Deployment charge testing

[jpoehlman]

We our first ground test of a deployment charge today. Test was in a fiberglass 3" ISQY Tomahawk by Rocketry Warehouse. Kit was purchased prior to the move to Mad Cow. Test was performed remotely via the Wi-Fi interface of a EggTimer Quantum. Test was a 2g fff charge inside a glove finger and wrapped with a few wraps of duct tape and 2x 2-54 nylon screws holding the NC.


Freeze frame from a SLO-Mo iPhone video.

Interesting to see the charge burn glowing thru the white fiberglass.

YouTube:
https://youtu.be/lNXAXADS7ME

(Could figure out how to embed the video in the post - Input welcome)

The charge blow the NC to the end of the 25' one bad hawk harness, but appeared to just get tight and only slightly pulling the rocket. Curious to hear opinions on the if this is just right, or a bit too much fff powder.

Thanks,

Jack

 

[Exactimator]

The is one of those questions where you'll get a range of answers.

For my experience, I aim for what you describe. A full stretch of the harness with a bit of a tug on the rocket at the end. If it barely eeks out the laundry, I think that's asking for trouble. If it yanks the rocket, tips over the test stand and digs a short trench with the body tube, that's too much. Just a nice tug to nudge the rocket is what I shoot for.

FWIW, I have a 3" Rocketry Warehouse Reaper 3 and use 1.9g of ffffg with three 2-56 shear pins on the main and PVC cap charge wells. Multiple flights with no issues.

 

[Handeman]

Just realize that your rocket body dug into the ground so all the momentum it had was stopped before the nose cone hit the end. In the air there will be no ground to stop the rocket so the momentum of the nose cone will be added to the rockets momentum and the shock on the cord will be higher then what you got on the ground.

I would recommend doing the test again and laying the rocket along the yellow top of the bench and let the rocket and nose cone fly apart. That should give you a much better simulation of what will happen in the air. I think you might be able to reduce the charge slightly.

BTW, if the rocket can't take dropping to the ground in a test like that, it probably won't hold up well on landing either. Of course the paint won't look as pretty on it's first flight, but.....

 

[Nathan]

. . .That's why I do my ejection ground tests before I start painting.

 

[Nathan]

But to answer the OP's question, in my opinion it sounds like your charge might have been a little too energetic. I would shoot for the nose cone going about 15 ft. You also want to be sure that the force of the charge is blowing out the chute, and not just the momentum of the nose cone pulling it out.

 

[Exactimator]

Just watched the video. Where are your main chute and nomex? Did I not see them or did you test without?

I recommend testing with the entire set up like you're going to fly it. Maybe substituting something for the chute so you don't accidentally damage it during testing.

 

[amarillo_rocket]

The main thing I look for is the parachute is out of the main tube by roughly 2/3 the lenght of my shock cord. In your case if your nosecone goes around 15ft-16ft (assuming your parachute is attached about 8ft from your nosecone or less) then you are more than safe in the air. In the air their is nothing the stop the nosecone such as hitting the ground and nothing to stop the remaining airframe from moving backwards so in the air ejection is more energetic than ground testing.

 

[jpoehlman]

Just watched the video. Where are your main chute and nomex? Did I not see them or did you test without?

I recommend testing with the entire set up like you're going to fly it. Maybe substituting something for the chute so you don't accidentally damage it during testing.

This test was without the chute or nomex, as it was just a first run. Since my AV bay is in the nose cone, the apogee charge will be to separate and pull out the laundry with a small drogue on the Nose cone. The main will be deployed via a cable cutter. I'll have dual altimeters and dual charges for main and apogee.

My one remaining challenge is to determine the best method to wire the e-matches on the cable cutters. I plan for the main to be on the loop 3' from the nose cone where the terminal blocks from the AV are located on the bulkhead. My best idea so far is to attach a length of stranded Cat5 wire to the harness, securing the bulkhead end to the rocket nut so there is no flex on going to the terminal block. Then solder the e-match wires to the Cat5.

This all needs to be fully tested in my next test!

Thanks for the input,

Jack

 

[jpoehlman]

Just realize that your rocket body dug into the ground so all the momentum it had was stopped before the nose cone hit the end. In the air there will be no ground to stop the rocket so the momentum of the nose cone will be added to the rockets momentum and the shock on the cord will be higher then what you got on the ground.

