# Are bicomplex numbers including tessarines ever used in spaceflight (as an alternative to quaternions)?

The answer to What are quaternions and how are they used to represent spacecraft dynamics? gives us a thorough overview of the topic and is worth a good read.

I saw the HNQ Why are quaternions more popular than tessarines despite being non-commutative? and wondered:

Quesiton: Are bicomplex numbers including tessarines ever used in spaceflight (as an alternative to quaternions)?

See Bicomplex number for more.

• The most obvious use of quaternions in spacecraft is to represent rotation (and related concepts such as orientation), and the top answer to the "Why are quaternions more popular" question notes that they can't model 3D rotations. The only other potential use I can think of would be some signal processing operation, and it seems unlikely for such an obscure detail to be generally known...it could be buried deep inside some library. Commented Feb 16, 2021 at 4:15
• The answer seems to be general to bicomplex numbers. I'm not sure they're an alternative to quaternions at all, they're just similar in some respects, being another type of hypercomplex number. Commented Feb 16, 2021 at 13:01
• This is a bit tricky: Once math equations are written down, the implementation of , complex numbers, quaternions, etc. comes down to multidimensional arrays of numbers with some specified formulas for multiplying, combining, etc. these arrays of what are, after all, rational reals because that's all a computer deals with (other than naming them "classes" as an attribute). Commented Feb 16, 2021 at 14:27
• @CarlWitthoft Heh. SW engineers wish computers handled rational reals. Unfortunately, floating point numbers aren't quite the same. It's of course all reduced to simple arithmetic operations supported by the CPU's ALU/FPU, but I think it's reasonable to consider the software to "use" the abstractions used in its development. A problem is that what's an abstraction at one level is an implementation detail covered by another abstraction at another. And the abstraction may not even be in actual code, but in a comment giving the higher level math explaining an implementation. Commented Feb 17, 2021 at 15:19
• The linked question at HNQ is nonsense. Quaternions are useful precisely because they don't commute, and they don't commute in the same way rotations in 3D space don't commute. Using a commutative algebra to represent 3D rotations doesn't make sense. Commented Feb 25, 2021 at 15:46