Electronics for Electric Propulsion

J. Palencia26 October 2017

Electronics for Electric PropulsionGridded Ion Technology

26 October 2017 Electronics for Electric Propulsion2

Contents

– About Crisa– Electric Propulsion Concept– Electric Propulsion System Elements– Electrostatic Thruster Operation– Power Processing Unit Overview– PPU Design Keys– Examples and missions

About Crisa


Overview

• Crisa: Computadoras Redes e Ingenieria S.A. (today subsidiary of Airbus Defence & Space)

• Allocated in Tres Cantos (northern of Madrid) employs about 400 qualified employees

• 30 years in Space business with more than 800 flight units delivered, Crisa has contributed to most of the European Space Agency (ESA) programs

• Activity: Strong reputation as high technology firm in Space (avionics for satellites, launchers and space vehicles) and Ground Segment both in civil and defence applications.


Product Lines


Applications


Electric Propulsion Concept


Spacecraft Propulsion Basic Concepts

Spacecraft propulsion is measured basically with two parameters:


Thrust (measured in Newtons):

Thrust Vemt

Specific Impulse (measured in seconds):It is the amount of time an engine can generate thrust, given a quantity of propellant whose weight is equal to the engine's thrust.

Thrust Ispmt

g

Electric Propulsion Overview

Electric Propulsion is based in the use of electrical power to expel the propellant at a very high speed.

Due to the high exhaust speed obtained the mass required to get a given thrust is much lower than using chemical rockets

On the other hand, due to the limitations of electrical power available in spacecrafts the maximum thrust that can be obtained is very low (hundreds of mN maximum)

A key parameter in electric propulsion systems is the thrust to electrical power ratio: W/mN

There are many types of electric propulsion engines but the more extended are the Electrostatic Thrusters


Electric Propulsion Overview


Engine Effective exhaust velocity (m/s) Specific impulse (s)

Space Shuttle Solid Rocket 2500 250Ion thruster 29000 3000

Advantages of Electric Propulsion:• Very high exhaust speed: fuel mass saving• Very high Specific Impulse: long time operation

Drawback:• Thrust level depends on electrical power available (usually low in spacecrafts)

Electric Propulsion Applications

• Station keeping manoeuvresEP saves fuel mass compared with other propulsion systems as cold gas, what allows to increase the number of payloads, instruments,…or reduce mass during launch

• Deep space scientific missionsEP allows to achieve very high speeds maintaining a low thrust level operating for very long periods of time

• Orbit RisingUsed together with Station keeping allows to remove additional propulsion systems however it delays the start of the spacecraft operation


Electric Propulsion System Elements


Electric Propulsion System


High Pressure Regulator

• Xenon is stored in a high pressure tank

• The High Pressure Regulator Controls the pressure of the Xenon supplied to the EP system

• Usually controlled by the platform directly


Xenon Flow Controller (Flow Control Unit)

• The FCU controls the amount of Xenon delivered to the Thruster and Neutralizer

• It is built of a set of open/closed valves (isolation valves) and flow restrictors (mechanical, thermal,…)

• It is usually controlled by the PPU but it can be also controlled by the platform directly


Flow Controller, courtesy of MOOG Bradford, NL

Thruster Assembly

• Composed typically by the Thruster itself

Generating the thrust

• And the Neutralizer

In charge of plasma neutralization


T5 Thruster courtesy of QinetiQ, UK

The Power Processing Unit (PPU)

The basic function of the PPU is to condition the power coming from the platform electrical system to supply the different elements of the Thruster and FCU following the commands received from the on board computer.

Usually the PPU is much more complex and includes:- Monitoring of the system- Full control of the system operation (start up, thrust control, switch off)- Monitoring of protections and Failure Detection Isolation and Recovery

The goal is that the EP system operation, in nominal conditions, is almost autonomous and OBC intervention is not necessary except for switch on/off the system.


Electrostatic Thruster Operation


Electrostatic Thruster

Electrostatic thrusters operation consists typically of three basic stages

1. Gas (propellan) IonizationUsually Xe is used as fuel

2. Ions AccelerationPositive ions (Xe+) are accelerated to exhaust velocity using electrical fields generating the thrust

3. Ions NeutralizationAn additional cathode emits electrons to neutralize the electrical charge of the ions expelled to avoid electrical charging of the spacecraft


Electrostatic Thruster


Differences between electrostatic thrusters

• Ionization process– Electrons bombardment– HF Electromagnetic field– Others…

• Generation of the electric field– Grids at different potential– Electrons cloud– Others…

The Gridded Ion Thruster

• It is characterized by the use of a set of grids (usually 2) with a high differential voltage applied between them to get the acceleration of the ions.

• Ionization process may be done in different ways.


Power Processing Unit Overview


PPU Basic Blocks

• CONTROLThe brain of the PPU, it communicates with the platform and perform all the control of the system

• FCU SUPPLIESProvide controlled and conditioned power to the elements of the FCU

• NEUTRALIZER SUPPLIESProvide controlled and conditioned power to the Neutralizer elements(Neutralization process)

• THRUSTER SUPPLIESProvide controlled and conditioned power to the Thruster elements(Ion generation and extraction process)


CONTROL

THRUSTER SUPPLIES

NEUTRALIZER SUPPLIES

FCUSUPPLIES

PPU

PPU Control

• The control of the PPU is usually performed by a uProcessor or a FPGA depending on the complexity of the program and the flexibility required by the system.

