The National Aeronautics and Space Administration (NASA) is progressing toward long-term lunar habitation. Critical to the design of a lunar habitat is an understanding of the lunar surface environment; of specific importance is the primary meteoroid and subsequent ejecta environment. The document, NASA SP-8013, was developed for the Apollo program and is the latest definition of the ejecta environment. There is concern that NASA SP-8013 may over-estimate the lunar ejecta environment. NASA's Meteoroid Environment Office (MEO) has initiated several tasks to improve the accuracy of our understanding of the lunar surface ejecta environment. This paper reports the results of experiments on projectile impact into powered pumice and unconsolidated JSC-1A Lunar Mare Regolith stimulant (JSC-1A) targets. The Ames Vertical Gun Range (AVGR) was used to accelerate projectiles to velocities in excess of 5 km/s and impact the targets at normal incidence. The ejected particles were detected by thin aluminum foil targets placed around the impact site and angular distributions were determined for ejecta. Comparison of ejecta angular distribution with previous works will be presented. A simplistic technique to characterize the ejected particles was formulated and improvements to this technique will be discussed for implementation in future tests

Cooke, William

Edwards, David L.

Moser, Danielle E.

Scruggs, Rob

NASA Technical Reports Server

[ .............
Comparison of Ejecta Distributions from Normal Incident
Hypervelocity Impact on Lunar Regolith Simnlant
David L. Edwards a, William Cooke a, Rob Suggs a, Danielle E. Moser b,
a M/S EV44 MSFC, AL 35812 USA
b Stanley/MSFC, AL 35812 USA
ABSTRACT
The National Aeronautics and Space Administration (NASA) is progressing
toward long-term lunar habitation. Critical to the design of a lunar habitat is an
understanding of the lunar surface environment; of specific importance is the
primary meteoroid and subsequent ejecta environment. The document, NASA
SP-8013, was developed for the Apollo program and is the latest definition of the
ejecta environment. There is concern that NASA SP-8013 may over-estimate the
lunar ejecta environment. NASA's Meteoroid Environment Office (MEO) has
initiated several tasks to improve the accuracy of our understanding of the lunar
surface ejecta environment.
This paper reports the results of experiments on projectile impact into powered
pumice and unconsolidated JSC-1A Lunar Mare Regolith stimulant (JSC-1A)
targets. The Ames Vertical Gun Range (AVGR) was used to accelerate
projectiles to velocities in excess of 5 km/s and impact the targets at normal
incidence. The ejected particles were detected by thin aluminum foil targets
placed around the impact site and angular distributions were determined for
ejecta. Comparison of ejecta angular distribution with previous works will be
presented. A simplistic technique to characterize the ejected particles was
formulated and improvements to this technique will be discussed for
implementation in future tests.
Key Words," lunar, regoligh, meteoroid impact, ejecta distribution, impact testing
1. INTRODUCTION
The series of tests discussed in this paper grew out of two focus areas, related to
exploration of the lunar surface. The primary goal of this series of tests was to
calibrate ground-based cameras utilized to observe and record meteoroid impacts
on the lunar surface. The other focus was the need to increase our understanding
of the ejccta environment on the lunar surface. NASA's Meteoroid Environment
Office (MEO) offered the opportunity to gather ejecta distribution data during a
series of test shots using the Ames Vertical Gun Range (AVGR).
https://ntrs.nasa.gov/search.jsp?R=20080014845 2019-08-30T04:17:01+00:00Z
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The AVGR is a 0.30 caliber light gas gun that can launch projectiles to velocities
ranging from 0.5 km/s to nearly 7 km/s. A very unique feature of the AVGR is
the ability to vary the gun's angle of elevation with respect to the target.
The angle of elevation of the gun can be varied in 15-degree increments from 0 to
90 degrees thus permitting oblique angles of impact. Impact events can be
recorded with a variety of high-speed imaging options.
NASA SP-8013 describes the ejecta environment subsequent to meteoroid
