Susan M. Motil, Angela R. Harrivel, and Gregory A. Zimmerli

Glenn Research Center, Cleveland, Ohio

Testing Microgravity Flight Hardware

Concepts on the NASA KC-135

NASA/TM—2001-210704

March 2001

AIAA–2001–0770
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National Aeronautics and

Space Administration

Glenn Research Center

Prepared for the

39th Aerospace Sciences Meeting and Exhibit

sponsored by the American Institute of Aeronautics and Astronautics

Reno, Nevada, January 8–11, 2001

Susan M. Motil, Angela R. Harrivel, and Gregory A. Zimmerli

Glenn Research Center, Cleveland, Ohio

Testing Microgravity Flight Hardware

Concepts on the NASA KC-135

NASA/TM—2001-210704 AIAA–2001–0770
Acknowledgments

We gratefully acknowledge the assistance of E. Hovenac, D. Minjares, and R. Skupinski for their effort in

designing and developing the KC-135 aircraft test rig. This work was supported by NASA

through the Microgravity Science Division.

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ideas from a technical peer group.

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analysis proceeds.

Available electronically at http://gltrs.grc.nasa.gov/GLTRS
NASA/TM—2001-210704 1 

TESTING MICROGRAVITY FLIGHT HARDWARE CONCEPTS ON THE NASA KC-135 

Susan M. Motil, Angela R. Harrivel, and Gregory A. Zimmerli 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Abstract

This paper provides an overview of utilizing the NASA 

KC-135 Reduced Gravity Aircraft for the Foam Optics 

and Mechanics (FOAM) microgravity flight project. 

The FOAM science requirements are summarized, and 

the KC-135 test-rig used to test hardware concepts 

designed to meet the requirements are described. 

Preliminary results regarding foam dispensing, 

foam/surface slip tests, and dynamic light scattering 

data are discussed in support of the flight hardware 

development for the FOAM experiment. 

Introduction

The Principal Investigator of the FOAM flight 

experiment is Professor Douglas Durian from the 

University of California, Los Angeles. The experiment 

is planned to be conducted on board the International 

Space Station in 2006. The objective of the FOAM 

experiment is to relate microscopic information about 

foam structure and dynamics to the macroscopic 

properties such as foam rheology and stability. 

Because primary interest is in wet foams, long duration 

microgravity conditions are needed so that the primary 

science goals can be achieved. Fulfilling the FOAM 

science objectives requires a general understanding of 

the effects of a low gravity environment on the behavior 

of foam and hardware functionality. Utilizing reduced 

gravity facilities, such as the NASA KC-135, can 

enhance the development process of space flight 

hardware.1

 Hardware that performs flawlessly in a 1g 

environment will often fail in a low gravity environment 

(g = 9.8 m/s2

 is the acceleration due to Earth’s gravity). 

A series of low gravity tests were conducted on board 

the NASA KC-135 Reduced Gravity Aircraft in support 

of the FOAM microgravity flight project. The FOAM 

test rig was developed to take advantage of the 

flexibility and availability of the KC-135, providing the 

team with otherwise unobtainable data. Design of the 

test rig (Figure 1) was modularized for versatility. 

Hardware components could be exchanged to meet 

specific test objectives. 

The KC-135 tests enabled the FOAM flight hardware 

development team to obtain quantitative results on 

sample cell surface treatments, laser light scattering, 

and video microscopy measurements. In addition, 

qualitative tests were conducted to observe the 

production of wet foams, and evaluate various foam 

delivery systems. 

In the first series of KC-135 flights, the FOAM project 

team conducted tests to determine compatible sample-cell 

surface treatments for future rheology tests, and to 

dispense foam in a controlled, repeatable manner in a low 

gravity environment. Test results were obtained on 

selected surface treatments and materials to determine the 

combination that allowed the least amount of slippage 

between the foam and the plate surface. Results of these 

tests will assist the project team in their selection of a 

material and surface treatment for the design of a 

rheometer sample cell fixture that will eventually contain 

the sample for the FOAM flight experiment. Initial 

observations on foam dispensing were evaluated, and lead 

the project team to further testing options. 

