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'[EE]: Sled propulsion'
2004\01\08@193006 by Jinx

face picon face
Does anyone have suggestions (and hopefully practical
experience) for methods of propelling a sled on a track ?

Sled weighs about 500g, hopefully much less but it will be
> 200g. Length of travel can be anything up to 2m. Shorter
is better but not important. It's required that the sled reach
full speed by the end of the track (obviously !). This speed
needs to be accurately set from 10m/s to 45m/s

My thoughts -

Air (how would speed be regulated ? Bleeder valve ?)
Electromagnetism (ie rail gun - ambitious ?)

Things like shotgun cartidges are out. Driving force has to be
something reasonably non-consumable (eg air, electricity) and
that can be reset fairly quickly and repeatably, say within 20s

Water would be too messy

I don't know if motorisation in some form would work. For example
direct drive with a stepper or clutching a flywheel

At the end of the track I thought the sled could be sent down a
downward curve, deccelerated with electromagnets or track
braking and then travel upside-down in a loop to get back to the
start position

TIA

==============================================
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- Wernher von Braun

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2004\01\08@193627 by Tal

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A firework maybe?

Tal

-----Original Message-----
From: pic microcontroller discussion list [spam_OUTPICLISTTakeThisOuTspamMITVMA.MIT.EDU]On
Behalf Of Jinx
Sent: Friday, January 09, 2004 2:30 AM
To: .....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU
Subject: [EE]: Sled propulsion


Does anyone have suggestions (and hopefully practical
experience) for methods of propelling a sled on a track ?

Sled weighs about 500g, hopefully much less but it will be
> 200g. Length of travel can be anything up to 2m. Shorter
is better but not important. It's required that the sled reach
full speed by the end of the track (obviously !). This speed
needs to be accurately set from 10m/s to 45m/s

My thoughts -

Air (how would speed be regulated ? Bleeder valve ?)
Electromagnetism (ie rail gun - ambitious ?)

Things like shotgun cartidges are out. Driving force has to be
something reasonably non-consumable (eg air, electricity) and
that can be reset fairly quickly and repeatably, say within 20s

Water would be too messy

I don't know if motorisation in some form would work. For example
direct drive with a stepper or clutching a flywheel

At the end of the track I thought the sled could be sent down a
downward curve, deccelerated with electromagnets or track
braking and then travel upside-down in a loop to get back to the
start position

TIA

==============================================
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- Wernher von Braun

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2004\01\08@194247 by Tal

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Second thought... a falling weight that kick the sled and give it a big push
to start?
the weight can be from hard rubber so the maximum force can be apply to the
sled with no damage to the sled.

Tal

{Original Message removed}

2004\01\08@195327 by Jinx

face picon face
> A firework maybe?

Hmmm, smoke, noise, injuries, cost ? A nice big red
chysanthemum would certainly have some ephemeral
entertainment value though, that's for sure. I think I may
have a little trouble for indoor use, regulation-wise

I'm leaning towards compressed air, but it's still a clean
blackboard so anything's possible

Maybe "rail gun" was the wrong choice of words. Don't
want to have to get in the car to fetch the sled back every
20s ;-)

Perhaps more along the lines of a linear motor, if it can
get the sled to top speed within a meter or two

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2004\01\08@200325 by Jinx

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> Second thought... a falling weight that kick the sled and give it a
> big push to start?

45m/s is 100mph. It could be possible if the weight was on a
rotating arm, eg propellor-style. rpm (and therefore circumference
velocity) would be quite easy to set accurately

Assuming you had a 1m diameter propellor, with one long arm
(counterweighted on the other side) to do the kicking. Get it
spinning to 15rps (=> 100mph). That gives you 60ms after it
passes the back of the sled to drop it so it hits the sled solidly
next time around. Not impossible, just wonder about the damage
from a sudden impact like that. I'd prefer it if the sled was
accelerated gradually, partly because there's stuff on the sled that
would probably need re-engineering to take such a whack

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2004\01\08@200537 by Tom

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Sorry, no ideas here but I certainly hope you will entertain us with
progress reports on this one.

45m/s huh? Around 100mph or so?

Have you worked out the acceleration numbers required here yet? About the
only suggestion that comes to mind is to include a video camera with the
setup so you can provide us with mpeg's or avi's. Please.

