Monday, November 16, 2015

A tragic weekend...

This weekend was tragic...

Not only for the horrible events in France, which are beyond belief, but also for an accident some deemed inevitable, but yet took over 55 years to occur...

A scout leader was killed on Saturday when a rocket came in ballistic and struck him in the head.

As I sit here writing these words, I must admit to feeling a bit shell-shocked. In the several hundred million launches of hobby rockets since things started back in the late 1950's, there have been a minuscule few accidents in which people have been hurt, but none seriously. My hobby was - and still is - just about the safest one on the planet. The safety code and best practices that have guided us throughout decades have enabled rocketeers to fly without major incident over and over again. So what happened?

Facts are sketchy, especially given the lack of knowledge the media have about hobby rocketry. Some initial reports said the scout leader was struck by a "bottle rocket"; others say it was a "homemade rocket." In all likelihood, it was a model rocket assembled from a kit, but even that is currently speculation. What appears to be consistent is that the rocket was launched at an annual Boy Scout event, the scout leader lost track of the model, and it came in ballistic, hitting him. A terrible case of an extremely low probability event having disastrous consequences. There is no evidence of a safety code violation, no indication that carelessness played a role - facts are few, and it will take time for them to come out, if they ever do.

However, I have been in the hobby a long, long time and attended many launches. And I know that with the increased popularity of high power rocketry, the safety emphasis has shifted to the large 10 foot beasties flying on L motors; you can just look at them and know that if one of those came in ballistic on a vehicle or, heaven forbid, a person, it would be a very bad day. Model rockets, because they are smaller, made of cardboard, balsa, and plastic, and use lower impulse motors, are not really considered much of a risk in today's environment. I myself have fallen into this mode of thinking on occasion - a few years back, I was greatly concerned that one of the high power rockets flown at the NASA Student Launch Initiative could make a hard landing among the crowd of spectators. calculations showed that this was a very low probability event, and it was easily mitigated by angling the launch rails so that a rocket coming in ballistic would impact down range, well away from the attendees. In my mind, big = dangerous, so I never bothered to quantitatively consider the risk posed by my model rockets.

That changed last night - after all, I am a scientist, and I need numbers. I wondered how one could quantify the danger posed by a model rocket coming in ballistic.

I quickly made up my mind to focus on the rocket, not the probability of a ballistic model hitting a person. That would be very variable, depending on the number of spectators, the size of the area in which they are located, and so forth. So I considered the energy possessed by a rocket coming down at terminal velocity, and how this energy compared to other non-rocketry impacts on humans. There were two obvious extremes - the case of being tackled in a football game, which seldom causes major injury, and being shot with a 0.38 caliber bullet, which is obviously very bad. I also looked at the U.S. military's lethality limit for explosive fragments, figuring they had put a lot of effort into establishing this. These numbers were fairly easy to find through Google, and here they are (I have converted the units from the obscure imperial system to metric, which is how I think - the imperial ones are in parentheses).

Lower lethality energy threshold for explosive fragments: 78.6 joules (58 foot lbs)
Energy of 0.38 caliber bullet: 163 joules (120 foot lbs)
Average energy involved in a tackle: 610 joules (450 foot lbs)

You can see that energy alone does not tell the whole story - if it did, the bullet would be much less lethal than a football tackle, which has the greatest energy of three. Obviously, there is another factor to be considered, and that involves the area over which the energy is applied. The tackle spreads out the energy over the cross-section of the human body, whereas the bullet concentrates it into a very tiny area. So let's compute a new number to characterize lethality, one in which we divide the energy by the area over which it is concentrated. Doing this for the tackle and the bullet gives

Lethality of 0.38 caliber bullet: 630,000 joules per square meter
Lethality of football tackle: 547 joules per square meter

This makes much more sense - it is over 1000 times easier to be killed by a bullet than a football tackle. So where do model rockets fit into this framework?

To figure that out, I used the Rocksim simulation program to generate results for 3 rockets - the Estes Alpha, Big Bertha, and the diminutive Mosquito. For maximum altitude, C6 motors were loaded into the Alpha and the Bertha, and an A10 into the Mosquito; the simulations were then set not to have an ejection charge so that the rockets would virtually crash into the ground at terminal velocity. I then used the mass of the rocket, its frontal area, and the impact speed to calculate the impact energy and lethality. Here are the results:

Estes Mosquito (A10 motor): 1893 joules per square meter
Estes Big Bertha (C6 motor): 14,500 joules per square meter
Estes Alpha (C6 motor): 25,200 joules per square meter

It was the difference between the Alpha and the Bertha that surprised me; even though the Bertha is bigger and more massive, the more streamlined Alpha had a larger terminal velocity and a smaller diameter, which accounted for its greater lethality. However, the main point is that model rockets can be dangerous if they come in ballistic, and the Alpha can have a lethality closer to that of a bullet than a football tackle, by a factor of 10 or so.

These numbers have convinced me to redouble my safety awareness at all launches, even if they involve "just" model rockets. In particular, I think there are three established practices to religiously implement:

  • Angle rods away from the crowd for ALL rockets. No one can be hit if the rockets are flying in the other direction.
  • Keep everyone 'heads up" whenever a model is launched, regardless of size.
  • Consider restricting motors to keep the models visible throughout the flight.

Hopefully, we will never have to give our sympathies to another rocket accident victim. The world is tragic enough.

1 comment:

  1. Nice article. Another thing is to make sure the flight line is parallel to the wind. This will minimize the number that are traveling over their heads. No?