What kind of damage can too much acceleration cause

[Funkworks]

I'm wondering what part would fail first, or what actually limits the acceleration a model rocket can take. Is it even structural? Is it only a cost or practicality issue? Are there any known records? What considerations need to be taken when trying to break g-force limits with a particular level of model kit?

Yeah. That's my question. I still haven't had my coffee today, which is my other concern, but I suppose this isn't the place to solve that one. So thanks.

 

[mbeels]

Speaking of acceleration, I don't have answers to your questions, but you may be interested in this launch. The motor burned out at 500' and it coasted to over 30,000 feet.

 

[dhbarr]

For Models, considering only G forces, nothing should really ever fail.

Two 80N-avg-thrust motors having less than the maximum propellant weight is the theoretical upper limit.

It's quite a boot to the rear, but aero forces are what will kink your airframe or flutter your fins off.

 

[Handeman]

I haven't done any really high G flights, 80 - 85 G on a I1299 Warp 9 is about the most I've tried. Tearing the Conformal rail guides off is the only issue I had. Besides the nose cone, body tube, coupler or fins failing, there could also be issues with electronics and batteries, how they're mounted, etc.

 

[dhbarr]

For Models, considering only G forces, nothing should really ever fail.

Two 80N-avg-thrust motors having less than the maximum propellant weight is the theoretical upper limit.

It's quite a boot to the rear, but aero forces are what will kink your airframe or flutter your fins off.

On further reflection we can probably get the peak acceleration a fair bit higher with, for instance, a trio ( or more ) of CTI 24mm loads having initiator pellets.

 

[mbeels]

Two 80N-avg-thrust motors having less than the maximum propellant weight is the theoretical upper limit.

I'm trying to understand this, where does this theoretical limit come from? I would have expected the maximum acceleration to only be limited by the weight of the rocket and how quickly the propellant can be consumed and sent out the back of the rocket (specific impulse and burn rate).

 

[dhbarr]

I'm trying to understand this, where does this theoretical limit come from? I would have expected the maximum acceleration to only be limited by the weight of the rocket and how quickly the propellant can be consumed and sent out the back of the rocket (specific impulse and burn rate).

Sorry, I assumed OP meant model == Model.

If we include HPR and Amateur, the three most common G-related failures I'm aware of are:
* load shift moving to instability
* sagging propellant to overpressure
* unsupported electronic components to deployment failure

 

[jderimig]

Speed kills, not acceleration usually. Except for the acceleration by ballistic impact.

 

[mbeels]

Sorry, I assumed OP meant model == Model.

Ah, got it, perhaps that is what the OP meant, and I was thinking too generally.

 

[prfesser]

Sorry, I assumed OP meant model == Model.

If we include HPR and Amateur, the three most common G-related failures I'm aware of are:
* load shift moving to instability
* sagging propellant to overpressure
* unsupported electronic components to deployment failure

Back when AT had their fire, they subbed out some of the propellant manufacturing to...someone else, I forget who. The J350 reloads made by that company had propellant that was a bit too elastic. The bottom grain sagged enough---even under modest acceleration, apparently---to choke the flow of hot gases and...CATO.

The quick fix was to use a sharp drill bit to enlarge (by hand) the core slightly.

Best -- Terry

 

[OverTheTop]

If you are running any electronics with a crystal for clocking that would be a likely culprit to fail. They are essentially a thin slice of quartz crystal, mounted and with a couple of wires attaching it inside the package. There are some MEMS-based oscillator modules nowadays that are resistant to shock by about an order of magnitude higher than crystals. Another alternative, if you don't need absolute clock stability, is to use the RC oscillators available in quite a few microcontrollers.

Some facts/figures here on the MEMS: https://www.digikey.com.au/en/ptm/s...place-crystals-with-mems-oscillators/tutorial

Putting some figures on maximum boost acceleration, I am designing a speedbird that recently simulated at 112G acceleration. I will be happy if the final product gets near that number.

 

[Funkworks]

Two 80N-avg-thrust motors having less than the maximum propellant weight is the theoretical upper limit.

Sorry, I assumed OP meant model == Model.

... perhaps that is what the OP meant ...

I'm still climbing the ladder. So, what I'm getting at is that OpenRocket says if I load my 225 g Rocketarium Viking-7 with two D21-7 (2-cluster), it will rise at 268 m/s^2, which about is 27 g-forces. Seems a bit much so I'm being cautious.

 

[dhbarr]

I'm still climbing the ladder. So, what I'm getting at is that OpenRocket says if I load my 225 g Rocketarium Viking-7 with two D21-7 (2-cluster), it will rise at 268 m/s^2, which about is 27 g-forces. Seems a bit much so I'm being cautious.

It's pushing that cardboard needle through our watery nitrogen soup that folds tubes in half, almost never kick.

Given you're using 2x 18mm motors to jam a 40mm tube into the sky, the only thing I would do is use water thin CA to stiffen any tube edges. That's more for ease of assembly and zipper resistance than anything else.

 

[Ez2cDave]

Back when AT had their fire, they subbed out some of the propellant manufacturing to...someone else, I forget who.

Best -- Terry

Terry,

Was that Scott Dixon at Vulcan Systems ?

