BP charge on Madcow Super DX-3

[rsbhunter]

Ok, I am trying to do my due diligence. I went to the rocketry calcuator.... put in the measurements of the length of bt ( with coupler figured) and dia, 3.9. The bp calculator shows like .79gm of bp for 12 psi. The bp charge on the hp 29-38mm Aerotech info shows 1.4gm of bp. ??? I have the means of weighing the bp to either charge....which one ??? Do some people just dump the 1.4gm of bp and launch?...Thanks for any suggestions...rsbhunter

 

[bad_idea]

I've seen a lot of 4" rockets fail to eject even with stock ejection charges, so unless you're running a stuffer tube up to a minimally sized parachute bay, I certainly wouldn't use less than your motor comes with. Ideally you'd ground test charges even for motor eject when working with edge cases like 4" tubes.

A friend had to do that after multiple ejection failures in a 4" rocket that very fortunately resulted in flat spins and a relatively undamaged rocket each time. In ground testing he found reliable amount of BP to add to the stock charge and got his L1 on the subsequent flight.

 

[cls]

Yep. Ground test using 1.5g. that'll be about right. You are right about test first, then fly as you tested.

 

[Art Upton]

Most folks use the full charge supplied by the motor. Sometimes you need more than that, like on really Fat rockets.*

* LOC Warlock comes to mind with folks flying a Pro38 motor. More powder needs added on top of the factory charge on top of the paper disk

@rsbhunter , remind us what motor your using again?

 

[tsmith1315]

Do some people just dump the 1.4gm of bp and launch?..

Yep.

Most folks use the full charge supplied by the motor.

I’ve never had a failure to separate using the supplied charge on single full length 4” LOC tubes. 29 & 38mm AT H-J motors. Just did it with my EZI-65 at NSL using a 38/240 motor.

I friction fit them, on the loose side.

 

[Capt. Eric]

In preparation for my L2 with dual deploy I did a few ground tests, starting low, and increasing the amounts in increments until I was happy with the force of separation. If you are at all unsure, I highly recommend doing this to remove any anxiety.

In your case, you may have to put a used motor in place and rig the charge above it, maybe running the ignition wires through the motor.

 

[drewnickel]

Ok, I am trying to do my due diligence. I went to the rocketry calcuator.... put in the measurements of the length of bt ( with coupler figured) and dia, 3.9. The bp calculator shows like .79gm of bp for 12 psi. The bp charge on the hp 29-38mm Aerotech info shows 1.4gm of bp. ??? I have the means of weighing the bp to either charge....which one ??? Do some people just dump the 1.4gm of bp and launch?...Thanks for any suggestions...rsbhunter

My Super DX3 ended up needing a bit extra (1.5 gm) to reliably separate, but I was using 3x #2-56 shear pins to retain sections of the rocket as I had it configured for dual deploy. As with anything else you have the ability to test on the ground, I'd recommend doing a deployment test. For something with simple motor eject, you can fish an e-match through a spent (DMS) or empty (reloadable) motor casing and test your ejection charges that way, just needing something like an Estes launch controller to set off the charge.

 

[MikeT]

Always ground test , well worth the effort.

 

[thequick]

For reference, I fly a similar sized 4” cardboard Madcow Batray pretty regularly (15-20 flights). I use 2g of ffffg black powder for deployment. I do not use shear pins, just a masking tape friction fit.

Also, the shock cord is 15’ nylon.

Your mileage may vary

 

[rsbhunter]

I wish I knew alot more about this than I do. The length of the inside of the bt is 10", if that helps. rsbhunter

Most folks use the full charge supplied by the motor. Sometimes you need more than that, like on really Fat rockets.*

* LOC Warlock comes to mind with folks flying a Pro38 motor. More powder needs added on top of the factory charge on top of the paper disk

@rsbhunter , remind us what motor your using again?

H220T...the fin can dimension is 3.90 dia, 10" length from forward bulkhead to bulkhead on upper bt.

 

[Art Upton]

I wish I knew alot more about this than I do. The length of the inside of the bt is 10", if that helps. rsbhunter

H220T...the fin can dimension is 3.90 dia, 10" length from forward bulkhead to bulkhead on upper bt.

I was one of those that when doing motor ejection on 4" put in the whole vial that was supplied.

