WO2005118394A1

WO2005118394A1 - Propulsion unit for spacecraft, servicing system for providing in-space service operations, and modular spacecraft

Info

Publication number: WO2005118394A1
Application number: PCT/EP2005/006069
Authority: WO
Inventors: Charalampos Kosmas
Original assignee: Intersecure Logic Limited
Priority date: 2004-06-04
Filing date: 2005-06-06
Publication date: 2005-12-15

Abstract

A propulsion unit (10) for attachment to a selected target spacecraft is provided, comprising a number of thrusters (12), a number of propellant storage tanks (20), and a docking facilitation system (14), said docking facilitation system (14) being set up to be removeably connectable to corresponding docking means of a further spacecraft. The docking facilitation system (14) provided for the propulsion unit (10) allows for in-space connection with another component, in particular an active component having control means and the like, rendering the formerly detached propulsion unit (10) controllable and maneuverable.

Description

Propulsion Unit for spacecraft, servicing system for providing in-space service operations, and modular spacecraft

The invention relates to a propulsion unit for attachment to a selected spacecraft. It furthermore relates to a servicing system for providing in-space service operations, to a modular spacecraft and to a method for operating a servicing system of this kind.

Spacecraft such as artificial satellites typically are intended to orbit a specific celestial body in order to provide their particular service or mission. In the majority of cases, satellites are used in the proximity of earth and in particular in a proximity the teller operation capability is not hindered by long electromagnetic wave propagation times. A spacecraft to be used in this configuration, i. e. in order to serve as a telecommunication satellite, a ground observation satellite or the like, typically is equipped with a number of thrusters or other propulsion means that may be used to keep the spacecraft in a desired target position and attitude. Satellites or other payloads especially for GEO orbit space applications typically used to be lifted into orbit in a two stage process, wherein in a first step the satellite is lifted from earth to an intermediate orbit by a rocket or other type of launcher. In the second step, by using a so-called apogee kick motor, which basically was a propulsion unit for the satellite, the satellite was moved to a stable or circular orbit.

However, in order to minimize the generation of space debris, in current stages of development the propulsion units or apogee kick motors have de-facto become designed as integrated or built-in propulsion units integrated into the satellite itself, and referred to as so-called Unified Propulsion Subsystems or UPS. Other approaches are not considered cost viable, and the mentioned configuration has become de facto standard design. However, in this setup the final mass of the satellite is increased by the weight of the propulsion unit itself, thus increasing the demand of fuel and other resources nee- ded to manoeuver the satellite in outer space and increases the requirements to the torquers and their fatigue.

Furthermore, nowadays many new satellites are equipped with a binary propulsion subsystem comprising two totally separate units. For the GTO to GEO transfer that requires a high thrust to pass the satellite quickly from the ratiation zones Van Allen and bring it fast to operation, a first propulsion unit is provided in addition to a second propulsion unit for the routine less fuel demanding and less thrust demanding station- keeping manoeuvers. The station-keeping manoeuver performing propulsion today is often based upon the concept of ion propulsion which depends on low thrust but high specific impulse Xenon gas. The GTO to GEO transfer is performed with so called hy- pergolic or storable propelants of high thrust but lower specific impulse. Accordingly, in view of the different performances and specific requirements the two propulsion modules are totally separate.

The fuel budget of the propulsion needed for the GTO to GEO transfer is calculated strictly for the need to propel the given satellite to GEO. After the intended performance, this spent propulsion module consist of mainly dry mass and it is deactivated completely. This propulsion unit typically consists of fuel tanks, pressurant tanks, one thruster, valves, filters, pipes, structure and harness and the associated sensors for pressure and temperature and heating elements plus cabling for power distribution and sensors. Consequently, it consists of non functional mass with respect to the remaining mission, but is carried uselessly by the spacecraft during its rest of lifetime penalizing each and every station keeping or repositioning manoeuver.

