Jimsphere
The "Jimsphere," although developed in the 1960's, remains the standard at all U.S. missile/launch vehicle ranges for obtaining accurate upper level wind data.
Large missiles and launch vehicles are very sensitive to wind shear, especially as they approach the area of maximum dynamic pressure, typically between 30,000 and 50,000 feet. Prior to every flight today, launch teams analyze a computer-generated flight profile that involves detailed specifications of the wind field through which the vehicle must fly.
In the early 1960s, however, no method existed for making high resolution measurements of the wind profile. At that time NASA was already developing the Saturn launch vehicles for the Apollo lunar landing program and it was essential that NASA also develop a meteorological sensor of superior aerodynamic stability to determine the vertical gradients of the wind before Saturn launches commenced.
The standard smooth-surface weather balloon could not do the job. The reason: the smooth balloon was subject to zigzagging or spiraling as it ascended, due to large air vortices that shed off the surface at various positions; this caused sporadic horizontal motions of the rising balloon that made accurate radar-tracking measurement of the balloon impossible.
After several NASA-sponsored studies failed to provide a suitable method, a NASA engineer came up with an answer. Dr. James R. Scoggins, today director of meteorological studies at Texas A&M University, then at Marshall Space Flight Center, took a simple approach to a complex problem: rather than invent a new system, change the characteristics of the existing system, the smooth surface balloon.
He bought some conical dixie cups and attached them to a balloon to "rough up" the smooth surface. The cones, or "roughness element," were intended to prevent the formation of vortices and thus damp the sporadic motions; additionally they increased drag. The combination of reduced lift and increased drag stabilized the balloon so that, when it entered a changing wind field, it would quickly assume the speed of the wind without zigzagging.
It worked; named the Jimsphere in honor of its inventor, the system was assigned to a company for refinement and production and Orbital Sciences Corporation (OSC), Fairfax, Virginia subsequently acquired the patent rights. More than 100,000 have been sold.
The Jimsphere now being produced is a balloon two meters in diameter, made of lightweight, flexible, radar-reflective materials. Ground radar, navaids or theodolites track position and collect wind data from the balloon at altitudes up to about 10 miles. Jimsphere data was used in the design of the Saturn vehicles, the Space Shuttle and other launch systems. The balloon has supplied pre-launch wind data for all NASA/Air Force ground-based rocket launches from Cape Kennedy, Kennedy Space Center and Vandenberg Air Force Base.[2]
[edit] Aerodynamics
Spherical balloons do not rise vertically in a calm atmosphere.[3]
The aerodynamic lift force, which acts primarily in the horizontal direction, is perpendicular to the drag force. Although the lift force is determined by the nature of the flow around the balloon, it cannot readily be evaluated.
The nature of the flow around a sphere may be altered merely by placing a wire around the equator. The wire acts as a boundary layer transition trip for the air moving around it, and it reduces the laminar separation of the flow. Surface irregularities on a superpressure balloon might be sufficient to induce transition at different points around the sphere. If this happens the separation area will move upstream or downstream on the surface of the sphere and not necessarily in the same direction at all points. This condition will cause changes in the lift and drag forces, with the result that the balloon might experience accelerations. An instant later the flow separation could change, accelerating the balloon in a different direction. These accelerations result in apparent winds due to the self-induced motions of the sphere itself.
The Jimsphere balloon[5] was conceived by James R. Scoggins of the NASA Marshall Space Flight Center to eliminate induced motions of previously used smooth-surfaced, 2-meter diameter, superpressure spheres which operated at supercritical Reynolds Numbers from ground level to approximately 11 kilometers altitude. Induced motions were not a problem at higher altitudes where the 2-meter diameter spheres operate at subcritical Reynolds Numbers conditions.
The Jimsphere Wind Sensor is a 2-meter diameter spherical superpressure balloon with large roughness elements (projections) randomly located on the surface. The projections stabilize the airflow over the surface of the sphere and control flow separation on the sphere during operation at supercritical Reynolds number conditions. The Jimsphere Wind Sensor is used to measure small-scale wind motions in the atmosphere between ground level and approximately 18 kilometers altitude. Position of the Jimsphere during flight is determined by tracking with an AN/FPS-16 or similar radar, and balloon (wind) velocities are determined from the position data. Flight time from ground launch to maximum altitude for the Jimsphere is less than one hour.
As part of the experimental program being conducted at the Marshall Space Flight Center to evaluate the aerodynamic characteristics of spherical wind sensors and to establish the optimum design, a series of tests was conducted in open air at night on August 2, 1963. The spheres were illuminated by two searchlights and time-lapse photographs were made by two cameras oriented 90" from the launch site at a distance of 375 feet. A chopper was placed in front of the lens to provide an image of the balloon at 1/2-second intervals. With this chopper arrangement the image was recorded for 1/4 of a second and then blanked out for 1/4 of a second.
A time-lapse photograph of the ascent of a Jimsphere is shown in the figure. This balloon configuration experienced smaller oscillations than the Rose through the entire ascent.
A comparison of the figures shows that the Jimsphere was considerably more stable than the Rose. A profile of the Jimsphere balloon’s horizontal speed and direction, computed from the time-lapse photograph, shows that the oscillations indicated by the Jimsphere have peaks which are approximately half those indicated by the Rose and that a reversal in the balloon’s direction did not occur. Although the exact influence of the roughness elements cannot be determined from these measurements, the response of the sphere is changed drastically by the roughness elements.
Other measurements made the same night gave similar results.
References
- ↑ 1.0 1.1 1.2 Adelfang, S. I.; Court, A.; Melvin, C. A.; Pazirandeh, M., A further study of Jimsphere wind profiles as related to space vehicle design and operations (pdf), August 1, 1970, Document ID: 19700027625, Report Number: LR-23535; NASA-CR-1640, Contract-Grant-Task Number: NAS8-30165
- ↑ Wind Monitor
- ↑ 3.0 3.1 Scoggins, J. R., Aerodynamics of spherical balloon wind sensors (pdf), February 15, 1964, Document ID: 19640016055, Report Number: NASA-RP-228. Originally published in Journal of Geophysical Research, Vol. 69. No. 4, February 15, 1964
- ↑ Luers, J. K.; Macarthur, C. D., Ultimate wind sensing capabilities of the jimsphere and other rising balloon systems (pdf), June 1, 1972, Document ID: 19720018842, Report Number: NASA-CR-2048, Contract-Grant-Task Number: NAS8-26600
- ↑ Eckstrom, C. V., Theoretical study and engineering development of Jimsphere wind sensor final report (pdf), July 1, 1965, NTRS Document ID: 19650024373, Report Number: NASA-CR-64966, Contract-Grant-Task Number: NAS8-11158