James F. Morrissey importance oi and Andrew S. Carten, Jr. thermistor mount configuration A. F. Cambridge Research Laboratories to meteorological Bedford, Mass. temperature measurements

Abstract tions. Thus, we are receiving more data than ever be- A description is given of the original rocketsonde ther- fore—thanks to more successful firings and improved mistor mount, consisting of a 10-mil bead suspended signal reception—but data quality has stayed at a low between two metal posts. The difficulties encountered to medium level. Recent evidence, described later in this with this mount and the subsequent development of article, confirms our belief that caution is in order. the superior "thin-film" mount are also described. The Today's rocketsonde is, for the most part, a more uncertainties associated with the use of the latter mount rugged version of the standard . This is only are outlined along with their effect on data acceptance. natural, considering both the effort which has gone into A different approach to the original problem is de- refining radiosonde and associated ground equipment scribed, which employs longer leads for dissipation of design and the success which has crowned that effort. heat conducted to the bead. The uncertainty associated In choosing a sensor for the rocketsonde, it was recog- with the long lead is shown to be minimal. Preliminary nized that the small bead thermistor has the necessary results of a series of 10 rocket flights are presented. response time to provide useful measurements to about These results tend to confirm the advantages of the 60 km. A 10-mil diameter bead, aluminized to minimize long lead mount. solar and infrared radiation effects, was selected. Initially, the thermistor measurements were masked 1. Introduction somewhat by vehicle problems (propulsion system, spin An indispensable tool of modern is the rate, separation charges). It became apparent before small sounding rocket. This probe enables test range long, however, that higher values of temperature were meteorologists to measure wind, temperature, and density being recorded than had been previously obtained by between 30 and 60 km, in support of launchings of other methods of measurement. This led several in- large missiles and space vehicles. Since meteorological vestigators to analyze the energy balances involved with sounding are relatively inexpensive, reliable, and the sensor as mounted. They soon realized that a dis- easy to launch, they make practical the operation of proportionate amount of the energy transferred to the cooperative networks such as the Meteorological Rocket bead was conducted down the relatively short (0.6 cm) Network (MRN). From the many synoptic observations leads from the very warm mounting posts (Fig. 1). These taken by MRN members, stratospheric and mesospheric posts were metallic and massive compared to the bead. circulation patterns have been deduced which offer new They, and the rocketsonde to which they were attached, insights into tropospheric circulations. There is, how- were heated on the ascending portion of the rocket ever, a noticeable aversion on the part of some atmo- flight, due to aerodynamic heating of the nose cone, spheric scientists to use the measured temperatures. This and retained that heat well after rocket separation problem is due partly to the history of these measure- and nose cone ejection. Wagner (1964) published a ments and partly to a large discrepancy between the group of corrections to apply to the measured tem- available theory and the measured values of the diurnal peratures. While these corrections remove considerable temperature wave in the ozone heating layer (Lindzen, bias, their use is somewhat questionable since the equa- 1967; Beyers and Miers, 1965). tions on which they are based depend upon an assumed As instrumentation developers, we regret the trend model of post temperature variation with time. Con- towards limited acceptance of the data, but we appre- siderable disagreement with this model is seen in a ciate the concern of the atmospheric scientists. We, report by Clark and McCoy (1964) on tests of a rocket too, would recommend caution in any application of instrumented to measure post temperature. the data where small temperature differences between In recognition of the metal mounting, post-short-lead various soundings or limited groups of soundings are conduction problem, a thin film mount was developed significant. The important consideration is that the at Atlantic Research Corp. under the sponsorship of sensor portion of the meteorological rocket probe has NASA (Drews, 1966). This mount employs a clear mylar not kept pace with the propulsion and telemetry sec- film, 1 mil (0.0025 cm) thick and stretched between two

684 Vol. 48, No. 9, September 1967

Unauthenticated | Downloaded 10/04/21 07:15 AM UTC Bulletin American Meteorological Society

FIG. 2. Film mount. Lower lighter half of picture is the mylar film. Two large white dots on film at end of wire are solder. Small dot suspended by bead wire off end of film is the 10 mil bead. (Reference dimension: Leads are approxi- mately 0.16 cm long).

