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TitleRam Accelerators: Proceedings of the Third International Workshop on Ram Accelerators Held in Sendai, Japan, 16–18 July 1997
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LanguageEnglish
File Size15.9 MB
Total Pages337
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Page 1

Ram Accelerators

Page 2

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Page 168

Analysis and experimental results of a fin stabilized
sub caliber projectile with a blunt step in the external
propulsion accelerator

J. Roml, D. Kruczynski2 , M. Nusca3
1 Professor, Lady Davis Chair, Department of Aerospace Engineering, Technion - Israel Institute of Technology,
Haifa, ISRAEL
2Utron Inc, Aberdeen, MD 21001, USA
"Research Engineer, Propulsion and Flight Division, Army Research Laboratory, Aberdeen Proving Ground, MD
21005, USA

Abstract. The paper presents the results of the first proof of concept test of the External Propul-
sion Accelerator using a subcaliber fin guided projectile. The fin guided sub caliber projectile is
fired in the 120 mm Ram Accelerator facility of ARL. The projectile body shape is similar to the
free flight projectile body except that the fins do not span the 120 mm tube diameter. The pro-
jectile body diameter is 52 mm and the body length is selected so that the flow over the projectile
will not be affected by reflected shock waves from the tube wall. A forward-facing step of a 1 mm
height is used. The projectile weight is 1.017 kg. The 120 mm RA facility of ARL was modified to
accommodate the sub caliber EPA projectile test. The first tube section was filled with nitrogen
at initial pressure of 2.0 MPa. The solid 6 m long accelerator tube section is followed by a clear
test section. This test section is a 1.8 m long plastic tube filled with the combustible mixture
of CH4+202+lOC02 also at 2.0 MPa. The projectile velocity history is measured by a Doppler
radar apparatus. The results of these measurements indicate that after the projectile is clearly
separated from the orburator the projectile decelerates in the nitrogen by about 4500 ± 2500 g's.
It is found that in the clear tube that is filled with the combustible mixture, the projectile picks
up thrust to cancel the drag after a flight time of about 0.4 ms. The projectile then reaches an
equilibrium velocity of 1905 m/s that is 1.59 times the detonation velocity of the mixture. The
projectile stayed at about this velocity, from 1907-1905 mis, for 0.7 ms until it exited the tube.
High speed photography is used to visualize the combustion on the projectile. The photographs
clearly indicate combustion on the projectile, probably initiated by the step and by the leading
edges of the root chord of the fins. Calculations using a CFD code are used to verify the position
of the combustion on the projectile. The results of this first test indicate that combustion was
initiated and stabilized on the rear part of the projectile. It was also shown in this first test that
thrust, sufficient to balance the drag, was generated by the external combustion.

Key words: External propulsion accelerator, Blunt step, Subcaliber projectile

1. Introduction

The development of in-tube chemical accelerators is motivated by the fact that the chemical
propellants are about three orders of magnitude more compact in weight and size than electro-
magnetic energy production and storage systems. Therefore, there is a great advantage to develop
accelerators of projectiles using chemical propellants. There are two methods for in-tube chemi-
cal accelerators utilizing premixed gaseous detonative mixtures; the Ram Accelerator (RA) and
the External Propulsion Accelerator (EPA). These in-tube chemical launchers for accelerating
projectiles to hypervelocity utilize, in the superdetonative mode of operation, the possibilities of
generating continuous thrust by initiating combustion-detonation in the premixed fuel/oxidizer
mixture by shock wave interactions. Developments in the research on the External Propulsion
Accelerator (EPA) were presented in Rom et al. (1995) and Rom (1996)

K. Takayama et al. (eds.), Ram Accelerators
© Springer-Verlag Berlin Heidelberg 1998

Page 169

168 Results of a fin stabilized subcaliber projectile

The first method proposed for an in-tube chemical launcher was the ram accelerator (RA),
originated and developed by A. Hertzberg and his colleagues at the University of Washington
(Hertzberg et al. 1988). The concept of RA, operating in the superdetonative mode, is based on
utilization of the scramjet cycle, where the projectile acts as a free centerbody and the tube as
an extended cowling. The sharp nosed projectile diameter is slightly less than the tube diameter
(typically 70% to 80%). Therefore, the nose shock wave is reflected from the tube wall into the
projectile centerbody. Under proper conditions this reflected shock wave initiates a combustion-
detonation process so that when the products of the chemical reactions are expanded on the rear
part of the projectile, thrust is generated. The projectile is centered in the launching tube by 4,
5 or 6 fins attached to the projectile. These fins span the short distance from the projectile body
to the tube wall.

