Impact experimentation on the NASA KC-135 Reduced-Gravity Aircraft was shown to be possible, practical, and of considerable potential use in examining the role of gravity on various impact phenomena. With a minimal facility, crater dimensional and growth-times were measured, and have demonstrated both agreement and disagreement with predictions. A larger facility with vacuum capability and a high-velocity gun would permit a much wider range of experimentation

Cintala, Mark J.

Hoerz, F.

See, T. H.

English

NASA Technical Reports Server

IMPACT CRATERING IN 
EXPERIMENTS ON THE NASA 
M.J. Cintala'. F. Hdrz'. and T.H. 
REDUCED-GRAVITY ENVIRONMENTS: EARLY 
KC-135 AIRCRAFT 
See2; 'Solar System Exploration Division, NASA Johnson 
Space Center; 2Lockheed EMSCO, Code C23; both in Houston, T X  77058 
As outlined a t  last year's workshop''2, a variety of reasons exist for  performing 
impact experimentation a t  reduced gravity-levels. The execution of such experiments, 
however, will require familiari ty with reduced-g environments, as well as with the 
idiosyncracies associated with the change in gravitational acceleration. Since there has 
been little low-g cratering experimentation in general3 and  no impact experimentation 
specifically performed in flight (although some ground-based work has been done4), a 
series of impact-cratering experiments has been initiated on the NASA KC-135 Reduced- 
Gravity Aircraft. The intent of this program is threefold: ( I )  to acquire experience in 
low-g experimentation, (2) to develop an  observational understanding of the changes in 
the cratering process a t  reduced g-levels, and (3) to collect scientifically useful data  in 
the process. This report describes the apparatus, the experimental environment on the 
KC-135, and  some early results regarding cratering in sand targets. 
The  Low-Velocitv Imuact Facility: The ideal impact facility would utilize a projectile 
accelerator capable of launching projectiles of a variety of types and sizes over a wide 
range of impact velocities. The  availability of funding for  this initial facility, however, 
required a more modest approach. The accelerator is a Sheridan 5mm pellet pistol, which 
has been modified to be mounted vertically and fired electronically. Impact velocities of 
-50-130 m/s (with 0.96g, cylindrical lead pellets) are  attainable by varying the 
pressurization in the gun's pump chamber. The impact chamber is a 51.8~51.8~45.8-cm 
aluminum-framework box, with tempered-glass walls to permit photography and viewing 
of experiments; its lid is hinged for  access to the interior. Since this facility does not yet 
support vacuum capability, all experiments to date have occured a t  aircraft-cabin 
pressure, typically 0.83 atmospheres. Data collection relies principally on photography 
(high-speed motion picture, still, and video cameras), while acceleration and atmospheric- 
pressure information is recorded digitally with a microcomputer. Projectile velocities are  
measured via interruption of infrared photodiodes separated by a fixed distance; an  
independent oscillator circuit is used for  timing. Experimentation during fl ight is 
invariably performed under somewhat hectic conditions due  to the compressed timeline 
necessitated by the aircraft 's high rate of fuel consumption; in  order to maintain 
efficiency a t  a level as high as possible, the computer is also employed as a n  event 
sequencer -- operating the cameras, f ir ing the gun, measuring and  recording projectile 
velocities, and  recording event times. 
Exuerimental Environment on the KC-135: Gravity levels ranging from -0.1 to -2 g's can 
be supported by the KC-135 for  tens of seconds; the fl ight profile is selected by the 
investigators in concert with the flight crew. Depending on a number of factors related 
to the overall experimental program on a given flight, up  to 50 parabolas can be 
accommodated on a typical 2.5-hour flight. Moderate- to high-frequency vibrations of 
noticeable amplitude a re  minimal during a typical parabolic maneuver, although low- 
frequency oscillations of varying amplitude around the targeted g-level are  not unusual 
(Fig. 1). While the pressurized cabin provides a comfortable, shirt-sleeve working 
environment, normally routine operations are  often made more diff icul t  due to the 
variable-g environment. Experimenters have been known to sleep well following a typical 
flight. 
Early Results: The early developmental flights have provided the opportunity to collect 
limited data  on crater dimensions and growth times as functions of g-level and impact 
velocity. A coarse-grained, polymineralic sand (1.57 g/cm3 and  32.5' angle of internal 
friction) was employed as the target. Comprised predominantly of quartz  and feldspar 
(see Table 1 for  its grain-size distribution), the sand filled the chamber to a depth of 15 
A-18 
C -  2- 
https://ntrs.nasa.gov/search.jsp?R=19890005637 2020-03-20T05:05:35+00:00Z
crn; comparison with earlier findings4 leads to the conclusion that the volume of target 
material was sufficient to obviate noticeable edge-effects during formation of the largest 
craters a t  lg. No anomalous phenomena that might have been related to the f ini te  target- 
depth were observed during flight, but this parameter remains to be examined in more 
detail before serious experimentation is conducted a t  the lower g-levels, especially when 
higher projectile energies a re  employed. A total of 27 shots have been performed to date, 
covering a range of 0.082 to 0.534 g's. Crater dimensions were obtained from 35mm 
photography af te r  completion of each experiment; crater growth-times were derived 
through analysis of ejecta-plume shapes from 16mm motion-picture photography a t  250 
frames/s, yielding a time resolution of 0.04 s. Due to limitations in attainable impact 
