NASA Mars Mission Faces Setback After Heat Shield Cracks Under Pressure | Hacker News Reader

NASA Mars Mission Faces Setback After Heat Shield Cracks Under Pressure

npr.org

76 pointsyeukhon40 comments

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.

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.

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.

Good point. Sometimes (often?) though the schedule is taken very seriously as a prediction of the future. The business case for the product is made because it will be ready for the holiday season, or such and such important trade show or whatever. Key people schedule vacations, transfers etc. and are unavailable to help finish the product once reality emerges from optimism.

Akin's Laws of Spacecraft Design number 41:

  41. There's never enough time to do it right, but somehow, there's always enough time to do it over.

http://spacecraft.ssl.umd.edu/akins_laws.html

Well, with respect to your airline scenario - most companies would design along with those industry specifications in mind from the outset.

Even without doing so, considerable reverse engineering can still be achieved. The case of the De Havilland Comet comes to mind - they had to basically redesign the entire fuselage construction from the ground up once they realised the single layer pressure cell was a major design flaw.

I think the De Havilland Comet is actually a poster child for exactly how badly things can turn out rather than a good counter-example. The original mistake was apparently quite small - square windows instead of round. By the time they finally fixed this and shipped a solid and reliable product it was too late. Hundreds of people had died, Boeing and Douglas had entered the market with better planes and ate their lunch. The company died as well. You could make a case with only a little exaggeration that this mistake killed the British aerospace industry and setup the US for decades of dominance advanced technology industries.

Margin.

> 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.

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.

Money is one factor, but so is time. A lot of these kinds of parts like heat shields and expensive instruments are effectively one-offs, where there may be 1, 2 or 3 of them made, and usually the test articles aren't fit for flight because they determine defects that are later fixed in the one that flies.

There are certain launch windows for Mars where Earth and Mars are closer together [1] that you really need to be ready for. If you miss it, you might have to wait 2 more years for the next window. Sometimes you can't get the necessary refit time no matter how much money you spend.

[1] https://en.wikipedia.org/wiki/Exploration_of_Mars#Launch_win...

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

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

https://www.nasa.gov/feature/jpl/results-of-heat-shield-test...

"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.

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.

Absolutely, and NASA is currently investigating this. There is the idea of magnetoshell aerocapture[0][1], which is to create a shell of magnetized plasma around the spacecraft which can be used to slow down the atmosphere. There are a couple reasons why this is attractive. For one, you can modulate the size of the magnetized plasma shell and how much drag you make. Aerocapture, that is getting into orbit via dissipating momentum with planetary atmosphere, is a very risky maneuver because things like solar flares can change the height of the atmosphere causing there to be too much(crashing into the planet) or too little drag(flying off into space). Being able to change the size of the shell fixes this problem. The other is that this scales better than traditional aerocapture methods. The authors show in [0] how this could enable 2 tons of asteroid to be delivered to earth orbit, faster delivery of payloads to Mars orbit, and more economical delivery of payloads to orbit around outer planets like Neptune. There's a lot of promise for this technology and it's currently being developed for a cubesat demonstration mission[2].

This does not harvest any energy though, however, a separate NASA project did investigate the feasibility of generating power and capturing Mars' CO2 atmosphere to make O2 oxidizer through aerocapture[3]

[0]https://iepc2017.org/sites/default/files/speaker-papers/iepc... [1]https://www.nasa.gov/sites/default/files/files/Kirtley_2012_... [2]https://www.geekwire.com/2017/uw-team-wins-nasas-nod-small-s... [3]https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/201700...

> 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

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

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

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

If I understand correctly the main energy transfer mechanism to the heat shield is thermal radiation, not conduction or kinetic effects. The plasma is separated from the vehicle by a boundary layer. So it doesn’t help that the really hot stuff isn’t directly touching the heat shield.

That definitely makes sense. I was thinking in terms of conduction, not radiation.

It has to become plasma before it's susceptible to EM deflection. So you'd still have to heat it to 4000 degrees through friction. And so you'd still need a heat shield.

...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.

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.

...or your test coverage is insufficient.

Also, a learning opportunity. If it had just worked, we would know less. Sometimes you have to blow up a few things to make progress--ask Elon.

>A successful test, identifying an flawed component design.

It could also be a flawed component. Just because the test failed doesn't mean the design is flawed.

true, thx (definitely a flawed writing component there)

Every inch of this is true, good tests are our success meter. /s

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

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

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)

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.

Space is hard.

Things get broke.

Your mission still

Must not choke.

Burma Shave

(H/T to the RhymeZone rhyming dictionary...)

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.

> 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" :)

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. :)

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.

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!

Maybe they forgot to convert inches into centimeters...

Reply on news.ycombinator.com