The second series of KC-135 testing was conducted to 

further test a “bag-in-can” foam dispensing concept, and 

to conduct laser light scattering measurements on wet 

foams. The FOAM test rig was reconfigured to 

accommodate a video microscopy system to observe the 

bubbles while conducting simultaneous laser light 

scattering measurements. Valuable data was successfully 

taken with the light scattering system during low g 

(~0.01g), Lunar-g (1/6 g), Martian-g (~0.4 g) and highgravity (1.5 – 2g) portions of the KC-135 flights. 
NASA/TM—2001-210704 2 

The KC-135 tests have provided the FOAM project 

team with essential information regarding foam

behavior. Flight hardware development for the FOAM 

experiment presents numerous challenges to the 

engineering team. Despite the short duration (~20 s) of 

low gravity provided by the KC-135, much can be 

learned regarding foam behavior, and assessment of 

hardware concepts.2

 Results from the KC-135 Reduced 

Gravity Aircraft will support the development process, 

and help verify the functionality of the hardware in 

meeting the FOAM science requirements. 

Foam Science Requirements

The FOAM microgravity experiment seeks to provide a 

fundamental understanding of foam rheology and 

stability in relationship to the foam bubble structure 

and dynamics. This is to be accomplished through the 

use of light scattering measurements, rheological 

measurements, and video microscopy on foams of 

variable liquid content. The developed instrumentation 

must meet the science requirements for all four of these 

elements. Throughout the experiment, the foam will be 

allowed to coarsen3

 and data will be collected in each of 

the required diagnostic techniques. 

Of particular interest is the loss of rigidity in foams with 

a high liquid content, i.e., wet foams. The study of wet 

aqueous foams is particularly difficult in normal gravity 

because the liquid easily drains out of the sample.4,5 In 

this particular experiment, the liquid content is required 

to range from 5% to 40%. The sample cell containing 

the foam must accommodate simultaneous video 

microscopy, diffusing-light spectroscopy, and 

rheological measurements. The sample cells are 

required to be loaded with foams without introducing 

large gas pockets and other inhomogeneities. In 

addition, the sample cell must be designed such that 

uniform shear deformation throughout the foam sample 

can be achieved. 

Figure 1.—FOAM KC-135 aircraft rig.
NASA/TM—2001-210704 3 

Rheological measurements are made throughout the 

foam coarsening process. A cone-and-plate sample cell 

geometry is desired because it will provide a uniform 

shear rate, and allows for easy cleaning of the sample. 

In addition, this geometry provides access to diffusinglight spectroscopy measurements at different foam 

thicknesses. 

Video microscopy will be used to record the gas 

bubbles at the surface. Bubble size and distribution can 

be determined through this method. Recordings are 

required throughout the aging process (coarsening) of 

the foam sample. 

These techniques have been demonstrated with dry 

foams in ground-based studies.6,7,8,9 Microgravity 

conditions are necessary to eliminate drainage for wet 

foams such that observations of the changes in structure, 

rearrangement dynamics, coarsening, and rheology that 

are expected as liquid content increases can be 

achieved. 

KC-135 Hardware and Test Results

Throughout the flight hardware development process, it is 

important to show that concepts meet the FOAM science 

requirements. The FOAM test rig was designed to be 

compatible with the NASA KC-135 Reduced Gravity 

Aircraft.10 It is a standard aircraft instrument rack that was 

reinforced and outfitted with interfaces for mounting in the 

KC-135. Handles and casters were installed to facilitate 

moving the rack from place to place. 

Most instruments used in the experiment were 

commercial off-the-shelf hardware and attached to the 

base plates. The base plates mount to the racks via 

adjustable shelves, which aid in the versatility of the test 

rig. All electronic components use 120 VAC power, 

which was compatible with the power supply from the 

KC-135 aircraft. 

The experiment was designed to be operated by one 

person. Tests conducted using this rig were foam 

dispensing, slip tests, video microscopy, and laser light 

scattering. Video data from all tests were recorded on 

digital 8mm tape. A color monitor was used to verify 

that the system was operating correctly. 

During the KC-135 flights the parabola numbers were 

noted, as each parabola provides near 0g, but each 

differs due to pilot technique and weather conditions. 

