Pergamon Int. J. Impact Engng, Vol. 20, pp. 63-68, 1997

©1997 Elsevier Science Ltd. All rights reserved Printed in Great Britain

0734-743X/97 $17.00+0.00

ACCELERATING DEVICES USING ENERGY OF P O W E R F U L E X P L O S I O N S

EVGENY N.AVRORIN, VLADIMIR Z.NECHAI, VLADIMIR N.NOGIN, VADIM A.SIMONENKO, OLEG N.SHUBIN, VALERY E .CHEREMAZOV,

YURY V.OLHOVSKY

]Russian Federal Nuclear Center - Institute of Technical Physics, 456770, P.O.Box 245, Snezhinsk, Russia

Summary--During the period of nuclear underground testing the idea arose at RFNC- VNIITF on simultaneous experiments investigating processes going on during the high velocity impacts using nuclear explosion energy for accelerating the impactors. Two types of the devices are described for experimental simulation of direct impact at which the velocity vector of the plane impactor is peq~endicular to its free surface. The both types of the accelerating devices were successfully tested. Velocities of the impactors were 2.5 up to 5 km/s. An accelerating device for simulating the oblique impact was successfully developed. At the "impactor" velocity about 4.5 km/s the value of the angle between the particle velocity vector and the ground boundau, was about 60 degrees.

INTRODUCTION

While addressing the problems of crater formation, meteorite impacts on spacecraft, and studying material properties under shock loading we are faced with the necessity of experimental simulation of high velocity impacts. Some non-traditional problems have arisen in association with the wide discussion of the problem of the asteroid menace. They include effects resulting from collision of large bodies - asteroids and comets - with planets, issues of affecting the asteroid by kinetic impact of massive body, as well as processes of destruction during spacecraft collision with the asteroid.

Recent discussions have been considering configuration of experiments performed during space missions to asteroids for studying their physical properties in which a special impactor or spacecraft impacts into an asteroid, and effects accompanying the collisions are recorded by additional spacecraft. To perform such experiments in space successfully it is necessary to pursue a comprehensive development on the Earth including simulation of the collision process itself (at least it is necessary to ensure sufficiently complete set of experimental data for calibration of apt computational methods).

In modern experiments various means to achieve high impact velocities are used: powder, electromagnetic and light gas ~, ,,s, ,,u" impactor acceleration by HE, rocket tracks (even using rocket launch), etc.. Along with certain advantages each of these methods has inherent drawbacks. Record impact velocities can be reached with electromagnetic and light gas guns. However, mass of the accelerated impactors is not high and decreases rapidly with growth of the attained velocity value. While using the electromagnetic guns we are faced with additional problems in determining the impactor state after completion of acceleration. While accelerating the impactors by HE there is a natural maximum speed limit. Besides that, if we need to pursue experiments with large devices (e.g., while accelerating plates tens of cm thick for simulating impact into an infinite semi-space) we have to use large amounts of HE that is not always feasible and convenient. The rocket track experiments are fairly expensive.

63

64 E.N. AVRORIN et al.

During the period of nuclear underground testing a great attention was paid at RFNC- VNIITF to issues of peaceful use of nuclear explosions, ira particular, for obtaining unique experimental data in the interests of basic research. At that time the idea arose on simultaneous experiments investigating processes going on during high velocity impacts using nuclear explosion energy for accelerating the impactors. The most feasible was a

• , ( i l l scheme of the so-called reversed configuration - when "the m~pactm~, body is at rest, and it is impacted by a layer of matter accelerated by a major shock of the nuclear explosion. To estimate capabilities of this method several types of accelerating devices were developed which are described in this work.

ACCELERATING DEVICES FOR DIRECT IMPACT S I M U L A T I O N

Operat ion of the devices described below is based on the effect of increase of the matter particle velocity when a shock reaches its free surface. The shock is created by an underground nuclear explosion, and the accelerating device is placed in its vicinity in a special excavated hole.

As we know when not a very powerful shock (in the region of acoustic approximation applicability) reaches a free surface of matter the particle velocity of this matter is doubled. For some substances the doubling occurs up to relatively high pressures in the shock wave, e.g., for iron it is up to 1.5 Mbar.

