The T5 Ion Propulsion Assembly for Drag Compensation on GOCE

Dr C H Edwards and Mr N C Wallace QinetiQ Ltd, Cody Technology Park, Ively Road, Farnborough, GU14 0LX, UK

C Tato Astrium CRISA, C/Torres Quevedo, 9 Tres Cantos, 28760 Madrid, Spain

P van Put Bradford Engineering B.V., De Wijper 26 4726 TG Heerle, The Netherlands

ABSTRACT Due to the nature of the gravity field measurements to be made by GOCE, the satellite must fly in a near-circular, sun-synchronous, dawn-dusk orbit at an altitude of around 250 km. At this altitude the residual air drag is significant, and must be actively compensated by onboard thrusters. In addition, the gradiometer instrument is sensitive to linear acceleration, which must also be precisely compensated for by the thrusters. The dominant component of the drag force is in the primary flight axis, requiring a propulsion system capable of continuous throttling between 1 and 20 mN, with a thrust resolution of 12 µN, a response rate up to 2.5 mN/s, and a thrust vector stability of better than ± 0.2 degrees. In addition to the drag compensation role, the propulsion system will also be operated at a fixed high thrust during orbit-raising for the long eclipse season. These stringent requirements can only realistically be met using a highly controllable ion propulsion system, which on GOCE is known as the ion propulsion assembly (IPA). The QinetiQ T5 Kaufman-type ion thruster assembly (ITA) is the heart of this system, and is ideally suited for this mission, having been designed for a nominal thrust of between 15 - 25 mN, and including solenoid magnets which allow the operating parameters to be efficiently and accurately controlled over the required thrust range. As such the QinetiQ T5 ion thruster represents an enabling technology for the mission. This paper presents an overview of the IPA system and its components, and is intended to provide the GOCE scientific users with an understanding of the technology, its performance, and the challenges, which have been overcome to provide it.

• INTRODUCTION The Gravity Field and Steady State Ocean Circulation Earth Explorer (GOCE) Mission is the first of a series of “Earth Explorer Core Missions” that the European Space Agency is to undertake in the frame of its “Living Planet Programme - Earth Observation Envelope Programme”. The mission objective of GOCE is to provide unique models of the Earth’s gravity field and of its equipotential reference surface, as represented by the geoid, on a global scale with high-spatial resolution and very high accuracy. This will not only help to advance knowledge of the Earth’s interior structure but will also help to develop a much deeper understanding in areas such as ocean circulation, ice sheet balance and thus climatology. The Agency has selected a Core Team consisting of Alenia Spazio as the GOCE Prime Contractor, with Astrium GmbH as the Platform responsible Contractor, Alcatel Space Industries as the Gradiometer Instrument responsible Contractor and ONERA as the supplier of the Gradiometer accelerometers and of the related support at Gradiometer Instrument and System Level. Astrium GmbH as the Platform Contractor is responsible for provision of the Ion Propulsion Assembly, including the overall system design, specification and procurement of the individual products of the IPA. The following products make up the IPA and are to be supplied by the indicated sub-contractors:

______Proc. Second International GOCE User Workshop “GOCE, The Geoid and Oceanography”, ESA-ESRIN, Frascati, Italy, 8-10 March 2004 (ESA SP-569, June 2004) • Ion Thruster Assembly (ITA) QinetiQ Ltd. including control algorithms QinetiQ Ltd. • Ion Propulsion Control Unit (IPCU) Astrium-CRISA including HV transformer and Ion Beam converter. Astrium GmbH and flight software QinetiQ Ltd. • Proportional Xenon Feed Assembly (PXFA) Bradford Engineering B.V. (including flow control algorithms) The main function of the IPA is to compensate in real-time for the drag force experienced by the satellite operated in the proposed GOCE orbit. The orbit is a near-circular, sun-synchronous, dawn-dusk orbit at an altitude of around 250 km, and leads to a drag profile on the spacecraft consisting of relatively large variations at a frequency equivalent to the orbital period, superimposed by smaller magnitude variations at higher frequencies up to 10 Hz.

• THE ION PROPULSION ASSEMBLY A schematic of the IPA architecture is shown in Figure 1. The system is a cold redundant architecture, comprising two Ion Thruster Assemblies (ITA), which are powered and controlled by two Ion Propulsion Control Units (IPCU), and fed propellant directly from the tank by two Proportional Xenon Feed Assemblies (PXFA). The Xenon Storage Tank and associated pipework complete the assembly, but these items are not discussed in this paper. The IPCU provides overall control of the system, receiving power, timing and enable commands directly from the spacecraft and thrust control commands from the Drag Free Attitude Control System (DFACS) via the MIL-1553 Bus. These control commands are interpreted by the IPCU, and converted into the appropriate demand signals for the ITA and PXFA using software and control algorithms defined and supplied by QinetiQ Ltd.

