Burned BT

OK..I flew my Foray twice on a C11-5.Thats the only 2 times its flown. Has a BT50 main tube. Thing is its allready cooked. The outside primer is bubbled and the tube is very tender at that spot.You can see the inside is chared and cooked. I figure I can slide a BT50 coupler in it and save it.My question is..should I have done this in the first place and is this normal for what I call a min dia rocket? Thing that bothers me is ,I have kits that have a BT50 main that run 24mm in them.Are they going to do the same thing after just a few flights on them. The kits dont call for any coupler/reinforcement above the end of motor.I know in the future any build designed like this ,Ill reinforce them above end of motor. Any thoughts,clues about this issue?

Using a stage coupler or two in front of the motor would help with the durability of Estes BT-50 tubing when used with 24mm motors. Remember that most Estes body tubes smaller than BT-55 have a wall thickness of only 0.013" rather than the 0.021" used in BT-55 and larger body tubes. For future 24mm minimum-diameter builds, you might consider using Semroc ST-9 tubing with a wall thickness of 0.024". Even in the Estes D and E Motor Mount kit, the motor tube is a heavy wall version of BT-50.

The other way of dealing with this is using D12s from 1996 or 1997 with weaker ejection charges. These motors have an 'A' or a 'B' for the year in the date code. However, you need to scrape some of the excess clay from the ejection charge cap to get reliable ejection. YMMV.

That charring is really odd, especially since the delay and ejection charges of C11s are recessed a good way into their cases. (IOW, the top of the grain is way down there, like it is in an 18mm B.) My thinking is that charring like that would only occur if the ejection charge fire was bottled up in that area for an extended period of time (relatively), perhaps by packing in the wadding too tight or by a tight-fitting recovery device. Was the parachute or shock cord also damaged? Early on, I used to wonder why the motor's ejection charge didn't cook the inside of the paper body tube, and why some rockets would remain intact for dozens, even hundreds, of flights. Eventually I realized that the ignited charge only stays in the tube for a fraction of a second before erupting out of the top. As soon as that takes place, outside air begins to cool the inside of the tube.

Your tube roasting could only have occurred if there was a sustained fire in that area, because in a normal ejection charge sequence the fire would have blown past it and exited the tube well before it caused any damage. So either the ignited charge was bottled up in the area long enough to do some damage or else there was some flammable object there that caught fire and burned. I have never experienced that phenomenon in any of my minimum-diameter rockets. I have made multiple flights of minimum diameter rockets whose airframes consisted of single wraps of 110 lb. card stock. They resisted charring during normal ejection charge firings, and so should your BT-50 if the ejection charge firing and recovery system deployment follows the normal sequence.

MarkII said:

My thinking is that charring like that would only occur if the ejection charge fire was bottled up in that area for an extended period of time (relatively), perhaps by packing in the wadding too tight or by a tight-fitting recovery device.
(snip)
Your tube roasting could only have occurred if there was a sustained fire in that area, because in a normal ejection charge sequence the fire would have blown past it and exited the tube well before it caused any damage. So either the ignited charge was bottled up in the area long enough to do some damage or else there was some flammable object there that caught fire and burned.

Click to expand...

I suspect that BP motors sometimes exhibit the hibachi effect which is normally associated with APCP motors. But I think that occasionally, the delay on BP motors will burn thru and ignite the ejection charge while leaving behind a still burning chunk of delay that cooks the inside of the rocket.

In this case, I'd bet both motors used came from the same pack and had the same unusual delay mode.

Doug

.

MarkII said:

That charring is really odd, especially since the delay and ejection charges of C11s are recessed a good way into their cases. (IOW, the top of the grain is way down there, like it is in an 18mm B.) My thinking is that charring like that would only occur if the ejection charge fire was bottled up in that area for an extended period of time (relatively), perhaps by packing in the wadding too tight or by a tight-fitting recovery device. Was the parachute or shock cord also damaged? Early on, I used to wonder why the motor's ejection charge didn't cook the inside of the paper body tube, and why some rockets would remain intact for dozens, even hundreds, of flights. Eventually I realized that the ignited charge only stays in the tube for a fraction of a second before erupting out of the top. As soon as that takes place, outside air begins to cool the inside of the tube.

