All Shook Up!
All Shook Up is how I certainly was when I learnt that Robert Sadler was part of the team that built the seismometers used on the Apollo missions. I spoke to Robert every two months from 2014 until 2021 before he passed away at age 94, and updated him of the Citizens Party's campaign. I post here the text of an article with the same title he wrote which was published in Sky and Space magazine Feb-March 2000. Thanks to Joseph Mate for the original scan! Cheers, Robert, you're in immortality!-DJM
All Shook Up!
With the advent of the
Apollo missions to the Moon, a new scientific discipline was born — lunar
seismology. But the seismometer designs of the day were not up to the job. Bob
Sadler describes how he helped solve the problem...
One of the scientific objectives of the US
lunar space programme of the 1960s, was to determine if the Moon had a similar
geological structure to that of the Earth, which hopefully would help settle
the debate over the Moon's origin. Geologists already had learned
much about the structure of the Earth (eg. a semi-molten core with a
thin outside crust) by using seismometers to measure earthquakes. By placing
several well-separated seismometers on the lunar surface, it was hoped to
duplicate this achievement on the Moon.
But in 1966 very little was known about the
Moon beyond what had been deduced from telescope observations from Earth.
Therefore, many assumptions had to be made about the anticipated lunar
environment in which the seismometers would have to work. It was assumed
that the surface would probably be made of rock, possibly crumbly or dusty, and
would experience temperature variations from night to day of possibly -100℃
to +120℃. It was not known what sort of seismic
activity could be expected, although no volcanic events had ever been observed,
so it was assumed that the Moon would be seismically quiet. Finally, since
meteoric activity could not be predicted, artificial 'shocks' would need to be
produced during and after the astronauts' visits in order to gather seismic
data.
The seismometers also would need to be many
times more sensitive than any previously designed for Earth use. At the same
time. they would have to withstand a much more extreme environment than any
Earth seismometer could possibly survive.
Tidal effects
As well as ‘moonquakes', NASA also wanted to
measure tidal effects. We are all familiar with the rise and fall of the
Earth's ocean tide where it reaches the shore. This is caused by the
gravitational pull of the Moon (and Sun) on the ocean, causing, in effect, a
watery bulge to travel around the globe under the Moon. What is not so
generally known is that there is a similar (but much smaller) bulge in the
actual crust of the Earth caused by the Moon's gravitational pull.
Measured on a flat plain such as the Nullarbor, this could be as much as a 30m
rise and fall. depending on the thickness of the Earth's crust at that point.
(The crust of the Earth is like an egg shell ... only 50 to 100 kilometres
thick, compared to the 6,375 kilometre radius of our planet.)
One way to measure this gravitational bulge
is to measure its 'slope' as it approaches. An instrument sensitive to tilt
would detect any 'out of vertical' motion, in the same way that our eyes can
detect the tilt of a buoy-mounted flagpole as an ocean wave approaches. By
this method, NASA geologists hoped to determine if the Moon had a 'core and
shell' structure like the Earth and thus perhaps a similar origin. To do this,
they needed a seismometer capable of measuring the tilt angle to an accuracy of
one arcsecond (1/1600 of a degree). To further complicate the design of a
such a delicate instrument, it had to be of absolute minimum weight, yet robust
enough to withstand the horrendous vibration of the Saturn rocket as it
ascended through the Earth's atmosphere ... described by one astronaut as like
driving a truck fast down a very rutted farm track, and ending with a train
smash. But in 1966 the Saturn rocket and its engines had not yet been built, so
all we had to work with was a computer-simulated vibration specification. This
meant we would have to build a prototype and put it on a vibration table to
prove the design was capable of surviving the trip to the Moon.
