NSTS 214921 identifies three legacy on-orbit operations of immediate interest: grappling, berthing, and docking. These three operations, from which most present-day concepts descend, can be abstracted as different mechanical means of one spacecraft asserting Control Authority2 over the trajectory and attitude of a cooperating spacecraft3:
A grapple operation occurs when one spacecraft acquires “local” control over another, as when an RMS seizes a free-flying payload. The two are in close proximity and have near-zero relative velocity4 at Initial Condition (IC). Control is by way of a non-Primary Structural joint. The scope of control is in the reference frame of, and in limited proximity to, the acquiring spacecraft. The scope does not necessarily extend to the full range of the acquiring spacecraft’s loading conditions. Orbital mechanics are not the primary determinant of the trajectory during these operations, although they might produce appreciable loads because of changes in the aggregate inertial tensor during subsequent local maneuvering of the acquired spacecraft.
A berthing operation occurs when one spacecraft acquires complete (global) control over another, as when an RMS payload was placed in the payload bay of the NSTS. The two are in close proximity and have near-zero relative velocity at IC. Control is subsequently asserted by way of a Primary Structural joint between the two spacecraft. Orbital mechanics are not the primary determinant of the trajectory during these operations, although they might produce loads.
A docking operation occurs when one spacecraft acquires complete (global) control over another by way of a controlled impact maneuver, as when a Progress vehicle attaches to the ISS. The two are in mutual free flight at IC, and “thump” into each other in order to actuate the latching mechanisms. Control is subsequently asserted by way of a Primary Structural joint between the two spacecraft. Orbital mechanics are the primary determinant of the trajectory during these operations, although targeting by direct line-of-sight might be adequate to the task.
The operational implementation of docking predates the other two means by many years, having been executed by both the US and Soviet human spaceflight programs several times. Grappling and berthing came into operational use with the NSTS (US Space Shuttle) program in the 1980’s.
Construed as above, berthing involves up to three participating items. Unfortunately, references to those items use terms inconsistently throughout the body of CBM-related documentation. By way of both identification and disambiguation, the participants are:
(1) An existing on-orbit element or segment. This item is sometimes referred to as the “target vehicle” because that term conforms to the operational vernacular in legacy docking maneuvers5. It is also referred to as the “base” because it typically has a far larger mass than the incoming element.
(2) An in-coming element being added to or relocated on the existing element/segment. This item is sometimes referred to as the “chase vehicle”, again because of the operational vernacular in legacy docking maneuvers6. In the context where “base” is used, this is often simply referred as “element” or, in the context of RMS operations, as “payload”.
(3) An item surrendering “control authority” over the incoming element during the operation. This item might, or might not, be part of either element. It might also be part of some intermediate system (e.g., the SRMS on the NSTS). This item is usually a telerobotic device but, as long as the IC is adequate, could be some combination of the Attitude Control, Reaction Control and Orbital Maneuvering Control systems on either side of the berth.
Footnotes- “Payload Bay User’s Guide”, Johnson Space Center (2000)[↩]
- Used here in the pilot’s sense of having sufficient positive margin in the spacecraft’s effectors to execute some specific maneuver.[↩]
- Contrast with “logical” means such as mere Command Authority.[↩]
- Sometimes referred to as “station keeping”.[↩]
- It is the one being approached.[↩]
- It is the one that does the approaching.[↩]