Safety First!
Spools scale well. They can be made bigger but keep in mind that the stresses placed on larger spools increase dramatically (particularly on the forwardmost disc). When considering the construction of larger spools, it becomes even more important to consider safety vs. durability. To safely recover spools without any recovery device (like a parachute), you must keep them light weight. That means that the discs must be thin and therefore will be likely to crack or break upon impact. Yes, even on their first flight. So if you lean more towards making your rockets indestructible tanks, you will almost certainly need to adjust your build techniques.
Big Spools on the Cheap
A cheap but very effective method of building larger spools is to use empty wire spools from your local home improvement or hardware stores. Head on over to Essence's Model Rocket Reviews for my Live Wire Spool rocket review for more information on how I made one for use with mid power motors.
Even Bigger!
A fellow club officer was looking for something neat to do with a large empty wire spool. We talked at our launch in January 2007 about what it would take to make something fun and memorable. Based on the size of what he described, a 38mm motor tube running through the center held in place with a couple of spare centering rings should be all that was needed. No launch lugs would even be needed--simply matching holes drilled in each disc would provide ample guidance.
The Specs
The basic dimensions are as follows: diameter = 10.5", length = 6.75", and weight = 1.0lb (sans motor). The only parts used were the network wiring spool, a piece of PML 38mm phenolic motor tubing, a pair of PML 3.9" to 38mm centering rings (so that the spool wouldn't have to bear the full load of the pressure while the spool is under thrust), and some metal clips to retain the motor casing.
Once the motor tube and centering rings were mounted, a pair of holes were drilled (one in each CR/disc) inline and as closely to the center as possible for the launch rod.
The Flight
When it came time to launch, we made the decision to use our away pad (200' from the launch line) to give the spool plenty of room to recover safely without endangering anyone or anything (even though based on the high power safety code states that 100' is adequate). Looking at the picture on the left above, notice that the spool has a couple of standoffs to prevent any chance of the motor blast coming back up and causing any unwanted problems.
Typical of a well designed spool, it didn't sit on the pad very long. The dense black smoke of the Blackjack propellant made it very easy to track the flight. After 400' or so, it came to an abrupt stall at motor burnout and immediately went into an end-over-end tumble that increased slightly in rotational speed on the way down. It came down at a more than acceptable speed and distance from the flight line.
He captured the flight on video and was kind enough to let me have it and post it here. It can either be viewed within this page (courtesy of YouTube) or you can download it here.
Where Do We Go From Here?
This same guy mentioned to me that he really enjoyed this big spool and that a 54mm motor powered pyramid is a future possibility! Check back here for updates.