Three systems might not fit into your design or be too expensive for what the project is. How do you combine the votes? The chip that is doing that work needs t

Three systems might not fit into your design or be too expensive for what the project is. How do you combine the votes? The chip that is doing that work needs to be absolutely immune from any bit-flips otherwise the voting is worthless... There are three main things to protect against with radiation hardening: lifetime dosage, latch-ups, and upsets. Chips can only go for so long being blasted with heavy particles until they cease to function properly. Smaller, newer technology is more affected (thicker traces and larger gates might last longer) by this so designers will tend to use older chip designs when making new radiation hardened stuff. This design choice also leads to less chance of latch-ups. Large particles can short out traces and cause the chip to halt and draw a lot of current. Larger tech makes this less likely to happen. Upsets are a lot different. You have to mitigate these a few different ways, an easy one being thick layers of insulating substrate to isolate gates better. This can also help with latch-ups too. Next are things like memory voting and error correction, methods that can be done in software. One layer up from that are redundant systems, very robust in terms of bit-flips but adds more complexity and cost to the system.