Keep having fun!
Tom


At 01:29 PM 1/9/04 +1300, you wrote:
>Does anyone have suggestions (and hopefully practical
>experience) for methods of propelling a sled on a track ?
>

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2004\01\08@201531 by Jinx

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> only suggestion that comes to mind is to include a video camera
> with the setup so you can provide us with mpeg's or avi's. Please.

OK, I'll see how many cameras end up in next door's garden ;-)

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2004\01\08@202819 by Andrew Warren

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Jinx <PICLISTspamKILLspammitvma.mit.edu> wrote:

> Assuming you had a 1m diameter propellor, with one long arm
> (counterweighted on the other side) to do the kicking. Get it
> spinning to 15rps (=> 100mph). That gives you 60ms after it passes
> the back of the sled to drop it so it hits the sled solidly next
> time around. Not impossible, just wonder about the damage from a
> sudden impact like that. I'd prefer it if the sled was accelerated
> gradually

   So put a spring behind your sled, and have the arm hit THAT.
   Shouldn't take too many tests to find the right spring rate and
   to create an rpm-to-sled-velocity lookup table.

   Dropping the axle would be easier if your arm weren't (heavily)
   counterweighted; it seems pretty straightforward to arrange
   things so the arm's centrifugal force would push it down.

   -Andy

=== Andrew Warren -- .....aiwKILLspamspam.....cypress.com
=== Principal Design Engineer
=== Cypress Semiconductor Corporation
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=== Opinions expressed above do not
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2004\01\08@202820 by Rafael Fraga

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Could you provide some background about these tiny sledges?   Just for the
rest of us down here who do not have any clue about them... ;)  My sledges
have dogs in front of them....
Thanks!

Rafael Fraga

{Original Message removed}

2004\01\08@203232 by James Nick Sears

picon face
Yeah this should be pretty radical.  If my numbers are right it looks like
about 51g to accelerate (linearly) from 0 to 45m/s in 2m.  I would think if
you were to have any hope of accurately controlling the speed in this realm
you are looking at a railgun type approach but I have no experience to offer
in this area.

Good luck.

Nick



{Original Message removed}

2004\01\08@203854 by Jack Smith

picon face
>Does anyone have suggestions (and hopefully practical
>experience) for methods of propelling a sled on a track ?

>Sled weighs about 500g, hopefully much less but it will be
> 200g. Length of travel can be anything up to 2m. Shorter
>is better but not important. It's required that the sled reach
>full speed by the end of the track (obviously !). This speed
>needs to be accurately set from 10m/s to 45m/s


If it's not important that the sled be independent of connections, you could
use a simple cable system, something like a San Francisco cable car.

I assume the sled will ride on rails. Between or below the rails, have a
continuously rotating cable on a pair of pulleys. Power it from a variable
speed motor, so you can set your desired speed quite accurately.  Have a
clutch / gripping mechanism with a solenoid to activate the friction
material that grips the cable. (Obviously the pulleys are outside the rail
length, so you have a free run of cable for the entire length of the rail.)

At the end of the run, a trip mechanism disengages the cable grip and some
braking system is applied. (I would look for something simple, like a
pneumatic damper, similar to what you might find on a screen door, but
larger. It isn't hard to build a pneumatic damper and it's easy enough to
control it by adjusting the exit orifice. You can even build one with
variable deceleration by having multiple orifices.)

There may be excessive acceleration at startup, so you could modulate the
gripping pressure and allow some degree of slip at the outset. Or, you could
have a hard clutch, i.e., off/on, but buffer it with a spring on the sled.

For return you could have the cable drive horizontal, with two clutch
mechanisms. When one cable is gripped, the sled goes out. Then the second
cable is gripped, it returns. You probably would want the return to be
gripped with a great deal of slippage to control the speed.

Or you could use one cable and one clutch, but stop and reverse the motor
(and slow its speed) for the return.


Jack

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2004\01\08@204308 by Sergio Masci

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What about a spinning wheel lowered onto the sled from above.
Apart from spinning, the wheel would only be allowed to move up and down.

I think there is a kids toy that uses wheels to kick toy cars along a track.

Regards
Sergio Masci

{Original Message removed}

2004\01\08@210839 by Dave VanHorn

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>
>I think there is a kids toy that uses wheels to kick toy cars along a track.

Hot wheels supercharger, introduced around 1970-ish.

