47th AIAA Aerospace Sciences Meeting Including The New Horizons Forum and Aerospace Exposition
5 - 8 January 2009, Orlando, Florida
AIAA 2009-1015
Lunar Surface Stabilization via Sintering or the use of Heat
Cured Polymers
Two potential methods for lunar surface stabilization are: 1) sintering the regolith into a
solid and 2) using heat or UV cured polymers. Sintering, a method in which powders are
fused into a solid, has been proposed as a way of building lunar launch pads, roads and other
building materials. Polymers are currently used by the military to stabilize sandy soils, and
adaptations of this technology may be effective for lunar surface stabilization. This paper
describes ongoing work at NASA Kennedy Space Center on these two technologies. A solar
concentrator has been built to provide the heat source for sintering. Various solvent free
polymers have been investigated. Results of physical testing, including load strength and
abrasion resistance, on field and laboratory prepared samples are presented.
Paul E. Hintze1
National Aeronautics and Space Administration, Kennedy Space Center, FL 32899
Jerry Curran2 and Teddy Back2
ASRC Aerospace, NASA Kennedy Space Center, FL 32899
I. Introduction
The vision for space exploration calls for NASA to return to the moon by 2020. The exploration mission
involves setting up a sustained human presence on the Moon, a broad mission that will require numerous disciplines
to create technologies, solve current known problems and anticipate new ones. One problem that has been identified
from the past Apollo missions is the issue of dust mitigation to protect people and infrastructure.1,2 There have been
numerous recent papers describing and cataloging dust problems during the Apollo missions that pertain to human
health and operations.2-6 Dust ejecta from a rocket plume can affect visibility during landing, erode nearby coated
surfaces and get into mechanical assemblies near the landing site. Videos taken during landing of later Apollo
missions, show regolith erosion during the landing process and astronauts have seen large amounts of regolith ejecta
during take off and landing (see videos at NASA image gallery, nix.nasa.gov). During the Apollo 12 landing,
visibility of the local topography was so obscured that there was concern that the lander could have touched down
on a boulder or crater.2 Dust erosion during landing can cause damage to nearby infrastructure, as shown by the
recovery of Surveyor 3 lander parts. The Apollo 12 lander landed 183 m away from the robot lander. There was
considerable dust accumulation on the craft and evidence of “sandblasting” and pitting, as a result of dust ejecta
during landing, on the returned tubing and optics.5,7
Dust transport has been caused by other human activities besides the launch and landing. There have been
reports of dust being kicked up by the rovers. When riding the Lunar Roving Vehicle (LRV) with a damaged
fender(s), dust was kicked up so badly that it immediately began to affect the space suits.2 Dust was also observed
around the ankles of the astronauts after walking. The dust is extremely persistent and adheres to all surfaces. Dust
caused some acute health issues for the astronauts. Cases of eye, nose and sinus irritation were reported during
several missions.4,5 The dust made it into the crew modules and caused problems with space suits and seals, and
other mechanisms. Zippers, connectors and helmets all experienced some degree of dust-caused malfunction.
This paper describes investigations of two methods for stabilizing the lunar regolith for dust mitigation: 1)
sintering the regolith into a solid and 2) using heat or UV cured polymers to stabilize the surface. Sintering has been
proposed as one way of building a Lunar launch/landing pad.8-13 A solar concentrator has been built and used in the
field to sinter JSC-1 Lunar simulant. Polymers are used by the military to build helicopter landing pads and roads in
sandy areas. The polymers are sprayed on a surface using a solvent. The use of solvents is not practical for lunar
use, but there are solvent free particles that would be practical. Commercially available, solvent free polymers are
being investigated to determine their viability to work in the same way as the terrestrial analog. Field and laboratory
testing involving these polymers is being performed.
1
2
Scientist, Corrosion Technology Laboratory, Mail Stop KT-E3, Kennedy Space Center, FL 32899
Engineer, Corrosion Technology Laboratory, Mail Stop ASRC-24, Kennedy Space Center, LF 32899
This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States.
Sintering is a method in which loose particles are heated, but not fully melted, until they bond together and form
a solid. Ceramics, from ancient clay pots to modern materials and composites, are made via the sintering or firing
process. Ceramic materials are traditionally made from local, natural products including silica and silicate materials,
with little or no pre-processing. Ceramics made from non-silicate materials, such as alumina or silicon carbide, are
often used as refractory materials or materials that keep their strength at high temperatures.
