Shock Waves (2000) 10: 235 240
Projectile acceleration in a single-stage gun
at breech pressures below 50 MPa
A. Sasoh, S. Ohba, K. Takayama
Shock Wave Research Center, Institute of Fluid Science, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
(e-mail: [e-mail], [e-mail], [e-mail])
Abstract. Experimental studies were carried out to investigate projectile acceleration in a single-stage gun
at breech pressures below 50 MPa. The gun was driven by firing either liquid or solid propellant. In-bore
projectile velocity was continuously recorded using the well-known, precise VISAR interferometer technique
so that accurate projectile acceleration data could be deduced. Both the attained projectile acceleration
and muzzle exit velocity depend upon the charge-to-mass ratio and the pressure at which the blow-out
disk ruptures. The results obtained from these experiments render information on the interplay between
propellant combustion and projectile acceleration for low in-bore pressure regimes, and they provide the
input data required for adequate numerical simulation.
Key words: Interior ballistics, Liquid propellant, VISAR, Acceleration, Pressure measurement, Combustion
1 Introduction by the propellant charge mass C or mc divided by the
projectile mass M or mp. In solid-propellant and liquid-
propellant guns, maximum breech pressures are typically
Very often, ballistic ranges and gun acceleration techniques
of the order of 300 to 500 MPa. However, experimen-
have been used in shock wave studies as, for example, in
tal data obtained for such high-pressure gun conditions
supersonic or hypersonic flight simulations, in hyperve-
are not easily scaled down to low-pressure firings. Fur-
locity impact studies, and even in medical applications
ther, although modern interior ballistic models can simu-
(Bellhouse et al., 1998). In Japan, a special permission
late both the low-pressure and high-pressure regimes quite
from a municipal government institution is required when
well, an accurate prediction of the behavior of a specific
using compressed air at pressures above 5 MPa or other
low-pressure propulsion system requires adequate input
gases at pressures above 1 MPa. Also, procurement and
data that are only attainable from experiment. As shall
maintenance costs for compressed-gas driven facilities are
be shown later, interior ballistic performance of the low-
relatively high encumbering the limited university bud-
pressure device used here is, for instance, affected by the
get. Thus the employment of different propelling means,
pressure at which the blow-out disk ruptures.
such as solid-propellant or liquid-propellant charges, is a
The present paper describes experiments that deal with
reasonable alternative. However, the employment of such
the behavior of a single-stage gun at breech pressures be-
chemical propulsion techniques imposes special safety reg-
low 50 MPa. Pressure histories in the gun chamber are
ulations, especially in university laboratories. For exam-
measured and projectile acceleration data are deduced
ple, maximum gun pressures are limited to about 100
from measurement of the in-bore velocity when firing dif-
MPa, whereas projectile muzzle velocities can range from
ferent propellants and varying both the propellant charge
about 0.1 km/s to several kilometers per second. Further-
and projectile masses as well as the rupture pressure of the
more, if the gun is used as a pre-accelerator for a ram
blow-out disk. Their effect onto projectile acceleration is
accelerator (Hertzberg et al., 1988) so that a fragile projec-
determined and discussed in view of the importance for
tile must be accelerated, then the peak acceleration should
numerical simulation.
not exceed 5 × 105 m/s2.
The two alternative gun systems firing either solid or
liquid propellant have been studied experimentally and
2 Experimental apparatus
theoretically in great detail (see, for example, Stiefel 1988,
Klingenberg and Heimerl 1992, and Klingenberg et al.
The experiments have been conducted in a single-stage
1997). For example, velocity limits for chemical propulsion
gun. The gun tube in-bore diameter was 25 mm and its
were proposed relating the attainable projectile muzzle ve-
length 2.0 m. The muzzle of the barrel was put into an
locity to the charge-to-mass (C/M) or (mc/mp) ratio given
evacuated chamber. Prior to the firing, the ambient pres-
Correspondence to: A. Sasoh sure inside the evacuated chamber was of the order of
236 A. Saroh et al.: Projectile acceleration in single-stage gun
Table 1. Ignition powder and main propel-
lant combinations
Notation Igniter Main propellant
LP SS (0.9g) LP 1846(10-40g)
SP BP (1.3g) NY-500(10-20g)
BP; black powder, LP; liquid propellant,
SP; solid propellant, NY-500; solid propellant
(single base, Nippon Oil & Fats), SS; solid
propellant (double base, Nippon Oil & Fats),
quantity in parentheses; igniter/propellant
mass
Fig. 1. Schematic illustration of propellant chamber, length
unit; mm
100 Pa. The projectile velocity in the gun tube was mea-
sured by means of the VISAR (Velocity Interferometer
System for Any Reflector; Barker and Hollenbach 1972,
Sasoh et al. 2000). The fringe constant was set to be
130.84 m/s/fringe.
