Did my first ground test -- Now what?

[4regt4]

First, a few specifics: 3" Leviathan clone, body tube reinforced with couplers. Estes plastic nose cone. 23oz weight.

Shock cord: 4ft of 550# Kevlar, followed by 3 ft of 1/2" elastic, followed by 7 ft of 550# Kevlar. Parachute attached to loop in Kevlar about 2 ft from the nose cone. ** Important: Using the same idea as what North Coast Rocketry does, I am using 6 ft of Kevlar in parallel to the elastic. I braid this Kevlar into 3 bundles, each also bound with some masking tape.

Using one of the online 4f calculators for 20" of 3" tube, it appeared that I needed somewhere in the neighborhood of 0.75 to 1 gram of powder. So I made 2 charges up with those values. For the purpose of the test, I bound up the 30" chute with a JLCR.

The 0.75g charge resulted in the nose cone and chute separating in a bit of a lazy manner, to the full length of the shock cord. However, the taped and braided sections of the shock cord (the section in parallel with the elastic) did not extend, except for perhaps 1/3 of the forward bundle. This indicates that the elastic stretched only a very minimal amount.

The 1.0g charge was much more energetic. It had a "shot out of a cannon" look to it. The taped and braided sections all extended, except for a tiny portion of one of the bundles. So the elastic stretched from it's "relaxed" 3 ft to nearly 6 ft. In the first 0.75g test, the rocket body didn't move. In the 1.0g test, the body tube "followed" the nose cone for about 9".

So.... What charge should I use? As I'm using motor eject, I think I should plan for events where the ejection isn't right at apogee. (I had a delay on an F39 that was 3.5 seconds too long once...) I'm guessing that more energy would be required if the rocket needs to separate while going some fair speed. So I'm inclined to use 1.0g, or something close to it, like maybe 0.9g.

Thoughts?

Hans.

 

[SolarYellow]

I'd probably do about 0.9 grams.

If you get an ammunition reloading scale, you can measure in tenths of a grain. There are about 15.3 grains to a gram, off the top of my head.

Another aspect to consider is the nose cone itself. There's a Doorknob build thread here where someone modifies the same nose cone you have to eliminate the wedge, making it full-diameter all the way down. This should lead to much stronger ejection, as the pressure is more or less contained until the full-diameter shoulder clears the BT and "uncorks" the pressurized tube. I have a few Estes 3-inch cones inbound and will be doing that to all of them. You may get the more authoritative ejection you want with a smaller powder charge.

 

[Worsaer]

I’ve had more than one example where multiple ground tests were successful, and I used the exact same charge and was unsuccessful in flight. Even though prepped identically for flight and testing, flight dynamics are different than a ground test. I’ve taken others advice to add 20% to 25% to the charge to compensate.

 

[David Schwantz]

first thing I would do is get rid of the elastic. It is stretching and limiting your deployment. You do not blow out the chute, the NC pulls it out after it is blown out of the rocket. Also, the elastic can cause your NC to bounce back into the airframe, damaging it or tangling the cords.

 

[4regt4]

I'd probably do about 0.9 grams.

If you get an ammunition reloading scale, you can measure in tenths of a grain. There are about 15.3 grains to a gram, off the top of my head.

Another aspect to consider is the nose cone itself. There's a Doorknob build thread here where someone modifies the same nose cone you have to eliminate the wedge, making it full-diameter all the way down. This should lead to much stronger ejection, as the pressure is more or less contained until the full-diameter shoulder clears the BT and "uncorks" the pressurized tube. I have a few Estes 3-inch cones inbound and will be doing that to all of them. You may get the more authoritative ejection you want with a smaller powder charge.

I'm using a milligram scale, resolution to 1 mg. It's kind of hard to dispense the powder to better than 0.01 accuracy, though. I either under or overshoot by a tiny amount.

The nose cone has been modified to eliminate the shoulder. It was cut out, a bit of coupler tube inserted, and then some more cardboard laid in to create a smooth exterior surface. A plywood bulkhead was put in for shock cord attachment, and a place to put a Firefly.

Hans,

 

[4regt4]

I’ve had more than one example where multiple ground tests were successful, and I used the exact same charge and was unsuccessful in flight. Even though prepped identically for flight and testing, flight dynamics are different than a ground test. I’ve taken others advice to add 20% to 25% to the charge to compensate.

