Discussion > True or False?
Caveat: I wasn't there, so consider this third-hand speculation with a sprinkling of assumption based on your loose failure description....
If you were using an altimeter that uses barometric pressure to determine relative altitude, the air pressure inside the av-bay would have stayed roughly the same as at ground level -- until it didn't, and then may have triggered the ejection early. If that happened under thrust (or high speed), things don't go well. If the altimeter never registered an altitude change and thus never fired ejection... also things don't go well. Been there, done that!
Also, sometimes you can pop the nosecone off, or even separate the sections apart, if there isn't a way for the air pressure to equalize between the inside and outside.
Now, I've never felt the need to put holes in the centering rings (nor have I flown high enough for that to matter), but I do make a general practice of having holes in the av-bay section and usually one just below the nose cone so that air pressure can equalize, and so that the altimeter is reading properly. They really don't have to be very large hole, *but* you do need to make sure that your altimeter holes are far enough back from the nose cone that turbulence isn't a factor -- if too close, turbulence can block air from actually entering the hole, negating its function.
-SS (TRA #13240 L2)
Scott Schnegelberger
Hey Rodney... I'm trying to remember your L2 attempt. I left about 1 on Sunday so I might have missed it. I don't usually use a lot of fiberglass on L2 projects, but I do lay a layer or 2 of glass on the air frame, particularly the lower half, and use JB weld on the centering rings and motor tube joints. I have started foaming the area between the fins as well with A/B foam. That does several things for you, stiffening the air frame, keeping the fin can & fins in alignment, and eliminating any movement in the booster section overall.... makes the back end very strong without adding a lot of weight.
Scott's right about the sections needing to vent, especially if your boost is going to be aggressive. If your boost phase is going to be fairly slow then it's not so much of an issue, but then if you're going to go past a few thousand feet then it gets even more important that the air frame not have a lot of high pressure air from ground level that can't vent from the air frame. It's the same reason that when you take a thermos of coffee on a small plane you'd better loosen the top, otherwise it'll go all over the cockpit when you open it at 5,000 feet!
Scott's right about the size of the vents as well, vent holes don't need to be large to do the job. About the only place you really need to be careful is in your avionics bay, where there's a formula for the size and placement of the holes to vent for the altimeter. Here's a link to Adept Rocketry's site talking about altimeter vents: http://www.adeptrocketry.com/staticports.htm
Hope this helps... dr
DOC R.
Hello Rodney,
I'm not sure of the specifics of you L2 rocket, or the motor you used so I'll stay on the general side. I generally vent the booster & payload sections to prevent premature separation in addition to shear pins. Like Doc said, they don't have to be huge. My L3 used 1/8" holes in each section in addition to the vent holes needed for the ebay. It is possible to over pressurize the fin can, but that would have to be a very aggressive L2 flight I would think.
The vent holes are to allow the internal air pressure to equalize with the external atmospheric air pressure that decreases the higher you go. So what they said is possible, but on a L2 flight I would say unlikely.
For construction I always use epoxy mixed with West Systems 406 colloidal silica to creat a fillet around the motor tube and centering ring and the centering ring and the BT.
John Bergsmith
Thanks for all the replies! I didn’t put in the post the details of the rocket but it’s the HyperLOC 835 that most everyone has seen fly many times at ROCC. I have flown this on H242 to K185 with no problems to over 6500’ in Orangeburg. This was the 8th flight on it and wanted to go higher and faster so I used a CTI K590 dual thrust. I think I just found the limit of the airframe (Somewhere between 900N and 1500N maximum thrust).
The payload bay was vented and nosecone had 3 2-56 screws, plus had 3 6-32 grade 5 metal screws attaching that bay to the Altimeter bay. I feel the upper part was very solid and I didn’t have any ejections till long after it come apart.
The bottom end was just wood and paper held together with epoxy. I did have some good fillet around the MM, airframe and centering rings but no glass or foam. The fins was epoxied and had fillet on each side.
I don’t want to beat a “Shredded Horse” to death. So, again I think it was just too much of a kick off the pad! I do plan to use some glass and foam next time as a piece of mind.
I do have another question! I have been using the standard hobby epoxy from Bob Smith. It seems to be brittle and is cracking around the fin fillets when landing (and I do measure in a cup with equal amounts). John you talked about using West Systems 406 colloidal silica, would this help keep them from doing that. And are you using the West Systems epoxy also.
Thanks,
Rodney Earp
Rodney Earp
> They talk to me about the lack of my building skills, about how I didn’t fiberglass my centering rings to the motor mount
> airframe or made fiberglass fillets for the fins.
My L2 had an overkill fin can but honestly there are L3 birds with wood glue and no fiberglass. Don't over or under engineer things. What diameter was the rocket? What weight? What motor? What thickness were the fins? Had the rocket flown before or was this the maiden voyage?
> But then they said I should have drilled holes up into the centering rings so the pressure could be equalize to the
> outside air, I got lost. They said that the air trapped in between the centering rings and the sudden increase in
> altitude created more pressure in between the rings and blew the airframe sides out. Now, I’m not a Rocket Scientist
> I just play one on the weekends but I don’t understand this. Where would that pressure come from I ask, the heat
> from the motor tube expanding the air, they said no. or the different in the air tripped and the lower air pressure
> at 2000’, they said no. I never did understand where they were coming from.
