Supersonic CFD of a Mars Disk-Gap-Band Parachute

Maysarah Sukkar

Sep 26

7 min read

Ansys Fluent · Onshape · SpaceClaim

Introduction and Background

In 1960, Clint Eckstrom developed the disk-gap-band (DGB) parachute [1]. Originally designed for high-altitude meteorology rockets, it has since become the parachute of choice for Mars lander missions. The DGB is well suited to supersonic speeds, has a low packing volume, and deploys stably. Advances in materials science, such as stronger Kevlar fabrics and threads, have also made it practical at higher speeds and larger scales.

Figure 1 – The disk-gap-band parachute: (a) component diagram, (b) sample canopy mesh

Supersonic stability matters most in Mars' thin atmosphere, where a lander must slow from hypersonic to subsonic speeds within an extremely short altitude window. Deployment typically happens between about Mach 1.5 and 2.5. The Perseverance parachute was designed for Mach 1.82, and Curiosity's for Mach 1.7 to 2.3 [2,3]. Understanding how drag varies across this range is therefore critical to mission design.

The modern design process for these parachutes begins in simulation, using FEA, CFD, and FSI to capture how the parachute interacts with the flow, followed by wind-tunnel and flight testing to validate those results. I've always been fascinated by lander design and robotic rovers, and wanted to understand how they decelerate safely and reliably. So for my CFD course final project, I took on the CFD portion of this workflow: a steady RANS simulation of a DGB parachute in Mars atmospheric conditions from Mach 1.5 to 2.5, in Ansys Fluent. The deliverables were a drag coefficient curve across the Mach range and contour plots of Mach number, pressure, temperature, and shock structure around the canopy.

Geometry and Meshing

The parachute consists of three key elements: a flat circular disk, an annular open gap, and a cylindrical band. I used the MSL nominal geometry ratios [4], defined relative to the nominal diameter D. The disk diameter is 70% of D, the gap height is 10% of D, and the band height is 20% of D, with the gap and band sharing the disk's outer diameter. Using a nominal diameter of 21 m, I modeled the parachute in Onshape as a surface body, with 20 suspension tapes connecting the disk to the band across the gap.

Figure 2 – Onshape surface model

I exported the model as a STEP file and prepared it in Ansys SpaceClaim, using the enclosure tool to build a cylindrical fluid domain around the assembly. The domain has a radius of 168 m (8D), an upstream extent of 168 m (8D), and a downstream extent of 420 m (20D). I then split the domain in half to run a symmetric half-body analysis, which kept the model within the student license limits. Finally, I assigned named selections to the boundary faces: pressure far-field, pressure outlet, wall, and symmetry plane.

Figure 3 – Half-symmetry fluid domain in SpaceClaim

In Fluent Meshing, I used the watertight geometry workflow with local sizing to target a 0.1 m cell size on the canopy surfaces. The global surface mesh used a 0.1 m minimum and 30 m maximum cell size, a growth rate of 1.2, and 2 cells per gap, with three boundary layers on the walls. The final polyhedral volume mesh has 134,832 cells with a minimum orthogonal quality of 0.11, well above the 0.05 acceptability threshold.

Figure 4 – Polyhedral mesh around the canopy

Solver Setup and Boundary Conditions

I used a density-based steady-state solver with the energy equation and the k-ω SST turbulence model with compressibility corrections. Since Mars' atmosphere is about 95% CO₂, I modeled the fluid as CO₂, using the ideal gas law for density, the NASA 9-coefficient polynomial for specific heat, Sutherland's law for viscosity, and constant thermal conductivity. I set the operating pressure to 0 Pa so I could specify the boundary pressures directly.

Boundary conditions: the inflow and far-field were pressure far-field boundaries at M = 1.5–2.5, T = 210 K, and P = 170 Pa, with 1% turbulent intensity and a turbulent viscosity ratio of 1. The outflow was a pressure outlet at 170 Pa and 210 K. The flat cut face was a symmetry plane, and the canopy was an adiabatic wall with Kevlar material properties.

For numerics, I used AUSM flux, Green-Gauss node-based gradients, second-order upwind flow discretization, and first-order upwind for the turbulence equations, with a Courant number of 5. Each case was initialized with standard initialization followed by Full Multigrid (FMG) initialization, which gave a much better starting flow field for supersonic cases and reduced the risk of early divergence. Each case ran for 500 iterations at Mach 1.5, 1.82, 2.0, 2.25, and 2.5. I used Mach 1.82 instead of 1.75 to compare directly against Perseverance literature values.

Results

Table 1 – Drag force and coefficient per Mach number

Mach Number	Drag Force (N)	Drag Coefficient
1.50	41,971	0.18
1.82	59,778	0.25
2.00	67,676	0.29
2.25	86,642	0.37
2.50	114,908	0.49

Figure 5 – Drag coefficient vs. Mach number

Drag force increased with Mach number, from 41,971 N at Mach 1.5 to 114,908 N at Mach 2.5. The computed drag coefficient rose correspondingly, from 0.18 to 0.49, following a quadratic trend with R² = 0.995. Contour plots of Mach number, static pressure, static temperature, and a Schlieren-style shock plot (dp/dx) were produced for each case.