http://www.bbc.co.uk/news/world-us-canada-29669205

It looks a little bit like a Space Shuttle for 4 year-old children, but this plane (the Orbital Test Vehicle, or X-37B) is unmanned and returned yesterday (Friday 17 October 2014) from a 2-year mission in space. What truly piques the interest is that almost all information on this vehicle, and especially on the specific reason for it’s flights, is top-secret.

The X-37B started as a Nasa project in 1999, before being handed to the military in 2006. It is built by Boeing (rather than Lockheed Martin, who are responsible for all the other Skunk Works projects) but is operated & paid for by the US Military. The first flight was in April 2010 & lasted 8 months.

Project Mission:

A re-usable test-bed for new sensors and other space technologies.

Dimensions:

Length: 9m (29ft)
Wingspan: 4.5m (14ft)
Height: 3m (9.5ft)
Mass: 5t (11,000lb)

So the X-37B is really small; it’s cargo bay is about as big as a small van, and that is tiny in comparison to the space-stations designed to house humans such as the International Space Station (ISS) or China’s space-lab Tiangong-1. However, whatever it does carry the USAF refuses to say.

Here is the X-37B on the launchpad some years ago, looking showroom shiny:

One key feature of the X-37B is that it is designed to be re-usable (just like the Space Shuttle, which had it’s first launch on April 12, 1981 & finally rolled to a complete stop on July 21, 2011). If you look closely at the top picture, you can see the black sections on the underside & leading-edge of the wings; those are clearly c/c ceramic plates (again, just like the Shuttle).

Any re-usable space vehicle has 2 difficulties:-

1. Getting into orbit
   It takes a fantastic amount of energy to accelerate a vehicle into even low orbit (defined as ~300km)
   (for Space Shuttle Columbia that was an altitude of 175 miles (282km) and speed of 17,500 miles per hour (7.8 km/s))
2. Returning to earth
   Meteorites burn up as they pass through the Earth’s atmosphere, and the situation is identical for space vehicles
   (the peak heat strains for Space Shuttle Columbia were at Mach 24.1 (~16,000 mph; ~30,000 km/h; ~8,000 metre/sec); altitude: 243,000 feet (74 km; 46.0 mi))

An Atlas rocket is used to lift the X-37B into orbit. That is the easy part! The next step is to not burn up in the Earth’s atmosphere as the orbiter returns to Earth.

he Earth’s atmosphere begins to be an issue at about 400,000 feet (120 km; 76 mi). At that point there is enough air that the passage of the orbiter causes such compression of that air as to raise the temperature of the nose, wing-edges - anything in it’s way will begin to glow white-hot. Now, on the way up the difficulty is to rise in spite of gravity pulling you back down. On the way down, the difficulty is to come down slowly, whilst gravity constantly causes an acceleration. So, there are 2 difficulties:-

1. Do not fall like a stone
2. Be able to deal with the heat of re-entry

A personal story:-

Every year in early August the Earth moves through the remains of an old meteor, and we in the Northern hemisphere are able to see the Perseids, which is an almost continuous rain of meteors. Fantastic! (if the skies are clear, with no moon).

Now, the reason that we can see those streaks of light is because small lumps of rock get heated to incandescence as they fall. However, one year I saw a Perseid meteor that acted like a Belisha Beacon, going “glow - dark - glow - dark” ... all across the sky. I got very excited! Clearly, that meteor had hit the atmosphere at just the right angle and, like a flat stone thrown expertly across a lake, was skipping across the atmosphere. Unfortunately I’ve only ever seen that once, but that is also exactly how the X-37B & the Space Shuttle before it stop themselves speeding up as they fall:- they skip across the atmosphere, slowing down as they go.

The final part of this puzzle actually uses a combination of 3,000 year-old technology (ceramics) & utterly modern technology (carbon fibre-reinforced carbon, also used for F1 brake pads). The beauty of ceramics--as discovered by the Chinese--is that they are incredibly hard + can be heated & not degrade nor rapidly conduct that heat (damn useful for plates & cups). The downside is that they are brittle & easily damaged by impact. c/c (a composite material consisting of carbon fibre reinforcement in a matrix of graphite) adds to the normal ceramic properties light-weight & retention of properties up to ultra-high temperatures. The downside is ultra-high cost (typically $100,000/sq ft).

We can see the culmination of a return to Earth and the effect on the X-37B’s ceramic panels in this next photo, which dates from 2012 & was shot in infra-red:-

This latest flight was the 3rd:

1. April 2010 - 8 month duration
2. March 2011 - 15 months duration
3. Wednesday 12 December 2012 - Friday 17 October 2014
4. (planned for) 2015

Most military orbital flights are pole-to-pole, because that allows all of the earth to be viewed as it rotates under the orbiter. However, the March 2011 mission was at an inclination of 42.79° (with respect to the equator). Since Tiangong was launched in September 2011 at 42.78°, at a similar height, and expected to be manned in early 2012, there was intense speculation that the Pentagon were using the X-37B to spy/listen-in on the Chinese vessel. The Pentagon have said nowt.

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Alex Kemp