4 ms·

This is absolutely not 27,000 joules. 27,000 joules would produce a temporary wound channel nearly large enough to destroy the ballistic gel block, and then it

This is absolutely not 27,000 joules. 27,000 joules would produce a temporary wound channel nearly large enough to destroy the ballistic gel block, and then it would travel through the entire block, sending the block a foot or two into the air. Based on the penetration of the projectile into the gel, but taking the obtuse nature of the projectile into account, I'm left to conclude this was more along the lines of 1000 joules... which makes me think the "50mph" that the author claims the CO2 provides might be more along the lines of 200mph, and that the magnetic acceleration aspect of the gun is actually doing nothing at all. In other words, we might be looking at a slightly high-powered potato gun being described by a misinformed author. Don't believe me? Take a look at the delay between the shot and the entry hole in the car door gif. That's commensurate with a projectile moving at freeway speed, not something with 27,000 joules, which for an aluminum projectile at that size, would be something like +4,000 feet per second. EDIT: Updated mph to take @avian's math into account - the ballistic gel penetration supports that as well.

hatsunearu 11y ago
It's probably 27kJ measured by the stored charge in the capacitor... which probably aren't delivered to the fullest extent. You can see the wires jump--that's inductive magic in action. To get any sort of quick magnetic pulse into the railgun, you need high voltage[1] and low transmission line inductance; neither of which are satisfied here. A guy I know works on high power pulse mode lasers and they achieve ridiculous rising edges with thin twisted pair wire. Lots of resistive loss but hey at least the current rising edge is nice and fast. [1]: a few kilovolts.
- mangeletti 11y ago
  Perhaps it is, but the inefficiency of the machine shouldn't really come into play when describing how powerful it is. Otherwise, I could call my car a 900 horsepower Honda Accord, based on the gas it burns, since the engine probably isn't even close to 30% energy efficient.
  - Synaesthesia 11y ago
    I think a lot of the energy is going to heat since the blocks of Aluminum melt and even vaporize.
  - hatsunearu 11y ago
    I didn't mean inefficiency, I meant that not all of the energy can be delivered to the projectile because the delivery is rate-limited because of electric properties. Also, there is inefficiencies in play, such as the melting aluminum like the other person suggested.
  - iaw 11y ago
    They actually typically report horsepower and torque measurements for cars at the engine and not at the wheels. There's an additional efficiency loss in the drive train not accounted for in marketing numbers.
- grecy 11y ago
  It's probably 27kJ measured by the stored charge in the capacitor... which probably aren't delivered to the fullest extent. When this was on Reddit yesterday the OP did a mini-AMA in the thread [1], and he confirmed this. 27kJ is the stored charge the capacitor bank. For various reasons, he's vary aware this is not all transferred to the projectile. [1] https://www.reddit.com/r/DIY/comments/3s4o4j/diy_250_lb_giant_mobile_railgun_27000_joules/ https://www.reddit.com/r/DIY/comments/3s4o4j/diy_250_lb_gian...
avian 11y ago
Let's see. The webm video of the car door shot is 30 fps, with each frame repeated 4 times. This suggests that original was shot at 30 fps and then reduced to 1/4 speed. The delay between the first flash from the gun and the entry hole appearing is 4 frames at original fps. At 30 fps, that gives flight time between 100 and 130 ms. I estimate that the distance from the gun to the car door is around 10 m. That gives speeds on the order of 100 m/s (200 mph). Energy depends on the mass of the projectile. At these speeds, 27000 joules would require mass in the range of kilograms, which is obviously not true. Still, it seems feasible to me that this gets better than highway speeds.
- danielvf 11y ago
  Paintball guns are usually limited to 300 feet per second in competition- that's 91 meters per second, which is about what you are measuring. A paintball weight 3grams, whereas the rail gun projectile is 11 grams. Paintball guns can shoot faster than is competition legal, and the much longer barrel could allow it to add more energy than it could with a heavy projectile in an equivalent paintball gun.
  - tomcam 11y ago
    Um, no. A paintball weighs about 3 grams [1]. [1] http://www.dtic.mil/dtic/tr/fulltext/u2/a473706.pdf http://www.dtic.mil/dtic/tr/fulltext/u2/a473706.pdf
    - danielvf 11y ago
      Thanks! Corrected. At least I wrote the wrong measure on both, so the ratio was still right.
  - simonh 11y ago
    They say that the paintball gun doesn't shoot the projectile fast enough for it to overcome friction from the rails, so without the magnetic field it wouldn't exit the gun at all, which seems plausible. So assuming they are trustworthy you can completely discount the energy provided by the painball gun when considering the energy given to the projectile.
PepeGomez 11y ago
I think it's because the bullet can't withstand the current/pressures and melts/vaporizes. Edit: also note that these are high speed camera recordings.

