jimzcatz
Boss, Carolina Rocket Mafia
I use 4-40 screws on 7.5" fiberglass I use 4 of them.
The force trying to separate the rocket at burnout is given below. If you just consider inertia forces you can set R=1
This force can be - or + depending on the mass ratios. If the mass of the lower section is more than 1/2 the mass of the total rocket then the joint is in compression after burnout. No shear pins necessary.
Fsep = a [ M / (1+R) - M1 ]
Where:
a = max deceleration [just prior to motor burnout]
M = total mass of rocket (mass NOT weight)
M1 = mass of lower section
R = drag ratio
Yes, that is the right calculation.A very naive question about deceleration (or inertial) separation force.
I started with this:
So,
Fsep = a [ M / (1+R) - M1 ]
Fsep = a [ M / (1+1) - M1 ] => inertial separation force only
Fsep = a [ M/2 - M1 ]
Given:
M1=24 lb, M=35 lb, a=-80 ft/sec^2
Fsep = -80 * [ 35/2 - 24 ] = -80 * [ 17.5 -24 ] = -80 * -6.5 = 520 lb ft/sec^2
Fsep lbf = [ 520 lb ft/sec^2 ] / [ 32.1705 ft/sec^2 ]
Fsep lbf = 16.1621
Given:
Coupler friction constant = 5 lbf
Net Fsep lbf = 16.1621 - 5 = 11.1621 [if >0 then shear pins needed]
Does this look right?
Yes, that is the right calculation.
I believe you are right. Let me check my original derivation to confirm.One more clarification. If drag is being considered, which is the correct ratio for R:
#1 CdA upper / CdA lower
OR
#2 CdA lower / CdA upper
To me, the calculation only make sense if the correct ratio is #2. Is that correct?
Disregard last answer. R = CdA upper / CdA lower. Note the direction of a (positive a is decelleration). If using imperial units, express a in g's, mass in lb and units for Fsep will be in lb.One more clarification. If drag is being considered, which is the correct ratio for R:
#1 CdA upper / CdA lower
OR
#2 CdA lower / CdA upper
To me, the calculation only make sense if the correct ratio is #2. Is that correct?
Most normal rockets with heavy motor cases in the lower section will not require shear pins for the apogee break. None of the rockets I fly do. What you can do with the Fsep equations is you can reality check what the drag ratio needs to be to be in danger of drag separation. Then judge for yourself if that R value could possibly happen.For what it is worth, calculating the different drag coefficients for different pieces of a single body is not easy. Most drag coefficients from look-up tables account for the entire object tested in a wind tunnel. You have to separate out pressure drag on the nose, shear drag on the nose, body and fins and base drag from flow separation. I have crude estimates in Rocksim for some of my higher performance rockets.
It is worth noting that over-pinning the joint for the drogue can have pretty big consequences. Also adding pins for that joint means larger ejection charges which adds another stress point if the shock cord gets pulled hard. Unless you are flying past Mach, I recommend a decent friction fit for the drogue joint and shear pins for the main. By decent, have the joint slightly stronger than the weight of the booster. This is just a rule of thumb and different rockets will necessitate different joints.
And even still that may not be adequate, be sure, config pins for worst case pressure differential and use a larger charge if need be.Can't remember if it was mentioned: but do put a vent hole in the BT so as the exterior air pressure drops the interior pressure does not push the ebay off.
If you just want to test for drag separation vulnerability you can just evaluate:
@jderimig John, Thanks for all of this. I'd like to make this a bit more practical and be able to evaluate this more easily, through Mathcad, or excel.Draggy rockets like tube fins or heavily nose weighted rockets would fall in the category of R values that could easily happen to create a drag sep value.
I use the component values in OR generally but if you can get the CdA at max deceleration that is better. However you are not trying to thread a needle here. If you decide not to use shear pins you want [m1 >> M/(R+1)] to be robust and should be insensitive to reasonable uncertainty around R.Questions:
- How does one determine CdA (or FdX) for each section? Grab the Cd at the parameters from component analysis in OR with selectors set to the appropriate values for the simulation (motor) we are using at the time of deceleration?
