Our Heroes are a small group of people around the level of the heroes of The Skylark Of Space. Operating on a budget of maybe ten million dollars, they've built a spacecraft with a mass somewhere around 75 metric tons, which can take off from an ordinary runway using jet engines and then, once out of the atmosphere throttle, the specific impulse up to 8000 seconds or beyond.
Mass: 75 tons
Mass Ratio: 3
Propellant: Water or Liquid Methane
Crew: 2, plus up to 2 passengers, plus 250 kg cargo
Powerplant: Proton-Proton Fusion, 5 GW output electron beam
Engines: 2x Thermal Turbojet/Rocket, 1x high-Isp-low-thrust plasma
The problem? They want to be able to fly this wonder around, land at various airstrips, not get chased by the Air Force in the air or the FAA on the ground or the news media in general, have access to spare parts and facilities, and not be forced to give up the secret of tiny lightweight high-power proton-proton fusion before they've built up a substantial space infrastructure to flee to.
Their cunning plan? Claim that they just legitimately developed a high-performance jet aircraft with a vastly less dramatic description (but fitting its appearance and atmospheric flight performance), then register their spaceship as an instance of that aircraft.
The question: What barriers are they going to face to this? "This is air traffic control, did you just accelerate to FIFTEEN TIMES your Vne?!" is obvious, unless they can drop off the radar somehow. And I imagine that somebody might want to do too close of an inspection at some point. How long are they going to be able to keep up the game, and how is it going to fall apart? What about third-world countries?