The Falcon Heavy is essentially three of the company's Falcon 9 rockets bolted together in a row, a design that makes it outwardly similar to United Launch Alliance's Delta IV Heavy. For those not up-to-date on SpaceX nomenclature, "Falcon” is an homage to the Millennium Falcon from Star Wars, and "Heavy" simply means the rocket can carry heavy things to space.
SpaceX revealed the Falcon Heavy in 2011, predicting it would fly as early as late 2013. Musk is known for his ambitious timelines, and it turned out the rocket's design was more difficult to perfect than originally thought. Initially, SpaceX considered equipping the Heavy with a complicated propellant crossfeed system, in which the two side boosters re-fill the center booster as the rocket ascends. When the side boosters are empty, they drop away, leaving the center core with a full fuel tank. This ultimately makes the rocket more efficient.
The crossfeed feature ended up being too complex, but even without it, the Falcon Heavy was a challenge to bring to fruition. Whereas the Delta IV Heavy has just one engine for each of its three boosters, the Falcon Heavy has nine, for a grand total of 27 engines that must all ignite and work in tandem without tearing the rocket apart. SpaceX's two disasters in 2015 and 2016 delayed things further: the 2016 accident damaged the company's only launch pad, Space Launch Complex 40, forcing SpaceX to rush to get pad 39A operational. But since there's a chance the Heavy flight will end in disaster and damage pad 39A, SpaceX also needed to get pad 40 operational again.
There's actually a further bit of controversy surrounding this point. Only pad 39A is outfitted for crew flights, which are expected to start later this year (an ambitious timeline, according to the Government Accountability Office). Should the Falcon Heavy damage 39A, how will that affect NASA's commercial crew program, which has been waiting to launch astronauts from American soil since 2011? It's a fair question, and you can bet NASA officials will be watching this demo flight with clenched teeth.
The hot end of a Falcon Heavy
This should be a real E ticket ride.