why dragon capsule has heat shield only from bottom side

The Dragon capsule, developed by SpaceX, has a heat shield on its bottom side (also known as the heat shield's "aft" or "heatshield backshell") because this is the part of the spacecraft that faces the most extreme heat during reentry into Earth's atmosphere. The design of the heat shield is specific to the spacecraft's reentry trajectory and the physics of atmospheric reentry. Here's why the heat shield is primarily on the bottom side:

1. Reentry Orientation: During reentry, the spacecraft enters Earth's atmosphere with its bottom side facing forward. This orientation is known as the "heatshield-first" entry. The heat shield is designed to withstand and dissipate the intense heat generated by the compression of air in front of the spacecraft as it rapidly decelerates in the atmosphere.

2. Heat Generation: The extreme heat during reentry is primarily generated by the compression of air in front of the spacecraft as it travels at high speeds. This is due to the fact that the air in the upper atmosphere is relatively thin, and as the spacecraft descends, it rapidly compresses the air in front of it. This compression results in a shock wave and a high-temperature plasma sheath, which can reach temperatures of several thousand degrees Celsius.

3. Heat Shield Material: The heat shield is made of materials that can withstand these high temperatures and effectively dissipate heat. Typically, heat shields use ablative materials that char and erode as they absorb heat, effectively protecting the spacecraft's structure and occupants.

4. Reentry Stability: Maintaining the heat shield-first orientation is crucial for the stability and control of the spacecraft during reentry. The shape and orientation of the heat shield help ensure that the spacecraft remains aerodynamically stable, allowing it to follow its planned descent trajectory.

5. Protection of Payload and Crew: The primary purpose of the heat shield is to protect the spacecraft's payload (such as scientific instruments or cargo) and, in the case of crewed missions, the astronauts inside, from the extreme heat and forces of reentry.

While the bottom side of the Dragon capsule has the primary heat shield, other parts of the spacecraft, including its outer shell and the sides, also have thermal protection to help manage the heat generated during reentry. However, the bottom side receives the most intense heat and is the primary point of contact with the high-temperature plasma created during reentry.