## January 21st, 2006

### You make my heart beat like a drum

For the programming class, I thought I'd make the first assignment a numerical estimate of the rocket equation, calculating rocket velocities based on propellant use, exhaust velocity, and the like. This seemed easy to program and a problem students could actually find interesting. Then I foolishly thought I'd ask them to compare program results to actual, Saturn V, rocket performance for its first stage, and see if any could explain the difference. (Asking freshmen ``why'' questions is a marvelous way to discover perspectives from wildly variant universes.)

Now. Rate of propellant consumption for the first stage is easy to find, but exhaust velocity of the F-1 rocket engine? Nobody says it, not even Mark Wade or the Saturn By The Numbers reference. I could calculate it from the thrust, but kept discovering that, by the rocket equation, with that exhaust velocity the Saturn V could have achieved a speed about half what it actually did by the end of the first stage's burn, and that not counting that gravity and air resistance slowing it down. To get the right speed, I had to suppose an exhaust velocity a healthy bit above what's considered best currently achievable.

Finally I tracked down the problem: I had swapped the numbers for the amount of propellant and for the dry mass (the upper stages and the un-burnable parts of the first stage). When I fixed that, suddenly, everything fell nicely into line. And it took only an hour of work on my part. My students better like this problem.

Another side effect of the recent heavy rains: there was a drop in people walking in to the blood donation centres. Blood supplies are low enough that elective surgery's being suspended, and new blood drives have swung into gear. And thus they had someone in a large blood drop costume wandering around campus handing out flyers, mostly to me. I'm sure he handed it out to other people, but he was very good at catching me.

Trivia: The Saturn V F-1 engine had 3,700 orifices injecting fuel into the combustion chamber, and 2,600 orifices for oxygen. Source: Stages to Saturn: A Technological History of the Apollo/Saturn Launch Vehicles, Roger E Bilstein, NASA SP-4206.

Currently Reading: The Best Short Stories of Ring Lardner, Ring Lardner.