Hybrid venting

[CarVac]

I'm planning on building a rocket using a Rattworks I90 sometime in the near future. I'm designing every aspect prior to building anything, but here's where I've gotten stuck:

What exactly do you do with the vent hole? Looking at photos of the motor, it's just a small hole in the side of the forward closure, but how do you ensure that the escaped nitrous oxide shoots out the side of the rocket instead of freezing the inside of the rocket and causing condensation and water damage? Does it require a tube to go between the motor and the outside? Does it matter if the motor is allowed to rotate?

I'm planning on using it in a 38mm body tube and my self-imposed design constraints (No airframe breaks, potentially removing the entire 29mm motor mount in case I want to fly 38mm motors, the charge needs to be at the bottom of a footlong section of tube right above the motor, I don't want to put wires through shock cords, the wires are coming from the aft end of the rocket forward) are forcing me to design some clever mechanisms for installing charge holders, but without knowing what to do with the vent hole I can't finalize my designs.

In case anyone is wondering, this will be a clustered rocket with an I90 in a 4-foot 38mm tube, boosted by two airstarted 29mm solid motors in nonremovable pods on the sides, with a drogue chute in one pod, a Raven altimeter in the other pod doing the deployment and airstarts, the main parachute above the I90 in the central body tube, and a tracker in the nosecone.

 

[troj]

If you're building the rocket with a 38mm mount, then built the vent tube into the side of the mount and run it down and out the bottom of the rocket -- I've done that, and it works well.

Otherwise you drill a hole into the airframe and after installing a motor, you insert a vent tube into the side, so that the gas is directed out the tube.

-Kevin

 

[CarVac]

That would work really well for me: I was planning on having the entire 29mm mount removable, with a Giant Leap ogive tailcone retainer to transfer thrust. At the front I would mount the charge canister, and have wires down one side. This way, I could have wires on one side, and the vent tube down the other side. Excellent!

Also, thanks for the quick reply. What sort of tubing would I use that would fit between 29mm and 38mm?

 

[troj]

Also, thanks for the quick reply. What sort of tubing would I use that would fit between 29mm and 38mm?

I don't recall what I used; it's a rigid white tubing with an OD that fits easily into the vent in the top of the motor. I then notched the centering rings on the adapter, ran the tube down the side, and held it in place with wraps of masking tape.

-Kevin

 

[billspad]

I don't recall what I used; it's a rigid white tubing with an OD that fits easily into the vent in the top of the motor. I then notched the centering rings on the adapter, ran the tube down the side, and held it in place with wraps of masking tape.

-Kevin

It's the same size as the fill tube, 1/8" o.d. I believe it's nylon.

I did something similar and I was concerned about the sharp bend causing the tube to kink so I made an elbow out of 1/8" i.d. copper tube.

 

[troj]

I did something similar and I was concerned about the sharp bend causing the tube to kink so I made an elbow out of 1/8" i.d. copper tube.

Leave it to a plumber to whip up some plumbing magic!

Mine did kink a little bit, but it didn't ever seem to cause any problems in venting. Your solution is better, though.

-Kevin

 

[billspad]

Leave it to a plumber to whip up some plumbing magic!

My hybrid rockets comply with the high power safety code and the plumbing code!

 

[karlbaum]

It is not difficult to insert the tubing through a hole in the side of the 38mm tube and into the motor vent hole. Although I think that if I were to make a short radius 90 with nylon tube I would just warm it up and bend it.

Your project is bold. I have a 54mm rocket with a 38mm motor mount for the sustainer and it has two 29mm boosters that drop off. It is a challenge to fly but rewarding.

 

[boomtube]

What is it with the people who fly Hybrid Rockets!!!???
Don’t they realize that in the time it takes for them to set up and launch their hybrid; five other people could have launched their AP or BP rocket!!!???
And let’s not forget the superior attitude we have to endure from the guys that build and launch hybrids.
Then there’s the stink of these things and the noise and ,and, and!!!

Ok; I’ve vented enough.

But seriously folks; these things are really neat to watch.

 

[CarVac]

@karlbaum

Thanks. This isn't quite as ambitious as your rocket, since I'm not dealing with pod separation. Also, I decided that having fins on the main tube for stability looked dopey compared to little tiny fins on the boosters. Anyway, it's a good challenge for me: my first airstarting, my first dual-deploy, and my first hybrid (obviously). Luckily, I can test them all independently. It can fly on a single solid motor to test the dual deploy, then try the airstarting, and finally graduate to flying on the hybrid. And it'll look awesome all the while.

@boomtube

Part of the reason is because
1. The motor is not expensive to fly: even with boosters I can fly an awesome-looking, high-performance rocket on $40 a flight.
2. It isn't easy: it's more interesting to work around all the problems than for everything to go smoothly. I'm an engineer at heart, and there's nothing on earth that I like to do more than design things.
3. Hybrids sound cool. You seem to want quiet rockets?????

I do have another question I guess I could ask here: what do I do with the igniters for the booster motors? I don't want the physical connection to them to be too weak lest they break, nor do I want them hanging off the rocket the entire flight. How do I ensure that the connection remains intact until they ignite their motors and then have them cleanly separate from the rocket, with minimal space taken outside the rocket?

 

[kramer714]

People ask me why do I fly hybrids? my answer - Geek Masochism

Mike K

 

[troj]

People ask me why do I fly hybrids? my answer - Geek Masochism

My answer was always "because I can".

-Kevin