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Boeing X-37: America’s Secret Space Plane?

A few days before the end of October 2019, a small aircraft like shape appeared above the Kennedy Space Center Launch Complex. The size of a van, but similar in appearance to NASA’s space shuttles, what landed that morning was the X-37, the U.S military’s mysterious space plane that had just spent nearly 778 days orbiting the Earth. Nothing out of the ordinary there I hear you say, except for one significant factor that is – we have almost no idea what it was doing up there.     

The story of the X-37 is shrouded in the kind of complex mystery that only a military operation can provide. With the space plane now run by the U.S Department of Defence, and more specifically the newly formed Space Force, almost all of what has happened on each of its five completed flights remains completely classified.

Theories have been hurled around ever since the X-37 blasted off for the first time on 22nd April 2010. Is it a spy plane being used to track or even interfere with other satellites? Are the Americans deploying some kind of devious weapon in space? Or are they simply testing a wide range of new technologies for the greater good of space understanding, as has been claimed? 

While much of what is going on with the missions are classified, we are slowly building up a clearer picture of this next-generation spaceplane. Whether its intentions are entirely honourable or not remains to be seen, but either way, we are looking at the future. 

Boeing X-37

With a large slice of information regarding the X-37 still well under wraps, it’s probably best to get straight to what we do know about this elusive space aircraft. There’s plenty of gaps throughout this story but we’ll do our best to piece it all together.  

The X-37 is also known as the Orbital Test Vehicle (OTV) and is a reusable robotic spacecraft. As I said, it carries more than a striking similarity to NASA’s Space Shuttle but on a slightly smaller scale. Like the Shuttles, the X-37 takes off on the back of a space vehicle, predominantly the Atlas V 501 rocket, but can also land like an aircraft. 

It is 8.92 metres (29 ft 3 in) in length and has a wingspan of 4.55 metres (14 ft 11 in). The maximum take-off weight of an X-37 is 4,990 kg (11,000 lb) – slightly less than an average African bush elephant – and has a payload bay dimension of 2.1 × 1.2 m (7 × 4 ft) – roughly the size of an average pickup truck bed – although what goes inside isn’t clear. 

The X-37 is solar-powered and comes with Gallium arsenide solar cells and lithium-ion batteries. It is designed to operate at altitudes ranging between 177 to 805 km (110 to 500 miles) above the Earth and it’s believed to have the ability to alter its orbiting altitude quickly, possibly to evade detection, though that’s far from being confirmed. 

Included on the X-37 is a thermal protection system, which still uses the same kind of silica ceramic tiles as the Space Shuttles, enhanced avionics, an autonomous guidance system and an advanced airframe. It first used a single Aerojet AR2-3 engine but this has apparently now been changed to a hypergolic nitrogen-tetroxide/hydrazine propulsion system and is designed to withstand speeds of Mach 25 (30,870 km/h-19,181 mph) upon reentry.

Now, for those who love our usual detailed technological descriptions, you may be a little disappointed, because that’s about all we know about the physical properties of the X-37. This is a spacecraft that the U.S military is happy to acknowledge, but good luck trying to get any more details out of them.    


The story of the X-37 begins back in 1999. At that point, NASA was in charge and selected Boeing to design and build a reusable unmanned space vehicle for the 21st Century. The next four years saw $192 million spent on the project ($109 million from NASA, $16 million from the U.S. Air Force and $67 million from Boeing) and in 2002 a full contract was awarded to Being, worth $301 million, as part of NASA’s Space Launch Initiative, which was a joint program between NASA and the U.S Department of Defence to look into the future of space travel. 

Initially, the X-37 was designed to travel within one of the space shuttles and deploy while in space, but it soon became clear this made little economical sense and the plans were altered so it could travel up solo on the back of an Atlas rocket. 