velocities, a variation of only a factor of 6 in impact energy was possible. For 
comparison, experiments with the JSC Vertical Impact Facility can be performed with 
impact energies covering almost 4 orders of magnitude. The limited 
range of projectile velocities resulted in craters with rim-crest diameters varying over an  
interval of only 11 to 18 cm. This limited size range nevertheless is sufficient to 
demonstrate well-defined gravity and  velocity (or energy) dependences (Fig. 2). The 
diameters of craters formed at  two fixed energy levels (and velocities, since the projectile 
mass was constant for  each shot) are presented for  a range of g-levels. Not only does this 
f igure illustrate the inverse relationship between crater diameter and  gravitational 
acceleration, but the relatively unscattered data attest to the suitability of the KC-135 as 
a scientific test bed: the precision of these data is comparable to those obtained in 
ground-based laboratories. The slopes of the two least-squares f i ts  -- which are 
statistically indistinguishable owing to the small number of data points -- are  very near 
the value of -0.165 obtained in ground-based  experiment^.^ The 
times required for  crater growth were obtained by counting the number of 16-mm frames 
from the time of impact to the time that the profile of the ejecta plume near the target 
surface changed from a continuous, concave-outward curve to a discontinuous 
intersection between the expanding curtain and the target surface. Data for  the same 14 
craters are  illustrated in Figure 3 and, once again, the small number of actual data points 
will not permit the assertion that the two slopes are  distinguishable. Confidence intervals 
notwithstanding, both f i ts  are  substantially different  f rom that obtained in the drop- 
platform  experiment^,^ which is included in the figure as the dashed line. The slopes of 
the f i ts  to the KC-135 data are  also outside the theoretical limits as determined through 
dimensional a n a l y ~ i s ; ~  the causes of these differences are not yet understood. The target 
sands, for  example, were different  in both sets of experiments represented in Fig. 3; i t  has 
been demonstrated that variations in bulk density and angle of internal friction can play 
a nontrivial role in  determining the outcome of a cratering event.6 The  drop-platform 
data4 were collected with impact velocities of 6.4 km/s, while those displayed here were 
a t  a maximum of 0.111 km/s. Finally, there is a small but undetermined effect  due to the 
atmospheric pressure differences between the two experiment series. On the other hand, 
the limits placed on the gravity exponent by the dimensional analysis are  -4/7 to -5/8, 
which is expected to hold "for all materials and impact  condition^."^ Clearly, more work 
will be required to determine whether these variations are  indeed real and, if so, to 
evaluate their possible causes. 
Summarv: Impact experimentation on the NASA KC-1 35 Reduced-Gravity Aircraft  has 
been shown to be possible, practical, and of considerable potential use in examining the 
role of gravity on various impact phenomena. With a minimal facility, crater dimensions 
and growth-times have been measured, and  have demonstrated both agreement and 
disagreement with predictions. A larger facility with vacuum capability and a high- 
velocity gun would permit a much wider range of experimentation. 
Crater Diameters: 
Crater Growth-Times: 
The authors would like to thank K. Caldwell, F. Cardenas, G .  Jolly, T.  Thompson, (all of LEMSCO, Houston), R. Williams 
(Aircraft Operations, NASA JSC), M. Fox, and R. Woyan (Kelsey-Seybold, NASA JSC) for their technical assistance and 
support during flight and in the altitude-chamber during this phase of the study. 
A-19 
References: 'M.J. Cintala (1986) SDace Station Planetolonv ExDeriments (SSPEX), NASA Conf. Pub. 2424, p.6-13. 
'R.A.F. Grieve (1986) Space Station Planetolow ExDeriments (SSPEX), NASA Conf. Pub. 2424, p.6-26. S.W. Johnson 
- al. (1969) J. Geophys. Res., 74, p.4838. 4D.E. Gault and J.A. Wedekind (1977) Impact and Explosion Cratering, D.J. 
Roddy, R.O. Pepin, and R.B. Merrill, eds., Pergamon Press, (New York), p.1231. 'R.M. Schmidt (1981) w, 62, p.944. 
%.M. Schmidt (1980) PLPSC 11, p.2099. 
3 
1 
>0.250-0.500 0.660 w 
>0.125-0.250 0.075 
- ~ 0 . 1 2 5  0.001 5 1.0 
> 
k 
Figure 1. An example of the acceleration history 0.5 
during an impact experiment on the KC-135. The g- 
level on the vertical axis was sampled 4 times per 
second; the time of gun firing and impact is indicated 
C3 
0.0 
Table 1. Grain-size Distribution 
of target sand 
Size (mm) Mass Fraction 
>0.500-1 .OOO 0.264 
L 
I 1 d . . t . . 1 . 
- DAY 2 ,  PARABOLA 13 
" " 1 " " 1 " " 1 ' " ' 1 ' " '  
KC- 135 REDUCED-GRAVITY AIRCRAFT 
n 
VI 
0, - 1.5 
'3i 0.2 
.- E 
.- 6 
Y 
a 
Figure 3. Crater formation-time as a function of g-level 
possesses the slope determined for high-velocity impacts 
for the same craters treated in Figure 2. The dashed line 
in the drop-platform experiments.' The slopes for the 
KC-135 data are beyond the limits suggested by 
dimensional a n a l y ~ i s , ~  but the reason for the disparity is 
unclear. 
t 
0 0.1 E 
ki 
El 
See the text for possible causes. 
t 
\ 
a 
- 
- 
- 
- 0 E = 3.50 xl07ergs 
a E= 5.92 x 107ergs - 
1 I 1 
- 
E Figure 2. Crater diameter as a function of g-level for 
two fixed impact energies. The lower energy impacts 
occurred at -83 m/s, while the others took place at -111 
m/s. These slopes can be compared with -0.165, the 
value obtained from drop-platform experiments.' Note 
c the modest scatter in the data, attesting to the stability 
of the KC-135 as a test bed. 0.5 e IO 0 0.05 0. I 
Gravity Level (gas) 
A-20 


Impact cratering in reduced-gravity environments: Early experiments on the NASA KC-135 aircraft

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