rands5

I want to make something like this. How do I get the rockets to fire all at once? I tried wiring rocket motors together myself, but they go off slightly at diffrent times. That could make my airplane flip. Also what would be the best way to make it fire remotely? Servos? or do they make something for this?

http://www.youtube.com/watch?v=uIcAXrHRY7M

Lorduss

Getting model rocks to go off at the same time is extremly hard, and how he managed to get such even fireing im not sure.
when you wired all the rocket ignitors togeather, did they all connect directly to the battery or togeather in a *string* of sorts?
they should all connect togeather at the battery for the most even firing, wiring anything in series or *string* or sorts allows one to get more voltage before the other, adding to the problem.
2X ignitors may also be needed per rocket, not everyone is made equell.
Something like this requires alot of trial and error, and infact if you watch mythbusters they did a episode or *myth* about a rocket powered swing, they could allmost never get both rockets to fire evenly. so obviously its a common problem, but fixable with enough effort.

as far as a remote firing, a servo could do it mechanicly by directly attaching 2 wires to complete the circut, there is probbly a way to do it without a servo directly but using the servo for a direct connection would make it simpler and just as effictive, i cant think of the correct RX powered switch at the moment but they exist.

foodstick

I absolutely can't believe how high the angle of attack was on those rocket engines, I would have never believed it would have worked without seeing it.

That first takeoff looked like a harrier for awhile !

rands5

I think I'm getting closer. I've been doing more testing. So it comes down to probability and averages. If I only have two rockets (one on each side), one of the rockets firing late causes a guaranteed roll because there will be a time when 100% of the lift force is on one side. If however, I have 6 rockets (three on each side) and one fires late then at worst I have a 1/6 imbalance. Also, it is likely that both sides will fire at different times, but the average force is still about the same on both sides at all times. So if you had 10 or more rockets, the effect of late ignitions is very diminished especially since it's likely to happen on both sides.

Secondly, there is the issue of a non firing rocket. That will also cause a guaranteed failure (especially with just two rockets). I of course have all the rockets wired together in parallel.  This has the effect of splitting the current between all of the rockets. So the more rockets I have, the more current it will take to launch. It is plausible that with variation in starters, the same amount of current might not fire all of the rockets. It will take a long time to heat up the starters to the correct temperature with a low current and that leaves enough room for variation in starters to cause one of them to need an extra second or two to fire. By that time, the airplane has already left.. If you are using a regular 9V battery, the maximum current you can output is very limited. So in order to fire multiple rockets you require a power source that can output a lot of current. I read through the comments of the youtube video and it turns out that the guy used a car batter to set off the rockets. I wonder what could be a better lighter source that you could keep on board?

Lorduss

lipos are known for there *cranking* power if you well, and with the size of some 3S batts that should take care of it for you. i would leave the battery on the ground but thats up to you.

foodstick

Maybe a high voltage portable drill pack? like a 19.2 volt one...

I still can't believe the way that thing comes up, not forward at the first launch. I didn't realize the electric motor was already running when it launched. It just seems wrong the way it comes up..

Thanks for the link !

Lightfoot

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That is about how they look when being launched with a high-start.

foodstick

well I mainly mean how it just lifts up out of the cradle the first foot or so...