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TITLE: Telemetry Analysis
A caiirciion ol articles on the historical, operational, doctrinal, and theoretical aspects ol intelligence.
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Some of the ways in which Soviet missile flight* and the missiles themselves can be reconstructed by monitoring their signals.
TELEMETRY ANALYSIS David S. Brandwein
cable linking It to tbe launchut. and tba "bird" nfts off to begin ia trip into space. Froa the moment the umbilical cable iaDitbe inrssfle's designers must rely on telemetry (measurements of key variables converted Into electrical signals and radioed to ground stations) for their observation of the perfoxrnance of its components.
While then night these multi-channel telexnetry signals are received at ground stations along the trajectory and relayed back to the control center, where tbe measurements are displayed, usually In the form of line traces, one for each channel, on long strips of paper. Anxious engineers dusteret of these "analogtoirst quick look Along with tbe records, the Instru-mcnUtion specialistsey to the assignment of the telemetry channels to as to Identify which trace is recording which kind of measurementist of cabbrariont, conversion factors fora given trace deflection Into so many units of pressure,flow rate, or other variable.
All this must be done by anyLunch facility, whether It be the US. Atlantic or Pacific cuisue range or the Soviet sites at Tyuratam or Eapustin Tar. Thus when we Intercept Sovietwe may be able to ose it to measure the performance of Soviet missiles. There are two very serious handicaps, however, first, tbe intercept usually coven only the time when the missile Is above the horizon of the place of inter ception, and second, we haveey to the diarmel assignmentsist of calibrations. The analyst can do little about tbe first banJkop, thisroblem for theHow we seek to overcome the second ooe and the rinds of information we get when we succeed are described below.
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Which is Which?
Id trying to identify tbe various Soviet measurements, we nuke use first of alt of tbe fact tbat certain basic measurements are required on any flight, regardless of what additional speeiali/ed ones may be called for. For Instance, the propulsion system will alwayseasurement of acceleration and one of thrust chamber pressure; and if the engine fa liquid-fueled, with gas-driven turbopumprthe prnpellants in, then we are likely to ace pain of measurements
and oiidizer and readings of gas generator pressure, turbine speed, and fuel and rrrirltrer flowiquid propeHanf nasslle stage with tbe rornrnon mstrumentation on the propulsion and propeUant feed systems Is pictured schematically fn Figuree thus know what to look for and can search the Soviet telemetry for counterparts of readingsisnle lights.
Iftentative identification Is made, wo can then apply various tests, based on the laws of physics and on reasonable design practice, to check itsrace suspected of being an acceleration measurement, for example, we cheek against the theoretical plot of acceleration against rimeonstant thrustyperbola governed by the equation
where acce lore tjon,
If tbe identification of acceleration is validated, the next step derives from the fact that the force producing tbe acceleration, the thrust of the rocket. Is proportional to the pressure fn the thrust chamber. If minor perturbations In the acceleration record, therefore, correlate very closely with some In another trace tbat does not have thecharacteristic, this second traceair candidate for the thrust chamber pressure.
From here on, the aiiah/ru gets more complex as wo delve deeper Into the system; but there Iseasonable expectation that, so longood sample ofvailable. It wul be possible to identify all the major rncasureroeotx. Of Ley Importance here is that tbe sampleajor transition period such as engine shutdown.iquid-fueled turbopump-fed engine, for example, the fact that the pressures In the propeJlant feed system drop to .zero at rhutoff in considerably lessecond while the turbine.
rotating at high speedreat deal of Inertia,econds to coasttop Is most useful for identification purposes.
