December 22, 2003
But it's not often I get to act like Horschack.Victor left a comment on my post about the upcoming Bowling Ball Loft contest. He was referencing part of the rules:
I took a look at the rules, and this intrigued me: Use a launch rail, tube or tower. Rods are prohibited due to past bad experience.
And he asked:
Can you explain the difference between the four launch pad configurations (apologies if my terminology is not correct) and what kind of bad experience they may have had with a launch rod? (I realize that might be speculation on your part.)
No speculation needed, I know exactly why they don't allow the use of launch rods in this situation. First a little background:
An unguided rocket (like we fly) has to be moving at a certain speed for the fins to have a stabilizing effect. Usually itÂ’s around 40mph, although a lot of different factors can make a difference one way or another. Since hobby rockets are launched very nearly vertical, we use different ways of making sure that the rocket stays pointed straight up until itÂ’s moving fast enough for the fins to take over.
All of these assume that the launch pad itself is stable. Good wide legs, low center of gravity, anchored to the ground or hefty construction; all of these combine to ensure that the launch pad wonÂ’t tip or tilt when the thrust of the motor kicks it. Attached to the launch pad itself will be the rod, rail, tube or tower.
The oldest method is the launch rod. Most commonly used for the smallest model rockets (1/8” x 36” long), it doesn’t scale up well but is still used - up to 1” diameter rods around 12 feet long. The problem is that when more power and weight are used, the rod tends to ‘whip’ which can fling a rocket off vertical. Not a biggie with a nine ounce model rocket, but it can be very dangerous with a nine pound rocket. A ‘launch lug’ is used, which is just a length of tubing glued to the rocket that slides loosely over the rail. On smaller rockets, the lug looks like a short piece of soda straw.
The launch rail is quickly becoming the standard method of launching bigger rockets. Made of extruded aluminum, the extra mass and shape of the rail makes for a much stiffer guide, which ensures that the rocket stays vertical as it launches. Instead of lugs, ‘rail buttons’ are used, which slide into the channel of the rail to provide the guidance. There's a picture of a typical rail in the extended entry.
A launch tower is primarily used in altitude contest launches. Instead of a lug or buttons attached to the rocket, the tower provides the guidance for a rocket by using three rods or rails spaced around the rocket body (between the fins). In its simplest form, a launch tower can be three parallel rods sticking up out of a coffee can full of cement. The main advantage is that since the rocket doesn’t have lugs or buttons, there is significantly less drag, which makes for higher altitudes. The main disadvantage is that a tower is only good for one diameter of rocket, unless some way of adjusting the guide rods is included, which adds to the complexity and cost. This elegant design here – by another Ted – allows for the three most common diameters of model rockets.
A launch tube is similar to the tower, except that the guidance is provided by the walls of the tube against the tips of the fins. Unlike a gun barrel, there is no back pressure assisting the liftoff. There are ways to use the ‘cannon’ method of launching as well, but it’s difficult enough that it’s not usually worth the effort and extremely rare to see it done.
Professional rockets use a variety of these methods, usually for the same reasons we do. The Super Loki Dart sounding rocket (this picture is of a scale model) is launched from an 12Â’ long tower (picture here along with some specs) that is spiraled like a gun barrel to provide spin and extra stability. The Loki reaches Mach 5 in a little less than a second, so staying straight is critical or the rocket will break into pieces.
Personally, I use launch rods up to about ¼” diameter – on anything up to about 2 pound rockets. I have rail buttons mounted on our larger rockets, and a lot of our rockets are rigged to use either, just in case a rail isn’t available. Given a choice, I’ll use the rail any time, because I’ve seen some scary flights caused by rod whip. more...
Posted by: Ted at
03:16 PM | category: Rocketry
Comments (5)
| Add Comment
Post contains 848 words, total size 5 kb.
December 21, 2003
Arizona High Power Rocketry Association (AHPRA) will be once again holding the bowling ball loft at LDRS (annual high power rocket launch featured in the Discovery Channel programs).
For LDRS 23 the Bowling Ball Loft class will be I-Lite (on the small end of the scale for 'I' sized motors). This was chosen to best suit the field size and waiver restrictions at the New York site (yes, safety matters to us).
In addition to the regular cornucopia of prizes AHPRA gets from vendors
there is the potential to win up to $1000 (One Thousand Dollars US) cash if
you set the new I Lite record during the contest.
Look here for more information.
Posted by: Ted at
10:49 AM | category: Rocketry
Comments (1)
| Add Comment
Post contains 153 words, total size 1 kb.
December 16, 2003
I won this kit in an online raffle, and she flies great on 'H' motors. She'll handle 'I' and 'J' motors as well, but I haven't tried them yet. Thanks to all the fins, once the motor stops burning and she's coasting upwards, you can hear her whistling. It sounds pretty cool. more...
Posted by: Ted at
09:47 AM | category: Rocketry
Comments (6)
| Add Comment
Post contains 76 words, total size 1 kb.
This is a Boy Scout event and does require registration prior to participation. REGISTER NOW! Call or visit your local Michael's store between January 2 and March 5, 2004 to register your troop.
To find the nearest Michael's store, call 800-642-4235 or visit www.michaels.com
Note: Registration doesn't start until January 2nd, despite the "Register Now" in the advertisement. If you know a scoutmaster or scout, pass the word along. As for the requirement of launching the rockets they build, contact your local rocket club, easily located at the National Association of Rocketry.
Posted by: Ted at
06:50 AM | category: Rocketry
Comments (4)
| Add Comment
Post contains 215 words, total size 1 kb.
71 queries taking 0.0897 seconds, 189 records returned.
Powered by Minx 1.1.6c-pink.
