How much BP for a 38mm dia (Go Devil 38)?

Both events looked fine to me. You want the main out with authority. Kurt

Are you using (4) nylon shear pins to hold your AV Bay coupler in your upper body tube?

I have two pins that penetrate the sustainer and the coupler and two that penetrate the forward body and the coupler. That second video shows how I shears all four and pulled the coupler out of the sustainer. Of course in real life the rocket will be free-falling with the coupler and forward section attached when the second charge goes off.

I think the events are pretty good. Frankly, I prefer charges to be a little stout- not quite to the point of blow it out or blow it up.

If anything, I might make the recovery harness longer. I fly at Bong Rec Area in Wisconsin- there are "some" trees- long harnesses aid in recovery.

You guys are telling me interesting feedback. You like my energetic charge. If I go with an energetic charge I could go with energy absorption in the shock cord to mitigate the risk of recontact. There is a thread in the High Powered Rocketry page on breakable Kevlar loops: https://www.rocketryforum.com/showthread.php?134230-Breakable-loop-in-Kevlar-shock-cord

In that thread they discuss breakable sewn loops. We call that frangible in the engineering world. A frangible thing is something designed to break. The system looses no integrity with the break and gains a kind of functionality.

One way to absorb energy is to loop your shock cord as you pack it and to tape your loops together. The taped loops break. I can practice that.

I can pull the shock cord straight, tear the tape and feel how much resistance it gives.

I can see if the extra bulk inhibits packing. My forward compartment is extremely tight. I fear I won't be able to fit much tape. I have to use a lot of force to mash the laundry when I push the forward compartment onto the coupler.

I can test it if it isn't too bulky and see how it absorbs the energy of my ejection charge.

I learn a lot from you guys and I'm glad your all there.

Thanks to all of you.

woferry said:

Bay Area Rocketry should be one of the vendors at LDRS, and Mike (owner) has a good selection of TFR chutes. Below a certain diameter (≤18"?) everything he has is thin mill I believe.

On my Go Devil 38 I use a Dog House small charge well on each end, each one holds ~0.6g along with the Firewire initiator, and I've always just filled them. Has worked fine, though I was only friction-fitting the nose and finding that sometimes the main would come out at apogee, other times during descent but before the altitude set in the altimeter. I have about 15' of shock cord in each bay. So I've drilled holes for shear pins for the nose and will be using them moving forward, I've never done any ground testing with this rocket as I couldn't fit my TRS in the bay, now that I have a Quantum I really should do a test just to see if I should perhaps go down a bit on the charges.

Click to expand...

Will,,
I contacted Bay Area and Mike said they would have the parachutes in his truck.

So I looked at the Dog House charge wells. What diameter is the 0.6 g well? I take it you ordered a 0.5g well since he has no 0.6g well, and you stuff that well full, is that correct? What size and what type chute did you pack in the front? Mine is a Top Flight 30 in Standard. I have to force my upper body tube onto the coupler. When I put the shear pins in I have to hold and squeeze the body tube into place. That is with a 10 ft cord.

Tom

Yeah, it's the smallest one with the terminals, 0.5g. Pretty sure I had poured BP in and back out to an empty Aerotech BP tubes and it read ~0.6 on the side of the tube. My avionics bay bulkheads are attached via a pair of eye bolts and a coupler nut, I offset the eye bolts as much as I could while keeping them within the bulk plate area, and this left enough room on each plate for one of these wells that fits and doesn't interfere with the turning of the eye bolts. Hopefully this picture helps.



I use a TFR 30" for the main and a TFR 12" for the drogue. I used a GLR chute protector for the main and I think the one for the drogue was from Sunward. The shock cord was from RW, think it was 1/8"? Here's the whole assembly as I fly it. Come to think of it, it may have been a 25' run that I split for the upper and lower shock cords, I'd rather not break the tape bundles to verify right now. The main goes in fairly easily with the cords tucked along side it, I do sometimes use some baby powder on the upper chute protector to ensure it slides more easily.

