Pressure relief holes in the airframe in the area of the motor tube?

[grizzly]

Pressure relief holes are recommended for other enclosed spaces in an HP rocket.

What about one per enclosed space around the motor tube? IE The three to four open spaces between the fins and the CR's?


Is the change in pressure dramatic enough to stress the CR epoxy over time? These spaces can be decent sized on the larger diameter rockets.

Another plus to foaming the fincan?

 

[Handeman]

Don't worry about it. Non-issue.

 

[brianc]

What about one per enclosed space around the motor tube? IE The three to four open spaces between the fins and the CR's?

The vent is to bleed off pressure so the airframe doesn't prematurely separate, not for relieving stress on the joints.

Like Handeman said 'forget about it...'

 

[grizzly]

The vent is to bleed off pressure so the airframe doesn't prematurely separate, not for relieving stress on the joints.

Like Handeman said 'forget about it...'

Yes, I understand this, but the airframe or nosecone seperate prematurely from a significant presure change and resultant force exerted on said airframe that is relived or greatly reduced by a vent hole.

The same undesireable forces also should be occuring in each airfilled chambers created beween the fin tabs and fore and aft CR's. Larger rockets, larger spaces, greater unwanted forces acting on airframe and CR joints.

We go through a lot of trouble to use internal and external fillets, top notch epoxy, proper prep of bonding areas, etc. Why no simple vent holes to reduce or eliminate undesireable pressure change effects in this area?

 

[CarVac]

Yes, I understand this, but the airframe or nosecone seperate prematurely from a significant presure change and resultant force exerted on said airframe that is relived or greatly reduced by a vent hole.

The same undesireable forces also should be occuring in each airfilled chambers created beween the fin tabs and fore and aft CR's. Larger rockets, larger spaces, greater unwanted forces acting on airframe and CR joints.

We go through a lot of trouble to use internal and external fillets, top notch epoxy, proper prep of bonding areas, etc. Why no simple vent holes to reduce or eliminate undesireable pressure change effects in this area?

Because the forces from air pressure are miniscule compared to the loads experienced on landing.

 

[grizzly]

Because the forces from air pressure are miniscule compared to the loads experienced on landing.

Yet we do try to minimize the potentially damaging loads sustained from landings by painstakingly picking the right size chute for the weight of the rocket and the condtions of the landing environment.

Do you know what those miniscule forces from air pressure are? What formulas would be appropriate?

Clearly, those forces are high enough to overcome the friction between a nosecone and the airframe plus the friction from drag at speed. I wouldn't call that force miniscule.

 

[CarVac]

Yet we do try to minimize the potentially damaging loads sustained from landings by painstakingly picking the right size chute for the weight of the rocket and the condtions of the landing environment.

Do you know what those miniscule forces from air pressure are? What formulas would be appropriate?

Clearly, those forces are high enough to overcome the friction between a nosecone and the airframe plus the friction from drag at speed. I wouldn't call that force miniscule.

Bare Necessities would have been the worst case of pressure separation since we intentionally sealed the airframe. If it had gone to apogee, then the separation force would have been a gently applied 175 pound force.

However, if your landing shock is 40 g with a 20 pound rocket, that is 800 pounds force applied suddenly.

 

[Handeman]

Yet we do try to minimize the potentially damaging loads sustained from landings by painstakingly picking the right size chute for the weight of the rocket and the condtions of the landing environment.

Do you know what those miniscule forces from air pressure are? What formulas would be appropriate?

Clearly, those forces are high enough to overcome the friction between a nosecone and the airframe plus the friction from drag at speed. I wouldn't call that force miniscule.

The only time the "forces" are going to open up a rocket is when the friction between the BT and nosecone are minimal. I've been flying 10 years now and I have never seen a separation that was caused because of internal air pressure building up due to lack of vent holes. Most premature openings occur because of a motor/delay issue, or because of drag separation at motor burnout, which is a different issue. If your fin can structure is strong enough to handle Mach speeds or a rough landing, it will be many times stronger then required to hold up to any air pressure changes in those sealed chambers.