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2004\01\08@213659 by Dave Tweed

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Jinx <EraseMEjoecolquittspam_OUTspamTakeThisOuTCLEAR.NET.NZ> wrote:
> Assuming you had a 1m diameter propellor, with one long arm
> (counterweighted on the other side) to do the kicking. Get it
> spinning to 15rps (=> 100mph). That gives you 60ms after it
> passes the back of the sled to drop it so it hits the sled solidly
> next time around. Not impossible, just wonder about the damage
> from a sudden impact like that. I'd prefer it if the sled was
> accelerated gradually, partly because there's stuff on the sled that
> would probably need re-engineering to take such a whack

Even with constant acceleration, it's going to take over 50G (506 m/s^2)
to get up to 45 m/s in a distance of 2m.

With a total mass of 500 g, this represents 506 J of energy, which needs
to be released in 89 ms, for a power level of almost 5700 W.

Suppose you have a cylinder containing a piston with 10 cm^2 area (about
1.55 in^2). You'll need a force of 253 N on this piston, or a pressure of
253KPa (36.7 PSI). If you have a large enough reservoir at this pressure
and a fast-acting, free-flowing valve, you should be able to drive this
piston at relatively constant acceleration.

However, don't forget to account for the mass and friction of the piston
and its 2m connecting rod. It would probably be a good idea to turn it
inside out and anchor the piston and use the cylinder to push the sled.
Then the walls of the cylinder could be relatively thin and light.

If you have a larger piston and/or higher pressure available, you can
use a lever of some sort to reduce the piston's stroke relative to the
motion of the sled.

-- Dave Tweed

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2004\01\08@213702 by James Nick Sears

picon face
So if your sled weighs .5kg neglecting friction you need about 57lbs of
force (253N).  I'm no jet propulsion expert (actually I know nothing about
it) but it seems that if you were to use compressed gas you would get at
best Pressure * orifice area of force (with losses for turbulence, etc).  So
if you could deliver gas at a regulated 120psi through a ~.4" diameter
orifice (.5" area) you would have in the neighborhood of what you want.  If
there is no friction or turbulence.

I think you can get small CO2 or N2 tanks for paintball guns and the like
that can hold into the thousands of PSI.  So at that range you would be
looking at more of a pinhole orifice which would allow less dropoff in the
acceleration (a more constant tank pressure).  Add a regulator for speed
control and a couple of solenoid valves (forward, reverse, stop) and you are
good to go.  Not sure how small/light you can get the tanks but if you found
something you could get a big bottle to refill them relatively easily
cheaply.

Maybe you could design a simple tank and have it custom machined to your
desired size.  I don't know how to go about figuring the required
strength/thickness/etc. though.  Definitely err to the heavy side!

Or what about a chemical reaction.  Like vinegar and baking soda but more
reactive.

Nick

{Original Message removed}

2004\01\08@215148 by Denny Esterline

picon face
> Or what about a chemical reaction.  Like vinegar and baking soda but more
> reactive.
>
> Nick

Rocket fuel?

I've read good things about Hydrogen Peroxide with a silver catalyst and
kerosene afterburners.

Seriously though, what's the purpose of such a thing? Mr. Tweed beat me to
the math, but dumping 8 horsepower into a brick in less than a tenth of a
second is a pretty non-trivial task.
-Denny

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2004\01\08@220642 by Liam O'Hagan

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Try a soda bulb. They're available from grocery stores for ~30c each, and
hold solid C02 in a metal bulb with a foil cap. They're used in old
fashioned soda syphons...

Pierce the foil and all that C02 converts to gas, with a resulting
signicicant temperature drop and much expansion of gases, very suitable for
propelling a device such as this. Advantage is that the force rises to its
maximum level very rapidly.

Disadvantage in a closed environment would perhaps be large quantites of C02
floating about ;)

If you want to add a little excitement use a "whipped cream bulb", which
contain nitrous oxide!