It is accordingly an object of the present invention to provide for a propulsion unit for a satellite or other payload that allows for particularly effective and low-cost manoeu- vering while at the same time avoiding an overload of space debris. It is another object of the invention to provide for an in-space servicing system, for modular satellites making beneficial use of the propulsion unit, and a still further object of the invention is to identify a particularly beneficial method to operate this servicing system. With respect to the redockable ejectable propulsion unit, this object is achieved by a propulsion unit for complementing attachment to a selected spacecraft, comprising a number of thrusters, a number of propellant storage tanks, and a docking facilitation system, said docking facilitation system being set up to be removeably connectable to corresponding docking means of a further spacecraft.

The invention is based upon the concept that in order to keep the generation of space debris low and at the same time to provide for very efficient manoeuvering capabilities, in contrast to state of the art systems of integrated designs, the propulsion unit should be designed as a separate, detachable unit that may be removed from the target satellite after performing its functions. In this concept, the propulsion unit after detachment from the satellite may be handled independently from the satellite, thus automatically decreasing the necessary effort and resources required to manoeuver the satellite unit. In contrast to propulsion units used as apogee kick motors in early applications that used to be a detachable unit to be separated from the satellite after completion of the final movement, the present concept avoids further contributions to space debris that might possibly hinder future space missions due to enhanced danger of collisions with used components by offering means for controllable handling of the propulsion unit after it gets disconnected from the modular satellite in consideration.

The propulsion unit after detachment may be positioned in space in reserved zones or even free outer space where it would not create debris problems. Alternatively, it may be reused for further propulsion missions for other satellites, especially after refuelling. In order to allow for this independent handling, the propulsion unit is equipped with a docking facilitation system that allows for safe approachingh, for electrical and/or mechanical connection to other spacecraft that may be used to manoeuver or control the propulsion unit. In particular, the docking facilitation system preferably comprises means for assisting the approach when connecting to the target spacecraft, such as re- troreflectors for approach locking, a video camera for manual backup, and/or a proximity laser for final close loop after locking. The docking facilitation system provided for the propulsion unit allows for in-space connection with another component, in particular an active component having control means and the like, rendering the formerly detached propulsion unit controllable and ma- neuverable. In consequence, the spent propulsion unit may be moved for disposal purposes to a space region where debris issues are not problematic (the so-called „gra- veyard orbit", agreed today to be 300 km above GEO). It may also be attached to other pieces of debris similarly equipped, thus allowing for a joint handling of those pieces for easier control and unitary and combined transportation to the graveyard. In particular, the docking facilitation system may be designed in order to allow for interconnection between a plurality of propulsion units, in order to achieve an arrangement of a stacked type in which several propulsion units as a combined stack may be manoeuvered to a predefined graveyard position. In this embodiment, with respect to the intended stak- king of a multitude of propulsion units, the docking facilitation system comprises so- called male and female connectors that may be used to connect to each other.

Alternatively or additionally, the propulsion unit equipped with the docking means may be connected to active components in order to render the propulsion unit reusable for other purposes.

The docking facilitation system in this context comprises a piece of equipment for the propulsion unit that allows for connection or docking of other components in outer space. The docking facilitation system in an examplary embodiment may comprise a mechanical system, preferrably based upon a clamping mechanism, a magnetic system or an electro-magnetic system. Preferably, the docking facilitation system comprises docking safeing means for mechanical latching to firmly attach the propulsion unit to the target spacecraft.

The docking facilitation system for the propulsion unit allows the propulsion unit to be connected with an active component having control means and the like, rendering the formerly (in old days) detached propulsion unit controllable and manoeuverable. In consequence, the used-up propulsion unit for disposal may be moved into a position in space where debris issues are not problematic. Alternatively, it might be attached to other pieces of debris for combined and thus more efficient further handling.

Alternatively, the propulsion unit via its docking system may be attached to an active component for reusing for other purposes such as a tanker base or a utility base for providing in-space refuelling and other service functions to other spaceship. In a particularly advantageous embodiment, the docking facilitation system comprises a propel- lant transfer port or unit, allowing for refueling of the propulsion unit, thus rendering it reusable for further missions or applications, and/or allowing for emptying the on-board fuel tanks of the propulsion unit to retrieve unspent fuel reserves for transfer to other spacecraft.

