Structure Health Monitoring for a High Altitude Space Shuttle

Exploration in the middle atmosphere, 20km to 100km above sea level, has been an interest since 1930’s. Currently, the middle atmosphere is used for military surveillance programs and interests in commercial communications. Recently, there has been an influx of companies that want to use the middle atmosphere for suborbital spaceflights, launching satellites, space transportation, and science missions launched from sub-orbit. However, there isn’t enough information on how structures respond in such high altitudes. That is why structural health monitoring (SHM) systems are used to monitor such situations. SHM is a dynamic tool to help engineers improve the safety and maintainability of critical structures, like bridges, buildings, and aircraft. SHM combines a variety of sensing technologies with an embedded measurement controller to capture, log, and analyze real-time data.[1] SHM is either used to calculate short-term extreme conditions, like earthquakes, or long-term monitoring to see if the structure performed its intended function.[2]

New Mexico Tech, a science and engineering-focused university in the United States, has a team of students working with a faculty member who have been approached by Near Space Corporation (NSC). NSC asked the team for an array of strain gauges and their corresponding electronics that can be used in structural health monitoring for their High Altitude Space Shuttle (HASS) that is being developed. The HASS system is an unmanned aerial vehicle that is capable of reaching heights of 30km and can maintain flight for days at a time.[3]

The HASS system is used to transport payloads carrying scientific experiments close to space as possible to achieve space-like conditions. Space-like conditions reduces obscuration, filtering, and distortion of the sky from affecting the payload. Another benefit of the HASS system is that it uses their patent-pending balloon launch system to release the shuttle at around 30km. The benefits of their balloon launch is that the payload does not experience high g-forces or vibrations from the launch and that the balloon can be taken off in winds up to 30kt.[3] Due to these factors, Near Space wants to have an active structure health monitoring system on the HASS during its long flights.

Usually, aircrafts are tested with SHM by placing strain gauges throughout the airframe and in sections that would like to be tested before assembly. That aircraft is then put through a series of tests and eventually flown, and throughout all these tests data is collected. On a smaller scale, a model wing of an aircraft can be constructed and the loads, stresses, strains, and twists that the wing may experience are placed on it. The advantage here is that max loads can be found on the smaller scale model without any endangerment or destruction of expensive parts.

The first problem that arises is that the HASS system is relatively small. This gives restrictions as to what gauges may be used, what locations they can be positioned, and the type and size of the data acquisition system which can utilized. While many strain sensors, including foil, semiconductor, and piezoelectric sensors will work, very few data acquisition systems are small enough for application in a small, HASS system. The second problem is that the strain system must wirelessly transmit a signal to the ground so that Near Space can monitor the HASS system in-flight and make active decisions regarding the system’s structural integrity.

For initial research, a cantilever beam experiment was conducted to verify a finite element analysis (FEA) model of the same beam. A 6061-T6 aluminum beam with a foil strain gauge was loaded with a range of weights, and the corresponding strains were measured. This data was then compared to a Solidworks rendering of the beam that was subjected to an FEA program, ANSYS Workbench. With the data being shown to be accurate enough, a Solidworks rendering of a wing of the HASS will then be subjected and recorded.

The team then researched strain sensors and their corresponding data acquisition systems. One possible solution is using a signal amplifier/conditioner in conjunction with a wired data logger. This system would utilize foil strain gauges. Specifically, the team’s research has revealed the SGA A/D Strain Gauge Amplifier and OM-CP Bridge120-25 Strain Gauge data logger. The strain gauge amplifier can handle up to four 350Ω strain gauges, making it well suited for the given application in this aspect.  It is also supplied with a waterproof, mountable case which may prove to be useful when installing it.[4]  The data logger is capable of storing up to about 33,000 readings at up to 20 readings per second.  To retrieve the data from the system, it is simply transferred to a computer via USB or PC serial.[5]

As of right now, the team has completed preliminary research and experiments about SHM and its uses. With the results from our cantilever beam experiment being accurate with our FEA model, we are now able to use software to model our stresses and strains on an aircraft wing when we are given the material information. With the limitation of the size of the HASS, we were forced to choose between two strain gauges that would give accurate results but would have a negligible effect on the whole system. Hopefully, the SHM on the HASS system will be fully implemented in the coming years.

References

  1. C. McDonald, Structural Health Monitoring, 2012, Accessed 22 October 2013.
  2. B. Dawson, Vibration condition monitoring techniques for rotating machinery, Shock Vib. Dig. 8, 1976.
  3. Near Space Corporation, Flight operations, Accessed 10 October 2013.
  4. Micron Meters, Accessed 22 October 2013.
  5. Omega, 2013, Accessed 22 October 2013.

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