> {Original Message removed}

2004\01\08@231330 by Jinx

face picon face
> Seriously though, what's the purpose of such a thing? Mr. Tweed
> beat me to the math, but dumping 8 horsepower into a brick in less
> than a tenth of a second is a pretty non-trivial task

And thanks to Mr Tweed for those maths and construction ideas. The
pressure he's talking about doesn't seem so high

Actually I do have a compressor for something else, which is maybe
why I keep thinking air-gun. A percussive force doesn't sit easily with
me, mainly because if it all works this sled could be fired 1000 times
a day most days. I'm sure metal fatigue would be a factor. Somebody's
eventually going to get a faceful of sled and I'll get a faceful of fist

I can't say exactly what the application is. It's legitimate though, but
a niche market. So no, you won't all want one ;-)

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2004\01\08@233859 by James Nick Sears

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----- Original Message -----
From: "Jinx" <joecolquittspamspam_OUTCLEAR.NET.NZ>
To: <@spam@PICLISTKILLspamspamMITVMA.MIT.EDU>
Sent: Thursday, January 08, 2004 10:13 PM
Subject: Re: [PICLIST] [EE]: Sled propulsion


> > Seriously though, what's the purpose of such a thing? Mr. Tweed
> > beat me to the math, but dumping 8 horsepower into a brick in less
> > than a tenth of a second is a pretty non-trivial task
>
> And thanks to Mr Tweed for those maths and construction ideas. The
> pressure he's talking about doesn't seem so high

The other thing to conisder is that for your max speed run you only need to
maintain power for about 88ms - more of course if go with retrorocket
braking.  What about a mag-lev track to cut friction losses?  Or have the
sled slide by an array of IR diodes and use a photosensor to trigger the
transition from rearward expulsion to forward expulsion for braking?

Nick

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2004\01\09@001347 by Jinx

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> What about a mag-lev track to cut friction losses?

That's definitely a possibility. The sled is the single most
important part of the machine and making it work efficiently
and effectively is very desirable. If the weight is borne by
a mag-lev then it only needs retaining wheels, a la rollercoaster,
to keep it on track

> Or have the sled slide by an array of IR diodes and use a
> photosensor to trigger the transition from rearward expulsion
> to forward expulsion for braking ?

Once the sled has reached terminal velocity there's no need to
slow it down instantly. If I can make it loop-de-loop (I'll have to
find out what radius it can follow at 100mph and friction will be
an important factor) then it'll be on the underside, braking section
of track, which won't take up much extra room

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2004\01\09@012654 by Steve Smith

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Pulse jet !

Small Loads o energy no moving patrz !

-----Original Message-----
From: pic microcontroller discussion list
[KILLspamPICLISTKILLspamspamMITVMA.MIT.EDU] On Behalf Of Tom
Sent: 09 January 2004 01:05
To: RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU
Subject: Re: [EE]: Sled propulsion

Sorry, no ideas here but I certainly hope you will entertain us with
progress reports on this one.

45m/s huh? Around 100mph or so?

Have you worked out the acceleration numbers required here yet? About
the
only suggestion that comes to mind is to include a video camera with the
setup so you can provide us with mpeg's or avi's. Please.

Keep having fun!
Tom


At 01:29 PM 1/9/04 +1300, you wrote:
>Does anyone have suggestions (and hopefully practical
>experience) for methods of propelling a sled on a track ?
>

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2004\01\09@013730 by Al Shinn

picon face
Try some spear gun rubbers - think bungee cord. Tune it by varying the
stretch. Or think of all the old weapons like crossbows, catapults,
trebochets etc. Also look up spud guns, there are some that are powerd
by just air with some sorta positive feedback diaphram valve and then
there are the ones driven by fuel/air explosions useing something like
hairspray for fuel.
--

Looking forward,
Al Shinn

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2004\01\09@025805 by ahid Sheikh

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I see this is no ordinary sled.
How about a tesla coil flash along the length of the acceleration
portion of the sled's track and have an inward shaped cone at the back
end of the sled to get the most propulsion from the thrust.

Check with the producers of back-to-the-future if they'll let you have
the flux capacitor. You may need power in the giga-watt range.

On second thought, can the sled's track be vertical? Make a really tall
channel and let the sled free-fall in it.

Shahid

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2004\01\09@034028 by o-8859-1?Q?Tony_K=FCbek?=

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Hi,
Jinx wrote:
>Does anyone have suggestions (and hopefully practical
>experience) for methods of propelling a sled on a track ?

Well certainly not experience (at least not for a >9.8m/s^2),
but I have an idea, not having read the entire thread this is what I would
suggest.