Depending of the type of fuel used in the propulsion unit and its specific design, various preferable embodiments are proposed. In certain configurations, i. e. if due to the design specifics the propulsion unit may be used as a temporary tanker or fuel storage base only, it preferably is equipped with fuel transfer ports that allow to empty the onboard tanks to a visiting spacecraft, thus retrieving the unspent fuel from the propulsion unit. If, however, type of fuel and design of propulsion unit allow for the use of the propulsion unit as a long-term accumulative tanker, it preferably is set up in order to allow for multiple access and interaction with servicing vehicles. In order to achieve this, the propulsion unit preferably is equipped with a data port for transferring data (in particular test data/control data) and/or solar panels for power generation. These preferably are used to maintain a desired fuel temperature, thus rendering the propulsion unit into a potential long term fuel storage base.

In a preferred embodiment, in the reuse mode for the propulsion unit, the docking facilitation system is designed to be controllable or remotely accessible, e. g. in a mechanical (clamp-type) or electro-magnetical design or a combination thereof.

Preferrably, the propulsion unit is used in the context of a modular spacecraft consisting of a payload and the (separable) propulsion module for orbit raising. In an embodiment preferred even further, the modular spacecraft is equipped with a satellite bus based upon ion propulsion (electrical or otherwise high specific impulse thruster as compared to chemical thrusters).

With respect to the servicing system for providing in-space service operations to a selected target spacecraft, the object identified above is achieved with a propulsion unit according to one concept as described above, and with a separable servicing vehicle, in which the docking facilitation system is set up to establish a removable connection between the propulsion unit and the servicing vehicle, said servicing vehicle being equipped with communication means for establishing a communication channel through said propulsion unit and modular spacecraft and a remote control module. In this setup, the servicing vehicle is considered the further or selected spacecraft the docking facilitation means are designed to connect the propulsion unit to. The servicing vehicle itself contains all relevant means and features to render it a completely independent and operable spacecraft, in particular on-board thrusters, manoeuvering equipment and communication means. In this setup, the propulsion unit may be designed without specific on-board communication means, accordingly saving the corresponding budgets in view of weight etc. After connection with the servicing vehicle, the on-board equipment of this unit is made available to establish control on the propulsion unit as well. In this system, the servicing vehicle may also be referred to as a utility agent.

In a preferred embodiment, the servicing system comprises a redockable ejectable propulsion unit which is equipped with a number of docking bays for temporarily containing said servicing vehicle.

In case the propulsion unit is intended to be docked upon by the servicing spacecraft in order to be moved to a graveyard position, in particular after consumption of all the onboard fuel reserves, the servicing vehicle preferrably is equipped with on-board propulsion means such as thrusters designed with sufficient capacity to propel the rejectable propulsion module to the intended graveyard position. When operating the servicing system as described above, the communication module preferrably is connected to the propulsion unit, and controls the propulsion unit. Accordingly, on-board communication means for the propulsion unit are rendered obsolete.

The family of new apogee kick motors (AKM) or propulsion units according to the invention is being defined to :

- Enable more efficient operation of the operational spacecraft by the ejection of the AKM from the beginning of operational period allowing so reduction of its dry and not functional any longer mass, which the AKM would represent at this moment.

- Enable batch transportation of AKM.

- Enable tide (stacked/combined) disposal of the AKM.

- Provide active means that ease docking of a utility agent before separation of the AKM.

- Enable docking after separation for further transporting purposes.

- Enable the additional functionality of the AKM to operate as a tanker spacecraft.

- Enable additional functionality of the AKM to operate as Utility Base.

- Enable cost efficient and often launching opportunities for porting of utility agents and/or fuel reserves to GEO

- Enable cost efficient and mass efficient solution for the disposal of the AKMs.

- Enable cost efficient repositioning of fuel reserves. - Enable collection of fuel of more than one AKM to a selected single AKM.

- Enable reuse of AKM for moon bound transportation missions.

Exemplary embodiments of the invention are disclosed in the figures. Fig.1 schematically shows a rocket system with a satellite payload. Fig. 2 through 5 each show a propulsion module for the modular satellite in Fig. 1.

The rocket 1 in Fig. 1 comprises a thruster 2 for launching purposes. In cargo bay 4, a modular satellite 6 comprising a payload (including a bus for station keeping) 8 equipped with a detachable propulsion unit 10 is positioned.

