Engineering Integrity and Safety Review

Last Update – January 2021

The technical review outlined in the Committee’s work plan evolved into a more focused assessment of the engineering integrity and safety of the first proposed flight testing the equipment for the SCoPEx project. This proposed equipment test flight would not be the experiment itself but rather a test of the SCoPEx platform without the release of any particles. Specifically, the research team would like to test the gondola’s horizontal and vertical control using a winch system and propellers, as well as the power, data, navigation, and communication systems. They would not release any aerosols, nor fly an aerosol injection/release system. 

The Advisory Committee recruited three scientists with expertise in balloon flight dynamics to review SCoPEx’s experiment plan. Based on the feedback from reviewers and responses from the research team, the Committee has found no significant or imminent safety concerns. In terms of this technical review, the committee agrees the research team has successfully met the requirements of this review.  

Reviewers of the SCoPEx Engineering Integrity and Safety of SCoPEx

Henry Cathey, Aerospace Division Director and UAS Flight Test Site Director at the Physical Science Laboratory, New Mexico State University

Rodger Farley, Founder and CTO of Farley Flight Aerospace LLC

Christer Fuglesang, Professor Astronautics at KTH Royal Institute of Technology, Stockholm, Sweden Director KTH Space Center 

 Documents Sent to Reviewers:

We specifically asked reviewers to evaluate the platform test only, not the overall research plan and design. Reviewers were asked to focus on the following questions: 

1. Are there technical and/or other issues that have not been identified (or identified properly) that could compromise the platform test or cause it to not yield the desired data?
2. Are there unidentified risks or risks that have not been appropriately evaluated in the plan or correspondence from the research team?
3. Are there material improvements or prior research which have not, in your experience, been included in the plan for the test which might improve the project?

Feedback and Responses

To protect the integrity of the review and encourage candid feedback on the research plan, we decided to keep reviewers’ comments confidential between the Advisory Committee and Research Team. We have therefore summarized the major themes along with responses from the SCoPEx Research Team that captures all of the feedback we received. 

Weather Conditions 

What are the limits on the weather conditions at altitude for launching the balloon for safe operation? Can you provide a wind profile and bounds that allow a successful test?

Response from SCoPEx Research Team: The trajectory analysis that SSC conducted was very promising. They analyzed data from 2018 and 2019 and did not find any days in June that would prohibit a launch. While some days provided <6 hours of flight time, there were no days with winds unsuitable for launch. Certain wind conditions may mean that we do not achieve the desired float time. However, even a 4-hour flight would provide significant time to test the platform. Our two-week launch window should provide ample opportunities for appropriate weather and launch conditions. SSC is responsible for all safety aspects of evaluating the weather and launch conditions. 

The wind and relative wind shear between the balloon and platform will affect the repositioning abilities of the payload. Since the platform test aims to test the maneuvering capabilities of the platform and it will not be searching for a plume, the absolute position of the payload is not a determining factor in the success of the engineering flight. The goal of the platform test is to demonstrate the capabilities of the platform. Wind data from this first flight will inform future flights and future flight plans. These requirements will be narrower for future science flights. Initial models have shown that the propellers can move the payload-balloon system up to 3 m/s horizontally, at an altitude of 65,000 ft, under wind conditions typically encountered in flight and based on historical data. 

Potential for Human Error 

Human operational or preparatory errors can happen. What measures will you take to reduce the risk of human error?

Response: We concur. There is no way to eliminate such errors, but we believe there are some institutional practices that make them less likely. We look to lessons from high-rise liability organizations drawn from the work of Gene Rochlin and collaborators. In particular, we will focus on making sure that everyone feels empowered to speak out when they see problems and on review processes that assume accidents will happen. 

Altitude Control

How will the research team maintain a relatively level flight during the test period? 

Response: The initial Statement of Work proposal specified an altitude of 65,000 ft. While this is a target altitude for science flights, it is not a firm requirement for the platform test. Our altitude requirements can be relaxed for the engineering flight. The platform will test the capabilities of the ascender, propulsion system, navigation system, power system, and flight computer, which can all be accomplished at higher altitudes. The ultimate target altitude for the flight will be selected by SSC, taking into account their flight train design. This is a part of the flight services provided by SSC and they have a strong and successful track record. Speed and altitude data will be available from our GNSS unit. The unit was selected for its high precision and accuracy. The GNSS unit and communication system data rate are sufficient for this application.  

