Shuttle Musings…


Today,
Slashdot is reporting that the
BBC has announced that recent investigation of the Columbia disaster has shown that the foam that blew off and contacted Columbia’s wing could have resulted in damage consistent with the
catastrophic loss of the spacecraft.

In the test a 0.8kg piece of foam was fired by a nitrogen powered gun at a panel that was decommissioned from Atlantis. It contacted the wing at 850km/hour, and blew a 40cm square
hole.

Immediately the "cogniscenti" of slashdot set up a huge cry that such a large velocity
was unwarranted: that there was literally no way that the piece could have accelerated to such a
high velocity in such a short span of time. Most are just ignorant assertions, but some people try
to do the math, such as this one:

As far as I can see, I’d imagine that the foam falls from the fuel tank/booster onto the shuttle wing. The rate of fall should be only the relative acceleration that the shuttle experiences during the fall. (Since both foam and shuttle are presumably moving at the same speed when it detaches from the launcher)

So the total acceleration should be the acceleration of the shuttle (max 3G at liftoff according to a couple of web sources) plus normal gravity – call it 4 G. At most, the foam could fall the full 56 meters of the shuttle/booster/tank height (and most likely substantially less than that).

So, a quick (and probably hideously wrong) calculation based on v^2=2 * Accel * Distance shows that the end velocity of a body falling 56 meters at 4g should be about 33 meters/second, or 119 kph (74 mph)

Of course, the factor that they are missing is the enormous drag caused by the very unaerodynamic chunk of foam insulation as it detaches from the tank. The relatively low
mass and high drag of such a chunk causes it to decelerate very rapidly.
When the insulation disconnected, Columbia was 81 seconds into its flight, travelling at
Mach 2. Initial estimates indicated that the impact speed could not have been more than
500mph.

But do the math! At a relative speed of 500mph, a 1kg chunk of insulation has momentrum
equal to 0.5 * m * v^2, which is 25,000 joules or so. What does this mean? Well, let’s say
that I weigh 100kg (dreaming, I know, but it makes the math easier). If I am travelling 50mph,
then I have virtually the same kinetic energy as that chunk of foam.

I suspect that if you threw me at the Challenger wing at 50mph, something bad would happen.

When in Crete, do as the other Cretins do…

It’s amazing how sometimes surfing leads you to interesting ideas that you’ve never had before. Like the mention of slashdot of pykrete, a combination of 14% wood pulp and water, which is frozen together to make blocks. During WWII, the British had a bold plan to build aircraft carriers several thousand feet long and capable of carrying 200 Spitfires and long range bombers.

Anyway, I found this to be interesting, but of no real practical use.

Surprisingly though, while perusing comp.graphics.rendering.raytracing, somebody mentioned that they were experimenting with padobe. What’s padobe? A form of
adobe that utilizes waste paper. It turns out that people are making various
slurries of water, paper, and cement and using them as inexpensive building
materials for home construction. I’ve been interested in alternative home building techniques for a while, but hadn’t heard of this construction technique.
I rather like the idea of do-it-yourself home construction using renewable and/or
waste materials, and some of the resulting homes are pretty amazing.
Nifty stuff. If you start at papercrete.com, you’ll get some ideas.

Fun with PyEphem…

I’ve been trying to tune up some of my telescopes for the upcoming Mars event. In late August, Mars will be closer to the earth than it has been in all of recorded history. While surfing around, I noticed that there was a Python package called PyEphem
which provided Python bindings for the popular Xephem program. It’s remarkably easy to deal with, in a few minutes I had a script which computed the apparent size and magnitude of Mars and printed them out. A much longer time was spent trying to understand how to use the gdchart module, but the results turned out the nice charts you see here.

The apparent brightness is in apparent magnitude, where negative numbers are brighter. When Mars is at its closest approach, it will outshine everything in the night sky except the moon. (I wasn’t able
to figure out how to get Python’s gdchart module to display the graph
with the yaxis reversed. I’ll keep at it.


Finding Nemo released


Well, it’s that time again, I’ve got another production credit on another movie.
Pixar is releasing Finding Nemo, starring a young clownfish named Nemo and his dad Marlin. Nemo is captured by divers and put in a fishtank in a dentist’s office, and the movie highlights their parallel adventures to be reunited. I can’t be impartial, but I think it’s a pretty good movie, and it looks beautiful. So get out there, drive our stock price up, and enjoy.


Narrowband Television

The problem with the web is that sometimes you hit a website that drags you off into a direction completely unrelated to anything you’ve really considered before.
This happened to me most recently when I encountered a site which described
the work of John Logie Baird, a Scottish engineer who designed a television system that transmitted 30 line images and was used in the first public broadcasts by the BBC. Some of these early transmissions have been restored and documented in a rather interesting looking book entitled Restoring Baird’s Image by Don McLean.

Curiously enough, there are still amateurs who are devoted in building replicas of these early mechanical televisions. Some links:

Moon picture…


Well, I didn’t try to take any pictures of the lunar eclipse, but I did try to take a picture of the post-full moon using the maximum zoom setting on my Nikon 4500. Exposure was for 1/60 second at f/5.1, and was a simple handheld exposure. The image was cropped from the normal 3.8 megapixel image. Not stellar, but not bad.


More fun with fingerprints

I have to admit that I’m a pretty big fan of CSI. Not the crappy new
CSI: Miami, but the original one. It gets Tivo’ed at my house all the time, as well as many CourtTV shows like Forensic Files.

The previous blog entry got me thinking about aquiring fingerprints clandestinely, and I thought about CSI. Often you see them place some fuming substance under a fume hood and it brings out fingerprints. I did a bit of digging, and found out that the fuming substance is cyanoacrylate glue, better known as Superglue. The fumes
interact with the fats and acids in fingerprints, and cling to them, resulting in
a white residue that is both permanent and can be easily photographed.

Using Matsumoto’s technique, one could probably construct fairly accurate forgeries of fingerprints, which is part of the reason why I detest these attempts
at biometrics.

If your interested in doing this yourself with minimal equipment, you can check out this link for the rather
simple instructions.

Fun with Fingerprint Readers

Bruce Schneier is the author of Applied Cryptography, the incredibly useful encylopedia of modern cryptographic algorithms. He also publishes the Cryptogram, an electronic
bulletin involving computer security, privacy and cryptography. Today I received the latest issue, which includes an article on fooling biometric fingerprint scanners. Tsutomu Matsumoto tried several techniques to create phony fingers that the fingerprint scanner would accept as real. Nifty!

Addendum: You can click here
for some slides that Matsumoto did detailing his work.

phlog — a photo weblog

Since Carmen and I both just got new digital cameras, we are taking a lot more photos and Carmen wanted me to create a simple way for her to put her photos on the web and to organize them. I used some of the ideas that I got from writing my primitive blosxom clone, and create a photo weblog script that I call phlog. It’s very simple, but will grow in functionality. Try checking out my gallery to see what it is all about.