The objective of this flight was to collect both air and particulate samples as part of two programs conducted in mid/late 50's: project GRAB BAG carried out by the US Air Force and project ASHCAN developed by US Atomic Energy Comission. Their goals were respectively the monitoring of soviet tests of nuclear weapons and to gather information concerning the nature, concentration and distribution of radioactive debris injected into the stratosphere during nuclear weapons tests. Starting in 1957 during the final stage of Grab Bag and the beginning of ASHCAN, payloads belonging to both programs often shared the same balloon flight.
In the figures on the left we can see the schemes corresponding to the Grab Bag (top) and ASHCAN (bottom) payloads. Next we will make a more or less detailed description of both.
The collection system for the program was developed by General Mills, Inc. and consisted of four major components: a lift stratospheric balloon, a collection bag, an armored vessel, and a control unit. The key concept behind the program was to suspend an uninflated envelope beneath a large helium filled balloon, and carry the envelope to high altitude to fill it with ambient air. An autopilot would then initiate the descent of the entire system to recover the samples on ground. The image at left shows a schematic description of the sampling system along with one of the balloons during descent.
The collection bag or sample balloon was a large balloon fabricated of the same material as the lift balloon. Two sizes of bags were used, depending on the collection altitude: 47.5 feet in diameter at altitudes above 80,000 feet, and 34.6 feet in diameter at lower altitudes. The base of the bag was modified to take a cylindrical fitting that was fitted with a collection blower of the centrifugal type, powered by a 24-volt direct-current motor. The blower would operate at the ceiling altitude of 80,000 feet for about two hours to fill the sample balloon with about 33,000 cubic feet of air. At this point, an auto pilot would release a predetermined amount of helium from the lifting balloon and initiate the descent of the entire system at a rate of about 400 feet per minute. As the system descended to about 10,000 feet, an axial fan located at the top of the sample envelope would be activated to transfer the air sample into a flexible armored vessel located just above the sample balloon.
The non-rigid armored vessel was designed to withstand the rigors of most landings and preserve the air sample. It consisted of three nested cylinders of flexible materials that, from the outside to the inside, were: a tough outer layer of plasticized nylon; an intermediate layer of woven nylon with very high tear strength, and; an innermost cylinder of polyethylene film that would serve as a gas barrier for the air sample. At takeoff the armored vessel hung in a deflated form around the steel cable and below an open parachute that would provide braking of the system descent if the rate exceeded about 2000 feet per minute or to prevent free fall of the equipment in case of failure. The transfer of the sample to the vessel during descent required a short period of time after which a valve at the base of the armored vessel was sealed.
All the operation of the different components of the system were managed by the electronic control unit. It consisted of a power supply, master control unit, and barograph, all housed in a large, insulated, red bag. An upward-viewing sequence camera suspended below the control unit provided a record of the expansion of the lift balloon and the deployment of the collection bag. The electronic control unit transmitted via telemetry the altitude of flight and key steps in the flight sequence, like starting of the blower and axial fan, auto pilot descent, rate of descent and finally the arming of the explosive cut down devices. When the control unit contacted ground, explosive devices severed the connection between the lifting balloon and the parachute leading to its collapse thus ending the flight.
During the entire flight, a recovery crew in an helicopter followed the course of the balloon visually or tracking the signals generated by a radio beacon. Once in the landing site they transferred the air sample with a four-stage compressor from the armored vessel to high-pressure cylinders for shipment to the laboratory. The air samples were then analyzed for the presence of specific isotopes that provided information of probable size, yield, and composition of the nuclear weapons detonated and the distribution of radioactive elements generated.
Near 1000 Grab Bag flights were performed until the program termination in 1958. A detailed history and complete flight record of the program can be found in the first link of external references below.
The official name of the initiative was Upper Atmosphere Monitoring Program but was nicknamed Project Ash Can because of the classical shape of the first sampling unit used. It was first initiated in 1956. The main instrument of the project was an impactor collector that was carried out by a stratospheric balloon to obtain samples at preset altitudes between 50.000 and 90.000 feet.
General Mills engineers designed also the first "Ashcan" filter sampler (which we can see in the images at bottom left) which was flown operationally from November 1956 until December, 1959. This sampler utilized a Torrington 403 blower to pull air through five square feet of low background, low pressure drop filter paper. Sampling rate depended upon altitude and voltage applied, but averaged between 500 and 600 cubic feet per minute. After discovery in the laboratory of some inconsistencies in the volumes measured, in later models of this instrument were incorporated 40-foot exhaust ducts to prevent re-entrainment of sampled air. Also in later years the collector unit was modified and improved many times.
ASHCAN program was run in a monthly basis, with regular flights from Alaska, Australia, Brazil, Panama, and in several sites of the United States. Also special missions were performed to analize certain events like the debris left by the reentry of soviet satellites carrying small nuclear reactors, or after volcanic eruptions.
The program was terminated in 1983.