Richard Feynman's NASA Space Shuttle Challenger Disaster Report Appendix

Personal observations on the reliability of the Shuttle     
 by R. P. Feynman, 1986.

Richard Feynman With Neil Armstrong at  the Rogers Commission Report Press Conference, June 1986.

Introduction

   It appears that there are enormous differences of opinion as to the
probability of a failure with loss of vehicle and of human life. The
estimates range from roughly 1 in 100 to 1 in 100,000. The higher
figures come from the working engineers, and the very low figures from
management. What are the causes and consequences of this lack of
agreement? Since 1 part in 100,000 would imply that one could put a
Shuttle up each day for 300 years expecting to lose only one, we could
properly ask "What is the cause of management's fantastic faith in the
machinery?"

   We have also found that certification criteria used in Flight
Readiness Reviews often develop a gradually decreasing strictness. The
argument that the same risk was flown before without failure is often
accepted as an argument for the safety of accepting it again. Because
of this, obvious weaknesses are accepted again and again, sometimes
without a sufficiently serious attempt to remedy them, or to delay a
flight because of their continued presence.

   There are several sources of information. There are published criteria
for certification, including a history of modifications in the form of
waivers and deviations. In addition, the records of the Flight
Readiness Reviews for each flight document the arguments used to
accept the risks of the flight. Information was obtained from the
direct testimony and the reports of the range safety officer, Louis
J. Ullian, with respect to the history of success of solid fuel
rockets. There was a further study by him (as chairman of the launch
abort safety panel (LASP)) in an attempt to determine the risks
involved in possible accidents leading to radioactive contamination
from attempting to fly a plutonium power supply (RTG) for future
planetary missions. The NASA study of the same question is also
available. For the History of the Space Shuttle Main Engines,
interviews with management and engineers at Marshall, and informal
interviews with engineers at Rocketdyne, were made. An independent
(Cal Tech) mechanical engineer who consulted for NASA about engines
was also interviewed informally. A visit to Johnson was made to gather
information on the reliability of the avionics (computers, sensors,
and effectors). Finally there is a report "A Review of Certification
Practices, Potentially Applicable to Man-rated Reusable Rocket
Engines," prepared at the Jet Propulsion Laboratory by N. Moore, et
al., in February, 1986, for NASA Headquarters, Office of Space
Flight. It deals with the methods used by the FAA and the military to
certify their gas turbine and rocket engines.  These authors were also
interviewed informally.

Solid Rockets (SRB)

   An estimate of the reliability of solid rockets was made by the range
safety officer, by studying the experience of all previous rocket
flights. Out of a total of nearly 2,900 flights, 121 failed (1 in
25). This includes, however, what may be called, early errors, rockets
flown for the first few times in which design errors are discovered
and fixed. A more reasonable figure for the mature rockets might be 1
in 50. With special care in the selection of parts and in inspection,
a figure of below 1 in 100 might be achieved but 1 in 1,000 is
probably not attainable with today's technology. (Since there are two
rockets on the Shuttle, these rocket failure rates must be doubled to
get Shuttle failure rates from Solid Rocket Booster failure.)

   NASA officials argue that the figure is much lower. They point out
that these figures are for unmanned rockets but since the Shuttle is a
manned vehicle "the probability of mission success is necessarily very
close to 1.0." It is not very clear what this phrase means. Does it
mean it is close to 1 or that it ought to be close to 1? They go on to
explain "Historically this extremely high degree of mission success
has given rise to a difference in philosophy between manned space
flight programs and unmanned programs; i.e., numerical probability
usage versus engineering judgment." (These quotations are from "Space
Shuttle Data for Planetary Mission RTG Safety Analysis," Pages 3-1,
3-1, February 15, 1985, NASA, JSC.) It is true that if the probability
of failure was as low as 1 in 100,000 it would take an inordinate
number of tests to determine it ( you would get nothing but a string
of perfect flights from which no precise figure, other than that the
probability is likely less than the number of such flights in the
string so far). But, if the real probability is not so small, flights
would show troubles, near failures, and possible actual failures with
a reasonable number of trials. and standard statistical methods could
give a reasonable estimate. In fact, previous NASA experience had
shown, on occasion, just such difficulties, near accidents, and
accidents, all giving warning that the probability of flight failure
was not so very small. The inconsistency of the argument not to
determine reliability through historical experience, as the range
safety officer did, is that NASA also appeals to history, beginning
"Historically this high degree of mission success..."

   Finally, if we are to replace standard numerical probability usage
with engineering judgment, why do we find such an enormous disparity
between the management estimate and the judgment of the engineers? It
would appear that, for whatever purpose, be it for internal or
external consumption, the management of NASA exaggerates the
reliability of its product, to the point of fantasy.

   The history of the certification and Flight Readiness Reviews will not
be repeated here. (See other part of Commission reports.) The
phenomenon of accepting for flight, seals that had shown erosion and
blow-by in previous flights, is very clear. The Challenger flight is
an excellent example. There are several references to flights that had
gone before. The acceptance and success of these flights is taken as
evidence of safety. But erosion and blow-by are not what the design
expected. They are warnings that something is wrong. The equipment is
not operating as expected, and therefore there is a danger that it can
operate with even wider deviations in this unexpected and not
thoroughly understood way. The fact that this danger did not lead to a
catastrophe before is no guarantee that it will not the next time,
unless it is completely understood. When playing Russian roulette the
fact that the first shot got off safely is little comfort for the
next. The origin and consequences of the erosion and blow-by were not
understood. They did not occur equally on all flights and all joints;
sometimes more, and sometimes less.  Why not sometime, when whatever
conditions determined it were right, still more leading to
catastrophe?

  In spite of these variations from case to case, officials behaved as
if they understood it, giving apparently logical arguments to each
other often depending on the "success" of previous flights. For
example. in determining if flight 51-L was safe to fly in the face of
ring erosion in flight 51-C, it was noted that the erosion depth was
only one-third of the radius. It had been noted in an experiment
cutting the ring that cutting it as deep as one radius was necessary
before the ring failed. Instead of being very concerned that
variations of poorly understood conditions might reasonably create a
deeper erosion this time, it was asserted, there was "a safety factor
of three." This is a strange use of the engineer's term ,"safety
factor." If a bridge is built to withstand a certain load without the
beams permanently deforming, cracking, or breaking, it may be designed
for the materials used to actually stand up under three times the
load. This "safety factor" is to allow for uncertain excesses of load,
or unknown extra loads, or weaknesses in the material that might have
unexpected flaws, etc. If now the expected load comes on to the new
bridge and a crack appears in a beam, this is a failure of the
design. There was no safety factor at all; even though the bridge did
not actually collapse because the crack went only one-third of the way
through the beam. The O-rings of the Solid Rocket Boosters were not
designed to erode. Erosion was a clue that something was wrong.
Erosion was not something from which safety can be inferred.