Figure 1 illustrates schematically the propellant cham-
ber. The chamber has a cylindrical inner diameter of 35 mm
and a length of 113 mm. It is separated from the evacu-
ated launch tube by a flat, cross-grooved blow-out disk.
The pressure at which the blow-out disk ruptured, pr, was
varied by using different material (stainless steel SUS304
or aluminum) and varying its thickness and the depth of
the cross grooves. Two piezoelectric pressure transducers
were recess-mounted from the inner wall. The pressure
measured near the igniter fuse head is marked  p1, that
Fig. 2. Pressure p1 vs. time t with different pr, liquid propel-
near the blow-out disk  p2. The difference between p1 and
lant
p2 curves is important for analyzing wave propagation in
the propellant chamber, as discussed in Sasoh et al., 1999.
Otherwise this issue is trivial, especially in the present
pressure (pressure on the projectile base) profile that ap-
study. The projectile-to-launch tube fitting is arranged so
proaches a rectangular shape (Klingenberg et al. 1997).
that the friction force developing at the interface (Sasoh
In the present bulk-loaded configuration with breech pres-
et al. 2000) is negligible.
sures below 50 MPa, the projectile acceleration character-
In this study, gun experiments were carried out us-
istics need to be re-examined.
ing two types of propellants: a single-base solid propellant
Figure 2 shows pressure histories obtained in the pro-
(NY-500, Nippon Oil & Fats Co.) and a liquid propellant
pellant chamber, p1, and measured when firing the liquid
(LP) based upon HAN (Hydroxyl Ammonium Nitrate),
propellant (LP). The time t = 0 corresponds to a pressure
LP1846 (Klingenberg et al., 1997). Data of the igniter
drop caused by the arrival of expansion waves that orig-
powder and propellant used are listed in Table 1. The liq-
inate at the blow-out disk upon rupture. In the present
uid propellant was primarily used because its combustion
study, the static blow-out disk rupture pressure, pr, was
rate is sensitive to pressure, and it is suitable for studying
varied from 10 to 40 MPa. The rupture pressure was mea-
the relation between propellant combustion and projectile
sured using a hydraulic test device. The scatter of the
acceleration. In the present study, a bulk-loaded type of
rupture pressure was Ä… 15 %. As seen in Fig. 2, the peak
LP firing was used. The solid propellant served to com-
value of the propellant chamber pressure was of the or-
pare the effect of liquid when opposed to solid propellant
der of pr. From the impetus of LP1846 = 899 J/g( Klin-
in this single-stage gun configuration.
genberg et al., 1997), the attainable propellant chamber
pressure without blow-out disk rupture for mc (propel-
lant mass) = 25 g was calculated to be of the order of 210
3 Results and discussions
MPa. However, the measured peak pressure is one order
of magnitude lower. The dependency of the peak pressure
3.1 Effect of blow-out disk rupture pressure
on rupture pressure pr implies that the combustion rate is
It is well known that liquid propellant, if used in a regen- so low that expansion waves caused by disk rupture and
erative liquid propellant gun, can have a favorable base subsequent projectile motion limit the peak pressure.
A. Saroh et al.: Projectile acceleration in single-stage gun 237
ab
Fig. 3a,b. Profiles of projectile acceleration a vs. x for the same conditions as in Fig. 2, liquid propellant, a: a b: a/amax
Figure 3a shows profiles of projectile acceleration a vs. The corresponding variations of p1 vs. time t are plot-
travel distance x from the breech. In this study, a is deter- ted in Fig. 5. For mp = 34g, the pressure increase after
mined by smoothing and differentiating the experimental- the blow-out disk rupture becomes about two times as
ly-measured projectile velocity profile in the launch tube. large as that obtained for the smaller mp. The larger mp,
Both the peak acceleration and muzzle velocity are strong- the lower a becomes, thereby decreasing the pressure drop
ly dependent upon pr; the muzzle velocities with pr = 10, due to expansion waves originating from the accelerating
20 and 40 MPa equaled 615, 800 and 937 m/s, respec- projectile; further, compression waves generated from pro-
tively. However, when normalizing the accelerations by pellant combustion can catch up with the projectile more
the respective peak values, the acceleration profiles (see rapidly, enhancing the base pressue. It follows from these
Fig. 3b) decrease in a similar fashion down to a/amax results that with decreasing mc/mp the piezometric effi-
