Perseverance is running on 110 watts ( https://mars.nasa.gov/mars2020/spacecraft/rover/electrical-p... ). The RAD5545 uses 20 watts of that budget. Tripling it means it doesn't have enough energy to power the rest of the rover.
The Mars MAVEN ( https://spaceflight101.com/maven/spacecraft-information/ ) is using a RAD-750 which uses 10 watts of power ( https://en.wikipedia.org/wiki/RAD750 ) (also used in the JWST).
The RAD-750...
> The processor can endure radiation doses that are a million times more extreme than what is considered fatal to humans.
And while you may argue "yes, that seems a bit excessive" - the goal isn't "it can run in those extremes" but rather...
> Also, RAD750 will not suffer more than one event requiring interventions from Earth over a 15-year period.
> “The RAD750 card is designed to accommodate all those single event effects and survive them. The ultimate goal is one upset is allowed in 15 years. An upset means an intervention from Earth — one ‘blue screen of death’ in 15 years. We typically have contracts that (specify) that,” said Vic Scuderi BAE Business Manager.
... because it can't be replaced easily.
From Wikipedia:
> The CPU can withstand an absorbed radiation dose of 2,000 to 10,000 grays
And a more recent computer that isn't designed for rad-environments will automatically be way more efficnent than a rad-hardened one.
If you try to do this without comparing the whole state, and only comparing the outputs of some control algorithm between the computers, one of them could have some internal state corrupted and remain unnoticed for a long time, long enough for other corruptions to happen on the other computers, and then you have no straightforward way to figure out which one to trust and which to reset.
The radiation in space is not dangerous because it just produces a bit of current that can flip a bit; it is dangerous because it is strong enough to bump a bunch out of their position in silicon crystal, causing permanent damage that will accumulate over time. And when your feature size is starting to measure in <100 atoms it would be easy for single high energy event to break whole core.
In the context of radiation hardening by design, this means that a cosmic ray strike could evaporate part of the 5nm trace or cause arcing to the next trace.
A lower voltage chip can be more easily permanently damaged by a cosmic ray. That's why the RAD750 has its radiation resistance measured in grays.
> The gray (symbol: Gy) is the unit of ionizing radiation dose in the International System of Units (SI), defined as the absorption of one joule of radiation energy per kilogram of matter.
This is the total absorbed dose - not the flux.
Additionally, this is for the entire system - not just the CPU. It's not just "here is a redundant CPU" but also "here is the entire system that the CPU is part of" that is redundant.
The JWST is using a RAD750 with 44 MB of ram... and yea, that's not a lot, we're not trying to ship super computers out there. The alternative that you're suggesting is suggesting putting those all in multiple out in space and checking them against each other constantly while the system is in use collecting data.
The power budget is slim and also coupled to the heat dissipation budget. The JWST has a tighter heat dissipation budget than power budget... and the Perseverance has a tighter power budget (total rover power budget is 110 watts).
The challenge I'd pose is "what other processor arrangement fits within its power budget?"
I'm also going to point out that I got the numbers wrong for Perseverance's power budget for its CPU. The single board is 10 watts, the cpu is 5 watts.
https://en.wikipedia.org/wiki/RAD750
> The CPU can withstand an absorbed radiation dose of 2,000 to 10,000 grays (200,000 to 1,000,000 rads), temperatures between −55 °C and 125 °C, and requires 5 watts of power. The standard RAD750 single-board system (CPU and motherboard) can withstand 1,000 grays (100,000 rads), temperatures between −55 °C and 70 °C, and requires 10 watts of power.
https://mars.nasa.gov/mars2020/spacecraft/rover/brains/
> Unlike people and most animals, the rover's brains - its computer - are in its boxy body. The computer module is called the Rover Compute Element (RCE) - there are actually two identical RCEs in the body so there is always a spare "brain."
Also - https://space.stackexchange.com/questions/50470/what-makes-i...
These are modern chips that are designed for that efficiency - it's just that there are other constraints too. You are unlikely to find a lower wattage processor that you can run with redundancy that is able to tolerate space travel.
https://link.springer.com/article/10.1007/s12567-016-0138-0
Radiation Effects and COTS Parts in SmallSats https://digitalcommons.usu.edu/cgi/viewcontent.cgi?article=2...
Note that the cubesat has an average lifespan of about a year.
if it was a good idea to have multiple redundant CPUs on these spacecraft, the spacecraft would have multiple redundant CPUs.
https://www.jpl.nasa.gov/news/press_kits/mars_2020/landing/m...
> Just like Curiosity, Perseverance has two main computers, or “brains”: One is active at any given time, while the other serves as a backup. Both use radiation-hardened RAD750 computers, and together, they’re referred to as Rover Compute Element A and B.
> Another RAD750 serves as a third “brain,” on Perseverance. Called the Vision Compute Element, it is equipped with a special card for analyzing images. This card has a Virtex-5 field programmable gate array (FPGA) and is the driving force behind Terrain-Relative Navigation and the Lander Vision System, which analyzes images of the landing site during descent and compares them to an onboard map to determine the rover’s position relative to the ground.
That "Virtex-5 FPGA" is an interesting one too...
https://www.xilinx.com/products/silicon-devices/fpga/virtex-...
Where it is listed as "Space-grade Virtex-5QV FPGA"
The RAD750 is a 5W processor and on a motherboard, is a 10W system.
https://en.wikipedia.org/wiki/RAD750
> The CPU can withstand an absorbed radiation dose of 2,000 to 10,000 grays (200,000 to 1,000,000 rads), temperatures between −55 °C and 125 °C, and requires 5 watts of power. The standard RAD750 single-board system (CPU and motherboard) can withstand 1,000 grays (100,000 rads), temperatures between −55 °C and 70 °C, and requires 10 watts of power.
Worth nothing here with Intel's Turbo Boost stuff this processor will draw a LOT more than 28W during normal operation - which makes this an awkward comparison.
Power constraints and heat dissipation constraints.
https://arstechnica.com/science/2019/11/space-grade-cpus-how...
> BAE RAD5545 is probably the most powerful radiation-hardened processor available today. Fabricated in the 45nm process, it is a 64-bit quad-core machine clocked at 466MHz with power dissipation of up to 20 Watts—and 20 Watts is a lot. A Quad Core i5 sitting in a 13-inch MacBook Pro 2018 is a 28 Watt processor. It can heat its thin aluminum chassis to really high temperatures up to a point where it becomes an issue for some users. Under more computationally intensive workloads, fans immediately kick in to cool the whole thing down. The only issue is that, in space, fans would do absolutely nothing, because there is no air they could blow onto a hot chip. The only possible way to get heat out of a spacecraft is through radiation, and that takes time. Sure, heat pipes are there to take excessive heat away from the processor, but this heat has to eventually go somewhere. Moreover, some missions have tight energy budgets, and they simply can’t use powerful processors like RAD5545 under such restrictions. That’s why the European GR740 has power dissipation at only 1.5 Watts. It’s not the fastest of the lot, but it is the most efficient. It simply gives you the most computational bang per Watt. The HPSC with 10 Watt power dissipation comes in at a close second, but not always.