  There was no way, without full understanding, that one could have
confidence that conditions the next time might not produce erosion
three times more severe than the time before. Nevertheless, officials
fooled themselves into thinking they had such understanding and
confidence, in spite of the peculiar variations from case to case. A
mathematical model was made to calculate erosion. This was a model
based not on physical understanding but on empirical curve fitting. To
be more detailed, it was supposed a stream of hot gas impinged on the
O-ring material, and the heat was determined at the point of
stagnation (so far, with reasonable physical, thermodynamic laws). But
to determine how much rubber eroded it was assumed this depended only
on this heat by a formula suggested by data on a similar material. A
logarithmic plot suggested a straight line, so it was supposed that
the erosion varied as the .58 power of the heat, the .58 being
determined by a nearest fit. At any rate, adjusting some other
numbers, it was determined that the model agreed with the erosion (to
depth of one-third the radius of the ring). There is nothing much so
wrong with this as believing the answer! Uncertainties appear
everywhere. How strong the gas stream might be was unpredictable, it
depended on holes formed in the putty. Blow-by showed that the ring
might fail even though not, or only partially eroded through. The
empirical formula was known to be uncertain, for it did not go
directly through the very data points by which it was
determined. There were a cloud of points some twice above, and some
twice below the fitted curve, so erosions twice predicted were
reasonable from that cause alone. Similar uncertainties surrounded the
other constants in the formula, etc., etc. When using a mathematical
model careful attention must be given to uncertainties in the model.

Liquid Fuel Engine (SSME)

  During the flight of 51-L the three Space Shuttle Main Engines all
worked perfectly, even, at the last moment, beginning to shut down the
engines as the fuel supply began to fail. The question arises,
however, as to whether, had it failed, and we were to investigate it
in as much detail as we did the Solid Rocket Booster, we would find a
similar lack of attention to faults and a deteriorating
reliability. In other words, were the organization weaknesses that
contributed to the accident confined to the Solid Rocket Booster
sector or were they a more general characteristic of NASA? To that end
the Space Shuttle Main Engines and the avionics were both
investigated. No similar study of the Orbiter, or the External Tank
were made.

  The engine is a much more complicated structure than the Solid
Rocket Booster, and a great deal more detailed engineering goes into
it. Generally, the engineering seems to be of high quality and
apparently considerable attention is paid to deficiencies and faults
found in operation.

   The usual way that such engines are designed (for military or
civilian aircraft) may be called the component system, or bottom-up
design. First it is necessary to thoroughly understand the properties
and limitations of the materials to be used (for turbine blades, for
example), and tests are begun in experimental rigs to determine
those. With this knowledge larger component parts (such as bearings)
are designed and tested individually. As deficiencies and design
errors are noted they are corrected and verified with further
testing. Since one tests only parts at a time these tests and
modifications are not overly expensive. Finally one works up to the
final design of the entire engine, to the necessary
specifications. There is a good chance, by this time that the engine
will generally succeed, or that any failures are easily isolated and
analyzed because the failure modes, limitations of materials, etc.,
are so well understood. There is a very good chance that the
modifications to the engine to get around the final difficulties are
not very hard to make, for most of the serious problems have already
been discovered and dealt with in the earlier, less expensive, stages
of the process.

   The Space Shuttle Main Engine was handled in a different manner,
top down, we might say. The engine was designed and put together all
at once with relatively little detailed preliminary study of the
material and components.  Then when troubles are found in the
bearings, turbine blades, coolant pipes, etc., it is more expensive
and difficult to discover the causes and make changes. For example,
cracks have been found in the turbine blades of the high pressure
oxygen turbopump. Are they caused by flaws in the material, the effect
of the oxygen atmosphere on the properties of the material, the
thermal stresses of startup or shutdown, the vibration and stresses of
steady running, or mainly at some resonance at certain speeds, etc.?
How long can we run from crack initiation to crack failure, and how
does this depend on power level? Using the completed engine as a test
bed to resolve such questions is extremely expensive. One does not
wish to lose an entire engine in order to find out where and how
failure occurs.  Yet, an accurate knowledge of this information is
essential to acquire a confidence in the engine reliability in use.
Without detailed understanding, confidence can not be attained.

   A further disadvantage of the top-down method is that, if an
understanding of a fault is obtained, a simple fix, such as a new
shape for the turbine housing, may be impossible to implement without
a redesign of the entire engine.

   The Space Shuttle Main Engine is a very remarkable machine. It has
a greater ratio of thrust to weight than any previous engine. It is
built at the edge of, or outside of, previous engineering
experience. Therefore, as expected, many different kinds of flaws and
difficulties have turned up. Because, unfortunately, it was built in
the top-down manner, they are difficult to find and fix. The design
aim of a lifetime of 55 missions equivalent firings (27,000 seconds of
operation, either in a mission of 500 seconds, or on a test stand) has
not been obtained. The engine now requires very frequent maintenance
and replacement of important parts, such as turbopumps, bearings,
sheet metal housings, etc. The high-pressure fuel turbopump had to be
replaced every three or four mission equivalents (although that may
have been fixed, now) and the high pressure oxygen turbopump every
five or six. This is at most ten percent of the original
specification. But our main concern here is the determination of
reliability.

   In a total of about 250,000 seconds of operation, the engines have
failed seriously perhaps 16 times. Engineering pays close attention to
these failings and tries to remedy them as quickly as possible. This
it does by test studies on special rigs experimentally designed for
the flaws in question, by careful inspection of the engine for
suggestive clues (like cracks), and by considerable study and
analysis. In this way, in spite of the difficulties of top-down
design, through hard work, many of the problems have apparently been
solved.

   A list of some of the problems follows. Those followed by an
asterisk (*) are probably solved:

   1.Turbine blade cracks in high pressure fuel turbopumps (HPFTP). (May have been solved.)

   2.Turbine blade cracks in high pressure oxygen turbopumps (HPOTP).