Your tube roasting could only have occurred if there was a sustained fire in that area, because in a normal ejection charge sequence the fire would have blown past it and exited the tube well before it caused any damage. So either the ignited charge was bottled up in the area long enough to do some damage or else there was some flammable object there that caught fire and burned. I have never experienced that phenomenon in any of my minimum-diameter rockets. I have made multiple flights of minimum diameter rockets whose airframes consisted of single wraps of 110 lb. card stock. They resisted charring during normal ejection charge firings, and so should your BT-50 if the ejection charge firing and recovery system deployment follows the normal sequence.

Click to expand...

No damage to chute or chord. As I said and you can see,it is cooked inside and by the bubbles on the outside ,you sure can tell it got hot. Ill throw a coupler in it to save it. I was bummed out when I found this when I was going to finish up the paint job.Im glad I did find it before flying it again. Dont think it would have taken another flight without some kind of major damage.Strange!!!!!!!!!!!

mwtoelle said:

Using a stage coupler or two in front of the motor would help with the durability of Estes BT-50 tubing when used with 24mm motors. Remember that most Estes body tubes smaller than BT-55 have a wall thickness of only 0.013" rather than the 0.021" used in BT-55 and larger body tubes. For future 24mm minimum-diameter builds, you might consider using Semroc ST-9 tubing with a wall thickness of 0.024". Even in the Estes D and E Motor Mount kit, the motor tube is a heavy wall version of BT-50.

The other way of dealing with this is using D12s from 1996 or 1997 with weaker ejection charges. These motors have an 'A' or a 'B' for the year in the date code. However, you need to scrape some of the excess clay from the ejection charge cap to get reliable ejection. YMMV.

Click to expand...

Thanks. I know in the future Ill either use coupler or heaver tube on builds like this.

plano-doug said:

I suspect that BP motors sometimes exhibit the hibachi effect which is normally associated with APCP motors. But I think that occasionally, the delay on BP motors will burn thru and ignite the ejection charge while leaving behind a still burning chunk of delay that cooks the inside of the rocket.

In this case, I'd bet both motors used came from the same pack and had the same unusual delay mode.

Doug

.

Click to expand...

Yes, but in a C11? I just checked a C11-7, and the clay cap was a full inch in from the top of the case. Yeah, I suppose its possible, but I've never seen it. I have pulled several spent 24mm Estes motors and found thick chunks of ash right at the top of the case, but I haven't seen any tube charring in those rockets. The stuff is blown free or is extinguished before it does anything to the tube. But there can be a rare exception to that rule, I suppose.

Scotty Dog said:

No damage to chute or chord. As I said and you can see,it is cooked inside and by the bubbles on the outside ,you sure can tell it got hot. Ill throw a coupler in it to save it. I was bummed out when I found this when I was going to finish up the paint job.Im glad I did find it before flying it again. Dont think it would have taken another flight without some kind of major damage.Strange!!!!!!!!!!!

Click to expand...

Oh, I saw that. Your photos clearly showed it. If everything else was normal, then I'm more inclined to go with Doug's explanation that it was an anomaly with the motor rather than a flaw inherent in the tube. I'm all for using ST-9, but as I said I haven't even seen this with rockets made from card stock, which have a single-layer wall thickness of 0.009".

MarkII said:

Yes, but in a C11? I just checked a C11-7, and the clay cap was a full inch in from the top of the case. Yeah, I suppose its possible, but I've never seen it. I have pulled several spent 24mm Estes motors and found thick chunks of ash right at the top of the case, but I haven't seen any tube charring in those rockets. The stuff is blown free or is extinguished before it does anything to the tube. But there can be a rare exception to that rule, I suppose.

Click to expand...

It's not the ejection charge that does it. It's the delay grain. If it somehow burns unevenly - maybe it burns thru in the center first ala drilled APCP delays - it could ignite the ejection charge and then continue burning afterward.

Keep in mind the delay is just more (but different) powder rammed on top of the propellant grain. If it somehow doesn't build up uniformly around the center of the motor, it could burn faster on one side such that the delay burns thru while still leaving behind a burning chunk of delay material. Once the ejection charge fires, it clears the way for the remaining delay to burn forward into the airframe.

Another possibility is that some material is ejected into the airframe where it collects and then burns resulting in a charred air frame.

There's a whole lot of stuff going on in there - it's surprising there aren't more charring issues. So it's not surprising that someone gets some excess cooking once in a while. I know the Alpha I built as a BAR back in 99 is quite thin just ahead of the motor tube due to the cumulative effects of ejection charges and whatever else comes out of the forward end of the motor.

Doug

.

