Putting the rocket on the launch pad: Then vs Now

[LaunchPad]

Back when I first launched my rockets, about 30 years ago, I just slid the launch lug on the stick and had the bottoms of the fins touching the launch pad.
Now it's suggested to wrap a piece of masking tape around the stick and not letting the fins touch the pad?
Why?
And will it effect on how high the rocket goes?

[north boy]

Scorched fins LoL

The blast pad on some pads are angled at 45 deg rather than flat. If the motor nozzle is sitting directly on the blast plate it can affect the take off of the rocket. There was a post on here months back where the rocket didn't even get off the pad as the motor basically stuck itself down onto the blast plate. Other guys on here will give you more technical info on it Im sure.

[KennB]

With age comes wisdom, even if it's borrowed wisdom.

Depending on the model and fin design, if you let the rocket sit all the way down on the rod, you could have an electrical short across the blast deflector. Now, that statement makes a lot of assumptions like you are using an electrical ignition system and a metal blast deflector.

Tape works but a clothespin (or clothespeg, we don't know where you're from) is adjustable and doesn't leave any residue on your "stick" (really, a wooden stick or is it a metal rod - I don't want to assume here, too).

There wouldn't be an effect on your final altitude by using a couple fewer inches (centimeters) on your launch stick. This only keeps your rocket going straight until it has enough speed for fin stabilization to take over. Allowable variance in the total impulse of the motor you are using would have a greater impact.

Hope all this helps.
Kenn

[LaunchPad]

With age comes wisdom, even if it's borrowed wisdom.

Depending on the model and fin design, if you let the rocket sit all the way down on the rod, you could have an electrical short across the blast deflector. Now, that statement makes a lot of assumptions like you are using an electrical ignition system and a metal blast deflector.

Tape works but a clothespin (or clothespeg, we don't know where you're from) is adjustable and doesn't leave any residue on your "stick" (really, a wooden stick or is it a metal rod - I don't want to assume here, too).

There wouldn't be an effect on your final altitude by using a couple fewer inches (centimeters) on your launch stick. This only keeps your rocket going straight until it has enough speed for fin stabilization to take over. Allowable variance in the total impulse of the motor you are using would have a greater impact.

Hope all this helps.
Kenn

Well, last time I launched, I didn't use anything. (Fins touched the pad.) A couple of the rockets did squiggle after lift off but still managed to go up pretty high.
Could this have been due to not using tape on the stick or a clothes pin? (Midstates here.)
Or is the squiggle just part of lift off sometimes and is it a cause for concern?

[KennB]

Well, last time I launched, I didn't use anything. (Fins touched the pad.) A couple of the rockets did squiggle after lift off but still managed to go up pretty high.
Could this have been due to not using tape on the stick or a clothes pin? (Midstates here.)
Or is the squiggle just part of lift off sometimes and is it a cause for concern?

LP,
There are lots of reasons a rocket could squiggle when it come off the launch rod. If it was a stick, the launch lug could have been dragging on the way up and you may not have had enough velocity for stability. Fin alignment could give you a tail wag; a marginal CP/CG relationship can cause a wobble that improves as the motor burns weight. Weather conditions can play into this, too.

Your best bet would be to find a club in your area (NAR if you are looking at low and mid power right now) and pick some brains with someone who can see what you're doing with what you've got and can show you some other ways to do things.
Kenn

[JStarStar]

I guess I have heard of people using wood dowels (a 'stick'?) instead of metal launch rods and my guess is if that is the case it could be part of the problem.

A dowel of the dimensions of a typical launch rod (1/8" dia. x 36" length) would probably be very very whippy. Plus almost all wood dowels I have ever seen (including hardwood ones) have somewhat of a grainy surface and my guess is they would be much more subject to friction/catching on the launch lug as the rocket takes off.

Either of these problems could make the rocket wobbly as it leaves the launch pad.

As far as why people raise rockets up on tape stand-offs or clothespin to get the motor surface off the blast deflector plate, at least in my experience it is usually to avoid short circuits. When you have a metal launch rod, metal blast deflector, two micro-clips and the igniter, that is a lot of bare metal surface to worry about. Getting the rocket up off the blast deflector reduces the chance of a short.

[MarkII]

What north boy and KennB said.

1. To prevent damage from the flame reflecting back up from the blast deflector.
2. To provide clearance for the clips, so that they don't come in contact with a metal deflector.
3. With a few rockets (such as saucers), giving them a "short rod" can help prevent whip as they leave the rod (as well as protecting them from the reflected blast).*

Mostly it's for the first reason.

As long as the rocket receives sufficient guidance from the remaining rod length, using a stand-off will have absolutely no effect on its eventual altitude.

*Saucers and pyramids usually don't need as much guidance before they achieve stability. (They do need some, though.) But because most of their mass is distributed perpendicular to the rod and they travel relatively slowly, they can cause a thin rod to flex if they have to travel on it for too much distance. (Causing the rod to flex, or "whip," during the launch is a very bad thing.) Placing the saucer a bit higher up effectively shortens the rod for it. Plus as you can surely imagine, this type of rocket is very vulnerable to damage from the reflected blast, so moving it up and further away from the deflector is necessary anyway.