Typically the only extra holes you need are vent holes in the sealed sections of payload that are designed to separate (excluding of course the altimeter's required venting). This is to avoid the SLIGHT change in pressure from forcing the rocket sections apart - particularly when not secured with sheer pins. The difference in pressure - isn't large, but over a large surface area the force can be reasonably large in the same way that ejection charges work. If you fail to vent these you run a very real risk of premature separation - no pun intended. The result is a rocket that likely shreds the parachute(s), goes unstable and generally ruins your day.
However the modest amount of altitude change you are describing doesn't sound like it would ever be sufficient to blow the sides out of the rocket between centering rings. Frankly none of my birds is a high altitude rocket and I never EVER drill excess holes in my centering rings. So far no rockets have spontaneously blown apart due to air pressure in between centering rings in the tail of any of my rockets.
> I haven’t read this anywhere or heard it before. I just don’t see how this can be true, can someone tell me if I’m
> missing something or is this BS.
My sincere hope is that it was just misunderstanding. Maybe they were talking about multiple things and this got mixed up. The lack of fiberglass in the tail could - repeat COULD - leave you with fins more easily torn off than the force you applied. If so then the rear of the rocket could have come apart rapidly and torn sections of the body out making it look like the thing exploded out. The rocket might have separated prematurely due to a lack of venting which could have happened at high speed resulting in "BAD THINGS" (see previous explanation for some of the bad things I predict... .)
If you have time and want to talk to someone about it in detail let me know. We can pour over a few rocket designs and talk thru the details.
I can tell you this. Crazy Jim has a number of smaller diameter rockets that he uses the "Injected Fillet" method with - there simply isn't room to easily add fiberglass reinforcement. It can be done - and people did it until people like Jim showed it was not needed and that the method he uses is easier and just as safe.
Take all talk of Fiberglass and Foam with a grain of salt. I have done both and I have done without both. The tradeoff for using them is a more "junk in the trunk" which means your stability suffers unless the rocket is longer or has more weight up front. Also fixing a foam filled fin can is a pain... ask me how I know. I no longer foam fin cans. I might be talked into it someday but after several experiments I don't see that the value outweighs the tradeoffs.
Brad Shea
> I didn’t put in the post the details of the rocket but it’s the HyperLOC 835 that most everyone has seen fly many times
> at ROCC. I have flown this on H242 to K185 with no problems to over 6500’ in Orangeburg. This was the 8th flight on
> it and wanted to go higher and faster so I used a CTI K590 dual thrust. I think I just found the limit of the airframe
> (Somewhere between 900N and 1500N maximum thrust).
> The payload bay was vented and nosecone had 3 2-56 screws, plus had 3 6-32 grade 5 metal screws attaching that
> bay to the Altimeter bay. I feel the upper part was very solid and I didn’t have any ejections till long after it come apart.
> The bottom end was just wood and paper held together with epoxy. I did have some good fillet around the MM, airframe
> and centering rings but no glass or foam. The fins was epoxied and had fillet on each side.
> I don’t want to beat a “Shredded Horse” to death. So, again I think it was just too much of a kick off the pad!
> I do plan to use some glass and foam next time as a piece of mind.
When you put the fins in - were there six fillets total per fin? Two outside, two inside on motor tube, two inside on inner surface of the body tube?
> I do have another question! I have been using the standard hobby epoxy from Bob Smith. It seems to be brittle and
> is cracking around the fin fillets when landing (and I do measure in a cup with equal amounts). John you talked about
> using West Systems 406 colloidal silica, would this help keep them from doing that. And are you using the West
> Systems epoxy also.
Most of the time I am fine with Bob Smith - which type? Slow, Fast? I tend to use nothing faster than 30 mins with fillets, and most of mine do not need crazy strength. I often use micro balloons to allow for greater flex and easier sanding. There's nothing wrong with silica or West Systems. I use US Composites mostly but that isn't a major preference. When using these types of epoxy the ratio's need greater precision and you need to use a scale and not measure by volume if possible.
The only time I have seen brittle epoxy I attributed to bad mixing on my part but I have heard it can also be bad chemistry from the factory. One way to test is to mix some up with whatever additive you plan to use and cast a long pencil sized piece on wax paper. Then test it - is it brittle or does it have a little give?
In any case, the brittle epoxy issue could have also caused your fin issues - if the epoxy was too brittle over time it may have weakened and heat soak may have contributed over many flights to issues as well. I tend to use JB Weld in my motor mounts because I trust it and I like it but your mileage may vary, there are plenty of rockets I have flown dozens of times with just Bob Smith 5 minute epoxy unaltered - however I haven't hit them 8-9 times before using a K in them!
Brad Shea
A lot of you saw me attempt my TRI Level 2 at Freedom Launch on Saturday that didn’t go so well. And we all know that when something goes wrong how people can tell you what the problem was. Well I when I went to retrieve the pieces I had guys analyzing the wreckage. They talk to me about the lack of my building skills, about how I didn’t fiberglass my centering rings to the motor mount and airframe or made fiberglass fillets for the fins. It’s all good I like input and most of what they said made since too me. But then they said I should have drilled holes up into the centering rings so the pressure could be equalize to the outside air, I got lost. They said that the air trapped in between the centering rings and the sudden increase in altitude created more pressure in between the rings and blew the airframe sides out. Now, I’m not a Rocket Scientist I just play one on the weekends but I don’t understand this. Where would that pressure come from I ask, the heat from the motor tube expanding the air, they said no. or the different in the air tripped and the lower air pressure at 2000’, they said no. I never did understand where they were coming from. I haven’t read this anywhere or heard it before. I just don’t see how this can be true, can someone tell me if I’m missing something or is this BS.
Thanks
Rodney