Question:
Just flew my Raven from Shadow Composites the first time. First flight in
standard configuration on an H-123 went beautiful. Then I tried another
flight in the dual deployment configuration just to see how the rocket would
fly with the extra 2 ft. of body tube also on an H-123. Rocket went
straight up like an arrow - and the way it went up it came down - straight
as an arrow. The rocket buried itself 19 inches into the hard dirt of the
dry lake bed and I had to use a shovel to dig it back out. To my amazement
the fins stayed on and only about 1 " of the front of the body tube got
shredded, where the nose cone had pushed into it. Still need to check it
for any hairline cracks and I have to make sure it's still straight, but it
looks like I'll be flying that rocket again soon.
I did figured out what went wrong: I greased the ejection closure just a bit
too much and the grease insulated the black powder from the ejection delay.
Well life and learn. I am glad nobody got hurt as there was a big crowd
watching.
From the sims I know the rocket hit the ground at 320 ft/sec. Knowing the
depth of 19" is there a way to calculate the deceleration and G forces
involved in that crash? I'd sure like to be able to figure that one out.
Anybody any ideas?
Answer:
Believe it or not, many things can be engineered to take this kind of
punishment. There are research vehicles used in space exploration known as
"hardlanders" and "penetrators". You can guess what they do. In a typical
penetrator, there's a narrow, heavy forward part and a wider aft part. The
two pieces are designed to separate on impact, with the forebody penetrating
several yards into the surface and the aftbody staying on the surface. A
cable connects the two, and the aftbody has a radio antenna for relaying
measurements. Typically, the structure and electronics have to be designed
to survive 20,000 G's or so. It's not really surprising that a carbon
airframe should survive an impact of 1000 G's as long as it comes in
straight. However, you should probably also check the tube for internal
delaminations because carbon composite is extremely strong but not very
impact-resistant. Try tapping a coin up and down the tube and listen for
changes in the sound.
Car, person, neither would have significantly slowed this rocket down. It
would have gone right through either of them, and then, perhaps, buried itself
only 12" in the hard dry lake bed.
It's like driving while drunk. You are taking a normally risky operation and
ratcheting the risk up a few notches until it become more easily life
threatening.
There is always a risk of damage or injury when one attends a rocket launch.
Probably less risk than attending a baseball game, but there's risk just the
same. It's a risky operation (just like driving), but making a non-frangible
rocket is ratcheting that risk up a whole bunch. A rocket should come apart
if it streamlines into a hard surface. No one should be proud that their
rocket survived intact after a 4000' lawn dart if it hits a very hard surface
like a dry lake bed.
Even more to the point, that rocket landed less than 10 feet of people who
were attending their first high power launch. They were safely behind the
flight line. Because this small rocket did not deploy its recovery system, no
one knew where it was until it appeared fence-posted into the ground right
near some spectators. There were no cries of "heads up" because no one knew
where it was, not until the sickening smack as the rocket needled, intact,
into the hard ground. Needless to say, the spectators left, swearing they
would never return. They'll always consider high power rocketry a life
threatening activity. There will be some word of mouth spreading of this
because it was a seriously frightening experience. It shouldn't have been
that way.
Had the rocket come apart when it hit (like it should have) these people would
have realized that it would have dented a car, or injured a person. Maybe
that would have scared them, maybe not. As it was, though, they were treated
to a graphic example of inappropriate over-building that resulted in a mistake
that could have seriously damaged a car, or killed a person.
A model rocket or high power rocket normally carries the ability to injure a
person or damage a car, but the potential for serious damage or deadly injury
should remain a long shot. That was not the case with this rocket. If this
rocket were to impact a person, the likely injury would be very severe or
deadly. It it were to impact any vehicle, the likely damage would be severe.
It's too much.
In my opinion, if anyone brags that their rocket can handle a 4000'
streamlined drop and survive hitting a cement like surface, they're not
proving their building ability, their proving their disregard for basic rocket
safety.
All I'm saying is that people need to think about it. Overbuilding isn't
necessarily cute, or funny, or idiosyncratic. It has the potential to be
negligently dangerous.