To estimate Fsep, If [a] is in units of g's then you want M in weight units, if [a] is in dimensional units like m/s or ft/s^2 then M should be in units of mass. To test for drag sep danger it doesnt matter if you are using [m1 >> M/(R+1)].
- Why does it matter if you use weight or mass...the ratios are still the same, no? (I get the units would be off in the result, but relative would be correct?)
I am not sure about the simple model (inertial forces only), there is a reason it is called drag separation. Always use m1 >> M/(R+1). If it fails that use shear pins. The Fsep equation can be used to help size the shear pins.
- What is the right boundary for when to use the simple model vs. Full? In this rocket case, the breakeven is right around R===.42.
No problem.Thanks for all the help!!!
Weight F2:=1.5
Weight F1:=3.5
Cd Upper := .046
Cd Lower := .514-.046=.468
I would add the BT drag to the nosecone drag and only use the fin set for the lower.@Handeman Thanks for the lifeline...I'm trying to work it all out and I'm still a bit confused. Rock sim does not separate NC drag from Body tube drag, so I'm using OR.
Here are the Drag characteristics from the component analysis:
View attachment 627557
This is motor deploy, so no booster, just the NC. Seems like the Body tube accounts for the majority of the drag and is larger than the NC by factor of 6. As you say, fins are de minimis...
Thoughts...?
I'm not arguing, I'm confused/trying to learn. Why add the body tube drag to the upper, when there is no upper, just the NC (there is no sustainer/payload section/tube). There is NC and airframe with fins...those are the only two parts.I would add the BT drag to the nosecone drag and only use the fin set for the lower.
Right...heavy sustainer/payload, and drag on booster...large separation force. I get it...I don't understand how the math works out the other way in my case (well I do, because of using the large Cd for the (booster"). Sigh...only if the upper section was several times as heavy as the lower.
I you have a rocket that is single deploy with just the nosecone coming off, I would ignore anything to do with drag separation unless you have so much weight in the nose cone that it's several times heavier than the whole rest of the rocket. The amount of air resistance on the nose cone is way more than anything the fins can produce so only a huge weight difference and momentum could possible cause an issue.I'm not arguing, I'm confused/trying to learn. Why add the body tube drag to the upper, when there is no upper, just the NC (there is no sustainer/payload section/tube). There is NC and airframe with fins...those are the only two parts.
Right...heavy sustainer/payload, and drag on booster...large separation force. I get it...I don't understand how the math works out the other way in my case (well I do, because of using the large Cd for the (booster"). Sigh...
Don't go down the TRF rabbit holes.
Don't forget to wave on the way down!
On rockets like this, I dont use screws, I use push in plastic rivets. Much easier to handle and without the threads they shear cleanly. Mc Master Plastic Rivet https://www.mcmaster.com/90136A530/ On my 6 inch rocket (the one on top of my car in my profile picture) I use 4 of these. There are larger diameters too.I’ve been using this Ejection Charge and Shear Pin Calculator for my fiberglass projects and its recommendations have always been confirmed in ground testing.
https://www.rimworld.com/nassarocketry/tools/chargecalc/index.html
However, I’m working on an 8” dia project that may end up using #6 maybe even #8 shear pins if one was to simply scale up the hardware used in smaller projects. But, this calculator does not make a recommendation of pins larger than #4. So, this raises the obvious question… Is it best practice NOT to scale up the hardware and instead use more pieces? So for instance if I use 3 or 4 (#4) nylon screws for pins on a 4” dia. airframe does this mean best practice is to use 6 or 8 (#4) on a 8 dia. project?
IOW is best practice to “scale up” (size) or “scale out” (count)?
One could crunch some numbers, maybe even perform some material testing, but I expect one would end up simply confirming the conclusions and experience of others and to a certain amount their own common sense.
On rockets like this, I dont use screws, I use push in plastic rivets. Much easier to handle and without the threads they shear cleanly. Mc Master Plastic Rivet https://www.mcmaster.com/90136A530/ On my 6 inch rocket (the one on top of my car in my profile picture) I use 4 of these. There are larger diameters too.
View attachment 627876
https://www.mcmaster.com/90136A530/
Anything can be a shear pin with enough force.That is not a shear pin.
Yes, especially on impact.Anything can be a shear pin with enough force.
That is not a shear pin.
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