Amount of black powder for ejection charge.

[DizzyUnicorn]

Hello, building a 4in Cherokee 98 from Wildman and want to ground test my ejection charges. First time using ejection charges, want to do it right.

Using https://rocketrycalculator.com/rocketry-calculator/bp-estimator/ to estimate BP required - with 2x 4/40 nylon shear pins.

Empty, the payload bay is roughly 10 inches between the bulkheads - so at 4in diameter, 10in length, 10psi and 2x 4/40 the suggested BP charge = .61grams.

When I stuff the bay with my harness, chute and chute protector, the entire cavity is basically filled (tightly) and the chute protector sits just on top of the ejection wells (Firebird3g) - so I'm wondering if I need to reduce the weight of the BP charge, given it's not an empty space, full (and densely so). This is a fiberglass airframe and I certainly don't want to blow a hole in the frame, maybe that's dumb and it doesn't work that way, but I wanted to ask and see what folks generally do about this. Like what is the appropriate adjustment factor, if that's even necessary. I would note, the rolled chute covers the vent holes from the inside - given the amount of space the recovery components occupy, I don't see a way to clear the vent holes....

Thanks!

 

[Upsrocketman]

In my 4 inch Iris, I run 1.5 grams on the drogue side and 2 grams on the main side. I would start small and work your way up. 61 grams of BP will likely blow the nosecone off and blow the rest of the rocket apart with it.

 

[DragonRocketry]

In my 4 inch Iris, I run 1.5 grams on the drogue side and 2 grams on the main side. I would start small and work your way up. 61 grams of BP will likely blow the nosecone off and blow the rest of the rocket apart with it.

I think that was supposed to be .61 grams. That is close to what VCT calculators says.

 

[jmasterj]

I'm confused. It looks like the recommendation in that calculator is .61g with the measurements you gave. That sounds way low with 2 4-40 pins, while 61g is obviously ludicrous.

 

[Rocketclar]

Yes, 61 g is much too high. I use 2 to 3 g of 4F BP for most of my 4" diameter rockets. All have been ground tested. I run calcs first and then test with my wife videotaping it. I've had to tweak the amount a number of times. I do weigh my BP out on a jeweler's balance. Also, I keep an Excel sheet with calcs and the test results so that the next time, I can prep the rocket based on known results.

 

[ep29030]

I ran a calculation on the website for 10 inch, 3.9 in diameter tube. At 10psi, it suggested 0.61 grams and 3 - #2 shear pins. It requires about 25 lbs of force to shear a #2 nylon screw. That charge generates slightly over 100 lbs of force on the bulkhead. Just based on experience I would have guessed about 1 gram of BP for your ejection charge. And as suggested, load up the rocket and do a ground test to verify that your charge will work. It is simple to do and gives you immediate feedback.

 

[DragonRocketry]

My 4" for the main are 1.5 gram main charge and 2 gram back up.

 

[DizzyUnicorn]

Yes, sorry, .61grams (not 61). My bad. Seems like I can start with that as a baseline and shouldn't worry about blowing up the airframe, even stuffed with all the recovery gear.

 

[waltr]

I use 2.0 gram for main in a 4" LOC cardboard tube and only 1 #2 shear pin.

Take above advice and ground test, then add a little more.

 

[cbrarick]

wildman wildthing. 4 inch DD rocket. 2.6 grams everywhere. I use 3 small sheerpins

 

[Donnager]

wildman wildthing. 4 inch DD rocket. 2.6 grams everywhere. I use 3 small sheerpins

Pretty much the same as all my DD 4" stuff that is around 6-7' long. I may tweak it a bit and put 2.6 on the bottom and 2.2 on the top, but it depends on the amount of junk in the rocket, and the void space.

I tend to err on the heavy side for charge sizes for the fiberglass stuff.

 

[cwbullet]

If I use a calculator, I always, ALWAYS ground test.

 

[OverTheTop]

I use
length x diameter x diameter x 0.006 = grams of BP
Length and diameter in inches. Treat the volume as empty.

I use that as a starting point, then ground test. Backup charge is around 50% bigger.

 

[Philip Tiberius D.]

I think it was Wernher Von Braun who said “Blow it apart or blow it up.”

 

[JoePfeiffer]

I always treat the volume as empty, and I can't remember one where I didn't end up needing more than calculated.