2 x Primary Power

2 x 1 PPS Nominal 1 x Status 2 x reg. / 5 x fixed current, IPCU 4 x fixed voltage to ITA 2 x HPC 1 x MIL-Bus nom. Nominal 1 x MIL-Bus red. ITA

6 x Signal 8 x Sec. Power

Neutraliser

PXFA 1 53B

5 1 x reg./ 2 x fixed Xenon Flow Xenon Tank Bus 1

L- 1 x reg./ 2 x fixed Xenon Flow I PXFA 2 M

Neutraliser

8 x Sec. Power 6 x Signal Redundant 1 x MIL-Bus red. ITA

2 x HPC 1 x MIL-Bus nom. Redundant 1 x Status IPCU 2 x 1 PPS 2 x reg. / 5 x fixed current, 4 x fixed voltage to ITA 2 x Primary Power

Figure 1 IPA Architecture The IPA products are mounted on the propulsion module panel, in the layout shown in Figure 2. The IPCUs are mounted on the inside surface of the panel, while the PXFAs and ITAs are mounted on the external surface. The ITAs are mounted together on a separate structure close to the centreline of the spacecraft. Each ITA is canted to a nominal angle of 2.4 degrees, and includes an adjustable mounting bracket, to ensure that the nominal thrust vector can be aligned with the spacecraft centre of gravity prior to launch.

Figure 2 Ion Propulsion Module Layout

• ION THRUSTER ASSEMBLY (ITA) The GOCE ITA is based on the existing T5mkV thruster design, as shown in Figure 3. It is of conventional Kaufman configuration, with a direct current (DC) discharge between a hollow cathode and a cylindrical anode used to ionise the propellant gas. The efficiency of this plasma production process is enhanced by the application of a magnetic field within the discharge chamber, which can also be used to provide accurate throttling. A 10 cm diameter grid system, forming the exit to the discharge chamber, extracts and accelerates the ions, to provide the required thrust. The thrust produced depends on the number and velocity of the ejected ions. The number of ions extracted is dependent on the ion density in the discharge chamber and the transparency of the grid system, and the velocity depends only on the accelerating potential applied to the ions. An external hollow cathode, referred to as the neutraliser, emits the electrons necessary to neutralise the space charge of the emerging ion beam.

SOLENOID

ISOLATOR ANODE

OUTER POLE

PROPELLANT HOLLOW CATHODE SCREEN ACCELERATOR KEEPER BAFFLE GRID GRID

CATHODE ISOLATOR SUPPORT INNER POLE

MAIN FLOW DISTRIBUTOR

DISCHARGE CHAMBER

INSULATORS

FERROMAGNETIC CIRCUIT

MAGNETIC PROPELLANT FIELD LINE

EARTHED INSULATOR CATHODE SCREEN NEUTRALISER ASSEMBLY

Figure 3 QinetiQ T5 mkV Thruster Table 1 ITA Key Parameters

Mass 2.95 kg (including adjustable mounting bracket) Dimensions ∅ 190 mm x 242 mm long (including adjustable mounting bracket) Grid ∅ 100 mm Thrust range 1 to 20 mN Thrust Noise 1.2 mN/√Hz @ 1 mHz to 0.012 mN/√Hz @ 100 Hz Power 55 W to 585 W (across thrust range) Specific Impulse 500 s to 3500 s (across thrust range) Total impulse capability > 1.5 x 106 Ns (under GOCE continuous throttling conditions) Cycle life GOCE requirement: > 1000 On/Off cycles T5 capability: > 8500 On/Off cycles Thrust vector stability < ± 0.1 degrees (across thrust range)

Beam divergence < 25 degrees (2σ half-cone angle @ 1 mN) < 12 degrees (2σ half-cone angle @ 20 mN)

The GOCE requirements have lead to challenges in terms of the grid design. In particular, the wide thrust range, coupled with the long lifetime requirement have necessitated a detailed optimisation of the grid design to be performed. The grid optimisation entailed detailed modelling of the ion extraction and erosion processes using an ion- optics modelling tool called SAPPHIRE. The grid modelling led to the adoption of a twin grid configuration, with the ion optics system optimised to the mission profile and lifetime requirements. The accelerator grid material was also changed to graphite, in order to provide additional operational margin whilst reducing mass

• ITA CONTROL ALGORITHMS The ITA is controlled using a set of algorithms, specifically designed to meet the challenging GOCE requirements. The control algorithm architecture is shown in Figure 4. The ITA uses three of its input parameters to control the output thrust. The flow rate and anode current are adjusted relatively slowly, in an open-loop mode, to provide a coarse control of the thrust, while the solenoid magnet current is adjusted quickly, in a closed-loop mode, to provide fine control for high accuracy and quick response.

