Drag augmented semi-controlled re-entry: Proof of concept
Uncontrolled re-entries of satellites and rocket bodies can pose an undue risk to humans on Earth due to surviving fragments. With current technologies, uncontrolled re-entries can be tolerated for spacecraft with a dry mass up to one ton. With proper design measures (“Design-for-Demise”), this limit can probably be extended to two tons. Above two tons, controlled re-entries, targeting for uninhabited areas (e.g. the South Pacific Ocean), have to be used in order to control the on-ground risk. Controlled re-entries can cause significant additional costs for a satellite project in the order of several million euros (development, manufacturing, launch, and operational costs).
One of the investigated self-removal technologies of the TeSeR project makes use of a drag sail deployment at quite low altitudes (<200 km). Such drag sails should be large enough to generate sufficient drag force for a semi-controlled re-entry. In contrast to the fully controlled re-entry, where the targeted impact zone has a length in the order of just a few thousands of kilometers, a semi-controlled re-entry is targeting for an impact zone with a length up to a couple orbits. Risk reduction is achieved by selecting impact orbit arcs with minimum population density.
In order to confirm the general feasibility of drag-assisted removal concepts resulting in a semi-controlled re-entry, the analysis was focused on the following three main topics:
- Residual lifetime estimations
- Identification of minimum populated ground-track arcs as potential targets for a semi-controlled re-entry
- Determination of thermal and mechanical loads on the drag sail after deployment
The length of the impact orbit arc depends on the residual lifetime until re-entry. Uncertainties for the atmospheric density are causing uncertainties for the residual lifetime in the order of up to ±20%. For example, 20 hours of residual lifetime corresponds to a re-entry window size of ±96 minutes or ±1 orbit.
The goal was to find solutions for one- or two-ton class satellites. Drag sails with an area of 100-200 m² should be feasible for such large satellites. Thus, the area-to-mass ratio of interest is about 0.1 m²/kg. The residual lifetime for such a system (until 90 km altitude) estimated with a numerical propagator is in the order of 50 minutes for drag sail deployment at 150 km altitude. In order to be safe, we decided to increase the residual lifetime uncertainty to ±50%. In this case, the achievable re-entry arc length is 100 minutes (now also including the time from 90 km down to ground).
Based on population density databases, a tool was developed to find the location of the minimum populated arc for a given length and orbit inclination. Figure 1 shows the result for a polar orbit. Compared to an uncontrolled re-entry, this semi-controlled re-entry would reduce the on-ground risk to only 0.4%.
However, it is quite unlikely that the drag sail will remain intact until an altitude of 90 km. Additional re-entry simulations showed that drag sails based on Nylon will probably fail already at about 145 km altitude due to exceeding of the maximum temperature limit of the sail material. Figure 2 shows the orbit evolution of the sample mission (one ton satellite, 100 m² drag sail) before and after the sail failure event.
Although the sail fails quite early, aerodynamic capture and residual lifetime reduction is achieved, especially if the sail is deployed at 150 km altitude. Nevertheless, the total time until ground impact is only slightly higher than previously estimated: 116 mins from deployment at 150 km altitude. This extends the minimum populated arc, but does not affect the achievable risk reduction. The needed extra arc length is added over uninhabited areas.