2. Need for another approach Although the film mount has effectively isolated the fast- responding bead from the hot, slow-responding rocket- sonde, it has by no means isolated the bead from its FIG. 1. Original thermistor mount showing short leads (0.6 cm) mount. In fact, the wire leads to the bead have been attached to metal mounting posts. shortened to about 1/3 of their previous length, and the bead is more closely coupled to its mount than ever before. fibreglass posts, to support the 10-mil bead (0.025 cm) Measurements taken with the previous post-mounted (Fig. 2). This mount overcomes many of the shortcom- bead were felt to be non-representative of the atmo- ings of the previous mount in that the bead is no sphere because of the errors contributed by the metal longer connected intimately to a massive thermal posts. In deciding now whether the film-mounted bead reservoir. gives an accurate representation of atmospheric tem- The low mass of the thin film allows it to dissipate its peratures, an analysis of possible error contributions by own residual heat and to come into equilibrium with the film mount must be carried out, particularly in view the at a much faster rate than was possible of the intimate physical connection between the mount with the posts. The poor thermal conductivity of the and the bead. Any useful theoretical model of this rela- film and its posts also inhibits the transfer of heat from tively complex mount must include a large number of the sonde to the thermistor leads. Electrical connection physical parameters. [Drews (1966) lists 30 or more between the sonde and the thermistor leads is achieved physical characteristics of the film, lead, or thermistors, by means of wide (0.25 cm), thin (1.5 X 10"* cm) plated although no values are given for the solder connections.] strips on the mylar film, which dissipate any heat con- The following partial listing of potential error sources ducted from the sonde. in the film mount points up the uncertainty of any The data obtained from this mount are significantly corrections applied and offers an explanation—on an more in agreement with data from other measuring instrumental basis—for differences observed from flight techniques. Fig. 3 shows a plot of temperature versus to flight. altitude with typical curves one might expect with the a) The four solder balls (two are clearly visible in Fig. two mounts described and the U. S. Standard Atmo- 2) used to attach the leads to the film contain more sphere 1962. This figure is shown only to demonstrate than 10 times the mass of the bead. Also, their how the uncorrected data from the thin film mount optical properties are quite variable. (These have more closely reflect the temperature profile expected. not been included in any of the analyses.) This mount is now standard for all ARCAS sounding b) The aerodynamic recovery factor of the film is rockets. A similar approach is being tried on the LOKI- sensitive to the angle of attack and varies from class dartsonde. about 1,2 to 2.4 (Schaaf and Chambre, 1961). This