   3.Augmented Spark Igniter (ASI) line rupture.*

   4.Purge check valve failure.*

   5.ASI chamber erosion.*

   6.HPFTP turbine sheet metal cracking.

   7.HPFTP coolant liner failure.*

   8.Main combustion chamber outlet elbow failure.*

   9.Main combustion chamber inlet elbow weld offset.*

  10.HPOTP subsynchronous whirl.*

  11.Flight acceleration safety cutoff system (partial failure in a redundant system).*

  12.Bearing spalling (partially solved).

  13.A vibration at 4,000 Hertz making some engines inoperable, etc.

   Many of these solved problems are the early difficulties of a new
design, for 13 of them occurred in the first 125,000 seconds and only
three in the second 125,000 seconds. Naturally, one can never be sure
that all the bugs are out, and, for some, the fix may not have
addressed the true cause. Thus, it is not unreasonable to guess there
may be at least one surprise in the next 250,000 seconds, a
probability of 1/500 per engine per mission. On a mission there are
three engines, but some accidents would possibly be contained, and
only affect one engine. The system can abort with only two
engines. Therefore let us say that the unknown suprises do not, even
of themselves, permit us to guess that the probability of mission
failure do to the Space Shuttle Main Engine is less than 1/500. To
this we must add the chance of failure from known, but as yet
unsolved, problems (those without the asterisk in the list
above). These we discuss below. (Engineers at Rocketdyne, the
manufacturer, estimate the total probability as 1/10,000. Engineers at
marshal estimate it as 1/300, while NASA management, to whom these
engineers report, claims it is 1/100,000. An independent engineer
consulting for NASA thought 1 or 2 per 100 a reasonable estimate.)

   The history of the certification principles for these engines is
confusing and difficult to explain. Initially the rule seems to have
been that two sample engines must each have had twice the time
operating without failure as the operating time of the engine to be
certified (rule of 2x). At least that is the FAA practice, and NASA
seems to have adopted it, originally expecting the certified time to
be 10 missions (hence 20 missions for each sample). Obviously the best
engines to use for comparison would be those of greatest total (flight
plus test) operating time -- the so-called "fleet leaders." But what
if a third sample and several others fail in a short time? Surely we
will not be safe because two were unusual in lasting longer. The short
time might be more representative of the real possibilities, and in
the spirit of the safety factor of 2, we should only operate at half
the time of the short-lived samples.

   The slow shift toward decreasing safety factor can be seen in many
examples. We take that of the HPFTP turbine blades. First of all the
idea of testing an entire engine was abandoned. Each engine number has
had many important parts (like the turbopumps themselves) replaced at
frequent intervals, so that the rule must be shifted from engines to
components. We accept an HPFTP for a certification time if two samples
have each run successfully for twice that time (and of course, as a
practical matter, no longer insisting that this time be as large as 10
missions). But what is "successfully?" The FAA calls a turbine blade
crack a failure, in order, in practice, to really provide a safety
factor greater than 2. There is some time that an engine can run
between the time a crack originally starts until the time it has grown
large enough to fracture. (The FAA is contemplating new rules that
take this extra safety time into account, but only if it is very
carefully analyzed through known models within a known range of
experience and with materials thoroughly tested. None of these
conditions apply to the Space Shuttle Main Engine.

   Cracks were found in many second stage HPFTP turbine blades. In one
case three were found after 1,900 seconds, while in another they were
not found after 4,200 seconds, although usually these longer runs
showed cracks. To follow this story further we shall have to realize
that the stress depends a great deal on the power level.  The
Challenger flight was to be at, and previous flights had been at, a
power level called 104% of rated power level during most of the time
the engines were operating. Judging from some material data it is
supposed that at the level 104% of rated power level, the time to
crack is about twice that at 109% or full power level (FPL). Future
flights were to be at this level because of heavier payloads, and many
tests were made at this level. Therefore dividing time at 104% by 2,
we obtain units called equivalent full power level (EFPL). (Obviously,
some uncertainty is introduced by that, but it has not been studied.)
The earliest cracks mentioned above occurred at 1,375 EFPL.

   Now the certification rule becomes "limit all second stage blades
to a maximum of 1,375 seconds EFPL." If one objects that the safety
factor of 2 is lost it is pointed out that the one turbine ran for
3,800 seconds EFPL without cracks, and half of this is 1,900 so we are
being more conservative. We have fooled ourselves in three ways. First
we have only one sample, and it is not the fleet leader, for the other
two samples of 3,800 or more seconds had 17 cracked blades between
them. (There are 59 blades in the engine.) Next we have abandoned the
2x rule and substituted equal time. And finally, 1,375 is where we did
see a crack. We can say that no crack had been found below 1,375, but
the last time we looked and saw no cracks was 1,100 seconds EFPL. We
do not know when the crack formed between these times, for example
cracks may have formed at 1,150 seconds EFPL. (Approximately 2/3 of
the blade sets tested in excess of 1,375 seconds EFPL had cracks. Some
recent experiments have, indeed, shown cracks as early as 1,150
seconds.) It was important to keep the number high, for the Challenger
was to fly an engine very close to the limit by the time the flight
was over.

   Finally it is claimed that the criteria are not abandoned, and the
system is safe, by giving up the FAA convention that there should be
no cracks, and considering only a completely fractured blade a
failure. With this definition no engine has yet failed. The idea is
that since there is sufficient time for a crack to grow to a fracture
we can insure that all is safe by inspecting all blades for cracks. If
they are found, replace them, and if none are found we have enough
time for a safe mission. This makes the crack problem not a flight
safety problem, but merely a maintenance problem.

   This may in fact be true. But how well do we know that cracks
always grow slowly enough that no fracture can occur in a mission?
Three engines have run for long times with a few cracked blades (about
3,000 seconds EFPL) with no blades broken off.

   But a fix for this cracking may have been found. By changing the
blade shape, shot-peening the surface, and covering with insulation to
exclude thermal shock, the blades have not cracked so far.

   A very similar story appears in the history of certification of the
HPOTP, but we shall not give the details here.

   It is evident, in summary, that the Flight Readiness Reviews and
certification rules show a deterioration for some of the problems of
the Space Shuttle Main Engine that is closely analogous to the
deterioration seen in the rules for the Solid Rocket Booster.