NASA was offering a fixed-price, fixed-time
schedule (six months) contract to design and build eight lunar seismometers,
with penalty clauses for being late, over budget or overweight (but there were
also performance bonuses). In addition to a difficult schedule, NASA also
insisted on regular design reviews (a licence to nit-pick) regular reports of
current design weight calculated from technical drawings, adherence to a
time-schedule they thought we should meet, and explanations of all design
features at their regular (time wasting) bi-weekly visits. They insisted,
without good reason, that the structural parts be made of beryllium, a very
toxic and expensive metal, very difficult to machine and with characteristics
that were not well known.
In addition to those difficulties, there was
the problem of insulating the instrument from the extreme temperature range so
that it would maintain accuracy and repeatability of results. Finally, there
was the problem of astronaut clumsiness. In an Apollo spacesuit, an astronaut
had severely limited vision due to his almost-black sun-visor. Also, he was not
able to bend over or move his arms easily. We had to consider these
capabilities, or lack of them, when designing the handling aspects of the
seismometer.
There was really only one American company
doing seismometer design at the time, and they had no space experience. This
was a small firm in Pasadena, California, founded by Carl Richter (of the
Richter Scale fame), who needed seismometers for his work as a geology
professor at the nearby California Institute of Technology. The company, Earth
Sciences, mainly did seismic instrument design for oil exploration, but had
been purchased by a large aerospace corporation, Teledyne, who managed to get
them this NASA contract. To fulfil the contract they hired a team of two
electronics engineers and two mechanical design engineers (of which I was one)
to add to their existing three draftsmen and two instrument designers.
Assistance on thermal and vibration analysis would come from consultants.
Each lunar seismometer package would
comprise four small seismic detectors: two for horizontal motions, one for
vertical motions, and one for strong motions that would push the others
off-scale ... plus all the electronics needed to transmit the data back to
Earth.
The seismometer
A seismometer is
based on Newtons famous principle, that a body will remain stationary until a
force tries to move it. Its resistance to that force is its inertia. In
its simplest form, a seismometer comprises a lump of heavy metal
(eg. lead) on the end of a pivoting arm. Attached to the heavy lump is a pen,
which is in contact with the surface of a slowly-rotating paper-covered drum.
Normally the pen would draw a straight line, but if an earthquake caused the
instrument to vibrate, the drum would dance around while the heavy lump tried
to stay still. The relative motion between the stationary pen and the shaking
drum would produce a wiggly line on the paper. The height of the wiggles
indicates the earthquake force, while the distance between wiggle peaks
can reveal such things as the distance from, and terrain traversed between, the
epicentre of the quake and the seismometer. By comparing recordings of the same
quake by instruments located at different points on the Earth's surface,
investigations of the crustal structure can be made.
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There would be no geologists on the Moon to
read the seisometer, so the drum and pen were replaced by an electrical
capacitor system — two fixed plates replaced the drum, and in the middle a
third plate was attached to a heavy weight, replacing the pen. The
capacitor was part of an electrical circuit which would be unbalanced by
movement of the plates. This unbalance was made proportional to the amount of
movement, and could be transmitted back to Earth.
Seismic Success
The Apollo Passive Seismic Experiment
studied the propagation of seismic waves through the Moon and provided our most
detailed look at the Moon’s internal structure. The Apollo 11 seismometer
returned data for just three weeks but provided a useful first look at lunar
seismology. More advanced units were deployed at the Apollo 12, 14, 15, and
16 landing sites and transmitted data to Earth until September 1977. Each
of these seismometers measured all three components of ground displacement
(up-down, north-south, and east-west).
If a seismic event is observed by three or
more seismometers, the time and location of the event can be determined.
Because seismic waves from distant events travel deeper into the Moon than
waves from nearby events, by measuring events at various distances from the
seismometer, one can determine how seismic velocities vary with depth in the
Moon. In turn, this information can be used to study the Moon's internal
structure. Most of the events observed by the seismometers were due either to
moonquakes or to meteoroid impacts. However, the third stages of several Saturn
V rockets and the ascent stages of several lunar modules were deliberately
crashed into the Moon after these spacecraft were no longer needed. These
crashes produced seismic events of known times and locations and helped to
calibrate the network of seismometers.
