Cluster Rocket max lift off weight safety margins

Thinking about going back to some cluster rockets, wondering about safety margins
When you start dealing with more than one engine, always possible one or more won't light. I believe commericial kits like Deuce's Wild are designed to be stable and flyable even if not all engines light.
Any rules of thumb or input from the experienced "Cluster-ers" out there?
Example: If you do a three engine cluster should it be designed so that it will still be flyable if only two light?
Thanks

The concern I have if one doesn't light isn't so much if it get off the pad, but rather 1) how much is it going to arc over, and 2) how far off will the delay timing be? That's what I'd like to hear about...

If you have three engines in a close triangle configuration it will be somewhat more tolerant of one not firing than if they're in-line and spaced out a bit, and one of the outboard motors doesn't light. You should design it so that there's at least a 3:1 thrust/weight ratio with two motors firing, but 5:1 would be better. The higher the thrust/weight ratio, the less effect that having one not fire will have, and the less horizontal it will go as a result.

You also need to pay close attention to your CG/CP position... clusters tend to get tail-heavy. Underpowered tail-heavy clusters tend to power-prang if one of the motors doesn't light.

a failed dual G76G


If you're going dual, make sure the motors are side by side parallel to the flight line, so if one fails it heads in a safe direction.

If you can cant the motor mounts to point through the rocket's CG, it will have less effect when one fails to light.

CTI motors light quicker and more reliably, in most cases.

AT isn't impossible to cluster, just take more care selecting ignitors. I've had success with green, white, and Blue thunder is a never-fail. Green is very hard to pull off.

The best way to do it, is a large central motor that can lift the whole mess by itself. Then use electronics to ignite the other motors on liftoff/ airstart them.

Be mindful if you mix motor sizes, larger motors will take longer to come up to pressure. If you mix BP/AP, the BP will light first.


Dual or triple, both lit on the pad is fun and easy. Just be aware of the risks and do anything you can to minimize them. (good igniters, blue propellant, watch motor position relative to flight line)

the loc viper 4 will fly just fine on 3 of 4 motors, just doesn't get as high. 2 will make it limp off the pad and you probably will recover if you use D12-5s. Not so much with one. That's generally a rebuild job unless it stays on the rod...

Yes, that is a wise approach and a good answer to your question.

BABAR said:

Thinking about going back to some cluster rockets, wondering about safety margins
When you start dealing with more than one engine, always possible one or more won't light. I believe commericial kits like Deuce's Wild are designed to be stable and flyable even if not all engines light.
Any rules of thumb or input from the experienced "Cluster-ers" out there?
Example: If you do a three engine cluster should it be designed so that it will still be flyable if only two light?
Thanks

Click to expand...

BABAR said:

Thinking about going back to some cluster rockets, wondering about safety margins
When you start dealing with more than one engine, always possible one or more won't light. I believe commericial kits like Deuce's Wild are designed to be stable and flyable even if not all engines light.
Any rules of thumb or input from the experienced "Cluster-ers" out there?
Example: If you do a three engine cluster should it be designed so that it will still be flyable if only two light?
Thanks

Click to expand...

That is normally how I approach circular clustered motors, However it doesn't work as well with in-line clusters which will fly fine on two out of three motors but will arc over badly. the farther apart these in-line motors are the quicker the model will rotate. ie and Estes SR-71 will arc over an hit the ground if one of the outboard boosters fails to ignite.
Symmetric clusters are much easier to balance by slightly angling all motor mount tubes or pods at the CG of the ready to fly model.

Best way to avoid having motor not light is to use a 12V Hi-amp/hr power supply with a RELAY ignition system, and Check each igniter before, and after insertion in the motors and one last time after the motors are hooked up on the pad.

Triceratops was launched at Space Center Houston on Saturday. It took (3 D12-3 s) to get Triceratops up a few hundred feet before it nosed over and flew horizontally for a few hundred more, which, as you can see in the launch video in the link below, was even more exciting.

https://upcyclingtextbooks.org/?p=1328

The rocket was recovered, mostly intact. One of the perpendicular pieces attached to the fin was broken off. But that's an easy fix. Triceratops will fly again at some point!

Have to say that video
https://upcyclingtextbooks.org/?p=1328
not one of my favorites.

Ah well, give others a chance to get smarter at my expense.

Actually glad no one got hurt as that was a 2lb rocket.
Craig, do we know if all three engines lit? Was the chute open at recovery (doubt)
Appreciate (now that the video is out there, anyway:blush the opinions of the experts on this flight. Seemed like the rocket got off the pad quite well (2 lbs 10 oz total weight with engines and recovery device, 3 D12-3), then ran out of newtons and arced over (can't tell on the video if it just gently arced over or had acute turn, but no good reason for acute turn)
In any case, delay clearly too short. The rocket has lot of fin area so I don't think stability an issue, think just underpowered. With three 24mm motor mounts, not sure how to power it up anymore (E engines add a little extra thrust at cost of a LOT of extra weight, don't think this would help.)
If you reeeeallly wanted to launch it again, wondering if using D12-0s would work, as on the video seemed the rocket was already arced over by time delay smoke was seen. Would be a quick up and pop flight. Definitely heads up.

Quick math (not necessarily right)
3 D12s give me max thrust 32 newtons each so 96 newtons (https://norwoodhobbyshop.com/Rockets/enginechart.html)
2 lbs 10 oz is 2.626 lbs for 11.68 newtons. So thrust to weight greater than 5 if I did it right (explains why it seemed to get off the pad well.)
But it is a draggy rocket and may have slowed down quickly once the initial thrust spike died.
Maybe those extra outboard fins hurt more than they helped.

Micromeister said:

That is normally how I approach circular clustered motors, However it doesn't work as well with in-line clusters which will fly fine on two out of three motors but will arc over badly. the farther apart these in-line motors are the quicker the model will rotate. ie and Estes SR-71 will arc over an hit the ground if one of the outboard boosters fails to ignite.
Symmetric clusters are much easier to balance by slightly angling all motor mount tubes or pods at the CG of the ready to fly model.

Best way to avoid having motor not light is to use a 12V Hi-amp/hr power supply with a RELAY ignition system, and Check each igniter before, and after insertion in the motors and one last time after the motors are hooked up on the pad.

Click to expand...

Everything Micro said is pretty much dead on balls. Angling the mmt's is good insurance and it helps with inline mounts as well( side by side by side). Best, IMO, is to plan on a single engine out scenario.