Motor ejection gasses and the Aerotech shock cord

[highflyer1968]

Just looking at my GForce rocket from Aerotech that I built a couple of years ago and I haven't flown it yet. I'm looking at it and I built it without the cooling mesh but did put the baffle in and used a chute protector. I am wondering if the elastic shock cord can take the heat from the hot ejection gases? It is attached to the baffle so that may be a point where it could fail. what are your thoughts about the shock cord?

 

[Handeman]

A piece of Kevlar shock cord would be better, at least to just above the top of the BT. I watched a G-Force shock cord break at ejection. The nose cone drifted off under the chute and the rest of the rocket went through the windshield of a pickup. Made a nice 4" diameter imprint in the dash.

I know some used a long drill to put a hole through CRs. Then a smaller stiff wire to pull the Kevlar through to the bottom and knot the Kevlar to keep it from pulling back out. This also allows replacement later if needed.

Just don't skimp on the BP in your ejection charges. I've seen more G-Forces than any other rocket just pop the nosecone and never eject the chute and then end up tilting down and lawndarting. I know the G-Force was designed for HobbyLine 29/40-120 G motors, but it always seemed to me that the 0.75g of BP in the reloads was marginal for the BT volume of the G-Force.

Good luck with the G-Force. That should still be a Class 1 rocket so could be a park flyer on those Hobbyline Gs. Now that all the soccer fields are empty with the stay at home orders, there's even more places to fly Class 1 rockets.

 

[highflyer1968]

A piece of Kevlar shock cord would be better, at least to just above the top of the BT. I watched a G-Force shock cord break at ejection. The nose cone drifted off under the chute and the rest of the rocket went through the windshield of a pickup. Made a nice 4" diameter imprint in the dash.

I know some used a long drill to put a hole through CRs. Then a smaller stiff wire to pull the Kevlar through to the bottom and knot the Kevlar to keep it from pulling back out. This also allows replacement later if needed.

Just don't skimp on the BP in your ejection charges. I've seen more G-Forces than any other rocket just pop the nosecone and never eject the chute and then end up tilting down and lawndarting. I know the G-Force was designed for HobbyLine 29/40-120 G motors, but it always seemed to me that the 0.75g of BP in the reloads was marginal for the BT volume of the G-Force.

Good luck with the G-Force. That should still be a Class 1 rocket so could be a park flyer on those Hobbyline Gs. Now that all the soccer fields are empty with the stay at home orders, there's even more places to fly Class 1 rockets.

So what do you think about using Kevlar for the entire shock cord? Is it as heavy as tubular nylon? I would need about 15 feet.

 

[JackC]

If it were a fiberglass tube, I would recommend using Kevlar for the entire shock cord. Although I've seen zippers on fiberglass tubes from Kevlar, they're never as bad as a zipper on cardboard tubing from Kevlar. I would suggest making a portion of the cord that never touches the lip of the tube as Kevlar and the rest should be tubular nylon. Tubular nylon has more elasticity than Kevlar and would be safer for your G-Force tube.

 

[highflyer1968]

A piece of Kevlar shock cord would be better, at least to just above the top of the BT. I watched a G-Force shock cord break at ejection. The nose cone drifted off under the chute and the rest of the rocket went through the windshield of a pickup. Made a nice 4" diameter imprint in the dash.

I know some used a long drill to put a hole through CRs. Then a smaller stiff wire to pull the Kevlar through to the bottom and knot the Kevlar to keep it from pulling back out. This also allows replacement later if needed.

Just don't skimp on the BP in your ejection charges. I've seen more G-Forces than any other rocket just pop the nosecone and never eject the chute and then end up tilting down and lawndarting. I know the G-Force was designed for HobbyLine 29/40-120 G motors, but it always seemed to me that the 0.75g of BP in the reloads was marginal for the BT volume of the G-Force.

Good luck with the G-Force. That should still be a Class 1 rocket so could be a park flyer on those Hobbyline Gs. Now that all the soccer fields are empty with the stay at home orders, there's even more places to fly Class 1 rockets.

I happen to have 18 ft of tubular nylon so I will use that and get a short length of Kevlar. I will only need maybe 4 inches if that.

 

[DaHabes]

I use Kevlar for everything these days, even LPR. I buy in bulk in different sizes to keep the cost reasonable. I coat the edge of the BT with CA to stiffen it and do successive layers of heat shrink plastic on the shock cord at the body tube junction to avoid zippers. Easy, peasy. Then Dog barf or chute protector depending on BT diameter.

 

[mikec]

FWIW, I've got two Aerotech rockets built stock with the cooling mesh and the elastic, and have yet to have a problem on either after years and dozens of flights. Many people are worried about the mesh, the elastic, the baffle on rockets that have one, and the piston on rockets that have one, but I think they are all generally workable.

That said, make whatever changes you want, they all have pluses and minuses but are generally fine too. My own experience is that kevlar has its own problems (lots more shock, zippering, etc.) and that elastic for smaller rockets has unfairly gotten a bad reputation in some cases (though snapback is always an issue and vendors often don't give you enough.)

 

[highflyer1968]

I use Kevlar for everything these days, even LPR. I buy in bulk in different sizes to keep the cost reasonable. I coat the edge of the BT with CA to stiffen it and do successive layers of heat shrink plastic on the shock cord at the body tube junction to avoid zippers. Easy, peasy. Then Dog barf or chute protector depending on BT diameter.

What size of Kevlar would I need? I can get .026" with a strength of 140 lbs, .045" strength of 225 lbs or .1" strength of 1500 lbs

 

[Handeman]

If you are going with kevlar, I would use the 1/8" tubular Kevlar. It's plenty strong, and easy to use. I use it as main and drogue cords on several L1 rockets.

 

[Initiator001]

If you build AeroTech kits per the instructions you should not have any problems with flying them.

The plastic used for the baffle is not heat resistant and some testing that was done at AeroTech with the baffle system not having the mesh caused the plastic to melt and the screw eye to pull out.

Try to put the cooling mesh back in. The system has always worked well for me.

 

[RocketTree]

I recently ordered some 5/8 tubular nylon for the GForce, not realizing how heavy duty it is.

Is it supposed to be simply tied on, or is there a better way to create a loop in the end of the cord?

Attempting dual deployment on this rocket and not able to find the factory style elastic cord.

 

[Handeman]

You can tie it on and save the weight of the quick links. Although with the weight of that heavy of nylon, I'm not sure it makes much difference.

When I use nylon close to the ejection charges, I wrap it in electrical tape 10 pack for $6.19 at Harbor Freight. When that even starts to look bad, I remove it and re-tape. It keeps the nylon undamaged.