Minimum dia build question

[north boy]

Hi guys I just built a scratch on open rocket. Its a min dia bt and the rocket is a 2 stage design. For it to be a min dia and the 29mm motor to fit and 2 stage it I'm trying to avoid the need of a coupler at the booster/sustainer join. Can I just have the motor over hanging by 30mm out of the sustainer to act as a coupler. Im using a F20-7 Econojet for the sustainer and a F20-3 Econojet for the booster. Is this possible ??
Id need to get my airstart setup to time the ignition of second stage correct. The motors have a 3.4 seconds burn time and apogee is 1800 meters ( about 5500 ft )

Any suggestions welcome. Its just a simple cardboard tube rocket design with 1mm fins not through the wall but will have heavy fillets. 31mm bt diameter with a 29mm internal dia.

[north boy]

Any body lol

[mikec]

Can I just have the motor over hanging by 30mm out of the sustainer to act as a coupler. Im using a F20-7 Econojet for the sustainer and a F20-3 Econojet for the booster. Is this possible ??

I would think so, assuming you can get the second stage to ignite using a timer or altimeter. You can't use the ejection charge of the first stage to ignite the second stage, at least not reliably, if that's what you were thinking.

[north boy]

Hi no I wasn't going to use the delay shot to ignite the second stage. It was through electronics bay through a timer. I was just a wee but unsure if 30mm over hang of motor was enough to be a firm secure joint. The booster stage has a 4 second delay and I think the rocket might be starting to tip at that point entering apogee so the timer was going to come in at a second to 2 seconds after burn out of booster. I dont know how to calculate the coasting period properly buy the burn time is 3.4 seconds on take off so a 5 second to 5.5 second of the launch pad would give a 1.6 to 2.1 second coasting time. I think that would be a safe calculation unless someone can point out any probs ?

[mikec]

I think that would be a safe calculation unless someone can point out any probs ?

What kind of igniter are you going to use for the second stage? The motor will probably take at least a second or two to pressure up. I would use zero delay between burnout and ignition, at least to start.

[north boy]

Ahhh Its a copper igniter that comes with the motor. I launched a G80 on a copper and it did have a delay on ignition!!! Last thing I want is it to be pointing down on second ignition Ill need to buy a battery for onboard ignition to so what size should I get?? A leight weight low amp hour lipo would be required I'd think but do you guys have any knowledge on what others use. Voltage size etc. Would a 2cell lipo do?? from lipo memory that would be about 7.4 volts I think (about 3.7 volts per cell I think) obviousely weight is an issue

[Adrian A]

A featherweight 29mm av-bay with a Raven could do the job, but routing the igniter down is tricky with this setup. I would want to have at least 50mm of overhang for the coupler.

See the airstart safety thread on Rocketry Planet for more safety information.

[north boy]

Hi Adrian, I read through the whole link. Thanks. What was the accident that happened??? it sounded like a serious accident that brought a lot of questions to amature rocketry. After reading that thread I feel slightly un happy about air starting a rocket now lol even though its a wee 29mm motor. That was back in 2007 and has the altimeters improved now with the safety aspects built into the new versions for better safer working results??? Or is it up to the user to try and install kill switches etc for safe handling. There was a mention on some one having to arm the rocket once everyone has vacated the launch pad to the safe distance. Why isnt it possible to just have a delay switch incorporated into the electronics for say 20 seconds delay working on a buzzer for a sound to tell you the delay timer is still functioning and once the buzzer goes off the 20 seconds has passed and the rocket electronics have armed and its ready to fly. This would give the final guy 20 seconds to run like hell in case it goes awoll lol.

[Adrian A]

The accident happened at LDRS 29, and there are legal issues not yet resolved. If a rocket isn't safe to be near it for arming, then it's also not safe to be around it for disarming, which can sometimes be necessary. If you put in an altitude condition into the flight settings for the airstart, then the only way to get a premature ignition is with a significant altimeter failure.

[Nerull]

Ahhh Its a copper igniter that comes with the motor. I launched a G80 on a copper and it did have a delay on ignition!!! Last thing I want is it to be pointing down on second ignition Ill need to buy a battery for onboard ignition to so what size should I get?? A leight weight low amp hour lipo would be required I'd think but do you guys have any knowledge on what others use. Voltage size etc. Would a 2cell lipo do?? from lipo memory that would be about 7.4 volts I think (about 3.7 volts per cell I think) obviousely weight is an issue

If you're planning to use a copperhead, you might need to launch a car battery in your rocket to get them to work. I'd suggest some low current ignitors more suited to the purpose of staging.

[north boy]

Ok thanks for all the advice. Thanks for the safety thread on Airstarts Adrian. Nerull I think an F motor would struggle with the 12volt battery lol. I'm going to have to solve the igniter problem because when I ignited the copper igniter it took a lot from the 12 volt battery