Maintaining the International Space Station: Fuel Usage and Orbital Adjustments

The International Space Station (ISS) requires constant maintenance of its orbit to ensure its functionality and safety. Unlike traditional spacecraft, the ISS does not require continuous engine firing due to its initial momentum. However, periodic adjustments are necessary to combat atmospheric drag and avoid collisions with orbital debris. In this article, we will discuss the specific fuel usage, types of propellents, and the process of reboost maneuvers to maintain the ISS's position in its orbit.

Orbital Dynamics and Maintenance

The ISS maintains its orbit by leveraging the principle of angular momentum. The gravitational pull of Earth attempts to pull the station, but the station's constant forward motion cancels out this pull, keeping it in a specific altitude. Unlike propulsion systems, the ISS primarily relies on solar panels for its power. However, to sustain its orbit, it must perform translational burns to counteract atmospheric drag and make adjustments when necessary.

Translational Burns and Propellents

Translational burns are essential for maintaining the ISS's altitude and position in orbit. These burns are performed by firing thrusters on various modules, including the Service Module, Progress modules, and the ATV (Automated Transfer Vehicle). The purpose of these burns is to alter the velocity vector of the ISS, thereby changing its altitude. The most common types of propellents used for these maneuvers are Unsymmetrical Dimethylhydrazine (UDMH) and Nitrogen Tetroxide (N2O4).

Reboosts and Altitude Control

Reboosts are critical for maintaining the desired altitude of the ISS. These burns are required because the Earth's thin atmosphere still exerts a small amount of drag on the station, causing it to lose altitude over time. The typical decrease in altitude is around 2 kilometers per month. Two types of reboosts are commonly used: single burn and two-burn.

Single Burn Reboost: A single firing of thrusters provides a delta-v that affects the vehicle throughout its orbit. This method can lead to changes in the eccentricity of the orbit. Two-Burn Reboost: This method involves a initial burn followed by a second burn at the 180-degree point to cancel out the initial delta-v, resulting in a new circular orbit at the altitude of the second burn.

The design envelope for the ISS is 280-460 kilometers, but reboosts are typically performed to keep the station around 400 kilometers to reduce the workload on visiting vehicles. Reboosts occur approximately once a month, with a typical delta-v of 2 meters per second and a thruster firing time of around 900 seconds.

Type of Propellents Used

During reboost maneuvers, the ISS primarily uses UDMH and N2O4. UDMH is an unsymmetrical dimethylhydrazine, and N2O4 (Nitrogen Tetroxide) serves as the oxidizer. These propellents are chosen for their stable and efficient performance in space environments and are used in various modules to ensure the station's safe and controlled trajectory.

While the specific consumption rate of these propellents is not publicly disclosed, it is estimated that the ISS consumes approximately 7000 kg of these propellents for altitude adjustments. The frequent reboosts are crucial for maintaining the ISS's operational status in the long-term.

Understanding the role of these reboosts and the specific propellents used is essential for the continued operation and safety of the International Space Station. As research continues, the methods and technologies for maintaining the ISS's orbit will undoubtedly evolve to ensure its operational efficiency and safety.