AFCRL LARGE IMPACTOR

This flight was part of the Joint Indo-U.S. Balloon Flight Program - 1961 a cooperative scientific launch campaign carried out between February and April 1961 by scientists from India and the United States in Hyderabad, India. The objective of the extended series of high-altitude balloon flights was to probe the tropical stratosphere using a wide variety of scientific instruments.

This particular flight was part of a series to mesure the vertical distribution of submicron particles in the stratosphere over India by means of a balloon-borne inertial impactor device known as the AFCRL Large Impactor which was basically a scaled-up version of the impactor air sampler developed by General Mills for the Air Force Cambridge Research Lab. modified by the addition of a backup filter. At left we can see an scheme of the flight train along with an image of the impactor at launch (click to enlarge).

The air-sampling apparatus consisted essentially of a motor-driven rotary air pump which drawed air through a series of jet impactors, thereby removing the solid particles from the air stream. In the large unit, four separate barrels were manifolded to the one pumping system and individually programmed during the ascent to provide a four-point vertical profile on a single balloon flight. Each barrel had a two-stage impactor and a third-stage polystyrene microfiber filter as a backup collector for the very small particles. The four sets of impactor jets for each altitude interval were individually designed and adjusted so that their first and second stages had a 50 percent collection efficiency at about 0.15 microns and 0.02 microns particle radius, respectively, for particles of 2 g/cm3 density, at mid-altitude for the sampling interval. These cutoffs were kept fairly uniform regardless of altitude by increasing the jet dimensions with altitude to compensate for the decrease in air density. A separate battery pack of silver peroxide-zinc cells supplieed electrical power to the various components of the air sampler.

The population of particles was determined after the flight by visually counting the particles under a microscope, and the numerical concentration was then known from the accurately measured volume of air sampled.

A series of four identical flights were performed during the India campaign with the following profile: ascent to 100,000 ft altitude, at or near a 500 fpm rate of rise; air sampling programmed to collect four samples during ascent from 30.000 to 50.000 ft, 50.000 to 70.000 ft, 70.000 to 90.000 ft, and 90.000 to 100.000 ft. The total time flight duration was 150 min from initiation at 30,000 ft. to the descent in parachute.

Balloon launched on: 4/20/1961 at 2:59 utc
Launch site: Begumpet Airport, Secunderabad, India  
Balloon launched by: General Mills Inc.
Balloon manufacturer/size/composition: Zero Pressure Balloon General Mills 161-1-1 (2.0 mil)
Balloon serial number: 417-2
Flight identification number: GMI Nº 2542
End of flight (L for landing time, W for last contact, otherwise termination time): 4/20/1961 at 7:50 utc (L)
Balloon flight duration (F: time at float only, otherwise total flight time in d:days / h:hours or m:minutes - ): 4 h 51 m
Landing site: 70 Miles NE of Hyderabad, India

Balloon was launched on April 20, 1961 at 8:08 local time from Begumpet Airport, near Secunderabad using dynamic method. Rate of rise to 97.870 ft was 765 fpm. Telemetered temperature on ascent indicated tropopause temperature to be -75° C at 54,000 ft. Pump rpm was obtained by telemetering and rpm was normal until float altitude was reached, when the batteries weakened rapidly. Pump rpm decreased rapidly and the motor finally stopped at 11:32 as the batteries reached a totally discharged condition. The flight was terminated at 12:41 and impact occurred 70 miles northeast from the launch site. The rate of rise was slightly above the desired rate and the batteries did discharge earlier than desired, but all functions were performed on the flight and collections of particulate matter were made on all four impactor collectors. The reason for early battery failure is undoubtedly due to the condition of the batteries upon arrival in India. Approximately half of the cells were badly damaged in shipment. Some of the cells were repaired and used on flights, but this resulted in somewhat less than desirable power supplies on these particular flights.

• Joint Indo-United States Balloon Flight Program - 1961 Scientific Report - Air Force Cambridge Research Laboratories (U.S.) - 1962