Data is provided, via an accelerometer system on board 

the aircraft, which measures the g-forces along three 

axes. This data can be correlated with the project flight 

data to differentiate Martian and Lunar g simulations 

from 0g parabolas, or to identify atypical noisy 

parabolas. 

Foam Dispensing Hardware

The FOAM flight experiment requires testing foams 

with various liquid contents. As the volume fraction of 

liquid is increased, the foam becomes wetter and 

behaves more like a simple liquid. In order to produce 

and conduct experiments on foams with various liquid 

fractions, a method of consistently dispensing foams at 

specific liquid contents in microgravity will be required. 

The simple task of dispensing and containing a stream of 

foam is challenging in the microgravity environment. In 

order to observe the various dispensing methods, a video 

camera mounted on the KC-135 rig was used to capture 

the results during the first series of tests. Videotape 

provided a convenient method of recording the 

qualitative, visual results. In addition, a microphone was 

employed to capture audio notes to eliminate the need for 

written observations during the flight. 

Low gravity tests were conducted to simply observe 

what happens as foam is dispensed from a can of 

shaving cream. Foam was generated using commercially 

provided aerosol cans, and a reusable bag-in-can 

system. Inside a can of shaving cream are a mixture of 

water, surfactants, propellant, and other additives. A 

typical propellant is A-31, which contains a propaneisobutane mixture that condenses at 31 psi. A dip-tube 

extends from the exit nozzle to the bottom of the can, 

where the liquid mixture is extracted. The placement of 

the tube relies on gravity to force liquid to the bottom of 

the can, and suggests that it will be ineffective in low 

gravity. When the foam is dispensed, the propellant 

vaporizes and forms bubbles of propane-isobutane gas 

in the water/surfactant mixture. A typical can of 

commercial shaving cream produces a foam having a 

liquid fraction of 7 to 10%. The liquid content of the 

foam was adjusted by varying the water/propellant ratio 

in the initial mixture. 

The reusable bag-in-can system was employed during the 

second series of tests. These were tethered directly to the 

rig for safety during testing and otherwise stored in the 

top shelf bin. This system consisted of a bladder (bag) 

filled with the foam mixture. In this design, a foil bag is 

filled with a water and surfactant mixture. The bag is presealed, but contains a nozzle and valve. The bag is placed 

in a can, which is sealed and then pressurized to a 

pressure below the vapor pressure of the propellant. The 

propellant is then forced through the nozzle into the bag 
NASA/TM—2001-210704 4 

by its own pressure, and finally the can is pressurized to a 

pressure well above the saturated vapor pressure of the 

propellant. By creating an overpressure environment on 

the outside of the bag, the propellant remains in a 

condensed state even as the sample is depleted. Since the 

gas is separated from the liquid phase, only the liquid can 

be expelled from the can.

Foam Dispensing Test Results

Our simple tests were to continuously dispense foam 

from an aerosol can during the low g parabolas, and 

record the observations by video. We found that foam

dispenses intermittently with pockets of gas, producing 

an unacceptable stream at the output. The gas pockets 

are ejected when the vapor pocket in the can reaches the 

dip-tube. In low gravity, the liquid in the can was 

observed to easily move away from the dip-tube, 

causing chaotic streaming of the foam out of the nozzle.

To circumvent the problem of a vapor pocket inside the 

can containing the foam sample, an alternative method 

of dispensing the foam using the “bag-in-can” method 

was tested. Tests in 1g confirmed that the bag-in-a-can 

method produced a foam stream irrespective of the 

orientation of the can. We did find, however, that in 

order for the technique to work, sufficient overpressure 

in the can was needed to keep the foil bag collapsed. A 

drawback of the added pressure is that the foam 

dispenses much faster. Nevertheless, we confirmed that 

this design also worked in a low gravity environment.

