Modern technology is far too advanced for there to be a conspiracy in which only a few people know. We use space dependent technology on a daily basis. Everything that relies on satellites must be faked. That means you have to have hundreds of engineers who are in on the conspiracy to develop and implement the fake satellite technology and ensure that all the technology developed based on satellites still works on the flat earth and that no one knows the difference.
Then there are the space programs like NASA. All of the astronauts need to be in on the conspiracy. You also have to have a significant amount of people to organize the fake shuttle launches and landings. Where does the shuttle go? You would have to say that they land it at some secluded location and then relaunch it for when the shuttle is supposed to land. That would require an enormous amount of people and funds. Then you have the space probes which are launched, and things like the hubble telescope. There are hundreds of engineers at NASA that operate these things and are constantly analyzing the data they produce. Either all of those people are in on the conspiracy or there is a significant amount of people who are faking the data at real time so that they don't know. NASA would have to pay all of its normal employees and provide the funding for the RE related projects, while still having a huge amount of money left over to pay all of the conspirators and pay for all the infrastructure needed to fake a round earth.
Then there is the problem of long distance transportation. The distance between two places on the flat earth could be greatly different than on the round earth. Don't you think pilots, for example, would notice this? Or are there people deceiving them?
These are just a few of the things that would need to happen for the earth to be flat. Do you really think this could be accomplished by a few people telling a lie?