Merely to haveew of the key identilications brings aintelligence benefit, because we can then relate thestudy to earlier ones and form an opinion on whether it isa scries or Isew vehicle orew model ofmissile.air sample of powered-flight telemetry,can usually say whether the vehicle is liquid- or whether it bas a
The single most Important meosureinent and the one roost useful fn the analysis Is tbe acceleration of the missile along its longitudinal an* Every so often we intercept the signal before first-stageand the trace looks like the example shown Inbut without any annotations exceptrom this record we would, know Immediately that thebad two main burning stages of which the first shot downeconds, that It then coasted foe five seconds until the second stage Ignited, and that this burned for aneconds to shut down. The lowhe record after second-stage shuldown would tell us that small vernier rocket engines (for fine regulation of burnout velocity) operated foreconds after main engine cutoff, and the ratio of this acceleration to that at main-engine cutoff would be the ratio of vernier engine thrust to total thrust Tbe short negative displacementeconds signals the firing of rctrcrockets to separate the rocket body from the payload
Note in the same figure tbe dotted line starting at second-stageand continuing the rryperboLc curve that would reach infinityeconds. Thisheoretical extension of tho acceleration,how it would rise If tbe missile had continued burning and losing weight at the normal rate of fuel consumption, so that when the weight cropped to zero tbe acceleration would become Infinite. The significance of this Is that It gives us an upper bound on the paylood weight In the formatio between the weight of the vehicle at burnout (payload plus empty rocket stage) and the weight of the propel Lints burned by tbe stage, these two weights befog proportional respectively to the time from burnout to infinite acceleration and the time from firing to burnout. Then, If through some additional analysis
it becomes possible to introduce an actual weight into the equation, an estimate can be made of the ratio of empty stage weight alone (largely tankage) to propel!ant weight, tbe stage can be sized, and the payload can be determined This process can now be repealed for the first stage, so that the complete weight history of the vehicle from liftoff to burnout becomes known.
Vibrations and Transition Times
the mart promise far determination of missile size, Li theof time-related functions such as vibrations and pressure transients.
Telemetry traces Identified as measurements of liquid level fn the propellent tanks have oorauaonalryather slow oaciTlatioo of low magnitude. This usually It indicative of wave action at the liquid surface, or slothing. Now tbe interesting tiling is that tho rate of these osciUaUons, which can be measured directly from the' traces, is dependent only on the diameter of tbe tank, the acceleration of tbe missile. and die shape of the tank bottom. Further, if tbeoccurs when the liquid is moreank dlprrH*rr away from tbe bottom, then the shape of tbe bottom bas no effect either, and we need to know only tbe acceleration toeasurement of the diameter of the tank.
Analysts have also noted an oscillation of higher frequency super-Imposed on measurements of pump Inlet pressures. Because amissile will bave Its tanks in line, feeding tbe propellant from the upper tank to tbe engineong pipe passing through or around the lower tank. It has been foundS. missiles this pipe acts somewhat like an organ pfne: the longer ft b, the lower the frequency, or pitch, at which it wiTJ vibrate. Tbe phenomenon enables us to get the length of the pipe by measuring the frequency and comparing it with that from known missiles, and this pipe length is essentially equal to the length of the lower propellant tank.
Another occasional observation, one that seems promising but has not yet pioveds that the entire missile vibrates at awhich gradually changes as the burning proceeds andtep change when tbe noseeparated at burnout Ttus is as ir should be, because the vibration frequency Is related to the stiffness of the missile, whichunction of the length,weight, and construction materials. Thus while the propellants are burning the missile weight and stiffness are changing continuously.
a so awn
'h* time It takes for the
but when the nose cone Is separated the weight and length change mstanfaocously, producing the step change in stiflnesx. Very uttJe Intelligence has so far been derived from this type of analysis, how-ever. The ballistic missile cannot be treatedimple hollowits complex, structure has to be considered in detail The study of transient phenomena is another area which gives
pronsise of providing Intelligence, Very recently ft has beenn .
hru. s, The relationship seems to bold for engines using dx&ercrat and operating at different chamber pressures. Furthermore, It seems to bold for Sovietas well; those whose thrusts have been estimated by other methods show pressure decay times that fall right on the curve described by the US. data. The precision with which we can read out the thrust fa quite peer, but the method does giveough cut at the size of the engineew missile, dfacrimlnating between,aturn-sire enginehan-sire one.
Liquid Leeel Mcaivrementt
A third approach which has been very useful fa analysis ofdata on liquid feveU. So fax all the major Soviet ballistic missiles have used liquid propeUants, and they are often equipped with instrumentation for measuring bow full the tanks arc. The tensors are usually installed in both tbe fuel and oxidixer ranks, and they allow us to monitor tbe efficiency of propeOant utilization. Ideally, oneissile to reach burnout with an excess of neither fuel nor oxidizer, and bow closely this Ideal fa approachedeasure of the effectiveness of the system.