On altimeter charges on 4", it was was 1.5grams+ more if needed

We didn't trust the calculators back then, too many people using them were getting incoming.

If a calculator is saying .79 that sounds 'a bit low' to me as well, but I'm from back then

 

[rsbhunter]

I agree...the "posted" bp weight for Aerotech G reloads is .7gm. I weigh each charge, and have found variances from .59 to .71. On a 2.65 mpr with a 24" internal length, I bumped up the charge to .74. It made ejection positive after a ejection failure. I have done the calculator for both dimensions (2.65x24" and 10"x 3.90) and the charge weights were REAL close....I have electric matches and 4f powder....how do I set up a test??? What do I do to the 240mm assy to not damage it when I test? Do I block the case off on the nozzle end?....Thanks for any help....rsbhunter

 

[Capt. Eric]

I agree...the "posted" bp weight for Aerotech G reloads is .7gm. I weigh each charge, and have found variances from .59 to .71. On a 2.65 mpr with a 24" internal length, I bumped up the charge to .74. It made ejection positive after a ejection failure. I have done the calculator for both dimensions (2.65x24" and 10"x 3.90) and the charge weights were REAL close....I have electric matches and 4f powder....how do I set up a test??? What do I do to the 240mm assy to not damage it when I test? Do I block the case off on the nozzle end?....Thanks for any help....rsbhunter

Here's a couple of videos (not mine. I just did a quick google search) showing some tests. Personally, I used saw horses and shot the nc into moving blankets on the ground. Make sure the back of the rocket is braced against a tree or something.




I would put a spent motor case in the mmt. You can run the ignition wires through the motor as well.

 

[rsbhunter]

Ok, I don't have a spent motor case, as this is for my level 1 cert. However, I do have electric matches that the wires fit through the hole in the ejection charge well...but that will leave the "igniter" head in the charge well??? Will that be a problem?

 

[bad_idea]

Ok, I don't have a spent motor case, as this is for my level 1 cert. However, I do have electric matches that the wires fit through the hole in the ejection charge well...but that will leave the "igniter" head in the charge well??? Will that be a problem?

Assuming an empty 29/240 case (if I read your motor correctly, and please, please do not ground test with a live motor), yes, that should work fine, though I'd instead run the ematch wires out a pressure vent hole in the rocket body to better simulate the charge blowing back through the spent (or empty in your case) motor as well as forward.

In reality, there should be considerable pressure still in the motor at the point where the delay grain burns through and ignites the charge, but in the few occasions when I've ground tested for motor ejection, I've been looking for as "worst case" a test as possible. I've basically made up a loose charge taped around an ematch, tucked the bundle into the charge well, run the ematch wires out a pressure vent, and pressed play.

 

[Capt. Eric]

Ok, I don't have a spent motor case, as this is for my level 1 cert. However, I do have electric matches that the wires fit through the hole in the ejection charge well...but that will leave the "igniter" head in the charge well??? Will that be a problem?

One of us is confused. I think it's me.

Are you using the motor ejection? Then you do not want to put the motor that you'll use in the rocket for this test. You'll have to rig something else to simulate a motor being there (with a small hole like the nozzle, and the ejection charge well in the mmt). That's why I suggested a spent motor. Doesn't have to be exactly the same, just something to the hole. You could use a wooden disk in the retainer cap, if you have a screw on type. Then you can place an ejection charge fwd of the disk (wires run through the "nozzle hole"). I'd use a small container to hold the charge. You can use a straw or even a rubber glove finger. Just make sure the powder is held close to the igniter tip.

If you are not using the motor ejection, then I assume you have a charge well on the AV bay or some other arrangement. You can fill the mmt, and use the charge well or whatever you plan on doing.

Does this make sense? It's a little late, so I might not be as coherent as I think I am.

 

[Art Upton]

@rsbhunter is using an H220 reload in a 29/240 case based on other threads.

@bad_idea is suggesting to put the match in the charge well of the forward closure and fold it over, so it does not run thru the hole and block it to 'simulate' of the delay was totally burned up, but it generally does have left over. So that would emulate worst case.

Put the BP power in the well after the ematch folded over and cover with masking tape. The issue is then where to the wires get out of the rocket to attach to a ignition system, say an electron beam from an Estes set?

Many HPR folks drill a vent hole in the booster body tube to keep air pressure locked inside the tube from pushing off the nose or payload bay before the charge fires. This is a perfect hole to put the wires out.