Testing the X-37A

The Scaled Composites White Knight was used to launch the X-37A on glide tests.
The Scaled Composites White Knight was used to launch the X-37A on glide tests.By Alan Radecki, is licensed under CC-BY-SA

Then in 2004, before a working prototype had even appeared, the X-37 program was transferred under the sole jurisdiction of the Defense Advanced Research Projects Agency (DARPA), an agency established back in 1958 in response to the launch of Sputnik 1 as a way of developing and focusing emerging technologies to be used by the U.S military. In an instant, the X-37 project became classified and a whole lot more mysterious in the process. 

In 2005, glide testing got underway with the aid of another unusual aircraft you don’t see much of. Known somewhat formally as the Scaled Composites Model 318 White Knight, or by its looser name, White Knight One, this is a jet-powered carrier aircraft often used to trial experimental space planes by essentially dropping them from a great height and hoping that they glide safely to the ground. 

The first part of the testing was a series of captive carry flights in which the X-37 remained attached to the White Knight throughout the entire flight to gauge its aerodynamic response. This was first done on 21st June 2005 with the X-37 prototype then undergoing structural upgrades over the following eight months. 

The aircraft was set to take its public bow on 10th March 2006 with its first free-flight, but an Arctic storm put paid to that idea. They tried again five days later, but this time high-winds led to the cancellation of its maiden free glide. Quite bizarrely, the next attempt on 24th March 2006 was also a failure as a data link problem prevented the X-37 from detaching from the White Knight. Things were not exactly running smoothly and the bumpy ride continued from there. 

On 7th April 2006, the X-37 completed its first free glide flight but during landing it managed to overshoot the runway, sustaining only minor damage but enough to prevent any further testing from taking place for the time being. While the prototype was under repair, the entire program up and moved from the Mojave Spaceport in Mojave, California to Air Force Plant 42 in Palmdale, California. Five further tests were carried out once the X-37 had been patched up and in two of those tests, the futuristic space plane glided successfully back down to Earth. In terms of testing it was far from faultless, but good enough.  

New Variant – X-37B

Just a month after the second successful test, the U.S Air Force announced that they would be pursuing their own variant of the experimental spacecraft, the X-37B, which is what is in use today. Now, we’ve got to be honest here, it’s not exactly clear how similar or how different the two variants are. Information coming out about the space plane slowed to trickle and then pretty much stopped. What had been used during testing was only a working prototype and as far as we know, NASA never got their hands on a finished model of the X-37A. I’d love to explain why, but I have a feeling that this decision was taken well above my paid grade as the Americans like to say. 

In 2010, with one X-37B complete, production began on the second X-37B vehicle which had its first launch in March 2011, but by that point, OTV-1 (the first X-37B) had already blasted off and was circling the Earth – which I’m just coming to.  

The involvement of the U.S Air Force has certainly thrown a blanket of secrecy over the entire project. According to officials, the X-37B series is,

“an experimental test program to demonstrate technologies for a reliable, reusable, uncrewed space test platform for the U.S. Air Force”

Broadly speaking, the program comes with a dual purpose, to test reusable spacecraft technology and to carry out experiments, such as avionics, flight systems, guidance and navigation, thermal protection, insulation, propulsion, and re-entry systems, which can be returned to Earth. It all sounds perfectly reasonable, doesn’t it?


Before we get into the murky world of space conspiracy theories, let’s have a look at the six flights involving the X-37B, all of which have gradually increased in duration. 

The first flight took off on 22nd April 2010 and landed back on Earth 224 days later on 3rd December 2010. Almost nothing was officially revealed regarding the mission but keen civilian observers on the ground were able to track it sporadically, however, during a two week period, it didn’t appear on their projected flight paths, leading to speculation that the X-37B’s orbit had been altered. 

The second flight left on 5th March 2011 and stayed in orbit for 468 days before again successfully landing. Again, what went on up there is a complete mystery to us and one which became even more puzzling when a spokesperson for the Air Force announced in November 2011 that the mission would extend beyond its anticipated length of 270 days due to ongoing experimentations. That’s 198 days overdue, so it must have been some experiments.  

