read john friedmann "Planning in the Public Domain" - I think that's where "rational planning" was coined. Is that not assigned?
I think rational planning was actually best explained by richard babcock, even though he didn't use the term rational planning, when he said in The Zoning Game (a great great great book) that planners traditionally assumed that ?above every town there exists a Platonic ideal zoning map, waiting to be dropped into place. This map shows for each piece of property the use or uses which will give to the sum of all property the greatest total value.? Likewise there would be an ideal street map that would serve this distribution of uses, and a public utility map, and a services map etc etc. Rational planning is scientific, or wants to be. the hypothetical 'rational' planner believes he can employ empirical tests of planning actions, tests of "efficiency" ... maybe using the kaldor-hicks model or the pareto superior models, which are often discussed in economic and legal theory. Also look at Komesar "Housing, Zoning and the Public Interest." It's all very "rational" ... with a clear, most efficient, solution.
A critique of this would start by saying the general material welfare should not necessarily be a planners priority, if it can even be defined. Complicating its definition are rival capital/material interests, ie the small property owner versus the large property owner. You have use value vs. exchange value, individual welfare, community welfare, regional welfare etc, which all may conflict. This critique brings you closer to what Webber and Rittel (read that?) called "wicked problems" - the idea that planning problems are wicked problems, they always involve zero sum games. It's the nihilism of the advocate planner, who believes the best a planner can do is to advocate for one party or another, but never really hope to find a best solution, because one doesn't exist.
I think an important thing to keep in mind is that for the most part planning theory is only a way of understanding planning history, most importantly how planning evolved from a pseudo-scientific "rational" process in the first half of the 20th century into a process based more on outreach, advocacy, and local political action in the second half. Unfortunately a lot of classes don't really contextualize it that way. Keep in mind that planning theory doesn't fully explain how any planner in particular acts, or inform anybody's actions, and it's not theory that can be tested, like scientific theory. Nobody says "I theorize that you are a rational planner, and so I conclude that your actions will be as follows." As it is it's mostly a hodge-podge of ideas borrowed from other disciplines across the social sciences, that form a retrospective intellectual history of planning. you may say "robert moses believed in the possibility of one best solution for nearly everybody, a science of planning, and jane jacobs brought this into question by arguing the existence of values in her community that could not necessarily be served by moses' ideas. These two reflect common attitudes in planning practice, for the purpose of this over-wrought academic paper i may call moses and people like him rational planners, and jane and people like her advocate planners." ok? and talk to your professor; if he's too busy to meet with you about an assignment he sucks and it's not your fault if you write a bad paper because this isn't easy stuff or fun stuff to do for a great majority of people.