Presented here is a preliminary interpretation of a recent experiment conducted on Space Shuttle Discovery (Mission STS 29) in which a stream of liquid supply water was vented into space at twilight. The data consist of video images of the sunlight-scattering water/ice particle cloud that formed, taken by visible light-sensitive intensified cameras both onboard the spacecraft and at the AMOS ground station near the trajectory's nadir. This experiment was undertaken to study the phenomenology of water columns injected into the low-Earth orbital environment, and to provide information about the lifetime of ice particles that may recontact Space Shuttle orbits later. The findings about the composition of the cloud have relevance to ionospheric plasma depletion experiments and to the dynamics of the interaction of orbiting spacecraft with the environment

Berk, A.

Elgin, J. B.

Kofsky, I. L.

Murad, E.

Pike, C. P.

Rall, D. L. A.

Satayesh, A.

Trowbridge, C. A.

Viereck, R. A.

English

NASA Technical Reports Server

 91-20726
PHOTOMETRIC ANALYSIS OF A SPACE SHUTTLE WATER VENTING
R. A. Viereck, E. Murad, and C. P. Pike,
Geophysics Laboratory, Space Physics Division, Hanscom AFB, MA 01731
I. L. Kofsky, C. A. Trowbridge, and D. L. A. Rail,
PhotoMetrics, Inc., Woburn, MA 01801
A. Satayesh,
Radex, Inc., Bedford, MA 01730
A. Berk and J. B. Elgin,
Spectral Sciences, Inc., Burlington, MA 01803
ABSTRACT
A photometric-photogrammetric analysis of intensified-
video images from AMOS and onboard Shuttle Orbiter
"Discovery" of a sunlit venting of supply water has shown
that the 1½ mm-diameter liquid stream breaks up within ~ 1
m to form ice/snow particles of two characteristic sizes,
much as was observed in earlier space-tank simulations.
Discrete droplets produced by the flash evaporation are the
principal feature in the photographs of the wake trail taken
from the bay and crew cabin; these particles have an
"average" diameter comparable with that of the initial
continuous flow. Unresolved submicron ice droplets
formed by recondensation when the evaporated water gas
overexpands dominate the images of the ~2½ km of trail
detectable by the groundbased tracking telescope-camera;
these particles sublimate at rates that we modeled from the
decrease in visible radiance of the trail by applying energy
balance arguments for spherical Rayleigh-Mie scatterers/
radiators in low earth orbit that undergo the (small) surface
roughening seen in laboratory experiments. The angular
spreads of the two types of ice particle are the same within
observational error, and the kinetic energy imparted by the
boiling-explosion of the superheated (in vacuum) quasi-
cylindrical water stream is about an order of magnitude less
than the injection energy.
INTRODUCTION
We present here a preliminary interpretation of a recent
experiment conducted on Space Shuttle Discovery (mission
STS 29-orbit 49, 16 Mar 1989) in which a stream of liquid
supply water (1) was vented into space at twilight. The data
consist of video images of the sunlight-scattering water/ice-
particle cloud that formed, taken by visible light-sensitive
intensified cameras both onboard the spacecraft and at the
AMOS ground station near the trajectory's nadir. This
experiment was undertaken to study the phenomenology of
water columns injected into the low earth-orbital environ-
ment, and to provide information about the lifetime of ice
particles that may recontact Space Shuttle several orbits
later (2). The findings about the composition of the cloud
have relevance to ionospheric plasma depletion experiments
(3) and to the dynamics of the interaction of orbiting
spacecraft with the environment (1).
EXPERIMENTAL
Pure and largely gas-free liquid water produced in the
vehicle's fuel cells was forced out through a 0.14-cm
diameter nozzle, which was heated to ~70 ° C to prevent its
blockage by icing; 19.4 g/s was vented within a few degrees
of the wake direction. The experiment was planned so that
the spacecraft would be directly solar illuminated just before
dawn, while the atmosphere below ~100 km altitude
remained in the hard earth's shadow. Discovery was in a
southwest-to-northeast circular orbit at 329 km that passed
almost directly over AMOS (the Air Force Maui Optical
Station), which is atop 3.0 km high Mt. Haleakala, HI (21 °
N - 204°E). The radiance distributions of the water trail
were measured for about 2 rain in three projections: to a
closed circuit zoom television camera in the open bay just
