7 Motor Cluster

[emlang93]

I am leading a rocket team in building a large rocket, 17 feet tall and 18 inch in diameter. We are wanting to use 6 ground start K motors surrounding a center air start 'L'.
The current plan is to use a custom built ignition system onboard the rocket. Will be redundant for its ignition circuits. Will also be using BKNO3 ignitors that will be doubled up with e-matches. (Had good luck with reliability of BKNO3 over the years)
The idea is a launch signal can be sent and start the ignition sequence. Then the ground starts will light and a timer will control the air start. We are planning for enough thrust so even if there are 2 motors not lit, the rocket will leave the pad stable. Just won't be as cool.
Now for the questions.
1) does this sound feasible?
2) has anyone done this?
3) what are some of the designs principles to consider with a large number cluster? Angle the motor exhaust outboard a little bit for stability?
4) any other suggestions?

Just want to see what people's thoughts are and any ideas for us to consider in design.

-Evan
TRA 16099 L3

 

[cls]

Safer to ground start the one central motor, and air start the others.

Think it through: what happens if say only 3 or 4 of the six Ks light? Offset thrust? Low rail exit velocity?

 

[emlang93]

Safer to ground start the one central motor, and air start the others.

Think it through: what happens if say only 3 or 4 of the six Ks light? Offset thrust? Low rail exit velocity?

Yes, I am concerned about offset thrust. With current estimates, we can handle 4 of the 6 starting and still be stable. Assuming the 2 that are out are not right next to each other.
Also putting trust in the onboard ignition to help ensure we have proper ignition even if some start later.

My concern with air start of the 7 is the uneven startup pattern. That will make offset thrust right?

Also planning on preforming a ground test to better understand the start characteristics of the motors and timing.

 

[Seatechnerd83]

I'd suggest enhancing your ignitors with some Quickburst ProCast, its 50 bucks including shipping, and it makes the ignitors much much more potent.

 

[waltr]

I'd suggest enhancing your ignitors with some Quickburst ProCast, its 50 bucks including shipping, and it makes the ignitors much much more potent.

He is using:
"Will also be using BKNO3 ignitors that will be doubled up with e-matches. (Had good luck with reliability of BKNO3 over the years)"
ProCast is BKNO3 (Boron Potassium Nitrite)

I agree this stuff works very well.


OP,
I would use a flight controller that can 'lock-out' the air-start if flight parameters are out of spec (tilt, speed, etc).
Then at least it doesn't fly into the next county if angled.

Also, Be SURE all deployments happens regardless to motors firing.

 

[Seatechnerd83]

He is using:
"Will also be using BKNO3 ignitors that will be doubled up with e-matches. (Had good luck with reliability of BKNO3 over the years)"
ProCast is BKNO3 (Boron Potassium Nitrite)

I agree this stuff works very well.


OP,
I would use a flight controller that can 'lock-out' the air-start if flight parameters are out of spec (tilt, speed, etc).
Then at least it doesn't fly into the next county if angled.

Also, Be SURE all deployments happens regardless to motors firing.

sorry missed that sentence, i know ProCast is BKNO3, i just am not very observant when it comes to words.

 

[jdsmith39]

For APCP/Sugar motors, I've done a few different things now. What I do now is by far the easiest with reloadable motors. What I've been doing as of late is:

• use e-matches (much more reliable than standard igniters)
• place triple 7 pieces into the top of the top grain of the motor. How much you put in, depends on the motor diameter. These will be held in place with a dab of super glue if needed.
  • 29mm - few slivers
  • 38mm - 1/4 of a 50/50 pellet
  • 54mm - 1/2 of a 50/50 pellet
  • 75mm Untested myself but others say to use a full 50/50 pellet.
• put in igniter pyrogen (I use QuickBurst) into the top grain around the triple seven piece(s)

The idea is, the e-matches fire, which ignites the triple 7. If the added igniter pyrogen didn't ignite from the e-match the triple 7 will ignite it. The igniter pyrogen will keep burning to help make sure the motors ignite even if the rocket leaves the pad and the igniters fall out before all motors have ignited. They should/will still ignite.