Dave

 

[SpaceEggs]

I've sent a rocket up at 99 G's, and that was 100% fine being made out of standard LOC parts (and that was on an obscenely high thrust experimental, not the kind of thing you're likely to encounter with commercial motors). As others have said, it's the velocity that will kill the rocket. Tubes are designed to take force along the axis of acceleration, so the actual upper limit where those would crumple from acceleration alone is probably very high. Airspeed over the fins will cause them to flutter, and at a certain velocity (dictated by thickness, material, root chord, shape, tab size, etc.) they will break off. Also, shockwaves above Mach 1 can cause problems for brittle airframes (Quantum tubing, which is PVC iirc).

 

[OverTheTop]

Forgot to mention. Heavy parts on electronics can become dislodged off circuit boards if they are not fixed down sufficiently and subject to large accelerations. Large electrolytic capacitors are great at this. I usually fix them down with a CA adhesive, with Loctite 401 being my usual choice.

GPS antennas can also rip off circuit boards. They are held on only by typically one solder joint in the middle, and conductive tape to the PCB around the outside of the patch antenna. A bead of epoxy around that edge helps with that, and people have also used heatshrink/cable ties to hold the part down.

 

[Speaknoevil]

Yep, antenna, capacitors, inductors, bulkheads, airframes, forged eyebolts, and finally Kevlar...

 

[Steve Shannon]

Terry,

Was that Scott Dixon at Vulcan Systems ?

Dave

Ellis Mountain - it’s the final article on this page: https://www.aerotech-rocketry.com/news.aspx?y=2008

 

[HASTE]

Speed kills, not acceleration usually. Except for the acceleration by ballistic impact.

Or is it rapid deceleration

 

[Nytrunner]

It's not the ballistic descent that re-kits a rocket.....

 

[5x7]

Don’t put a Loki 38-1200 case in a cardboard motor adapter no matter how much epoxy you use, unless you want to fly the motor....without the rocket.

 

[Nytrunner]

Don’t put a Loki 38-1200 case in a cardboard motor adapter no matter how much epoxy you use, unless you want to fly the motor....without the rocket.

That's funny. What fails? the bulkhead, or does the tube delaminate?

That's a good question actually, what's those adapter limits?

 

[heada]

My first time with a Warp-9 motor (J1999 in a 4" PML AMRAAM), I forgot to set the mach delay. Deploying recovery at near mach is bad for the rocket, m'kay. The sudden stop can be on the way up just as easy as the way down. I ended up being able to reuse a few fins and the motor casing.

 

[boatgeek]

I've flown my L1 rocket (54mm LOC tubing, ~1200 grams all up, built like a tank) on a lot of motors. The wrinkles in the body tube didn't show up until after the G250 launch (~20 G). It still flies, though I'm not putting an H410 into it.

With a high thrust motor like a VMax or Super Thunder, I think you could get low power tube to fold over. Whether that's technically aero forces from drag or G forces from thrust is probably a fair question, but may be into Captain Technicality territory. I agree that fins are fine as long as they don't flutter off at the speed of balsa, and nose cones are almost certainly OK up to ludicrous speeds.

 

[Sabrina]

I'm still climbing the ladder. So, what I'm getting at is that OpenRocket says if I load my 225 g Rocketarium Viking-7 with two D21-7 (2-cluster), it will rise at 268 m/s^2, which about is 27 g-forces. Seems a bit much so I'm being cautious.

Aerodynamic forces - air flowing over balsa fins at 200+ mph - will be the biggest concern. The other concern will be - recovery. I recommend a 10 ft long shiny mylar streamer - not a parachute.

 

[Handeman]

Or is it rapid deceleration

That's the old saying referring to automobiles, "Speed doesn't kill, it's the sudden stop!"

 

[5x7]

That's funny. What fails? the bulkhead, or does the tube delaminate?

That's a good question actually, what's those adapter limits?

The cardboard motor tube was torn out of the centering rings. The glassine surface gives up its hold on the glue. I think the centering rings on the adapter were broken also because the thrust ring is too large to fit through. I was flying with a friend and we each bought our own aero pack 38 to 54 right after the launch, and didn’t even tell each other before we each bought one.

 

[SpaceManMat]

Usually airframe failure occurs just above the motor / top CR. This is usually the weakest point with the highest loading, I believe this referred to as column buckling.

With the cardboard tubes the spirl winding of the tube becomes apparent from the outside as it is pushed to near its limit. This happens due to the cardboard being compressed. I assume boatgeek was talking about.

 

[Dane Ronnow]

Speaking of acceleration, I don't have answers to your questions, but you may be interested in this launch.

I stumbled onto this post in an old thread when I was searching for information on acceleration (specifically, how much is too much?), and I've got to say that out of all the youtube rocketry videos I've watched—and I've watched a lot of them—the one you posted is the best I've ever seen. Love it. Thanks.

 

[Ez2cDave]

I'm wondering what part would fail first, or what actually limits the acceleration a model rocket can take. Is it even structural? Is it only a cost or practicality issue? Are there any known records? What considerations need to be taken when trying to break g-force limits with a particular level of model kit?

The weakest component will always fail first, whether it be structural failure or components being damaged in electronic devices.

You will always be limited in acceleration by three factors, Total Mass, Aerodynamic Drag, and Motor Performance . . .

"Speed Costs Money . . . How Fast Do You Want To Go ?"

Dave F.