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What goes up, must come down – Re-entry and it’s many challenges

Yesterday was the anniversary of Apollo 17′s splashdown so I thought it would be a good time to talk about the ironically fatal difficulties of returning through the blue layer which supports all life on earth.

It was an early, yet not often talked about, observation in space literature that a consequence of going to space would be that you would have to come back from space. This, unsurprisingly, is quite difficult. A spaceship travels very fast to remain in orbit in the vacuum of space. When it comes back into the comparatively thick atmosphere the immense stresses a spacecraft is put under could easily be enough to rip it apart. Even then, if you could design a structure to sustain this force the things inside the capsule could be destroyed by the huge heat supplied as the craft slows down. So how did engineers view this problem, and what did they do to overcome it?

What causes things coming into the atmosphere to get hot?

Space can be considered a vacuum. As an object moving very quickly starts to enter the atmosphere it starts to compress the air it collides with. When the air is compressed by something moving at this speed it gets hot, very hot and this heat is transferred to the object moving through it. Some may remember a demonstration in science class of using a sudden compression of air with a piston in a tube to ignite a cotton bud, or know that a diesel engine works by compressing the air/fuel mixture in the chamber. It’s much the same concept.

Nearly all the kinetic energy of a spacecraft is converted into heat. As a thought experiment, let’s say we put you into a ballistic re-entry path. You are a human who weighs 50kg travelling at 17’500 mph. You have 153 million joules of kinetic energy which is enough to turn half a ton of ice into steam. Clearly spacecraft weigh much more than you and me, so we can see magnitude of the problem the engineers faced.  

In the early ponderings of spaceflight, the dominant vision of a spacecraft was of a plane launched from rockets or even a runway. One of these spaceplanes, the Silverbird (above), designed by Eugen Sänger and Irene Brendt was a suborbital spacecraft that re-entered the atmosphere by “skipping”. This method would incrementally slow down the craft and extend its flight time using lift. After each skip, the heat generated during would be radiated into space. This theoretical method of re-entry lasted for many years until the 1950s when the NACA (the predecessor to NASA) labs showed it wouldn’t have been as effective as a method of direct re-entry. Under direct re-entry the temperature would be higher, but for a shorter period and thus was deemed more manageable. With the mounting pressures of the Cold War and looming space race, NACA decided to abandon the spaceplane model in favour of the blunt body capsule design we associate with the space race today.

But the craft is still dealing with large amounts of heat, how did they try to deal with this? One of the first ideas engineers designing ballistic missiles, such as NACA’s H. Julian Allen and Alfred J. Eggers in the 1950s, tried was a heat sink. The principle was that a material of high melting and sublimation point could absorb all the energy of re-entry as heat without reacting with oxygen at the very high temperatures it was subjected to. Copper, beryllium, graphite, and an alloy called Inconel X were shortlisted and subjected to a series of tests measuring their suitability as heat sinks. Graphite had the best thermal qualities, but was readily oxidized in air. As such, a large amount of copper was chosen to be the heat sink on the first warheads on missiles designed by General Electric despite the extra mass required.

Whilst the US Air Force were conducting their tests and usage of heat sinks the US Army was leading the way in ablative heat shields. An ablative heat shield is a semi-passive thermal protection system. A heat shield is constructed from a material which is designed to sublime at the high temperatures of re-entry. The gas it creates forms a boundary layer of gas which protects the craft and is jettisoned as the craft moves through the air. These heatshields proved much better at protecting the contents of the craft were lighter than the heat sinks originally posed by the air force and all future warheads and some other space craft still use the design today.

As the space race heated up as the Cold War continued the challenges faced by engineers grew even larger. Not only would they now need to be able to return a capsule at a much higher velocity (i.e. from circumlunar orbit) NASA would need the pilots to be able to control their craft through re-entry and perform a much more accurate landing. Ablator heat shields remained the main method of heat rejection, but grew in complexity and size. To control the descent accurately astronauts could now offset the centre of gravity of their ships and as such change it’s pitch to lead to much more accurate landings. However, despite their best predictions, NASA scientists could not be sure of the conditions of lunar re-entry without attempting it.

In the years after Apollo the main challenge became reusability. The space shuttle needed to go to space and return many times in it’s lifetime and so ablative heat shield technology wasn’t suitable. Instead, the famous (or infamous) tiles of the Space Shuttle were developed. These tiles are very good insulators and were incredible at absorbing heat. Today, and in future the problems of space flight will be returning not just the payload or capsule to Earth, but fuel tanks and engines used to get up there.

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“Why don’t you just light this candle?” – Alan Shepard, whilst sitting on top of 30 tonnes of explosives

It took me a while to decide how to begin this blog, but a recent anniversary seemed like a good place to start explaining what this is all about. Pictured is the launch of Apollo 17. The 14th December marked the 45th anniversary of Eugene Cernan closing the hatch of the lunar module finishing a mission where two men spent nearly 3 days on the surface of another world. Only 11 years before, Alan Shepard sat on top of a rocket designed to carry a nuclear warhead with a failure rate in use of one in every three attempts. Through posts as close to the anniversaries of the key moments in this story I hope to celebrate the pioneering men, women, and (occasionally) animals that dreamt and worked to explore worlds beyond our own pale blue dot.