Wednesday, May 07, 2008

Rocketeering Redux

Last Autumn we shot for the Moon and ever-so-slightly missed the target. Truthfully, we didn't so much miss the target as come up short - by about 1.2 billion feet less the 30 feet our rocket managed to travel.

It was all great fun, but there was effort in constructing the launcher mechanism and the rocket, itself. We've got too much skin in the game to walk away so we're taking things to the next level this Summer - faster, stronger, more dangerous. We have 2 simple goals:

1. Fly higher. A lot higher.

2. Get rocket-eye-view photographic evidence.

Let's start with number 1. In thinking about how we could fly higher, we turned the question around and wondered, "Why didn't we fly higher last year?". The answer was clear - pressure. We couldn't supply enough air pressure inside the rocket to give us maximum lift. The fact is, a 2L soda bottle - which acts as the 'engine' for the rocket - is capable of withstanding internal pressure upwards of 150psi. This limit, however, is more of a design goal for manufacturers of these bottles and can be dangerous for rocketeering.

Here's the physics: A liquid-filled 2L bottle that exceeds its pressure limit will rupture - but not explode. Since there is minimal air in the bottle, there is no scenario whereby high-pressure air is looking for low-pressure air and creating an explosion in the process. In the world of water rockets, the bottle is about 2/3 full of air. Exceeding the pressure limit here can be A Very Bad Thing in that you'll be rebuilding your rocket, maybe rebuilding your launcher, and maybe picking shards of rocket-stuff out of your belly. There are better ways to die, I think.

After some Googling, we've determined that 100psi is a good target to help us increase our rocket altitude. It is possible to throw a rocket 300 feet in the air at this pressure. In our flights to-date, we have been using a bicycle pump to pressurize the rocket and managed to squeak out a measly 30-40psi. A bicycle pump is just not powerful enough to achieve our 100psi target. In the heady world of high-pressure water rockets, some manner of powered air compression is de rigeur. I decided we needed something small enough to be carried into a field and powerful enough to manage launches at 100psi. In short, we needed some manner of portable battery-powered air compressor.

And here's a fun fact: Under the right conditions, a water rocket launching at 100psi or better creates enough launch thrust to (theoretically) break your finger.

Kismet shined us last week in the corporeal form of the Canadian Tire flyer. The Airman Inflation Station - a portable, rechargable, handheld air compressor - was on sale at half-price. We picked one up and charged it up overnight - with this past Monday evening slated for backyard pressure testing. Jediboy acted as ground crew and set up the launcher and rocket as if we were going fly. We uncoiled the 20 feet of tubing from the launcher and took our rather inadequate cover behind a cedar tree. The Airman, screwed onto the air valve at the end of the launcher tubing, hummed to life in our hands.

The gauge's needle swept smoothly across 20, 30, then 40psi. The pace slowed down at 50 and it was a full minute to get to 80psi. Things seemed to stall out and I feared that maybe the Airman wasn't up to the task. But the needle crept higher still. Over a grand total of perhaps 4 minutes, the gauge was showing 100psi while the rocket remained intact - if not turgid - on the launcher. The only sign of stress was 2 small drops of water tracing a path from the O-ring that seals the rocket 'engine' on the launcher. Success!

While the temptation to pull the launch cord was almost overwhelming, we knew it would not be smart to launch our reborn rocket from our backyard. So, our next step will be to move everything over to our neighbourhood school's field for a real test. We're waiting for good weather - dry, little wind - for this next step. So many other things can go wrong with launching a water rocket: launcher collapses, rocket veers off verticle, parachute fails to deploy. And all of these risks get magnified as rocket thrust increases.

We can't wait.