James F. Morrissey importance oi and Andrew S. Carten, Jr. thermistor mount configuration A. F. Cambridge Research Laboratories to meteorological rocket 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 radiosonde. 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 meteorology 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 rockets 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 atmosphere 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 685 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.