Tinkerbell Design Notes 

by Eric D. Tarkington, 17Oct96 (Back to Home Page) 

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 Diagram 

Tinkerbell 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)
        System Bus(2.2)
        Grapple System(2.3)
        Propulsion System(2.4)
            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)
    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)

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2. Tug 

The Tug is the core subsystem of Tinkerbell. 

2.1 Framework 

The structural frame of the tug gives it rigidity, provides the underlying support for the grapple system, skin, and interior components, and provides the conduit for the System bus, as well as the skin camera video bus. It is made from off-the-shelf aluminum extrusion (or easily fabricated equivalent), and it supports a crushing force of 200 kg (~441 lb.) along any line through the center of mass. 

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. 

2.2 System Bus 

Most of the System bus is physically embedded in the framework. 

The system bus includes: 

  • Power 
  • Control 
  • Video (two channels) 
Few lines are required, because the control portion of the bus is serial, rather than parallel.  The control protocol is designed to permit control of a wide variety of device types on the control bus, in addition to the grapples, jets, environmental controls, and cameras that are included in the initial spacecraft configuration. 

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. 

2.3 Grapple System 

The tug grapple system physically and electrically connects to other flight modules (payloads). Physically, each grapple is directly attached to the framework, and is capable of retaining the rigidized 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.) 

The Grapple 

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. 

2.4 Propulsion System 

The propulsion system is based on pressurized nitrogen gas. The propulsion system provides positional information to the operator, and controls the translation and rotation of the tug. 

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.) 

2.4.1 Guidance 

As a design goal, the guidance system is based on inexpensive optical gyros, but could also use GPS, image analysis, or other means. Fiberoptic gyros are preferred because of light weight, absence of moving parts, and cost. Gyros are devices on the control bus. 

2.4.2 Reaction Control 

The reaction control system operates jets disposed around the skin of the tug. Each jet includes a valve that simply vents spacecraft interior pressure to space. An embedded computer, the Reaction Control Computer, controls the jets. 

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). 

2.4.3 Pressure Vessel 

The Pressure Vessel is the high-pressure nitrogen storage that is used to replenish gas vented through the jets for propulsion. The pressure vessel is designed to hold gas at a pressure of up to ? psi. A pressure sensor reports the pressure within the pressure vessel to the reaction control computer. 

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. 

2.5 Tug Control Computer 

The control computer is the master device on the system bus. The control computer routes data to and from other devices on the system bus, thus managing all control of devices in the tug. 

2.6 Control Transmitter/Receiver 

The control transmitter/receiver is the radio interface with the remote control unit. It carries data and control signals (for the ultimate use of the operator) between the remote control unit and the tug. 

2.7 Video Transmitter 

The video transmitter is a device on the system bus.  It carries two video channels concurrently.  (It is possible that the video might be encoded as a digital stream carried via the control transmitter, thus eliminating the need for a separate transmitter in the design.  The early design tradeoff study will determine which approach is most cost-effective.) 

2.8 Skin Camera System 

Six wide-angle, fixed-focus, black-and-white video cameras are disposed about the skin of the tug, permitting maneuver of the tug without the main video camera subsystem. Each skin camera is a "boresight" camera for a grapple, and may be used to guide the grappling operation. These cameras are switchable inputs to the internal Video Transmitter system. The switcher and the transmitter are devices on the System bus. 

2.9 Electrical Power System 

The electrical power system supplies electrical power to devices on the power and control bus. It is designed to power Tinkerbell for eight hours of continuous operation in shade. It may include solar cells on the skin of the tug. The battery component is capable of recharging from the system bus. 

2.10 Environmental Management System 

The environmental management system is key to the use of off-the-shelf components, which must have a suitable ambient temperature range (roughly 0 C to 40 C) and pressure (15 psi) to operate without modification or special lubricants. 

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.

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3. Main Video Camera 

Main Camera - 3 Views 

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. 

3.1 Picture/Optics Unit 

This is the camera optics, electronics, and zoom/focus/aperture controls. There is no provision for pan or tilt (the flight unit will be maneuvered to achieve these functions). 

3.2 Camera Control Computer 

The camera control computer is a device on the control bus of the tug. It makes zoom/focus/aperture control available via the control bus.  It also controls the lighting system. 

3.3 Light Booms and Headlight 

Light booms are hinged at the port and starboard of the camera face.  They are deployed to provide controllable, good-quality lighting, at an offset from the camera. 

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. 

3.4 Grapple Fixture 

The grapple fixture physically and electrically connects the main video camera  subsystem to a grapple on the tug. 

Grapple Fixture Diagram 

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. 

3.5 Electrical Power System 

The electrical power system supplies electrical power to the main video camera subsystem. It is designed to power the subsystem for a period of time equal to the duration of the power system in the tug.  The battery component is capable of recharging from the system bus. 

3.6 Environmental Management System 

The environmental management system within the main video camera has a role and design similar to the system in the tug. As with the tug, the environmental management system is key to the use of  inexpensive, off-the-shelf components. 

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. 

3.7 Framework 

Like the tug, the main video camera subsystem has a strong interior frame, capable of withstanding launch stresses.

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4. Remote Control Unit 

The remote control unit is the operator interface to the tug, and through the tug, to the other subsystems.  Ideally, this unit is comprised of all commercial off-the-shelf units for the four components listed below. 

4.1 Control Transmitter/Receiver 

The control transmitter/receiver is the radio interface from the remote control unit to the tug system bus. 

4.2 Video Receiver 

The video receiver is the radio interface which receives the two video channels transmitted from the tug. 

4.3 Laptop PC 

The laptop PC is a space-qualified commercial laptop that provides the operator interface (based on the GUI).  The intentionally slow progress of events in Tinkerbell operations makes it feasible for the operator to use keyboard commands and mouse or trackball manipulation to operate the craft, but joystick control may be added during tradeoff studies. 

4.4 Video Window PC Card 

The video window PC card plugs into the laptop PC, and permits the received video to be displayed in a window on the laptop computer's display screen. 

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5. Berthing Unit 

The berthing unit is the unit that supports the tug in the shuttle cargo bay during launch, serves as the stowage for the flight unit (tug and main camera) aboard the space station or the shuttle, and resupplies consumables to the tug when it is not in service. The berthing unit minimizes the time requirement for EVA service of the flight unit. 

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. 

5.1 Berth Control Computer 

The berth control computer controls the other components of the berthing unit.  When Tinkerbell is berthed, the berth control computer becomes a device on the system bus of the tug. 

5.2 Grapple Fixture(s) 

The berthing unit may include several specialized grapple fixtures.  One grapple fixture is designed to recharge the tug with nitrogen and with electrical power (two main roles of the berthing unit after deployment of Tinkerbell).  Other specialized grapple fixtures may be used to hold the tug to the berthing unit during launch. 

5.3 Gas Reservoir 

The gas reservoir is a large, high-pressure nitrogen tank with provisions for recharging the tug. 

5.4 Solar Power Collector 

The solar power collector uses solar panels to accumulate electrical power to recharge the tug and any attached battery-powered manipulator on the tug. 

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