Note: CTI motors, already have triple 7 pellets in them.

This method has been very reliable for me. I've been using it for a couple years now.

 

[rfjustin]

For APCP/Sugar motors, I've done a few different things now. What I do now is by far the easiest with reloadable motors. What I've been doing as of late is:

• use e-matches (much more reliable than standard igniters)
• place triple 7 pieces into the top of the top grain of the motor. How much you put in, depends on the motor diameter. These will be held in place with a dab of super glue if needed.
  • 29mm - few slivers
  • 38mm - 1/4 of a 50/50 pellet
  • 54mm - 1/2 of a 50/50 pellet
  • 75mm Untested myself but others say to use a full 50/50 pellet.
• put in igniter pyrogen (I use QuickBurst) into the top grain around the triple seven piece(s)

The idea is, the e-matches fire, which ignites the triple 7. If the added igniter pyrogen didn't ignite from the e-match the triple 7 will ignite it. The igniter pyrogen will keep burning to help make sure the motors ignite even if the rocket leaves the pad and the igniters fall out before all motors have ignited. They should/will still ignite.

Note: CTI motors, already have triple 7 pellets in them.

This method has been very reliable for me. I've been using it for a couple years now.

@emlang93, so this post above from @jdsmith39 I think is the most reliable way of getting a cluster of 54mm going at the same time. I have lit (3) K-1100's and (3) K-2050's reliably this way.

More info here: https://www.rocketryforum.com/threads/clustering-composites-question.170547/#post-2232202

 

[StreuB1]

Yes, I am concerned about offset thrust. With current estimates, we can handle 4 of the 6 starting and still be stable. Assuming the 2 that are out are not right next to each other.
Also putting trust in the onboard ignition to help ensure we have proper ignition even if some start later.

My concern with air start of the 7 is the uneven startup pattern. That will make offset thrust right?

Also planning on preforming a ground test to better understand the start characteristics of the motors and timing.

You have just identified what you view as the worst case scenario and given (6) motors in a ring, you have a rather not insignificant probability that if (2) motors fail to light, they will be adjacent to each other. That means that if you have (2) motors fail, your worst case scenario is more likely than not.

Now, if your central motor is already firing when the rest fire, you have far less off-axis thrust acting on the rocket than if the center motor was not firing.

One thing that really must be taken into account is the mass of the vehicle, the distance from the radial motors to the centerline of the vehicle, and the applied moment by a single motor. This will tell you how much control authority ONE motor will have on the entire rocket. Depending on the rocket and the motors, it might fly almost perfectly on a SINGLE radial motor firing.

This is a probability problem acting on a control authority problem.

Another thing to consider is, how well will the rocket fly on JUST the central motor OR how well will it fly on (2) of the radial motors firing? I would base my entire project on the probability that either ONLY the central motor fires OR that only (2) of the radial motors fire. If you cannot achieve a minimum safe altitude and safe attitude flight on these, then you need to do some rethinking.

 

[1MTRNTENF]

I am leading a rocket team in building a large rocket, 17 feet tall and 18 inch in diameter. We are wanting to use 6 ground start K motors surrounding a center air start 'L'.
The current plan is to use a custom built ignition system onboard the rocket. Will be redundant for its ignition circuits. Will also be using BKNO3 ignitors that will be doubled up with e-matches. (Had good luck with reliability of BKNO3 over the years)
The idea is a launch signal can be sent and start the ignition sequence. Then the ground starts will light and a timer will control the air start. We are planning for enough thrust so even if there are 2 motors not lit, the rocket will leave the pad stable. Just won't be as cool.
Now for the questions.
1) does this sound feasible?
2) has anyone done this?
3) what are some of the designs principles to consider with a large number cluster? Angle the motor exhaust outboard a little bit for stability?
4) any other suggestions?

Just want to see what people's thoughts are and any ideas for us to consider in design.

