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SOLSTICE Hybrid Drive: A Turning Point in Aircraft Propulsion

Gavin Kutil and Team

The purpose of this document is to provide a brief overview of the SOLSTICE Hybrid Propulsion System design phase and the current standings at the cessation of the fall semester, 2010.

Introduction and Background

The Standalone-electric Optimized Lifting System, Transitional Internal Combustion Engine, or SOLSTICE, represents a turning point in how aircraft will be flown in the future. The goal of this project is to design, manufacture, integrate and test a hybrid combustion/ electric propulsion system specifically for small aircraft. Currently, hybrid propulsion systems for aircraft are nearly nonexistent. However, their possible uses in multiple fields of aviation would allow for many further benefits. An aircraft that is able to utilize a standard combustion system during normal flight and then transition to a primarily electric motor during landing would greatly alleviate the noise pollution that is rampant at today’s airports. A transitional hybrid engine would also allow for greater fuel efficiency (similar to a hybrid car) by decreasing fuel expenditure, allowing the combustion engine to operate more efficiently, while the electric motor provides any necessary remaining power to fly the aircraft. This technology will showcase increased safety for general aviation applications, where engine failure is the root cause of an inexcusable number of accidents.

The SOLSTICE hybrid propulsion system (HPS) is one facet of a larger program known as Hyperion. The Hyperion team is comprised of teams from the University Stuttgart in Germany and from the University of Sydney in Australia, as well as an undergraduate and graduate team from the University of Colorado. Under the leadership of the CU graduate team, Hyperion is designing a model blended wing-body aircraft (seen in Figure 1) with the intent of developing a cleaner, greener and quieter aircraft system. The SOLSTICE undergraduate team is responsible for the propulsion system of this new aircraft concept.

The Hyperion team plans to fly the modeled aircraft in multiple flight modes, including cruise, quiet and dash modes at the end of the spring semester, 2011 through the utilization of the SOLSTICE hybrid engine design. To achieve these different operational modes, SOLSTICE controls two engines within the hybrid system either individually or in tandem; dash mode utilizes both the internal combustion engine and the electric motor, while quiet mode uses the electric motor only, and cruise uses the combustion engine alone. SOLSTICE is designing the control system to operate these flight modes along with the physical HPS.

System Configuration

The SOLSTICE hybrid propulsion system makes use of two separate engines, one electric motor (EM) and one internal combustion engine (ICE) to drive the propeller. The patent-pending gearbox converts the inputs of these two engines into one output torque, turning the propeller shaft. The overall system is constrained in mass and volume as it is necessary for the HPS to fit inside the Hyperion aircraft. During the fall semester design phase, multiple analytical models were produced (thermal, structural stress and strain, and power) in order to ensure that the system would provide the necessary power to successfully fly Hyperion aircraft. Figure 2 below illustrates the propulsion system configuration to be manufactured in the spring semester, 2011.

This configuration was designed iteratively over the course of the semester utilizing a systems engineering approach, working from broad to specific requirements. Initially, the team worked with advisors and clients to develop a preliminary goal and set of objectives. As is common practice, top level requirements were devised based on these objectives and on certain parameters such as the overall aircraft mass restrictions. During the Preliminary Design phase, these requirements were broken down to a subsystem level, where enough detail was defined in order to narrow down the selection of components. Trade studies were performed and the design continued with those selected. The overall system architecture was continually observed, allowing for a constant systems engineering perspective. The results of these studies and their impact on the design were presented for a panel of aerospace engineering faculty in a Preliminary Design Review (PDR). In the following Critical Design phase, parts were selected and prototyping of major risks in order to find mitigating solutions was performed. The system architecture was juxtaposed with itself from the preliminary design phase during the Critical Design Review (CDR), where the team presented the complete system, its components and the analysis behind them.

This educational process goes hand-in-hand with aerospace industry practices. Project definition is the first and foremost step to the design process even with the existence of a multitude of unknowns at the time. This process—and the process to follow in the spring semester—has invaluably prepared the SOLSTICE team for any future engineering endeavors.

Financial Budget

This program was initially provided a fixed budget with which to go about designing and manufacturing the HPS for the Hyperion aircraft. This allows for a learning experience involving business constraints, common in all design projects.

In order to better simulate work performed in the aerospace industry, weekly time sheets are composed by each team member, comprised of work details as well as the time in which these details are performed. Over the course of the fall semester, the SOLSTICE team accrued over 1,710 hours of engineering design work. To obtain a more realistic industrial representation, an hourly wage of $30 was assumed, with overhead rates of 100%. After removing the allotted spending budget it was calculated that the program would have cost about $100,000 for one semester of work. A similar cost will occur during the Manufacturing and Operation phase in the spring semester.

Program Risks

A critical component to any project’s success is the ability to identify and mitigate risks opposing the validation and verification of the project, in this case, of the HPS. As the project continues, it is important to decrease risks in both possibility and consequence, moving them toward the darker area seen in Figure 3. The major risks are identified in both the Preliminary and Critical Design phases in order to illustrate the mitigations made by the team throughout the design process. The preliminarily high risk of structural failure has been mitigated as the initial design was further analyzed and constraints were applied. The gearbox manufacturing, though decreased in risk possibility due to further analysis, is still high; this will be mitigated through prototyping of the gearbox system in the spring semester. Based on lessons learned from the HELIOS heritage project, the team is confident in its ability to mitigate this risk.

Program Schedule

The following summation schedule maps the major deliverables from the end of the fall semester, 2010 through the completion of the spring semester, 2011. This schedule allows for nearly three weeks for full system testing and verification to determine system success through ground testing. No flight testing is necessary for the success of the SOLSTICE program, though it is a desired stretch goal, provided the aircraft assembly for which the Hyperion graduate team is responsible is completed on schedule, and allows for integration of the HPS to the aircraft.


The SOLSTICE team thanks Dr. Jean Koster and Dr. Donna Gerren for their continued advice on this project. The team also appreciates the strong support and guidance received from Cody Humbargar and the entire Hyperion team (Scott Balaban, Derek Nasso, Andrew Brewer, Julie Price, Chelsea Goodman, Eric Serani, Derek Hillery, Alec Velazco, Mark Johnson, Richard Zhao and Tom Wiley). We also thank the international partner teams from the University of Stuttgart in Germany and from the University of Sydney in Australia who helped make this project a fascinating and unique learning experience. Funding for this project is provided in part by The Boeing Company, eSpace Inc., and NASA under grant NNX09AF65G.