Ejection Charge for Large (12.75") Rocket

[rharshberger]

I find it hard to believe Sportsmans Warehouse doesn't carry the ffff black powder...

Give them a call..

Teddy

Our local Sportsmans Warehouse does not carry BP due to local regulations. I know where to buy it but its a 20, 75, or 140 mile trip depending on where I have to go.

Edit: those are one way mileages.

 

[RocketFeller]

I find it hard to believe Sportsmans Warehouse doesn't carry the ffff black powder...

Give them a call..

Teddy

I just did. They have FF and FFF, but not FFFF... I may have to go up to Cabela's or BPS.

Again, back to post #7. Your results with 15 psi pushing on a 12.625" piston are as below. And again, I have to ask, why would anyone need nearly 1900lbf (30 to 50 shear pin equivalents) in separation force?!?

Ejection Charge

Deployment Section Measurements
Width: 12.625 (in)
Length: 38 (in)

Pressure: 15.00 (psi)
FFFF BP Amount: 36.81 (g)
Force Produced: 1877.78 (lbf)

Number of Nylon Screws (Shear pins):
49 x 2-56
or
30 x 4-40
or
20 x 6-32

Generated by Rocket Calculator

Available on the Apple App Store​

I see - so maybe we should start ground testing using the 5 psi figure.

Our local Sportsmans Warehouse does not carry BP due to local regulations. I know where to buy it but its a 20, 75, or 140 mile trip depending on where I have to go.

Hopefully I can get it within forty miles each way.

 

[jsdemar]

For the ground test, make sure you have the volume filled with the equivalent of the parachute and harness. I'm guessing it will be about half full.
An 18 grams charge seems like more than enough for that volume, based on my experience.
I'm assuming you're using sheer pins (four #4's ?) and will have them in place for the test?
(I browsed the whole thread but I may have missed it if this was already discussed).

Depending on how heavy the nosecone is, it may be worthwhile to add a small pilot 'chute at the base of the cone to help pull all the laundry out. Learned that one the hard way.

 

[bobkrech]

Just a few comments that you may find interesting.

1.) The separation force for a 15 psi charge is 1877 pounds = 8356 N.

2.) The separation velocity, delta V = sqrt[2d x (F/M)] where d is the length of the separation shoulder, F is the separation force, and M is the mass of the separating component. For example if the shoulder length is 12" ~ 0.3 m and the mass is 22 pounds = 10 kg, the separation velocity is sqrt (0.6 x 835.6) = 22.4 m/s = 73 fps = 50 mph.

3.) The reason for bringing this up is that this velocity is a bit higher than most smaller rockets because the shoulder length is ~= to 1 diameter of the airframe, and the ejection distances in a ground test will be quite long and the recoil forces and shock cord jerk will be strong because of the high mas and velocity.

4.) An internal pressure difference equivalent to 1 mile AGL between the inside and outside of the rocket represents ~310 pound load so compartment needs some venting and shear pins are required to prevent premature separation.

5.) The boom from the ejection charge will be loud so be prepared for it. Not something to do in your back yard unless you have a back 40.......

Bob

PS I see Tim and I think alike. :wink:

 

[Bat-mite]

No experience, but what I hear is that FFF will work. Make sure it is packed tightly since the grains are a bit less refined. Volumes of online calculators assume 4F, so make sure you ground test and note your amounts as 3F, in case you switch to 4F down the road.

 

[RocketFeller]

For the ground test, make sure you have the volume filled with the equivalent of the parachute and harness. I'm guessing it will be about half full.
An 18 grams charge seems like more than enough for that volume, based on my experience.
I'm assuming you're using sheer pins (four #4's ?) and will have them in place for the test?
(I browsed the whole thread but I may have missed it if this was already discussed).

Depending on how heavy the nosecone is, it may be worthwhile to add a small pilot 'chute at the base of the cone to help pull all the laundry out. Learned that one the hard way.

Good point, we will put a towel or something equivalent in weight to the chute in the nomex blanket for the test. We are planning on a flame proof RM drogue on the nose.

Just a few comments that you may find interesting.

1.) The separation force for a 15 psi charge is 1877 pounds = 8356 N.

