Tuesday, November 10, 2009

The Weight of Software

Throwing shit into outer space has always been a battle between weight and thrust with gravity playing the referee.

You have to be able to generate more thrust than your payload weighs, and you have to keep doing that until the ballistic trajectory of your payload exceeds the diameter of the object you are trying to orbit.

But in order to generate the thrust you need propellant. For rockets that is liquid hydrogen and liquid oxygen, which is very heavy and requires refrigeration, pressurization, fuel pumps, electronic monitoring equipment, triple redundancy, fail safe switches and more moving parts than any other machine made by man to keep it from blowing up like a bomb. All of which weighs something and requires more thrust.

So everything you launch into orbit has to be just strong enough to get the job done, but not a single bit stronger.

The modified Atlas ballistic missile that launched John Glenn into orbit had an aluminum skin thinner than a dime. It relied on the pressure of the hydrogen and oxygen fuel tanks to keep it rigid. It was basically a big, highly explosive, metal balloon with a really ballsy guy sitting on top.

That part of the rocket that looks white? It's actually the same color as the silvery aluminum parts. It's just covered with frost due to the frozen hydrogen and oxygen aboard. All of that frost carries a weight too. Everything has to be calculated to the last ounce.

See all of those big, huge, ice chunks falling off of the Apollo 11 Saturn V booster during the launch? They all weighed something and had to be factored into the weight of the booster by the engineers.

The Grumman Lunar Module that Neil Armstrong piloted to the surface of the moon with less than 10 seconds of fuel left had a skin about as thick as the heavy duty aluminum foil you use for your turkey on Thanksgiving.

So one of the design requirements that aeronautical engineers impose is to determine the weight of each component of the rocket stack early in the design so they know how much thrust will be required.

As software became a part of rockets to control navigation and fuel consumption, the aeronautical engineers wanted to know the weight of the software.

The programmers responded that it didn't weigh anything.

One day an aeronautical engineer came back all upset. He was holding a deck of punch cards and was insisting that this was what the software weighed.

To which the programmers responded, "No, you don't understand. We only use the holes."