Chris Voorhees is the founder and president of First Mode, a Seattle-based company that is designing and building technology for extreme environments off and on planet Earth.
Chris has decades of experience in the implementation of robotic systems for the exploration of deep space. His notable experience includes his work as a mobility systems engineer for NASA’s Spirit and Opportunity rovers and lead mechanical engineer for NASA’s Curiosity rover. For his efforts, Chris received NASA’s Exceptional Achievement and Exceptional Engineering Achievement medals.
Today, Chris oversees the design, development, and deployment of engineered solutions for missions around the globe and throughout the solar system. First Mode is also focusing on significant problems on Earth including the challenging issues of sustainability for the natural resources sector.
In this episode of the Aerospace Engineering Podcast, Chris and I talk about:
his background in engineering, including his time at NASA’s Jet Propulsion Laboratory
his past work on Mars rovers
why we should go back to the Moon
the space projects First Mode is currently involved with
and First Mode’s growing engagement in the hydrogen sector
“If you’re trying to put these structures into orbit, every gram counts. Not just every pound but every gram…So you are making structures that are operating at their margins.” — Dr Chauncey Wu, NASA Langley Research Center
Today’s conversation features Dr Chauncey Wu, who is a research engineer at NASA Langley Research Center in Hampton, Virginia. Chauncey has worked at NASA for more than 30 years, predominantly in the field of structural mechanics, and has been responsible for designing and testing a number of space structures that have been launched into space. Some examples of his work include structural analyses on the LITE telescope that was launched into space in 1994, as well as the optimisation of rocket propellant tank structures, and conceptual design studies of lunar lander vehicles and habitat structures for the colonisation of the Moon. In this wide-ranging conversation, we discuss:
Chauncey’s path to NASA as an undergraduate student
The history of NASA and the cultural shift compared to its predecessor, the NACA
The reason why rocket science is so hard
Chauncey’s recent research on a new type of lightweight composite material: tow-steered composites, which could be a game-changer for rocket booster designs
For many years engineers have been trying to harness mechanical work from thermal energy by taking advantage of the crystallographic phase change of shape memory alloys (SMAs). SMAs can exhibit strains of up to 8% actuated by a transformation of the internal crystal structure from martensite to austenite as the metal is heated. This solid state phase change causes a shearing of the internal structure that deforms the material. By introducing additional internal stresses the alloy can be “trained” to transition between two states by applying temperature changes. One of the most well-known projects of the past is the Smart Aircraft and Marine Propulsion System demonstration (SAMPSON), intending to demonstrate the potential of SMAs in tailoring the geometry of jet-propulsion systems through a series of experiments.
Boeing variable geometry chevron, flight testing (1)
One experiment investigated the utilisation of bending actuation of SMAs to optimise the compromise between noise-mitigation at take-off and landing (noise levels are strictly regulated by civil agencies), and maximum thrust at cruise altitude. To achieve this Boeing formed the trailing edge of the exhaust nozzles on commercial turbo-fat jet engines in a triangular “chevron” shape (Figure 1) designed to be reconfigurable by actuation of embedded SMA beam components. The “Variable Geometry Chevrons” (Figure 2) feature NiTi (60% Ni and 40% Ti by weight) SMA beam elements encased in the composite chevrons in a complex 3-D configuration to induce the necessary bending moments to force the chevrons inwards into the bypass flow at low altitudes and low speeds where the engine temperature is high. The intruding chevrons cause a disturbance in the bypass flow, inducing a broader diffusion and mixing of the hot exhaust gases with the cooler bypass flow. Thereby the shear stress between the two different-velocity flows is decreased leading to a reduction in the noise level.
FEA analysis of Boeing Variable Geometry Chevron with SMA strips shown (1)
At higher altitudes and high speeds where the engine temperature is low, the chevrons relax and straighten-out. This guarantees a smooth exit flow that decreases the pressure difference between the inlet and exit of the engine and thus increases the engine thrust. In the original work published by Mabe et al. (2005) the system is designed for both autonomous operation as well as controlled actuation using heaters installed in the engine casing with a closed loop controller to maintain optimum in-flight tip immersions. A parametric study showed that during cruise marginal immersion helped to reduce shock cell noise with negligible thrust penalty.
NASA developed an active bending chevron system by embedding tensile pre-stressed NiTinol SMA strips on one side of the neutral axis of the composite laminate. Actively controlled thermal excitation thus causes the SMA actuators to attempt recovery of the pre-strain constrained by the bond to the host material. The resulting asymmetry in thermal stress causes a moment that deflects the structure. The aerodynamic load due to engine flow and the strain energy stored in the deformed host composite are used to restore the structure to the un-actuated configuration.
The simple design appeals by its lightweight construction with low part count and opportunity to be fully integrated into an autonomous morphing system. The “Variable Geometry Chevron” demonstrates the excellent potential of SMA’s to be integrated in composite laminates to provide internal actuation for smart structures.
References
(1) DJ Hartl & DC Lagoudas (2007). Aerospace applications of shape memory alloys. Proc. IMechE Vol. 221 Part G: J. Aerospace Engineering
