another thread noted that the hardened CPU was not fast enough to do the required sensor fusion and responses. In the case of a severe fault, the 801 can be restarted fast enough (while in mid flight!) to not crash the vehicle.

I think the primary reason is that Ingenuity is not considered essential part of the main mission, so they could use non-qualified COTS parts like Snapdragon and Linux.

The Helicopter is also not operated in space, so radiation might be less of an issue. It's intended only as a flight demo anyway.

The Martian atmosphere and magnetosphere are both substantially worse at blocking radiation than those of Earth, so I'd imagine radiation would, at best, only be marginally less of an issue than, say, in interplanetary space.

If memory serves, the radiation environment of the Martian surface is not too much worse than LEO. If so, radiation would probably not be that much an issue.

More likely just too power hungry

apparently list price for the RAD750 is $200,000! could you not externally harden the snapdragon for less?

That's not how hardening chips works.

You need a specialized fab manufacturing, etching and packaging process for this and it's not like regular fabs are cheap to begin with. Plus you're working from the start with much larger nodes with dedicated cell libraries so everything has to be designed from scratch to fit that node which means you can't reuse consumer off the shelf designs very easily.

Then there's the lack of economies of scale in building such custom parts in small numbers. I imagine if Apple would only order 100 5nm chips per year from TSMC, the unit price would be equally eye watering.

I've only read this doc quickly [1]. I would have guessed a lead case of a particular thickness would do the trick. But I don't know the thickness needed, maybe too much?

[1] https://nepp.nasa.gov/files/25295/MRS04_LaBel.pdf

In theory you could just cover everything in lead and call it a day, IF, lead wouldn't also be one of the heaviest metals in the world, which kinda goes against the mantra of space travel.

Actually it doesn’t work that way (solely based on mass). Radiation doesn’t just get stopped by lead, it generates secondary particles. Lead happens to be effective against x-rays, which is why we tend to think of “lead shielding” but it’s not effective against all types of radiation. So it would help but you couldn’t just call it a day, and other materials are more mass efficient.

yeah, but it'd only be 3/8 as heavy on Mars!

seriously, that's something I haven't thought much about. when designing rovers, do they calculate solely on the weight of what it will be on the destination, or limit it to weight limits of escaping earth's gravity well?

Rover designer here. The weight at the destination is important to characterize performance but it’s not a big driver of design. For example, once you consider losses due to friction inside gearboxes, there’s no big difference to size a motor. You want to be able to test on Earth easily anyway, so you wouldn’t design without a margin to allow that. I suppose if you designed a rover for Uranus you might take a different approach.

Launch mass is important as a constraint, but the launch environment is the main design driver for mass. The rover must be much stiffer than the rocket to not “couple” it’s response. The first vibration mode (think tuning fork) of a rocket is about 20Hz so the spacecraft inside needs a first vibration mode higher than 40Hz. Something inside the spacecraft similarly needs a first vibration mode higher than 60Hz or 80Hz, although you can make exceptions based on analysis.

But that’s not all, the sustained load on components during launch can easily be 10 to 30 times gravity (30G) and the instantaneous load can be 100G. You can’t just add material for these kinds of loads, it would be an endless feedback loop because adding mass decreases stiffness. Look closely and you’ll see that every deployable or movable part of the rover is locked down by a mechanism until it has landed.

It's not just the mechanical weight at the destination and the work required to escape earth's gravity (which is enormous). Mass of the rover also means more fuel to speed up and slow down if there is any delta-V changes en-route, the heat energy that needs to be dissipated during re-entry, the forces on parachute and lander as well. So really, that mass penalty gets paid over and over... and not often linearly.

Weight on board the rocket is the most important consideration as payload weigh is the limiting factor.

Lead coating a critical and embedded component like a CPU, or even a SOC, has gotta be barely significant for the weight of the payload.

I'm guessing the faster CPU is just not necessary for the core rover, so via KISS, use the proven chips.

Ha, if you think that's a lot, I've got some news for you. That might be the price for just the CPU, but the price for the avionics package (basically a RAD750 with the necessary boards to manage power and IO) is a whole lot more than that. (I'm probably not allowed to name the exact number, but you're going to need to add a zero for sure.)

no way I'd be surprised by it. When you only sell 4-5 a year, its going to be pricey!