When is cardboard not enough??

[curtthedirt]

I’m just wondering if my scratch build could handle an 54mm L motor?? I’m gonna be attempting my L-2 cert. @ LDRS 40. Probably gonna go w/K-550W but I haven’t ruled out slapping an L motor for the attempt. The 1st 4 ft of the rocket is super heavy duty cardboard tube 5 3/8 ID I found somewhere a decade ago & saved it thinking someday I’ll make a rocket out of that. The 2 upper sections are regular 5 3/8 ID BT I ordered from PML both 24 inches long. The CP from my calculations is 32inches from the bottom of the rocket.
( 1st 48 inches is heavy duty cardboard )
Aside from everyone quacking I should use a smaller impulse motor for my flight. Can I get some knowledgeable info on this?? Can cardboard alone handle a L motor??

 

[OverTheTop]

My L2 rocket was a Binder Design Velociraptor. Cardboard airframe and takes any L motor I have thrown at it.

 

[user 30299]

Cardboard can handle M motors. Go look at all the cardboard kits made by Loc Precision that handle M motors.

But it's all about how you build the motor tube, body tube and fin assembly. Fins thru the wall? Thrust plate? (which can be plywood)
Also a coupler of proper length and fit at the body tube joints.

But add up your component weights and run a simulation. You may find that cardboard rocket, with some BT sections,
will be a hard push on the K motor.

If you have a sim file - please post it.

 

[Theory]

there is no "magic power level" where any material becomes inappropriate.

91.7% of it is construction technique and flight profile. no doubt cardboard kits can take an L, M, or even an N.

remember flights that "take a turn" off the rail, those that spin, wiggle etc. all place significantly more stress on the vehicle than those that are dead straight.

lots to consider.

 

[David Schwantz]

The concern I would have is the age of the tube. Granted cardboard can take a lot, but I have seen older tubes shatter when used as MMT.

 

[FlyBy01]

there is no "magic power level" where any material becomes inappropriate.

91.7% of it is construction technique and flight profile. no doubt cardboard kits can take an L, M, or even an N.

remember flights that "take a turn" off the rail, those that spin, wiggle etc. all place significantly more stress on the vehicle than those that are dead straight.

lots to consider.

^^^^^^This.

 

[Glasspack]

Very nice to read...
I too am building my LVL 2 model still. Its built using the pieces from a Binder Design Cobra Kit.
It is mostly done but still has lots of finish work and testing to due. My Sim Files on "RockSim" are very cool....
It has been Simulated at Mach 1.2 already, BUT, I will go for that much later after my Cert.

Intersting note is what QFactor said......... my only concern right now is the Coupler connection
between the lower airframe and the upper airframe. I have to make it tight but not too tight and
now that it seems too late (glued in place).....I worry that it is nNOT long enough.

Thanks all !!

 

[user 30299]

Very nice to read...
I too am building my LVL 2 model still. Its built using the pieces from a Binder Design Cobra Kit.
It is mostly done but still has lots of finish work and testing to due. My Sim Files on "RockSim" are very cool....
It has been Simulated at Mach 1.2 already, BUT, I will go for that much later after my Cert.

Intersting note is what QFactor said......... my only concern right now is the Coupler connection
between the lower airframe and the upper airframe. I have to make it tight but not too tight and
now that it seems too late (glued in place).....I worry that it is nNOT long enough.

Thanks all !!

Your coupler description is a little confusing.

Is the coupler glued in place to add two tubes together and lengthen the body tube?

Or is the upper airframe (body tube) a large payload area, and you just have the one separation at apogee
between the upper & lower airframe (body tube).

Or is the coupler your avionics bay for maybe a traditional dual-deployment setup; separation at apogee, then
a separation at some altitude for Main deployment?

What is the tube diameter? What is the length of the coupler's section that goes into the body tube?

 

[mtnmanak]

Cardboard shouldn't be an issue, but a CP that is 32" from the base of the rocket sounds a bit high. How small are the fins? It sounds like you have about 8 ft of body tubes, so if the overall rocket is 10'-11' long, your CP is about third of the length of the rocket. I have a bunch of rockets that are similar in diameter/length with relatively small fins and the none of them have a CP that is a third of the length of the rocket. The closest I have come to that kind of CP is the LOC Saturn V with its little tiny fins and even then, the CP was not a third of the length of the rocket and it still needed about 5 pounds of nosecone weight to make it stable with any motor that was going to get it off the pad safely. 54mm L motors are pretty heavy. I am not aware of any commercial 54mm L's that weigh less than 5 pounds. It sounds like you may need some pretty significant nosecone weight to overcome that CP with an L.

 

[Glasspack]

Your coupler description is a little confusing.

Is the coupler glued in place to add two tubes together and lengthen the body tube?

Or is the upper airframe (body tube) a large payload area, and you just have the one separation at apogee
between the upper & lower airframe (body tube).

Or is the coupler your avionics bay for maybe a traditional dual-deployment setup; separation at apogee, then
a separation at some altitude for Main deployment?

What is the tube diameter? What is the length of the coupler's section that goes into the body tube?

