[meteorite-list] Structural and Thermal Model of BepiColombo Arrive at ESTEC

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 9 Aug 2011 16:39:16 -0700 (PDT)
Message-ID: <201108092339.p79NdGMc020210_at_zagami.jpl.nasa.gov>


Structural and Thermal Model of the Mercury Planetary Orbiter arrives

European Space Agency
August 5, 2011

This is the first entry in the BepiColombo Test Campaign Journal, a
series of articles covering the main events during the integration and
testing of the Structural and Thermal Models (STM) and the Proto-Flight
Models (PFM) of the four major components that make up the BepiColombo
composite spacecraft stack. The journal is expected to run until the
completion of PFM testing in the fourth quarter of 2013; shortly after
this, the BepiColombo Launch Campaign Journal will begin.

* *

The BepiColombo Mercury Planetary Orbiter (MPO) STM arrived at ESA's
European Space Research and Technology Centre (ESTEC) in Noordwijk, the
Netherlands, during the night of 29/30 July. It had travelled by road
from the Turin facility of Thales Alenia Space, the MPO Prime
Contractor, where most of the spacecraft integration was performed. The
journey lasted nearly six days, because the unusually wide load could
only travel at night and had to follow a prescribed route. The orbiter
travelled in a specially built transport container that maintained it in
a temperature and humidity controlled nitrogen atmosphere and provided
protection against vibration and shocks.

On Saturday 30 July, the exterior of the transport container was cleaned
in an airlock at the ESTEC Test Centre and moved into a cleanroom. Once
it had reached thermal equilibrium overnight, the container was opened;
all personnel then left the cleanroom while the nitrogen from the
container dispersed. The MPO was then lifted out of the transport
container and mounted via its +Z face (the 'bottom' when it is mounted
on the launcher) on a ground handling trolley. The trolley allows the
spacecraft to be positioned with its Z-axis (the vertical axis in launch
configuration) anywhere from vertical to horizontal and to be rotated
360?? about its Z-axis, allowing full access.

A number of additional integration tasks are now being performed,
including the installation of the outer, high-temperature thermal
protection blankets. These tasks and preparations for the MPO
thermal-vacuum test will continue for almost three weeks; the MPO will
then be installed in the Large Space Simulator (LSS), beginning on 30
August. Closing of the LSS main chamber after MPO installation and
connection of all the test harnesses is currently scheduled for 9
September, followed by pumping and outgassing to achieve the required
vacuum of around 10-9 bar. Thermal-vacuum testing is expected to last
until the first week of October, with the MPO scheduled to leave the LSS
chamber by mid-October.

Harsh Thermal Environment

The BepiColombo MPO will face a particularly challenging thermal
environment while in orbit around Mercury. Not only will it be strongly
illuminated by the Sun, it will also orbit closer to its host planet
than previous spacecraft and will therefore experience much higher
levels of infrared radiation on its nadir-pointing panel.

To cope with these demands, the MPO is fitted with two sets of
multi-layer thermal insulation blankets; a special 30-layer
high-temperature blanket covers a more conventional 10-layer blanket.
The outer blanket employs a special fastening technique to avoid the use
of mounting holes in the blanket and the protrusion of stand-offs that
might be illuminated by the Sun. To minimise conductive coupling between
the two blankets, they are kept 15 mm apart and their facing surfaces
are highly reflective, to minimise radiative coupling. In total, the MPO
is fitted with 66 kg of thermal blankets.

The MPO is equipped with a very large radiator to transfer the heat
generated by its internal systems to deep space. Heat from the
electronics units inside the spacecraft is carried to the radiator by 93
heat pipes, the majority of which are embedded in the internal
structural panels. The radiator takes up the entire nadir-facing panel
of the spacecraft. It is protected from infrared radiation coming from
Mercury by polished titanium louvres that reflect the incident radiation
into space. The louvres will reach a temperature of around 400 ??C, while
the radiator will operate at 60 ??C.

A Testing Challenge

Testing to ensure that the MPO design will withstand the thermal
environment in Mercury orbit poses a challenge for the LSS. Reflectors
in the Sun simulator that focus the radiation from the 19 lamps onto the
spacecraft have been adjusted to concentrate the radiation and achieve
the highest ever level of illumination ??? 10 solar constants. To be able
to maintain a sufficiently low temperature on the thermal shrouds that
surround the MPO to simulate the cold of deep space, the flow rate of
the liquid nitrogen that cools the shrouds has been increased by a
factor of six, to 5000 l/hr. An additional shroud has been installed to
cool the MPO radiator.

Both Hot and Cold

It might seem odd that a spacecraft in orbit around Mercury and so close
to the Sun has to be able to withstand extreme cold, as well as extreme
heat. For all practical purposes, Mercury has no atmosphere (it has a
surface-bound exosphere at a pressure of around 10-14 bar), which means
that, both in orbit and during its journey through interplanetary space,
the only heat transfer mechanism for the spacecraft is radiation; the
convection cooling of hot surfaces experienced at Earth's surface is
absent. The result is that those parts of the orbiter that are
illuminated by the Sun or Mercury become extremely hot, since there is
no way for them to dissipate the incoming energy other than
re-radiation. Those areas of the orbiter that are in shadow or pointed
away from the infrared sources radiate thermal energy into deep space,
which has a temperature of about -270 ??C, and get very cold. The effect
of these large temperature differences is one of the aspects of the MPO
design that the thermal-vacuum testing is designed to investigate.

During the two weeks of thermal-vacuum testing, temperatures in and on
the MPO will be monitored by around 550 thermocouples, connected to a
dedicated thermal data handling system.

Future events

Scheduled Date Activity

12 September 2011 Arrival of Magnetospheric Orbiter Sunshade and Interface
                        (MOSIF) STM at ESTEC
14 September 2011 Arrival of Mercury Transfer Module (MTM) STM at ESTEC
15 November 2011 Arrival of Mercury Magnetospheric Orbiter (MMO)
                        Structural Model at ESTEC

About BepiColombo

BepiColombo is Europe's first mission to Mercury. It is scheduled to
launch in 2014 and arrive at Mercury in late 2020. It will endure
temperatures in excess of 350 ??C and gather data during a one year
nominal mission, with a possible one-year extension. The mission
comprises two spacecraft: the Mercury Planetary Orbiter (MPO) and the
Mercury Magnetospheric Orbiter (MMO). During the journey to Mercury, the
MMO will be shielded from the Sun by the Magnetospheric Orbiter Sunshade
and Interface (MOSIF), which also provides the interfaces for both the
MMO and the MPO. The fourth component of the composite spacecraft stack
is the Mercury Transfer Module (MTM), whose primary task is to provide
solar-electric propulsion for the journey to Mercury.

BepiColombo is a joint mission by ESA and the Japan Aerospace
Exploration Agency (JAXA), executed under ESA leadership.
Received on Tue 09 Aug 2011 07:39:16 PM PDT

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