Most ceramic objects, except for some
glasses, are made by forming the fine ceramic
particles into a shape and performing a heat
treatment to cause the particles to adhere. The
sintering process proceeds as shown in Figure
1. In sintering, the particles are heated to
below the melting point, and solid diffusion
takes place between the particles, filling the
pore space and forming the bond.
Sintering is an ideal method for surface
stabilization because it uses in situ materials
and only requires a heat source. In addition,
the Lunar regolith has many properties that
lend themselves to processing to form
ceramics, and consequently, there have been
many ideas on how to use the lunar regolith as
structural ceramics. Lunar rocks are made up
Figure 1. Interparticle bond formation during the sintering
mostly of silicate minerals (>90% by volume).
process. Plate A shows the loose powder with initial
Common silicate minerals include pyroxene,
contacts. Plates B and C show the progression of the
plagioclase feldspar and olivine.
Oxide
interparticle bonds, grain boundary growth and pore
minerals such as ilmenite and spinel are also
shrinkage. Plat D shows the final product with minimal
found.6 These silicate minerals are some of
pore volume.
those traditionally used in the manufacture of
ceramics.14
The military currently uses polymers to stabilize sandy surfaces for helicopter pads and roads.15,16 The
technology is relatively simple and uses a water soluble polymer that is sprayed over the area to be stabilized. The
water evaporates leaving a durable polymer surface. Although polymers dispersed in a solvent are not practical for
use in this way on the Moon, there are many solvent free polymers that cure with the application of heat or
ultraviolet (UV) light. These polymers come in either solid or liquid form. We choose to investigate the solid
polymers to minimize any difficulty that might occur when spraying a liquid in a vacuum. The solid polymers come
as a powder, with particle sizes on the order of a micron. The powders can be spread on a surface in a number of
ways including an electrostatic spray. The polymers can also be mixed with lunar regolith to form a composite.
There are many commercially available solid polymer powders that could be used. They include organic and
inorganic polymers that are tailored with different desirable properties such as flexibility or temperature resistance.
II. Stabilization Methods
A. Sintering with a Solar Concentrator
A solar concentrator with a 1 m2 collection area has been constructed for field testing at KSC. The solar
concentrator consists of a large Fresnel lens mounted on a frame that allows the lens to move and follow the sun.
The focal point of the lens is pointed downward to allow for rastering across a surface. The highest measured
temperature generated by the solar concentrator has been 1350°C, higher than is necessary to melt JSC-1A lunar
simulant. Solar sintering is a promising technique since it gets its power from the sun (1380 W/m2), can be
lightweight, is inexpensive and is a relatively simple technology.
Initial experiments using the solar concentrator
have focused on evaluating how thick a surface can
be sintered and how best to sinter large areas. The
first tests involved simply focusing the light on a
bed of JSC-1. When this is done the top surface
quickly melts at the focal point. Within two to
three minutes, a combination of melting and
sintering occurs to a depth of about 6 mm.
Continued heating after this time does not increase
the thickness of the sintered area at the same rate.
The focal point of the solar concentrator can be
rastered back and forth over the surface of a bed of
JSC-1. At the focal point, JSC-1 quickly melts, but
the thickness of this melted product is only 1 or 2
mm. In addition, the density of JSC-1 decreases on
melting and the melted area seems to contract on
itself. This results in a weak bond in between the
melted areas formed on successive passes.
At present, using a solar concentrator as a heat
source for sintering must be considered promising
because it is capable of achieving high
Figure 2. The 1 m2 solar concentrator built at NASA
temperatures in a short time, without any electrical
KSC.
power. Two main problems that require future
work have been identified: 1) a solar concentrator
consisting of a single lens must move to follow the sun while keeping the focal point at the desired area and 2) it is
difficult to heat to great depths or wide areas. To address the first problem, a solar concentrator that has the
collector and applicator decoupled from each other could be used. Greater depth of sintering could be achieved by
sintering the surface layer by layer, or continuously adding regolith on top of a heated area. This has been
successfully performed and solid forms greater than 1 in3 have been made. Sintering wider areas would be facilitated
by better temperature control. There is a large temperature gradient between the melted area and the surrounding
areas when the simulant melts. These temperature gradients cause the cracking between passes of the solar
concentrator. Keeping each pass of the solar concentrator at the same temperature would ensure that the sintered
product produced on each pass was the same.