0.5. The acceleration is attributed mainly to the expand- ciency, see below, becomes higher.
ing gas that drives the projectile. In all of the three shots, As seen in Fig. 6, the higher mc/mp, the higher the
the travel distance reached when the projectile accelera- peak acceleration becomes. Here, a  piezometric efficiency ,
tion dropped to half of the maximum (50 % peak) was ·, is defined by (Klingenberg et al. 1997)
as short as 0.2 m (8 calibers), which is one order in mag-
x
Å»
adx
nitude smaller than the total length of the launch tube 0
· a" , (2)
Å»
(2 m). amaxx
where x designates the value of x where a attains 15% of
Å»
its peak value. From Fig. 6, · = 0.43 for mc/mp=1.56, and
3.2 Effect of charge-to-mass ratio
· = 0.54 for mc/mp = 0.74. In other words, the acceler-
ation profile with the lower mc/mp is flatter; this charac-
The mass of the propellant here is designated by mc and
teristic is preferable when launching a fragile projectile or
the mass of the projectile by mp.
increasing an effective acceleration length.
In Fig. 4, the profiles of a net force mpa vs. x are plot-
Figure 7 displays the variations of p1 vs. t as measured
ted for two different mp with a constant mc (= 25g). For
with different mc keeping mp constant ( mp = 16.2g).
mp = 34g (mc/mp = 0.74), the net force is higher than
With mc =30 g (mc/mp =1.85), it takes longer to build
for the other case (mp = 16g, mc/mp = 1.56) down to x
up the pressure needed to rupture the blow-out disk, be-
= 1.2 m. The force, pbaseA (A; tube cross-sectional area,
cause a larger amount of energy is necessary for decom-
pbase; projectile base pressure), is calculated using the fol-
posing thermally the liquid propellant. However, after the
lowing relation (Klingenberg et al., 1997).
blow-out disk rupture, p1 continues to increase with mc =
30 g due to continued gas production, whereas it starts to
p2A
pbaseA = . (1)
decrease with mc = 15 g because of the lower combustion
1+mc/(2mp)
rate. The corresponding acceleration profiles a vs. x are
Here the measured time variations of p2 are numerically shown in Fig. 8. As expected from Fig. 7, the peak value of
smoothed. Although detailed variations of the forces de- the acceleration, amax, becomes much higher for the larger
termined from the measured velocity profiles are not nec- mc, while · is lower. The dependency of · on mc/mp is
essarily reproduced by such a simple relation, the differ- the same as in Fig. 6.
ence in mpa between these two mc/mp ratios is of the Figure 9 shows the projectile acceleration profiles in
same order as in pbaseA. two operations where mc/mp is kept almost constant. It
238 A. Saroh et al.: Projectile acceleration in single-stage gun
Fig. 4. Profiles of mpa vs. x with different mp, liquid propel-
Fig. 6. Projectile acceleration profiles a vs. x with two different
lant
mp, liquid propellant
Fig. 7. Pressure p1 vs. t with two different mc, liquid propel-
Fig. 5. Pressure p1 vs. t for the same conditions as in Fig. 4
lant
In summary, in order to increase · a small value of
should be noted that those two operations are not com-
mc/mp is preferable. With a constant mc/mp and pr, ·
pletely scaled; the dimensions of the propellant cham-
increases with increasing the masses.
ber and pr are constant. As seen in the figure, the mea-
sured muzzle velocities are almost equal. With the smaller
masses (case A, mc/mp =15.0g/16.2g), the initial accel-
3.3 Solid propellant
eration level becomes higher. In the propellant chamber
of a constant volume, the attainable pressure for the case
B (mc/mp =31.9g/34.5g) is twice as high as that of the Interior ballistics of solid propellants have been inten-
case A. Therefore, the ratio of pr to the attainable pres- sively studied. The results of the previous investigations
sure in the case B equals half of that of the case A. The and their applications to numerical computations are well
acceleration level for 0.15mhigher for the larger masses (case B). However, basic accel- results with the solid propellant are presented in order to
eration characteristics for x >0.15m are similar to each compare it to the liquid propellant behavior. Figure 10
other, and those agree well with the corresponding esti- shows the projectile acceleration profiles a vs. x measured
mations based on Eq. (1). In other words, even with a with the solid propellant. Being normalized by the respec-
constant pr, the mc/mp similarity in the acceleration pro- tive peak values, the acceleration profiles nearly equal each
file is basically fulfilled, except for the case immediately other. The length at half maximum is 0.4 m  longer than
after blow-out disk rupture. that with LP, see Fig. 3b.
