For the last 22 months, Boeing’s not-so-secret plane, the X-37B, has been flying over our heads, but it is certainly not the first secret plane launched by the company’s Phantom Works division.
After nearly two years of continuous flying, the craft is due to touch down at Vandenburg Air Force Base in California on 21 October.
All that is known about the X-37B is that this unmanned aerial vehicle (UAV) had been a concept of US space agency NASA as far back as 1999, but after years of not knowing what to do with it, the craft was transferred over to the US Air Force with Boeing’s first prototype flying in 2010.
All of this secrecy behind the project goes back to the fact it is part of Boeing’s Phantom Works, which is responsible for all of the company’s most advanced and secret craft.
But what other Phantom Works craft has the company produced and what has happened to these very experimental planes?
Another craft still in development is the futuristic X-51 Waverider, which also underwent its first test flight back in 2010 and while also unmanned, it certainly stood out for its potential speed.
The name Waverider derives from the fact that compared to conventional craft, the X-51 uses shock waves generated by the ultra-fast scramjet engine which from testing showed a speed of Mach 5.1, or 6,200 km/h.
Having undergone successive tests, the last of which was in 2013, the general understanding is that the US Air Force is keen to adopt the technology not in aircraft, but as a high-speed strike weapon (HSSW), or missile.
Potentially entering service in the mid-2020s, the once-secret project could eventually be attached to advanced fighter jets of the time.
On the complete opposite end of the scale, Boeing had also been working on a project that would have led to a plane called the Pelican.
This behemoth would have been 400 feet long and have a wingspan of 500 feet to accommodate large cargo loads. This plane would use the ‘ground effect’ to fly just above the surface at a height of 20-50 feet.
It was certainly expected to be able to carry huge loads across enormous distances – 1,200 tonnes of cargo across 18,000 kilometres.
It could, however, have also gone airborne if it was required, at a height of 20,000 feet, but would see its range reduced by two-thirds.
Despite its potential use, nothing has been heard of the craft or any prototypes being created since 2002.
Initial sketches of the Boeing Pelican plane that never made it into development
Aircraft design can be a strange issue, as the plane that is soon to make its return landing, the X-37B, is actually a follow on from a previous project, the X-40.
Launched in 1998, the X-40 was the test bed for the X-37B with a similar design, but was incapable of orbiting the Earth like its older brother.
The craft was only really designed to test its gliding ability and was dropped from a height of 9,200 feet, 4 kilometres away from its target runway.
An image of the X-40 during a test flight. Image via Wikimedia Commons
The X-45/ Phantom Ray
Perhaps the craft most akin to what would be seen in a Hollywood action film, the unmanned X-45, is similar in design to the US Air Force’s existing B-2 Spirit flying wing stealth aircraft, but has the potential to be one of the most powerful drone aircraft in the skies.
First tested back in 2002, the craft that has now been dubbed the ultra-cool Phantom Ray has seen upwards of US$1bn or more in funding to provide a number of small but powerful missiles for whoever the target may be.
The status of the 900km/h aircraft is unknown since it last conducted a test flight back in 2011, having been launched off the top of a Boeing 747 previously used to carry NASA space shuttles to the launch pad.
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