impact _. The graph shown in Figure 1 indicates approximately a 4 order of
magnitude increase in the number of ejecta particles of mass, m, from the baseline
primary meteoroid impact flux of mass, m, as defined by the GrQn model 3. The
design of a long-term habitation structure to survive the ejecta environment
described in NASA SP-8013 would require excessive mass, making it difficult
and potentially cost prohibitive to launch and deliver the structure to the lunar
surface. A better understanding of the hmar ejecta environment is required to
optimize the lunar habitat design.
'1o s
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Figure 1. Estimation of the lunar ejeeta flux e_mpared to the prediction of primary impact flux
from the Grfln model.
2, EXPERIMENT
The purpose of this series of experiments was to collect information enabling the
characterization of the cjecta angular distribution resulting from a hypervelocity
impact into simulated regolith. These ejecta characterization experiments were
secondary experiments in the AVGR, and as such were dependent upon the test
parameters required by the primary experiment: the calibration of video cameras
from the primary impact flash. Therefore one of the constraints of the ejecta
characterization experiment was that the ejecta experiment could not influence,
bias, perturb, or otherwise contaminate the calibration of the video camera.
The experiment set-up, shown in Figure 2, consisted of placing sheets of
aluminum foil in two specific locations, identified as position "D" and position
'B" around the periphery of the AVGR vacuum chamber. The 0.17 mm thick
foils were positioned such that the ejecta would impact the foils at near-normal
incident angles. In each case, the projectile was fired vertically (90 °) into each
powered regulith simulant The projectile in each case was 6.35 mm diameter,
0.29 g Pyrex sphere. The incident projectile velocities ranged from 2.5 km/s to
5.18 km/s.
AI Foil Pos_ion B
1.2 m
,20._lmpac t Window
l! Site
AI Foil AccessDoor
Figure 2. Top view schematic of the, test chamber specifically indicatlng the positions of the
aluminum foil detectors D and B.
3.0RESULTS
As shown in figure 3, the foil targets were divided into 1° wide horizontal bands
after the impact test was performed and the number of penetrations in each band
was counted. The 1° band was approximately 1 inch ( 2.54 cm). The number of
penetrations was documented as penetrations / unit area. The unit area is the area
defined by the 1° band and 12 inches (30.48 em), which is approximately 12 in2
( 77.42 cm2).
t-------- 12 in --------_
1°
Figure 3. Schematic showing how the foils were divided into 1° bands, 12 inches
wide.
3
V Pl t
The JSCI-a targets produced more ejecta penetrations than the pumice target
given equivalent incident projectile mass, composition, and velocity. The impact
test using powdered pumice performed at 2.5 km/s did not produce ejecta that
penetrated the foil detectors. The impact test using JSCI-a produced a significant
number of ejecta penetrations. Projectile impact velocities of 3.78 and 5.18 km/s
produced primary ejecta that penetrated the foil targets. The results from counting
penetration in foils are shown in Figures 3 and 4 and indicate a preferred ejecta
angle ranging between 30°to 43 ° with respect to the horizontal.
I
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L
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5 ..........
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3 .....
2 .....
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60
Figure 3. Angular distributions of pumice ejecta with sufficient velocity to penetrate the aluminum
foi] detectors.
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J
100
80
60
40
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le, m
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I IL
I"
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Degrees from Horizontal
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Figure 4. Angular distributions of JSC l-a ejecta with sufficient velocity to penetrate the aluminum
foil detectors.
4. SUMMARY AND DISCUSSION
The Ames Vertical Gun Range (AVGR) was used to accelerate spherical Pyrex
projectiles of 0.29 g to velocities ranging between 2.5 km/s and 5.18 km/s. Impact
on the powered pumice target occurred at normal incidence.