-Evan
TRA 160

 

[1MTRNTENF]

I think everyone is missing one critical factor here. Six K motors and one L is not enough power for a rocket this size. I'm sure with high thrust motors you could get it to fly, kind of a pop shot, but to do it justice you need at least a central M.
I'd go with 8 outboards.
Prep of your motors is key to get reliable ignition. No single use motors, especially for the outboards.

 

[3stoogesrocketry]

I think everyone is missing one critical factor here. Six K motors and one L is not enough power for a rocket this size. I'm sure with high thrust motors you could get it to fly, kind of a pop shot, but to do it justice you need at least a central M.
I'd go with 8 outboards.
Prep of your motors is key to get reliable ignition. No single use motors, especially for the outboards.

I might* agree with you if the OP listed a weight. But to just say 20kns of power is not enough for a 17 foot 18 inch rocket is inaccurate, unless there is a specified altitude.

 

[1MTRNTENF]

I might* agree with you if the OP listed a weight. But to just say 20kns of power is not enough for a 17 foot 18 inch rocket is inaccurate, unless there is a specified

I might* agree with you if the OP listed a weight. But to just say 20kns of power is not enough for a 17 foot 18 inch rocket is inaccurate, unless there is a specified altitude.

Yeah, you're right. My imagination took me to Sona Tube construction and K1100's. The real question was about the cluster configuration. Regardless of the other variables I still don't like the L for the central motor.

 

[emlang93]

Thank you for all the input and conversation. A few changes.
1. Scaling the rocket down to 16 inch diameter.
2. Increasing center motor to a M1350
3. Ground starting all 7 motors. Air start is just to risky and would rather have the stability of the center M running on the pad.
4. Current weight estimation is 130lb (Starting to buy material and we are on track for this weight or maybe a little under)
5. Simulations show 4,000 +/- 500 feet with this configuration
6. 20 foot launch tower. simulations show you can lose the M and 2 K's on startup and it will still be stable off the pad.

 

[3stoogesrocketry]

Thank you for all the input and conversation. A few changes.
1. Scaling the rocket down to 16 inch diameter.
2. Increasing center motor to a M1350
3. Ground starting all 7 motors. Air start is just to risky and would rather have the stability of the center M running on the pad.
4. Current weight estimation is 130lb (Starting to buy material and we are on track for this weight or maybe a little under)
5. Simulations show 4,000 +/- 500 feet with this configuration
6. 20 foot launch tower. simulations show you can lose the M and 2 K's on startup and it will still be stable off the pad.

It really sounds like your on the right track here. I would suggest that you ground start the M1350 ( that motor will lift a 16 inch 100 rocket ) and airstart your outboard motors using a first motion piece of electronics. This way you know with 99 percent certainty (motors do self disassemble at times) that your rocket is going up safely. If you are using aerotech motors, you can do the center M and two or two outboard L1000's at first motion, then airstart the remaining outboard motors or your choice at a predetermined time. This seems like a fun project

 

[emlang93]

It really sounds like your on the right track here. I would suggest that you ground start the M1350 ( that motor will lift a 16 inch 100 rocket ) and airstart your outboard motors using a first motion piece of electronics. This way you know with 99 percent certainty (motors do self disassemble at times) that your rocket is going up safely. If you are using aerotech motors, you can do the center M and two or two outboard L1000's at first motion, then airstart the remaining outboard motors or your choice at a predetermined time. This seems like a fun project

All good points.
The custom ignition board is running on a RP Pico and each motor has independent control over starting. The thought right now is to start the center motor, wait about half a second for pressure rise, then start lighting the outboard motors. A few advantages but one is the lower peak current draw on the electronics.
It might not be a bad idea to bump up some of the outboards to L. Will see what it does. Out here in Alabama we can not go very high so also watching that too.
For the motors I would personally rather use CTI but right now 7 cases appear to be impossible to find and over budget. So we will make due with the single use motors and get the BKNO3 to work. As afar as the unscheduled disassembly, it is a fear of mine manly with the M so first motion is a good idea. We will see what happens.