
NASA Mars Mission Faces Setback After Heat Shield Cracks Under Pressure - yeukhon
https://www.npr.org/sections/thetwo-way/2018/04/27/606349244/nasa-mars-mission-faces-setback-after-heat-shield-cracks-under-pressure
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billforsternz
This is one of the reasons engineering is a difficult field, not just edge of
the envelope engineering like this but routine product development. Sometimes
it amazes me that anything ever gets shipped.

Think about airliners. After billions of dollars of development, you have to
pass critical safety tests (eg evacuation time tests). What happens if you
fail the test in such a way that you can't fix it without more-or-less
starting from scratch? How can anyone sign off on the development schedule?
Yet they do and somehow or other eventually products tend to get shipped.

Back in the day when I was a working engineer (embedded systems, telecom
systems) I decided there was basically an unspoken conspiracy of silence. The
pointy heads would demand a schedule. The engineering staff would provide it.
There would be a generous allocation for all kinds of testing. But sometimes
the results of testing would require new board layouts or other things that
were basically catastrophic for the schedule (I'm thinking things like
electromagnetic compatibility testing - stuff that wasn't in my wheelhouse
thank god). The schedules would never factor in serious go-arounds like this
yet it could and did happen and was one reason schedules were rarely met.

To me the unspoken conspiracy was that we know we can't really control the
development trajectory, but we'll sign off on it anyway. Because what else are
you going to do? Apologising later is more practical than seeking permission
for an open-ended schedule.

~~~
lutorm
To me, a "schedule" implies intent, not a promise. (Just like a "budget".)
There always exist factors out of your control that can blow them, and
everyone hopefully is cognizant of that fact.

Thinking that a schedule or budget is ironclad is the same thing as thinking
you can predict the future.

~~~
gmueckl
The intent is important because it becomes the lid on the project. Without it,
it is far harder to stay on track and make meaningful progress in finite time.
In a good company everyone involved knows the risks that may make the schedule
and budget slip and acts accordingly. Done right, the pressure does not rise
unduly shortly before a milestone. It should rise earlier, but also to a much
lower level. And a good manager will know when enough is enough and move the
deadline before it becomes too stressful.

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kodablah
> Engineers at NASA's Jet Propulsion Laboratory are working with Lockheed
> Martin, the contractor that built the heat shield, to figure out why the
> shield cracked. [...] A NASA spokesman says the agency doesn't have an
> estimate of how much it will cost to replace the Mars mission's heat shield.

Though I know it's often a counterproductive pursuit, I am curious where blame
lies here. Did Lockheed build to NASA specs? I assume so, but say that it was
determined that a manufacturing misstep that Lockheed made. Does the contract
for a piece of equipment like this include indemnification for Lockheed, the
opposite (that is must pass the tests to get paid), or is there no explicit
accountability? I know sometimes things happen, I'm more curious about the
money trail and preventative measures like any postmortem.

~~~
jpollock
Blame doesn't matter. All that matters for money is "who accepted the risk".

In a lot of contracts, the buyer lists the tests the object needs to pass
prior to acceptance. Then there will be a chunk of money attached to passing
that.

If it fails, then they don't get paid (that chunk).

After that, there may be more negotiation into who pays for the rework, but
that's also typically in the contract.

Usually the acceptance payment can be a large fraction of the total contract
(30-50%).

The same thing is done in just about every contract with a vendor. Even house
renovations. I typically leave 30% riding on the final inspection passing.

~~~
kodablah
> All that matters for money is "who accepted the risk". In a lot of contracts
> [...]

Right. I'm asking what is the case here.

~~~
jpollock
[https://www.nasa.gov/feature/jpl/results-of-heat-shield-
test...](https://www.nasa.gov/feature/jpl/results-of-heat-shield-testing)

"The structure was originally tested in 2008 and was one of two heat shields
manufactured in support of the Mars Science Laboratory mission, which
successfully landed the Curiosity rover on Mars in August 2012."

It was a spare from a previous mission. I would expect the risk was Nasa's,
and they are only out the cost of testing.

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aphextron
In theory, shouldn't we be able to deflect the plasma generated during reentry
with an EM field? I get that active generation would have all sorts of
complexity/power/weight costs. But could it be possible to engineer some type
of material which can passively convert thermal energy to a powerful enough EM
field that would envelop the spacecraft and work as a "deflector dish"? I just
feel like there has to be a better solution to reentry than ablation. It's the
main thing that sticks out in my mind which kills true reusability.