Another method for operating the chemical in-tube accelerator is based on the utilization of
the external propulsion cycle, proposed by Rom (1990). In EPA the projectile is fired into the
launcher tube that is filled with the premixed fuel/oxidizer mixture. However, here the projectile
diameter is only about 25% to 40% of the tube diameter. For this sub caliber projectile there
is no interaction with the tube wall over the complete length of the projectile. The combustion-
detonation is initiated and is confined to the rear part of the projectile only by aerodynamic means.
A forward facing step or ramp on the projectile shoulder or a blunt leading edge of a ring wing
positioned on the center/rear part of the projectile are used for initiating combustion-detonation.
By the interactions of the detonation wave with the nose shock wave an "external combustion
chamber" is produced. This external combustion region is confined by the contact layer generated
at the intersection point between the nose shock wave and the detached detonation wave ahead of
the step. The contact layer separates the high temperature reaction products from the outer layer
of non reacting flow. The pressure imposed on the contact region is determined by the pressure
jump behind the transmitted oblique shock wave. This pressure is higher than the free stream
pressure and is increased as the flow Mach number is increased. Thus the aerodynamically confined
combustion region is filled with the hot chemical reaction products compressed to higher pressure
by the transmitted shock wave. The hot compressed combustion products are then expanded on
the rear part and into the base region of the projectile, producing thrust on the projectile.

The sub caliber projectile can be launched into the large accelerator tube to fly freely in the
combustible gas mixture. There, the projectile is stabilized aerodynamically by the use of fins.
The fins can be sized to insure a large enough margin of stability. These fins can also be set to
induce some rolling rotation motion in order to maintain the projectile on a trajectory close to
the tube centerline.

The aerodynamic stabilization of the small projectile in a free flight trajectory along the
centerline of the accelerator tube imposes special requirements for the design of the projectile.
This design will require relatively large fins and a forward center of gravity position. There are
also requirements for a smooth sabot separation in order to minimize the initial disturbances to
the flight trajectories. It seemed advantageous to start the tests for proving the concepts of the
EPA using a projectile that is guided in its flight in the accelerator tube. Therefore large span
fins are attached on the subcaliber projectile in order to center the subcaliber projectile in the
accelerator tube. The projectile is centered by wide span fins attached to the subcaliber projectile
body extending to the tube wall (fin guided). The body diameter of the subcaliber projectile, for
this application, can be in the range of 35% to 45% of the launcher tube diameter. It should be
noted that the fin guided EPA projectile is centered in the launching tube in a similar manner to
the fin guiding in RA. However, the use of the small diameter body, in the case of EPA, eliminates
the strong interaction with the tube wall that can cause high drag forces on the projectile. The
present paper presents the results of the first proof of concept test using this subcaliber fin guided
projectile. This first test firing was performed at the ARL at the Aberdeen Proving Ground, MD

Page 336

344 Key word index

Ram acceleration 65, 79, 125, 151
Ramjet-in-tube 3
Real gas effects 39, 125
Rectangular projectile 105

S
Sensitivity 223
Shock-induced combustion 271
Shock waves 215,333
Space launcher 3
Starting process 97, 111, 181,

189, 205, 215,
223,305,325

Stoichiometric hydrogen 295
/ air mixtures
Subcaliber projectile 167
Subdetonative mode 65, 135
Superdetonative mode 89
Supersonic combustion 25
Supersonic projectile 255

T
Thermally-choked mode 25, 119, 125
Thansdetonative mode 31

U
Unstable shock-induced 243
combustion
Unstart 97, 305

V
Velocity 55
Venting 181

Page 337

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