At what point does model rocketry have to morph into HPR?

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majordude

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I've been in the hobby since 1973. I grew up on Estes and Centuri. Back then it was all paper, plastic, and wood.

But the last few kits I bought were fiberglass, carbon fiber, and metal tipped nosecones. Some of these "indestructible" kits are even designed for short-thrust E, F, and G motors.

I was wondering...

At what point is it nearly impossible to hold a rocket together made of paper, plastic, and wood? J? K? L?

Why I'm asking...

I've seen too many over-engineered and under-executed, "indestructible" rockets plant themselves balls-deep into the ground. I have to think that if that same lawn-dart hit an RV, car, or person it wouldn't be pretty.

And I've been thinking...

Why use "deadly force" instead of "reasonable force"? Why use a gun when you could use a taser? Why use fiberglass instead of paper tubing? Why use a metal-tipped nosecone instead of a plastic cone?

Then again...

I have to admit that I have no data behind my thoughts. I mean, if you jump into water from a meter it's fun. If you jump into water from 18 stories up, it's deadly. Maybe a balsa nosecone with an "E" bounces off your car... but with a "K" it pierces the roof. I don't know.

What are your thoughts?
 
For the other part, if it's aerodynamic and massy you're going to have to a real bad time, heavy cardboard or light CF
 
E=MC^2.... the speed, not the weight, is the major component of the rockets "potential" to do serious damage.

That being said, look at the statistics. Model Rocketry is super safe.

Be the steely eyed missile man... the stat's say your concern is misplaced.
 
E=MC^2.... the speed, not the weight, is the major component of the rockets "potential" to do serious damage.

That being said, look at the statistics. Model Rocketry is super safe.

Be the steely eyed missile man... the stat's say your concern is misplaced.

No, it is the mass and the velocity that determines the damage. The Mythbusters showed several times that objects with low mass do not do much damage, for example ping pong balls.
 
No, it is the mass and the velocity that determines the damage. The Mythbusters showed several times that objects with low mass do not do much damage, for example ping pong balls.
E=MC^2.... the speed, not the weight, is the major component of the rockets "potential" to do serious damage.

That being said, look at the statistics. Model Rocketry is super safe.

Be the steely eyed missile man... the stat's say your concern is misplaced.
Which is why I went with lower Cd, higher M.

But regardless, agree that rocketry risks are fairly well managed by being in the middle of nowhere, roping off the flight line, and keeping the pointy end away from the squishy bits etc.
 
Statistics say were safe.

I say were just standing on a very large dartboard and weve been lucky.
 
No, it is the mass and the velocity that determines the damage. The Mythbusters showed several times that objects with low mass do not do much damage, for example ping pong balls.

Re-read my quote....

E=MC^2.... the speed, not the weight, is the major component of the rockets "potential" to do serious damage.

"Major Component", as in squared.

You, and a television program, can't say
to the theory of special relativity.

The Mythbusters were testing the lethality of a ping pong ball... what did they do to it to make it more lethal? They shot it out of an air cannon at 1,100 mph. Because they know E=MC^2, and thus for a fixed mass the way to increase it's lethal energy is to increase it's speed. https://www.youtube.com/watch?v=msgfm4DHiyc
 
Build what you like to build. I like to build FG. I like to build big. It's what is fun for me. Build what is fun for you. Overbuilt? No such thing. Build rockets that meet your personal objectives and put the motors in them that meet your personal objectives. There is no one-size-fits-all. It's a hobby.
 
Momentum equals mass times velocity. The special theory of relatively? Really?
 
Sometimes more advanced materials can equate to lighter (safer) rockets.

Our upscale Dragonfly (5x - 12.75" diameter with a 5' span) is made out of carbon fiber, balsa, and foam. It weighs about 45 pounds on the pad.

A similar rocket built from sonotube and plywood would easily weigh double or triple what ours does, meaning that it would have double or triple the kinetic energy in at a given speed. It would also need a lot more motor and a lot more parachute.
 
E=MC^2.... the speed, not the weight, is the major component of the rockets "potential" to do serious damage.

That being said, look at the statistics. Model Rocketry is super safe.