hatsunearu 11y ago

It's probably 27kJ measured by the stored charge in the capacitor... which probably aren't delivered to the fullest extent. You can see the wires jump--that's inductive magic in action. To get any sort of quick magnetic pulse into the railgun, you need high voltage[1] and low transmission line inductance; neither of which are satisfied here. A guy I know works on high power pulse mode lasers and they achieve ridiculous rising edges with thin twisted pair wire. Lots of resistive loss but hey at least the current rising edge is nice and fast. [1]: a few kilovolts.

mangeletti 11y ago
Perhaps it is, but the inefficiency of the machine shouldn't really come into play when describing how powerful it is. Otherwise, I could call my car a 900 horsepower Honda Accord, based on the gas it burns, since the engine probably isn't even close to 30% energy efficient.
- Synaesthesia 11y ago
  I think a lot of the energy is going to heat since the blocks of Aluminum melt and even vaporize.
- hatsunearu 11y ago
  I didn't mean inefficiency, I meant that not all of the energy can be delivered to the projectile because the delivery is rate-limited because of electric properties. Also, there is inefficiencies in play, such as the melting aluminum like the other person suggested.
- iaw 11y ago
  They actually typically report horsepower and torque measurements for cars at the engine and not at the wheels. There's an additional efficiency loss in the drive train not accounted for in marketing numbers.
grecy 11y ago
It's probably 27kJ measured by the stored charge in the capacitor... which probably aren't delivered to the fullest extent. When this was on Reddit yesterday the OP did a mini-AMA in the thread [1], and he confirmed this. 27kJ is the stored charge the capacitor bank. For various reasons, he's vary aware this is not all transferred to the projectile. [1] https://www.reddit.com/r/DIY/comments/3s4o4j/diy_250_lb_giant_mobile_railgun_27000_joules/ https://www.reddit.com/r/DIY/comments/3s4o4j/diy_250_lb_gian...

avian 11y ago

Let's see. The webm video of the car door shot is 30 fps, with each frame repeated 4 times. This suggests that original was shot at 30 fps and then reduced to 1/4 speed. The delay between the first flash from the gun and the entry hole appearing is 4 frames at original fps. At 30 fps, that gives flight time between 100 and 130 ms. I estimate that the distance from the gun to the car door is around 10 m. That gives speeds on the order of 100 m/s (200 mph). Energy depends on the mass of the projectile. At these speeds, 27000 joules would require mass in the range of kilograms, which is obviously not true. Still, it seems feasible to me that this gets better than highway speeds.

danielvf 11y ago
Paintball guns are usually limited to 300 feet per second in competition- that's 91 meters per second, which is about what you are measuring. A paintball weight 3grams, whereas the rail gun projectile is 11 grams. Paintball guns can shoot faster than is competition legal, and the much longer barrel could allow it to add more energy than it could with a heavy projectile in an equivalent paintball gun.
- tomcam 11y ago
  Um, no. A paintball weighs about 3 grams [1]. [1] http://www.dtic.mil/dtic/tr/fulltext/u2/a473706.pdf http://www.dtic.mil/dtic/tr/fulltext/u2/a473706.pdf
  - danielvf 11y ago
    Thanks! Corrected. At least I wrote the wrong measure on both, so the ratio was still right.
- simonh 11y ago
  They say that the paintball gun doesn't shoot the projectile fast enough for it to overcome friction from the rails, so without the magnetic field it wouldn't exit the gun at all, which seems plausible. So assuming they are trustworthy you can completely discount the energy provided by the painball gun when considering the energy given to the projectile.

PepeGomez 11y ago

I think it's because the bullet can't withstand the current/pressures and melts/vaporizes. Edit: also note that these are high speed camera recordings.