Friday, August 10, 2012


Today is one of those Fridays when the last thing that I want to do is study.  However, there are no models for me to design, no airfoils to sand, and certainly no wind tunnel experiments to participate in.

So, I spend my time alternately reading through NACA (pre-cursor to NASA) technical reports and figuring out what song I want to listen to next on Youtube.  Don't I have the exciting life?

While reading about the development of retropropulsion in the 1950's and 1960's, I was amazed by how little they understood about the things that most of the aerospace industry takes as fundamental knowledge.  For instance, one 1957 paper is devoted to understanding detached shock waves from different aerodynamic shapes.  I tried to imagine what it was like before that knowledge and how they would even design experiments and aerodynamic shapes to study when they had no idea what that shape would mean for even basic aerodynamics.  It was fascinating to realize how much I really do stand on the shoulders of giants - wonderfully, broad-shouldered aerodynamic genius giants.

Then I got to wondering, how did they ever discover the blunt body shape?

Making something aerodynamic or streamlined is not a foreign concept, even to those of us less engineerically-minded (yes, I did create my own word).  When we buy cars, we expect them to be smooth and sleek.  When we build our pinewood derby cars (or imagine that we did) we try to make the front as narrow and smooth as possible so it can slice through the air.

As aerodynamicists tried to make airplanes and rockets travel farther and faster, they designed vehicles with more and more narrow noses to help cut down on drag.  For instance, look at the nose of the Bell X-1 (below) which was the first aircraft to break the sound barrier in 1951.
Source: Air Force website
However, at that same time, scientists were starting to realize that these extremely sleek and fast rockets were having some significant heating problems coming back down to normal atmosphere, so much so that they would melt.

It was H. Julian (Harvey) Allen who proposed the solution.  He thought of the idea in 1951, published it in 1953 but it didn't become known to the world until after it was de-classified in 1957(8).

His solution is actually quite simple - don't try to decrease drag.  Instead, we should try to increase it as much as possible.  Using a big drag-heavy aerodynamic shape actually pushes the shock ahead of the vehicle.  With the shock pushed ahead of the vehicle and not in contact with the vehicle itself, the bulk of the heating occurs in that air cushion between the shock wave and the vehicle, leaving the vehicle itself within temperatures that our materials can handle.

When he first came up with the idea suggested a shape much like a cannon ball, people scoffed.  Then he did some tests and found that his theory was correct.

I did some research behind H. Julian Allen.  It turns out he was a clever man who was interested in a number of subjects varying from classical music to Asian culture to international cuisine.  Reading about him inspired me.  My engineering professors in undergrad always said that well-rounded people made the best engineers.  It seems, in this case certainly, they were right.

It makes me hope that my varied interests in just about everything will help in my own pursuits in this field.  ;)

In any case, here is my shout out to one of the smartest guys in the field of aerospace!

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