Some Insights from an Expert
a discussion with a NASA aerospace engineer familiar with the space shuttle
reaction control system, I learned that the thrusters never generate any
light while operating, but they always emit a small cloud of unburned propellant
just before the thruster fires and a much larger cloud immediately after
the thruster shuts down. The post-burn cloud may be visible, but only when
reflecting sunlight. The pre-burn cloud is never visible to the human eye
but might be detected by a light-sensitive camera. Any light flashes seen
in space shuttle videos cannot be from a thruster unless they coincide
with the beginning or end of a rocket burn. The consequences of this information
in regard to two videos of apparently anomalous objects taken by shuttle
video cameras are described.
described in previous articles here and elsewhere, several objects in the
STS-48 video of Sept. 15, 1991 seem to react to a flash of light by changing
course. According to James Oberg and others associated with NASA, the flash
of light was caused by the firing of a small reaction control system (RCS)
thruster on the space shuttle. Oberg has asserted that:
RCS jets usually fire in 80-millisecond pulses to keep the shuttle pointed
in a desired direction, under autopilot control (usually once every few
minutes). These jets may flash when they ignite if the mixture ratio is
not quite right. Propellant also tends to seep out the feed lines into
the nozzle, where it accumulates, freezes through evaporative cooling,
and flakes off during the next firing. The ejected burn byproducts travel
at about 1000 ft/sec. One pulse usually emits about a quarter pound of
propellant in a fan-shaped plume.
I've written several articles pointing out flaws in the interpretation
of the light flash in the STS-48 video as the product of a thruster firing,
I had no reason to question the above description of thruster operation
because Oberg had a long career as a flight officer in the Mission Control
Center at Johnson Space Center, and frequently appears as an expert on
spaceflight for TV news and on the lecture circuit. But it turns out his
description is wrong or at least misleading on several important points.
recently had the opportunity to discuss various aspects of the space shuttle's
RCS propellant supply system with a NASA aerospace engineer who was involved
in the design, testing, and performance evaluation of the RCS from the
nearly the beginning of the shuttle program. Unlike Oberg, this engineer
observed tests of thruster firings close up on a routine basis. Most of
our discussions were related to work I am doing and was focused on the
ingenious system that dependably supplies fuel and oxidizer to the rockets
under weightless conditions in space as well under Earth's gravity during
reentry. However, he also described what happens on the business end of
an RCS thruster when it fires.
Propellant Behavior Before, During, and After a Thruster Burn
of the space shuttle's rocket engines use the same "hypergolic" propellants,
meaning two different chemical compounds that ignite on contact without
the need of any ignition source such as an electrical spark. The fuel is
monomethyl hydrazine (MMH) and the oxidizer is nitrogen tetroxide (NTO).
This is fairly common knowledge and can be found on many web sites
important fact about the RCS thrusters that I learned from the NASA engineer
is that the two valves that simultaneously open to admit MMH and NTO into
the rocket combustion chamber are not located immediately next to the combustion
chamber walls. The valve mechanisms cannot withstand the intense heat generated
by combustion(about 3500¡ Celsius). So the valves are mounted on
a "thermal standoff" plate some distance from the combustion chamber and
are connected to it by tubing, as shown in the diagram of a vernier RCS
thruster in Figure 1.
1: Diagram of a vernier reaction control thruster of the type alleged to
have fired to cause object motions in the STS-48 video. The green regions
indicate the "dribble volume," which is the section of tubing between either
of the propellant valves and the combustion chamber.
the thruster starts firing, the propellants are briefly exposed to the
vacuum of space after flowing out of the opened valves until they reach
the combustion chamber and ignite.While exposed to vacuum, some of the
liquid propellants boils off into space and then immediately freezes into
"microscopic snow," as this engineer called it. In the case of the small
thrusters, this happens so quickly over the short distance from the valve
to the combustion chamber(about 2 inches) that the amount of "snow" generated
is too small to be seen.
the valves are closed to shut the thruster down, small amounts of propellant
are trapped in the tubes between the valves and the combustion chamber.The
engineer called this volume of trapped propellant the "dribble volume,"
perhaps because it was observed to just dribble out of the thruster after
shutoff during ground testing under atmospheric pressure. But in the vacuum
of space, the "microscopic snow" also forms after shutoff just as it does
at startup. But the dribble volume is large enough that the snow generated
can be seen as a white plume in reflected sunlight. It is totally
invisible without some external source of illumination.
