bepicolombo mission and the solar electric propulsion...

ESA UNCLASSIFIED - For Official Use

BepiColombo Mission and the Solar Electric Propulsion System (SEPS)

Neil Wallace

17/10/2018

ESA UNCLASSIFIED - For Official Use Neil Wallace | 17/10/2018 | Slide 2

BepiColombo Mission to the planet Mercury

Launch: 02:45:38 BST, 20th Oct 2018


BepiColombo – why explore Mercury?

• Mercury is the missing piece to better understand the evolution of our Solar System – it is a planetary ‘odd-ball’

• Data that can be acquired from ground based observations is very limited

• Two space science missions have previously explored Mercury:

• Mariner 10 (flyby – 1974)

• MESSENGER (orbital mission, 2011/2015)


BepiColombo – why explore Mercury

• The BepiColombo mission will place two independent spacecraft, each containing a suite of scientific instruments, in different Mercury orbits to follow-up on the MESSENGER findings and investigate:

• Origin and evolution of a planet close to the parent star

• Interior structure, geology, composition, surface composition and craters

• Vestigial atmosphere (exosphere): composition and dynamics

• Magnetized envelope (magnetosphere): structure and dynamics, dual spacecraft mission to separate inner and outer fields,

• Origin of magnetic field


BepiColombo – Main challenges

• Mercury is innermost planet in the solar system, 0.3AU from the Sun:

• Spacecraft needs to survive the extreme thermal environment with different s/c surfaces exposed simultaneously to 15kW/m2 (10 Solar constants), planetary surface temperatures up to 450°C and 4K deep space

• Large amount of energy needed to manoeuvre between Earth to Mercury orbits

• Forces a unique spacecraft configuration employing high specific impulse (SI) electric propulsion (EP)


MTM Mercury Transfer Module

MPO Mercury Polar Orbiter

MMO Mercury Magnetospheric Orbiter

MOSIF MMO Sun-shield and interface


BepiColombo – spacecraft configuration


Launch mass: 4,100 kg • MPO P/L: 80 kg

• MMO P/L: 45 kg

• MPO: 1,150 kg

• MMO: 285 kg

• MTM: 1,160 kg, Xenon fuel: 580 kg

• MTM Chemical fuel: 160 kg

• MPO Chemical fuel 670 kg

Delta-V • 4000 m/s electrical cruise

• 80 m/s chemical cruise

• 1,000 m/s chemical orbit descent

MTM Propulsion: • 4 x 145 mN ion engines,

• Isp = 4,200s, 290 mN max

• 16 x 10 N thrusters, 8 x 22 N thrusters

Dimensions: • Overall height 6.3m

• Span: 30.4 m

• Solar Array: MTM: 13,200 W, 2 x 21 m2

• Solar Array: MPO: 2,000 W, 8.2 m2

MPO Propulsion: • 8 x 5 N thrusters

• 8 x 22 N thrusters


• The spacecraft employs a high temperature multi-layer insulation (MLI), includes a titanium ceramic outer layer to protect all spacecraft surfaces exposed to solar illumination or Mercury albedo

BepiColombo – Thermal solutions (MPO)


• Cannot encapsulate the entire spacecraft because the MPO also needs to reject waste heat generated within its own interior –

• Solution a radiator that allows simultaneous heat rejection from the planet and from the spacecraft



• The MTM also employs a sun-shield to shadow the underside of the spacecraft and the ion thrusters

• The shape of the MTM is designed to have the side radiator panels always in shadow

• The high dissipation electrical units are located on these panels directly mounted to heat-pipes to spread the heat across the entire panel

BepiColombo – Thermal solutions (MTM)



• The MTM also employs a sun-shield to shadow the underside of the spacecraft and the ion thrusters

• The shape of the MTM is designed to have the side radiator panels always in shadow

• The high dissipation electrical units are located on these panels directly mounted to heat-pipes to spread the heat across the entire panel


• The orientation of the MTM and MPO Solar arrays are constantly varied as a function of solar distance to reduce the energy density and maintain array within thermal design limits

• The leading edge of the array and the yoke are also shielded to prevent direct solar illumination



BepiColombo – the journey to Mercury

Professor Giuseppe ‘Bepi’ Colombo (1920 – 1984)

University of Padua, Italy http://www.esa.int/spaceinvideos/Videos/2017/07/Animation_

visualising_BepiColombo_s_journey_to_Mercury


BepiColombo – MPO and MMO orbits

• MPO: polar orbit for global coverage (1500 x 480 km)

• MMO: highly elliptical for magnetosphere coverage (11639 x 590 km)

• The inclination for the MPO & MMO orbits are the same to restrict the ΔV

• The initial MMO perigee of 590km was chosen such that the MPO perigee drops to 480 km during the MPO orbit insertion burns.


