But I'm looking for an objective, non partisan explanation of whether or not Trickle Down economics has been effective in the past
People debate whether economics is a hard or soft science, but I think it's fair to say that even if it's a hard science, conclusions can still differ and, at the end of the day, economic policy certainly can, and often is, partisan to some degree.
All that said, there's a lot of economists that point out that trickle-down economics is not a real thing:
We find that increasing the income share of the poor and the middle class actually increases growth while a rising income share of the top 20 percent results in lower growth—that is, when the rich get richer, benefits do not trickle down.
-- Dabla-Norris et al., 2015
A recent book published by the Organization for Economic Cooperation and Development (OECD) supports the IMF study’s assertion that inequality suppresses economic growth. Both studies relied heavily on the Gini Coefficient, a measurement of income distribution in which a score of 0 represents a society in which all wealth is shared completely equally and a score of 1 a society in which all the wealth belongs to a single person (currently, the United States has a high Gini Coefficient of .4, falling only behind Chile, Mexico, and Turkey in this measurement of inequality for OECD countries.) The OECD study found that an increase in inequality on the Gini scale of two points corresponded to a 4.7% drop in GDP.
More recently, there was this ad-hoc survey of executives during a talk by Gary Cohn (Trump's chief economic adviser) asking if they will take money gained from corporate tax cuts and re-invest (ie, "trickle them down"). The response was, to put it bluntly, underwhelming.
And this article points out a key aspect:
Trickle-down economics, in its pure form, was never tested.
-- The Balance
That last article points out some past situations where tax cuts claiming to be 'trickle down' policies were enacted in a time where we did see economic growth, there were side effects, and several other factors, so it's impossible to say that the tax cuts did what they were promised.
As such, there is no proof that trickle-down economics works, though there is plenty of data showing that increasing the income gap isn't good for economies.
Of course, that doesn't mean the theory isn't valid...it's just that we have never (and may never) be able to validate it.
Ironically(?) if the current republican tax bill is passed into law, it may prove difficult for them to argue trickle down policy works as we currently have a growing economy. It'll be tough to show it works unless there is some level of exponential growth compared to the already upward trending current curve, and if it drops and/or the economy significantly slows, it'd put another black mark on arguments for that particular theory.