OF A MAGNETIC FIELD STABILIZED ARC-JET
A. C. Robotti and M. Oqqero
School of Aerospace Enqineerinq
Turin Polytechnic
1. Preface
In a previous Note  on the same subje,t, we described the ex-	 /28 *
4
perimen til results obtained with an electric arc rotating under the
influence of a magnetic field in ambient air. The continuation of
the research has been directed at gauging the possibility of practical
use of this type of arc as a means of heating a working fluid, with
a view toward possible astronautical applications.
The tests described herein were therefore carried out with a j
device similar to the above, but closed at the ends and equipped with
a convergent-divergent type nozzle suitable for transforming the heat
energy of the working fluid arc into the kinetic energy of the jet, L
which is the one which is of interest for purposes of propulsion.
2. Tes	 eting D vicei
 1 shows a schematic cross section of the test device: the 	 s
chamber, which also includes the discharge nozzle, is of copper and
is enveloped by a pressurized water-jacket; at its base it has the
connections for the propellant gas and the pressure device; an insu-
lating support on the outside of the brass jacket carries the winding
that creates the magnetic field, consisting of 12 turns of series
wound copper for the arc current.
A great deal of attention was given to the thermal stress.borne
* Numbers in the margin indicate pagination in the foreign text.
- 3 -
i
Fig. 1 - Schematic cross
section of the jet. 'rl
k
k
R
y
1
®RBG@NAI R, ..... ' !I'#
OF POOR QuU^` Lulv'u
the arc: contrary to expectation, the greatest
nozzle but on the area struck by the arc, even
be rotating rapidly, It is therefore necessary
this area in order to increase the heat exchange /?9
,a
s
by the chamber due to
stress is not on the
though the latter may
to provide Finning in
surface.
r
The water-cooled central electrode is concave, a form offering
the greatest stability among the various types previously studied.
Here, too, the material used is copper.
1
.4
!;
^, rt
In this series of tests, as in the preceding ones, the source
of electrical energy was a metadyne with variable current controlled
by the excitation.
The wiring and the plan for the cooling unit are also the same;
th,, only difference is that the propellant in the present tests is
nitrogen, which is supplied by a set of cylinders with reducer.
- 4 -
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f 	
^`
OF pOC)ft QUAL 6 V
Nitrogen was chosen so as to be able to experiment with a prat:
tically inert gas on the electrodes from the chemical point of view,
without needing to have recourse to excessively costly rare gases
or to hydrogen, which does not lend itself well to experiments in a
closed environment. Compressed air, which was used previously in the
tests in ambient air with the device without a nozzle, proved unsuita-
ble inasmuch as it caused the formation of slight oxide crusts that
clogged the nozzle from time to time, thereby affecting the results.
3. Measurement of Thrust
From the point of view of propulsion, the measurement of the
thrust is fundamental inasmuch as it permits calculation of the unit's
specific thrust sand its electrical/kinetic yield, which are at the
basis of every application of this kind.
	
