[meteorite-list] Hot Stuff: The Making of BepiColombo

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed, 1 Jun 2011 18:05:15 -0700 (PDT)
Message-ID: <201106020105.p5215FkJ008556_at_zagami.jpl.nasa.gov>

http://www.esa.int/esaSC/SEMZEUISDNG_index_0.html

Hot stuff: the making of BepiColombo
European Space Agency
1 June 2011

For BepiColombo, ESA has had to extend the limits of existing design
standards and develop altogether new design concepts as well. How to
begin building a spacecraft that needs to endure sunlight 10 times more
intense than in Earth orbit, with surfaces hotter than a kitchen hot
plate ??? high enough, in fact, to melt lead?
 
Back in late 2000, when the mission was first selected, no-one knew for
sure.
 
1. Achieving a close-up view of Mercury
 
BepiColombo will be the third mission to visit the innermost planet
after NASA's Mariner 10 in the 1970s and the current Messenger.

It is three spacecraft in one: ESA's Mercury Planetary Orbiter (MPO),
Japan's Mercury Magnetosphere Orbiter (MMO) plus ESA's additional
Mercury Transfer Module (MTM) to convey the other two across
interplanetary space.

But BepiColombo will be taking a much closer look than its predecessors:
Mariner 10 only flew past while Messenger has entered a
highly-elliptical Mercury orbit. While MMO will also follow an
elliptical orbit, the planet-mapping MPO will orbit much more tightly,
coming as close as 400 x 1500 km from Mercury's heat-radiating surface.

In certain orbital positions, when the orbiter comes between the Sun on
one side and Mercury on the other, it will have to endure temperatures
as high as 450 degrees C.
 
2. Technology making the mission possible
 
"A considerable team of researchers was involved in making the mission
feasible," comments Jan van Casteren, BepiColombo Project Manager. "An
exceptional amount of technology development and demonstrations has been
needed across a variety of fields."

Just reaching Mercury presents a major challenge: a new generation of
highly efficient electric propulsion was required, capable of achieving
the tens of thousands of hours of thrust needed to enter orbit.
 
3. How the spacecraft keeps its cool
 
Then comes the problem of thermal management, which drives the
spacecraft design. Slice through MPO and you would see a complex
labyrinth of heat pipes. Previously used on a variety of missions, these
sealed pipes work like a closed-loop version of human sweat glands,
containing liquid whose evaporation carries excess heat from MPO???s
sunward-side to radiating plates facing deep space. The liquid then
condenses, allowing the process to begin anew.

The heat pipe concept helps keep MPO's interior within room temperature.
What was new was the 2 x 3.6 m size of the radiator, and the operational
constraints it faced: "Its radiating plates must remain cold and shaded
for it to work," explains Ulrich Reininghaus, BepiColombo Spacecraft
Development Manager.

"If they ever come into sustained contact with sunlight, or the infrared
radiation emitted from Mercury's surface then they would stop working."

The mission had to develop a unique set of coated louvres that prevent
the radiator "seeing" the hot planet below while not preventing its own
radiation escaping to cold deep space.
 
4. Searching for material solutions

Expelling internal heat only goes so far, however; much better if it
never makes it inside the spacecraft at all.

The real technical challenge has been finding new materials for
everything on the outside of the spacecraft in particular - including
antennas, the solar array and its associated Sun-tracking sensors and
mechanisms and again the radiator and protective multi-layer insulation
(MLI) - which would be able to withstand the Sun's tenfold increase in
brightness and associated temperature extremes.
        
"We began a critical materials technology programme for BepiColombo at
the start of 2001," comments Christopher Semprimoschnig, head of the
Materials Space Evaluation and Radiation Effects Section of ESA's
Materials and Components Technology Division.

"We've kept busy for approaching a decade, gradually qualifying
materials. It's been a huge challenge because we had no previous
experience of such a harsh environment. The closest we ever came was
with Venus Express, though that meant handling two solar constants
rather than 10."
 
5. Testing, testing...
 
ESA's Materials and Processes engineers were involved because they had a
good understanding of what materials could be candidates, as well as of
related fields that might offer useful "spin-in" technologies, such as
protective coatings on jet engine turbines.
 
It took years to develop the laboratory facilities required for testing,
however, adapting existing facilities wherever possible. "When you
increase the light and heat intensities you are operating with 10 or 20
times compared to before, then failures can happen," Christopher says.

"We had to deal with melted lamp holders, melted reflectors, but we
gradually managed to build some representative simulation chambers like
our Synergistic Temperature Accelerated Radiation (STAR) facility."