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Unauthenticated | Downloaded 10/04/21 07:15 AM UTC Vol. 48, No. 9, September 1967 angle is continually varying, and there is no known way of determining its value at any one time on meteorological rocketsonde flights. The consequence of this is several degrees Celsius uncertainty in the film temperature at 60 km. c) The film exhibits greater sensitivity than either the bead or the leads to both solar radiation and infra- red radiation. In the case of solar radiation, this is a consequence of two geometric factors. The film, which is essentially a flat plate, can expose half its total surface to direct solar radiation whereas the leads and bead can be effectively irradiated on only one-third and one-fourth of their respective surface areas. The other factor is that cylindrical leads and the spherical bead by reason of their shapes are much more efficient at transferring heat to their surroundings than is the film with its flat plate configuration. This is to say they have a higher heat convection coefficient. Solar radiation thus changes the temperature of the film by about 10C at 60 km as opposed to about 2° for the leads and bead. However, due to the nature of the flat plate geometry of the film, the heat convection coefficient of the film is variable with angle of at- tack, while the bead and leads are relatively in- sensitive to attitude changes, thus introducing fur- ther uncertainties. In the case of the infrared wave radiation, the relative sensitivity of the film compared to the bead FIG. 3. Temperature versus altitude. Typical data from and leads is a consequence of its lower heat con- original mount (Fig. 1) and film mount (Fig. 2). vection coefficient mentioned above coupled with producing a large (> 30°) angle of attack for the its higher emissivity in the infrared (0.8 for 1 mil sensor (Murrow, 1966). mylar versus approximately 0.15 for the aluminized The sensitivity of the mylar film in the film bead and the leads). This results in about 12° of mount to both solar and infrared radiation cou- cooling for the film as opposed to less than 1° for pled with its closeness to the bead is believed to the bead or leads at 60 km. introduce systematic effects in measured tempera- The exact effect on the measured temperature tures due solely to changes in the solar elevation, caused by the gradients thus generated (and this albedo, and infrared surface temperatures from one is true of gradients due to aerodynamic heating time of the day to another, or from one location also) is not precisely predictable due to the other to another. This variation is independent of true factors discussed in this article and the large num- diurnal wave effects which are also sensed when ber of physical parameters which prove significant present. In the data gathered with the film mount in the analysis. in its operational configuration, there have been d) Another complication in determining how much significantly larger diurnal effects in the 50-km re- energy is actually transferred from the bead by gion than can be supported by the presently avail- conduction down the leads to the mount is that theories of this region. Some investigators have the leads are in the boundary layer of the film attributed these large observed diurnal changes as for an indeterminate amount of their length. indicating a true atmospheric tide; others suspect e) Finally, none of the investigators appears to have instrumentally-caused errors (Lindzen, 1967). Con- considered molecular conduction from the film to cern for errors caused by radiation effects on the the bead although some of the mounts have the sensor has caused some investigators to set up tests bead as close as one mean free path at 60 km and to determine the magnitude of these effects by the average spacing is only two to three mean free paired soundings at sunrise and sunset. Finger and paths at this altitude. This proximity undoubtedly Woolf (1967) recently reported on such a test. contributed to some of the inconstancies noticed (Unfortunately, the thin film mount employed in in the data as low as 30 km due to wash from the their test was surrounded by a protective metal mount because photographic records have demon- ring. This ring is not normally employed opera- strated that the parachute system is still capable of tionally and the test data may thus not be repre-

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FIG. 4. Temperature corrections for daytime and night- time with estimated error for long lead mount and film mount for a nominal Arcasonde fall rate approximating a W/CdA = 0.05. sentative of data taken with operational mounts.) Note: At White Sands Missile Range, a mount simi- lar (but not identical) to that described by Drews is used (Ballard, 1967).

3. Long wire mount Another solution to the original problem of heat being FIG. 5. Long lead mount. Ten mil bead is light dot at top conducted along the leads from the posts is to make the of picture. Coils supporting bead are the 1 mil leads (2.5 lead length longer. A program using this technique was cm long). Posts at bottom serve as electrical and mechanical initiated at AFCRL a few years ago. Up to the last few connections. Cross wires at bottom are part of r.f. short. months, this program has been mainly concerned with allows the sensor to be analyzed as a sphere (bead) sus- Laboratory investigations such as that carried out at pended by two infinite rods (leads). The following is a MIT under contract to evaluate the effects of radiation list of some of the salient features of this mounting con- on the bead and the leads. In this work, Thompson and figuration: Kiely (1967) derived a mathematical model and obtained 1) Small solar (2C) and infrared effect (0.5C). excellent agreement with the model in laboratory tests. 2) Relative insensitivity to angle of attack both from This model was modified (Morrissey and Harney, 1966) changes in the recovery factor and changes in the heat by including aerodynamic effects and long-wave radia- convection coefficient. tion effects to provide corrections for both the long wire 3) Easy mathematical description allowing for less un- and film mount (Fig. 4). Another laboratory investiga- certainty in the corrections. tion was carried out earlier at AFCRL to evaluate the 4) Faster response at high altitudes. lead conduction and time response of both the film and 5) At the higher altitudes, the wire becomes more im- the long wire approach (Ritscher, 1964; Harney, 1966). portant in the sensing and the data from this mount Other tests performed at AFCRL on a mount employing should be more meaningful for comparison with other this concept (Fig. 5) have demonstrated the mechanical countries that fly tungsten wire sensors. suitability of the mount for rocket application and the ability of effectively shunting any r.f. energy when em- 4. Flight tests ployed on the Arcasonde. Recently, a one-day series of 10 Areas rockets was flown One of the principal reasons for preferring the long by Capt. David C. Westhorp of AFCRL from Eglin AFB, lead approach is the physical simplicity of the mount Florida. These contained 6 long wire (2.5 cm) mounts and the subsequent ease of mathematically simulating and four film mounts. A complete analysis has not been it (Pearson, 1964). In designing the mount, we have performed, but two significant factors are evident from theoretically determined the lead length necessary to iso- the data. First, there is considerably less periodic oscil- late the bead from the mounting posts and incorporated lation on the flights with the wire mount as compared a safety factor. Presently, we are using more than 2.5 cm with those of the film mount. This oscillation is usu- of lead length which limits any post conduction effect to ally attributed to changes in exposure to solar radiation less than a few tenths of a degree Celsius. This isolation with changing attitude. Second, the long wire mount re-