The propulsion unit 10 is designed such that before detachment from the modular spacecraft 8, in order to avoid unnecessary debris in a particularly cost- and fuel-effective manner, it may be connected to/by other active components for gaining control and manoeuverability. For these purposes, as is shown in the various embodiments in Figs. 2 through 5, the propulsion unit 10 is equipped with a docking facilitation system 14. This system comprises, among potentially other elements, a connector 13 to the other module of the spacecraft 6, i. e. the payload 8 with the bus system. The connector 13 comprises an electrical part for transmitting signals and/or power and a mechanical part for attaching and/or fuel transfer. The docking facilitation system 14 further comprises an avionics module 15 used for proximity operations, comprising e. g. retroreflectors, a video camera for manual backup, and/or a proximity laser for final close loop after lok- king.

Still further the system 14 comprises male and female (16) mechanical connectors designed to allow for stacking of a plurality of propulsion units 10. Connectors 13 and 14 may be combined.

Version one (SINGLE USE AKM mode) comprising.

• a configuration of one, two/three/four thrusters 12, axisymmetrically placed • a docking bay 26

• the docking bay being centrally located (to the Center of Gravity CoG) of the propulsion unit 10.

• Optionally - solar panels 24 mounted in the periphery of its body

• Optionally - omni-directional antenna elements mounted on a central point on the upper side (towards the spacecraft)

• Optionally - permanent magnet based docking mechanism (male), single- point or multi-point, preferably at the side of separation with the satellite, so that it can be used after the separation for docking permanently to another passive space object equipped with permanent magnetic material. Or temporarily to an object possessing electromagnet based docking.

• Optionally additional magnet based docking mechanism in the opposite to the satellite direction (female), symmetrically, to allow stacking of multiple spent propulsion units 10 for batch transportation and/or tidily disposal.

• The docking bay 26 being in the opposite direction from the satellite it caries, to allow a Utility Agent to dock before the separation (ie before it become uncontrollable).

The docking facilitation means comprise:

• Visual aid means to assist automatic docking of a utility agent like retro-reflectors.

• Passive mechanical means to enable fail safe docking of a utility agent,

• Optionally - electrical connection to obtain telemetry information from the propulsion unit 10.

• Optionally - power connection with the propulsion unit 10 to obtain power generated by solar panels mounted on the surface of the propulsion unit 10.

• Optionally - proximity laser.

• Optionally avionics (antenna) for exchanging signals with the utility agent at approach. These signals can be converted to appropriate telemetry that is routinely transmitted by the satellite. • Optionally a camera for capturing live image video of the approaching utility agent. The video stream can be feed to the satellite for transmission directly on earth through its standard downlink transmission capabilities.

• Optionally a capturing mechanism (as proposed in copending patent) that can extend lengthily enough to capture the utility agent and pull in the bay cavity to dock.

The propulsion unit 10 until its separation from the spacecraft has stable attitude, power and telecommand feed from, telemetry feed to and all the elements that can characterize it as cooperative-target, by the terminology of the OOS. The propulsion unit 10 can even move by the help of the spacecraft through active thrusts or use of the momentum wheels to perform collision avoidance manoeuvres if a risk is detected during approach of the docking spacecraft (utility agent, servicing vehicle). It can also perform a formation-flying process to synchronize the approach with the utility agent to increase the speed and accuracy of the docking operation.

Another preferred embodiment of the SINGLE USE AKM can incorporate a porting cavity that can carry a plurality of utility agents of either chemical propellants or of Electrical propulsion. In the latter case the utility agent can port for disposal of the propulsion unit 10 after separation at a desired orbit in a slow but economical way and then separate from it to become available for performing other OOS missions,

Version two (Non autonomous PASSIVE TANKER mode) comprising in addition to the previous:

• Oversized propellant storage tank(s) 20 to allow for surplus propellant quantities to become available after the completion of the GTO-to-GEO transfer,

• Pipe connector interface serving as a propellant transfer port 22 in the dok- king bay for filling or draining propellant.

• Optionally oversized pressurant gas tank(s) to allow for surplus quantities to become available, • Optionally pipe connector interface in the docking bay for fill/drain pressurant gas.

In this configuration the PASSIVE TANKER can be used before separation to provide fuel supplies to one or more visiting utility agents in successive refuelling sessions.