The time needed to sinter a pad to a given depth can be calculated. Table 1 shows rough estimates of the time
needed to sinter a 100 m2 pad to a depth of 2.5 cm using a 1 m2 solar concentrator. The energy, Q, needed to sinter a
pad can be calculated with the equation Q  mcT , where m is the mass, c is the specific heat (800 J/(kg °C) for
basalts) and ΔT is the change in temperature. The mass of regolith to be sintered was calculated using the 2.5 m3
volume of a 100 m2 pad sintered to 2.5 cm depth, and a regolith density of 1.5 g/cm36. The temperature change
needed for sintering was assumed to be 1000 C for this calculation. The energy needed to perform this much
heating is calculated to be 3 x 109 J. The wattage collected with the solar concentrator, 1380 W, can then be used to
calculate the time needed to apply this much energy. This leads to a time of 27 days to sinter this area assuming
100% efficiency. The efficiency would decrease from heat loss of the regolith, inefficiency in the collector and nonideal conversion of the solar power into heat. The 67% efficiency level shown in Table 1 for the solar concentrator
takes into account loss in reflectance17 in the solar concentrator and a worst case albedo, of 24%,6 used to
approximate the loss during conversion of solar power to heat. The efficiencies do not take into account any losses
from coupling the heat source to the regolith, nor do they account for heating any more or less than the first 2.5 cm
of regolith. These times are upper bounds, as they represent a 100% efficient transfer of the power from the heating
method into heat in the regolith.
Table 1. Time needed to sinter a 100 m2 launch pad to a depth of 2.5 cm.
Efficiency
Time in Days
Heating method
Solar Concentrator (1 m2
area)
100%
27
67%
40
B. Heat and UV cured polymers
We are investigating the use of UV or heat cured polymers for surface stabilization. The heat cured polymers
are powders, that when heated melt together and cure. We have been investigating products that are commercially
available powder coatings used in various industries including high temperature applications. The powders do not
contain or require a solvent, and can be applied by an electrostatic spray or by any other method that distributes it
over a surface. Abrasion testing is being performed on the powder coatings by themselves and in various mixes
with JSC-1 ranging from 10 – 50% powder by weight.
A few demonstrations showing that the polymer can be cured with the solar concentrator have been performed.
Both 33% and 50% polymer:JSC-1 mixes have been cured with the solar concentrator. The cure temperature for
these polymers are 200°C for 10 minutes. This temperature was achieved by keeping the sample above the focal
point of the solar concentrator and monitoring the temperature. A small area about 6 cm in diameter and 0.5 cm
deep was solidified in this way. This demonstration shows the ease with which the polymers can be used to form a
solid surface.
The drawback of using polymers is the mass. Testing is currently underway to identify the amount of polymer
needed to cover an area, so that accurate masses can be calculated. Spread rates of up to 31 kg for a 100 m2 area
have been evaluated. This spread rate corresponds to a thickness of about 200 μm of polymer. Typically, the
polymer is mixed with an amount of lunar simulant to form a composite.
The three commercially available polymers we have tested have been evaluated for curing in a vacuum. Initial
tests have shown that the polymers flow and form a film under vacuum. However, the film was found to be more
brittle and seemed to take longer to flow than when the same experiment was performed under ambient conditions.
It is unknown why the vacuum affects the curing process, but it is possible that the commercial products contain a
small amount of a flowing aid that is not effective under vacuum. A polymer without any additives is being
formulated in our labs to evaluate the effects of additives on the flow and cure properties of polymers under vacuum.
III. Physical Testing
C. Load Bearing tests
The strength of the surface treatment for a given thickness is a key parameter for the system as it will determine
how long sintering will take or what mass of polymer will be needed. A load bearing strength test was conducted to
compare the different treatment methods. The load system, Figure 3, consisted of a 6 inch diameter dish filled with
JSC-1A Lunar simulant. The surface treatment was put in the center 3 inch diameter area of the dish. Force was
applied using a ¾ inch diameter piston. In the case of sintered test specimens, the specimens were sintered in a
crucible and then placed on the bed of simulant. The polymers were applied and cured directly on the simulant.
Both sintered and polymer samples
failed in tension with the sample
cracking around the piston and the
cracks spreading radially from this
circle as shown in Figure 3.
Figure 4 shows the loads at failure
for
some
different
polymer
applications and thicknesses of
sintered simulant. The load at failure
of uncompressed JSC-1A was found
to be 20 psi. The results shown here
are for one polymer applied with
different spread rates ranging from
0.08 to 0.31 kg/m2. All polymers had
strengths ranging from 20 – 80 psi.
