# What is the difference between “body drag”, “frictional drag” and “pressure drag” for astronaut or aerobot atmospheric locomotion in microgravity?

Complaints below my answer to Would a higher air pressure on the ISS or elsewhere make it easier to “swim” in microgravity? about my spherical-cow estimate of how fast an astronaut can accelerate by "swimming" in air and how that scales with the density of the atmosphere (in ) include terms like "body drag", "frictional drag" and "pressure drag".

I suspect the main reason my estimate was high is that I assumed the velocity of the backwards underhand motion of the astronaut was half of the world's record for the speed of a thrown ball. I should have done some frame-by-frame photogrammetry of the YouTube video instead.

Question: What is the difference between "body drag", "frictional drag" and "pressure drag" in the context of astronaut or aerobot atmospheric locomotion in microgravity?

These concepts will certainly become more important as evolve over time, as well as crewed missions take place in extended missions in larger spacecraft (e.g. Mars migrations), and in designing small robotic devices that fly around in microgravity environments.

## 1 Answer

Drag is a resultant vector that accumulates from integrating all forces in contact with the body over the entire surface. For the most part, this comes in two forms: pressure (normal to the surface) and shear stress (tangent to the surface). The key thing to obtain drag is to extract the components of these forces aligned with the direction of the flow.

Imagine a flat plate in two orientations: one aligned parallel with the flow, one perpendicular to the flow. The flat plate perpendicular to the flow has almost entirely pressure drag. The flat plate aligned parallel to the flow has almost entirely skin frictional drag.

Depending on how the aerobot or astonaut is oriented with respect to the bulk flow of the fluid (or fluidized particle bed), you will obtain different drag components.

• Thanks this is very helpful and instructive! Since the complaints were about scaling, do they both scale with density in the same way? With velocity? – uhoh Jul 12 '20 at 3:01