/30
Elsewhere thrust is an entity that is very difficult to measure
directly,	 especially when it is of very limited intensity due to
many other parasitic thrusts (cooling water,	 feed gas)	 which can cause
appreciable errors.
,h
Fig. 2 - Device for measuring thrust.
Jrnp rlNr}i.r"{^,	 p (1) Extensimetric strips
We therefore resorted to indirect measurement,	 fixing the jet
yr"	 on a flat surface so as to make it deviate 900 and then gathering
all of the variations of momentum.
The device is shown in Figure 2, while Figure 3 shows the wiring
for the reading; the deflector is a small block of graphite held in a
small bowl which in turn is attached to a thin plate; the sensitive
parts are two strain gauges stuck on the thin plate and acting as two
branches of a Wheatstone bridge. The bridge is completed by other
external resistances and the imbalance is read on a very sensitive
galvanometer. The device is calibrated with suitably arranged weights,
- 5 - J
u() pOOR ^1G^Wi^.ITI
great care being taken to prevent the calibration from varying with
y	 the temperature,	 and for this reason we have to properly insulate the
device from the possible projection of particles during operation.
i
The calibration is always checked before and after each series of
tests.	 The device is mounted at the side of the engine and is exposed
to the remote-controlled jet at the appropriate time. 	 While the thrust
readings are being taken the graphite block	 is deteriorating rapi-ly,
eroding at the center and developirg a slight concavity;	 it is there-
fore changed after every two or three tests so as to avoid results
that are affected by errors. This precaution in fact proved to be 	 /31
excessive inasmuch as we experimentally proved that if the concavity 	 4
i
did not reach very high values,	 the variations in thrust were very
slight with reference to the block,	 and practically negligible.
-----------
6 1
	 600"n
----- ---
j
LIP6J
n
8' -----
  cone
IIIIF­^
____-1
Fig. 3 - Thrust Detector Wiring Diagram. 1 = Jet; 2 = Sensitive
Element; 3 = Screened Cable; 4 = Feeder Detector;
V = Voltmeter; G = Galvanometer; T = Switch.
Great care was taken to see that the distance of the block from
the nozzle remained constant for all. of tr-e tests and a preliminary
calibration was also made to check the effect on the device due to
attraction by the magnetic field (extremely deak) generated by the
fixed windings carrying the current; but there was no noticeable effect.
- 6 -
—	 a
Device for measur-
ing thrust in
operation.
PXlCr M
QUALIre
Figure 4 shows the thrust detector during a measurement.
fT
Durinq the tests it is very interesting to study the motion of
the arc in the chamber, but since direct observation -is impossible
with the present device, the very luminous cathode spot is observed
indirectly, reflected on a white screen positioned at a suitable dis-
tance above the nozzle; this behaves like the aperture of a camera
obscura and gives a rather clear image of the cathode spot, permitting
its motion to be studied eves when the device is under pressure and
therefore closed.
4.	 Test Procedure
The device is usually primed with a wire inserted between the
two electrodes; when the circuit is closed the live wire vaporizes
and primes the arc.
At first, when the rate of gas flow is nil, th? arc rotates
rapidly around the central electrodee, which can be observed on the
screen. Then, with the incrFase of gas flow and pressure in the
chamber, the motion of the cathode spot becomes more irregular and
7 -
q	 tr `
tends to stabilize at two or three points which appear more luminous.
By using suitable goggles it is possible to discern the Mach waves
distinctly if the chamber pressure is at least 4 kg/cm 2 . The dart-	 4
A
shaped jet is 4 to 5 cm long; after that the zone where it mixes with Z.32
the surrounding air begins, giving it the characteristic foxtail
appearance.
The dart has considerable mechanical-thermal energy: It rapidly
	 s,
melts and vaporizes any metal that touches it, and even the graphite
block for measuring thrust quickly becomes incandescent inasmuch as
it: is exposed to the jet, and if the reading is prolonged it is con-
	 E
sumed, taking on a concave appearance.
5.	 Experimental Results
The principal aim of the present series of tests is to evaluate
the arc-jet as a possible propellant. we therefore sought essentially
to determine the parameters that are of interest for this purpose.
The first object in particular of the study was specific thrust.
In this first investigation, however, we did not inquire into the
maximum absoaute value possible but instead investigated the behavior
of the specifi^ thrust with variation of the two independent variables
available to the experimenter: the current and the rate of gas flow.
To this end it was necessary to operate with a possibly neutral
gas with a well-defined chemical composition that would not react with
the electrodes and not undergo alterations due to pressure and tempera-
ture (besides dissociation and ionization). Nitrogen was therefore
chosen, essentially for reasons of cost and safety, even though the
specific thrust obtained with it is nearly one third that of hydrogen
with the same chamber_ temperature, because of the molecular weight.
Using the two variables, electric current and rate of gas flow,
various series of tests were carried out on the gas flow/constant power
- 8 -
C,
p F
	
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OF POOR QUALITY
ratio, with a view toward d,-^termininq their effect on specific thrust.
I
tsm 250 s ,	 ^
'its. 005 '	
.0,04 g/I'w
^' I	 I	 J a 0 GAS glIr
I
t,
140
120
^d
	 100
80
60
40
20
0
0	 2	 4	 6	 P /p
	 8
Fiq. 5 - Variation of specific thrust and
chamber pressure
The experimental results are given in Figures 5 through 7. In
the diagrams the unbroken lines indicate the theoretical curves of
the variation of specific thrust in relation to pressure, presupposing
the use of the appropriate nozzle, for the two values of 250 s and 190 s
for P c/P e = Co .
6.	 Analysis of Results	 /33
Examination of the Figure 5 through 7 diagrams immediately brings
to the fore a very important fact, which is that the specific thrust,
pressure being equal, is affected hardly at all by the gas flow/power
supped  ratio. In fact, passing from a ratio of 0.04 g/kW to 0.09
g/kW, no noticeable improvement is seen.
It nevertheless must be noted that in regard to regularity of
functioning the gas flow/power ratio does have appreciable importance
inasmuch as the device, chamber pressure being equal, is much more
stable electrically and in terms of pressure oscillation when the power
supplied is increased,
- 9 -
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ORIGINAL PAC-1^: 
fo
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OF pOoR QUALITY
This behavior can be explained if we consider the mode of func-
tioning of the device. In the chamber there is an arc rotating at
a certain velocity in a nitrogen atmosphere which, given the ratio
between the cross section of the nozzle and that of the cha.aber, can
be considered calm.
The cross section of the arc is very small compared to that of
the chamber and therefore we can reasonably suppose that the greater
part of the gas is not touched by the arc but is heated only by con-
vection and radiation. Under these conditions the exchange of heat
between the nitrogen and the arc is essentially a function of the
electrical energy dispersed in the arc column.
An increase of current causes an increase in temperature in the
gas column, resulting in a decrease in voltage inasmuch as the con-
ductibility of the gas increases with the temperature, as is well-
known. Consequently, the power dispersed in the arc column is nearly
constant with variation of current, while the excess power is absorbed
by the elctrodes, which locally heat up more and therefore increase
their resistance.
i,
14C
12C
lot
ec
6C
4C
	 007g1A'w
20
0	 — I
0
	