In my case I epoxied the avionics bay coupler to the upper tube and separate at the nose. I still used the vent band because I determined the upper bay would be too short to comfortably fit the main without the extra inch or so and I didn't have the space to spare below either, I have my M38E epoxied in about 8.5" from the top, which I had determined should leave enough room for the 6GXL case and MC38 retainer while barely leaving enough room for the drogue. I had a panic during building an ordered another 2' tube section from RW thinking I might need to lengthen the upper airframe, that turned out fine and in hindsight I wish the lower airframe was maybe 3-6" longer. The other thing I considered was lengthening the bottom (below the fins, between the motor closure flange and the normal bottom of the airframe) if I couldn't get a motor to fit, but I ultimately concluded that I think I could get it all to fit. It can certainly handle 6G, the real test will be the first time I fly a 6GXL load, I may still need to add a tiny ring to the bottom for that case. Generally speaking there isn't an inch to spare in this rocket, except that I haven't flown the 6GXL yet (just got L2 cert'ed last month) so there has been space between the motor and M38E, though I've found I can fit an AltimeterThree in that space so even it hasn't gone totally unused so far. There's even an Eggfinder in the nose, so that space is fully utilized as well.

Nice set-up Will. You didn't mention shear pins so I take it you friction fit everything?

bill2654 said:

Nice set-up Will. You didn't mention shear pins so I take it you friction fit everything?

Click to expand...

See post #30 on this thread. I hadn't done shear pins but will be for the nose moving forward. The motor is screwed into the M38E using cut all-thread and a coupler nut (I have different lengths for different motor cases). My highest flight was on this rocket with an I366R (just over 8500'), the main popped out at apogee and the rocket blew over a mile away before landing. The Eggfinder took me right to it but that was not fun, so I've drilled holes to shear pin the nose and will be pinning it from now on. I do have a strip of tape around the avionics bay coupler that shows in the picture, it doesn't really make for a friction fit but it does keep it from wobbling. I have a vent hole above the M38E in the lower airframe yet my first flight had a pressure separation of the lower airframe, it seems it was the laundry that plugged the vent hole leading to the separation. I also had friction-fit (very tightly I'll add) the motor on the first flight and it was hurled from the rocket during the somersault after the early chute deployment. I did a second flight on the exact same motor with no changes except to re-pack the laundry avoiding the vent hole and using AeroPack's threaded rods for motor retention and that flight was a success, so since then I've been careful when packing the laundry to ensure the holes are not obstructed (i.e. lining the shock cord bundles up with the vent holes rather than the chute protector). I won't be friction-fitting a motor ever again either, I got my casing back but it landed not far from the flightline which left me feeling very uneasy, I won't make that mistake again.

Thanks Will and Tom. You guys have been very helpful. I may finally get mine up in the air next month, weather permitting.

Will,
Do you think I need vent holes in my sustainer and forward section even though I use shear pins? I don't want the pressure in my bays popping my shear pins before the altimeter wants to do it.

I re-did my main compartment test. I tried 0.5 gm and it did not open. I tried 0.64 gm and it opened. The blue tape on the shock cord worked to reduce the shock from the deployment. I was happy with that. When I opened the chute however I found opposing gores of the chute were melted together. That chute would never have opened.

One new thing I tried today, I had my 12 inch chute protector cut down and made into four 6 inch chute protectors. I figured since I could not get my chute in with the full 12 inches of protector maybe I could with 6 inches. It sure would beat leaving the chute protector out and trying to use blue recovery wadding paper. I had at least three attempts with the blue paper and two more with the 6 in chute protector. Maybe the chute melted in one of the previous attempts.

Here is the test. I initially jumped to a conclusion that this latest test melted the chute. It could have been the previous attempts,

<https://www.youtube.com/watch?v=mM6ILRU-SQ4&feature=youtu.be>

Now I have to do it again and see if it melts together again. I have 20 e-matches I bought from Crazy Jim (thanks for that). I will try his method of using the tips of nitrile gloves to encapsulate the black powder. That seems shaky but this way I get to check it out on the ground instead of in the air. My biggest problem is that I am running out of time. I have full weekends and my work is going to demand overtime. I have products I must get out by June 7 or I must go late to LDRS. That would be a disaster!

