International Berthing and Docking Mechanism

IBDM fact sheet
IBDM Engineering Model
Description
Role:	Part of SNCs Dream Chaser to supply cargo to the International Space Station
Prime:	QinetiQ Space under ESA contract
Dimensions
Outer width:	1.42 m	4.66 ft
Passageway:	0.8 m	2.62 ft
Mass:	< 325 kg	< 716.5 lb
Performance
Endurance:	At least 210 days
Capture success rate:	> 98% using IDSS initial conditions
Avionics
Redundancy:	Hot redundant (switch-over in less than 150ms) + passive safe mode
Interfaces:	28 V DC or 120V DC and 28V DC
	UART or MIL1553

The International Berthing and Docking Mechanism (IBDM) is the European androgynous low impact docking mechanism that is capable of docking and berthing large and small spacecraft. The development of the IBDM is under ESA contract with QinetiQ Space as prime contractor.^[1][2]

History

The IBDM development was initiated as a joint development programme with NASA JSC. The first application of the IBDM was intended to be the ISS Crew Return Vehicle (CRV). In the original Agency to Agency agreement, it was decided to develop an Engineering Development Unit (EDU) to demonstrate the feasibility of the system and the associated technologies. NASA JSC were responsible for the system and avionics designs and ESA for the mechanical design. However, since the cancellation of the CRV program, the two Agencies have independently progressed with the docking system development.

The IBDM is now designed to be compatible with the International Docking System Standard^[3] (IDSS) and is hence compatible with the future ISS International Docking Adapters (IDA) on the US side of the ISS.

The European Space Agency has now started a cooperation with SNC to provide the IBDM for attaching this new vehicle to the ISS in the future.^[4] NASA expects to select commercial contractors to resupply the International Space Station in September 2015.^[5]

Design

The IBDM provides both docking and berthing capability. The docking mechanism comprises a Soft Capture Mechanism (SCS), and a structural mating system called the Hard Capture System (HCS), explained in more detail below. The IBDM avionics runs in hot redundancy.

Soft Capture System

The SCS utilizes active control using 6 servo-actuated legs from RUAG Space (Switzerland) which are coordinated to control the SCS ring in its 6 degree of freedom. The leg forces are measured to modify the compliance of the SCS ring to facilitate alignment of the active platform during capture.^[6] A large range of vehicle mass properties can be handled. Mechanical latches achieve soft capture.

Hard Capture System

The HCS uses structural hook mechanisms to close the sealed mated interface. QinetiQ Space has developed several generations of latches and hooks to come to the final hook design.^[7] SENER (Spain) will be responsible for the further development and qualification of the HCS subsystem.

Advantages

The key feature of IBDM is that it is a fully computer controlled mechanism having in the following SAFE advantages:

1. Smooth: IBDM provides the capability for low impact docking and berthing. Low impact docking means that the contact forces during mating are greatly reduced and as a result this alleviates the large loads that the structure is required to withstand and it eliminates the need for impact energy to achieve spacecraft interface alignment and capture. Low impact berthing means that the SCS mating forces are smaller than the IDSS requirement, making it possible to perform berthing within the force limits of the robotic arm on station.
2. Autonomous: the IBDM comprises autonomous fast switch-over from primary to redundant lane in case of a single failure and from redundant lane to safe mode (described below) in case of two failures. This switch-over is performed at IBDM level and does not require the vehicle avionics to be in the loop.
3. Flexible: a large range of vehicle mass properties can be handled, as the SCS uses configurable active control, making it possible to be used for a wide variety of applications ranging from explorations to resupply missions.
4. Effective: a very high capture success rate (> 98%), enabled by an agile force-sensing controlled capture mode. As a result, IBDM can cope with the full range of initial conditions specified in the IDSS.
5. SAFE: the IBDM comprises a backup or safe mode in which the Steward platform behaves like an electro-magnetic damper. This mode is used if two failures have occurred in the IBDM avionics - or on command by the hosting vehicle - or in case no electrical power is applied to the IBDM, and hence provides a safe situation for both the vehicle and the ISS, even during the docking phase.

Application

Dream Chaser

The American company Sierra Nevada Corporation (SNC) is developing the Dream Chaser, which is a small reusable spacecraft that is selected to transport cargo and/or crew to the ISS. The European Space Agency has started a cooperation with SNC to potentially provide the IBDM for attaching this new vehicle to the ISS in the future.^[4] The IBDM will be mounted to the unpressurised cargo module, which will be ejected before reentry.

Status

The IBDM development has successfully passed the Critical Design Review (December 2015). An engineering model of the mechanism and its hot-redundant avionics has been developed and successfully tested (March 2016). The performance of the system has been verified at the certified SDTS docking test facility at NASA JSC.^[8] A proposal for the qualification and flight manufacturing of the complete IBDM system has been submitted to ESA (April 2016).

References