Z	 8
Fig. _6 - Variation of specific thrust and
chamber pressure
- 10 -
4:7
oil NOOWI. (_11J1%
Under these conditions it would be advanta qoo l i.o to work with
the least possible current, corresponding exactly to that absolutely
necessary for maintaining the arc. It must nevertheless not be for-
gotten that there are rather appreciable turbulences in the chamber
which; if too close to the limit, can very easily cause the arc to
break, as experience has shown.
Figur N s 5 through 7 indicate another interesting point: The
	
/34
specific thrust which at low temperatures is very low increases much
more rapidly than might be expected, exceeding the corresponding
value at 250 s in relation to infinite expansion (for chamber pressures
greater than 6 kg/cm 2	 This behavior can in part be explained by
taking into account the fact that the nozzle was designed for an ex-
pansion ratio of 6, so the Jot becomes hyperexpanded with less than 6
atmospheres in the chamber. Holv,,ever, it is quite probable that anoth-
er phenomenon of a different natire has come into play here.
Is
140
120
100
80
60
40
"'0
0
0	 2	 4	 6	 PC/Pe	 a
Fig, , 7 - Variation of specific thrust and
chamber pressure
- 11 -
SW
ISw /90 S
00,9	 W
-JA"17
rA
60"
We -)aid that the gas is heated through exchange by convection
and radiation between the arc column and the surrounding nitrogen.
This exchange increases with the pressure and more heat is drawn;
the arc reacts with an increase in voltage which
	 (current being equal)
is equivalent to dispersing more energy in the column of conductor gas.
Thus it is clear that while an increase in dispersed energy
(chamber pressure being equal) increases only the losses in the coplor,
the greater power supplied goes only to heat the cooling water; while
conversely the increase in absorbed energy caused by increasing the
pressure goes effectively tn heat the gas,
7.	 Conclusion
The abovomentionod hypotheses represent an attempt to explain the
arc's behavior tinder pressure with gas flow. There are many parameters
which in -his firs- series of investigations were negligible and on
which perhaps the behavior of the arc-jet depends in an appreciable way.
The task of the next experiments will be to investigate these
phonomena for the purpose of drawing some reliable conclusions.
Ferraris, who made the necessary electrical equipment available, and
to his co-worker Ing. A, Abete, who actively cooperated with the writ-
ers in carrying out the experiments described herein.
- 12 -
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Ayy^
9	 ^..a^^ '- r
RIP^^R1vNC^i;
( ) A. C. Robotti. & M, Oggero: "Ricerche -verimentali sulla propul-
nione elettrica. Nota TV; Ricerche eull'arco elettr.ico rotantr
in camps magnetico in vista di una aua applicazione in propul-
Pori apaziali" (iaxperimental reacarch on electric propulsion.
Note IV: Research on a rotating electric: arc in a magnetic field
in view of its 1,ossible application in apace propel:lanta) , in
Ric. aci., 32 (TT-), 2(1-27 (1962).
T
i
^t.•atar,tuti•
T!, h zs.'mental rosearch on electric propulsion. Note d: Experimental study of a magnetic
.,tAbU1zed are-jet, (Odgmal erienhflc f alw)
'f'him paper tleserihve n iiuuibot . of etperinients earried ntit nn it tiiagrietie field
stabilized tiro-jet, tixing, nitrogen nH it propellant.
The reprodurdnn of the S"u"ndry, "mnth"ting tnuhor and perindlcal, is authurUed,
- 1.3
	 -
f


Experimental research on electric propulsion. Note 5: Experimental study of a magnetic field stabilized arc-jet

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