## 2 Literature Review
### Systems Engineering
#### 2.2.1 Average Mass Budget Method  
A straightforward approach to gaining insight into the mass budget of spacecraft within the scope of our research area is to refer to the average mass budget table. Average mass budget is predicted based on historical data from existing satellites. The average mass budget is the first step for making initial estimates as a starting point before the iterative process of detailed subsystem design [25]. Table 1 below represents the average mass budget by spacecraft types. It shows the mass budget distribution among various subsystems.  
While performing LCA, the impacts can be measured in terms of midpoint indicators. Midpoint indicators serve as intermediate measures of environmental impact, indicating alterations in the natural environment resulting from emissions or resource utilization [26]. Spacecraft manufacturers can perform the midpoint indicators calculation in a straightforward way since the data on component level are readily available for them. The results are more accurate since a detailed component-level mass budget is available for the manufacturers who are interested in performing their own assessments. However, there is no publicly available method for predicting the footprint of a satellite manufacturer since the exact subsystem/component-level mass budget data are held by the respective satellite manufacturers. Hence, the preliminary mass budget prediction techniques used in space systems engineering are adopted for this research. Table 1 represents the average dry mass distribution among various subsystems, and it is categorized according to the type of spacecraft [25]. Since we are focusing on a low Earth orbit constellation, column 3 (LEO prop %) in Table 1 is our primary focus. The average mass budget table represented in Table 1 is used by during initial phases of mission design by systems engineers and mission design and analysis engineers to estimate the preliminary budget based on the requirements. A user can use the same table (LEO with propulsion) to calculate the midpoint indictors with the help of ESA's Life Cycle Inventory (LCI) database. However, it is necessary to partition it for our case study because of two reasons. One is that we need to partition it according to ESA LCI database, and the other reason is because the \"propellent mass 22%\" in LEO in the propulsion category in Table 1 is more generic and is not suitable use for our case study since Starlink uses a Hall-effect thruster [27]. The propellant mass also affects the mass of the tank. Hence, partitioning is necessary for the propulsion subsystem and propellant for our case study. The new SMAD (Space Mission Analysis and Design) list shows that all spacecraft used for the average mass budget estimation for LEO with propulsion [28]. If we look closer, the mass of propellant varies from 2% to 76% [25]. With Hall-effect thrusters, the quantity of propellant required for one year of station keeping in geosynchronous Earth orbit (GEO) is approximately 0.2% to 0.4% of the total mass, contrasting with the higher range of 1.5% to 3% typically consumed by traditional chemical propulsion systems [29]. Knowing only the total mass of Starlink, propellant mass calculation is needed. Therefore, by subtracting the propellant mass from the total mass, the total dry mass can be obtained. From the total dry mass, it is feasible to partition it according to the average mass budget table and space systems' budget allocation techniques.