Be the steely eyed missile man... the stat's say your concern is misplaced.

You’re correct that the kinetic energy is proportional to the square of the velocity, but the equation for kinetic energy at any of the velocities we’ll ever achieve is e = 1/2 m * v^2.
Conservation of energy and conservation of momentum must both be considered when analyzing the potential for damage.
 
Momentum equals mass times velocity. The special theory of relatively? Really?

Thanks for pointing that out -- it was bugging me but I didn't want to be the pedant.

Lots of things going on, but I'd argue that, all other things being equal (rockets with the same geometry falling from a great enough height), the higher mass rocket will have the greater potential to do damage than the less massive rocket.

The OP seems to be talking about heavy and robust rockets falling ballistically. The particular worry seems to be a rocket with a hardened nose-cone hitting a person or vehicle.

In this case, penetration would be an elastic rupture -- the stress at impact exceeding the ultimate strength of the target (the car roof, the bystander's skull, etc.). We'd figure the stress at impact in force per unit area. The force would be the time rate of change of momentum during the collision. Impulse divided by the duration of the impact, in a first order approximation.

The momentum of the falling rocket -- that is to say the impulse delivered to a stationary object struck by the falling rocket -- will be the product of its mass and velocity. Its velocity, however will depend upon its mass. The acceleration of the falling rocket will depend upon the difference between its weight and the drag force. So, for two rockets with the same geometry falling ballistically from the same height, the heavier rocket will have greater mass and will ALSO have a greater velocity at the moment of the impact.
 
So, for two rockets with the same geometry falling ballistically from the same height, the heavier rocket will have greater mass and will ALSO have a greater velocity at the moment of the impact.

Two objects falling with the same drag coefficient will achieve the same speed regardless of mass.
 
Somewhere on TRF there is a picture of a plastic nosecone that punctured the metal roof of the Animal Motor Works trailer. I believe it was a low power rocket that lawn-darted (or should that be trailer-darted?)
 
Somewhere on TRF there is a picture of a plastic nosecone that punctured the metal roof of the Animal Motor Works trailer. I believe it was a low power rocket that lawn-darted (or should that be trailer-darted?)

Like this?

AMW2.jpg AMW3.jpg AMW1.jpg
 
Two objects falling with the same drag coefficient will achieve the same speed regardless of mass.

Um, no.

Terminal velocity is achieved when there is sufficient altitude to do so, and the object accelerates to the point where the drag force at that altitude equals the objects weight at that altitude. As altitude increases, the objects weight goes down very very slightly, but the air pressure goes down much faster. So drag decreases much faster than weight. So terminal velocity increases with increasing altitude, again, under the assumption that you have enough altitude available to achieve terminal velocity at your altitude of interest.

But mostly it won't matter. Get hit on the head by most any HPR rocket except the smallest fattest lightest coming in ballistically and the construction methods will be irrelevant to you.

The up part is optional. The down part is not. Make sure your recovery method is good before flying.

Gerald
 
You are correct - I stand corrected. While it is true that acceleration due to gravity is unaffected by mass, I was incorrectly creating a corollary to Galileo's principles of falling to the extent that two falling objects with the same drag coefficient behaved the same as two falling objects in a vacuum, but I see that is not the case. Galileo's principle only applies when there is no air resistance.
 
E=MC^2.... the speed, not the weight, is the major component of the rockets "potential" to do serious damage.

The equation you listed shows the relationship between energy and mass. Essentially, they are different forms of the same thing. E=MC^2 has nothing to do with model rocketry. E=MC^2 is most relevant to nuclear bombs, nuclear reactors and the explaining how stars work.
 
Um, no.
Terminal velocity is achieved when there is sufficient altitude to do so, and the object accelerates to the point where the drag force at that altitude equals the objects weight at that altitude. As altitude increases, the objects weight goes down very very slightly, but the air pressure goes down much faster. So drag decreases much faster than weight.

Thanks. The rocket doesn't even have to reach terminal speed. The speed at any part of the descent will depend upon weight -- here, because I am actually a pedant.
fallingrocketOCD3.jpg

I am not sure that this version of the drag force is right for a model rocket, but the result is similar if we assume that the drag force goes as speed, rather than speed squared.