claim that significantly more unburned propellant is expelled at the end
of a thruster burn than at the beginning appears to be supported by the
telemetry records for the combustion chamber pressures of the Discovery's
thrusters during the STS-48 mission, which I obtained from the NASA FOIA
office.Figure 2 shows a plot of the combustion chamber pressure versus
time for the thruster burn proposed as the cause of the light flash in
the STS-48 video. At the beginning of the burn, the pressure quickly rises
from 0 (vacuum) to 110 psi, but after the thruster is shut off, the pressure
stops its rapid decline and tails (or dribbles) off again to zero. This
tail region evidently is the pressure generated by the unburned liquid
propellant trapped in the "dribble volume" evaporating into space. The
record of STS-48 thruster firings I have is for a time period of 45 minutes
with many burns recorded for all of the shuttle's vernier RCS thrusters,
and every single one of them has the same steep rise at ignition and the
same small "tail" after shutoff. Since it is a feature of every thruster
firing, an associated emission of a visible plume of propellants is most
likely to be present, too.
2: Plot of combustion chamber pressure versus time for the firing of the
L5D vernier thruster proposed as the source of the light flash seen in
the STS-48 video on Day 2, 21:28:19.5 Mission Elapsed Time.
engineer likened the plume of small ice particles to the smoke that pours
out of the barrel of a gun after the muzzle flash. Then he added "but
there is no 'thruster' flash." Quite to the contrary of what might
be construed from Oberg's assertion, there is no "flash" in the sense that
the propellant itself generates little if any light at all during a burn.
The unburned liquid propellant can be said "flash" to a vapor after thruster
shutoff, but this refers only to the rapid phase change from liquid to
gas, not to light emission. While Oberg may have been aware of this fact,
his description was unclear about the meaning of the word"flash" and I
doubt the meaning was apparent to many of his readers.
Oberg has asserted that:
should also be pointed out that as all experienced observers of shuttle
TV images realize, the visible flare of these jet firings is only an occasional
and sporadic feature of their actual firings, which at other times -- especially
in periods of smooth, stable propellant flow -- can be invisible.
evidence of the combustion chamber pressures indicates that there is nothing
at all "sporadic" about this phenomenon. The numerous photos of shuttle
thrusters emitting plumes provide more evidence that they are predictable
result of a thruster shutoff. I have been puzzled for a long time about
why the rocket combustion gases were so easy to see when they are supposed
to be nearly invisible. The puzzle is apparently solved: these photos show
jets of microscopic snow at the end of the firing cycle in reflected sunlight.
In Figure 3, plumes from two primary RCS rockets on the left can clearly
be seen. The dull reddish glow on the right is apparently from an aft-firing
primary thruster, making a total of three thrusters simultaneously emitting
plumes of what must be unburned propellant – an unlikely coincidence if
such emissions were sporadic rather than routine consequences of thruster
3: Photograph showing plumes of unburned propellant emitted by three RCS
the event that there is any instability during a thruster firing that causes
the fuel-to-oxidizer ratio to be out of balance, the unburned propellant
would be unlikely to form snow as it exits. According to one reference,
"Hydrazine ... is technically stable to about 250 C."At the 3500 C temperatures
in an RCS combustion chamber, any excess MMH fuel would be converted to
a gas, decompose into simpler component gases such as water and methane,
and finally exit the thruster nozzle unseen along with the combustion products.
blockage of propellant flow long enough to intermittently halt combustion
and cause "snow" to be generated in mid-burn could be extremely
dangerous, according to the NASA engineer. If the thruster were not shut
down immediately, such a malfunction could potentially lead to an explosion
and even loss of the vehicle.
the light of this new information, it seems that several things that have
been written about the STS-48 video have to be reconsidered concerning
the behavior of space shuttle thrusters. Another video taken during the
more recent STS-102 mission also is reexamined here because of its similarities
with the STS-48 video.
time-lapse composite of video frames before and after the light flash is
shown in Figure 4. Several objects move almost immediately when the light
flash occurs. Their position at the time of the flash is indicated by the
red dots in the image. The light flash and movements of the objects has
been attributed to the orbiter's L5D thruster (left side firing down).