Thanks Q

Sorry, I should have maybe included the info:

The coupler is ACTUALLY the Avionics bay with a 1" Switch band. There is about 4" insertable on AFT end,
and about 3" insertable on the FWD end. The Lower part will be Shear pinned in place and the upper part
attached to the upper payload bay with removable rivets to allow nose cone to seperate.
Here is a Pic of her so far..... For many reasons..... She is waaaayyyyy behind schedule !!

4" Dia Model, 90.4" long

 

[cwbullet]

I have found the failure acceleration or velocity of multiple tube materials. It is often the construction techniques and not the tubing that fails. Fins flutter and break. Tubes of all types can spread. As with the human body, it is often the rate of change in speed or acceleration and deceleration that causes the damage to tubes Or fins.

 

[user 30299]

Thanks Q

Sorry, I should have maybe included the info:

The coupler is ACTUALLY the Avionics bay with a 1" Switch band. There is about 4" insertable on AFT end,
and about 3" insertable on the FWD end. The Lower part will be Shear pinned in place and the upper part
attached to the upper payload bay with removable rivets to allow nose cone to seperate.
Here is a Pic of her so far..... For many reasons..... She is waaaayyyyy behind schedule !!

4" Dia Model, 90.4" long

Thanks for the explanation and the picture. The 3" length on the top end, and the rivets, should be fine. As long as you have a snug fit with the coupler and the body tube. If the fit is a bit loose, some well-placed masking (painter's) tape can help with a snug fit.

It's all about the construction technique.

 

[curtthedirt]

Cardboard can handle M motors. Go look at all the cardboard kits made by Loc Precision that handle M motors.

But it's all about how you build the motor tube, body tube and fin assembly. Fins thru the wall? Thrust plate? (which can be plywood)
Also a coupler of proper length and fit at the body tube joints.

But add up your component weights and run a simulation. You may find that cardboard rocket, with some BT sections,
will be a hard push on the K motor.

If you have a sim file - please post it.

I don't really have a computer right now so haven't done any simulation. The 54mm motor tube & 3 centering rings are not only epoxied but I put 2 stainless steel all thread rods thru all 3 CR w/ nuts & washers on both sides of all C Rings. The construction is solid. Sounds like my cardboard body tube will be fine.

 

[ZEDL1]

As CW and others have offered, it is the construction that feel will be the determining factor. I've seen many good and not so much outcomes over the years.
I don't know if you are able to do this, but you mentioned this Glasspack, the coupler is quite often the weak link. If you are able to reinforce it with a layer of 'glass cloth inside, that would be a good idea. Better may be a FWFG or CF coupler.
I've seen (too) many M flights over the years fail due to the coupler not being able to handle loading. The airframe and payload sections could, but the coupler, not so much.

 

[rharshberger]

As CW and others have offered, it is the construction that feel will be the determining factor. I've seen many good and not so much outcomes over the years.
I don't know if you are able to do this, but you mentioned this Glasspack, the coupler is quite often the weak link. If you are able to reinforce it with a layer of 'glass cloth inside, that would be a good idea. Better may be a FWFG or CF coupler.
I've seen (too) many M flights over the years fail due to the coupler not being able to handle loading. The airframe and payload sections could, but the coupler, not so much.

Most people seem to forget that LOC offers a upgrade for there coupler in the form of the LOC "Stiffy" insert. The stiffy coupler goes a long way toward making a coupler thats probably stronger than the airframe.

 

[GalantVR41062]

I have a Madcow Frenzy 4" cardboard kit I built, 54mm mmt and tail cone retainer.

I have done simulations with the Aerotech 54mm L1000W DMS (motor weight 77oz, max thrust 283lbs), my built weight is similar to yours.

I have 1/4" plywood fins that are airfoild. I have only flown the rocket once on a Aerotech J415w to just over 6,000 feet. I think my simulated velocity on the L1000 was mach 1.4 and just under 10,000 feet. The CTI L265 mellow simulates higher altitude but I was concerned with the rail exit velocity.

I think: pressure seperation, fin alignment/flutter and or other flight anomalies will result in rapid deconstruction of a 4" cardboard rocket on a L motor.

 

[G_T]

Late to the party...

About M3. At that point the stagnation temperature is getting pretty close to the autoignition temperature of cardboard. Beyond that, it's flame on!

There may be a lesser limit depending on whatever glue is used in the cardboard. If you hit the glass transition temperature of the glue then the strength will drop. I'd argue that would be too fast. Ditto for other types of glue which may break down, melt, or have other forms of degradation.

Anything below these limits can be solved by throwing more cardboard at the problem. That's a structural design problem.

Above that speed you have a materials engineering problem. And a bit of a race condition - what happens first? The rocket slows down before the cardboard heats enough and therefore doesn't ignite? The rocket gets high enough that there is not enough oxygen to burn the cardboard so it doesn't light? Or too bad, so sad, flambe?

Gerald

PS - I'd argue that paper phenolic tubes such as could be used for motor liners and similar tubes are not cardboard for the sake of this discussion, but are instead phenolic tubes that are internally reinforced with cellulose. Different beast.

 

[sharkbait]

The biggest limiting factors with cardboard rockets that I have found besides M3 and poor construction techniques are:

1. The bird lands on one of two big rocks we have at our field.

2. The bird hangs in a tree for a couple days before I can get back to get it or find it and it rains.

3. I have to slam on my brakes or take a turn a little too fast on my to or from the field and my range box or one of my big FWFG birds crushes it.