I love that they supplied 4 citations for the opening assertion: "Propulsion is a critical subsystem of many spacecraft [1,2,3,4]". Glad I didn't have to take that on faith :)
I work on compilers and I have lines in papers like "runtime performance is important for many heavily used services [1,2,3,4]". The reason you do this is to preempt some annoying reviewer whose area is some other dimension of the same problem (eg correctness).