For experimental simulation of direct impact at which vector of plane impactor velocity is normal to its free surface we addressed two types of accelerating devices:

The first one (a layered system) involves aluminum impactor having a shape of truncated cone fixed on a wall of the excavated hole through a pad of substance with low dynamic rigidity P0c~ (polyethylene, polystyrene, etc.) and accelerated by a set of waves resulted from propagat ion of shock of nuclear explosion (Fig. l). The pad provides a smooth acceleration of the metallic impactor without formation of spalls. As computat ions showed, non-spall acceleration without the shock mitigating pad is impossible for impactors having thickness interesting from practical point of view.

2 ' , , , - !

I

I i

4 ".h 7 , r , - ; , 2 / '

L . . . . . . . . . . .

Fig. 1 The first type of accelerating device for experimental simulation of direct impact

The impactor shape is chosen proceeding fiom minimum weight of the experimental unit. Value of angle between plane surface of the impactor and its lateral conic surface

Accelerating devices using powerful explosions 65

corresponds to the angle of lateral rarefaction for aluminum and ensures propagation of the first shock through the impactor without disturbing influence of the lateral rarefaction.

Advantage of this type of accelerating device is capability to predict the impactor condition at the moment of the impact and, hence, possibility to use the obtained experimental data for calibration of computations including those using elastic-plastic models. Scale of flows formed during a nuclear explosion enables to place in the excavated holes the impactors of larger dimensions than those used in laboratory explosive experiments. This enables to simulate impact with an infinite semi-space more adequately.

To reduce cost of the experiment and to speed up the preparatory work we also considered a simplified accelerating device in which a concrete layer of a prescribed thickness served as the shock mitigating pad.

The second type of accelerating device involves a ground surface of the excavated hole which is polished or made even with a grout and covered with relatively thin layer of aluminum accelerated in the process of rarefaction wave propagation from the free surface after propagation through it of the shock from the nuclear explosion (Fig.2). The cover is needed to exclude formation of micro-jets which may appear at micro-irregularities of the surface, heterogeneity of the ground and the concrete layer, etc..

I' t • /

i ;° ~ . . . . . . . . . . . . . . . . . P ~ t ~ 5 / , °

\i A /

k

i i °

Fig.2 The second type o~ accelerating device for experimental simulation of direct impact

Advantages of this type of accelerating devices involve comparative simplicity of manufacture and great adequacy of experimental simulation to conditions of impact with an infinite semi-space. Note that unlike the first type of accelerating device the impacting matter subject to powerful shock is not strong. The impact is thereby simulated in hydrodynamic mode. However, taking into account the high impact velocities this approximation is justified in some cases.

The both types of accelerating devices have been worked out successfully. The impactor velocities made up from 2.5 up to 5 km/s. During the development the following parameters were verified experimentally:

- parameters of shock propagating in the ground directly before it entered the accelerating device;

- parameters of the first shock propagating through the impactor; - velocity of the impactor flight; - parameters of shock excited in a plane metallic receiver after impact with the

impactor. It was shown that the impact conditions implemented in the experiments are close to

those of impact with a freely flying impactor. It turned out that using this method it is

66 E.N. AVRORIN e t al.

also possible to ensure a good planeness of the impactor surface. In particular, in the devices of the first type at the plate thickness about 20 cm lag of separate parts of the impactor free surface didn't exceed several mm at diameter 60 cm.

ACCELERATING DEVICES FOR SIMULATION OF OBLIQUE IMPACT

The most interesting and difficult for experimentation is simulation of an oblique impact at which vector of the impactor particle velocity is oriented at some angle to its free surface. A specific problem posed by us was to obtain in the experiment the flow in which the particle velocity vector fi at absolute value u23...5 km/s is oriented at an angle to the boundary of scattering ground significantly different from 90 ° .

At first glance we can attain turning of the particle velocity vector of the scattering ground with respect to its free boundary when the shock reaches the free surface at some angle or. Assess capabilities of such arrangement using the acoustic approximation. In this case we assume that when the shock reaches the free surface the component % of the particle velocity vector normal to initial position of the free boundary doubles whereas component ut which is parallel to the free surface doesn't change (fig.3).