IPCU

Anode Anode Current Demand Filter Control Schedule

Flow Rate Flow Rate Control PXFA Demand Filter Schedule ITA Achieved Thrust Thrust Thrust Demand Error Magnet Control Gain Function

Measured Thrust

Beam Telemetry

Figure 4 Control Algorithm Schematic A thrust demand is input to the IPCU from the spacecraft DFACS at a frequency of 10 Hz. Within the IPCU software, the raw demand signal is filtered to provide smoothed demand inputs for the flow rate and anode current controls. The IPCU then uses the control algorithm schedules to calculate the anode current and flow rate demands for input to the ITA and PXFA respectively. The PXFA uses a separate control loop to ensure that the flow rate is provided to the ITA with high accuracy. The details of these control schedules have been determined from extensive ITA performance testing on the pre-verification model thrusters. Fine control of the output thrust is provided by closed loop control of the solenoid magnet current. The IPCU software compares the raw demand with the measured thrust, and the resulting error signal used to drive the magnet current. The measured thrust is calculated directly from the ion beam current and voltage telemetry, with a correction factor applied to take into account physical effects on the actual thrust such as beam divergence, doubly charged ions and neutral propellant flows. Because of the interactive nature of the three control parameters, the magnet current gain is a complex function of thrust demand, thrust error, flow rate and anode current. The details of this function have again been determined from PVM ITA test data, and will be optimised and verified during IPA system testing.

• ION PROPULSION CONTROL UNIT (IPCU) The IPCU has been designed specifically to meet the GOCE requirements, utilising EADS Astrium-Crisa’s extensive experience in space power. The design is presented in the functional schematic of Figure 5, and provides the following functions:

• Control Electronics – provide TC/TM communication with the spacecraft via the two MIL-1553 interfaces, timing synchronisation with the spacecraft using a PPS signal, and implements the PXFA interface. • AC Inverter – converts the DC spacecraft power into two AC power outputs for the low voltage (LV) and high voltage (HV) power supplies. • Ion Beam Converter (Astrium GmbH) - converts the DC spacecraft power into the HV DC source required for the ion beam. • LV Control – provides auxiliary DC/DC conversion for internal IPCU functions and provides TM/TC links between the Control Electronics and the LV Supplies and HV Control. • LV Supplies – implements the LV power supplies, interfacing directly with the ITA. • HV Control * – provides auxiliary DC/DC conversion for internal IPCU functions and provides the TM/TC link with the LV Control • HV Supplies * – implements the HV power supplies, interfacing directly with the ITA. • Note that the HV Control and HV Supplies functions are housed within an electrically isolated box.

Table 2 IPCU Key Parameters

Total IPCU Mass 16.7 kg Dimensions 300 mm x 250 mm x 200 mm (approx.) Input Voltage Range 22 – 37 V, extended input range to 20 V without degradation Maximum Input Current 37 A @ 22 V IPCU Electrical Efficiency Beam converter 92 – 95 %, other supplies > 92 %

Figure 5 IPCU Functional Diagram

• PROPORTIONAL XENON FEED ASSEMBLY (PXFA) The Proportional Xenon Feed Assembly provides accurately metered Xenon propellant flow to three separate flow components within the ITA. A photograph of the breadboard model and a general functional schematic of the system are presented in Figure 6. The PXFA is split into pressure control and flow control sections. The pressure control section receives Xenon from the tank at high pressure (between 5 – 125 bar) and regulates it down to a pressure of exactly 2.50 ± 0.05 bar. This section also includes particle filters, pressure transducers for monitoring the tank and supply pressures, and isolation valves to allow switching between primary and redundant units. The flow control section receives the regulated pressure Xenon and provides accurately metered flow to the ITA main flow, cathode and neutralizer. The cathode and neutralizer flows are metered to fixed levels using advanced viscosity/thermal controlled passive flow restrictors, but the main flow must be varied over a wide range to allow the thruster to be throttled efficiently. This variable flow function is provided by a combined flow control valve and flow sensor system, which allows the flow sensor output to be used for closed loop control of the flow control valve.

Table 3 PXFA Key Parameters

Mass 7.5 kg Dimensions 150 mm x 200 mm x 350 mm (approx.) Input Pressure Range 5 – 125 bar Regulated Xenon Pressure 2.5 bar Main Flow Rate 0.01 - 0.63 mg/s ± 5% Cathode Flow Rate 0.11 ± 0.007 mg/s Neutraliser Flow Rate 0.041 ± 0.006 mg/s Micro-disturbance level < 1.1 x 10-6 m/s2/√Hz

PHigh = 125 - 5 bar

PR1 Incl. Inlet Filter

PLow = 2.5 (+/- 0.05) bar

LPT1 P TP1

GP1

F FS1

IV1 FCV1

TP2

FR1 FR2 IV2

Neutraliser Main Cathode Figure 6 Bradford Engineering B.V. PXFA Photograph and Schematic

The main challenges for the PXFA design have been the provision of low level, variable, and highly accurate main flow, and the elimination of significant micro-disturbances. The main flow has made use of new proportional flow control valve and flow sensor technologies, incorporated in a closed loop control system. Elimination of micro- disturbances has been achieved by designing isolation features at component and flow control section level, as well as performing highly accurate modal-stiffness modelling of the complete system.