Now on aa 900-1000 foot flight, vent holes are rarely needed, but don't hurt.

The other option is putting the ematch head thru the hole of the ejection well and snake the wires out the bottom of the motor, easy pleasy

TEST like Capt. Eric showed then

Most users are just pouring the power that comes with the H220 reload vail into the ejection well and flying. They don't test unless they ever get to Dual Deploy using altimeter charges

 

[rsbhunter]

@rsbhunter is using an H220 reload in a 29/240 case based on other threads.

@bad_idea is suggesting to put the match in the charge well of the forward closure and fold it over, so it does not run thru the hole and block it to 'simulate' of the delay was totally burned up, but it generally does have left over. So that would emulate worst case.

Put the BP power in the well after the ematch folded over and cover with masking tape. The issue is then where to the wires get out of the rocket to attach to a ignition system, say an electron beam from an Estes set?

Many HPR folks drill a vent hole in the booster body tube to keep air pressure locked inside the tube from pushing off the nose or payload bay before the charge fires. This is a perfect hole to put the wires out.

Now on aa 900-1000 foot flight, vent holes are rarely needed, but don't hurt.

The other option is putting the ematch head thru the hole of the ejection well and snake the wires out the bottom of the motor, easy pleasy

TEST like Capt. Eric showed then

Most users are just pouring the power that comes with the H220 reload vail into the ejection well and flying. They don't test unless they ever get to Dual Deploy using altimeter charges

I was thinking of running the wires thru the charge well hole, match head in the powder well with the black powder, tape over. The wires do reach out ot the motor , so good on ignition. I'm going to start with .7gm . I was thinking that it was a light charge, but with the shock cord, parachute "burrito" in the bt, the volume has to be reduced, so we will see. I did find some short grain insulators, and 3 cover the inside or the case. Should the hole in the forward closure be blocked with paper to keep the bp in?? I know they are open in a regular firing. Thanks again for ALL of the help and advice. rsbhunter

 

[Capt. Eric]

Should the hole in the forward closure be blocked with paper to keep the bp in??

yes, I think that's a good idea. And ensure the powder is around the igniter tip, then cap it with wadding and tape to keep the powder in place.

 

[Art Upton]

I was thinking of running the wires thru the charge well hole, match head in the powder well with the black powder, tape over. The wires do reach out ot the motor , so good on ignition. I'm going to start with .7gm . I was thinking that it was a light charge, but with the shock cord, parachute "burrito" in the bt, the volume has to be reduced, so we will see. I did find some short grain insulators, and 3 cover the inside or the case. Should the hole in the forward closure be blocked with paper to keep the bp in?? I know they are open in a regular firing. Thanks again for ALL of the help and advice. rsbhunter

I would never put a .7 gram charge in a 4" rocket. More like a 54mm rocket.

Things "Up There" are not like the ground. The idea Is Not to use the least amount of powder to eject. The idea is to use a good amount of powder that won't bust your rocket.

I Have Seen flights that did ground test till "It Worked" , but "In Coming" when flown. Even an L3 attempt. That was a sad day , even for me witnessing it

Edit: People that fly dual altimeter/charges many time use "Hail Mary" charges on the backup. "Blow it out, or Blow it Up"

 

[rsbhunter]

I like that mantra!!! Been my theory on life!! I want atheist to get everything out of the tube, so I'll go up till it's a clean ejection...rsbhunter

 

[2-0turbo]

I put together the following list that I printed and have in my range box. In a time long, long ago, motors came with ejection charges and you pretty much just flew it. With all of the dual deploy and online calculators out there, everyone wants to get this down to the tenth (or hundredth?) of a gram. Here is what I've compiled for commercially available motors:



The Estes values are a little iffy--all of this is use at your own risk. But the AT/Quest numbers should be pretty accurate. You can certainly use a calculator and come up with a precise value, and test to make sure it works. But for a lot of rockets, just flying a DMS motor, the above values usually work. If you dancing somewhere around these values, you're probably going to be okay. Or if you're not sure, compare the rocket you're flying to a "normal" rocket and just pick a value off this list.

I just noticed I don't have 54mm motors on here--I think they also use 2.1 grams. I don't have one laying around I can measure.

-Matt (still friction fits motors with masking tape)