TomSmith58 said:

Will,
Do you think I need vent holes in my sustainer and forward section even though I use shear pins? I don't want the pressure in my bays popping my shear pins before the altimeter wants to do it.

Click to expand...

I'm not the expert here, I've always vented until now and am going the shear pin route for the first time. In the end I think it's a fairly simple math problem, but right now I'm too lazy to figure it out. With any luck someone else will chime in, and if cwbullet is listening hopefully it's something the online calculator he's working on can help with.

The reason for venting is because without it you're trapping a volume of air at ground level pressure and rapidly taking it to an area of lower pressure, so the trapped air is going to want to expand (and the higher/faster the worse things get). With sufficient venting the air will go that route, without it the pressure could cause the rocket to separate wherever it's designed to do so, which can obviously be bad at the wrong time. Shear pins instead of venting means that pressure will be pushing against the pins, so as long as it's below what it takes to shear the pins I can't imagine it would be a problem and the vent holes could be avoided. In a rocket this small the air volume in the rocket isn't that large, and the surface area of the bulkhead is also small, so I imagine (again without doing any of the math) the shear pins wouldn't even be close to their limit. You get to bigger airframes (length and diameter) and there could be a lot more force on the pins from the air pressure delta (though the larger mass of everything probably means more/larger pins anyway).

So I suspect you'd be fine, but without doing any of the math it's just a gut feeling. And as I said I'm just dabbling with shear pins myself.

I could be wrong, but pins _and_ venting not either/or. No vents makes sad barometers ( although accel only, timers, etc )

TomSmith58 said:

I re-did my main compartment test. I tried 0.5 gm and it did not open. I tried 0.64 gm and it opened. The blue tape on the shock cord worked to reduce the shock from the deployment. I was happy with that. When I opened the chute however I found opposing gores of the chute were melted together. That chute would never have opened.

One new thing I tried today, I had my 12 inch chute protector cut down and made into four 6 inch chute protectors. I figured since I could not get my chute in with the full 12 inches of protector maybe I could with 6 inches. It sure would beat leaving the chute protector out and trying to use blue recovery wadding paper. I had at least three attempts with the blue paper and two more with the 6 in chute protector. Maybe the chute melted in one of the previous attempts.

Here is the test. I initially jumped to a conclusion that this latest test melted the chute. It could have been the previous attempts,

<https://www.youtube.com/watch?v=mM6ILRU-SQ4&feature=youtu.be>

Now I have to do it again and see if it melts together again. I have 20 e-matches I bought from Crazy Jim (thanks for that). I will try his method of using the tips of nitrile gloves to encapsulate the black powder. That seems shaky but this way I get to check it out on the ground instead of in the air. My biggest problem is that I am running out of time. I have full weekends and my work is going to demand overtime. I have products I must get out by June 7 or I must go late to LDRS. That would be a disaster!

Click to expand...

I can't tell for sure with your testing, but in my experience one of the worst things for a chute is a faulty charge test. In fact, I am always ready to run to the rocket after a charge test and pull it open and shake everything out. Otherwise you end up with a confined volume filled with smoldering dog barf (recovery material) giving you, as you have surmised, a high probability of a melted chute.

BTW, you are dealing with one of the tougher recovery challenges in HPR. It is much easier to consistently separate a large frame rocket (4" to 8") than it is a 29mm or 38mm as you have less piston area you are pushing on coupled with a tighter fitting payload area.

Finally, I don't understand the blue recovery wadding (probably need to read the thread in more detail), but I would not forsake the Nomex protector. There are many here that are experts in packing tight like CJ (you mentioned) and Griffin. Personally on these small dia. rockets I have gone to a deployment "pouch" of my own design that I sew together for any rockets <3" in diameter.

dhbarr said:

I could be wrong, but pins _and_ venting not either/or. No vents makes sad barometers ( although accel only, timers, etc )

Click to expand...