That's kind of what I was wondering. So perhaps even the 1.0g level may be borderline.

A bit off my original topic, but I've had one lawn dart (Estes Executioner) and one ejection that was barely adequate (AT Cheetah) on 24mm E20 motors. Those supposedly have 0.7g in them.

Hans,

 

[4regt4]

first thing I would do is get rid of the elastic. It is stretching and limiting your deployment. You do not blow out the chute, the NC pulls it out after it is blown out of the rocket. Also, the elastic can cause your NC to bounce back into the airframe, damaging it or tangling the cords.

I've had extremely good luck with the elastic. If you read my configuration, the elastic is only deployed *after* both the nose cone and chute have left the airframe. And I think my overall shock cord is long enough that I'm not worried about a snap-back. I have good quality video of several flights where I could see the entire ejection event, and there were no issues like that.

In the case of this ground test, the nose cone did not pull out the chute. The chute followed the nose cone out of the tube with slack in the cord in between. I got good video of that. It wasn't until both were clear of the body by about 5 ft before the cord between was pulled taught.

Hans.

 

[waltr]

I’ve had more than one example where multiple ground tests were successful, and I used the exact same charge and was unsuccessful in flight. Even though prepped identically for flight and testing, flight dynamics are different than a ground test. I’ve taken others advice to add 20% to 25% to the charge to compensate.

Same here.20" tube sounds
Thought is that in the air dynamics are different than ground testing.
I now ensure the charge is large enough for what ever happens in the air.

3" x 20" tube may need over 1gm. Maybe 1.2gm.

 

[boatgeek]

Do you have redundant altimeters? If so, I would probably use the .75g as the primary and 1g as a backup.

 

[4regt4]

Do you have redundant altimeters? If so, I would probably use the .75g as the primary and 1g as a backup.

No, it'll be motor ejection. But I'm going to do a baby test flight (maybe 350 ft) tomorrow. It should be marginally stable on an E28-4 at 41fps off the rod. Based on what I'm hearing, looks like I'll go with 1.1g to 1.2g. And cross my fingers...

Thanks all,
Hans.

 

[John Taylor]

First, a few specifics: 3" Leviathan clone, body tube reinforced with couplers. Estes plastic nose cone. 23oz weight.

Shock cord: 4ft of 550# Kevlar, followed by 3 ft of 1/2" elastic, followed by 7 ft of 550# Kevlar. Parachute attached to loop in Kevlar about 2 ft from the nose cone. ** Important: Using the same idea as what North Coast Rocketry does, I am using 6 ft of Kevlar in parallel to the elastic. I braid this Kevlar into 3 bundles, each also bound with some masking tape.

Using one of the online 4f calculators for 20" of 3" tube, it appeared that I needed somewhere in the neighborhood of 0.75 to 1 gram of powder. So I made 2 charges up with those values. For the purpose of the test, I bound up the 30" chute with a JLCR.

The 0.75g charge resulted in the nose cone and chute separating in a bit of a lazy manner, to the full length of the shock cord. However, the taped and braided sections of the shock cord (the section in parallel with the elastic) did not extend, except for perhaps 1/3 of the forward bundle. This indicates that the elastic stretched only a very minimal amount.

The 1.0g charge was much more energetic. It had a "shot out of a cannon" look to it. The taped and braided sections all extended, except for a tiny portion of one of the bundles. So the elastic stretched from it's "relaxed" 3 ft to nearly 6 ft. In the first 0.75g test, the rocket body didn't move. In the 1.0g test, the body tube "followed" the nose cone for about 9".

So.... What charge should I use? As I'm using motor eject, I think I should plan for events where the ejection isn't right at apogee. (I had a delay on an F39 that was 3.5 seconds too long once...) I'm guessing that more energy would be required if the rocket needs to separate while going some fair speed. So I'm inclined to use 1.0g, or something close to it, like maybe 0.9g.

Thoughts?

Hans.

I don't like your shock cord idea. Only use Kevlar 4-5 times the length of the rocket.
Tie the cordage into little bundles along the length of the shock cord. The length will dissipate the energy before it reaches the end of the cord.