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Unauthenticated | Downloaded 10/04/21 07:15 AM UTC Vol. 48, No. 9, September 1967 quires less time than the film mount to get rid of the TABLE 1. residual heat at ejection. These times are presented in Table 1 in the order of their apogee heights since the Time to dissipate residual heat Altitude response time is a function of height. Film mount In Fig. 6, the temperature data from these rockets are Wire mount presented along with data from a similar time period 195 K ft 12 sec taken at Ascension in April 1966 with the film mount. 195 K ft 10 sec There appear to be significant differences in the con- 199 K ft 20 sec sistency of the data and the phase that one might assign 205 K ft 12 sec to the diurnal wave. It is recognized that the data sam- 222 K ft 21 sec ple is not large enough to draw extensive conclusions; 224 K ft 18 sec consequently, future tests are being planned for com- 225 K ft 20 sec 226 K ft 40 sec parison purposes which will cover a couple of diurnal 228 K ft 36 sec cycles. 228 K ft Missing data

5. Continuing program In addition to the planned rocket test program, an ex- tensive laboratory and field investigation is continuing to determine the accuracies of the various mounting configurations. A laboratory program to determine the actual recovery factors of both the film mount and wire mount for subsonic Mach numbers with various atti- tudes at reduced pressures has been initiated. High alti- tude scientific balloons are being used for comparison testing of various rocketsonde mounting configurations and the rod thermistors used on balloon-borne radio- sondes. These tests are being carried out to resolve the temperature differences seen in the overlap between rocket and balloon tests. Also, it is hoped that a more useful balloon sensor might be developed for the higher altitude balloons in anticipation of their operational usage.

6. Conclusions The "film mount" introduced a couple of years ago rep- resented a significant improvement over previous mounts in that it effectively isolated the bead from heat con- ducted from the sonde and hot mounting posts. Unfortu- nately, the means chosen for doing this brought about an even more intimate relationship between the bead and its mount. The basic complexity of attempting to analyze this bead mount relationship and the sensitivity FIG. 6. Variation in temperature versus time for both the film of the mount to both aerodynamic and radiation pa- and long lead mounts. rameters leads us to conclude that a long lead mount is temperature measurements at high altitudes. NASA Con- preferable and will supply more representative data in tractor report, CR-533, 42 pp. both corrected and uncorrected form about the temporal Finger, F. G., and H. M. Woolf, 1967: Diurnal variation of variability of the atmosphere. temperature in the upper stratosphere as indicated by a meteorological rocket experiment. J. Atmos. Sci., 24, 230- References 239. Ballard, H. H., 1967: The measurement of temperature in Harney, P. J., 1966: A similator for rocketsonde temperature the stratosphere and . J. Appl. Meteor., 6, 150- testing. Aerospace Systems Conference (I.E.E.E.), Seattle, 163. Washington (not published). Beyers, N. J., and B. T. Miers, 1965: Diurnal temperature in Lindzen, R. S., 1967: On the consistency of thermistor mea- the atmosphere between 30 and 60 km over White Sands surements of upper air temperatures. J. Atmos. Sci., 24, Missile Range. J. Atmos. Sci., 22, 262-266. 317-318. Clark, G. Q., and J. G. McCoy, 1964: Rocketsonde measure- Morrissey, J. F., and P. J. Harney, 1966: Corrections for rock- ment of stratospheric temperature. U. S. Army Electronics etsonde temperature measurements. Conference on Dynamic Research and Development Activity ERDA-242, 16 pp. Structure of the Free Atmosphere, El Paso, Tex. (unpub- Drews, W. A., 1966: A thermistor arrangement to improve lished).