Surface-Slip Hardware

The rheology measurements planned for the flight 

experiment require that the surfaces of the rheometer in 

contact with the foam sample provide a no-slip condition, 

so that the sample is sheared in a uniform and consistent 

manner. The purpose of this KC-135 experiment was to 

test various surface treatments with wet foams, and 

compare the results with ground-based observations. The

plate materials tested included smooth and sandblasted 

glass, smooth and sandblasted acrylic, acrylic randomly 

scored with a knife edge, sandblasted quartz, and 

sandblasted polycarbonate. The bottom plate was held 

fixed while the top plate was translated horizontally by a 

motorized linear stage at 375 μm/s. The top plate holder 

was designed for easy change out of the top plate 

material. Slip tests were performed using a 1 mm gap. A 

sponge was located below the two plates that caught fluid 

pulled from between them during the high-g pull out 

portions of the flights. This hardware configuration 

allowed for observations of foam slippage on the plate 

surface while creating a stress on the foam via movement 

of the top plate.

A CCD camera was rigidly attached to the moving stage 

and focused on the inside surface of the moving plate. 

This camera has a very small sensor head, with its 

remaining electronics and power supply located 

elsewhere. This feature allows the camera and lens to be 

located where space and weight may be limited, as is 

often the case with rigs designed to be flown on the 

KC-135. The camera was mounted vertically. A ring 

light was used to illuminate the sample. The camera 

output was routed to both the monitor and a camcorder 

for digital recording to tape. 

An extension tube was used on the nozzle of each foam 

dispenser to inject the foam sample between the plates 

at the beginning of the 0g period. The stage was set in 

motion, and the image of the bubbles at the inside 

surface of the top plate were observed and recorded. 

The plates were removed and cleaned between samples. 

Voice notations were recorded with the images, which 

identified the plate material, translation velocity, and the 

liquid content of the foam used. 

Surface-Slip Test Results

We measured the apparent slip at a surface as a function 

of shear rate and foam liquid content using various 

surface treatments, in both 1g and low g. Ideally, a layer 

of gas bubbles would remain stuck to the walls of the 

container, providing a no-slip condition. Thus, surfaces 

that promote non-wetting of the liquid are preferred. 

The smooth glass and sandblasted polycarbonate 

performed the worst, as they consistently showed slip of 

the bubbles at the surface. The sandblasted quartz and 

randomly scored acrylic performed the best, and often 

showed that the bubbles remained fixed at the surface 

during the 15 second test. The other materials exhibited 

intermediate performance. 

The slip condition was tested with foams having a liquid 

content that ranged from 10% up to 40%. There did not 

appear to be any correlation between slip speed and 

liquid content. Nor did we observe any difference in the 

slip behavior in 1g and low g. One problem we did 

observe was that the slip behavior was not always 

repeatable. Occasionally, the bubbles would not slip 

even on the intermediate performing materials. 

Sometimes, the bubbles would not slip until the foam 

had been sheared for several seconds. 
NASA/TM—2001-210704 5 

More tests will be needed to fully quantify the slip 

performance under higher shear rates and for longer 

times, but clearly the sandblasted quartz and randomly 

scored acrylic performed the best, and the liquid content 

and gravity had little, if any, effect on the performance. 

Video Microscopy and Diffusing Wave Spectroscopy 

Hardware

In addition to the rheology measurements planned for 

the flight experiment, the FOAM experiment will use 

Diffusing Wave Spectroscopy (DWS) to characterize 

the microscopic bubble motion. DWS is a dynamic light 

scattering technique that is very sensitive to 

microscopic motion of the foam bubbles. Even in a 

static aqueous foam sample (not subjected to shear), the 

bubbles at any given location can be observed to 

undergo rearrangement events. These rearrangements 

occur to relieve stresses in the foam that build up as the 

foam coarsens. The DWS technique is sensitive to the 

frequency and duration of these events. The faster the 

decay of the intensity autocorrelation signal, the more 

active are the bubbles in the foam sample. 

Reconfiguration of the experiment for the second series 

of KC-135 tests was easy due to the modularity of the 

rig design. The FOAM aircraft test rig was equipped to 

accommodate Diffusing Wave Spectroscopy (DWS) and 

video microscopy within an enclosure (Figure 2), 

similar to experiment configurations developed by the 

Principal Investigator.11 The video and CCD cameras 

were relocated, and the enclosed shelf was added. 