2.) The separation velocity, delta V = sqrt[2d x (F/M)] where d is the length of the separation shoulder, F is the separation force, and M is the mass of the separating component. For example if the shoulder length is 12" ~ 0.3 m and the mass is 22 pounds = 10 kg, the separation velocity is sqrt (0.6 x 835.6) = 22.4 m/s = 73 fps = 50 mph.

3.) The reason for bringing this up is that this velocity is a bit higher than most smaller rockets because the shoulder length is ~= to 1 diameter of the airframe, and the ejection distances in a ground test will be quite long and the recoil forces and shock cord jerk will be strong because of the high mas and velocity.

4.) An internal pressure difference equivalent to 1 mile AGL between the inside and outside of the rocket represents ~310 pound load so compartment needs some venting and shear pins are required to prevent premature separation.

5.) The boom from the ejection charge will be loud so be prepared for it. Not something to do in your back yard unless you have a back 40.......

Bob

PS I see Tim and I think alike. :wink:

Thanks, I will definitely start small, no need to use way too much and break something during the first ground test!

The NC has a 10" shoulder. It fits fairly loose, which may help vent the expanding air as it goes up. I can also could drill a vent, either through the centering rings or the body wall. I plan to use four 4-40 shear pins to avoid drag separation.

Luckily we do have a lot of room - we are trying to get an FAA waiver for the ~1,000 acre property directly across the road from my house!

No experience, but what I hear is that FFF will work. Make sure it is packed tightly since the grains are a bit less refined. Volumes of online calculators assume 4F, so make sure you ground test and note your amounts as 3F, in case you switch to 4F down the road.

If I can't get some 4F I might get some 3F just to test charge cups and such.

 

[bobkrech]

........

Thanks, I will definitely start small, no need to use way too much and break something during the first ground test!

The NC has a 10" shoulder. It fits fairly loose, which may help vent the expanding air as it goes up. I can also could drill a vent, either through the centering rings or the body wall. I plan to use four 4-40 shear pins to avoid drag separation.

Luckily we do have a lot of room - we are trying to get an FAA waiver for the ~1,000 acre property directly across the road from my house!

......

In big rockets internal pressurization due to altitude gain is the problem, not drag separation. A very small increase in altitude generates a large internal pressurization separation force. Since you can't rely on random slip-fits to vent a large chamber, you either need to drill some vent holes near the base of the NC, or you need to secure the nosecone with shear screws, or do both.

https://web.archive.org/web/2011081...rials.org/datastore/cord/Shear_Pins/index.php shows that (4) 4-40 nylon screws shear at ~152 pounds load over 125 square inches area = ~1.25 psi.

https://web.archive.org/web/2011081...rials.org/datastore/cord/Shear_Pins/index.php is a calculator used to determine atmospheric parameters. Sea level is ~14.7 psi so subtract 1.25 = 13.45 psi which is equivalent to an altitude change of ~2400' AGL and is just enough to shear the (4) 4-40 screws. I don't know enough about your flight profile to advise you on your shear screws, but (4) 4-40 screws may not be enough without additional vent holes.

jsdemar has lots of experience with large rockets so I would respect his suggestions about the BP charge weight and the use of a pilot chute on the NC.

Bob

 

[webtech]

I found FFFFG BP at a local gun shop. It will be hard to find for sure. But I wouldn't use anything else.

As for 1900+lb of force, that's what it would take to blow 30+ shear pins. I think it's way too much.

you will have to figure out the amount of acceleration force you will have on with the nose cone to retain it on. then work from there.

then ground test it.. start small if you wish.. then work up.

the online calculators are good to get you in the right direction. temperature will affect burn rate of powder.. also how you have things pack..
so in your calculations, it's factoring an empty tube.. which isn't 100% true, with recovery 'stuffs' added..

so try it and see what happens.. but if it blows up, i didn't say it would work

 

[webtech]

and maybe 15 psi for a target is too much for what you need..

its a thought

 

[Dwatkins]

and maybe 15 psi for a target is too much for what you need..

its a thought

I will guarantee 15psi in this size rocket is to much . Start at 5psi!