~~~
larkeith
> passively convert thermal energy to a powerful enough EM field

This is much easier said than done. If such a conversion were simple, we would
not use turbines in nuclear or geothermal plants. You might be interested in
looking into thermocouples [1], however.

Also of note, any such conversion must by nature exploit an energy gradient,
which means any energy spent redirecting particles in such a manner would heat
up the interior of the spacecraft by an equivalent amount.

[1]
[https://en.wikipedia.org/wiki/Thermocouple](https://en.wikipedia.org/wiki/Thermocouple)

~~~
DoctorOetker
perhaps instead of trying to capture the energy: a (set of) heat pipes to the
colder side of the craft?

if the temperature difference between hot and cold side is too large one might
need multiple heat pipes in series each operating at their own temperature.
The hot side heat pipe might only act as a heat pipe as it nears its operating
temperature, i.e. it might be solid during travel, then melt during the first
phase of entry, and only reach both vapour and liquid phase -thus enabling it
to work as a heat pipe- during the hottest part of re-entry

~~~
jotm
That doesn't make sense for the size of the current crafts. You'd just end up
with a lot of heat on all sides of it instead of only the front, imo

~~~
DoctorOetker
right the temperature on the back side would be higher than on current craft,
but the backside would be able to radiate heat no?

alternative idea:

the solid aborbs heat (cools the shield) to melt, then absorbs heat to warm up
and boil, and the boiling gas is used as retro-firing rockets, any decrease in
momentum due to the retro-rockets is less future heat absorbed (since the
craft is slowing down faster)

think controlled steam explosion

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toss1
...Aaand, that's why we run the tests!

Better to fix it in a component/unit test than to lose the entire project

A successful test, identifying an flawed component design.

~~~
mattnewport
It's also worth pointing out that if your tests don't fail occasionally you
may be over-engineering or being too cautious in your design and planning.

~~~
Sharlin
...or your test coverage is insufficient.

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geuis
Space is hard. Goes to show they have thorough and extensive testing and
caught this now rather than losing the mission in 2020.

~~~
mattnewton
Definitely this. Better to lose a shield prototype than a fancy rover you
already spent millions to hurl into space.

~~~
mattashii
The article isn't 100% clear on this, but to me it reads that the actual heat
shield that they were going to use in the mission broke (not a prototype).
Which is still much better than the alternative (losing a multi-billion dollar
project in Rapid Unscheduled Disassembly)

~~~
duskwuff
Yep. Much better for the heat shield to break in a test chamber on Earth and
have to be rebuilt, than for it to break during Mars atmospheric reentry and
end the mission.

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pfdietz
The heat shield did not crack, the structure on which the heat shield would be
mounted cracked. The PICA tiles had not been installed yet.

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smaili
> The Mars 2020 mission's heat shield was undergoing stress-testing when it
> developed a crack that appeared around its entire circumference. The shield
> is designed to protect the rover as it enters the Martian atmosphere. "The
> test was designed to subject the heat shield to forces up to 20 percent
> greater than those expected during entry into the Martian atmosphere," NASA
> said in a statement.

Gives a whole new meaning to the term "stress testing" :)

~~~
maxander
Hate to be that guy, but I’d imagine this is closer to the _original_ meaning
if the term. Mechanical parts required to perform under pressure are much
older than servers. :)

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JarlUlvi
This is actually success - testing proved that a widget wasn't ready to be
space qualified for a challenging mission. It broke in testing, enabling
engineers to know about that they need a better design.

Now, they're paying for a new shield, and all the engineering work that goes
into that, rather than preparing the funeral for some astronauts and all the
horribleness that goes into the loss of team members.

~~~
ISL
Agreed, with the caveat that the loss would be a rover (and the resources,
humanity, and time poured into the effort), not any human passengers.

Testing did its job, and the shield did not!

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exabrial
Maybe they forgot to convert inches into centimeters...