For the science flights, the altitude will be controlled via a vent valve on the balloon and a ballast hopper. To simplify the system for the platform test, the first flight will be conducted without the vent valve at the top of the balloon. This limits the achievable altitude precision on this flight. Improved altitude control is a technical specification that will be developed in future flights.  

Propulsion

Are the propellers for this flight selected and tuned for this pressure altitude?

Response: The propellers were selected and optimized for 65,000 ft, however, they can be run at atmospheric pressure (at a lower RPM) or at higher altitudes (with lower thrust). 65,000 ft is a target for the science flights but this does not preclude the platform from safely operating at other altitudes. The feedback loop will have to be tuned for different altitudes. We have modeled propeller performance with a computational fluid dynamics study, which provides initial input to tune the feedback loop. 

Ascender

Previous flights of this kind have experienced issues with the ascender mechanism when exposed to significant changes in temperature and pressure, leading to issues retracting the mechanism before termination. How will the research team prevent the ascender from getting stuck if the payload hangs lower than intended from the balloon or twists in the event of early termination?  How can the balloon land safely in this event scenario?

Response: The SCoPEx team worked with Atlas Devices to identify an existing ascender unit that could be modified for stratospheric operation. It is rated for a 10 g termination load and has been modified to operate under stratospheric conditions. It has been specified to lift our 600 kg payload, survive the parachute shock load at termination, and have an operating temperature range of -80 C to 50 C. Additional testing of the structure can be conducted in our cold, low-pressure chamber. The double rope path was selected in order to provide a mechanical advantage and keep the forces under the load limit for the standard Atlas unit. A spreader bar was added to the design to minimize the chance of the rope twisting up. We judged that this was a lower overall program risk than working with Atlas to develop a new design that could handle the full 600 kg payload on one rope.

As part of the concept of operations the payload will be retracted before the flight is terminated. We agree that one risk is that the payload does not retract properly. This could be due to the system twisting up or an issue with the ascender. We have discussed the possible need to terminate the flight while the payload is fully extended. While this is not desired, SSC has not found any safety issues with this configuration.  

If at termination the payload is hanging lower than intended, no attempt will be made to adjust the position at the last minute. The flight could be terminated with the payload in the fully or partially extended position for two reasons: early termination means there is not enough time to retract the payload or there is a concern with the performance of the rope or ascender and the SCoPEx team chooses not to retract the payload.

If the payload descends in the extended position, the gondola will land a bit before the parachute. As the extended rope and then parachute reach the ground, there is a chance for the payload to be dragged by the parachute due to the prevailing winds. Dragging could damage pieces of the structure, particularly the propellers and booms extending from the structure. However, due to the forested areas of northern Scandinavia, the payload is more likely to be caught in a tree which would prevent it from being dragged along the ground. 

Materials

What considerations have been made for ensuring proper operation of the mechanical features of the mechanisms? Are the cables, grease, gear systems, pumps, motors, and bearings tested to handle extreme temperatures and higher pressure? What is the rationale for using composite landing legs?

Response: The ascender rope is a static rope made from Technora, an aramid fiber with high strength and chemical resistance properties. It also has appropriate thermal properties. A variety of ropes have been tested with the ascender and several were ruled out because they were not stiff enough or the sheath did not work well with the ascender jaws. 

Care has been taken to select components, grease, and epoxies appropriate for the cold flight temperatures. Specifically, the contract with Atlas devices is for a specially modified ascender for a stratospheric flight. Additional testing of specific components can be conducted in our cold, low-pressure chamber or sent out to testing facilities.  

The only pump on the payload will be a part of the POPS (Portable Optical Particle Spectrometer), a light-weight instruments which was designed with aerial platforms in mind. Any failure from this pump would not pose a safety risk to the rest of the payload.  

Bonding was selected because machining carbon fiber components is both difficult and weakens the material. Instead, the attachment points were selected to be bonded ferrules. The specific epoxy was selected for its strength and cold temperature properties.  

Carbon fiber was selected for the landing legs in order to build a lighter weight structure. The modular design of the payload means that any carbon fiber components damaged upon landing can be replaced before the next flight. The propellers and legs are considered to be consumable parts.