4: Time-lapse image of objects in the STS-48 video created from a composite
of video frames spaced at 1-second intervals.
there was no serious malfunction during the STS-48 mission, if the light
flash observed were from the L5D thruster, it could only mark the end of
the thruster burn, not the beginning. Figure 5 shows a graph I made for
an earlier article concerning this light flash. There is a faint "pre-flash"
that might seem to fill the role of a very brief jet of "snow" at the start
of the thruster firing, assuming the camera, which was set to for low light
levels, could detect light that is too faint to be perceived by the human
eye.The pre-flash occurs 0.4 seconds before bright "main flash" that seems
to have caused the objects in the video to move. But the L5D thruster firing
supposedly responsible for the flash was 1.2 seconds in duration, so the
pre- and post-burn flashes should have been 1.2 seconds apart, not 0.4
seconds. Worse for the thruster theory, the exhaust exits the nozzle at
a speed of 3500 meters per second. If it is assumed that the exhaust plume
hit the objects just as the thruster shut down, the objects would have
to be over 4 kilometers away from the shuttle, since that is the distance
the exhaust plume would travel in 1.2 seconds before impinging on the objects.
5: STS-48 video average frame brightness over seven seconds plotted at
1/30-second intervals. Start time corresponds to the video display clock
time of 20:39:23.Objects in the video changed course at the time of the
highest intensity peak in this graph.
is a third pulse that occurs roughly 1.3 seconds after the main flash that
might be suspected to be the rocket's shutdown flash, but again it is much
fainter than the main flash, which shouldn't be the case if the third pulse
was the light from the dribble volume of fuel escaping after thruster shutdown.
And there should be only one or two light pulses, not three.
a post-burn jet of "snow" might explain one seemingly anomalous characteristic
of the light flash I noted in my previous article: the three light pulses
occurred during a period of elevated background brightness in the image
that lasted for at least 5 seconds, which is much longer than the 1.2-second
duration of the thruster firing. The jet of snow at shutdown probably travels
at a much slower rate than the 3500m/sec speed of the (invisible) plume
of combustion products during the thruster burn. Moving at a slower rate,
it would disperse much more slowly than combustion products and thus linger
as a diffuse cloud, perhaps reflecting enough sunlight to raise the background
brightness for several seconds. So this one characteristic of the post-burn
plume from the thruster might support the thruster hypothesis. But all
its other characteristics make the thruster hypothesis seem even more preposterous
than it did when I wrote the earlier article, which assumed that thrusters
can "flare up" sporadically, as Oberg would have it.
relationship between thruster firings and "flashes" of light reflected
from unburned combustion products also has relevance to a video taken during
Space Shuttle Discovery's STS-102 mission in March 2001.As in the STS-48
video, a light flash occurs in the STS-102 video and a slow-moving object
abruptly changes course. This is shown in the time-lapse composite of Figure
6. Another object appears at the top of the frame and seems to pursue the
slower-moving "main" object. A high-speed object also appeared to be pursuing
Object M1 in the STS-48 video, but was moving too fast to be captured in
the time-lapse image of Figure 4.
6: Time exposure overlay of frames at 1-second intervals from STS-102 video.
The red shows the position of the object when the light flash occurs.
a previous article I noted that the time display in the Mission Control
Room, which appeared briefly in the STS-102 video, indicated that the light
flash occurred at least 5 seconds before the closest thruster firing, which
occurred at 12:30:39 GMT.Since it is impossible for the thruster firing
to be the cause of an event that preceded it in time, it could not have
been the cause of the light flash – assuming the mission control time display
event occurred in the STS-102 video that has been suggested as support
for the "prosaic" thruster hypothesis.This event was a second flash of
light that occurred 1.3 seconds after the first flash. As it happens, 1.3
seconds was also the amount of time that elapsed between the start of the
12:30:39 firing suspected to be associated with the light flash and the
next firing at 12:30:40.3. While I assumed this was probably a random coincidence,
the argument has been made that the elapsed times are the same because
the flashes came from the thruster at the beginnings of the two
firings. But as previously noted, the NASA engineer said that the visible
plumes of unburned propellant come at the end of a thruster firing
and not at the beginning. In this case, the elapsed time between the end
of the first and second firings was 0.96 seconds (12:30:39.548 GMT and
12:39:40.508 GMT) – significantly less than the 1.3 seconds between the
observed light flashes.