• It is in charge of:Communications with platform, telecomand and telemetryExecute the sequencing required to control the EP System (start up, switch off)Execute the closed loop required to keep the thrust level constantMonitor the system and execute recovery actions or report to the system in case of

failures


CONTROL

PPU

PPU Control


CONTROL

PPU

FCU Supplies

Typical FCU elements are:• Valves• Heaters• Restrictors

Depending on the type of elements to be driven the PPU may need to provide:• Constant DC Voltage / Current• Programmable DC Voltage / Current• Specific Voltage / Current patterns

Simplest design solution (not always possible):• Single DC Voltage supply + one switch per element to control• Switches controlled by the FPGA (PWM mode)


FCU Supplies


Typical driving figures:Voltage < 32 VCurrent < 0.5 A

Neutralizer Supplies

The neutralizer is a cathode that has to be initially heated and once the discharge process starts, a constant current has to be provided to maintain it.

The control of the neutralizer requires typically two power supplies:• One for the heater element (Heater supply)• Another one supplying the cathode itself (usually called Keeper supply)

The Heater supply powers basically a resistive load with a constant current (quite simple)

The Keeper supply powers the cathode (plasma load) what requires a constant current and wide bandwidth current source, additionally some cathodes may need high voltage pulses to start the discharge process.


NEUTRALIZER SUPPLIES

PPU

Typical driving figures:Heater supply: < 50 V & < 5 AKeeper supply: < 50 V & < 5 A (Initial pulses: 300 to 450 V < 100 mA)

Thruster Supplies

Two supplies are required to generate the electrical field to extract the positive ions• One providing positive high voltage (Beam supply) to the screen grid and discharge chamber• One providing negative high voltage (Accelerator grid) to the Accelerator grid supply

Both are constant DC voltage supplies with relatively wide bandwidth as the load is plasma

The Beam supply provides most of the power delivered (about 80 to 90%) at high voltage


Typical driving figures:Beam supply: 1 to 2 kVAccelerator supply: - 250 V to -1 kV (< 100 mA)

THRUSTER SUPPLIES

PPU

Xe+

e-

Screen gridAccelerator grid

Beam Supply

Accel Supply

Thruster Supplies

Ionization of Xenon requires additional power supplies

The features of these Power Supplies will depend on the ionization method required by the thruster:

• Electron bombardment requires an additional cathode within the ionization chamber• HF Electromagnetic field requires a radio frequency generator


THRUSTER SUPPLIES

PPU

PPU Design Keys


Key Electronics Design Issues

Main problems the Electric Propulsion Electronics design has to address

• Loads characterization / simulation

• Grounding / Noise

• High Voltage management

• Thermal issues

• Testing



Loads characterization / simulation

• Most of the power supplies have to provide energy to plasma loads

• Behaviour of plasma as electrical load is quite complex (even in steady state conditions the impedance is not constant, it suffers many transients)

No reliable mathematical / simulation models

• Power supplies have to be tested against the real load to validate / adjust the design



Grounding / Noise

• Grounding of EP Systems is specific

• Part of the electronics referenced to Neutralizer return (floating) not referenced to platform but sharing the same mechanical case

• Neutraliser return may oscillate tens or hundreds of volts DC and AC.

• Plasma is also a source of noise in the range of kHz that is injected to the power supplies



High Voltage Management

• Voltages above 250 V are difficult to manage in space applications specially in the range of kV

• When pressure falls to vacuum it has to go through partial pressure

• In vacuum Triple Junction effect is the enemy



High Voltage Management / Potting


• Withstands partial pressure

• Not affected by triple junction

• Not reparable

• Poor thermal behaviour

• High cost

• Quite massive


High Voltage Management / Open design - distance


• Less massive than potting

• Reparable

• Better thermal behaviour

• Affected by triple junction

• Does not withstand partial pressure

• High cost


Thermal issues

• Derived mainly from the high voltage isolation

Use of “plastic materials” increase also thermal isolation

Use of “ceramic materials” is better from thermal point of view but has mechanical issues



Testing

• Load behaviour very complex, simulation using hardware is usually not realistic

• Validation of PPU requires to couple with the Thruster

• Only possible in vacuum conditions (for Thruster operation)

Limited available facilities

Very expensive tests


Examples and Missions


GOCE MISSION

• Goal: Gravity field measurement and steady-state Ocean Circulation observation

• Launched: March 2009 for 20 Months mission, mission over in 2013 after 56 Months operation

• Orbit: 260 km

• Mass: 1100 kg including 40 kg of Xenon

• Electric Propulsion: Mission: Drag compensation, Ion thrusters commanded in closed loop 2 x 20mN T5 Thrusters (by QinetiQ) commanded by 2 PPUs (by Airbus DS) with a resolution of 100uN


GOCE PPU

IPCU: Ion Propulsion Control Unit

• Power delivered: < 1 kW

• Beam voltage: 1.2 kV

• Mass: 17 kg


GOCE MISSION


ESA Courtesy ESA Courtesy

BEPI-COLOMBO MISSION

• Goal: Mercury observation, get scientific data during 1 year.

• Launch: October 2018, 7 years cruise navigation until arriving to Mercury in 2025

• Mass: 4100 kg

• Electric Propulsion: Mission: Cruise navigation 4 x 145mN T6 Thrusters (by QinetiQ), two simultaneously commanded by 2 PPUs (by Airbus DS)


BEPI-COLOMBO MISSION


BEPI COLOMBO PPU

PPU: Power Processing Unit

• Power delivered: 5 kW

• Beam voltage: 1.85 kV

• Mass: 47 kg



Thanks for your attention

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Electronics for Electric Propulsion