Designing for Titan return velocity

January 2021

Q. A junior engineer has been tasked with designing a small reentry body that will return hydrocarbons from Saturn’s moon Titan. He runs the design by his hypersonic aerodynamics professor, predicting that the small nose radius will keep drag low during the reentry at return velocity and permit precise targeting of the landing zone. “Well,” says the professor, “even if you start with a sharp reentry body, at best you’ll end with a blunt one.” What does the professor want the former student to realize?

Draft a response of no more than 250 words and email it by noon Eastern Jan. 18 to aeropuzzler@aiaa.org for a chance to have it published in the February/March issue.

A TEACHING MOMENT: We asked you why a student who wrote a chapter titled “Reduction of turbulent flow for optimized maneuverability” should not try flying his prototype aerobic aircraft. There was no winning response so we asked professor Clint Balog of Embry-Riddle Aeronautical University to provide an explanation:

In theory, laminar flow would be preferred if it could be maintained along the entirety of the airfoil. However laminar flow, although it induces less frictional drag, tends to separate along the airfoil under real-world conditions. This separation occurs when the boundary layer does not have enough momentum to resist the adverse pressure gradient along the airfoil. Flow separation results in loss of lift, as well as increasing the “pressure” drag. Turbulence generators, called vortex generators, are even sometimes used to “trip” the boundary layer to make it turbulent. Turbulent boundary layers have much larger momentum, and thus they are able to resist the adverse pressure gradient, enabling the flow over the wing to reach the trailing edge of the wing without separation. As a result, lift force is preserved, and pressure drag is avoided (which may have a much greater effect than frictional drag). So that turbulent flow isn’t just for executing maneuvers, or for tightening up a turn, although in some circumstances it can accomplish those things, it is for every aspect of lift generation. So in reality an optimized turbulent boundary layer flow is preferred.