Experimental investigation of the flow over a toroidal aerocapture ballute

Abstract

NASA proposals for future planetary spacecraft involve the use of aerocapture to save propellant mass needed to enter planetary orbit. One concept that is being studied uses inflatable structures (ballutes) towed behind the spacecraft to provide the necessary drag while minimizing vehicle heating and flow unsteadiness. Experiments of the tandem body configuration of a toroidal ballute towed behind a spacecraft were performed in the T5 Hypervelocity Shock Tunnel. The test gases studied were carbon dioxide, nitrogen and hydrogen to simulate actual future missions to Mars, Titan and Neptune, respectively. Stagnation point heat transfer measurements were made and were found to compare well with theoretical estimates. In addition, flow visualization shadowgraphs were used to confirm that the flow was steady. The carbon dioxide and nitrogen tests successfully matched both the Reynolds number and enthalpy similarity criteria for the specific NASA missions. The hydrogen shots, however, failed to match the enthalpy criteria for the intended mission and care must be taken when using those results.

Experimental Expansion Tube Study of the Flow over a Toroidal Ballute

“…3 Generally good agreement between experiment and theory is observed for low Reynolds number, high Stanton number measurements. Larger discrepancies are observed where Stanton numbers are lower.…”

“…The nominal value of the separation between the centers of the spacecraft and the ballute, d, was 18.6 mm, scaled from the previous study. 3 Depending on the inner diameter of the toroid, it is possible for the flow through the toroid to become choked. To investigate such a phenomena, one experiment was conducted at each of the five test conditions.…”

“…The expression for the stagnation point heat flux is 13 q = 0.57Pr −0.6 ρ e µ e K (ρ w µ w /ρ e µ e ) 0.1 (h e − h w ) (8) which applies for a two-dimensional cylinder, and is used as a first estimate for the toroid, which is a wrapped-around cylinder. The value of (ρ w µ w /ρ e µ e ) is taken to be 1.26, as used previously, 3 and the effect of wall enthalpy is ignored in comparison to the total enthalpy. Subscript e represents conditions at the edge of the boundary layer, which can here be substituted by the stagnation conditions.…”

“…Figure 2 shows the main ballute model used, which was scaled from the model used in the shock tunnel study. 3 A metal ring (42-mm overall centerline diameter and 6-mm cross-sectional diameter) was used to represent a rigid toroidal ballute, and a sphere (4.76 mm in diameter) mounted on a cylindrical rod was used to represent the leading spacecraft. The nominal value of the separation between the centers of the spacecraft and the ballute, d, was 18.6 mm, scaled from the previous study.…”

“…This paper details the experimental investigation of moderate-and high-enthalpy flow over a spacecraft trailed by a toroidal ballute and follows an earlier study that examined flows relevant to entry into the atmospheres of Mars, Titan, and Neptune. 3 The earlier study was conducted in a free-piston-driven reflected shock tunnel that generates a high-temperature, high-pressure, near-stagnant reservoir of test gas before expanding the gas to high freestream velocities via a steady expansion in a supersonic nozzle. The current study was performed using a superorbital expansion tube.…”