elapsed time between the end of the first and second thruster firings does
not agree with the elapsed time between the first and second light flash,
assuming the visible flashes mark thruster shutoff, as the NASA engineer
asserted. In that case, a rocket firing can be ruled out as the cause of
the object's motion in the STS-102.
if it is assumed that the light pulses come at both the beginning and the
end of a thruster burn and that the startup pulse can be brighter than
the shutoff pulse, Figure 7 shows the actual brightness curve still cannot
be matched to a thruster firing. The first peak at time A (when the "main
object" in the video changes course) might correspond to the startup pulse
of the thruster burn and the second peak at time B might correspond to
the "shutoff" pulse. But the peak at B would be 0.37 seconds into the 0.48-second
burn, assuming it started at time A in Figure 7. This would indicate the
burn was prematurely interrupted and that a "snow" of unburned propellant
was being expelled from the thruster rather than the hot gases of combustion
more than 1/10 of a second before the intended thruster shutoff. Such a
premature interruption of the firing could signal a potentially serious
problem with the propellant supply, such as the ingestion by the thruster
of a large bubble of helium. Helium is the gas that is used to pressurize
the propellant so that it will flow to the thruster immediately when the
fuel and oxidizer valves are opened. A small amount of helium is always
in solution with the liquid propellants, but it is never supposed to form
large gas bubbles in the propellant lines.
7: STS-102 video average frame brightness over 2 seconds plotted at 1/30-second
intervals. The length of time from A to D is close to the 1.28-seconds
between documented thruster burns. The length of time between A and C is
the duration of the first thruster burn assumed to start at time A. The
rise in brightness to the peak near B might seem to correspond to the first
burn's "shutoff" light pulse but that peak comes a tenth of a second before
the assumed shutoff time.
are only two cases that I know of in which objects in a space shuttle video
react to a light flash with a radical change in course: STS-48 and STS-102.
The information I've received from an expert in the shuttle's RCS propulsion
system provides a compelling refutation of Oberg's argument that thruster
firings were the cause of the objects' behavior in both cases.
assertion by Oberg that is incorrect is that propellant seeping out of
propellant lines and freezing in the nozzle is a routine occurrence on
the orbiter. To the contrary, it is an indication of a potentially serious
leak according to the NASA engineer. If such a leak is detected the caution
and warning system notifies the crew that a "Fail-Leak" condition has occurred.
The leaking thruster is then isolated by shutting valves upstream of the
leak. There is no record that I know of that any such failure occurred
during either STS-48 or STS-102, at least during the time when the videos
were taken. Based on Oberg's statements, it had seemed to me that in both
the STS-48 and STS-102 videos the high-speed "projectiles" seemingly pursuing
slower-moving objects might be explained as ice chunks expelled from the
nozzle during the rocket burn. But as the presence of such ice in the thruster
would indicate a possibly serious problem with the shuttle propulsion system,
this explanation no longer seems feasible.
Oberg stated that the speed of the RCS rocket exhaust gases is about 1000
feet per second. Their actual speed is 3500 meters or 11,482 feet per second.
That is ten times faster than the speed he cited. This error is of no great
importance to the question of apparent anomalous objects in shuttle videos.
But it is one more indication that while Oberg may well be an expert on
many aspects of space flight, he evidently has no particular expertise
or experience with the RCS propulsion system.
the NASA engineer I spoke with was a legitimate authority on the space
shuttle's RCS rockets, it should be apparent from reading this article
that I have not simply made an appeal to authority here just because what
he said severely undermines the "prosaic" explanations for these space
shuttle videos. Instead, I've tried to verify his assertions to the extent
feasible by checking independent references and data sources such as the
RCS combustion chamber pressure records for STS-48. Everything I've found
was consistent with what he told me.
The brightness curves for both the STS-48 and the STS-102 videos do not match what would be expected for reflected light from unburned RCS propellant if it is assumed that the light flash comes only at the end of a thruster burn as asserted by the NASA engineer I discussed the question with. Even if it is assumed that a light flash can be detected at the beginning of a thruster firing by a light-sensitive camera, the brightness curves in both videos still do not match what would be expected for a thruster burn.