Avionics

   By "avionics" is meant the computer system on the Orbiter as well
as its input sensors and output actuators. At first we will restrict
ourselves to the computers proper and not be concerned with the
reliability of the input information from the sensors of temperature,
pressure, etc., nor with whether the computer output is faithfully
followed by the actuators of rocket firings, mechanical controls,
displays to astronauts, etc.

   The computer system is very elaborate, having over 250,000 lines of
code. It is responsible, among many other things, for the automatic
control of the entire ascent to orbit, and for the descent until well
into the atmosphere (below Mach 1) once one button is pushed deciding
the landing site desired. It would be possible to make the entire
landing automatically (except that the landing gear lowering signal is
expressly left out of computer control, and must be provided by the
pilot, ostensibly for safety reasons) but such an entirely automatic
landing is probably not as safe as a pilot controlled landing. During
orbital flight it is used in the control of payloads, in displaying
information to the astronauts, and the exchange of information to the
ground. It is evident that the safety of flight requires guaranteed
accuracy of this elaborate system of computer hardware and software.

   In brief, the hardware reliability is ensured by having four
essentially independent identical computer systems. Where possible
each sensor also has multiple copies, usually four, and each copy
feeds all four of the computer lines. If the inputs from the sensors
disagree, depending on circumstances, certain averages, or a majority
selection is used as the effective input. The algorithm used by each
of the four computers is exactly the same, so their inputs (since each
sees all copies of the sensors) are the same. Therefore at each step
the results in each computer should be identical.  From time to time
they are compared, but because they might operate at slightly
different speeds a system of stopping and waiting at specific times is
instituted before each comparison is made. If one of the computers
disagrees, or is too late in having its answer ready, the three which
do agree are assumed to be correct and the errant computer is taken
completely out of the system. If, now, another computer fails, as
judged by the agreement of the other two, it is taken out of the
system, and the rest of the flight canceled, and descent to the
landing site is instituted, controlled by the two remaining
computers. It is seen that this is a redundant system since the
failure of only one computer does not affect the mission. Finally, as
an extra feature of safety, there is a fifth independent computer,
whose memory is loaded with only the programs of ascent and descent,
and which is capable of controlling the descent if there is a failure
of more than two of the computers of the main line four.

   There is not enough room in the memory of the main line computers
for all the programs of ascent, descent, and payload programs in
flight, so the memory is loaded about four time from tapes, by the
astronauts.

   Because of the enormous effort required to replace the software for
such an elaborate system, and for checking a new system out, no change
has been made to the hardware since the system began about fifteen
years ago. The actual hardware is obsolete; for example, the memories
are of the old ferrite core type. It is becoming more difficult to
find manufacturers to supply such old-fashioned computers reliably and
of high quality. Modern computers are very much more reliable, can run
much faster, simplifying circuits, and allowing more to be done, and
would not require so much loading of memory, for the memories are much
larger.

   The software is checked very carefully in a bottom-up
fashion. First, each new line of code is checked, then sections of
code or modules with special functions are verified. The scope is
increased step by step until the new changes are incorporated into a
complete system and checked. This complete output is considered the
final product, newly released. But completely independently there is
an independent verification group, that takes an adversary attitude to
the software development group, and tests and verifies the software as
if it were a customer of the delivered product. There is additional
verification in using the new programs in simulators, etc. A discovery
of an error during verification testing is considered very serious,
and its origin studied very carefully to avoid such mistakes in the
future. Such unexpected errors have been found only about six times in
all the programming and program changing (for new or altered payloads)
that has been done. The principle that is followed is that all the
verification is not an aspect of program safety, it is merely a test
of that safety, in a non-catastrophic verification. Flight safety is
to be judged solely on how well the programs do in the verification
tests. A failure here generates considerable concern.

   To summarize then, the computer software checking system and
attitude is of the highest quality. There appears to be no process of
gradually fooling oneself while degrading standards so characteristic
of the Solid Rocket Booster or Space Shuttle Main Engine safety
systems. To be sure, there have been recent suggestions by management
to curtail such elaborate and expensive tests as being unnecessary at
this late date in Shuttle history. This must be resisted for it does
not appreciate the mutual subtle influences, and sources of error
generated by even small changes of one part of a program on
another. There are perpetual requests for changes as new payloads and
new demands and modifications are suggested by the users. Changes are
expensive because they require extensive testing. The proper way to
save money is to curtail the number of requested changes, not the
quality of testing for each.

   One might add that the elaborate system could be very much improved
by more modern hardware and programming techniques. Any outside
competition would have all the advantages of starting over, and
whether that is a good idea for NASA now should be carefully
considered.

   Finally, returning to the sensors and actuators of the avionics
system, we find that the attitude to system failure and reliability is
not nearly as good as for the computer system. For example, a
difficulty was found with certain temperature sensors sometimes
failing. Yet 18 months later the same sensors were still being used,
still sometimes failing, until a launch had to be scrubbed because two
of them failed at the same time. Even on a succeeding flight this
unreliable sensor was used again. Again reaction control systems, the
rocket jets used for reorienting and control in flight still are
somewhat unreliable. There is considerable redundancy, but a long
history of failures, none of which has yet been extensive enough to
seriously affect flight. The action of the jets is checked by sensors,
and, if they fail to fire the computers choose another jet to
fire. But they are not designed to fail, and the problem should be
solved.

Conclusions

   If a reasonable launch schedule is to be maintained, engineering
often cannot be done fast enough to keep up with the expectations of
originally conservative certification criteria designed to guarantee a
very safe vehicle. In these situations, subtly, and often with
apparently logical arguments, the criteria are altered so that flights
may still be certified in time. They therefore fly in a relatively
unsafe condition, with a chance of failure of the order of a percent
(it is difficult to be more accurate).

   Official management, on the other hand, claims to believe the
probability of failure is a thousand times less. One reason for this
may be an attempt to assure the government of NASA perfection and
success in order to ensure the supply of funds. The other may be that
they sincerely believed it to be true, demonstrating an almost
incredible lack of communication between themselves and their working
engineers.

   In any event this has had very unfortunate consequences, the most
serious of which is to encourage ordinary citizens to fly in such a
dangerous machine, as if it had attained the safety of an ordinary
airliner. The astronauts, like test pilots, should know their risks,
and we honor them for their courage. Who can doubt that McAuliffe was
equally a person of great courage, who was closer to an awareness of
the true risk than NASA management would have us believe?

