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Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
rp181, ".2% efficiency is dead easy. With some tuning, you should be able to break 1%."
I think by .2 he means 20% as it is in normal number form. .2% is really .002 not just .2...
1.2x.2%=2.4J 1.2x.2=240J
I think you should use 10% as a model efficiency, still difficult to reach but doable without having to use special projectile materials and advanced pulse circuitry. It'll have to extremely well tuned and heavier projectiles would be better.
Registered Member #1513
Joined: Sat May 31 2008, 12:08PM
Location: Russia Klin
Posts: 48
wrote ... Do you think the guys at NASA just decided to slap together parts and build a space shuttle?
I think your coilgun is much simplier than Shuttle :) . I'm sure that you can figure it out in your mind, and no need in 3D modelling. Almost all guys who had made this coilguns haven't modelling them before building
wrote ... I think you should use 10% as a model efficiency
Why he should use 10%, if only 1 man in the whole world has reached this efficiency after a lot of studying and experiments? I think using 5% will be more fair
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
Who said only one man has reached that? I have reached 10%, and I know for a fact the person who used a iron powder shell has too. What is wrong with modeling it? There is no harm at all. Is, somethings will change, but its still better to have a idea.
Sorry about the .2 misunderstanding. Do you plan on portable of just a system?
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
Wasting money and time in failing time after time is worth the effort of modeling. Especially with a coilgun that is more complex than just a pen tube with some hookup wire wound around it. Plus, 3d modeling is plain fun! :)
Are you going to model the coil dimensions with something like Barry's calculator? That would help a lot with tuning and thus getting a great efficiency.
Registered Member #1062
Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529
I never wrote up that coilgun, and it is disassembled now, as the railgun is priority. Modeling does help, but keep in mind the values change as the projectile moves. Take a look at FEMM, its actually quite easy once you take 1/2 hour to read the tutorial.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
kromsson wrote ... The energy of your capacitors is 1/2 x 6 x 2.000/1.000.000 x 450 x 450 = 1.215 J = 1.2kJ Assuming efficiency of 0.2 (which is VERY difficult already) you have 1.2 x 0.2 = 0.24 kJ of kinetic energy to the projectiles.
OK so far, but...
A projectile will have a volume of (converting inches to cm) : 1/2 x pi x 1.27 x 1.27 x 6.35 = 16 cm3 I assume you will use iron (induction coilgun) with a weight of about 8.000 kg/m3. This means your projectiles will weigh about 0.13 kg each.
BZZZZZT! Your factor of half s/b quarter, so it's 8 cm^3 and 64 grams.
E = 1/2 x m x u2 ===> u = 2 m/sec = 6 fps. For this matter you need greater capacitance, and a LOT smaller and lighter projectiles.
That would be the velocity for 130 kilograms! u = sqrt(2E/m) gives 61 m/s with the mass you stated, and 86 m/s with corrected value. The more conservative goal of 5% efficiency (muzzle energy 60 J) would yield 43 m/s. Launched vertically at that speed, a projectile without drag would climb 96 m (from E = mgh) and fly for more than 8 seconds. [edit]I have made pumpkins fly longer than that, with almost 2 kJ from bursting 2 liter soda bottles pressurized with CO2, but that is a story for another day.
When working this kind of problem in a spreadsheet calculator, I generally use separate text cells for variable names and their units of measurement. One way to avoid magnitude errors is to have all physics formulas and their I/O in columns that use basic SI units (kilograms, farads, cubic meters, pascals...) exclusively. A different column can have the I and O values in practical, intuitive units (inches, grams, microfarads, square millimeters ...). Merging unit conversions into physics formulas invites mistakes.
On the other hand, the auto loading of the magazine and the revolving and the cooling of the coils sound VERY interesting, good luck and I would be VERY interested to see the final project!
Registered Member #2360
Joined: Sun Sept 13 2009, 05:43PM
Location: Kennesaw, Ga USA
Posts: 14
with 2 pneumatic pistons
How about using solenoids instead for the pneumatic pistons. This way you won't have to run an air hose to the gun also. The gun sounds sweets though, good luck and post pictures.
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Concept Coilgun
