Burn BP motors from the wrong end?

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

ThirstyBarbarian

Well-Known Member
Joined
Feb 11, 2013
Messages
12,221
Reaction score
7,428
I had my MDRM with the fin pods at a launch recently, and a lot of people asked me if there were outboard motors in the pods. There aren't, but someday I would like to make a rocket with that design. I had a few discussions with different people about strategies for lighting such a cluster, and probably the best way to do it would be using an airstart --- get the rocket up and moving fast on a single central motor then airstart the outboards once it is going. You would probably also need to do an electronic deployment, so in case the outboards did not light, you could still have deployment at apogee.

That makes for a more complex rocket with electronic airstarts and electronic deployment. One person suggested just designing the pod motors for effect, and not really using them to propel the rocket. And one thing he suggested would be to use zero-delay black powder booster motors, and insert them into the mount backward and light them from the non-nozzle end. That way you would get smoke and flame from the pods, but the thrust would be almost zero, so the pod motors would not have a chance of disrupting the flight if they didn't light properly. You could light everything on the pad, and you could still use motor ejection. It would make things much simpler and still look cool.

First, would such a use of a motor be in violation of safety code or illegal? Second, if it is not in violation, then would it work? Also, if the central motor did not light and the outboards did, would the rocket be burned by the outboards?
 
First, would such a use of a motor be in violation of safety code or illegal?
I honestly don't know. There's probably some generic wording about "use only as intended" that will get you, but I'm just guessing.

Second, if it is not in violation, then would it work?
Quite possibly, but a fair amount of flame comes out so there is a concern there (don't ask how I know)

Also, if the central motor did not light and the outboards did, would the rocket be burned by the outboards?
Almost certainly; see above.
 
If you are using all BP motors you can light them all at the pad no problem. If they are widely spaced in pods all you need either a good clip whip or high end and long commercial igniters you can wire together. A cluster box or a nice, hot, high power relay launch system is a big help. With a clip whip and the old Q2G2's we would light eight widely spaced motors easily with an old 12v system using a fully charged car battery. I would not turn a D12 0 upside down, it is just wrong and would offend the rocket gods. Get AMPED UP and use all the god given thrust at your disposal. It is a violation of the code because of improper use and just plain bad form. It would burn the living crap out of you rocket, no nozzle with a big flame that would burn for quite a while. Think D11 P in the pods and a nice F 15 - 4 in the center.
 
If the pods were long enough, you could insert plugged motors (D11-P's) far into the tube and that would likely diminish the thrust very drastically (krushnic effect). I have no idea how one would quantify that...


Sent from my iPhone using Rocketry Forum
 
I'm not planning to retrofit this rocket with outboard motors, because the pods are already filled with foam, and there is an internal wooden tab that runs from the fin edge all the way through the pod. So this rocket is a done deal, and the question about burning a motor from the wrong end was more theoretical. I'm not planning to do it, and I think there are good reasons not to.

But I do have another MDRM kit sitting in the garage, and I could make something similar with lessons learned from this one. Basically, what I want is reliability for the flight --- if there isn't enough thrust for it to leave the pad safely, then it doesn't leave at all.

This rocket is heavy enough that I don't think it would move if I used 24mm BP motors in the pods and the central 29mm composite failed to light (definitely would not go with 18mm motors). And since that would be the reason for putting them in backwards, it probably just makes more sense to put them in right.
 
If you are using all BP motors you can light them all at the pad no problem. If they are widely spaced in pods all you need either a good clip whip or high end and long commercial igniters you can wire together. A cluster box or a nice, hot, high power relay launch system is a big help. With a clip whip and the old Q2G2's we would light eight widely spaced motors easily with an old 12v system using a fully charged car battery. I would not turn a D12 0 upside down, it is just wrong and would offend the rocket gods. Get AMPED UP and use all the god given thrust at your disposal. It is a violation of the code because of improper use and just plain bad form. It would burn the living crap out of you rocket, no nozzle with a big flame that would burn for quite a while. Think D11 P in the pods and a nice F 15 - 4 in the center.

I don't think all BP motors would lift it. The rocket came in a 61 oz in the end!

I saw another rocket with outboard pod motors disassemble itself in spectacular fashion at the launch this weekend. I missed the announcement and turned around just in time to see the rocket doing a high-speed wrestling match with itself at the pad before busting into pieces. I'm not exactly sure what happened, but it looked like multiple redline motors in the outboards. I don't know if there was a central motor or if it failed to light or what, but that rocket took itself apart in no time flat. My thought is it's probably most reliable for the central motor to be what flies the rocket, and the pod motors be there mostly to look cool, but not big enough to cause trouble.
 
Someone else mentioned using a pull pin to light the outboards. I'm not familiar with this, but they described it as like a grenade pin tied to the pad or the ground with a string, and after the central motor successfully launches the rocket, it reaches the end of the string, pulling the pin, and closing a circuit to light the outboards.

Is that legit?
 
Have you thought about just putting smoke bombs in the pods and using igniters to start the fuses?

Flight Safety. I will not launch my rocket at targets, into clouds, or near airplanes, and will not put any flammable or explosive payload in my rocket.
 
I don't think all BP motors would lift it. The rocket came in a 61 oz in the end!

Use small motors, such as 1/2A6s or 1/4A3s, in the pods. They aren't going to move the rocket if they light first. If you're using them with a composite as the main motor, they'll most likely ignite before the core motor. But, the tracking smoke from them may provide the effect you desire.

-- Roger
 
I think pull pins are not acceptable.
But we can build some low thrust/high smoke ex motors and launch it at Springfest!
 
Use small motors, such as 1/2A6s or 1/4A3s, in the pods. They aren't going to move the rocket if they light first. If you're using them with a composite as the main motor, they'll most likely ignite before the core motor. But, the tracking smoke from them may provide the effect you desire.

-- Roger

Very good suggestion.
 
I don't think all BP motors would lift it. The rocket came in a 61 oz in the end!

I saw another rocket with outboard pod motors disassemble itself in spectacular fashion at the launch this weekend. I missed the announcement and turned around just in time to see the rocket doing a high-speed wrestling match with itself at the pad before busting into pieces. I'm not exactly sure what happened, but it looked like multiple redline motors in the outboards. I don't know if there was a central motor or if it failed to light or what, but that rocket took itself apart in no time flat. My thought is it's probably most reliable for the central motor to be what flies the rocket, and the pod motors be there mostly to look cool, but not big enough to cause trouble.

That is over 3.8 pounds of high powered rocket! Must build light and strong for BP motors. Max liftoff on 3 D 12's and 1 F15 is 63 oz. If they all lit it would be so low and slow all the old farts would be overcome with shock and awe.
 
That is over 3.8 pounds of high powered rocket! Must build light and strong for BP motors. Max liftoff on 3 D 12's and 1 F15 is 63 oz. If they all lit it would be so low and slow all the old farts would be overcome with shock and awe.

The current pod rocket is the ultimate low and slow beast. It's been flying on G77, G78, and G79 motors to about 400 feet, and a 4-second delay is just right. People enjoy the flight because they can easily see the whole profile, including the deployment of a 54" spherachute right before their eyes. It's fun! I had a few people tell me it was their favorite flight of the day because they could actually watch it, as opposed to the minimum diameter HPR birds that finish their flights in the Gamma Quadrant.
 
Back
Top