Solving the problem for the speed of a rocket falling through still air for which density and/or viscosity vary with altitude is left as an exercise for the student. <smile>
 
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You&#8217;re correct that the kinetic energy is proportional to the square of the velocity, but the equation for kinetic energy at any of the velocities we&#8217;ll ever achieve is e = 1/2 m * v^2.
Conservation of energy and conservation of momentum must both be considered when analyzing the potential for damage.

Thanks for that Steve. I guess the point I was making is since the velocity is squared, velocities impact on energy is much greater than mass.
 
Thanks for that Steve. I guess the point I was making is since the velocity is squared, velocities impact on energy is much greater than mass.

As Labrasca points out, the mass of the object has a non-trivial effect on the resulting vterm (all things being equal). R'feller's carbon dragonfly will develop less kinetic energy in a ballistic reentry than a fiberglass dragonfly of the same dimensions and create less impact energy since it has the same drag characteristics but less mass.

That being said, upon apogee failure, would you prefer the carbon or fiberglass dragonfly like to land on your vehicle? :cool:
 
Thanks. The rocket doesn't even have to reach terminal speed. The speed at any part of the descent will depend upon weight -- here, because I am actually a pedant.
View attachment 344765

I am not sure that this version of the drag force is right for a model rocket, but the result is similar if we assume that the drag force goes as speed, rather than speed squared.

Solving the problem for the speed of a rocket falling through still air for which density and/or viscosity vary with altitude is left as an exercise for the student. <smile>

Aerodynamic forces tend to go as the square of the speed, neglecting Reynolds numbers and Mach factors.

Gerald
 
Aerodynamic forces tend to go as the square of the speed, neglecting Reynolds numbers and Mach factors.

Gerald

I always neglect Reynolds numbers -- I didn't like fluid mechanics much when I was a student, and my affection has not increased in the years since completing my degree.

But yes, neglecting the negligible, and making the usual simplifying assumptions, and with the appropriate application of Galilean idealization... my scribbles above might add up to a sufficiently close approximation of the behavior of a falling rocket to make the point we were both trying to make; the Mythbuster's ping pong ball was never going to go fast enough to to require a relativistic correction for the impulse delivered to Jamie Hyneman's belly. <g>

I think that is probably enough of a thread-jack for me. I've got stuff to do in the shop.

To the OPs Original Post: there was recently a discussion among some of the members of the club with which I launch, regarding rockets built exclusively from Estes components, launched on high power motors. Several of the PSII models are easily H or I capable, if assembled carefully.

The idea of flying very big motors in paper rockets is actually very appealing. Somewhere between the Pinewood Derby and the engineering challenges we set for 2nd year Mech. E.s and Civ. E.s - build a bridge out of uncooked spaghetti, build a tower out of balsa, etc.

I have some 4 inch LOC tube. I am resolved. If I get my L2 cert, I will use this tube to build something of not-space-aged materials and launch it on a K or L.
 
I've seen too many over-engineered and under-executed, "indestructible" rockets plant themselves balls-deep into the ground. I have to think that if that same lawn-dart hit an RV, car, or person it wouldn't be pretty.
And I've been thinking...
Why use "deadly force" instead of "reasonable force"? Why use a gun when you could use a taser? Why use fiberglass instead of paper tubing? Why use a metal-tipped nosecone instead of a plastic cone?

In short - because it's more challenging and more fun to engineer & build a rocket that goes higher and faster.
Because that is what this hobby is all about!

Yeah, slow'n'low is always easier, cheaper, and safer.
No question about that.

But is it more fun?
For many - sure, low-power is plenty fun enough.
For others - we, occasionally, want new and greater challenges.

The key is controlling the risks to preserve safety, which goes hand in hand with common sense and rocket durability.
No-one wants to invest weeks of efforts and 3/4/5+ figures of $$$s to produce a ballistic descent. That is both unsafe, unwise, and all around wasteful.

Thus we have organizations like NAR and TRA, as well as this board, to share the knowledge about best practices to always fly safely, as high and as fast are you dare.

a
 
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