Trampoline Deorbiting System

Version 1.1
Copyright ©2017 by Paul Niquette. All rights reserved.
A typical Trampoline Deorbiting Mission Sequence will be used to summarize the proposal.
 
Trampoline Deorbiting System Mission
                      Sequence

[1] Final Launch Burn
The spacecraft is delivered to the parking orbit by the main engine acting in prograde.
Deploying Trampoline[2] Transposition in Parking Orbit
This maneuver is required to enable the main engine to act in the retrograde direction.
[3] Deploy Trampoline and Transfer to
      Derelict Orbit

The trampoline is stowed within the aerodynamic shape of the launch configuration.  See detailsIn its deployed configuration, bungies are stretched across the trampoline above the platform to absorb contact shocks. 
captured derelict[4] Derelict
Depicted in the sketch above is a typical defunct satellite with cylindrical body and solar panels.  Here is a 'top view' of its capture.
[5] Capture Derelict
The trampoline arms are spring-loaded to retract against the derelict.
Deorbiting Burn[6] Deorbiting Burn
Here we see the most distinctive feature of the Trampoline Deorbiting System: The deorbiting burn acts in compression against the derelict.  The capturing mechanism is not required to grasp the derelict firmly to support extreme tensions during the deorbiting burn.  The thrust can thus be regulated to prevent collisions with on-orbit satellites during delivery to the disposal orbit.
[7a] Releasing Trampoline
[7b] Atmospheric Disposal
The manufacturing cost for the Trampoline Deorbiting System is dominated by the service module, which is designed to be reusable. Each derelict is delivered to the disposal orbit along with the trampoline module, which is designed to be 'sacrificial'.
Recovery of Service Module[8] Recovery Burn
The main engine of the service module can be used to change its orbit to target the recovery zone.
[9] Reentry
The atmospheric reentry is managed to limit the ablation of the heat shield and thus assure  re-usablity of the service module.
[10] Splash-Down
The service module needs to be equipped with a flotation collar for recovery at sea along with lighting and robust locator beacon for all-weather retrieval.


Design Features of the Trampoline Deorbiting System

Push not Pull

By far, the most distinctive feature of the Trampoline Deorbiting System is how the design gives full recognition to a fundamental reality in space technology -- that rocketry can provide only push not pull.  Based on Internet searches, one can find any number of proposed debris capturing mechanisms that require the subsequent towing of a derelict on the end of a cable into an atmospheric drag orbit

towingHere is a sketch of a derelict captured in a butterfly-net and being towed during a deorbiting burn.  Notice that a single, main engine cannot be incorporated in the design, since it would direct its flames along the center-line.  Instead multiple, laterally-displaced thrusters must be accommodated.

Moreover, during retrograde burns, the captured derelict will spontaneously get drawn to the side so that its center of mass will align with the thrust vector.  That can bring the towing cable into conflict with the blast from a thruster.  By the way, since deorbiting burns are not necessarily continuous, the cable will go slack from time to time, and any induced lateral movements can permit the derelict to swing undamped in deleterious ways.

Keep in mind that the derelict is apt to be more massive than the spacecraft used for its deorbiting, which imposes considerable forces on the cable and on the capturing mechanism -- tensile forces that are more difficult to manage than the compression forces against the platform of the Trampoline Deorbiting System.  For the proposed system, misalignments can be readily compensated by conventional thrust vectoring of the single main engine on the service module.


Tumbling Derelicts

 

tumblingOne of the most perplexing technical challenges is designing an active system for capturing a derelict that is tumbling in space.  Here is how the proposed Trampoline Deorbiting System would take on that challenge.

 

As noted in step [3] of the mission sequenceIn its deployed configuration, bungies are stretched across the trampoline above the platform to absorb contact shocks.  Prior to step [5], the service module is to be operated in remote-robot mode, commanded by operators in central control based on real-time video information. 


The service module with the trampoline deployed would first take up a position in the plane of rotation of the tumbling derelict.  The trampoline is then moved in gradually so that its bungies engage protuberances on the derelict.  The exchange in momentum on the rebound will slow the rotation of the derelict while repelling and rotating the the trampoline with the service module attached.  All such motions are autonomously opposed by thrusters on the service module.  Repetitions may be necessary before capture of the derelict can be completed.


Deorbiting Maneuvers

Once the capture is complete in the derelict orbit, the ensemble (trampoline with derelict, plus service module) will need to conduct the most appropriate deorbiting maneuvers.

 

From a derelict orbit, which is typically circular, a retrograde deorbiting burn that produces a small ∆V will change the orbit of the ensemble to an eccentric ellipse, with its perigee on the opposite side of the earth.  The ensemble will then return back up to the firing point, which will be at its apogee, having twice traversed the orbits of working satellites.


Taking into consideration the Oberth Effect, a larger ∆V would be used to put the perigee into atmospheric drag zone below 200 km in elevation.  The resulting apogee will be lower than the firing point, which reduces the likelihood of collisions in between. 


The preferred strategy is to program the service module to execute deorbiting burns at perigee, effectively adding main engine thrust to the atmospheric drag, which hastens the final outcome while reducing collision risks.  Accordingly, a large enough ∆V at perigee will effectively circularize the orbit within the atmospheric drag zone.


Electrical
Requirements

One does not see solar panels on the service module.  Here's why.  The duration of a typical mission for the Trampoline Deorbiting System
will be less than 24 hours... 

Operations
Hours
Launch to Parking Orbit
3 to 5
Transfer to Derelict Orbit
2 to 3
Capture Derelict
4 to 5
Deorbiting Burn
3 to 4
Recovery Burn
1 to 2
Reentry and Splash-Down
2 to 4
Estimated Total  
15 to 23

That means electrical power for communications and control, for solenoids and motorized operations, including valves and pumps for main-engine functions -- all electrical demands can be readily met by properly sized storage batteries.  This feature greatly simplifies the mechanical design of a fully recoverable service module. 


Details of Trampoline Operations
  • Trampoline DetailsAs part of pre-launch preparations, the trampoline arms are mechanically retracted by their spring-loaded returns and latched for stowage inside the aerodynamic fairings. 
  • After the transposition maneuver in the parking orbit, the trampoline arms are extended pneumatically from a pressure vessel inside the service module.
  • While capturing the derelict, the pneumatic pressure is relieved or selectively reinstated by valving  remotely commanded from the ground-based control center.
  • Releasing the trampoline after the deorbiting burn also opens the pneumatic line from the service module, which permanently maintains the spring-load arms against the derelict.
Solvers, you are invited to submit queries about web-base information sources or to make technical recommendations for the Trampoline Deorbiting System or -- best of all -- to share your own deorbiting system designs here.


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