You can certainly put vent holes in your rockets if that makes you feel better. I would suggest making them through the aft CR rather then on the BT.

Personally, I'm not convinced that vent holes are needed on any but the most high performance/high altitude rockets. I think vent holes have become known as a "best practice", but I'm not sure they really contribute much to reducing problems with flights. The one place I can see them helping is that by reducing the internal pressure, it should lessen the likelihood of drag separation occurring.

I think it would be interesting to see flight data that measures the internal and external air pressures during flight and plot that against altitude. I suspect that even tight joints bleed off quite a bit of pressure from inside during flight.

 

[scatsob]

I agree that it is probably not a problem, but I did it on my L3 rocket (a 5.5" sandhawk with a 75mm motor mount). It's not going to hurt anything if you do.

 

[grizzly]

I agree that it is probably not a problem, but I did it on my L3 rocket (a 5.5" sandhawk with a 75mm motor mount). It's not going to hurt anything if you do.

So what did your L3 mentors say about the additional vent holes? Did you include them as part of the original paperwork?

 

[grizzly]

I would also like to see that data. Again, I am referring to a very large, high performance rocket here. There are no tighter joints on a rocket than the ones between the fins and the CR when properly filleted. They are essentially sealed and waterproof chambers with no ability to bleed anything. They are also very large on 6 inch diam and up rockets.

 

[scatsob]

I honestly can't remember , that was back in 2003. If you do the math though we are not talking about much force, as mentioned above.

So what did your L3 mentors say about the additional vent holes? Did you include them as part of the original paperwork?

 

[Sailorbill]

Pressure at sea level is 14.7 PSI, about 11.8 @ 6500 and approx. 7 at 19,500 feet. So at 6500 ft ASL your air frame will see a difference of 2.9 lbs. per sq. in. or 7.7 psi at 19,500 ASL. If you don't build your rockets at sea level you will need to adjust for that. Then you throw in temperature changes and it gets even more complicated.
For me it is a non issue as my rockets tend to be built pretty strong and leaky. :wink:
Do I use a vent hole? Yes, one 1/8" hole in the recovery section.
My L3 rocket went to around 11K AGL with quite a few sealed areas and received no apparent damage.
I wouldn't worry about it. Just my :2:

 

[Nerull]

I honestly can't remember , that was back in 2003. If you do the math though we are not talking about much force, as mentioned above.

If you do the math you are potentially looking at quite a lot of force. A well sealed 4 inch rocket launched to one mile will experience 32 pounds pushing it apart. That's quite a bit more than it takes to separate most rockets. At 2 miles it goes to 60 pounds.

 

[scatsob]

Certainly, you are correct. However the OP is talking about the dead space in the fincan and booster section of the rocket where the pressure is held by well epoxied bulkheads and such. You should always vent the recovery section.

If you do the math you are potentially looking at quite a lot of force. A well sealed 4 inch rocket launched to one mile will experience 32 pounds pushing it apart. That's quite a bit more than it takes to separate most rockets. At 2 miles it goes to 60 pounds.

 

[CarVac]

If you do the math you are potentially looking at quite a lot of force. A well sealed 4 inch rocket launched to one mile will experience 32 pounds pushing it apart. That's quite a bit more than it takes to separate most rockets. At 2 miles it goes to 60 pounds.

Separation forces are one thing, but 32 pounds is NOTHING compared to the forces resisted by the centering rings from motor thrust.

 

[JLRockets]

Last year, Jackson put together a vent hole calculator to insure he was properly venting his N5800 rocket. You use it by inputting pressure data from rocksim and data for volume calculation.

The worksheet will give an estimate of the maximum force generated within the space from the change in air pressure. You can take this number and treat it as the maximum force on the epoxy joints. And then, you can test your attachments to see if they meet the strength requirements.

Perhaps you can also tweak the worksheet to accommodate changes to air temperature within the cavity due to motor heating, and other factors relating to a fin can cavity.