As you only have 2 m to reach final speed and I assume at that time
you want to have no remaing acceleration and fairly good control of
the speed, I think this might work:

An 4 stage acceleration device consisting of:

* initial launcher, this is just to get the sled moving and
 enter the remaining three acceleration 'zones'. This could be
 just by pushing it by hand, using an rubber band, spring or something similar.

* Three acceleration zones, each with an motor running (pwm) and configured
with two interconnected flywheeels acting as a 'pinch' on the sled.

I think the best accuracy would be achived when the first acceleration-pincher
would be set to reach 0.4-0.5 final speed and the second at 0.9-1.0 of final speed.
Then the last one would be set to exact 1.0 of target speed and just do the final correction. I don't see an major problems with this configuration, getting hold of flywheels
with sufficient friction should be quite easy, linking thewn in pairs and getting
them to pinch the sled should also be quite easy (rubberband, spring, etc) the
only thing needed trimming and tinkering would be the pwm control.

Actually I sometimes help a friend with servicing and repairing tennis trainers,
they work almost exactly like this, getting the tennis ball in pinch between two fast moving
(aluminium) wheels gets it upto speed *fast*.
Speed is controlled by pwm, the accuracy is so-so as there is only one wheel acting upon
the ball. But is works ok for a trainer where the speed in wanted to wary to some degree.

/Tony

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2004\01\09@035312 by Jinx

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> Try some spear gun rubbers - think bungee cord

That did cross my mind. If you measured the pull, the twang
should be quite repeatable, even as the rubber ages

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2004\01\09@041221 by Denny Esterline

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I'm ashamed of myself for not coming up with this sooner. This has been
done. You can buy one retail at major sporting goods stores for a few
hundred bucks, a baseball pitching machine.

-Denny



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2004\01\09@050733 by Jinx

face picon face
> I once saw a "sled" that was "suspended" on an air cushion

Like air hockey - could be a viable alternative to mag-lev

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2004\01\09@062209 by Jinx

face picon face
> I'm ashamed of myself for not coming up with this sooner

Shame, thy name is Denny

> This has been done. You can buy one retail at major sporting
> goods stores for a few hundred bucks, a baseball pitching machine

Boy, did I ever get side-tracked ;-) but it's all food for thought

Found the ball's weight, 5.12oz (145g), Rule 1.09

mlb.mlb.com/NASApp/mlb/mlb/official_info/official_rules/objectives_1.
jsp

(must remember to read MLB rules and definitions before
next World Series so I know what the hell the commentators
are talking about)

AFAIK there isn't a pneumatic pitcher for baseball. The two
types I've seen are spring-operated (a revolving disc stretches
a large extension spring until a trip point is reached, then the
ball is thrown by an arm) and contra-rotating pinch wheels, one
of which I know is a Jugs used by NZ Cricket in Auckland

http://www.thejugscompany.com/products/baseball.cfm

Now there's even one for soccer balls

http://www.thejugscompany.com/products/soccer.cfm

I've seen a spring-operated boot on TV. My brother the soccer
coach went ooooh and asked me to make him one. I didn't

But getting back to air..........

There is a pneumatic type for tennis balls, The Lob-ster

www.tennis-ball-machine.com/catalog/Tennis_Ball_Machine_Superstore/Lo
b-ster_401_Tournament_Model_Tennis_Ball_Machine

(The Lob-ster is quite cheap compared to the Prince units, which
can be up to US$3500 and I don't think they're pneumatic)

According to

www.itftennis.com/shared/medialibrary/pdf/original/IO_1404_original.P
DF

a tennis ball should weigh only 2oz (57g)

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2004\01\09@085726 by Darrell Wyatt

picon face
How about a pneumatic / hydraulic cylinder and some
mechanical linkage for the required length?

D.

Artificial Intelligence is no match for Natural Stupidity.





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2004\01\09@153844 by John Ferrell

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Sounds like a cross bow to me...

John Ferrell
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Phone: (336)685-9606
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----- Original Message -----
From: "Tal" <EraseMEkooterspam012.NET.IL>
To: <RemoveMEPICLISTEraseMEspamEraseMEMITVMA.MIT.EDU>
Sent: Thursday, January 08, 2004 7:34 PM
Subject: Re: [EE]: Sled propulsion


> A firework maybe?
>
> Tal
>
> {Original Message removed}

2004\01\09@155747 by Jinx

face picon face
> Sounds like a cross bow to me...

I wish, I really really wish, it was that simple

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