   Let us make recommendations to ensure that NASA officials deal in a
world of reality in understanding technological weaknesses and
imperfections well enough to be actively trying to eliminate
them. They must live in reality in comparing the costs and utility of
the Shuttle to other methods of entering space. And they must be
realistic in making contracts, in estimating costs, and the difficulty
of the projects. Only realistic flight schedules should be proposed,
schedules that have a reasonable chance of being met. If in this way
the government would not support them, then so be it. NASA owes it to
the citizens from whom it asks support to be frank, honest, and
informative, so that these citizens can make the wisest decisions for
the use of their limited resources.

For a successful technology, reality must take precedence over
public relations, for nature cannot be fooled.

                                   Space Shuttle Challenger Disaster. Date: January 28th, 1986

"Tractor Beam" Laser Maglev using 30µm Graphene Paper Sheets.

Here we demonstrate an effect which looks like the classic tractor beam seen in science fiction films and tv shows. We have made pieces of thin graphene which can levitate over a bed of rare earth permanent magnets can be ‘pushed’ around or made to spin using a handheld, focusable 5mW 390-405nm laser beam.

The phenomenon demonstrates visibly how it is possible to convert light, either from a laser beam or focused sunlight, to mechanical energy.

This demonstrates simple principles of transferring energy, as well as how motors and engines can operate on the principle of the magnetic spin degrees of freedom to create reversible heat cycles where no atoms or molecules are moved, offering a possible alternative way to harness solar energy.

It also demonstrates the principles of a maglev, how in certain materials diamagnetism can be used to levitate objects off a track of magnets.

Considering that this principle works at room temperature, unlike current high temperature superconductors,  it could be useful as an easy demonstration kit in schools as no cooling liquid, such as liquid nitrogen, is needed.

By depositing ferromagnetic nanoparticles on the graphene thin films it was also found that the films could bend in exposure to the UV light and return to their original position when the exposure stopped, demonstrating a weak hysteresis in the material for potential use as a optical memory material.

The fact that a low-powered laser can move the grapheme from a reasonable distance in a vacuum could mean it could become a practical tool for movement of thin films of grapheme for manufacturing purposes in the not too distant future should graphene become useful in industrial and consumer electronics, perhaps eventually moving to replace silicon as a transistor material in the not too distant future.

Although the band gap in graphene cannot be controlled as easily as it can be in silicon, hence the universality of silicon in transistor and memory technology, researchers are working on ways to create a stable bandgap in graphene via doping and structuring of deformations in thin films of the material. 

Due to the thin nature of the films, robotic tweezers used for holding silicon wafers are relativtive clumsy devices for holding thin films. Hence, using lasers to transport the films at least on assembly lines fitted with magnets could be an attractive way to avoid damaging fragile circuitry etched on the graphene paper. Other functions, such as frictionless motors guided by lasers, may also be applicable from this technology.

Nature of Magnetism

Magnetism is the direct result of electron spin, which can be imagined as a distinct directional arrow attached to each particle. In magnetic materials, when all of these individual arrows point in the same direction they produce the cumulative effect of a magnetic field – e.g, the north/south orientation of magnets.

Down on the nanometer scale, at billionths of a meter, electron spins rapidly communicate with each other. When a spin flips inside a magnet, this disturbance can propagate through the material as a wave, tipping the neighboring spins in its path.

This coherent spin wave, called a magnon by the convention introduced in 1930 by Felix Bloch, is theorised to be what creates the magnetic transport properties in high-temperature superconductivity.T hermal excitations of magnons also affects the specific heats and saturation magnetisation of ferromagnetism, leading to the famous specific heat formula derived by Bloch

In the case of the thermally excited magnons, the Bose-Einstein energy distribution, U,  is valid for a magnon of frequency ω :

Where the integral in is taken over the 1st Brillouin Zone.

In the limit of low temperatures, for the density of states:

The resulting specific heat of the magnons is then found to be:

Energy spectra of magnons in superconductors for example is determined by inelastic neutron scattering experiments. The specific heat of magnons as derived by Bloch in also contribute to the heat conductivity in certain materials, in particular diamonds and have contributions in other carbon atom crystal structures too such as graphene.

The flux of magnons are particularly strong in high temperature superconducting materials, so strong in fact that they reject external magnetic fields entirely, creating the Meissner effect which expels the magnet from the superconductor causing the magnet to levitate above the superconductor once the material has been lowered below a critical temperature Tc.

Developments in superconducting materials technology have produced superconductors which can enter the superconducting phase above the boiling point of liquid nitrogen allowing for table top demonstrations of the Meissner effect.

                    

                                                            

When the temperature of the superconducting material is above the critical temperature, the magnetic field penetrates the superconductor freely. When it is lowered below the critical temperature, the magnetic field is rejected from entering the superconductor causing the magnet to levitate above it.

Diamagnetism

Diamagnetism is defined by the generation of a spontaneous magnetization of a material which

A diamagnetic substance is one whose atoms have no permanent magnetic dipole moment, so the spins of the outer electrons are in a inhomogeneous state. When the orbital motion of electrons of any atom changes it results in diamagnetism.

When an external magnetic field is applied to a diamagnetic substance,(such as water, bismuth, ect), a weak magnetic dipole moment is induced in the direction opposite to the the applied field. This is known as Lenz’ Law in electromagnetism.

In quantum mechanics, the magnetic dipole moment is oriented in the direction of the spin angular momentum of the electrons, hence a change in the magnetic dipole moment experienced by them changes the spin orientation of the electrons to oppose the field in quantised units, represented by arrows, to oppose the external field in the same proportion.




Classically, we can say diamagnetism is the speeding up or slowing down of electrons in their atomic orbits and this results from the changing of the magnetic moment of the orbital in a direction opposing the external field. From any interpretation of the physics, diamagnetic materials repels the external magnetic field.

In the following order of diamagnetic constant, here are a few significant diamagnetic elements from the periodic table.

•             Bismuth

•             Mercury

•             Silver

•             Carbon (Graphite, Graphene, Carbon Nanotubes, Fullerenes, ect)

•             Lead

•             Copper

All materials are somewhat diamagnetic, in that a weak repulsive force is generated by in a magnetic field by the current of the orbiting electron.  This causes the electrons to not uniformly align by themselves and instead form small domains, where only a few electrons have their spins aligned in a particular direction.  This repulsive force is overcome by the strength of the external field.  In this case, all of the electrons in the material will align uniformly in the direction of the applied field lines. This is known as ferromagnetism.