The polymers were applied with two
methods. The first method, A, was
spreading the polymer over the
surface with no other treatment.
Method B consisted of spreading the
Figure 3. Picture of the load bearing strength system including 6
polymer over the surface, working the
inch diameter dish filled with JSC-1A Lunar simulant, 3 inch
polymer into the top few millimeters
diameter sintered area and 0.75 inch diameter piston
of the surface, followed by gently
compressing the disturbed surface. Changing from Method A to Method B doubled the bearing strength for the
polymer spread at a 0.16 kg/m2 rate, and it is expected that similar improvement would be found for the higher
spreading rates. The thickness of the polymer spread at 0.31 kg/m2 would be about 200 μm if applied to a flat
surface, although it is thicker in practice since it is mixed with simulant. This is considerably smaller than the
thinnest sintered sample.
Sintered samples ranging in thickness from 2.5 to 6 mm were tested and found to have strengths between 130
and 310 psi. Because gravity on the moon is 1/6 that of earth, structural dead loads will be reduced by 5/6 and
components should have 6 times the load bearing capacity of conventional materials on Earth.18,19 Using a 1/6
reduction in thickness, the moons highways could be 2.6 to 4.3 cm, as compared to the 15 to 25 cm used on Earth.
The sintered test samples are considerably thinner than that value, however, the bearing strengths of these samples
are in excess of the Apollo lunar module bearing pressure of 4.6 kPA (0.67 psi) meaning they would support the
dead weight of the landers. This does not suggest that they would withstand the force from the ignition of the rocket
or the forces that might occur if a footpad hit at an angle and did not properly distribute its weight.
600
Load at Failure / psi
500
400
300
200
100
0
JSC-1
0.08
kg/m2
polymer
0.16
0.16
kg/m2
kg/m2
polymer
polymer
method A method B
0.31
kg/m2
polymer
2.5 mm
thick
sintered
4.2 mm
thick
sintered
6.0 mm
thick
sintered
Figure 4. Load bearing strength of polymers, in blue, and sintered surface treatments, in
green, as compared to JSC-1A by itself.
D. Abrasion Resistance
Abrasive blast resistance testing was performed on polymer and lab and solar sintered samples. The abrasive
test consistent of a blasting gun that sprayed 500g of silicon carbide blast media at the sample. The sample was
masked to a 1.5 inch diameter area. The samples were weighed before and after blasting to determine weight loss.
Pure polymers, polymer composites consisting of 30 or 50% polymer mixed with JSC-1A simulant, lab sintered and
solar sintered samples were tested. The results are summarized in Figure 5.
Two polymer systems are shown in the results. Polymer A is a polyester and polymer B a silicone. The polyester
had significantly better abrasion resistance when mixed with lunar simulant, as did all other organic polymers tested.
The silicone had better abrasion resistance than the organics when pure, and did not show significant improvement
when mixed with simulant.
Lab sintered and solar sintered samples were tested. Solar sintered samples were prepared by starting with a thin
lab sintered sample followed by sintering additional layers of simulant on top using the solar concentrator. The
thickness of the solar sintered portions of the sample ranged from 2 – 3 millimeter. The solar sintered samples did
as well as the lab sintered samples. This is encouraging in that the solar sintered samples appear to have a glassy
phase on their surface and it appears that this glassy phase is as strong or stronger than the sintered areas.
45%
40%
Weight loss, %
35%
30%
25%
20%
15%
10%
5%
0%
100%
50%
Polymer A
30%
100%
50%
30%
JSC-1A JSC-1A
Lab
Solar
Sintered Sintered
Polymer B
Figure 5. Weight loss measurements as a result of abrasion testing.
E. Modulus of rupture
Modulus of rupture was measured with a three point bend apparatus. The polymer samples 10 mm wide, 2 mm
thick and the span was 30 mm long. The strain rate was 0.1 mm/sec. The 2 mm thickness of the sample
corresponded to the approximate thickness of the polymer samples in the load bearing strength tests. Sintered JSC1A samples were 5 mm thick, but otherwise had the same dimensions. Modulus of rupture was also performed on 5
mm thick polymer samples for a direct comparison to the sintered samples.
The polymers showed difference in modulus of rupture depending on the amount of simulant loading, but the
change was dependent on the polymer. Polymer A lost strength when the amount of polymer was decreased from
50 to 30%, while polymer B had the opposite behavior. The 5 mm thick polymer samples are considerably stronger
than the 5 mm thick sintered sample, but this amount of polymer would have a considerable mass if it were to be
used over large areas.
