Dealing with airstart igniters

[CarVac]

In a reply to my own thread on hybrid vent tubes, I asked this question but got no response, so here I am starting another new thread specifically for this topic.

When setting up an airstarted motor, how do people make the connections to the igniters? They have to be secure enough to maintain continuity during takeoff and at speed, but they shouldn't be strong enough for the igniters to keep hanging off the rocket once the airstarts light. They should be at least somewhat easily replaceable, or so I would think.

For my next planned rocket, this is a tough issue. The rocket has a 38mm central tube which can't have wires routed through it. (Treat it like it has a 38mm motor inside.) The two airstarted 29mm boosters will be in minimum diameter pods fixed to either side of the central tube, with an altimeter in one of them ahead of the motor.

How do I make the electrical connections to the airstarts in an aerodynamic, lightweight, secure, yet impermanent connection? I don't want too much thickness, though I'm not averse to putting a slightly bulkier connection in the valley between the pods and the central tube as long as it's aerodynamically symmetrical and not too heavy.

I'd like to hear both about what you people have done on your rockets, whether it's applicable to mine or not, and what people would suggest that I do. Just spit out any crazy ideas you have; I will make them work if I have to. I can do the specific designing; I just need your experience guiding me to make this happen.

 

[dixontj93060]

Combination of twisted wires and tape worked for me.

 

[mikec]

When setting up an airstarted motor, how do people make the connections to the igniters? They have to be secure enough to maintain continuity during takeoff and at speed, but they shouldn't be strong enough for the igniters to keep hanging off the rocket once the airstarts light.

I've used 100-mil headers and crimp pins with some success. See
https://www.seattlerobotics.org/encoder/apr97/basics.html but obviously don't use the locking type.

 

[xenon]

When setting up an airstarted motor, how do people make the connections to the igniters? They have to be secure enough to maintain continuity during takeoff and at speed, but they shouldn't be strong enough for the igniters to keep hanging off the rocket once the airstarts light. They should be at least somewhat easily replaceable, or so I would think.

I've done this two ways:

On rockets with enough room, I use a small tube running up the side through the centering rings. Usually I use e-matches and pyrodex pellets, so I'll leave enough lead length hanging out of the back of the rocket to reach the top of the core when the igniter is inserted (on the pad...). Then the e-match gets wired directly to the avionics. I have never considered using a release of any sort that allows them to fall away after the motor lights and never had an issue. This had worked on D's-K's.

The other way that I've done on a minimum diameter rocket was tape wire. This is the stuff I've used and it has worked well: https://www.amazon.com/dp/B003BWZY78/?tag=skimlinks_replacement-20

You can find longer rolls if you need it (last project used 32' per flight). I've attached it directly to the motor case, but I've also seen it attached to the inside of the tubing. With this stuff at least, it's not a bad idea to put another layer of tape on the outside after applying it for additional protection. For connections, I just stripped off a few inches and wrapped it around the igniter wire and crimped it with a pliers, although I've seen some people use double sided terminal blocks with good success.

For my next planned rocket, this is a tough issue. The rocket has a 38mm central tube which can't have wires routed through it. (Treat it like it has a 38mm motor inside.) The two airstarted 29mm boosters will be in minimum diameter pods fixed to either side of the central tube, with an altimeter in one of them ahead of the motor.

How do I make the electrical connections to the airstarts in an aerodynamic, lightweight, secure, yet impermanent connection? I don't want too much thickness, though I'm not averse to putting a slightly bulkier connection in the valley between the pods and the central tube as long as it's aerodynamically symmetrical and not too heavy.

IMO, I'd use 1/8" or so tubes running between the main tubes and use long igniter leads that will reach the altimeter such that there is no connection. With 2 outboards in nearby pods, you'll need to make sure that the igniter connections for those isn't going to get smoked/burned through by the central motor. This would be harder with tape wire as the connection itself takes up more room than twisting 2 normal wires together. There's enough room on 75mm motors to stuff the connection in the exit cone of the motor, but with 29's, that's probably not the case with tape wire.

 

[Adrian A]

I have run fine-gauge magnet wire alongside the motor. More recently I've been using head-end ignition instead, replacing the delay grain with high-temperature epoxy.

 

[CarVac]

With the magnet wire: how do you connect that to the ematch? Do you solder them directly onto ematch heads that you buy, or to a stiffer wire that holds the ematch up inside the motor during boost?

Is it possible to do head-end ignition in Aerotech cases without ruining a forward closure? It seems much more convenient to do in Cesaroni motors, where you dispose of the forward closure and so can glue directly to it. I want to be able to fly the rocket cheaply, which means using 29/40-120 motors in the boosters and reusing all of the hardware. Do you use the included cardboard delay grain liner and cast an epoxy slug in the middle with the igniter head sticking out?

Also, since high-temp epoxies tend to require elevated cure temperatures, is the igniter safe to put in a curing oven?

And furthermore: when doing head-end ignition, do you put the cap on the motor?

 

[bobkrech]

Modifying a certified motor for head end ignition is not allowed at NAR or standard TRA launches.

Bob

 

[Reinhard]

With the magnet wire: how do you connect that to the ematch? Do you solder them directly onto ematch heads that you buy, or to a stiffer wire that holds the ematch up inside the motor during boost?

Is it possible to do head-end ignition in Aerotech cases without ruining a forward closure? It seems much more convenient to do in Cesaroni motors, where you dispose of the forward closure and so can glue directly to it. I want to be able to fly the rocket cheaply, which means using 29/40-120 motors in the boosters and reusing all of the hardware. Do you use the included cardboard delay grain liner and cast an epoxy slug in the middle with the igniter head sticking out?

Also, since high-temp epoxies tend to require elevated cure temperatures, is the igniter safe to put in a curing oven?