Other materials, however, have stronger repulsive qualities that overcome their natural diamagnetic qualities.  Hence, domains do not form in these materials and the spin directions are truly random. In the presence of an external field, the electrons which will try to spin in opposite directions,

Some Ferromagnetic Elements

•             Iron

•             Nickel

•             Cobalt

•             Gadolinium

•             Dysprosium

Some Paramagnetic Elements

•             Uranium

•             Platinum

•             Aluminum

•             Sodium

•             Oxygen

Diamagnetic Levitation occurs by bringing a diamagnetic material in close proximity to material that produces a magnetic field.  The diamagnetic material will repel the material producing the magnetic field.  Generally, however, this repulsive force is not strong enough to overcome the force of gravity on the Earth's surface.  To cause diamagnetic levitation, both the diamagnetic material and magnetic material must produce a combined repulsive force to overcome the force of gravity.  There are a number of ways to achieve this:

Placing Diamagnetic Material in Strong Electromagnetic Fields

Modern Electromagnets are capable of producing extremely strong magnetic fields.  These electromagnets have been used to levitate many diamagnetic materials including weakly diamagnetic materials such as organic matter. 

A popular educational demonstration involves the placement of small frogs into a strong static electromagnetic field.  The frog, being composed of primarily water, acts as a weak diamagnet and is levitated.

This procedure is completely safe for the frog as the magnetic field is nowhere near strong enough to affect the chemical or electrical processes in complex organic life, with spiders and mice having no signs of illness or injury. With a large enough electromagnet even a human could be levitated, in principle.

Placing Magnetic Material in Strong Diamagnetic Fields

In the case of the superconducting  maglev the diamagnetism is induced by superconductivity in the ceramic material. Superconductors in the Meissner state exhibit perfect diamagnetism, or superdiamagnetism, meaning that the total magnetic field is very close to zero deep inside them (many penetration depths from the surface). This means that the magnetic susceptibility of a superconductor is negative

                  

The fundamental origins of diamagnetism in superconductors and normal materials are very different. In normal materials diamagnetism arises as a direct result of the orbital spin of electrons about the nuclei of an atom induced electromagnetically by the application of an applied field. In superconductors the perfect diamagnetism arises from persistent screening currents which flow to oppose the applied field (the Meissner effect); not solely the orbital spin.

Placing Diamagnetic Material in Strong Magnetic Fields

Advancements in the development of permanent magnets and diamagnetic materials such as pyrolytic graphite have produced a simple method of diamagnetic levitation by simply placing a thin piece of pyrolytic graphite over a strong rare-earth magnet. The pyrolytic graphite is levitated above the magnet.

Placing Magnetic Material in Diamagnetic Fields with a Biasing Magnet

The last method, and most easily duplicated by the average individual, uses a combination of readily available rare-earth magnets and diamagnetic material such as carbon graphite or bismuth.  Through the use of a biasing or compensating magnet, a small rare-earth magnet can be levitated above a piece of diamagnetic material.  For added stability, the small magnet is generally placed between two pieces of diamagnetic material.  Below is a diagram of this method:

All of these demonstration apparatus have been, more or less, mainstream in their incorporation into various niche areas. The study of how to manipulate these effects into technology which has easy to replicate applications is sometimes hard to do as it requires sensitive equipment and the payoff may not be obvious. Superconductivity, for example, when first discovered was hard to achieve and as such it was hard to incorporate outside of a very narrow field of study and application. We are only seeing how much potential the field has today and even so it is only the tip of the iceberg.

In studying these effects and using existing technology we have set up a demonstration of how some of the physics behind these stationary demonstrations can create a dynamic device which could be useful for some applications in microelectronics assembly and miniature robotics to name a few areas. In it, we combine lasers, magnetism and graphene making it a useful gadget in explaining some physics on the side of a weird toy if nothing else!

Reversible Heat Cycles in Graphene – Levitating Laser Driven Motion Devices.

In 1984, the famous American Physicist Richard Feynman gave a talk called “Tiny Machines”. This was to be a follow up of a previous talk he gave in 1954 at Caltech on Nanotechnology titled “There’s Plenty of Room at the Bottom”. Since many advances in microscopic manufacture had been released between 1954 and 1984, Feynman felt that he could give even more insight. In this talk he mentioned, in a brief question and answer session, that the internal mechanism of heat transfer in an internal combustion engine on the nanoscale of atoms could be accomplished using reversible cycles of quantum spin.

“All concept of heat in heat cycles depends on random motion of particles being transferred, however it does not have to be the motion of atoms or molecules vibrating like in a classical, combustion heat engine but it could be the heat of quantum spin, that is the magnetisation directions of the atoms are swishing up and down although the atoms and molecules themselves are remaining in place, offering a different scale at which we can get heat operating systems and reversible cycles.” – Richard Feynman, 1984.

Here, using graphene thin films in diamagnetic levitation, we have developed, what is in effect, the magnetic-based reversible engine that Feynman had alluded to being possible back in the 1980’s.

Graphene, like its parent compound Graphite, is a strongly diamagnetic material, so it can be levitated in the ~1.2 Tesla magnetic fields of permanent NdFeB magnets, rather than superconducting electromagnets.

Graphene is an excellent conductor of heat and electricity. Hence any small changes in temperature or electronic interactions in one region will quickly dissipate to the rest of the material.

Graphene therefore has excellent photothermal properties, so that even a weak light source , such as a focused laser, can heat up the graphene film in one region instantly, which affects its magnetic susceptibility, making it tilt and hence move.  

The fact that graphene also releases heat rapidly allows the process to be instantly reversible, which is what allows the film to move so responsively. Hence, the graphene film does not simply collapse onto the magnet as it loses its magnetic susceptibility, it simply tilts in one region and stays levitating on the region opposite from the laser focus as the heat dissipates before it reaches there.

The sum of the forces, ∑F(B)  , exerted from the magnetic dipoles, ∑m1, on our magnetic chessboard grid on another sum of dipoles in our levitating film, ∑m2, separated in space by a vector r can be calculated using

Where is the ΦB external magnetic field of each dipole magnet in the chessboard grid.
                                                   
Our sheet, which has a real mass, M0 , in zero field, is levitating with an effective mass M0  in a balance of magnetic, ΦB, and gravitation fields, ΦG.

So the sum of gravitational forces ∑F(G)  and the magnetic forces ∑F(B) are therefore balanced:

So every time we change the dipole moment of the levitating film, m2 , we change the effective mass, M2, of the levitating film in the field balance.

