Prop malfunction.
What a disgusting way to pass off the deaths of seven people, just because your closed mind can't accept the truth.
I'm pretty sure that independent people also found pieces of Challenger.
http://www.nytimes.com/1996/12/18/us/challenger-parts-wash-ashore-almost-11-years-after-explosion.html
They found pieces of a prop.
So you're ACTUALLY telling me that they made an ACTUAL shuttle that COULD FLY....... but it was just a prop..... that malfunctioned? Okay. Assuming that you're correct, I'll look at this from your perspective. They spent millions of dollars on fake math and engineering to find what a spacecraft should look like and do on a spherical earth. They spent more on fake math of how the orbits would work, and then published all of this completely useless math that works on a round earth that doesn't exist. So they've already invented an entire new planet, reality, and several new types of math, along with inventing quite a few new laws of physics. They then build on this by making several hugely expensive rockets, many of which fail on launch, just to.... hit the dome? They then recruit astronauts, brainwash them or whatever, and send them somewhere and back down, either tricking them or keeping them quiet. They continue to send up more and more complex rockets, again publishing huge volumes of work, all of which checks out flawlessly, all with the assumption of the globe earth, also publishing limited amounts of pictures and videos and broadcasting them live. They then go on the Apollo Program, hiding the astronauts somewhere (They got in the rocket, they didn't get out, it launched and nothing came down but parts that hit the ocean), for several days, sending back hundreds of pictures of our moon, then they fake a landing there (They would need some kind of.... zero gravity chamber?..... to simulate the movement of the dust in the videos and there is too much footage for a plane in a parabolic arc) They then get their funding lowered due to Vietnam and low public interest, and start to work on the shuttle "Prop", which can obviously fly, as many have seen, and caries a fuckton of fuel, so it's going SOMEWHERE. They continue through test flights, not any great accomplishments, but the shuttle does its job, and it does it well. You then have the Colombia disaster, where Shuttle Columbia disintegrates over Texas, after re-entering with a loose heat shield. (No idea what the flat earthers think about how this happened) You start building the International Space Station (That's an argument for another day), and then you lose Challenger, shortly after liftoff. The shuttle program gets shut down a bit later, and now they're going to space courtesy of the Russians, with unmanned probes being launched every so often, and returning a lot of data (Let me guess, they just INVENTED unique characteristics for every planet). SO for this conspiracy to work you need the government of every country to collaborate, you need somewhere to put the Apollo astronauts for a few days, as radar would detect any return, you need to somehow keep International Space Station astronauts in the sky for months when space doesn't exist, and invent an entire new reality and make several books worth of calculations based on it. That's a bit..... far fetched, to say the least.