One of the shelves was enclosed with an interlocked 

door on the front since a 100mW frequency doubled 

Nd:YAG laser was used in the DWS tests. The interior 

of the enclosed shelf was lined with blackout material to 

absorb ambient light and keep the expanded laser beam 

from escaping into the plane cabin. An optical 

breadboard was cut to match and bolted to the shelf, 

and optical components were installed directly to it. 

These included the laser head, and an electronically 

controlled shutter. The shutter allowed the laser to 

remain on during video microscopy, which was done 

with a white light. The laser beam was directed into a 

microscope objective used to expand the beam. A 

mirror was used to give the beam more distance to 

expand, as space was quite limited on this shelf. A 

negative lens was used between the mirror and the foam 

sample to further expand the beam. A single mode fiber 

optic probe with a collimating lens, coupled to an 

avalanche photo diode, collected diffuse transmitted 

light for DWS analysis. A CCD camera and white light 

were used to record video images. 

One foam sample was flown per flight. The sample cell 

was removed after each flight and refilled just before 

the next flight. It was delivered to the plane just before 

take-off, bolted in place and immediately tumbled until 

data was taken during reduced gravity. 

The samples were prepared by adding water to shaving 

cream, and mixing the sample thoroughly. The foam 

sample was contained in a disc-shaped cell, 70 mm in 

diameter by 3.5 mm thick (13.2 cc), with randomly 

scored acrylic windows and an anodized aluminum 

spacer ring. The windows were sealed against the 

spacer using o-rings. To prevent drainage of liquid in 

wet foams, the sample was continuously tumbled about 

its central axis except for those periods during which 

DWS data was collected. 

 

Figure 2.—Block schematic for video microscopy and laser light scattering.
NASA/TM—2001-210704 6 

The tumbler is simply a stepper-motor-driven device 

and rotates the sample at a rate of once every 3 to 

5 seconds. Ground-based tests showed that this 

technique keeps the excess liquid dispersed for several 

hours. At 38% liquid content, liquid begins to collect at 

the bottom after only two hours, even with sustained 

tumbling. The samples were aged in-situ for 1 to 1.5 

hours while continuously tumbling. 

At regular intervals, the laser shutter was closed and the 

avalanche photo diode turned off so the white light 

source could be turned on. Video images of the bubbles 

were recorded during these parabolas both for 

qualitative observation and to measure the bubble size. 

The laser power and tumbler speed could be easily set 

without opening the enclosed shelf. A set of switches on 

a front panel were used to control the laser, shutter, 

avalanche photo diode, white light, camera and tumbler. 

They were located across the front of the rig, in easy 

reach of the operator seated in front as they worked 

through the test procedures. 

Diffusing Wave Spectroscopy Test Results

We measured the intensity autocorrelation function in 

several foam samples of various liquid content, as a 

function of g-levels. The DWS data was acquired with a 

correlator card installed in the docking station of a 

laptop PC. At the start of the 0g portion of a parabola, 

 the tumbler was turned off. Approximately 5 seconds 

later, a correlation function measurement was initiated. 

Each correlation measurement lasted 12 seconds (the 

low g parabola lasts about 20 s). Temperature (around 

25 C) was not controlled, and the environment is 

acoustically noisy. 

DWS data was collected from several different g-levels: 

0g (closer to tens of milli-g, during the top of the 

parabolic loops); 1g (during level flight of the aircraft); 

2g (during the bottom of the parabolic loops; the “2g 

pullout” is actually somewhere between 1g and 2g, and 

varies); and also Lunar g’s (1/6 g) and Mars g’s (0.4g). 

There is a clear dependence of the characteristic decay 

time of g(t) on the g-level (Figure 3). The data also 

shows that as the liquid content of the foam increases, 

the effect of gravity becomes more pronounced. We 

interpret the faster decay times to the drainage of liquid 

in the foam, which induces bubble motion. These results 

will be useful for estimating the effect of acceleration 

disturbances (such as thruster firings) during the 

microgravity flight experiment, and will also be used to 

bridge the gap between ground-based data and 

microgravity data. 