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Unauthenticated | Downloaded 10/04/21 07:15 AM UTC Bulletin American Meteorological Society Murrow, H. H., 1966: High altitude parachutes. Presented Ritscher, G., 1964: Unpublished results of laboratory investi- at Stratospheric Circulation Course at Texas Western Col- gation on heat transfer of bead thermistors. lege, El Paso, Tex. (not published). Schaaf, S. A., and P. L. Chambre, 1961: Flow of Rarified Pearson, P. H. O., 1964: An investigation into the response Gases. Princeton, N. J., Princeton University Press, 52 pp. and corrections to a thermistor and a platinum wire re- Thompson, D. C., and D. P. Keily, 1967: The accuracy of sistance for temperature measurement in thermistors in the measurement of upper air temperatures the upper atmosphere. Technical Note PAD 83, Depart- /. Appl. Meteor6, 380-385. ment of Supply, Australian Defence Scientific Service, 29 Wagner, N. K., 1964: Theoretical accuracy of meteorological pp. rocketsonde thermistor. J. Appl. Meteor., 3f 461-469.

(Continued from news and notes, page 663) Tango plus three hours: An Air Force C-130 begins four hours of and wind measurements at about 29,000 Time table for eyewall seeding experiment ft (8.8 km). An RFF DC-6 arrives for five hours of observa- Although 14 planes make 17 flights during a Stormfury eye- tion at 12,000 ft (3.6 km). The RFF C-54 starts a 4-hr patrol wall experiment, there are never more than seven operating at 1000 ft (0.3 km). in the hurricane at one time. The following is a typical time Tango plus four hours: The Air Force WB-47 photographs table for Stormfury aircraft during an eyewall seeding. and observes the storm for two hours from 35,000 ft to Four hours before seeding: The first aircraft, a Navy Super 40,000 ft (10.7 km-12.2 km). Constellation, enters the storm at 1000 ft (0.3 km) above MSL Tango plus seven hours: A Navy Super Constellation takes and begins monitoring air flowing toward the center. over the patrol at 1000 ft (0.3 km) to continue until 14 hours Three hours before seeding: A DC-6 from ESSA's Research after the initial seeding. Flight Facility (RFF) begins a 5-hr flight at about 12,000 ft Tango plus nine hours: An RFF DC-6 resumes observations (3.6 km), measuring various aspects of the hurricane. The at 12,000 ft (3.6 km). RFF WB-57 arrives to monitor outflowing air between 35,000 Tango plus ten hours: The RFF WB-57 returns for two hours ft and 40,000 ft (10.7 km-12.2 km) for two hours. of data gathering at 35,000 to 40,000 ft (10.7 to 12.2 km). In the two hours prior to seeding: Two Navy Super Con- Dr. Robert M. White, ESSA administrator, and Capt. E. T. stellations fly into the storm to remain for nine hours. One, Harding, commander of Naval Service, have overall flying at about 10,000 ft (3 km), makes radar observations responsibility for the direction of Project Stormfury. Project and vertical soundings of the atmosphere. The others, at director is Dr. R. Cecil Gentry, director of the National 6000 ft (1.8 km), provides airborne control of all project Hurricane Research Laboratory, Miami, Fla. The assistant aircraft flying in the storm. project director and Navy project coordinator is Capt. R. J. At the signal "Tango": A Navy Intruder begins the seeding, Brazzell, USN, officer in charge of the Fleet Weather Facility, which is repeated every two hours for eight hours. Jacksonville, Fla. Harry F. Hawkins is alternate to the project director, and Cdr. J. D. McGill, USN, is alternate to the assistant project director. An advisory panel, consisting of five prominent scientists, provides expert advice to project officials on all scientific aspects of the program. Present members are Dr. Noel E. LaSeur (chairman), Florida State University; Dr. Edward Lorenz, Massachusetts Institute of Technology; Dr. Daniel F. Rex, National Center for Atmospheric Research; Dr. James E. McDonald, University of Arizona; and Dr. Jerome Spar, New York University.

FIG. 1. The hurricane model. The primary energy cell (convective chimney) is located in the area enclosed by the FIG. 2 broken line. (More news and notes on page 705) 689

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