forward of the spacecraft's tail, 18 m from the nozzle; to a
similar camera handheld to point out of a crew cabin
window a few m forward of and above the nozzle; and to
the ground station, where an ISIT low-light-level video
camera with 55 cm-diameter objective lens, 0.4 ° by 0.3 °
field of view, and S-20 spectral response (FWHM 0.4-0.65
t_m) precisely tracked the spacecraft. Examples of the
imagery from the three intensified electronic cameras are in
Figure 1.
Discovery came into direct sunlight at 62 ° elevation
southwest of the ground station, and the water trail
remained above background until it moved to about 5 ° in
the northeast. The solar scatter angles, aspect angles to the
676
https://ntrs.nasa.gov/search.jsp?R=19910011413 2020-03-19T19:04:17+00:00Z
Figure 1. Views of the sunlit water/ice particle trail from
onboard Discovery (a and b) and from AMOS
with a 0.37°-wide field (c). a) is a projection to
crew cabin window W1 and b) is a view from
the zoom camera mounted just forward of the
vertical stabilizer. In c) the solar-scatter angle is
48 ° , and the equivalent stellar magnitude of
Orbiter (whose bloomed image is at the head of
the water trail) is + 3.6.
trail's symmetry axis, zenith angles, and slant range to
AMOS are in Figure 2. Both onboard cameras could view
the essentially-backlit water cloud (the rising sun being
toward the northeast) beyond about 5 m from the ejection
nozzle. The bay camera, whose maximum field of view
encompassed most of the detectable length of the trail, was
at times pointed in azimuth to include its vanishing point.
The groundbased camera's projected field varied between
about 11,4 times (at culmination) and 5 times (near the
horizon) the maximum above-threshold longitudinal extent
of the optically thin sunlight-scattering volume.
DATA
The video images represent a series of illumination/viewing
conditions of a time-stationary physical phenomenon. We
analyzed radiometrically the AMOS scene at zenith angle
61° and azimuth 70 ° (Figure lc), in which the high
differential cross-section for forward scattering of sunlight
(scattering angle 48 ° ) and moderately elongated sight path
(angle to long axis of cloud 331A°) resulted in the best video
signal/noise ratios. Discovery, whose visual magnitude at
this range (640 km) is about +31,4, appears as a strongly
bloomed, photocurrent-saturated feature at the head of the
quasiconical extended water-containing volume. Figure 3
plots the relative brightnesses along the long axis of this
trail (the instrumental baseline is subtracted, and a linear
dependence of photocurrent on cathode irradiance
characteristic of is its assumed), and these brightnesses
summed over pixels along lines transverse to this direction.
(This latter, rapidly decreasing quantity is the sterance per
unit longitudinal path.)
The two onboard cameras show principally a flow of
densely-packed discrete particles, with a relatively broad
size distribution indicated by the variability of the video
photocurrents from particles at sensibly the same range.
This cloud diverges at about the same angle as the trail in
the AMOS images (see Figure 1), and its "edge" projects
back to an apex within a meter from the nozzle. We
measured the longitudinal velocity as 23 m/s 4-30% by
following individual strong sunlight-scatterers in successive
video frames, finding the variation among particles to be
less than the precision of this measurement. The mean
cross-track speed in the essentially-perpendicular view
directions to the two onboard cameras can be seen to be
about 1Athis axial speed, which indicates that the transverse
momentum imparted to these particles is small compared
with their wake-directed momentum. Many of these
particles flicker, with characteristic periods near _A s, and
are therefore more likely tumbling ice/snow crystallites than
spheres; indeed, erratic dropIet shapes as well as a broad
"size" distribution have been observed in a laboratory
simulation of the venting of water into near-space (4).
ORIGINA[ PAGE
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Figure 2. Solar scatter angle (r/l), aspect angle to the trail
axis (r/z), zenith angle of Discovery and the trail
(r;3), and slant range to Discovery from AMOS
(solid line) during the water venting.
Approximately 100 of these large particles per m path can
be resolved in the video frames from Discovery's camera.
This number would be expected from the laboratory finding
(4-7) that the mean diameters of the explosion-product ice
particles are comparable with the diameter of the ejection