 

[RocketFeller]

In big rockets internal pressurization due to altitude gain is the problem, not drag separation. A very small increase in altitude generates a large internal pressurization separation force. Since you can't rely on random slip-fits to vent a large chamber, you either need to drill some vent holes near the base of the NC, or you need to secure the nosecone with shear screws, or do both.

https://web.archive.org/web/2011081...rials.org/datastore/cord/Shear_Pins/index.php shows that (4) 4-40 nylon screws shear at ~152 pounds load over 125 square inches area = ~1.25 psi.

https://web.archive.org/web/2011081...rials.org/datastore/cord/Shear_Pins/index.php is a calculator used to determine atmospheric parameters. Sea level is ~14.7 psi so subtract 1.25 = 13.45 psi which is equivalent to an altitude change of ~2400' AGL and is just enough to shear the (4) 4-40 screws. I don't know enough about your flight profile to advise you on your shear screws, but (4) 4-40 screws may not be enough without additional vent holes.

jsdemar has lots of experience with large rockets so I would respect his suggestions about the BP charge weight and the use of a pilot chute on the NC.

Bob

Our flights will be between 1,500' and 5,000', more-or-less. The lower flights will be taking off from around 1,500' ASL and the higher flights will be taking off from around 4,600' ASL. It sounds like I definitely will need a vent hole or two, and possibly a couple more shear pins. I assume that it is better to use more shear pins rather than larger ones?

We are actually planning to use the drogue as a pilot chute, attaching it to the top loop of the main. The main will be bundled up inside the Nomex blanket with the drogue hanging out (hence the flame proof chute). When the cable-cutters release the main the idea is that the pilot chute will pull it out ASAP.

I found FFFFG BP at a local gun shop. It will be hard to find for sure. But I wouldn't use anything else.

As for 1900+lb of force, that's what it would take to blow 30+ shear pins. I think it's way too much.

you will have to figure out the amount of acceleration force you will have on with the nose cone to retain it on. then work from there.

then ground test it.. start small if you wish.. then work up.

the online calculators are good to get you in the right direction. temperature will affect burn rate of powder.. also how you have things pack..
so in your calculations, it's factoring an empty tube.. which isn't 100% true, with recovery 'stuffs' added..

so try it and see what happens.. but if it blows up, i didn't say it would work

and maybe 15 psi for a target is too much for what you need..

its a thought

I'm thinking it is. I really don't want to mess up our airframe, so I will definitely start out with something closer to 5 psi.

I will guarantee 15psi in this size rocket is to much . Start at 5psi!

That sounds reasonable.

 

[bobkrech]

(8) 4-40 shear screws will be good to 5,000 AGL which is +2.5 PSID without extra venting. (That's equal to ~300 pounds of force which ~ what it will take to shear 8 screws. You will want to develop twice that internal load with BP to make sure you have margin so +5 PSID would be generated with ~12 gram of BP which is ~1/3 of the original BP estimate in an empty volume and that's what most folks are recommending. You have some stuff in there so the actual pressure will be higher. Certainly if the 12 gram BP charge isn't good enough, 18 grams will be.

I think John is suggesting that you attach a drogue directly to the NC so that if the BP charge is not energetic enough to pull the NC completely away from the airframe or the NC slows down aerodynamically and does not stretch out the shock cord, the drogue will inflate and make sure the shock chord is fully extended.

Bob

 

[RocketFeller]

(8) 4-40 shear screws will be good to 5,000 AGL which is +2.5 PSID without extra venting. (That's equal to ~300 pounds of force which ~ what it will take to shear 8 screws. You will want to develop twice that internal load with BP to make sure you have margin so +5 PSID would be generated with ~12 gram of BP which is ~1/3 of the original BP estimate in an empty volume and that's what most folks are recommending. You have some stuff in there so the actual pressure will be higher. Certainly if the 12 gram BP charge isn't good enough, 18 grams will be.

I think John is suggesting that you attach a drogue directly to the NC so that if the BP charge is not energetic enough to pull the NC completely away from the airframe or the NC slows down aerodynamically and does not stretch out the shock cord, the drogue will inflate and make sure the shock chord is fully extended.

Bob

Thanks, Bob. I wonder if it is better to go with more shear pins, or vent the tube? Venting would be simple and would theoretically use less BP, but would you also lose ejection pressure measurably?

I did get what John meant, I have used drogues like that quite a bit with smaller rockets. The main chute on our rocket is attached directly to the NC, so as long as the nose separates the drogue should be in the airstream.