Modulus of Rupture, kgf/cm2
700000
Polymer A
600000
500000
400000
300000
200000
Polymer A
Polymer B
100000
0
30%
50%
30%
50%
JSC-1A
2 mm thick
30%
50%
5 mm thick
Figure 6. Modulus of rupture results.
IV. Conclusion
Lunar surface stabilization for dust mitigation is an important aspect in developing the infrastructure for a
permanent lunar outpost. There are many different concepts for achieving this goal, each with advantages and
disadvantages. In this paper, we have presented some physical and structural properties of two potential solutions,
sintering with a solar concentrator and the use of polymers. The physical and strength requirements must come out
of the lunar architecture and include such parameters as the forces applied during launch and landing. The
properties of the stabilized surface produced by different methods must be used in conjunction with other
requirements such as mass to orbit, power and cost to determine viability. The initial experiments presented here
suggest that solar sintering would be promising for the preparation of large areas and polymers would be more
desirable in small areas where ease of application concerns would outweigh the mass.
References
1
Cooke, D.,"Exploration Strategy and Architecture" in 2nd Space Exploration Conference 2006.
2
Gaier, J. R.,"The Effects of Lunar Dust on EVA Systems During the Apollo Missions",NASA/TM-2005213610/REV1,2007.
3
Khan-Mayberry, N.,"The Lunar Environment: Determining the Health Effects of Exposure to Moon Dusts" in 16th
IAA Humans in Space Symposium Beijing, China, 2007.
4
Noble, S.,"Assessing the Dangers of Moon Dust" in 2nd National Conference on USGS Health Related Research
Reston, VA, 2007.
5
Wagner, S. A.,"The Apollo Experience Lessons Learned for Constellation Lunar Dust Management",NASA/TP2006-213726,2006.
6
Heiken, G. H., Vaniman, D. T. & French, B. M. (eds.) Lunar Sourcebook A User's Guide to the Moon Caimbridge
University Press, 1991.
7
Carroll, W. F., Blair Jr, P. M., Hawthorne, E. I., Jacobs, S. & Leger, L., "Returned Surveyor 3 Hardware:
Engineering Results". NASA-SP-284 Analysis of Surveyor 3 material and photographs returned by Apollo
12, 1972 15-21.
8
Allen, C. C., Graf, J. C. & McKay, D. S., in Engineering, Construction and Operations in Space IV 1220-1229
(1994).
9
Meek, T. T., Vaniman, D. T., Cocks, F. H. & Wright, R. A.,"Microwave Processing of lunar materials: potential
applications" in Lunar bases and space activities of the 21st century (ed. Mendell, W. W.) Washington,
DC, 1985.
10
Taylor, L. A. & Meek, T. T., "Microwave Sintering of Lunar Soil: Properties, Theory, and Practice". Journal of
Aerospace Engineering 18, 2005 188-196.
11
Ishikawa, Y., Sasaki, T. & Higasayama, T.,"Simple and Efficient Methods to Produce Construction Materials for
Lunar adn Mars Bases" in Space 92 1992.
12
Allen, C. C.,"Bricks and Ceramics" in Workshop on Using in situ Resources for construction of Planetary
Outposts (ed. Duke, M. B.) Lunar and Planetary Institute, Albuquerque, NM, 1998.
13
Allen, C. C., Hines, J. A., McKay, D. S. & Morris, R. V., in Engineering, Construction and operation in space III
1209-1218 (1992).
14
Smith, W. F., Principles of Materials Science and Engineering (McGraw-Hill, New York, 1990).
15
Rushing, J.,"Development of a Dust Mitigation System for the U.S. Marine Corps" in Lunar Regolith Community
of Practice Webinar Series, http://tia.arc.nasa.gov/lunrcop/ 2008.
16
Environmental Products and Applications, I.,"in http://www.rhinosnot.com/.
17
Fend, T. et al., "Comparative assessment of solar concentrator materials". Solar Energy 74, 2003 149-155.
18
Ruess, F., Schaenzlin & Benaroya, H., "Structural Design of a Lunar Habitat". Journal of Aerospace Engineering
19, 2006 133-157.
19
Advanced Automation for Space Missions" in 1980 NASA/ASEE Summer Study (eds. Freitas Jr., R. A. &
Gilbreath, W. P.) 1980.