A. Saroh et al.: Projectile acceleration in single-stage gun 239
Fig. 10. Projectile acceleration (normalized) profiles a/amax
vs. x with two different pr, solid propellant
Fig. 8. Projectile acceleration profiles a vs. x for the same
conditions as in Fig. 7
Fig. 11. Pressure p1 vs. t with LP and SP
Fig. 9. Projectile acceleration profile a vs. x for a constant
differences appear in their combustion rates and their sen-
mc/mp, liquid propellant
sitivities to pressure. In the case of the solid propellant the
effective acceleration length is longer since the acceleration
continues to be boosted by the higher rate of combustion
The p1 vs. t curves obtained when firing using the liq-
even after the blow-out disk ruptures, in the case of the
uid or the solid propellant are shown in Fig. 11. In the case
liquid propellant the combustion effectively ceases to en-
of the liquid propellant, the first pressurize of p1 is due to
hance the projectile acceleration after disk rupture.
combustion of the igniting smokeless powder. The pressure
is kept almost constant ( 5.5 MPa) during the thermal
decomposition process. Then it rises up again due to the
4 Conclusions
combustion of the main propellant charge. However, the
pressure increase rate using the liquid propellant is much
lower than that of the solid propellant. In the case of the In controlling the gun performance under the present low-
solid propellant, its combustion rate is fairly high. Even af- pressure conditions, the interaction between the propel-
ter the blow-out disk rupture, the projectile acceleration is lant combustion and the projectile motion is the key issue.
boosted by the higher rate of combustion, thereby increas- The liquid propellant combustion rate is so low that the
ing the effective acceleration length, see Figs. 3b and 10. breech pressure are significantly decreased through expan-
The propellant chamber pressure overshoots significantly sion waves generated by the projectile motion; the muzzle
the static blow-out disk rupture pressure before expansion velocity is sensitive to the blow-out disk rupture pressure.
waves due to the projectile motion arrive at the location Suppressing the peak projectile acceleration by decreas-
of the measurement. ing the charge-to-mass ratio, the piezometric efficiency
Generally, the solid and liquid propellants have ba- is improved. Such an operation is important for launch-
sically similar operating conditions. The most important ing fragile projectiles like the devices needed for the ram
240 A. Saroh et al.: Projectile acceleration in single-stage gun
accelerators. The results presented in this paper provide Hertzberg A, Bruckner AP, Bogdanof DW (1988) Ram accel-
erator: A new chemical method for accelerating projectiles
experimental data available for constructing and refining
theoretical/numerical gun operation models for the rele- to ultrahigh velocities. AIAA J 26:195 203
Klingenberg G, Heimerl JM (1992) Gun Muzzle Blast and
vant operating conditions.
Flash, Progress in Astro & Aero, AIAA, Vol 139, Chap
2
Acknowledgements. The authors are grateful to Professor E.
Klingenberg G, Knapton JD, Morrison WF and Wren GP
Zaretsky, Ben-Gurion University, Israel, for his essential assis-
(1997) Liquid Propellant Gun Technology. Progress in As-
tance in preparing for the VISAR measurement during his stay
tro & Aero, AIAA, Vol 175, Chaps 1 4,7,8
at Shock Wave Research Center as a visiting professor. We also
Krier H, Summerfield M (eds, 1979) Interior Ballistics of Guns,
appreciate Dr. Carl Knowlen of University of Washington, WA,
Prog in Astro & Aero, AIAA, Vol 66, Part I
U.S.A, for his valuable suggestions and proofreading the paper.
Sasoh A, Ohba S, Takayama K (1999) Investigation on utiliza-
We would like to express our gratitude to Dr. O. Onodera,
tion of liquid propellant in ballistic range experiments. J
Messrs. H. Ojima and H. Ogawa for their helps in conducting
Jpn Explosive Soc 60:205 211
the experiments, and Messrs. M. Adachi, M. Kato, K. Taka-
Sasoh A, Ohba S and Takayama K (2000) Quantitative effects
hashi and K. Asano for their efforts in manufacturing various
of projectile-launch tube wall friction on ballistic range op-
items. We also appreciate Asahi Chemical Industry Co., Ltd.,
eration, AIAA J, in press
Oita, Japan, for its supplying us with the liquid propellant.
Stiefel L (ed, 1988) Gun Propulsion Technology, Progress in
Astro & Aero, AIAA, Vol 109, Chaps 3, 4, 8, 9, 13, 14
References
Barker LM and Hollenbach RE (1972) Laser interferometer for
measuring high velocities of any reflecting surface. J Appl
Phys 43:4669 4675
Bellhouse BJ, Quinlan NJ, Ainsworth RW (1998) Needle-less
delivery of drugs, in dry powder form, using shock waves
and supersonic gas flow. In: Houwing AFP and Paull A
(eds) Shock Waves, Proc 21st Int Symp on Shock Waves,
Panther Publ & Printing, Fyshwick, ACT, Australia, Vol.
1, pp. 51 56