And writing 0.61 is substantially clearer than .61

 

[DizzyUnicorn]

Thanks for your comment re: treating volume as empty.

And saying "I've always needed more than calculated" is substantially clearer than "I can't remember one where I didn't end up needing more than calculated."

 

[Dave S.]

Joe - Considering the volume as empty is worst case in a way. In reality, you have recovery parts taking up some of that volume and boosting the pressure up somewhat.

The fact that you have to add even more BP than the calculator says to makes me think either the pressure is being under-calculated, or you just typically need more pressure than initially thought to get the ejection to occur correctly. The weight of the components being pushed apart is going to vary a lot from rocket to rocket too, so that pressure you are trying to get to is going to vary a lot too.

I have never done dual deploy, but I would think this all means to just ground test and ground test again first to make sure you are going to have things work right in an actual launch.

 

[b.wieting]

I think you're at the right starting point with the calculator. Ground test, maybe you'll need more 4F to get "separation with authority".
Calculate but verify works well. E.g. My 4" Polecat V2 separates nicely with ~ 0.9g 4F tho the "laundry" pretty much fills the volume.

 

[JoePfeiffer]

Thanks for your comment re: treating volume as empty.

And saying "I've always needed more than calculated" is substantially clearer than "I can't remember one where I didn't end up needing more than calculated."

"I can't remember one..." is, however, more accurate than "I've always needed more than calculated".

 

[DizzyUnicorn]

"I can't remember one..." is, however, more accurate than "I've always needed more than calculated".

 

[JoePfeiffer]

Joe - Considering the volume as empty is worst case in a way. In reality, you have recovery parts taking up some of that volume and boosting the pressure up somewhat.

Exactly. I'm deliberately taking that worst case.

The fact that you have to add even more BP than the calculator says to makes me think either the pressure is being under-calculated, or you just typically need more pressure than initially thought to get the ejection to occur correctly. The weight of the components being pushed apart is going to vary a lot from rocket to rocket too, so that pressure you are trying to get to is going to vary a lot too.

The calculated amount does always seem to break the shear pins, which is what the calculators are all based on. That doesn't seem to be enough to actually eject the laundry, however, which is what I'm looking for.

I have never done dual deploy, but I would think this all means to just ground test and ground test again first to make sure you are going to have things work right in an actual launch.

And this is the real takeaway: the calculator is a starting point, and (importantly) it's a starting point that, based on my experience, won't cause damage. But keep ground testing until you've got enough to eject with "authority".

 

[Troy3003]

I have got to a point where I just round up to the nearest whole number once I get it to a suitable separation charge. Say if it takes 2.7 grams I automatically just round up to 3 grams. Most of my rockets are fiberglass so I don’t feel nervous with a slightly higher charge.

 

[crazyed]

Some backyard ejection charge testing goes a long way to figuring how much you need for the particular rocket after you have your best estimate.
Blow it up or blow it out and use quality parts / kits.

 

[jsdemar]

One factor usually missing in the BP calculators is the combustion efficiency. Even at low altitudes, some containment is needed to get better heat transfer to all the BP particles. At higher altitudes (>10Kft MSL), significant containment is required.

There are many approaches people have used: several layers of electrical tape over the charge well; vinyl tubing or surgical tubing with ziptied or hot-glued ends; a small "cannon" made from copper pipe with epoxy cap. Positioning the ematch on top of the BP instead of buried within the BP seems to help as well.

As many have mentioned, ground testing and extra BP are good practices. For high altitude, take a look at this article by Jim Jarvis:
https://cdn.imagearchive.com/rocket...high-altitude-deployment-charges-May-2013.pdf

 

[SDramstad]

