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“It suddenly struck me that that tiny pea, pretty and blue, was the Earth. I put up my thumb and shut one eye, and my thumb blotted out the planet Earth. I didn't feel like a giant. I felt very, very small.” – Neil Armstrong (1930-2012)

Fresh Reads from the Science 'o sphere!

Saturday, December 19, 2009

Counterintuitive Science: Fast Speed, Fat Shape

In popular science fiction, fast spaceships are often shown as streamlined, sharply-pointed vehicles: such as the X-wing in Star Wars or the Colonial Viper in Battlestar Galactica.

Sleekness has long been associated with speed, at least since the dawn of rocket science in the early 20th century.

This is hardly surprising because an aerodynamic shape is necessary to attain high speed on Earth, becoming increasingly important at speeds over 200 km per hour.

By the 1950s, human beings were on the verge of space travel, and the popular conception of a spaceship then (and even now!) for both professional engineers and the general public alike, was the sharp-nosed spaceplane.

A good example of this was the X-15 hypersonic research plane.

Striking resemblance to a you-know-what.

However, the reality that awaited spaceflight enthusiasts was somewhat less svelte...

How did needles turn into fat cones and bells?

It turns out that pointy-nosed spaceships perform well on their way out of the atmosphere, but not when they have to come BACK.

The re-entry speed of a vehicle coming in from low Earth orbit is about 27,000 km per hour (over 7 km per second!) or about 25 times the speed of sound.

Clearly, the vehicle has to lose a lot of speed in order to descend safely into the atmosphere, but how should this be done?

It is impractical for an Earth-launched spacecraft to reduce most of that speed using retro-rockets, since the large amount of fuel required becomes an additional burden to the launch vehicle.

So the returning vehicle must decelerate mainly by atmospheric friction using the atmosphere itself, and this is where the pointy-nose shape becomes a disadvantage.

At hypersonic speeds, a sharp object generates only a thin shockwave, allowing the intense heat of friction compression to come very close to the surface of the object contact the leading surface of the object. Thus, during early wind tunnel tests, the noses of the test vehicles simply melted away.

No known material could withstand such high temperatures.

However, when a blunt object is subjected to hypersonic speeds, due to much higher drag the air molecules ahead of the object cannot move away fast enough. A thicker shockwave forms, acting as a cushion of air that shields the leading surface from much of the intense heat, and lowering peak temperatures to within the limits that can be tolerated by existing materials.

Thus, only with the development of fat re-entry vehicles did human orbital spaceflight become a possibility.

Initially, Russian designers used a cannonball shape for their Vostok space capsule, which could safely re-enter the atmosphere in any orientation, but had a steep ballistic trajectory that was very harsh on the cosmonauts.

They later developed the "bell on a bowl" shape for their Soyuz, while US designers developed the "cone on a bowl" shape for their Mercury, Gemini and Apollo spacecraft. These shapes have a similar function - to provide some lift and self-righting ability, allowing the spacecraft to re-enter with a shallower and more comfortable trajectory.

For 20 years these fat and aesthetically displeasing spacecraft had the counterintuitive honour of being the fastest manned vehicles in history.

Not everyone was satisfied with this and there were numerous designs of spaceplanes (eg. Sänger, Hermes) to replace them, but most of them were unable to proceed beyond test phases.

Then, with the arrival of the US Space Shuttle (1981) and the Russian Buran (1988) the age of spaceplanes appeared to have finally arrived, though with their fat noses and thick bodies neither of them can really be considered sleek-looking. Unfortunately, Buran was cancelled after just one flight and the Space Shuttle is slated to be retired next year.

So for the foreseeable future at least, the vision of a sleek needle-shaped spacecraft stays bogged down in the realm of fantasy, while the cutting edge of real manned space exploration is delivered by the venerable, and fat, space capsule.

Would you like to know more?
- How the Spaceship Got Its Shape (Air & Space Magazine)


Anonymous said...

Interesting article, I found more informations about Buran on this site.

Wolf said...

Actually, it's a bit more complicated than that.

The bulk of the heat generated during re-entry of the capsules is due to air compression beneath the leading face of the capsule, and not air friction.

Because this compressed air layer is mostly stationary relative to the capsule, there is no air flowing immediately next to the surface, and hence no friction at the surface.

The deceleration of the capsule is also a result of this compressed air layer - the compressed air exerts an upward force on the capsule. Kind of like a cushy bed mattress slowing you down as you jump into it.

Lim Leng Hiong said...

You are correct. Article edited for clarity!