 

[bobkrech]

Thanks, Bob. I wonder if it is better to go with more shear pins, or vent the tube? Venting would be simple and would theoretically use less BP, but would you also lose ejection pressure measurably?

I did get what John meant, I have used drogues like that quite a bit with smaller rockets. The main chute on our rocket is attached directly to the NC, so as long as the nose separates the drogue should be in the airstream.

Venting will be either pressure neutral or a pressure loss, so you will not require any less BP. Regardless whether the chamber is vented or not, or you use shear screws or not, you will want to use ~12 grams of BP to generate ~5 psid to get the NC and chutes moving away from the airframe.

You do not waste any pressure when you shear the screws because the pressure is still going to push on the NC base until it clears the airframe. The trapped volume is 67 liters and a 1 cm^2 vent hole (1/2" diameter) has a conductance of 10 liters per second). A single 1/2" hole would have a time constant of Tc = 67/10 = 6.7 seconds and so you probably would need (2) 1/2" holes or (8) 1/4" holes to get the TC to ~ 3 seconds and I don't know how much extra BP you would need to add to compensate for the extra venting.

I think it's much simpler to use 12 grams of BP and (8) 4-40 screws and call it a day for flights to 5000' AGL.

Bob

 

[Bat-mite]

Just to be clear, pressure separation is separate and different from drag separation. A vent will reduce the likelihood of pressure separation (air inside airframe is more dense than air outside airframe), but will not eliminate drag separation (forward half of rocket continues forward at burnout while aft section starts to fall downward).

 

[bobkrech]

Absolutely correct. Internal pressurization separation and drag separation are 2 different issues, however drag separation also requires the sectional density of the forward section to be higher than that of the aft section.

For example dart rockets use drag separation to drop the booster from the dart. The dart is held in a conical taper joint by inertial forces. Upon motor burnout there are no net inertial forces holding the rocket together and separation occurs because the dart has a higher sectional density than the burned out booster which decelerates faster than the dart. The retained momentum of the high density dart is far greater than the low density booster, so the booster slows down very fast due to drag while the dart doesn't. (Typically darts have a sectional density of ~7 psi and the burned out booster has less than 2 psi.)

This is why in rockets with balsa or blow molded plastic nose you don't get drag separation if the NC is only friction fit. The sectional density of the aft portion of the rocket is much higher than the NC so when the power is cut, the NC would slow down faster than the aft part so the rocket will not separate due do drag, however if the rocket compartments are not vented and the apogee exceeds several thousand feet AGL, separation can occur, but it is due to internal pressurization pushing the NC out of the rocket, not aerodynamic drag slowing down the aft section.

Bob

 

[RocketFeller]

In our circumstance it looks like we have to deal with both drag and pressurization forces. Our nose is heavy and our booster is light, and we have a large internal volume. Looks like venting and lots of shear pins are in order...

Does it make a difference if the vent holes go out through the aft end rather than the body tube?

 

[Bat-mite]

In our circumstance it looks like we have to deal with both drag and pressurization forces. Our nose is heavy and our booster is light, and we have a large internal volume. Looks like venting and lots of shear pins are in order...

Does it make a difference if the vent holes go out through the aft end rather than the body tube?

Do you mean in the thrustplate? I would think that would work, provided you put vents in every CR so that there is an airflow from the forward end to the aft end.

 

[RocketFeller]

Yes, I was thinking of running a vent tube (thinking BT-20) through the two centering rings and thrust plate. There is about twelve inches between the forward CR (base of parachute compartment) and the thrust ring - I could just drill holes through both, but there is a camera going into the fin-can void area and I don't want to expose it to ejection gases.

I know aesthetics shouldn't trump function, but a bunch of holes (or a couple big ones) wouldn't look very nice, in my opinion...

 

[Bat-mite]

If aesthetics is your only concern, put the hole in the back. :wink:

 

[AlphaHybrids]

Based on experience with larger rockets (12"-16" diameter) I would use a Z-Pard aluminum piston type device. I was part of a scale Delta II and it used traditional ejection charges and shear pins. It was a bit finicky to ground test and feel confident during flight.

On another large 16" rocket we transitioned to using a dual Z-Pard system. We used just over 4 grams total (2 per device) for a 16" rocket that had 24" of open space and the nosecone weighed 35 pounds. We had very reliable and consistent separation of our 6 #4 shear pins and separation of the parts.

Edward