I would recommend doing the test again and laying the rocket along the yellow top of the bench and let the rocket and nose cone fly apart. That should give you a much better simulation of what will happen in the air. I think you might be able to reduce the charge slightly.

BTW, if the rocket can't take dropping to the ground in a test like that, it probably won't hold up well on landing either. Of course the paint won't look as pretty on it's first flight, but.....

I noticed the fins digging in as well. Given that this is a fiberglass rocket and nose cone with a metal tip, there is a fair amount of mass for each component, roughly 7 lbs total. I'd be concerned that if the body tube and fin section where not secured, that they would end up being propelled a significant distance as well. Effectively, if the mass ratio is 2/3 in the body and 1/3 in the nosecone and I have a 20' harness, the body would go 6.6' and the nose cone 13.3'. Agreed, this would be an optimal test and closer to what will happen in the air, but that's a lot of uncontrolled parts moving around. I don't have the space in the back yard to do this with a reasonable safety margin. No fear that the rocket would not be able to handle the event though.

On the next test, I may try with the rocket suspended or in some other way retained but allowed some movement.

Thanks for the input.

Jack

 

[Bat-mite]

Sorry if this was already covered. Did you test with the motor in? If not, lots of your pressure escaped through the motor hole. Always test with the rocket in RTF condition. If you don't want to build the motor, that's okay. Shove some moist rags or towels into the motor casing and use that.

 

[jpoehlman]

Sorry if this was already covered. Did you test with the motor in? If not, lots of your pressure escaped through the motor hole. Always test with the rocket in RTF condition. If you don't want to build the motor, that's okay. Shove some moist rags or towels into the motor casing and use that.

Good point! I used a motor with a sealed forward closure. One of the motors I'm considering is an AeroTech I65P, just not sure about the thrust to weight ratio after the initial thrust. Any thoughts on sustained thrust near 1:1?

 

[Bat-mite]

Good point! I used a motor with a sealed forward closure. One of the motors I'm considering is an AeroTech I65P, just not sure about the thrust to weight ratio after the initial thrust. Any thoughts on sustained thrust near 1:1?

Sealed closure works. Just need to stop up the hole.

Thrust:weight only matters until the rocket is moving sufficiently fast to be aerodynamically stable when the rocket gets free of the pad. As soon as propellant starts burning, your CG starts moving forward anyway, and your stability increases. What you really care about is speed off the pad. How fast does the sim say you are going just off the pad?

 

[jpoehlman]

How fast does the sim say you are going just off the pad?

I just updated my Open Rocket SIM with actual weights - Total 3710g / 8.2 lbs with motor
I65-P: 6' rail - 27.8 mph
I65-P: 8' rail - 31.1 mph

J425-14A: 6' rail - 44.1 mph

This seems to be reasonable I think.

Jack

 

[jpoehlman]

I completed another test this evening, this time included a chute simulator (towel wrapped with chute protector).

Charge was reduced to 1.5g of FFFg powered and appears to have just about the right energy to me:

[YOUTUBE]xXxwcJ1xEXs[/YOUTUBE]

While I was at it, I tested the Archtype now Prairie Twister cable cutter system:

[YOUTUBE]BDmSdU7lg7Q[/YOUTUBE]

It did exactly what it was suppose to do.

I'm thinking my fully dressed flight configuration should be:

Drogue / Apogee: 1.5g primary 2.0g secondary
main: 2x cable cutters with recommend .1 ml charge

My local club moved the high power launch schedule around so now the next DARS HP launch is this coming weekend. Looks like the Tomahawk will should be ready to fly.... now can I get anything else ready with Electronically controlled deployment?

Jack

 

[ksaves2]

EggTimer TRS right? I think Archtype is no longer available and Prairietwister Rocketry took it over. Kurt

 

[jpoehlman]

EggTimer TRS right? I think Archtype is no longer available and Prairietwister Rocketry took it over. Kurt

How'd you guess on the EggTimer TRS? The tones in the cable cutter test?

The deployment test was actually done with a Quantum using the Test interface. The 10 second countdown is great!

Yes, Archtype is no more and now can be had from Prairie Twister Rocketry along with Rocket Nuts (which I'm also using on the Tomahawk): https://ptrocketry.blogspot.com/p/cable-cutter.html

Flight computers will probably be the Eggtimer TRS and a Perfect Flight SL100 each powered via a Eggtimer Wi-Fi switch

Jack