We do so by heating it with a light source, in our case a focused 405nm 5mW laser.

The fundamental reason why the graphene paper responds so instantly to light is based on the theory of magnetic deflagration, which is a way of saying that the changes induced to the magnetic dipoles spread like wildfire, changing the overall magnetic susceptibility in the region affected by the heat of the laser.

The local heating induced by the laser sets off a chain reaction of magnetic dipole reversal in the diamagnetic material, which in a quantum mechanics viewpoint, we can represent as a sum of uniform units of spin angular momentum pointed in the direction opposite to the externally applied magnetic force.

The chain reaction should therefore propagate itself by exchanging momentum between the neighbouring dipole moments in a gated but entirely stochastic (i.e. random) fashion in a similar way to how a wildfires spreads

The local heating induced by the laser sets off a chain reaction of magnetic domain reversal, in a similar way to how runaway chemical or physical reactions or even how forest wildfires spread. The magnetic domain reversal changes the diamagnetic constant in the region, hence lowering the levitating film slightly in the direction of the directed beam.

Due to the lowered magnetic susceptibility in the heated region, this region is not contributing to the levitation however the mass is still distributed the same. The overall magnetic dipole reversal therefore increases the overall effective mass in the region heated, hence lowering the levitating film slightly in the direction of the directed beam.

We can think of this like a lever where the side that is lowered is now considered to be the “load” the entire film is bearing.

Since the film is levitating in free space above the magnets, the fulcrum is in the centre of gravity of the film. In this case the work is applied on the side of the fulcrum opposite to the load applied and the resistance son the other side. Hence the direction the film is pushed is towards the load by the force of gravity.

As the graphene “lever” rotates around the fulcrum, the points farther from the fulcrum move faster than points closer to the fulcrum. Therefore a force applied to a point farther from the fulcrum must be less than the force located at a point closer in, because power is the product of force and velocity. Hence by directing the beam toward the edge of the film, we create the load at the edge, which in turn means the force is delivered at the opposite edge and this causes the points to rotate much faster around the fulcrum hence moving the film rapidly.

The responsivity and reversibility of the cycle induced comes from the fact that the levitating diamagnetic graphene film, in the presence of the 1.2 Tesla magnetic, has a metastable magnetic susceptibility. In other words, the direction of the electron spin in the matieral is in a metastable state within the external magnetic field.  This makes sense, energetically, as the levitating graphene film is in a higher energy state than the non-levitating film. Hence, when heat energy is delivered to a magnetic domain, at threshold energy, it then flips from a higher-energy metastable state to a lower-energy stable state, from which energy is released as heat. If enough energy is released, the temperature of the surrounding domains will rise to the point that they also flip, releasing more heat. This can spark a chain reaction that rapidly causes the magnetization of the entire region affected to reverse direction.

So the energy released when a spin changes its spin orientation depends on the external magnetic field. If the field is weak, the energy is small and the heat simply dissipates. In this case, the spins reverse gradually by thermal diffusion, typically over a period of a few milliseconds, as the heat from the applied heat pulse thermally diffuses along the crystal structure.  Above a critical magnetic field, however, the energy released by one spin flipping provides enough activation energy to flip adjacent spins. In this case, a wave of spin flips will propagate through a magnetic domain in the material at a constant speed, reversing all the spins much faster than thermal effects can take action, about 1000 times faster than thermal diffusion, such that the magnetic susceptibility can be changed significantly in the heating region.

The reversibility of the cycle also comes from the fact that the movement of spin waves is severely limited at room temperature, and thermal effects will quickly overwhelm the spin wave with interference away from the heat source. Hence, the heat will dissipate quickly away from the region heated, making the whole cycle reversible.

Applications beyond demonstration.

As with any technology, the applications are only limited by the constraints of physics, engineering and our own necessity and imagination.

Already I can think of uses in contactless assembly manufacturing of graphene circuits from reduced graphene oxide on an assembly line configuration. Laser scribing of graphene oxide to graphene has of course huge potential in the microelectronics industry over the next decade and beyond. This can be done with both UV and IR Laser Etchers.

                        Graphene Circuits, etched by Infrared Laser on Graphene Oxide paper

In the following video, we reduce Graphene Oxide (GO) to Graphene in an interdigitated circuit pattern using a home-built robotic UV laser etching machine.

Perhaps, using a weak UV “Tractor Beam” laser, it might be possible to have contactless assembly line of graphene circuit manufacture.

The delicacy of such films, along with how graphene produced from reduced graphene oxide is vulnerable to defects, could be a main application for which these devices could be designed for, aside from their use in demonstrations.

As shown in Atomic Force Microscope scans of graphene grown from reduced graphene oxide, there is a huge amount of defects. Hence, for small scale manufacture, a way to move the substrate around without damaging it may be crucial.

(A) Contact-mode AFM scan of reduced graphene oxide etched on a graphene oxide paper substrate
(B) Height profile through the dashed line shown in part A.
(C) Histogram of platelet thicknesses from images of 140 platelets. The mean thickness is
1.75 nm.
(D) Histogram of diameters from the same 140 platelets.

Although other techniques for growing graphene exist, growing graphene by reducing graphene oxide is the most efficient method and currently the easiest. Hence developing new manufacturing technologies for optimizing its production is well worth doing.

In other experiments, levitating micro-sized graphene wheels on circular rare-earth magnets have been driven to speeds of about 200 rpm with UV lasers.

Just like in the force-load-fulcrum explanation of operation the wheels move based on the analogous torque-load-axis, hence the laser directed at one end of the wheel will create a torque of motion at the opposite end towards the strain induced by the load created as the laser lowers the magnetic susceptibility on one side.

The torque exerted by one magnetic dipole moment m1 on another m2 separated in space by a vector r can be calculated using the equation of torque under a field:

Hence the laser directed at one end of the wheel will create a torque of motion at the opposite end towards the strain induced by the load created as the laser lowers the magnetic susceptibility on one side

This may be of importance in the field of nanorobotics.


Another application which could be combined with this technology is using laser operated tweezers based on graphene oxide films coated with ferromagnetic nanoparticles which, under application of a laser, can create a phase change in the magnetization of the particles by lowering the magnetic susceptibility of some of the particles to the point at which the direction of magnetization becomes disordered (at the Curie point) in the particles exposed to the laser.