On 11th December 2012, USA-240 began with an Atlas rocket carrying the third X-37B mission into orbit. This time the space plane stayed up for just short of 675 days before returning to Earth. Whatever the American Air Force was doing up there, it was taking longer and longer. 

The fourth flight set off on 20th May 2015 and spent nearly 718 days in space. And as you would have it, we got a little information about this mission. The Air Force announced that it would be testing an experimental hall-effect thruster, an ion thruster in which the propellant is accelerated by an electric field, to be used on future Advanced Extremely High-Frequency satellites. 

NASA also announced that it was sending something up. Included on the X-37B was a materials science payload called METIS, which was thought to be similar to the Materials International Space Station Experiment (MISSE) carried out on the ISS to test the effects of long-term exposure of materials while in a space environment.  

The fifth flight was slightly different in a few ways. Firstly, it was the first X-37B flight to use Space X’s Falcon 9 rocket instead of the Atlas and it was the first mission where the Air Force revealed that cubesats (tiny mini-satellites) had been deployed while in space. Apparently, three such cubesats were released but with the announcement not coming until after the X-37B had landed, it left some questioning whether the U.S had broken the Convention on Registration of Objects Launched into Outer Space, a convention that came into being in 1976 which mandated nations to register objects being sent into orbit before they leave Earth. 

The mission length stretched to nearly 780 days with the Air Force also announcing that it would be testing an experimental oscillating heat pipe but failed to explain why this mission was deployed into an orbital inclination of 54.5° – higher than all of the previous missions. 

The sixth flight is currently ongoing and blasted off using an Atlas Rocket on 20th March 2020. This was the first mission under the control of the new Space Force and a few days into the mission a secondary satellite, FalconSat-8, was deployed. This came with some of the most complex experiments to sent up to date, including,

  • MEP (Magnetic gradient Electrostatic Plasma thruster), a novel electromagnetic propulsion system
  • MMA (MetaMaterials Antenna), a low power, high-performance antenna
  • CANOE (CArbon NanOtubes Experiment)
  • ACES (Attitude Control and Energy Storage), a commercial reaction wheel modified into a flywheel
  • SkyPad, off-the-shelf cameras and GPUs integrated into a low power package

What is it up to?

The X-37B has now been in operation for over 10 years and over the six missions they’ve now spent over 3,200 days, and counting, orbiting around the Earth – that’s 8.7 years slowly moving around our planet. And with so little information coming from the U.S Air Force, it’s hardly surprising several theories have emerged. 

One of the more logical is that the Air Force is testing some kind of advanced surveillance sensors, possibly intending to deploy small satellites in low orbit capable of taking high-resolution pictures of subjects on the ground. The fact that the X-37Bs appear to orbit at a lower altitude than the ISS and the deployment of the three cubesats on the fifth flight could support this theory.

Others believe that the X-37B program is much more offensive than defensive and might even one day be used to attack or disable other satellites in space. Heather Wilson, former secretary of the Air Force, appeared to suggest during the Aspen Security Forum in July 2019, that the X-37B could change the shape of its orbit and use the Earth’s atmosphere to manoeuvre quickly. Her comments, using the word adversaries several times, seemed to suggest a potential attacking nature, but it’s difficult to base anything substantial on that. 

But then again, maybe we’re all just too suspicious these days. Maybe the U.S Air Force is carrying out perfectly peaceful experiments for the greater good of humanity. But come to think about it, that sounds about as plausible as the X-37B attacking other satellites in space. Whatever they are doing, the Americans are almost certainly testing technology that sits at the cutting edge of of space engineering.

There is still a mountain of unanswered questions regarding the X-37B and it’s unlikely we’re going to get any answers anytime soon. What we know is that space technology is barrelling forward at a breakneck speed, but where it’s going and what it’s used for, those are the real questions.

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