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Registered Member #4237
Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117
Hello!
I'm having a bit of a hard time trying to figure out the optimal distribution of the energy stored in my cap bank. There's 4000 joule that needs to be distributed as efficiently as possible into 4 coils. The projectile will already have some speed as it enters the first coil, from a kicker coil powered by a smaller cap bank. As I understand it, I'll need to give more and more energy to each coil, since the faster the projectile is moving, the more efficiently I can transfer the electrical energy into kinetic. This makes perfect sense, but I am not sure what the best distribution of the energy will be. Do you guys know of any research done on this that I can read?
Registered Member #1525
Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294
Alrighty, I gave a high-level answer to this in your other thread, but your question is legit and deserves a more detailed answer.
A coilgun is a linear accelerator, that means in theory a sequence of an arbitrary number of coils with an arbitrary energy distribution imparts the same end velocity on the projectile as any other arrangement, including one long coil so long as the same overall external energy is applied. So, in terms of pure math, it doesn't matter.
However, in the wonderful world of magnetodynamics, nothing in reality is so simple. All sorts of dynamic loss factors change the way the projectile interacts with a given coil, distorting this linear behavior. As simple hobbyists with limited time and budget, we account for these factors by including a measure of efficiency in our calculations.
Unfortunately there is no formula or reliable way to predict efficiency, we must take a guess. We improve this guess by looking at what other hobbyists have measured from their designs similar to what we want to build. Once we have some idea of what efficiency we expect from each coil stage in our design, we want to distribute the energy in a way that results in the best overall energy transfer.
After we come up with some good guesses for efficiency for each stage, we can smartly choose our energy distribution. In the course of your research, you will find that the first stage always has the poorest efficiency, and subsequent stages (if properly tuned) will experience higher and higher efficiencies until they reach a plateau and efficiency begins falling again (suckback worsens as projectile speed increases). It follows that you would want to use only a small amount of energy in the first stage, and put more and more energy into later stages. But wait, there is a problem with this! You also discover that the more energy you dump into a single coil, the lower the efficiency is! So putting too much energy into your later stages defeats the purpose of having later stages. Also, as your projectile passes more stages, it speeds up, allowing less time per stage for energy to be transferred, once again limiting how much you may efficiently dump into subsequent stages.
MORAL OF THE STORY, do some research to find what efficiencies you can expect for each sage, and find a balanced energy distribution that puts less energy in early and late stages, and more in the middle where it counts the most. Lastly I must stress the importance of proper design and tuning. After your initial efficiency guess and energy distribution, design each coil carefully to meet the timing and energy requirements (that image file I gave you shows how to do that part). Then, as you build, check and tune each stage to make sure that it's performing close enough to what you predicted.
Good luck! If you need help guessing what efficiencies you may get for a given stage, give me some specifics and I'm happy to help you out.
Registered Member #4237
Joined: Tue Nov 29 2011, 02:49PM
Location:
Posts: 117
Wow, thanks a lot for the reply! Very helpful :) Only problem I'm having is that there seems to not be a lot of coil guns built in my power range (4 to 5 kJ) that I can read about. But I'll search on!
EDIT: I have changed the design, by the way, there is now 6 coils instead of 4 (plus the kicker coil, 7 in total). But as I understand it, that might not be enough!
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How to figure out the best energy distribtuion in a multistage coilgun?