The Curie Point refers to a characteristic point at which a material is at the boundary of having its magnetic dipole moments as being in the ferromangetic regime.

In Ferromagnetic materials, the magnetic moments are aligned at random at temperatures above the Curie point, where the magnetic suceptability is lowered. and become ordered at temperatures below the Curie Point, where magnetic susceptability is highered.

As the temperature is increased towards the Curie point, the alignment (magnetization) within each domain decreases. Above the Curie point, the material is purely paramagnetic and there are no magnetized domains of aligned moments

Ferromagnetic nanoparticles consist of intrinsic magnetic moments which are separated into domains called Weiss domains. This can result in some nanostructured ferromagnetic materials having no spontaneous magnetism as domains can be engineered to potentially balance each other out.
The position of nanoparticles can therefore have different orientations around the surface than the main part (bulk) of the material. This property directly affects the Curie Temperature as there can be a bulk Curie Temperature T_B and a different surface Curie Temperature T_S for a material.

This allows for the surface Curie Temperature to be ferromagnetic above the bulk Curie Temperature when the main state is disordered, i.e. Ordered and disordered states occur simultaneously.

The change in magnetic field orientation of the nanoparticle coating in turn causes the magnetic dipoles in the graphene oxide molecules to change shape slightly, making the crystal lattice longer in one dimension and shorter in other dimensions, hence creating a stress in the material and inducing a change in shape. This process is reversible, and as such has a hysteresis. Hence this allows the material to have a shape-memory.


This may eventually allow for a new scale at which motorized systems can be pushed to work at. Lack of electrical components would also mean liquids provide no damage to the function aside from increased turbulence which would require a more powerful laser to work against.

Bilayer paper-like materials based on graphene-based platelets and ferromagnetic nanoparticles can also be engineered to strongly deform in response to other force-bearing stimuli such as differentials in temperature.


Light-responsive graphene oxide (GO) + ferromagnetic nanoparticle composite fibres fibers can be prepared by the positioned laser deposition of ferromagnetic magnetite (Fe3O4)  nanoparticles onto a GO substrate.
The ferromagnetic nanoparticle layer is about 10 µm thick.

Magnetite nanoparticles, on the order of 1 and 100 nanometers, are important for research in superparamagnetic systems, at which the nanoparticles have very large magnetic susceptibilities which allow for fast manipulation of the magnetization of the system, which can occur using light absorption.

When exposed to UV laser light, the asymmetric (Fe3O4) /(GO) fibers display complex, well-controlled motion/deformation in a predetermined manner.

The bilayer material also has other interesting physical properties. For example, the surface of the graphene oxide layer is magnetically conducting but electrically insulating while the surface of the ferromagnetic nanoparticle layer is electrically and magnetically conducting. This electrical asymmetry could come in handy for applications.

For this more research is needed for how the actuators deform under certain stimuli. Also on the to do list: study curling response times and how stable the devices are.

These fibers can function not only as a single-fiber flexing actuator under UV laser alternation but also as a new platform for micromechanical systems, (MEMS), Nanomechanical systems (NEMS) optoelectronics, smart textiles, ect.

All in all, these gadgets are fun to design and build and along with it having applications for a science demonstration it may also be a welcome tool for contactless graphene manufacture, which I will hope to demonstrate online shortly using laser scribing of graphene.

Over the coming weeks and months I will hope to put up a few more design specifications, matlab simulations and other things so that if anyone wants to replicate this device themselves (maybe even doing it cheaper which would be impressive!). I will also be planning, very soon, to sell, for the moment at least, ten of these in small version kits (more if people are interested). Each one cost around $70 to build, for the rare earth magnet tiles, focusable laser and about 4 pieces of graphene paper thrown in. The large kit would obviously be more expensive, due to the sheer number of magnets, and was designed for manufacturing and positioning of the films and testing them with various lasers for etching circuits.

If there is a significant interest from people wanting to buy these I will make more. An added bonus of this would be that as they are bought, the price will go down in the manufacture as the demand would be, as the economists say, a financial security in and of itself. If there is no real interest then I will just update the blog with the specifications and that will be that.

Further Reading (for the Biophysics and Medically inclined):

03/08/12 - Magnetic Levitation Detects Proteins, Could Diagnose Disease 

magnetic levitation could also find use in diagnosing disease. Researchers at Harvard have shown that they can detect proteins in blood using MagLev.

The researchers, led by George Whitesides, use levitation to detect a change in the density of porous gel beads that occurs when a protein binds to ligands inside the beads.

The lower the bead levitates, the more protein it holds. The method could work for detecting disease proteins in people's blood samples in the developing world: The magnets cost only about $5 each, and the device requires no electricity or batteries.

Because the beads are visible to the naked eye, researchers can make measurements with a simple ruler with a millimeter scale."

To analyze the interactions of a protein, bovine carbonic anhydrase, with small-molecule inhibitors, he and his colleagues attached the inhibitors to the inside surfaces of colored diamagnetic beads, with a different color for each of the five molecules. They added the beads to a cuvette containing the protein in a paramagnetic gadolinium(III)-containing buffer, and allowed the protein to diffuse into the beads over several days.

The researchers placed the cuvette between two magnets oriented with their north poles facing each other at the top and bottom of the cuvette. Three forces acted on the beads. The magnets pushed the beads from the bottom of the cuvette. Meanwhile, the gadolinium ions moved toward – and displaced the beads away from -- the magnets. And, finally, gravity pulled the beads downward. These three forces balanced out so that the beads levitated at a height determined by their density. Because high-affinity inhibitors bound more protein, their beads had higher densities and levitated lower. By measuring the distance from the bottom of the cuvette to the beads, the researchers could calculate the binding affinities of the inhibitors.

The researchers realized that their method could also measure concentrations of a protein in a sample if they measured the height of beads coated with a single high-affinity inhibitor. As a proof-of-principle experiment for disease protein detection, the investigators used this method to measure concentrations of a protein added to blood samples.

The main advantages of the levitation system are its low cost and portability. Rare-earth magnets cost only about $5 each, and the process required no electricity or batteries. Because the beads are visible to the naked eye, researchers can make measurements with a simple ruler with a millimeter scale and it would not be difficult to perform such measurements automatically with a computer. Moreover, with the increasing demand for lab-on-a-chip devices with biological fluid and particle scanning capability for versatile medical scanners, the applications of science like this could be very useful indeed for medical diagnostic technology.