And furthermore: when doing head-end ignition, do you put the cap on the motor?

I would heed Bob's advice and operate the motor as certified.

Should you still decide to go for head end ignition, with all its implications, make sure to let someone knowledgeable check your design before you fly it. It is easy to mess up the forward seal with modifications. This is, in my opinion, particularly true for the AT hobby line casings.

Reinhard

 

[Adrian A]

With the magnet wire: how do you connect that to the ematch? Do you solder them directly onto ematch heads that you buy, or to a stiffer wire that holds the ematch up inside the motor during boost?

Is it possible to do head-end ignition in Aerotech cases without ruining a forward closure? It seems much more convenient to do in Cesaroni motors, where you dispose of the forward closure and so can glue directly to it. I want to be able to fly the rocket cheaply, which means using 29/40-120 motors in the boosters and reusing all of the hardware. Do you use the included cardboard delay grain liner and cast an epoxy slug in the middle with the igniter head sticking out?

Also, since high-temp epoxies tend to require elevated cure temperatures, is the igniter safe to put in a curing oven?

And furthermore: when doing head-end ignition, do you put the cap on the motor?

Like Bob said, the head end ignition discussion only applies if you're a level 2 or 3 member of Tripoli.
I haven't tried it with Aerotech motors. In theory you should be able to dig out the delay grain material and replace it with JBWeld + ignitor and seal it up normally. Removing the delay grain material from the CTI forward closure is important, because it's easy to get the delay burning along the length of the ignitor, and get blow-by. That happened to me one out of 2 times I've tried it. I haven't fully tested my new method yet.

 

[troj]

Clarifying what Adrian already knows - his head end ignition approach is only permissible at a Tripoli Research launch, and if the flier is Level 2 or above, as the change turns it into a Research motor.

 

[karlbaum]

I use standard 22AWG twin wire for my igniter and run it between the body tube and motor tube. It is stiff enough to hold itself up in the motor. I do not insert the ignigter in the motor until on the pad. Before launch I leave the igniter taped to the outside of the rocket. That is one problem I have with head ignition is that I want to be able to insert the igniter on the pad for safety reasons.

My electronics bay is in the forward section that separates from the sustainer section. So I learned to make a "breakaway" in the wiring so that when they separate at apogee and the igniter wires aren't yanked out of the ebay causing damage. All I do is a twist and tape connection.

 

[Adrian A]

pad. Before launch I leave the igniter taped to the outside of the rocket. That is one problem I have with head ignition is that I want to be able to insert the igniter on the pad for safety reasons.

That's why I use a separate screw switch on just the ignitor circuit when I'm using head end ignition. The ignitor is electrically isolated that way, and almost as safe as if it weren't installed.

 

[karlbaum]

That's why I use a separate screw switch on just the ignitor circuit when I'm using head end ignition. The ignitor is electrically isolated that way, and almost as safe as if it weren't installed.

I guess "almost as safe" is where I have the problem. If you are dilligent to keep the circuit open you will probably never have a accident. I just know that I make errors and I try to eliminate the possibility of errors. That is the application of Murphy's law. I would also question if head ignition is compliant with the Tripoli Safety Code.

2-12.4 A person shall install an ignition device in a high power rocket motor at the launcher or within the area designated by the RSO. The rocket shall be pointed in a safe direction during and after installation of the ignition device.

 

[CarVac]

It seems that the consensus is "twist and tape" is sufficiently secure. I plan on testing that by first flying with igniters hooked up to the altimeter but no booster motors: just checking to see if they lit. If that doesn't work, I'll just use permanent wiring and trim the caps so that they don't stay hanging on the igniters for the duration of the flight.

I'll keep head-end ignition in mind for once I get level 2 certified and get the opportunity to go to a research launch.

 

[mikec]

I would also question if head ignition is compliant with the Tripoli Safety Code.

One reading of the TRA code is that all of section 2-12 only applies to the launch ignition system. Otherwise how do onboard electronics meet requirement 2.12-1 for "remote control"? Installing igniters of a multistage rocket at the pad, if it's practical at all, could easily be more dangerous than installing them in a designated RSO area as the rules also allow.

Without wanting to start an argument, the TRA code is overly long and vague to be effective IMHO.

 

[CarVac]

You could always carry your rocket with partially-assembled motors up to the RSO station and then install the forward closure with igniter. But it would be a pain if you had, say, nine airstarts (delta-II anyone?), especially since you would need to deal with a lot of wiring once that was done, or have a lot of space above the motor to crumple the length of wire needed to hang the forward closure below the bottom of the motor tube. And you'd have to worry about dust.

Overall, the rules make it quite a pain to do head-end ignition.

 

[Adrian A]

You could always carry your rocket with partially-assembled motors up to the RSO station and then install the forward closure with igniter. But it would be a pain if you had, say, nine airstarts (delta-II anyone?), especially since you would need to deal with a lot of wiring once that was done, or have a lot of space above the motor to crumple the length of wire needed to hang the forward closure below the bottom of the motor tube. And you'd have to worry about dust.

Overall, the rules make it quite a pain to do head-end ignition.

In the rockets I build with head-end ignition, I have more than an hour of prep required between forward closure installation and being ready to take my rocket to the pad. The open switch I use on the ignitor, in addition to the altimeter arm switch and altitude check, gives me a lot of piece of mind when I'm doing that prep. Based on the discussion I've been involved in on the Tripoli list, nothing in the Tripoli rules makes head-end igniton any harder except the fact that it makes it a research motor. The board and others are well aware that head-end ignition systems are typically assembled well in advance of use at the pads, and there hasn't been any push to change that that I'm aware of.

 

[bobkrech]

You can also have a remove before flight shorting pin so even if the pyro circuit is powered up, the igniter is shorted out and will not fire.

Bob