This was in reference to the parachute portions of the rocket, the avionics bay absolutely requires venting (and more ventilation than the other airframe sections) for barometric altimeters. In that case it's not because the pressure will separate anything (whatever transfers the shock load through the av bay should hold it together) but so that the altimeter can actually get a reading. For the other rocket sections it's purely to avoid premature separation (same goal as the shear pins, so it could be either/or).

So for me on my Go Devil 38 it's:
a) 1 vent hole in the motor section (because I'll typically fly an AltimeterThree in there unless the motor leaves no space for one)
b) 1 vent hole in the lower airframe (to prevent pressure separation of the drogue parachute section, note that I'm flying a non-bypass M38E so there's no opening between the motor section and top of the lower airframe, if I had gone with the M38BE instead I possibly could have gone with just this hole and skipped (a))
c) 3 vent holes in the avionics bay (for deployment altimeter air pressure sampling, I also lined up one of these holes with the blue LED on my magnetic switch so I can easily tell where to pass the magnet and visually confirm that power is on/off)
d) 1 vent hole in the upper airframe (to prevent pressure separation of the main parachute section, though now that I'm using shear pins I could have probably skipped this hole. I'm not going to bother trying to fill it at this point).

Technically, (a) should be more venting like (c) since again it's for a barometric altimeter, I wasn't interested in even more holes and I've found the A3's logs match that of the SLCF I fly in the av bay so I don't seem to need anything more there. There's likely another opportunity for venting around the motor casing out the bottom of the airframe since the motor fits with some play. But it is important not to over-do the venting in the sections where a charge will go off, because you do want the charge to be able to build enough pressure to separate the rocket, which obviously takes more pressure if there are also shear pins to break. Ground test, ground test, ground test!

This BP calculator might help, as it shows some pressures and correlates it to altitude as pressure on the nose cone.

https://www.google.com/url?sa=t&rct...S6Li6XMgKVR8OyZkMqBIhg&bvm=bv.122676328,d.cGc

The caveat with vent holes is to make sure that any recovery laundry does not plug the holes, and ground test again if your initial work up did not include the venting effect of the holes.

blackjack2564 said:

For a almost identical size rocket, I use .5 apogee [20ft of 1/8 kevlar] & .7 main. [15ft of same kevlar]
NC is held on with 1 2-256 shear-pin & fit is rather snug.

PS: since there is only 1 altimeter, I use 2 matches per charge.

Click to expand...

Crazy Jim
How do you do the two charges ? Just jam two wires in each hole of the terminal block on the altimeter?

Uncrazy Tom

TomSmith58 said:

Gents,
I am at that stage. How much BP for the two charges? It is a 38mm. The available volume in the sustainer is 5.5 inches long. There is a 15 ft shock cord and a 12 in chute.

In the forward body it is 6.7 inches. There is a 10 ft shock cord and a 30 in chute.

The formula in Modern High Powered Rocketry 2 says:
BP (grams) = 0.006*D_compartment^2*L_Compartment

That comes out to 0.074 g for the sustainer and 0.090 grams for the forward compartment.

Does that sound right to you?

I will begin testing tomorrow.

Click to expand...

It takes on average 50 lb(f) to shear (2) 2-56 nylon shear screws. The cross-section of the airframe is 1.77 sq.in. That means an average of 50 lb/1.77sq.in.=29 psi. To insure the screws shear you want at least 25% more or ~>=35 psi.

You will need a minimum of 0.25 g for the smaller compartment and 0.3 g for the larger compartment. If the gases are cooled by the heat capacity of the airframe, extra BP is required. O.5 g and 0.6 g are not unreasonable BP weights.

As was stated earlier, thin mil chutes are the way to go. I use ironing board fabric from JoAnn's as a chute protector, aluminum side towards charge.

Bob

Soooooooooooo, Flying this at LDRS? If so report back here when done.:wink: Kurt

Sad ending. Lake Stake. The altimeter didn't work. I still haven't opened up the coupler to look at it. I am too busy at work. I'll post what I find in the High Powered section