Generalized Separation of an Object Jettisoned from ISS
- Paper number
IAC-06-B6.2.09
- Author
Dr. John Bacon, National Aeronautics and Space Administration (NASA)/Johnson Space Center, United States
- Coauthor
Dr. Evgeny Menkin, ARES Aerospace, United States
- Year
2006
- Abstract
The purpose of this paper is to provide generalized description of behavior of a jettisoned object relative to the International Space Station (ISS). The study covers all credible thermospheric density conditions; ballistic number ranges; jettison angles and velocities. In the future absence of the U.S. Space Shuttle to return large cargos, jettison will become an increasingly viable option to dispose required large hardware. Since jettison of objects is a collision risk the the ISS itself, the goal is to determine the conditions which will guarantee a minimum safe separation of an object from the spacecraft. Analysis covers three distinct time periods:
- The first few seconds – initial departure trajectory must not cross path with the ISS structure
- During the first orbit, the relative motion of the object must clear ISS structure in X, Y, and Z axes
- Long term relative ballistic effects must not cause the jettisoned object and the ISS to drift back within a safe keep-out sphere, defined to be 200 meters.
These three constraints, as well as limitations of the crew, EVA hardware and ISS performance lead to the development of a policy that defines jettison conditions for generically safe ballistic separation of such objects. The policy also addresses other safety concerns, such as the ability to track all such objects and limitations on the candidate objects such that they do not impose undue risk to the general public when they finally re-enter the atmosphere.
Summary of Analyses:
1) No credible object will naturally separate from the ISS within the specified first-orbit minimum clearance corridor by drag effects alone. To separate naturally within specified keepout zone in minimum density atmosphere, jettisoned object would need to have a minimum ballistic number (BN) of 1.26 m2/kg (which is comparable to the EVA cotton towels in tumbling mode) All objects of higher BN than ISS will separate and continue to separate naturally, however in most cases will not clear specified minimum corridor unless augmented by initial jettison velocity.
2) Objects will clear the ISS outside of the given first-orbit range if jettisoned with a translational velocity greater than 5 cm/sec within +/-45 degree angle. Parametric charts covering all conditions are included in the paper. Vertical separation in the Z (Earth radial) axis is the limiting close approach under most conditions. Larger separations are possible with higher jettison velocities or lower cone angles relative to the ISS velocity vector.
3) It is shown that for objects denser than the ISS, forward jettison is preferred, and for lighter objects, aft jettison is preferred. However, due to operational limitations, aft jettison of denser objects, or forward jettison of light objects, may occasionally be necessary. It is shown that the minimum aft-jettison velocity required for an infinitely massive object is 6.5 cm/sec in a maximum atmosphere, and 65 cm/sec in minimum atmosphere to allow the object to separate for longer than the required minimum separation time. Such velocities will always satisfy the fist-orbit separation minima. Objects with ballistic numbers closer to that of the ISS will need less initial ΔV to meet the requirements in the event that they are thrown in directions that ultimately lead to reconvergence.
4) Velocity imparted to a jettisoned object by an EVA crewmember is limited by foot restraint maximum allowed torque, speed of motion of crewmember’s arms and the amount of rotational velocity put into an object due to the force being applied on a vector that does not intersect the object’s center of gravity. Under worst-on-worst assumptions of crew and structural performance, it is shown that all objects weighing less than a metric ton will separate from the ISS and will meet the requirements within specified angles. Energies and velocities diverted into rotational motion of the object strongly depend on the distance between the center of gravity of the object and the vector of the applied force. However, theoretical limits are encountered to such energy partitioning, which allows bounding safe cases to be developed.
- Abstract document
- Manuscript document
IAC-06-B6.2.09.pdf (🔒 authorized access only).
To get the manuscript, please contact IAF Secretariat.