It can optionally have additional features to become an AUTONOMUS PASSIVE TANKER:

• A pipe-work configuration (with special valves that can enable the propulsion storage subsystem to survive unattended).

• A thermal protection blanket that can enable the propulsion storage subsystem to survive unattended.

• Optionally pipework to enable flushing out the liquid content of pipe segments that are expected to freeze when powered off and unattended.

Alternatively it can have other optional features to become an AUTONOMUS ACTIVE TANKER:

• Solar panel mounted 24 on the surface of the body.

• Minimal thermal control functions.

• Momentum wheel for orientation to ease in the docking process.

• Avionics to transmit telemetry in the proximity.

Version three (PORTING AKM) comprising the version two above (TANKER) and

• A number of docking bays 26 sufficiently large in size to encapsulate and port one active spaceship for use as utility agent or communcation unit for performing servicing functions and the like to other spaceships.

• Has a configuration of one/two/three/four thrusters 12

• In the case of a single thruster 12 two/three/four cavities or docking bays 26 shall be available axi-symmetrically at the bottom of the propulsion unit 10.

• In the case of double thrusters 12 one or two or three cavities or docking bays 26 shall be available. In the later case the central one can be placed exactly at the centre having complete refuelling dock apparatus while in the other two cavities would cater only for porting purposes. In this later case these utility agents can be equipped with electrical propulsion means.

The propulsion unit 10 may be beneficially used together with a modular spacecraft that has been designed to make use of a rejectable AKM or propulsion unit 10. In this configuration, fuel trading between the spacecraft and the propulsion unit 10 is considered. For this purpose, the spacecraft preferrably is equipped with easily connectable fill/drain valves, preferably by using quick couplers, and an appropriate dockig interface of a utility agent for performing fuel trading. This concept takes into consideration that almost 50% of the satellites have a major malfunction before their nominal end-of life. These malfuctions are associated in many cases with failure of electronics or failure of an electrical or another subsystem, while the propellant storage subsystem still functions. In these cases it would be beneficial to exploit the unused propellant reserves of the said malfunctioned satellite towards another satellite that needs additional fuel. So to speak a fuel trading capability is required. Equipping any satellite with the necessary easy to connect fill/drain valve would render such operation possible. This would also facilitate the ground operations that would further enable automation of the hazardous operation of refuelling.

Claims

1. Propulsion unit (10) for attachment to a selected spacecraft, comprising a number of thrusters (12), a number of propellant storage tanks (20), and a docking facilitation system (14), said docking facilitation system (14) being set up to be remove- ably connectable to corresponding docking means of the selected spacecraft.

2. Propulsion unit (10) according to claim 1 , in which said docking facilitation system (14) comprises an electrical connector and a mechanical coupling system.

3. Propulsion unit (10) according to claim 1 or 2, in which said docking facilitation system (14) comprises a coupling system designed for stacking several propulsion units (10) for joint handling.

4. Propulsion unit (10) according to one of the claim 1 through 3, in which said docking facilitation system (14) comprises a propellant transfer port (22).

5. Propulsion unit (10) according to one of the claims 1 through 4, in which said docking facilitation system (14) further comprises a data transfer port.

6. Propulsion unit (10) according to one of the claims 1 through 5, further comprising a number of solar panels.

7. Modular spacecraft comprising a payload and a satellite bus for station keeping, and further comprising a propulsion unit (10) according to one of the claims 1 through 6.

8. Servicing system for providing in-space service operations, comprising a propulsion unit (10) according to one of the claims 1 through 6, and further comprising a separable servicing vehicle, in which said docking facilitation system (14) is set up to establish a removable connection between said propulsion unit (10) and said servicing vehicle.

9. Servicing system according to claim 8, in which said servicing vehicle is equipped with communication means for establishing a communication channel to a remote control center through the propulsion unit (10).

10. Servicing system according to claim 8, in which said propulsion unit (10) comprises a number of docking bays (26) for temporarily containing said servicing vehicle.

11. Method for operating servicing system according to claim 8 or 9, in which said communication module is connected to said propulsion unit (10), and command signals for said propulsion unit (10) are processed and converted into triggering signals in said servicing vehicle.