Ground testing basics

[Josh Nyegard]

Hello all. I am level 1 certified, somewhat experienced. I am curious about ground testing to determine the appropriate amount of ejection charge needed. The build I've got going is a 4" Madcow Phoenix Missile, almost complete. My concern is whether or not the standard ejection charge included would be sufficient for this given it may have around 14-16oz of nose weight added to it (I have a removable nose weight system). So my questions are as follows:

1. Should I need to ground test to determine the needed ejection charge, or is the standard amount good?
2. What materials are needed for ground testing? I know a launch controller, black powder. What kind of igniter? How do you hold the black powder?
3. What does the setup typically look like, how is it arranged? How do you run the wires, plug the motor hole, etc?

Any help that can be provided will be much appreciated!

Fyi: I am in Aiken, SC

 

[Hobie1dog]

read this article:
https://www.apogeerockets.com/education/downloads/Newsletter587.pdf

 

[Zeus-cat]

Ejection charge calculator:

https://rocketrycalculator.com/rocketry-calculator/bp-estimator/

 

[Josh Nyegard]

Alright. Thank you for the responses. I will take a look.

 

[Wrightme43]

read this article:
https://www.apogeerockets.com/education/downloads/Newsletter587.pdf

Thanks man, that helps alot!

 

[David Schwantz]

Plug MMT with old used motor. I use vials to hold my BP. Latex glove tips work. Just about anything that will hold it and then shatter or burst. I just use a battery to set them off. Assemble rocket as if it was going to fly, chutes, cords, Avbay. Install shear pins or tape on joints, which ever way you need to keep from separating. I run the ejection wires out a vent hole. I'll set rocket in a stand, horizontal. Set off drogue chute deployment first, if DD, then main. Otherwise, just set off main chute deployment. You want it to blow apart but not extend to end of shock cord and yank on the fin can. Just keep adjusting BP till you get that.

 

[Bob Yanecek]

Plug MMT with old used motor. I use vials to hold my BP. Latex glove tips work. Just about anything that will hold it and then shatter or burst. I just use a battery to set them off. Assemble rocket as if it was going to fly, chutes, cords, Avbay. Install shear pins or tape on joints, which ever way you need to keep from separating. I run the ejection wires out a vent hole. I'll set rocket in a stand, horizontal. Set off drogue chute deployment first, if DD, then main. Otherwise, just set off main chute deployment. You want it to blow apart but not extend to end of shock cord and yank on the fin can. Just keep adjusting BP till you get that.

Start on the low end and video the results.
This will help identify where any leaks are.

 

[jahall4]

Lots of good information here. This is my favorite way, no extremal wires, no launch controller, no hassle. You'll find that if you contain the BP to burn all (or almost all) the online calculators will be spot on. If more than a test or two is required w/o repeatable results something is wrong beware! BTW... this type of test is just to confirm the size of the charge. I separately component test the altimeter in a makeshift vacuum chamber.

 

[cwbullet]

Just a small tip. If you plan to do it on a launch field, ask when and where. I have had fliers just do it without asking. That might get you dismissed from the field.

 

[Steve Shannon]

1. Should I need to ground test to determine the needed ejection charge, or is the standard amount good?
2. What materials are needed for ground testing? I know a launch controller, black powder. What kind of igniter? How do you hold the black powder?
3. What does the setup typically look like, how is it arranged? How do you run the wires, plug the motor hole, etc?

1. Yes, determining whether the mass of the ejection charge is sufficient is the most common reason to ground test.
2. a. Launch controller is nice, but a battery with long zip cord and alligator clips works fine. You don’t need something fancy, but you do want a little distance between you and the rocket and definitely don’t point the nosecone towards something you like. And don’t lay the rocket on something that is flammable. I’ve seen many people burn holes in sleeping bags trying to protect their paint jobs.
2. b. Use an electric match. They’re low current and made especially for setting off blackpowder fireworks.
3. Put a motor case or spent motor in the motor mount. Run the electric match leads through the motor and out the nozzle end, over to the battery. I just lay the rocket on the green grass or over a sawhorse. I know people who hang them on the clothesline. Point the nosecone in a safe direction, especially since yours is weighted. Remember to hook up the shock cord and include the parachute. The nosecone must have enough inertia to yank the folded parachute out into the air!!!
Expect the rocket body to recoil, especially since your nosecone is weighted.

Good luck!

 

[NateB]

This is how I do it. Rocket is braced and pointed a safe direction. As mentioned by others, there is a motor case in the mount. I pull the match wire out of the end of the rocket or a vent hole, whichever is most convenient. A few altimeters have ground testing modes which is nice too.

 

[David Schwantz]

During flight the rear of the rocket is not supported against recoil of the charge
. This is why I lay mine horizontal in a stand.

 

[AcadiaRockets]

View attachment 1713196260317.mov
I did some ground testing on my Extreme darkstar. I had intermittent success with the laundry exiting the payload tube.
I tested 4 times with same amount of powder (3g) but on two tests the chute "burrito" folded itself in half an jammed in the end of the tube.I ran out of time to experiment further but the theory right now is my "burrito" was too long and thin (not really THAT log and thin - it looked like a burrito) that the bottom part of the burrito comes up alongside the top part and was very tight. I wish I had taken a picture of the folded in half burrito plugging the tube.
So given some more testing - I am going to (and this seems counterintuitive) pack my "burrito" shorter, wider, and a bit tighter in the tube thinking it will be less likely to fold itself and act more like cannon wadding. Have others seen this?

 

[David Schwantz]

IMHO, when ground testing rocket should as if it was being flown, not supported by a backstop to limit recoil. There is no backstop in the sky. Also since body moved forward when NC ejected, looks as if your chute got stuck on something in the tube.

 

[rcktnut]

Nice looking rocket you have there. I would try shortening up the distance between your chute and nose cone. I don't go by the 1/3 the length of your shock cord recommendation. I attach all my chutes about 18 in. from the nose cone.