Tug Satellite

Active Debris Removal (ADR) requires dedicated solutions for each scenario. I want to propose an architecture for deorbiting high altitude (above 500 km) abandoned rocket second stages. Their higher potential energy and large mass would yield immense amount of space debris if they collide with one another.

I approached the problem like intercepting a missile. The mission should be conducted rapidly and with precision. The orbit and tumbling rate of a large second stage can be determined more precisely from Earth than smaller space debris. The mission trajectory would be planned to intercept the target with the Tug Satellite (TugSat). As a result, the TugSat, when released from the deploying rocket, would not require much thrust to catch the target. It would approach from the zenith so that once engaged it can push the target towards Earth.

The maneuverability of the TugSat would be improved by placing the positioning thrusters on its center of gravity (CG) to replicate an interceptor's Divert and Attitude Control System (DACS). This would allow the alignment maneuvers to be conducted without introducing oscillations or other instabilities. TugSat would aim for the nozzle of the targeted upper stage. Before the mission, the abandoned upper stage's nozzle and its aft section would be analyzed, and the TugSat's nose cone and the supporting rods would be designed accordingly. The passive conical inverse-contour nose of the TugSat would be designed to fit perfectly with the target's nozzle. It would be flexible and filled with a non-Newtonian fluid. This allows the nose to deform as a compliant fluid to self-center during entry, then instantly solidify into a rigid, non-slip structural interface under the shear stress of engine thrust. Meanwhile, a tripod of three extended support rods would engage directly with the target rocket's reinforced aft structural ring to lock the gimbaled nozzle from moving sideways. Once the docking to the nozzle is complete and the tumbling target is aligned towards the Earth, TugSat's main engine would be fired to push it towards the Earth to deorbit it.

I chose the target's nozzle as the docking point because it is aligned with the CG of the target and is designed to withstand and transfer the thrust forces applied on it. The gimbal movement would be nullified by the extended support rods.

TugSat’s aft main engine would be rigidly fixed without complex gimbal machinery, relying entirely on the frozen target interface and the CG-mounted thrusters to steer the combined stack. TugSat's aft main engine would use a pressure-fed RP-1 + LOX system to provide instant ignition responsiveness. This high-density combo allows a more compact de-orbiter, which is an important requirement. The LOX on board would also be used in the gaseous oxygen (GOX) thrusters mounted on the CG of the TugSat, simplifying the design. The mission would be planned to be completed in under an hour. This would allow the TugSat to utilize on-board batteries and negate the need for protruding solar panels.

To validate this coupling mechanism prior to full-scale orbital deployment, the nose-to-nozzle contact mechanics can be verified via a scaled suborbital microgravity test. Utilizing a suborbital flight profile, such as Blue Origin’s New Shepard, a 1:8 scale replica of the target engine bell and the TugSat nose can be tested during the three-to-four-minute weightlessness window. Tiny gas thrusters on the target mockup would initiate an unpowered tumble in the true vacuum of space. The scaled TugSat prototype would then utilize its CG-aligned thrusters to match the rotation, insert its non-Newtonian nose cone, and deploy the stabilizing rods against the mockup frame. This test bed provides the empirical data required to analyze the mechanical self-centering forces, the fluid's solidification under shear stress, and the rigid locking performance against a loose gimbaled target without full orbital launch overhead.

TugSat, like an interceptor, must accomplish its rendezvous in a minimum amount of time. However, this tight window also includes a terminal deceleration phase that is not required for a missile intercept. This constraint dictates that the TugSat remain highly compact, completely free of structural protrusions, and reliant on a lean propulsion and maneuvering system. This architecture marks a fundamental departure from standard Active Debris Removal proposals that rely on slow, protracted engagement maneuvers; instead of spending days matching orbits, TugSat substitutes complex tether, net, or robotic arm architectures with a rapid, high-precision intercept.