There is a large difference in charge between blow it out and blow it up. When I first started doing dual deploy I ground tested until I found the smallest charge that worked. Thought that was the thing to do but..... I started losing rockets when the charge did not eject the chute. It finally dawned on me that the charges I was using worked under perfect conditions. Unfortunately, we dont fly under perfect conditions. Couplers get dirty and things jam up. I started upping my charges a bit and my failure rate dropped to almost nothing. So my rule of thumb is this. If the calculator calls for .61 then I use 1 gram for the primary charge and 1.5 for the backup charge. Dont get hung up on trying to measure charges out to 2 decimal points. It is not that exact. The other variables that come into play will make that kind of accuracy a complete waste of time. Temperature and humidity can make a big difference in what it takes to separate your rocket not to mention the altitude it goes to and the acceleration of the rocket jamming things together. So my recommendation is to use the calculator as a starting point and then up it by maybe 25 to 50 percent for a safety factor. And round it to either a full gram or at a quarter gram increment. So 1/4,1/2,3/4, or 1gram. Yes, way too much is a bad thing. If you need 1 gram in a rocket and then load it with 5 grams instead it will probably cause problems. Most likely your nose cone will part company with your shock cord. But I have never seen a fiberglass body tube split from an ejection charge.

 

[crazyed]

The smallest change isn't small when measuring BP. But if you are worried about the smallest measurement causing failure then you might want better components..
Ground testing is the only way to know for sure on any particular rocket. The calc gets you close but everything about the rocket build and loading of recovery devices etc changes your calculation.
So ground test
And don't put 5 grams of BP in your charge can when you only need 1. Lol

 

[DizzyUnicorn]

There is a large difference in charge between blow it out and blow it up. When I first started doing dual deploy I ground tested until I found the smallest charge that worked. Thought that was the thing to do but..... I started losing rockets when the charge did not eject the chute. It finally dawned on me that the charges I was using worked under perfect conditions. Unfortunately, we dont fly under perfect conditions. Couplers get dirty and things jam up. I started upping my charges a bit and my failure rate dropped to almost nothing. So my rule of thumb is this. If the calculator calls for .61 then I use 1 gram for the primary charge and 1.5 for the backup charge. Dont get hung up on trying to measure charges out to 2 decimal points. It is not that exact. The other variables that come into play will make that kind of accuracy a complete waste of time. Temperature and humidity can make a big difference in what it takes to separate your rocket not to mention the altitude it goes to and the acceleration of the rocket jamming things together. So my recommendation is to use the calculator as a starting point and then up it by maybe 25 to 50 percent for a safety factor. And round it to either a full gram or at a quarter gram increment. So 1/4,1/2,3/4, or 1gram. Yes, way too much is a bad thing. If you need 1 gram in a rocket and then load it with 5 grams instead it will probably cause problems. Most likely your nose cone will part company with your shock cord. But I have never seen a fiberglass body tube split from an ejection charge.

This is great, thank you

 

[crazyed]

Of all the single use motors for sale . I haven't seen a one that indicates how much BP is contained in the deployment charge. Why ? Not needed . In a single use motor the same amount of BP is used in a 18 mm rocket as in an 1.5 inch rocket same length.
The amount of BP could be adjusted but no need , it's the integrity of the parts that keep it flying and coming down safely.
And you won't " blow it up" unless you built it wrong.
Sure you can calculate how much BP you should or could use , but you won't know until you ground test your build.

 

[Donnager]

Joe - Considering the volume as empty is worst case in a way. In reality, you have recovery parts taking up some of that volume and boosting the pressure up somewhat.

The fact that you have to add even more BP than the calculator says to makes me think either the pressure is being under-calculated, or you just typically need more pressure than initially thought to get the ejection to occur correctly. The weight of the components being pushed apart is going to vary a lot from rocket to rocket too, so that pressure you are trying to get to is going to vary a lot too.

I have never done dual deploy, but I would think this all means to just ground test and ground test again first to make sure you are going to have things work right in an actual launch.

There is some charge pressure reduction, simply by the loose mass of recovery gear, and it does influence the ejection. The loose mass and dog barf can absorb heat, and how the laundry and the powder is packed may contribute. The ground test shows if/how your methods work.

I have a few of these 4" DD rockets. Can 1.7g break them? probably. Will 2.2 do it reliably? in most cases, Yes (according to ground testing and actual use). A lot depends on the tightness of the assembly, the gear packing, and how much friction there is aside from the shear pins.

I've crashed a 54mm body that used 1.5g on the drogue side. Just puffed smoke, and did not separate. This was primarily because of how tightly I packed the chute, as best I can tell, but it had very little free space. I only mention this because the calculator called for 0.6g. I had previously tested 1.4 and it worked admirably. I've since flown that one with a 1.7g drogue charge and better packing and had no issues.