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Note: This document is in an incomplete, draft form. It is under development.
Tinkerbell is the codename of a tiny, teleoperated spacecraft, a proposal for the Canadian space program.
Keywords: free flyer, free flying, remote control, remotely controlled, unmanned spacecraft, teleoperated, small satellite, video inspection, microsatellite, nanosatellite.
Tinkerbell is designed to do remote-controlled servicing and inspection of equipment on orbit, in the near neighborhood of the Space Station or the Space Shuttle. The initial free flyer, comprising a tug and a main camera module, is roughly 2 feet long. It is operated by astronauts in a shirtsleeve environment, using a control unit based on a laptop PC.
1. Decomposition DiagramTinkerbell comprises a Tug, a Main Video Camera, a Remote Control Unit, and a Berthing Unit.
The following decomposition diagram shows how the Tinkerbell system breaks down into parts. It is also a set of links to information about each component -- click any component to be taken to the numbered section that gives its description. In the description, click the numbered section heading to return to the decomposition diagram.
Tinkerbell(Link to Gazette Article) Tug(2.) Framework(2.1) System Bus(2.2) Grapple System(2.3) Propulsion System(2.4) Guidance(2.4.1) Reaction Control(2.4.2) Pressure Vessel(2.4.3) Tug Control Computer(2.5) Control Transmitter/Receiver(2.6) Video Transmitter(2.7) Skin Camera System(2.8) Electrical Power System(2.9) Environmental Management System(2.10) Main Video Camera(3.) Picture/Optics Unit(3.1) Camera Control Computer(3.2) Light Booms and Headlight(3.3) Grapple Fixture(3.4) Electrical Power System(3.5) Environmental Management System(3.6) Framework(3.7) Remote Control Unit(4.) Control Transmitter/Receiver(4.1) Video Receiver(4.2) Laptop PC(4.3) Video Window PC Card(4.4) Berthing Unit(5.) Berth Control Computer(5.1) Grapple Fixture(s)(5.2) Gas Reservoir(5.3) Solar Power Collector(5.4)
The requirements that apply to the frame far exceed the base requirements of the Tinkerbell mission, because the frame design is reusable and extensible, to enable the spacecraft to offer very short implementation cycles on modifications to support other missions. The frame and all component mountings are sturdy enough to survive launch mounted in the Space Shuttle cargo bay. Dimensions of the frame are dependent on the size of interior components, but the target size of the enclosing cylindrical skin is about 12" in diameter and 18" in length.
The system bus includes:
The video portion of the bus carries two channels concurrently, thus permitting operation of a stereo camera or the use of two different views. Devices that access the system bus are designed to use either channel. The tug alone has six video sources (one for each grapple), and the main video camera has another. The number of sources that may be switched to a video channel is large (32 or 64?). Tradeoff studies may lead to selection of digital video for the video bus.
The physical construction of the bus allows easy connection to the grapple system, and easy connection of internal components during fabrication.grapple fixture against a withdrawing force of 200 kg (this is a concept number, and other physical characteristics are commensurate). There are six grapples: fore and aft (roll axis), port and starboard (pitch axis), top and bottom (yaw axis).
(The drawing below is conceptual only, but has the features that would be needed: self-orienting, bus-extending, snare-equipped, and supplied with a camera and lights. It is 3" high and 3" wide.)
As with the frame, the requirements that apply to the grapple system far exceed the base requirements of the Tinkerbell mission.
Electrically, each grapple extends the System bus. In terms of control, each grapple is a device on the control bus, and grapples or releases under control of the tug control computer.
The maximum acceleration available along any vector is 1 cm/sec2.
The reaction system issues an alarm to the control bus if the tug exceeds a velocity of 10 cm/sec2 along any vector.
(The tug could also have momentum wheels to stabilize the tug during station keeping, but it is presumed in this version of the design that the reaction control system can be manipulated finely enough for stabilization.)
The reaction control computer is a device on the control bus. It is capable of calculating jet pulses in response to translation and rotation rate commands, and of maintaining station, using a continual feed of orientation data from the guidance unit (either by direct signal, or indirectly through the control bus). The reaction control computer continually sends its operational and sensor data to the tug control computer.
The reaction control computer monitors a pressure sensor for the pressure vessel.
The arrangement of jets is a matter for study: it must support operation of the tug with the main camera head and one other payload grappled, so positions relative to the grapple fixtures must be chosen to allow modes of operation that will not plume the payload (not because of potential harm from a light puff of low-pressure nitrogen, but because of errors in propulsion).
An electronically operated recharge valve and mating fixture (controlled by the reaction control computer) is placed on the surface of the tug, in a modified aft grapple. The recharge valve allows nitrogen to be pumped into the pressure vessel from the Berthing Unit or other source.
Spacecraft are typically heated to 100 C on the sunward side, but tend to operate at a temperature deficit amounting to perhaps -30 C (check these numbers). The main environmental control mechanism is a pressurized enclosure, a heater, a thermostat, and a circulating fan. A cylindrical skin is anchored to the framework, and encloses the interior components of the tug.
Nitrogen gas is used to pressurize the interior of the tug, and to distribute heat evenly around the interior.
Rad hardening is an additional function of the skin.
The main video camera subsystem is grappled by the tug (using the tug's front grapple), and extends the tug's System bus, so that the camera subsystem's devices are part of the tug control system. Like the tug, the main video camera is a small spacecraft.
Each light boom is an independent manipulator under the control of the camera control computer. Yaw (and, possibly, focus) controls permit the lights to be harmonized on the center of camera focus.
An additional light, the headlight, is placed close beside the camera as an alternative light source that can operate when the light booms are not deployed. (It is also possible that one or more "pen" lights will be needed in extreme close proximity to the camera lens, for use when the camera is operating in macro mode.)
Light intensity on all of these lights is controlled by the camera control computer.grapple on the tug.
Because the grapple fixture extends the tug's system bus via the grapple, the main video camera may operate as one or more devices on the system bus.
The main environmental control mechanism is a pressurized enclosure, a heater, a thermostat, and a circulating fan. A cylindrical skin is anchored to the framework, and encloses the interior components of the tug.
Nitrogen gas is used to pressurize the interior of the tug, and to distribute heat evenly around the interior. Since there is no propulsion system in the main video camera, nitrogen is a very lightly used consumable (leakage is the only cause of loss), but a small high-pressure gas reservoir and pressure gauge are included as a device on the system bus.
The berthing unit is not part of the initial, proof-of-concept, version of tinkerbell. It is possible that the berthing unit may be abandoned in favor of service equipment within the shirtsleeve environment of the space station or the shuttle.
This design assumes that a servicing system for the flight unit will not be acceptable if it penetrates the mothership cabin's pressure containment.