Figure 3:—DWS measurements of the intensity autocorrelation function g(t) as a function of delay time, obtained 

on the NASA KC-135. The foam sample contained 34% liquid content. The legend indicates approximate g-levels. 

As the g-level increases, the characteristic decay time becomes shorter due to drainage and bubble motion. 
NASA/TM—2001-210704 7 

Concluding Remarks

The flight hardware development process requires an 

understanding of the experiment and what to expect in a 

microgravity environment. Engineering challenges can 

be overcome through a test program in both 1g and low 

g. Although not all aspects of experimental design can 

be foreseen utilizing low gravity testing environments 

due to time limitations, many problems can be corrected 

prior to space flight through the use of low gravity test 

facilities. By conducting tests on the KC-135 reduced 

gravity aircraft, we are able to foresee or debug 

problems that may also occur during a microgravity 

mission. Testing hardware on the KC-135 low gravity 

aircraft offers a relatively low-cost method of testing 

concepts before the flight hardware design is finalized. 

The KC-135 is a versatile and unique tool, which allows 

researchers to perform experiments in a low gravity 

environment. The duration of microgravity makes it 

possible to complement the ground-based research, and 

help prepare us for our future on the International Space 

Station. 

Acknowledgements

We gratefully acknowledge the assistance of 

E. Hovenac, D. Minjares, and R. Skupinski for their 

effort in designing and developing the KC-135 aircraft 

test rig. This work was supported by NASA through the 

Microgravity Science Division. 

References

 

1. V. Pletser, “Microgravity Research During Aircraft 

Parabolic Flights: The 20 ESA Campaigns,” ESA Bulletin

Nr. 82, May 1995. 

2. C.A. Winter and J.C. Jones, “The Microgravity Research 

Experiments (MICREX) Data Base,” NASA Technical 

Memorandum 108523 Volume I, November 1996. 

3. D.J. Durian, D.A. Weitz, and D.J. Pine, “Scaling Behavior 

in Shaving Cream,” Phys. Rev. A 36, 306–316 (1987). 

4. R. Bruinsma, “Theory of Hydrodynamic Convection in 

Soap Films,” Physica A 216, 59–76 (1995). 

5. G. Verbist, D. Weaire, and A.M. Kraynik, “The Foam 

Drainage Equation,” Journal of Physics: Condensed 

Matter 8, 3715–3731 (1996). 

6. D.A. Weitz, J.X. Zhu, D.J. Durian, H. Gang, and D.J. Pine, 

“Diffusing-Wave Spectroscopy: The Technique and Some 

Applications,” Physics Scripta 49B, 610–626 (1993). 

7. D.J. Durian, “Relaxation in Aqueous Foams,” Bull. Mat. 

Res. Soc. 19, 20–23 (1994). 

8. D.J. Durian and D.A. Weitz, “Foams,” Kirk-Othmer 

Encyclopedia of Chemical Technology, 4 ed., edited by 

J.I. Kroschwitz (Wiley, New York, 1994), Vol. 11, 

pp. 783–805. 

9. M. Tabor, J.J. Chae, G.D. Burnett, and D.J. Durian, “The 

Structure and Dynamics of Foams,” Nonlinear Science 

Today (1991). 

10. L.G. White, “Operations User’s Guide; JSC Reduced 

Gravity Program User’s Guide,” JSC-22803 Rev. D, 

NASA Lyndon B. Johnson Space Center, October 2000. 

11. D.J. Durian, D.A. Weitz, and D.J. Pine, “Multiple Light 

Scattering Probes of Foam Structure and Dynamics,”

Science 252, 686–688 (1991). 
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Susan M. Motil, Angela R. Harrivel, and Gregory A. Zimmerli

Testing Microgravity Flight Hardware Concepts on the NASA KC-135

This paper provides an overview of utilizing the NASA KC-135 Reduced Gravity Aircraft for the Foam Optics and

Mechanics (FOAM) microgravity flight project. The FOAM science requirements are summarized, and the KC-135 testrig used to test hardware concepts designed to meet the requirements are described. Preliminary results regarding foam

dispensing, foam/surface slip tests, and dynamic light scattering data are discussed in support of the flight hardware

development for the FOAM experiment.