6. A life in the Sun

ESA's materials experts needed to predict the end-of-life condition of
all the materials in question. How might specific mission-critical
properties change after years of intense solar glare? Would reflective
coatings discolour, MLI crack, solar arrays lose electrical performance
or thermal emittance - the crucial ability to radiate away heat?
 
:Total exposure will be something like 100 000 equivalent Sun hours,"
explains Christopher. "Traditionally we boost illumination levels for
accelerated lifetime testing. But a move up from 11 solar constants to
30 or 40 is not so easy.

"The accuracy is uncertain, due to non-linear effects - the materials
might unexpectedly fail for some unknown reason."

End-of-life estimates for Venus Express offered a starting point: the
five-year-old mission remains in good health, showing the team's
original estimates had been broadly accurate.
 
7. Less air for more precise testing
 
In such an extreme environment, everything degrades, like plastic left
out in the Sun. But precisely how critical properties degraded over time
in orbit needed to be exactly understood.

For example, it was found that when test items were removed from their
vacuum chamber then their subsequent exposure to air would induce
chemical interactions with radicals within the material.

"These radicals are basically degradation products, so this alters the
state of degradation," says Christopher. "A day later when a measurement
is done, their condition could be very different. So if we extrapolate
from these results we would get a performance curve, but the real curve
would end up being much worse.

"So we set up a system to make measurements while still in vacuum,
saving us time and letting us assess changes much more reliably."
 
8. Testing to breaking point and beyond

ESA needs to be sure that its chosen materials would function reliably
for years on end.

"We are going to the limit of a material's performance, seeing what
happens when it breaks down," Christopher adds. "The result is a wealth
of information that could be of interest to many other industries as well."

The programme continues to qualify all the materials needed for the
mission, currently standing at around 75% complete.
 
The MLI covering the bulk of the spacecraft's surface is foreseen to be
a woven ceramic fabric. There are multiple layers kept apart by spacers,
designed to be as light as possible - some of the layers have less than
a tenth the thickness of printer paper, just 7.5 micrometres across.

"The result is much lighter than metallic foil but also more brittle,"
says Christopher. "Now we need to look at processing issues: how to
stack it, what shapes can it fit around and how to handle it without
damage and release of particles."
 
9. A way to save the solar arrays
 
Solar cells became the single most challenging material question.
Dramatic degradation in solar cell performance was detected: just one
simulated month saw a 20% power loss.

"This failure brought the mission to the brink of cancellation,"
recounts Christopher. The effect was due to a combination of material
degradation from ultraviolet radiation and high temperatures driving
down cell efficiency.
 
A combination of protective coatings and carefully solar array tilting
offers a workable solution. If the solar arrays directly face the Sun
then they would heat up and fail. So instead they stay tilted at an
optimum angle. Their power production stays lower, but so does the
temperature.

The main antenna also requires protective coatings, though for a
different reason. It is made of thin titanium for maximum performance:
it needs to perform highly accurate radio science experiments to
determine how spacetime curves around the Sun.

By itself it would warm up like any other metal in the Sun - to as high
as 700 degrees C. But temperature-driven deformations have to be prevented. A
specially-tailored coating should keep its temperature 300 degrees C lower while
allowing electromagnetic signals to pass through freely.
 
10. Spacecraft-level testing has begun
 
A test model of Japan's MMO arrived in the Netherlands in mid-September
2010 for testing in the Large Space Simulator (LSS) at the ESTEC Test
Centre, the largest vacuum chamber in Europe. The LSS's Solar Simulator
was carefully adjusted to attain 10 solar constants, its light beam
being brought into much tighter focus.

"To safely remove the resulting heat from the chamber walls we installed
an extra thermal shroud with a more than six times greater flow of
liquid nitrogen than the existing system," explains Alexandre Popovitch,
overseeing modifications. "That required around 5000 litres of liquid
nitrogen per hour of each two-week test."
 
There were two sets of tests, one with MMO free-spinning - as it will
operate during its active life - then one with an ESA sunshield that
will keep it cool as it rides as a passenger to Mercury.

This summer, test models of Europe's BepiColombo spacecraft will go
through the same experience. Follow-up versions incorporating any
lessons learnt will be ready for evaluation in 2012, with the launch of
BepiColombo scheduled for 2014.

The materials team, meanwhile, is looking forward to ESA's Solar Orbiter
mission - destined to venture even closer to the Sun.
Received on Wed 01 Jun 2011 09:05:15 PM PDT