I had the same thing on my Vagabond including bubbled paint after the first flight. It's not min diameter but BT-50 inside BT-60. I noticed on the spent motor that the motor burned out the top at an angle, burning a U shaped hole out of the rim of the casing. I realize we are talking about C11's here but I have seen this on two D12's I have used. The first even had about a quarter of the clay cap still intact. I didn't know it was uncommon so I didn't keep it for evidence.

MarkII said:

Oh, I saw that. Your photos clearly showed it. If everything else was normal, then I'm more inclined to go with Doug's explanation that it was an anomaly with the motor rather than a flaw inherent in the tube. I'm all for using ST-9, but as I said I haven't even seen this with rockets made from card stock, which have a single-layer wall thickness of 0.009".

Click to expand...

Wasnt implying you didnt see it the first time. Im a repeater,exspecially when Im excited. Ya know, "hey did ya see that".."ya I saw it" ...DID YA SEE THAT!" .."Yes Scott I did see that! I think maybe I should start a log,maybe take time to inspect things closer after each flight. Anywhoo..thanks guys for your input on this.

I had bubbled paint on the first (and only) flight of my Centurion with a C6-5. Having read Mark II's first explanation, I understand it now. My 'chute didn't come all the way out and his remarks would account for something like that. Glad I read this thread, because I was wondering about that.

YES. The Hibachi Effect. Black Powder motors burn in a dome like shape inside and the delay does indeed break through to the ejection charge in the center, leaving the edges to burn for quite a while after ejection charge. This delay afterburn is like a highway flare and if you watch it you will be astounded and horrified. A 24mm motor has a really large delay afterburn and a BT-50 tube has the same thin wall as a BT-20 or BT-5, but since the 24mm motor afterburn is so intense, the damaging effect is worse.

Always use a coupler or other internal tube protection or use a thick walled version of 24mm tubing if building a minimum diameter 24mm motor rocket.

https://www.payloadbay.com/video-7978.html

plano-doug said:

It's not the ejection charge that does it. It's the delay grain. If it somehow burns unevenly - maybe it burns thru in the center first ala drilled APCP delays - it could ignite the ejection charge and then continue burning afterward.

Keep in mind the delay is just more (but different) powder rammed on top of the propellant grain. If it somehow doesn't build up uniformly around the center of the motor, it could burn faster on one side such that the delay burns thru while still leaving behind a burning chunk of delay material. Once the ejection charge fires, it clears the way for the remaining delay to burn forward into the airframe.

Another possibility is that some material is ejected into the airframe where it collects and then burns resulting in a charred air frame.

There's a whole lot of stuff going on in there - it's surprising there aren't more charring issues. So it's not surprising that someone gets some excess cooking once in a while. I know the Alpha I built as a BAR back in 99 is quite thin just ahead of the motor tube due to the cumulative effects of ejection charges and whatever else comes out of the forward end of the motor.

Doug

.

Click to expand...

plano-doug said:

I suspect that BP motors sometimes exhibit the hibachi effect which is normally associated with APCP motors. But I think that occasionally, the delay on BP motors will burn thru and ignite the ejection charge while leaving behind a still burning chunk of delay that cooks the inside of the rocket.

In this case, I'd bet both motors used came from the same pack and had the same unusual delay mode.

Doug

.

Click to expand...

+1 to what Doug said... the delay probably burned unevenly, touching off the ejection charge and blowing the chute out while part of the delay continued to burn inside the casing-- with the nozzle being so small, the flame shoots out forward and roasts the tube-- the gases expand and cool fairly rapidly and it's not a particularly vigorous flame, but it creates enough localized heat to really burn and blister the tube, especially a min-diameter one...

Good luck! OL JR

pick1e said:

I had the same thing on my Vagabond including bubbled paint after the first flight. It's not min diameter but BT-50 inside BT-60. I noticed on the spent motor that the motor burned out the top at an angle, burning a U shaped hole out of the rim of the casing. I realize we are talking about C11's here but I have seen this on two D12's I have used. The first even had about a quarter of the clay cap still intact. I didn't know it was uncommon so I didn't keep it for evidence.

Click to expand...

I had a D-12 I got back one time with the clay cap INTACT-- just a small "pie shaped" piece of the cap blown out on one side... :y: So it happens...

The larger the motor diameter the more likely it is to happen... so the 24 mm motors are more prone to this than the 18's and much more than a 13...