The ejected particles were detected by thin aluminum foil targets placed around
the powered targets in a vacuum chamber maintained at a vacuum level of 0.5
Tort during the impact test. The pumice was fine grain, 60 _tm diameter particles
and the JSC l-a was a larger grain powered target with average grain size of
. The results presented in this paper indicate that a peak ejection angle for
penetrating ejecta is approximately 38 ° offthe horizontal for pumice and 41 °
to 43 ° for the JSC 1-a. Previous work by Yamamoto resulted in a peak ejection
angle of approximately 30° (Yamamoto, 2002, p.92). Yamamoto et al used a
"staple-shaped" copper projectile with impact velocities ranging from 243 m/s to
272 m/s and impacted a target consisting of soda-lime particles with a nominal
diameter of 220 ram. Cintala et al. performed a series of impact tests using
spherical aluminum particles accelerated to velocities ranging from 0.8 to 1.92
krrt/s (Cintala, 1999). The incident projectiles had a nominal diameter of 4.76 mm
and impacted coarse-grained sand with grain sizes ranging from 1-3 mm.
5
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Cintala provides extensive detail for characterizing the ejecta angular and size
distributions and recorded ejecta angles ranging between 38° and 55 °. Cintala,
Yamamoto, and this work used varying techniques to determine ejecta
distributions, with Cintala and Yamamoto also providing ejecta velocities.
This experiment made no attempt to measure ejection velocities. Speculation on
anticipated ejecta velocities was aided by referencing Yamamoto's work, which
states "In the case of the vertical impact of the projectile, most ejecta have
velocities lower than 24% of the projectile speed."(Yamamoto, 2002, p.87) If
Yamamoto's 24% prediction can serve as a guide for the higher velocities in this
experiment, then using 24% of the incident projectile speeds of 2.5, 3.78, and
5.18 km!s provides upper threshold ejecta speeds of approximately 600 m/s,
907 m/s, and 1243 m/s, respectively. Using the penetration equation given in
NASA SP-8013 (page 7) describing threshold penetrations of"single thin ductile
metal plates", we get a penetration threshold velocity for the aluminum foil of
approximately 988 m/s. This equation is:
t = K1 p i/6 m0352 v O.87s (1)
where t is the thickness of the foil penetrated, K1 is a constant, p is the mass
density of the ejeeta ( 1.3 grn/cm 3 for pumice), m is the mass of the ejecta particle,
and V is the ejecta velocity. This calculation assumes all ejeeta particles are 60
_tm diameter pumice. IfYamamoto's 24% prediction holds true for this body of
work, then there is an explanation of why the impact in pumice at 2.5 km/s did not
produce ejecta that penetrated the aluminum foil and why the 3.78 and 5.18 km/s
impacts did produce foil penetrating ejecta. This explanation needs to be
confirmed by a future experiment.
The lunar meteoroid ejecta environment needs to be better characterized. This
work, integrated with other activities being initiated by NASA's Constellation
program, can lead to a more accurate understanding of the ejecta environment. To
build on this existing database, additional tests at the AVGR are scheduled. These
tests will include varying the regolith target to begin to understand the differences
in mare and highland regolith by using foil detectors of various thicknesses to
gain information on ejeeta velocity distributions. Detectors will also be placed at
various distances from the impact site to better characterize the ejeeta dispersions.
REFERENCES
I. Dine J., ed. http:/Avww.nasa, aov/,_enters/anms/research/tech_ology.onepa_ers/t.an_e.
_x.htrnl cited 5 May 2007 (2007)
2. NASA SP-8013 Meteoroid Environment Model - 1969 tNear Earth to Lunar
Surface], March 1969.
3. Cooke, W. Unpublished presentation of lunar Impact ejeeta analysis, 2007
Cintala M.J., Berthoud, L., H6rz, F., "Ejection-velocity distributions from impacts into coarse-
grained sand", Meteoritics & Planetary Science 34, 605-623 (1999).
Yamamoto, S. "Measurement of Impact Ejecta from Regolith Targets in Oblique Impacts", Icarus
158, 87-97 _2002)
6







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Comparison of Ejecta Distributions from Normal Incident Hypervelocity Impact on Lunar Regolith Simulant

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