nozzle.
INTERPRETATION
We interpret the discrete sunlight scatterers seen by
Discovery's onboard cameras to be the product of the
cavitational bursting of initially-coherent water streams in
vacuum, which results from the rapid formation of bubbles
("steam') in the superheated liquid (4-8). A cloud of much
smaller (tenths-micron diameter) particles that has also been
identified in laboratory tank experiments is not
unambiguously detectable against these bloomed images of
the close-lying large droplets. This second component of
the flow is formed by recondensation of water molecules
that had been evaporated from the original and flash-
exploded stream after this gas becomes overexpanded. On
the basis of the following observations, we judge these
much smaller particles to be responsible for the major
fraction of the radiances of the trail that were measured by
the groundbased camera (Figure lc).
[1] Thermodynamic arguments (3) show that most (~70%)
of the water mass remains in the large ice particles, while a
laboratory simulation (6) indicated that the small
recondensate particles contain less than 2%. We applied
standard light scattering theory for 0.2 /_m and
0.7 mm radius ice, finding that despite the great differences
in both the fraction of initial water in and the absolute
scattering amplitudes of the two populations, the much
larger number of small particles results in considerably
more visible radiance over the length of water trail
detectable by the AMOS camera. (That is, the particles
whose circumference is comparable with the photon
wavelength have higher scattering "efficiency".)
[2] The surface brightnesses of the trail measured from the
ground exhibit a very weak dependence on the solar-scatter
angle, which is a behavior characteristic of Rayleigh rather
than geometric scattering (particle large compared with the
photon wavelength). Specifically, the fractional increase in
the axial brightnesses beyond about 1 km from the nozzle,
corrected for sight path length, remains small as the sunlight
scattering angle decreases from 107 ° (when the trail was at
AMOS zenith) to 48 ° (to which Figure 3 refers). This
small change is consistent with the differential cross-
sections of submicron particles, while in contrast scattering
of visible light per unit solid angle from mm spherical ice
drops would increase by about a factor 50 over this range of
angles.
[3] The dependence of axial and crosswise-integrated
radiance of the optically thin trail on distance from
Discovery (Figure 3) is consistent with a simple model of
the energy balance of the smaller ice particles. We applied
standard Mie radiation theory with the known dependence
of the real and imaginary components of the index of
refraction of ice on wavelength to calculate the sublimation
rate of smooth spheres in vacuum, taking into account the
energy they absorb from earthshine and the incident solar
flux and the energy they lose by thermal emission and
sublimation. (Collisional heating and recondensation can be
readily shown to be negligible.) The equilibrium 166K
temperature is reached within less than a second after the
particles form.
Since the emissivity of weakly-absorbing spheres is
proportional to their radius r when r is less than ~1 _m,
their radiance decreases exponentially with time, or distance
x from the spacecraft. That is, r of submicron particles
with axial velocity v (which we took as the velocity of the
large particles, in view of the similar cone angles) varies as
exp [-ax/v], where a (= 0.004 s-! for smooth spheres) is
the fractional loss rate due to sublimation. Thus the
scattering in the "geometric" particle size range, which we
take to be where the cross-section at fixed angle varies with
r2, would exhibit an exp [-2ax/v] dependence. In contrast
scattering in the Rayleigh size range, where the cross-
section varies as r2"r 4 = r6, would vary as
exp[-6ax/v]. The "break" near x=1400 m in Figure 3 is
due to the transitioning of the radius of the subliming
particles from the geometric to the Rayleigh-scatter regime.
(The familiar Mie oscillations in this transition region are
678
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B) INTEGRATED RADIANCE
700 10013 1300 Ir_8 |900 22o0 ,?-.._8
_otonce From Orl_Ler X (_|
Figure 3. a) Relative radiance along the line of maximum
brightness extending out from Discovery in the
AMOS image (Figure 1 c), with background
subtracted (and a linear relationship between the
video photocurrent and scene radiance assumed).