The worksheet is here:
https://www.rocketryforum.com/showt...ze-Calculator-Spreadsheet&p=406284#post406284

 

[dixontj93060]

Judy beat me to it. I was going to share my pressure calc spreadsheet done for my Mach Madness analysis (again based on David's initial work). The Google Docs link is: https://docs.google.com/file/d/0B4LURzRDtzD3bnh6VF9tSlBZbFE/edit?usp=sharing and it is an Excel sheet. Just cut/paste your Rocksim simulation data in the blue area of the spreadsheet and you can see the pressure graphs (you can duplicate/delete rows as needed if you want, but you really only care about the ascent stage of flight anyway). I think you will find that the gap between fins and pressure build up on those particular CRs is not an issue.

 

[CCotner]

I think one issue that has been neglected is that rockets with large amounts of volume between the body tube and the MMT are not going to be high-performance rockets. Without lots of aerodynamic stress on the fins, the only major load on sport rocket fins is probably the landing; and at landing, the pressure difference inside to outside will be ~0 again. I don't see how vent holes here would be useful. However, assuming they are small (<1/8" diameter, which is plenty), they won't make any difference on structural integrity (again, because of the low loads expected on this kind of vehicle), so if you want to do so, then go for it.

 

[grizzly]

Thanks for the spreadsheets guys.

If you input a 10 inch diameter rocket, ignoring the volume taken up by the motor tube and fin tabs, you get nearly 300 lbs of force acting on the airframe and CR joints at 10,000 ft. That seems like a pretty big force to me.

Also, factor in the possible thermal expansion at the joints from the heat generated by a big M motor, plus action of the heated air in that space. Finally, the heat weakening the epoxy in the fillets and joints if low temp epoxy was used (West/aeropoxy).

Just some thoughts...not disagreeing.

 

[CarVac]

Thanks for the spreadsheets guys.

If you input a 10 inch diameter rocket, ignoring the volume taken up by the motor tube and fin tabs, you get nearly 300 lbs of force acting on the airframe and CR joints at 10,000 ft. That seems like a pretty big force to me.

Also, factor in the possible thermal expansion at the joints from the heat generated by a big M motor, plus action of the heated air in that space. Finally, the heat weakening the epoxy in the fillets and joints if low temp epoxy was used (West/aeropoxy).

Just some thoughts...not disagreeing.

300 pounds force is 1200 Newtons... that's less than most M motors.

 

[grizzly]

300 pounds force is 1200 Newtons... that's less than most M motors.

Point taken, but it is a force that is in addition to the force of the motor, and with a properly sized and placed vent hole or 4, a force that can be effectively eliminated.

 

[scatsob]

See above. Also when you get into rockets that large people are bolting all or most of the booster section bulkheads together with all thread. My father and I have flown an 8" D-Region tomahawk on Kosdon O-6000's (over 10kN initial thrust) to 16-18k feet and never had a problem. Like everyone is saying, landing is the hard part as we have had to fix the fins on a few occasions.

 

[CarVac]

Point taken, but it is a force that is in addition to the force of the motor, and with a properly sized and placed vent hole or 4, a force that can be effectively eliminated.

I wouldn't say it's in addition to the force of the motor, because most rockets burn out much lower than 10,000 feet AGL.

Also, think about this: with a moderately weak epoxy, you have, say, 2000 psi shear strength. If you have a single square inch of bond area, that theoretically gives you 2000 pounds of resistance. On a 10" rocket with 1/4" fillets on each side of the centering ring, you get 0.5"*10"*pi*2000psi = 31,400 pounds force strength on that joint. 300 pounds is a drop in the bucket there.

 

[dixontj93060]

I wouldn't say it's in addition to the force of the motor, because most rockets burn out much lower than 10,000 feet AGL.

Also, think about this: with a moderately weak epoxy, you have, say, 2000 psi shear strength. If you have a single square inch of bond area, that theoretically gives you 2000 pounds of resistance. On a 10" rocket with 1/4" fillets on each side of the centering ring, you get 0.5"*10"*pi*2000psi = 31,400 pounds force strength on that joint. 300 pounds is a drop in the bucket there.

This! (Just about to post the same thing)