SOLAR ELECTRIC PROPULSION SYSTEM (SEPS)


Solar Electric Propulsion System (SEPS)

• 3 xenon tanks – 580kg of xenon

• High pressure regulator (bang-bang regulation)

• x4 T6 ion thruster, each mounted to independent gimbal mechanism

• x4 xenon flow control units

• x2 Power processing units

• Interconnecting harness and pipework


`

Anode

Solenoid

Earth screen

Xe flow

NEUTRALISER ASSEMBLY

Neutraliser

Xe flow

Cathode

Xe flow

Main flow

CATHODE ASSEMBLY

Screen Grid

Accel Grid

Baffle

Discharge Chamber

Backplate and Inner pole

Cathode Keeper

Front Pole

Cathode Tip

Insulators

Feromagnetic Circuit

Stainless Steel

Titanium Alloy

Magnetic Field Line

Molybdenum

Carbon

Tantalum

Solar Electric Propulsion Thruster (SEPT)



• The four thrusters are clustered together, recessed into the MTM structure

• Each thruster is mounted on a gimbal mechanism that allows the thrust vector to be adjusted for the different thruster combinations, s/c CoG migration, momentum wheel off-loading etc.



Video of thruster pointing mechanisms testing can be found at: http://www.esa.int/spaceinvideos/Videos/2017/07/Mercury_Transfer_Module_electric_propulsion_thruster_steering_test



• The four thrusters are clustered together, recessed into the MTM structure

• Each thruster is mounted on a gimbal mechanism that allows the thrust vector to be adjusted for the different thruster combinations, s/c CoG migration, momentum wheel off-loading etc.


FCU- xenon flow control unit

• Development and qualification by Moog-Bradford Engineering

Images courtesy of Bradford Engineering

PP

IV1

IV2FCV1 FCV2

LPT1 LPT2

FR1 FR2 FR3

F

HTR

Main Flow

Cathode Flow

Neutraliser Flow

T1

T3

T2


Power Processing Unit (PPU) - architecture

• The thruster requires 5 supplies:

• Anode • Constant current

• Solenoid/Cathode Heater • Constant current

• Accel Grid • Constant voltage

• Keeper/Neutraliser Heater • Constant current

• Beam • Constant voltage

-ve

Anode +ve

Accel. Grid

Neutraliser Keeper

Power Processor Unit (excluding FCU drivers)

Solenoid +ve

-ve

Beam Supply

Cathode

Neutraliser

Main Flow

Sol

enoi

d / H

eate

r Sup

ply

Cathode Heater

+ve

Neutraliser Heater

-ve

Hea

ter /

Kee

per

Sup

ply

-ve

-ve

Splic

e Pl

ate

Thruster

Spacecraft Ground


Images courtesy CRISA

• Each PPU effectively contains 2 systems

• Discharge, Accel & Neutraliser Supplies – DANS – Anode, Cathode Heater/Solenoid, Accel Grid

and Neutraliser Heater/Keeper

– Anode, Cathode Heater/Solenoid are HV referenced and contained within internal Faraday Housing

– PPU contains 2 sets DANS

– Each DANS can be switched to 2 thrusters

• Beam Supply Unit (BSU) – Parallel Beam Supply Modules (BSMs)

• FCU Drivers – Drivers can be switched to 2 FCU



Images courtesy CRISA

• Modular Beam Supply

– Provides a fault tolerant system

• SEPS PPU contains 4 BSMs

– SEPS requires 145mN minimum which is guaranteed with 3 BSMs

• Unit interfaces directly to heat pipes

• Unit footprint = 800mm x 420mm

DANS x2

BSU




• Each T6 thruster and its associated xenon Flow Control Unit (FCU) can be operated via either of the two cross-strapped Power Processing Units (PPU)

• Cross-strapped arrangement allows simultaneous operation of any two thrusters even in the event of the failure of any one system element

BSM1a

BSM1b

BSM1c

BSM1d

PPU#1

TSU#1

TSU#2

BSM2a

BSM2b

BSM2c

BSM2d

PPU#2

DANS#3 TSU#3

TSU#4

DANS#4

DANS#2

DANS#1

SEPT#1

SEPT#2

SEPT#3

SEPT#4



• Each T6 thruster and its associated xenon Flow Control Unit (FCU) can be operated via either of the two cross-strapped Power Processing Units (PPU)

• Cross-strapped arrangement allows simultaneous operation of any two thrusters even in the event of the failure of any one system element

BSM1a

BSM1b

BSM1c

BSM1d

PPU#1

TSU#1

TSU#2

BSM2a

BSM2b

BSM2c

BSM2d

PPU#2

DANS#3 TSU#3

TSU#4DANS#4

DANS#2

DANS#1

SEPT#1

SEPT#2

SEPT#3

SEPT#4


SEPS – coupling tests


SEPS – full system coupling at spacecraft level

• The SEPS was enabled and each thruster operated in discharge only mode

• Verified every SEPS electrical and fluidic interface

• High voltage testing also performed by enabling beam and Accel grid supplies


BepiColombo – meets the launcher


BepiColombo – mission animation

Animation of mission can be found at: http://www.esa.int/spaceinvideos/Videos/2018/06/BepiColombo_launch_to_Mercury

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