Then angle 0 between the vector of the particle velocity of the scattering ground and its free surface is defined by a ratio of the particle velocity behind the shock front u to the shock velocity D and angle of shock surface inclination with respect to the ground free surface a:

tg0 = cos(arctg( 12 tgcz))

u sino~. (1 + 3cos 2 o r ) '/2 - sin(arctg( 1 tgot)) D 2

Fig. 3 shows the dependence 0(¢x) for one specific case D=6.7 kin/s, U=2.5 km/s (that corresponds to point on Hugoniot for shale at p=470 kbar). For values

0°<(z<50 ° the 0 deviations from 90 ° don't exceed 5 °. At the same time large values of et exceed the angle of lateral rarefaction that causes turning of the shock fiont, angles at which the shock reaches the fiee surface being less than ct that leads to scattering of the wall perpendicularly to its fiee surface. Values of the ratio D/u~2...3 are typical of majority of materials in the considered range of particle velocities behind the shock front, and dependencies 0(o 0 for other ground types and other values of pressure behind the shock front are similar. Thus, we haven't succeeded to obtain flows in which the angle between direction of the particle velocity and the free surface of the scattering ground differs significantly from 90 ° using just turning of the fiee boundary with respect to the shock front. The similar result was also obtained in 2D calculations.

As the calculations showed it is possible to attain the necessary flow parameters when the shock is incident on the free surface of wall having a special shape of "oblique" wall (Fig.4). The flow formed in this case is in some sense a modified shaped jet. In this case the wall shape was selected with calculations so that at the scattering velocity about 5 km/s the angle between the particle velocity vector and the ground free boundary in sufficiently large area of the flow was 60 ° .

Accelerating devices using powerful explosions 67

I

1 v ] , ,

'<7/'_-~ . . . . . . .

\,

Fig.3 Dependence 0(or) for one specific case D=6.7 kin/s, U=2.5 km/s.

q,O,~-t

~ o n c P s / B J J

. I p,p~ ,

~ g C7 * / '~2 , - r , U ' I

t _ _ ', J j i

Fig.4 The accelerating device [or experimental simulation of oblique impact

The accelerating device under consideration was successfully tested. At the impactor velocity ~4.5 km/s we experimentally registered almost fiat area of the free surface in which value of angle between the particle velocity angle and the ground boundary was .~ 60 ° .

68 E. N. AVROR1N et al.

DISCUSSION OF RESULTS

While running the impact experiments using tile above devices we are faced with several problems.

The first one is associated with influence of the explosion shock front sphericity on the impactor flight parameters. Use of powerful explosions for accelerating the impactors is of practical interest if dimensions of the impactors significantly exceed those which can be accelerated by other methods. Therefore, this problem can be addressed by increasing the explosion yield E since the distances at which the prescribed shock parameters are attained are proport ional to E ~/3. In this case as it was shown in calculations and verified by experiments at the explosion yield more than -10 kt the accelerating devices of practically interesting dimensions of_>l m appear to be at such distances from the explosion center that the front sphericity doesn't strongly influence parameters of the accelerating devices of all the three types.

The second problem arises if actual explosion yield in experiment differs flom the expected one. It turned out that the above devices remain workable over a rather large range of yield. So, for instance, parameters of flow formed at the "oblique" wall change little when the particle velocity in the shock changes within the diapason _+0.5 km/s (certainly, other than the "impactor" velocity itself).

To demonstrate capabilities of the method under consideration an experiment was pursued at R F N C - V N I I T F simulating impact of iron meteorite into ground. The meteorite was simulated with iron cylinder having diameter and height equal to 20 cm. The impact velocity made up 5 km/s. A system of pin switches registered parameters of a crater formed as a result of the impact.

In terms of tile impact physics the most interesting are experiments with impact velocities exceeding~10km/s at which the evaporation process is significant. In principle, such velocities are attainable using the accelerating devices similar to those described above. The maximum velocities in this case are limited by evaporation of the ground in the shock of the explosion (since it is necessary to provide conditions simulating free flight o f "co ld impactor"). Therefore, apparently, for the arrangement in the form of the ground wall the limiting velocities of the" impactor" flight will be a b o u t - 1 0 km/s. For metallic impactors using shock mitigating layers this velocity can be somewhat higher.

In the case of oblique impact into a barrier the largest interest is posed by the impact angles when, e.g., oblong craters are formed. If the impact occurs at such the velocities that it is of explosive nature, then these angles must be extremely small - much less than those attained in our accelerating device, i.e. 60 ° . Such impact angles in the devices of the "oblique" wall type are hardly feasible.

Nevertheless, to our mind, for many problems the accelerating devices described above can be of great interest. Currently due to absence of research underground explosions consideration of this method besides the historical interest has also a certain fundamental meaning. It shows basic capabilities of experimental study of high velocity collisions (velocities up to 5 km/s and more) of heavy bodies (100 kg and more). We hope that peaceful applications of nuclear explosions, in particular, for basic research will be estimated in the future at their true worth, and in international research projects the described method for simulating high velocity impacts will find its niche in addressing a large number of problems.