Later! OL JR

Just a reminder that Black Powder Motors do not have delays that 'burn unevenly'. The motor burns from the bottom to the top. The burning face of the propellant forms a dome shape. Different motors have different shaped domes (some deep and some shallow and almost flat, but never fully flat). The center of the dome reaches the delay and the delay burns very slow and also burns in a dome shape, with the edge along the outer wall remaining and still burning after the top of the dome breaks through and ignites the ejection charge.

The remaining edge of delay material continues to burn - the "delay afterburn'" - and this causes the Hibachi Effect. The flame coming out the top of the motor is amazing. I've static tested literally hundreds of Estes D12 motors (432 minimum in one series of tests). Thin BT-50 does not stand a chance. That is why they built the old Delta Clipper kit from a thicker wall tube since it was designed to use D12-0 to D12-7 and the only way for the upper stage to survive is if it had a thicker wall tube. Ditto for the 'probe' in the large Star Wars Star Destroyer rocket.

Watch the video.

Yes, if the clay cap blows off only partially, it can direct the delay afterburn flame off to one side and that can be "bad". I have not seen that in recently manufactured motors. Of course, YMMV.

shreadvector said:

YES. The Hibachi Effect. Black Powder motors burn in a dome like shape inside and the delay does indeed break through to the ejection charge in the center, leaving the edges to burn for quite a while after ejection charge. This delay afterburn is like a highway flare and if you watch it you will be astounded and horrified. A 24mm motor has a really large delay afterburn and a BT-50 tube has the same thin wall as a BT-20 or BT-5, but since the 24mm motor afterburn is so intense, the damaging effect is worse.

Always use a coupler or other internal tube protection or use a thick walled version of 24mm tubing if building a minimum diameter 24mm motor rocket.

https://www.payloadbay.com/video-7978.html

Click to expand...

perhaps we should go Back to the Past circa 1955, and use a removeable rolled piece of paper inside the body tube to protect the body tube from the heat of the ejection charge, as Orv Carlisle did . A piece of nomex paper would be perfect and would last for a long time too.

Terry Dean

shockie said:

perhaps we should go Back to the Past circa 1955, and use a removeable rolled piece of paper inside the body tube to protect the body tube from the heat of the ejection charge, as Orv Carlisle did . A piece of nomex paper would be perfect and would last for a long time too.

Terry Dean

Click to expand...

A rolled up piece of 110 lb. card stock would work, and it would be trivial to replace. I'm still puzzled about the fact that I haven't seen this phenomenon in 11 years of flying rockets, though. That includes paper rockets made from a single layer of card stock, flown on 24mm Estes motors.

I thought that the "hibachi effect" was a problem that is mostly seen in drilled (adjusted) delays in composite single-use and reloadable motors. The drilled out core results in a thinner delay grain "bulkhead."

I started doing the following after I had a jamb up of recovery wadding, shock cord and parachute once.
I use TiteBond II for just about everything when it comes to balsa and paper tubes.
I coat the inside of the body tube with a good coat of the stuff. It doesn't burn, it adds a little weight yea, but...
It also adds strenght to the tube.
First thing I do when I start a rocket, rather a kit or scratch build, I thin TiteBond II about 20% and give 3 coats to outer side, sand w/180 or 120, add additional coats as needed and finish sand w/220.
When I comes time to add the shock cord mount, I coat the inside of the tube with an unthinned coat of TiteBond II. Both sides of the body tube are then coated with glue. Adds Weight, but, my therey is if you are adding wieght, it should have a second purpose like adding strength.
Primer is just that, a primer, not a filler. All the sprirals and humps and bumps are smooth before priming. Takes less primer, less weight for asthetics where strength isn't added and gets way less shrinkage on finish coats.
TiteBond II is sandable if you don't go too fast and heat it up. If heavy sanding is required, keep moving around so it gives it time to cool, then come back.
Primer has no effect on the glue unless it's latex based. Which I doubt anyone uses.
As a side note, again, kit or scratch build, I take the balsa even before the fins are removed and coat it with TiteBond II. First coat thinned about 20%, 2 or 3 more coats of unthinned. Do both sides at the same time or you will get warpage. Stand and end and let dry.
I then use my ROS at the lowest speed w/220 and it smooths out like glass, like as a shaved taco. Just the way you like it.
Once the fins are mounted and filleted, one thinned coat on the edges and one coat of unthinned and it's ready for primer.
Just my way of doing things from trial and error over the years. May or may not work for everyone, but it works for me. I'm patient, so dry time and sand time isn't a problem for me. May be for others.
Good Luck!
JEA