b) Radiances summed along lines transverse to
this long axis. The data are contaminated by
video blooming from Discovery's image when
x,,.K.500m.
"largely washed out by the polychromatic sunlight
illumination.)
Figure 3 indicates that the radiances measured from AMOS
can be described by geometric scattering over the first
~1400 m and by Rayleigh scattering over the remaining
~1000 m (beyond which the trail's brightnesses fall below
the camera's threshold), when a small correction for the
progressive roughening of the particles is included in the
calculation. The correction assumes a linear increase in
sublimation rate with time, so that the radius decreases
about as exp [-ax/v-bx2/v2]. That somewhat-larger ice
droplets in vacuum indeed develop irregular surfaces that
lead to more rapid sublimation was observed in laboratory
experiments in which the chamber walls were held at low
temperatures (9). The roughening required to achieve the
fit of the data shown to an energy-balance model that
includes this effect increases the rate of decay of mean
radius by sublimation from the submicron particles by up to
only a factor 2, which is in general agreement with the
laboratory findings. The small downward curvature of the
radiances in the two scattering regimes is also predicted by
the progressively-increasing surface roughness that we
assumed.
These calculations also showed that sublimation would
result in a virtually negligible decrease in the sunlight-
scattering cross-sections of one-ram ice droplets over the
~100-sec length (2_h kin/23 msq in the spacecraft's frame
of reference) of the visible trail. The substantial measured
decrease in total sterance per unit longitudinal path shown
in Figure 3 and a calculation of the absolute mean radiances
from solar scatter with the observed 100 such particles per
m path, both further show that these much larger ice
droplets could be responsible for only a minor fraction of
the optical signal at AMOS.
The transition to Rayleigh scattering near x = 1.4 km
indicates that the mean particle radius at this distance from
Discovery's nozzle is between 0.1 and 0.3 pro. Our
sublimation model gives an estimate of the mean initial
radius as 0.3 + 0.15 t_m. The corresponding radius
inferred in a space-simulation chamber (6), in which the
water stream had 1.6 mm diameter, 20°C temperature, and
a somewhat higher total flow rate, was 0.08 + 0.03 #m.
CONCLUSION
The cloud of discrete particles that results when ram-
diameter continuous water streams are vented into the low
earth-orbital environment is found to have three
components. Macroscopic (from laboratory observations,
also mm-diameter) ice/snow particles are created by the
explosive bursting of the supersaturated high-vapor pressure
liquid, producing irradiances that were readily detectable in
the images from the intensified video cameras onboard
Discovery. The narrow cone angle in which these particles
appear indicates that this flash evaporation imparts relatively
little momentum compared with the longitudinal momentum
of these particles. Much smaller droplets that theory
indicates result from condensation of gaseous water produce
radiances that dominate the images from the much more
distant groundbased ISIT telescope-camera, in a volume
with essentially the same angular spread as the large
particles. The ~25% water gas, which is not detectable by
either type of camera, is evolved from vaporization and
sublimation of the liquid and solid (3, 7, 8). We have
estimated the mean diameter of the submicron particles
from an analysis of the dependence shown in Figure 3 of
the relative brightnesses from scattering of sunlight at
visible (S-20 photocathode response) wavelengths on
distance into the wake of the spacecraft. The surface
roughening that has been reported from laboratory
experiments is interpreted as accelerating the sublimation
rates of these small particles by factors up to about 2.
679
ACKNOWLEDGEMENTS
The success of this experiment is due to the support
provided by a number of USAF, NASA, and AMOS
personnel. We thank especially D. J. Knecht, J. P. Bagian,
J. F. Buchli, Lt T. Hols, L. Dungan, J. Baird, Maj E.
Imker, G. Ashley, and L. Twist.
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5. Curry, B. P., Bryson, R. J., Seibner, B. L., and Jones,
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Photometric analysis of a space shuttle water venting

Photometric analysis of a space shuttle water venting

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