Orion spacecraft and radiation

[Luke Pemberton]

This is a major reason why FPGAs (field programmable gate arrays) are so popular in spacecraft. You only have to design and qualify one (or a few) parts, and then you can use them for lots of things. The microprocessor is a similar idea, but many circuits still have to operate faster than you can practically do them in software on a computer.

That's provoked a question, although I may have missed the answer in the previous discussion. Is there a balance between the electronics on a vehicle those needed for the mission versus electronics that can remain terrestrially bound? I would imagine that there are various advantages to carry out computing on Earth, e.g. reducing weight and not having all your eggs in a basket that is on the other side of the solar system.

[ka9q]

But most importantly, with FPGAs you only need to develop, test and space-qualify a very limited number of hardware designs that can then be customized to perform many different functions that would otherwise require many different parts, all of which would have to tested and space-qualified.

The Apollo Guidance Computer (Block II) used only one type of integrated circuit: a dual 3-input NOR gate. As we all learned in digital logic design, you can build anything from just NOR (or NAND) gates if you have enough of them. In that case I doubt radiation susceptibility was the major driving factor to use a single part as much as availability, testing and stockpiling considerations; the integrated circuit industry was still in its early infancy.

[ka9q]

That's provoked a question, although I may have missed the answer in the previous discussion. Is there a balance between the electronics on a vehicle those needed for the mission versus electronics that can remain terrestrially bound? I would imagine that there are various advantages to carry out computing on Earth, e.g. reducing weight and not having all your eggs in a basket that is on the other side of the solar system.

Absolutely; this is a classic example of a systems design problem, one hardly unique to space systems.

Engineering can be defined as the art of making tradeoffs. Tradeoffs between hardware and software, between reliability, schedule, power, maintainability, cost, and many other criteria.

Production scale is an important factor, because you have to trade off the cost of NRE (non-recurring engineering, i.e., design and testing) vs the incremental costs of production. My last employer makes literally billions of chips for mobile phones, so they hire a lot of engineers to eke out every last fraction of a penny in production costs. This usually means cutting chip area to the bare minimum, to put as much in software as possible, and to automate production to the greatest possible extent.

Space systems are at the other extreme; they're built in such small quantities that the NRE dominates the bottom line and there's little need to cost-reduce either the design or the production line. Indeed, there's a strong bias against doing this because of the real or perceived risk of decreased reliability.

Sometimes you can't decide on the basis of hard data, such as when you're doing something for the very first time, so you have to follow your experience-driven instincts.