# Hypersonic Inflatable Aerodynamic Decelerator entry to Mars' atmosphere

Is it known whether or not a Hypersonic Inflatable Aerodynamic Decelerator inflation and entry into Mars' atmosphere would produce more violent vibrations and lateral movements than was experienced by the Apollo CM's atmospheric entry.

• @Paul Look up Hypersonic Inflatable Aerodynamic Decelerator. – Bob516 Dec 12 '18 at 2:50

Hard to say since HIAD has not been tested on Mars yet. However, we can use Apollo CM and Mars Science Lab as to perhaps make some sort of analogy.

Apollo 10 CM reentered Earth at a speed of 11km/s at an altitude of approx. 122km. MSL entered Mars at 5.9km/s at an altitude of 125km MOLA.

Now, both vehicles (ballistic coefficients, L/Ds, etc.) are different and so are the atmospheres (many properties change similarity parameters, e.g. density, speed of sounds, etc.) they are entering, therefore, their trajectories (entry angles, speeds, trajectory profiles, etc.) are wildly different. But, to compare, Apollo CM had a peak acceleration of about 6g's; MSL encountered peak reentry deceleration of 12.6g's and peak lateral loads of 0.492g's. I could not find Apollo nor MSL vibration data.

Atmospheric perturbations, either random or correlated, during flight, even ballistic flight, cause the vehicle to experience bumpiness, i.e. how bumpy the ride is depends on how these perturbation affect the vehicle. As a simple example, assume there is no wind shear, so atmospheric perturbations could be due to density fluctuations. Ballistic coefficient is a function of density, so it follows that any density perturbation will result in a proportional perturbation in drag force:

$$BC = m/ (C_d A)$$

and

$$F_{drag} = 1/2\rho v^2AC_d$$

so

$$BC = \rho m v^2 / (2F_{drag})$$

therefore $$\Delta F_{drag} = mv^2 / (2BC) \Delta\rho$$

The promise of HIAD is, among other features, much lower ballistic coefficients, making changes in density have a larger impact on the drag it experiences. Coupling that with the density variations observed in Mars will give you a good idea of the magnitude of drag force perturbations the vehicle will experience.

Vibrations are also caused primarily by how the flow reacts due to the presence of the body. Assuming a traditional HIAD architecture, it will have a more abrupt change in geometry as soon as the flow moves past the shell, causing more separation and recirculation. Vibrations could also arise from the HIAD itself oscillating due to the large pressure differentials (large pressure in front of the HIAD, i.e. behind the bow shock, then much lower fluctuating pressure in the recirculation zone behind it).

Lastly, lateral vibrations are primarily due to asymmetric flow conditions in addition to density perturbations discussed above. The vehicle is nearly symmetric, sure, but it has to maintain a specified angle of attack profile so it can follow its computed guidance and control profile. Angle of attack causes complex asymmetric flow conditions on the backside of the shell (recirculation, etc.). Compared to a traditional capsule, HIAD will have larger recirculation zones, so I would expect this to be a larger factor.

Again, not an exact answer, but through analogies and logic, I this this qualitatively addresses your question.