Information on the shapes of propeOant tanks can be gained from the level censors. When tbe liquid Is up In tbe main cylindrical portion of the tank the rate of drop in level wiD bo constant, but as soon as the level enters the tank bottom It will start droppingaster rate, and the precise way ft falls with time- willesult of tbe geometry of the tankelliptical, spherical, or some other shape. Thus If weood record of the changing liquid level near burnout we may be able to determine the shape of tbe bottom. This fa not jost an interesting academic exercise, because if we know the geometry of the bottom along with the time needed to empty first the cybndrical part of the rank and then the bottom,
we can calculate the ratio of length to diameter and exert some leverage on the sizing problem.
Another Important product of liquid level analysis is measurement of the volumetric ratio of oridizer to fuel. If, for instance, the level sensors show twice asrop ia one tank as in the other, and if wo make tbe reasonable assumption that the two bave the same diameter, then we know that two volumes of one propellant are burnedingle volume of the otlser. We would therefore know that
. at volumetric ratios knveratio would yield efficient combustionitric acid system. This kind of miormatiou, whenby other data such as specific impulse (thrust per unit of propellant flow rate) allows one to narrow the choice of propcEant combinations significantly.
Up to this point we bave been talking mostly about measurements in the form of ratios, becausehas been very difficult to determine arxsolute magnitudes. Two notablealibration have been achieved,liquid level sensors and accelerometers. These are described below.
Tbe level sensor which has been calibrated Is of tbe "hump" type, io called because Its trace looks like this:
The calibration became possible when tank length for the vehicle was determined by an independent method The total burning time was known, as well as the time it took for tbe instrument to cycle through eight bumps. Then the ratio of these two times could simply be multiplied by tic tank length to give the drop in level represented by the cycle. Having the calibration, we could now obviously turn ft around and use it to measure tank length on any other Soviet missile which might use the instrument.
Acceleration traces have been cabbratcd by two techniques. The first Is quite complex, requiring the history of the missile's acceleration and velocity to be reconstructed from fts powered flight trajectory
by untiring known or
estimated data such as probable launch location. sUging and bumou, times, burnout position and velocity, ratio of acceleration atto acceleration at burnout, pitch program, drag coefficient, and the" ratio otacuum to sea-level thrust
The second technique, much simpler, can seldom be employed because it requires an mtercept of telernetry before lift-offi Such mterren-it* are obtained only rsseli '
*aes; wo receive on pad
telernetry only when speciil atmospberic conditions cause the rfgnah) to be ducted along the earths surface. If we do receiveignal, and If an accelexorneter is registering, then it will be reading one(the acceleroroeter measures gravitational effect rather than acceleration proper) and by comparing this to its reading when the missile Is under power we can calculate the acceleration at any time.
Whicheversed, if the accelerometcr is calibrated then we can go back to the equation for acceleration presented, earlier,
and determine tbe magnitude of the constants K, and K, Now E,
Is the initial weight of the missile stage divided by the flow'rate,
whilehe specific impulse, tbe thrust of the missile divided by
the flow rate. Taking the last term first, the specific
figure of meritocket engine reflecting principally the chemical
energy available In tbe propellant combmation, and It wul be different
for different ptopellants. Further, if we have (from liquid level
sensor analysis) an Idea of the ratio In which the propeUants are mixed,
we canretty good stab at Identifying the propeUants. Then
if by other methods we bave sized the propellant tanks, we can now
calculate tbe propellant flow rate, which multiplied by tbe constants
Kj and Xj gives respectively tbe initial weight of the missile stage and Its thrust.
oped that these explanations will bave given tbeetter understanding of some of the mysteries of telernetry analysis and Its usefulness In acquiring missile mtelllgence. Perhaps he will also appreciate more the fascination at holds for its devotees. One should note here the cumulative effectuccessful analysis: one breakthrough leads to another, that to another, and so on.an erroneous coodunon will propagate errors, and in this respect this Intelligencerobable no different from anyOriginal document.