I usually try to put the tank in my car trunk (Mustang.. barely fits) sideways. I used to load my tanks valve backwards, but after thinking about what would happen if I were rearended, I stopped doing that.
Now about the front/back question. This is my take. When the valve flys off, it's propelled by about 3000psi*1inch^2= 3000lbs for a fraction of a second. Maybe .01seconds? The tank, on the other hand, is propelled by about the same force(steadily diminishing as air drains) until the tank drains out, which would probably be a few seconds. Which means, that the valve, if it hits something, like the back of your seat, will lose most of its energy and will probably bounce back. The tank however, is continuously being propelled, and even after hitting something, will tend to continue to exert force by itself. So, it seems to me, that the tank is much more dangerous. (And the initial energy will be larger too with the tank.)
This is what I infer from freshman physics anyway..
Now about the front/back question. This is my take. When the valve flys off, it's propelled by about 3000psi*1inch^2= 3000lbs for a fraction of a second. Maybe .01seconds? The tank, on the other hand, is propelled by about the same force(steadily diminishing as air drains) until the tank drains out, which would probably be a few seconds. Which means, that the valve, if it hits something, like the back of your seat, will lose most of its energy and will probably bounce back. The tank however, is continuously being propelled, and even after hitting something, will tend to continue to exert force by itself. So, it seems to me, that the tank is much more dangerous. (And the initial energy will be larger too with the tank.)
This is what I infer from freshman physics anyway..