Hacker News Reader

Comment by arijo | Hacker News Reader

> Another interesting thing SpaceX is doing is to use consumer-grade chips in triple redundancy configurations instead of using $100,000+ radiation-hardened aerospace/defense grade chips.

This has been known in the high availability and safety systems industry for a while and a good book to learn these reliability engineering techniques is "Reliability Evaluation of Engineering Systems".

The book is available on amazon: https://a.co/d/1nH824K

View on HN

One downside of using non-rad hard parts is degradation from TID (gamma) and latch up effects. You can have chips monitoring other chips to reset whenever they latch up but TID is mostly permanent. The good thing is that TID in LEO, where SpaceX mostly operates, is relatively lower than GEO so they can get by with mostly commercial parts. It's not like the big defense contractors haven't figured out the same thing, they do fly stuff using commercial parts as well, they are just slower to adopt the same culture. SpaceX and the companies that built components using commercial parts are building the new-space industry.

Could you surround your components with gamma-shielding materials and get away with off-the-shelf parts deeper in space?

"To block gamma rays completely, you need about 13.8 feet of water, 6.6 feet of concrete, or about 1.3 feet of lead."

https://www.epa.gov/radiation/radiation-basics#:~:text=Gamma....

I would wish you would use proper units. ;-)

But would using redundant systems separated in space connected with each other not offset the chance that they all would be affected at the same time? This is actually not rocket science .. just hard engineering and hardware/software design for redundant systems which is also usable on the ground.

Would they need to be blocked completely? Maybe a much thinner shielding would still produce a significant benefit?

(Though likely not of course ;)

High energy gammas have a relatively low cross section, most are going to pass right through the chip. If you add a too little shielding, or don’t layer shielding appropriately you are going to stop more gammas but produce lower energy x-rays from the shielding, which have a higher cross section, potentially increasing your chip dose.

Would it be possible to create a "skip" EM shield that does the opposite - increasing the energy of the gamma rays thereby reducing the likelihood of stopping them?

No idea how. Energies of most chemical bonds / electrons around atoms are not very high, not sufficient to emit proper gamma rays AFAICT. High-energy gamma rays are produced in nuclear reactions. While "clean" nuclear reactions that emit only gamma rays and not neutrons do exist, they are very high-energy and thus hard to initiate, and I don't think it would be easy to capture the energy of incoming gamma efficiently enough.

Yeah, the problem is getting the EM and M fields to interact. I'm not sure creating gamma rays would help.

Yes, you can attenuate TID effects with reasonably thin aluminum shielding

Just put the electronics in the middle of the header tank.

Can it even be blocked completely? Every layer of material geometrically reduces the proportion of rays going through. Or am I wrong about that?

No, that's correct. Of course there's still some level of reduction beyond which the gamma rays don't matter, but where you want to place it is somewhat arbitrary.

A box with 1.3’ walls seems doable, actually, depending on how small the chips are. Might still be cheaper and more effective than specialized chips. But I know nothing, so am probably wrong.

I think the trouble is such a cube would weigh 12,400 pounds (a sphere maybe more reasonable at 6,510lb - without any room for electronics inside)

https://www.wolframalpha.com/input?i=+lead+cube+with+side+le...

Hah, beat me to the nerd snipe. Moreover, that sphere would cost $10k to make and, at a launch price of $1500/kg, cost $4.5 million to launch into orbit.

The aim for the launch price of the entire rocket is to be around 5 million (once it's fully re-usable and in production). Basically the price of fuel and maintenance.

So something might be off with your assumption of 1500 usd / kg.

Yes, it's based in the real world. This was the Falcon 9 launch price that I could come up with in the amount of time I was willing to spend on a shitpost. I agree that launch prices will continue to come down, but launchers will always be mass-constrained and launching lead spheres into orbit will never be a practical solution.

I agree.

IIRC the CPUs are much less susceptible to damage when powered-off ? So have a bunch of them in cold standby or even as additional pluggable modules on missions with humans on board & swap to good ones when needed? :)

How long until they can build that massive box, stage it in orbit, and pick it up/put it down as needed?

I can't decide if I'm joking or not.

Lead is extremely dense so carry 1.3' walls of lead is probably more expensive than just having more redundancy or using better quality chips.

If the only thing that effectively shields these processors from radiation is lead, concrete etc (per earlier comments), what design changes / quality improvements can compensate?

Does liquid fuel protect as well as water? Suspend the computers in the center of the fuel tank.

You don't need to block gamma radiation completely to increase the electronics reliability :)

Maybe you could improve the system availability considerably by a bit of gamma radiation protection combined with some more parallelism of the components ..

Usually partial blockage is worse, because you end up with a spray of secondary particles instead of a single ray.

Maybe the secondaries could be blocked by different/lighter materials ? Basically a Whipple Shield for radiation. :)

Stopping power is basically correlated with mass.

Makes intuitive sense, thanks for the insight.

A second layer blockage for the secondary particles wouldn't have to be as dense or am I missing important physics?

(I guess a lot of gamma radiation would still reach this second layer so please ignore my question :)

Keep adding layers until you get to 1.3 feet of lead and it’ll work.

haha thanks for the correction - I was under the Turtles All the Way Down mindset :)

Isn't that more like how you make bombs than armors more effective - with backside spalling and secondary fragmentations?

can you block a hemisphere? the other 2pi steradians are shielded by the earth...

This may be true for high energy particles, but the majority of TID damage is done by higher flux lower energy, for which shielding is often viable!

The point is that shielding turns a single high energy particle that would otherwise strike and probably destroy a single transistor, into a veritable spray of lower energy particles causing bit flips or worse all over the circuit. This spray of particles can be stopped... with 1.3 feet of lead shielding.

More water in orbit sounds like a good idea to me

In reading Musk's biography,

"Hollman also found that creativity got him a long way. He discovered, for example, that changing the seals on some readily available car wash valves made them good enough to be used with rocket fuel."

"Elon Musk" by Vance pg 123

I have seen some people who decide to keep moving forward with whatever they have at the time. Sure what they produce is way less than perfect, but what they produce is way ahead of what everybody else is doing.

Perhaps the key is to be relentless, and resourceful.

There's a Lowes across the street from 1 Rocket Rd in Hawthorne.

I doubt any Lowes parts made it to space, but you know some went into test articles

Considering the political views of Elon Musk, it might be worth noting that his biographer is not the same Vance who is currently running for election as vice-president of the USA!

From an article for this I remember one more interesting side effects of this approach - the flight computer ends up as a generic x86/ARM board that the engineers can just have on their desk during development. Previously the dev boards would use the same rad hard chips and would be as expensive and scarse as flight hardware, resulting in much harder development & engineers having much less experience with the real hardware.

Aviation pioneered the use of redundancy in order to survive failures.

The Fukushima and Deepwater Horizon disasters show that this knowledge has not penetrated other industries.

Plenty of industries know about redundancy.

But it turns out, it doesn't matter how many redundant backup diesel generators you've got if a 45-foot wave comes along and they're all left underwater.

... and you put all of them in the basement :/

Thank you very much for the reference!

You're welcome :)

Reply on news.ycombinator.com