The experiment proved several important
scientific results.
Knowledge of the Moon's internal structure. Like the Earth, the Moon has a crust,
mantle and core. The lunar crust is rich in the mineral plagioclase and has an
average crustal thickness of 60-70 kilometres, which is about three times the
average crustal thickness on earth. The lunar mantle lies between the
crust and the core and consists mostly of the minerals olivine and pyroxene. The
core is mostly composed of iron and sulphur and extends from the centre of the
Moon out to a radius of no more than 450 kilometres; ie., the core radius is
less than 25% of the Moon's radius, which is quite small. In comparison
the Earth's core radius is 54% of the Earth's radius.
The source of lunar seismic sources. More than 1.700 meteoroid impacts were
recorded by the seismometer network, with impactor masses estimated to be
between 0.5 and 5,000 kilograms. Most moonquakes occur at depths of 800 to
1,000 kilometres. These occur at monthly intervals at about 100 distinct
sites, indicating that these moonquakes are caused by stresses such as changes
in lunar tides as the Moon orbits the Earth. These moonquakes are quite small,
mostly with Richter scale magnitudes less than 2. The amount of energy
released by earthquakes, in a typical year is about 10 million times larger
than that released by moonquakes in a year. Only a few near-surface moonquakes
were detected.
Attenuation of seismic waves. Meteoroid impacts cause heavy
fracturing in the upper 20 kilometres of the lunar crust. These fractures in
turn cause scattering of any seismic waves that travel through these regions.
Below 20 kilometres, seismic wave scatterring decreases as a result of either
the closure of these fractures, due to the increasing pressure, or because of a
change in chemical composition of the crust. In the mantle, seismic
waves are attenuated much less on the Moon than they are on the Earth.
Attenuation of seismic waves is enhanced at higher temperatures and also in the
presence of water, so the low levels of attenuation on the Moon indicates
a cold, dry interior. Because the Moon is smaller than Earth, it is expected to
have cooled more rapidly, producing a cold interior. The absence of water may
be due either to the failure of the Moon to accumulate water when it formed or
to subsequent loss of water to space. Below 1,000 kilometres depth, seismic
wave attenuation increases. possibly indicating the presence of a small amount
of molten rock.
Information courtesy of the Lunar and
Planetary Institute.
It was thus possible to
detect moonquakes, their intensity and direction by comparing the signals
from the north-south and east-west horizontal detectors, and with identical
instruments left at other locations on the lunar surface.
The horizontal and vertical motion detectors
were mounted on a 'gimbal platform', which was capable of being tilted in any
direction by two small electric motors (stepper motors) which rotated in
discrete steps (An example of such a motor is the one in an electric clock,
which makes the second hand move one step per second.) Through suitable
gearing, the motors could be made to tilt the platform in steps of one
arcsecond. which was the angle necessary to measure the ‘bulge tilt'. The
motors were controlled by the output from the very sensitive detectors, which
would move the capacitor blade as the instrument tilted. By transmitting back
to Earth the number of 'stepping impulses' needed for each motor
to re-centre the capacitor blades, the tilt of the bulge would be measured, and
hence the lunar crustal structure (if any) would be detected (which
it was, though different from that of Earth).
To prevent damage during launch, the arms of
each of the three detectors were firmly clamped against a fixed stop using
small stainless steel bellows. After manufacture and testing, each instrument
was placed in a 'protected mode' by inflating the balloons using an inert
gas introduced through an interconnecting tube made of stainless-steel
hypodermic needle tubing. The tube was sealed with a small
electrically-operated explosive device which blew a hole in the pressurised
tube after deployment on the lunar surface by the astronauts.
