Newton’s 3rd law is tricky

Many people have trouble really understanding Newton’s laws.  Including, apparently, the Chinese government, which is building a prototype engine that violates Newton’s third law.  The most straightforward critique describes the problem as a violation of conservation of momentum, but a basic consequence of Newton’s third law (that a closed system cannot produce a net force on itself) is equivalent to conservation of momentum (a closed system cannot spontaneously accelerate).