It is amazing how many antigravity projects (NASA, Airforce, Navy, Army, foreign nations, privately funded etc.) have been conducted without success. Unfortunately no documents have been made available as to the theoretical approaches, proposed and conducted experiments, and outcomes if any, until Project Greenglow.
The BAE Systems-funded Project Greenglow is well documented by Ronald Evans in his 2015 book Greenglow & the search for gravity control. As was typical of the scientific community its announcement was not well received. One American physicist said at that time, "One can only conclude that at the higher levels of these organizations there are people who don't have a very sound grounding in fundamental physics." I must say, these physicists were not very adventurous, and worse, didn't want others to attempt something that might overturn their worlds. Did the universal failure of all these antigravity projects point to the need to overturn legacy physics?
For me, the most important questions that needed to be answered was: why did all these antigravity projects fail? Incorrect theory? Incorrect experimental methods? Incorrect experiments? Insufficient budgets?
Considering Project Greenglow did not produce any positive outcomes, is valuable in itself as it gives an insight into why other hush-hush antigravity projects failed. The book explains how scientific thought evolved over the centuries to what it is today, by trial and error, by proposing mathematical models, by testing against empirical data, by the iteration between theory and experiments.
Universal failure points to two different lessons. First, the dialog between theory and experiments had failed. That some theories could not provide Engineering Feasible experiments while others proposed experiments that failed to show gravity modifying effects. Experimental failure could be due to either incorrect experiments derived from an incorrect theory or a working theory that overestimated the observable effects.
Second, maybe different approaches are warranted. There are two I can think of. An approach to eliminating what could not work and a means of generating new hypotheses to gravity modification. The former is easier. It requires reviewing our legacy theories and finding the faults in these theories. For example, wormholes and time machines as we know them are not feasible as the exotic matter they are based on cannot exist because exotic matter leads to perpetual motion machines. Quantum foam does not exists and therefore Casimir forces are produced by a new unrecognized phenomenon that is easily mistaken for quantum foam. The Alcubierre Warp Drive is based on erroneous axioms. All string theories have been invalidated due to their erroneous Tidal Axiom.
The latter approach is much more difficult to achieve, not because it is theoretically more difficult but because the "system" is not conducive. It is about budgets. Who wants to be poor? Contemporary physics as a "system" funds status quo theories only. Therefore, to have a reasonable income as a physicists you have to research what is being funded. Therefore, new, unfamiliar concepts go unfunded. The budget problem is symptomatic of a leadership problem noting the American physicist comments above. That is we don't have a physics leadership that is willing to think outside the box, and so we don't. Unfortunately that perpetuates failed exotic matter & string theories.
In this light we must congratulate BAE Systems for having the courage to fund Project Greenglow, knowing well the ridicule they would and did get. America, where is your Greenglow?