To help the astronauts carry the
device, special lugs were fitted that attached to one of the astronauts’
special carrying sticks. For levelling, the instrument was housed in a
three-legged support cradle — stable on any rough surface — which could be
tilted in any direction. Because an ordinary bubble-level would not work in
such hot conditions, a simple but ingenious device was incorporated that told
the astronaut when the instrument was level. It was a black ball resting
in a saucer-shaped dish, in the centre of which was painted a round black spot
the same size as the ball. When the astronaut looked clown at this device he
simply had to tilt the instrument until the ball and spot merged into one.
Fine tuning was done by using the stepper motors and feedback from the
capacitors.
Putting it to work
Initial testing of the seismometers proved
to be a problem because of their extreme sensitivity. They were mounted on the
company seismic test platform, whist was a large, heavy concrete block sitting
in a sandpit — the sand absorbed unwanted vibrations and the inertia
of the concrete block contributed to its seismic ‘quiet’. But we found
that vibrations from local street traffic, not detectable with earthbound
seismometers, were a real problem on this ultra-sensitive lunar model. At one
point a mysterious rhythmic seism (earth tremor) was identified, purely by
accident, when a technician’s wife phoned from home to tell him of unusually
high waves breaking on the beach outside her window. By asking her to tell him
when each wave broke, he found that it coincided with the readings on his lunar
test instrument, 40 kilometres away!
As mentioned above, the design work was done
from a computer vibration specification, since in 1966 the rocket and engine
had not been built. Our original prototype did not pass the tests on the
vibration table, so we hired a consultant from the University of California
to analyse the design. Unfortunately, he was unable to improve it enough
to make it survive testing.
The panic button
was pushed and the consultant and l were flown to meet with the project
engineers from Rocketdyne (engines), Martin Marietta (rocket shell) and NASA
(the 'customer’). As computer design in 1966 was an esoteric art the consultant
and I managed to pick enough holes in their design assumptions to force them to
reduce vibration specifications to a point where we knew our prototype design
would pass. History confirms that we were correct, for all the seismometers
that went to the moon performed to the requirement. The solution to keeping the
instrument's temperature to plus or minus 1℃ when the outside
temperature varied from -100℃ to +120℃, was solved by using
the same sort of blankets now used for rescue purposes ... several layers of
shiny aluminium-coated mylar. Final adjustments were made with an internal
thermostat and small solar-powered heater.
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Checking the lunar seismometer data, including the impacts of spent rocket stages. |
We managed to complete the design and testing within the time and financial budgets, meeting NASA specifications for performance and weight.
The first seismometer was taken to the Moon
in 1969 by the Apollo 11 crew, and operated for only a few weeks. Subsequent
missions carried longer-living versions; they also took along a hand-held
'thumper' device to create artificial mini-moonquakes ... described as a bit
like simultaneously firing both barrels of a shotgun at the lunar surface.
Other tests included having discarded Apollo lunar landers crash onto the
Moon's surface at several hundred kilometres per hour, producing moonquakes.
Unfortunately, after several years of
operation, NASA turned off the radio receivers in Houston because they had run
out of money. But by that time the scientists had a much better idea of the
origins and structure of the Moon, so the lunar seismometer project was a resounding
success.
Bob Sadler was born in
England in 1926. His career in engineering and design took him to Canada, the
USA and New Zealand, during which time he worked on early computers, printers
and other mechanical devices as well as the lunar seismometers. Now retired and
living in Queensland, he became an Australian citizen in 1992.
The Original Article, thanks to Michael Abdilla from the Space Association of Australia for providing this link at the State Library of Victoria
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| Pages 14 and 15, Feb-March 2000, Sky & Space magazine |
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| Pages 16 and 17, Feb-March 2000, Sky & Space magazine |
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| Page 18, Feb-March 2000, Sky & Space magazine |
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| Buzz Aldrin with the seismometer at Tranquility Base, the Apollo 11 landing site. Caption at NASA: Neil took this picture at about 111:06:34 (Mission Elapsed Time-DJM]. Buzz has now deployed both the east and west solar panels on the seismometer. He is looking toward the LM [Lunar Module], perhaps to get a reference for his alignment. |
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