Investigation and Measurement of Balloon Dynamics at the Apex and Base of a Scientific Balloon, 15-R8577
I. Steve Smith, Jr.
Inclusive Dates: 07/10/15 – Current
Background — The large balloon reflector (LBR) (Figure 1) is a 10-meter aperture (20m diameter), inflated, spherical THz antenna designed to fly on a large scientific balloon at 120,000 to 130,000 feet. The realization of a large, near-space, 10-meter class reflector for THz astronomy and microwave/millimeter-wave remote sensing and telecommunications has long been a goal of NASA and the DoD. The LBR is one of 12 concepts selected in 2013 out of ~550 submissions by the NASA Innovative Advanced Concept (NIAC) program to conduct a fast-paced design study. The LBR team consists of SwRI, University of Arizona (UA), Applied Physics Laboratory (APL), and Jet Propulsion Laboratory (JPL). The concept was originally conceived between UA and SwRI.
LBR has won both Phase I and II funding under the NIAC program. During our Phase II mid-term review on April 27, 2015, the review panel wanted to see quantitative evidence that locating LBR within the carrier balloon instead of tethering it below the payload gondola provided a sufficient pointing stability advantage to warrant the additional operational complexity. Since there is little or no data for the balloon apex dynamics or simultaneous data between the apex and a suspended payload, additional data was needed. In late May, NASA's Science Mission Directorate authorized a free piggyback balloon test flight so we could obtain the required data. This was contingent on us getting flight instrument packages flight ready in time for the flight in late August; the next opportunity would not occur until a year later. They indicated that once we have this data in-hand, we would be in a strong position to propose and obtain funding to fly a LBR engineering model.
Approach — The objective of this project was to develop two flight packages to obtain simultaneous dynamics data from both the apex of a balloon and its suspended payload under the same flight conditions. This data would then be used to quantitatively answer the question of whether it was better to place the LBR at the balloon apex or from the payload. To accomplish this task, we needed to:
- Construct small, efficient instrument packages using COTS hardware to be flown at the apex of a scientific carrier balloon and on the payload gondola suspended below it. The packages will leverage the experience and hardware gained in recent SwRI efforts and will contain accelerometers, inclinometers, temperature sensors, and cameras. The instrument packages will be used to measure the flight dynamics data (three-axis accelerations, rotations, magnitudes, rates, frequency, modes, etc.).
- The measured dynamics data will be used to constrain and validate numerical models. We will perform a differential analysis of the relative merits of locating LBR in the top of the stratospheric balloon or tethered below the payload gondola. These results will be used to position us for the next phase of development and funding.
Accomplishments — We were able to successfully design, fabricate, test, and integrate two LBR sensor packages (LBRSP): an up-looking balloon apex and a down-looking gondola sensor. As part of the package, we were also able to integrate a UA star camera (Figure 2). Both packages, apex and gondola, were able to link and transmit data to each other via an onboard Wi-Fi for data redundancy in the event one was damaged or lost.
The NASA piggyback flight was conducted from Fort Sumner, N.M., on September 4, 2015. An image of the balloon and gondola can be seen in Figure 3. The LBRSPs were able to acquire, record, transmit, and receive data for the entire flight lasting a total of 7.5 hours, including the ascent, float, and descent. All instrumentation was successfully recovered.
Initial data analysis has been completed adequately to successfully fulfill the objectives of this effort. The apex data shows that there are several oscillation modes at the apex of the balloon, but all of these modes have periods longer than 5 seconds. The strongest modes are at 8.2 sec., 22.5 sec., and ~300 sec. These are relatively low frequencies that are easier to compensate in a pointing system. The gondola jitter is much more pronounced than the apex. The gondola shows several short period oscillations between 2.0 and 0.8 seconds that are much more difficult to compensate with a pointing system. In summary, from a pointing control point of view, it is preferable to have the LBR at the balloon apex than suspended from the gondola.