Further OT:
Water rocket might be useful for some cases when placing Amateur Radio=20
antennas. Quite a few of us are using a variant of the PVC "Spud Gun" to=20
put lines over trees to erect wire antennas. The scheme works pretty=20
good but the projectile could cause property damage or even injury if=20
the operator gets careless.  An empty soda bottle seems to be a safer=20
projectile.

Any Urls appreciated, I will Google chase a little later.

A tennis ball Launcher (Google "HyperDog") works well for shorter=20
ranges. I am still looking for a suitable barrel/tennis ball=20
configuration that is available in this area.
I daydream about a tennis ball catapult but have (so far) resisted any=20
serious experimenting along those lines.

On 2/28/2012 5:21 AM, RussellMc wrote:
> I have substantial experience using PET softdrink bottles for high=20
> performance water rockets. HP here means they may pull 100g off the=20
> launcher and if you want to see them ascend you MUST look up before=20
> they launch. Photograph the launch and you find that the exiting water=20
> falls upwards from about 2 metres above the ground until Brennschluss.=20
> An optimally filled bottle is about 1/3 water and 2/3 air. CofG moves=20
> backwards during most of expulsion and then forwards again towards the=20
> end (makes sense when you work through it) with maximum instability=20
> occurring late in thrust phase when CofG is at rearmost point. Thrust=20
> tends to last 10's of milliseconds depending on pressure, fill%,=20
> orifice etc. At 100g that's enough for the bottle to "almost vanish"=20
> at Apogee when viewed from ground level. Toys they are - but serious=20
> ones. Energy stored approaches that of a 38 Magnum shell. Less=20
> destructive on impact, fortunately. Smaller capacity bottles take=20
> substantially higher pressures. It's both due to hoop stress being=20
> proportional to diameter and because to some extent the blank they=20
> blow them from does not seem to increase in mass at the same rate as=20
> bottle capacity so you get a thicker wall per capacity. A 1.5l Pepsi=20
> bottle will hold 100 psi for months, will tolerate about 130 psi=20
> without too much distress, and starts to grow larger while watched at=20
> 150 psi. A bottle that is overpressured will produce magic smoke=20
> equivalent in a stunningly impressive manner. This is to be avoided if=20
> you are trying to launch a water rocket. A small nick in a bottle with=20
> a sharp implement will not result in a small hole and a gradual=20
> pressure release. Transition from small nick to no bottle is closer to=20
> instantaneous than the ear numbed brain can quantify. Self=20
> deconstructing bottles will generally throw plastic only small=20
> distances - probably usually under two metres and often under one. But=20
> occasionally a bottle neck or similar may be thrown substantially=20
> further. Safety glasses are a good idea. Other protective clothing=20
> optional but you are unlikely to sustain major injuries from fragments=20
> if eyes are protected. YMMV. Ears can take some minutes to regain=20
> useful function. A well streamlined 1.5l bottle with nose cone and=20
> fins and no aerobraking will impact at 75 kph+. Being in its way would=20
> be unwise.
>> and taping a tantalum cap to the outside, setting it up somewhere, and a=
pplying reverse voltage to the cap, when the neighbours are having a partic=
ularly noisy party ...
> Anything that mechanically compromised sealing of bottle at 100 psi +
> would rapidly result in while bottle shredding. Rockets do not fly
> well in that mode.
>
> My adult children both left home about 5? years ago. When I
> occasionally meet their friends they still enthuse about the Guy
> Fawkes nights we used to hold :-)
>

--=20
John Ferrell W8CCW
Be thankful we're not getting all the
government we're paying for. - Will Rogers


--=20
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