Need clustering advice on brand new scratch build with side tube fins

[cbwho]

I'll start a new thread to avoid confusion.

I am putting this sort of rocket together. The equal marks are tubes, angles are fins.

Side pod boosters will act as tube fins when not in use. To adjust the nose weight the body tube will be sectioned such that the top most body tube forms part of the nose cone. In which removable Play-doh can be installed for extra weight.

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My questions:
Is there a lot of planning needed to determine the size/delays of the side motors? All three motors be the same size? Or can I use lower power engines for the side and a more powerful longer lasting one in the middle? I plan on all 3 to be 18mm cases.

 

[spigalau]

Personal experience is that not all delays are equal - we had a flight on 3 x 18mm C12-6 and one of the motors let loose after 2 seconds.



The other issue with have seen on the 18mm composite motors is that they can fail to ignite. The Black powder motors are far easier to ignite.

I would go with 18mm 'glider' (plugged) motors for the side pod boosters and just use the delay off the single central one.

 

[teepot]

I use booster motors with zero delay in the side boosters. I also mix the power and size, ie, 1 F15-6 and 2 E12-0's or 1 G40-10 and 2 C6-0's etc.

 

[cbwho]

Personal experience is that not all delays are equal - we had a flight on 3 x 18mm C12-6 and one of the motors let loose after 2 seconds.



The other issue with have seen on the 18mm composite motors is that they can fail to ignite. The Black powder motors are far easier to ignite.

I would go with 18mm 'glider' (plugged) motors for the side pod boosters and just use the delay off the single central one.

What an open launch site! I only get open launch sites on frozen lakes. Only a few more weeks left!

 

[jqavins]

As the others have said, I'd use only the center engine for ejection, and one way or another "nullify" the side pods' engines for that purpose.

Using plugged engines is obviously the best way to do that, if there are plugged engines available that you want to use.

The discontinued Aerotech 18 mm SU engines had ejection charges under paper caps that could be removed; this was a vendor approved procedure, so it was allowed. I don't know if removing the charge is feasible or allowed with Q-Jets.

Otherwise, the conventional wisdom, as I understand it, says to vent the side pods' tubes just below their nose cone shoulders, and glue the cones in.

What some people do, which is against the safety code so I am not-recommending-it-even-though-it's-a-really-good-idea-but-no-I'm-really-not, is to plug the engines yourself by pouring some epoxy into the top. That's technically modifying the engine in a non vendor approved manner, so against the rules. So you definitely shouldn't do it, no matter how well it works. (Nudge nudge wink wink.)

 

[cbwho]

Hopefully some day I will have a successful cluster light?

This is 3 A motors wired together. Of the ignitors had an open connection...

My second attempt.

 

[BABAR]

View attachment 452696


Hopefully some day I will have a successful cluster light?

This is 3 A motors wired together. Of the ignitors had an open connection...

My second attempt.

Hmmmmm..... Fly-By-Wire is usually reserved for high tech fighter planes. Looks like the technology is a bit premature for model rocketry.

 

[BABAR]

I'll start a new thread to avoid confusion.

I am putting this sort of rocket together. The equal marks are tubes, angles are fins.

Side pod boosters will act as tube fins when not in use. To adjust the nose weight the body tube will be sectioned such that the top most body tube forms part of the nose cone. In which removable Play-doh can be installed for extra weight.

|\
|==
|=====|==|>
|==
|/

My questions:
Is there a lot of planning needed to determine the size/delays of the side motors? All three motors be the same size? Or can I use lower power engines for the side and a more powerful longer lasting one in the middle? I plan on all 3 to be 18mm cases.

Hard to scale your design as provided!

few things.

outboard cluster motors are cool. They also range from near zero risk when they are abutted centrally right up against the main body tube to extremely high risk/danger don’t go there if they are far off axis AND NOT canted medially, I think through the CG but @Daddyisabar and @jflis are more expert on that subject. An exception is when the outboard motors are very low impulse compared with the central main motor or motors, in which case they are mainly for show and not for significant contribution to lift off velocity or altitude (think 13 mm outboard with at least a 24mm D main.)

next, tube fins. If you indeed want the tubes to be a significant factor in stability of the rocket, they can’t be much longer than the motor casing length to width ratio. Tubes that are much longer than wide even when OPEN progressively (as that length to width ratio gets higher) stop functioning as fins and start functioning as cylinders, good for drag but not much else. Perhaps more importantly, if you are thinking of a dual purpose rocket, “sometimes I will fly with tubes open and functioning as fins, sometimes I will plug them with motors”, you got a problem. If the rocket flown WITHOUT motors is dependent on the tube fins for stability, you have squared your skywriting probability when you 1. Plug those tube fins with motors and simultaneously 2. Add tail weight with those same motors. Put another way, the second configuration moves Cp forward and Cg backward. Think either more NON-tube fin area or adding nose weight.

next and likely more controversial, assuming you are going with 18 mm outboards and depending on your launch site rules, the easiest and most efficient way of dealing with ejection charges (or burn through forces of zero delay motors, which are less than ejection charges but NOT negligible) is to use zero delay motors, loosely friction fitted, and eject them. This is perfectly legal by NAR rules, don’t know about Tripoli, but may NOT be allowed by local clubs or landowners (the motors are biodegradable, but I am told they can mess with farm equipment, like sod farms.). I say most efficient because with zero delay motors they eject as soon as they burn out, reducing the mass of the model (which assuming the main is still burning should give you more altitude) AND the force of ejection itself according to Newton actually provides a kiss of thrust as the motor flies out the backside.

hope you get three straight trails and a short recovery walk!

 

[jqavins]

View attachment 452696


Hopefully some day I will have a successful cluster light?

This is 3 A motors wired together. Of the ignitors had an open connection...

My second attempt.

Looks like I need to finish my stalled electronics project. I have a working proof of concept for a launch controller that remotely measures your cluster resistance, and calculates how many igniters are there, not broken if parallel and not shorted if series. It also reports the "raw" resistance. It's been sitting in the proof of concept stage for a few years. So I'd better get a protoype finished and publish the plans. (I'm not planning a product; I'll be content to keep using a prototype.)

 

[cbwho]

Looks like I need to finish my stalled electronics project. I have a working proof of concept for a launch controller that remotely measures your cluster resistance, and calculates how many igniters are there, not broken if parallel and not shorted if series. It also reports the "raw" resistance. It's been sitting in the proof of concept stage for a few years. So I'd better get a protoype finished and publish the plans. (I'm not planning a product; I'll be content to keep using a prototype.)

That's a great idea. I could do some ohm tests and use that to double check.

 

[cbwho]

Hard to scale your design as provided!

few things.

outboard cluster motors are cool. They also range from near zero risk when they are abutted centrally right up against the main body tube to extremely high risk/danger don’t go there if they are far off axis AND NOT canted medially, I think through the CG but @Daddyisabar and @jflis are more expert on that subject. An exception is when the outboard motors are very low impulse compared with the central main motor or motors, in which case they are mainly for show and not for significant contribution to lift off velocity or altitude (think 13 mm outboard with at least a 24mm D main.)

next, tube fins. If you indeed want the tubes to be a significant factor in stability of the rocket, they can’t be much longer than the motor casing length to width ratio. Tubes that are much longer than wide even when OPEN progressively (as that length to width ratio gets higher) stop functioning as fins and start functioning as cylinders, good for drag but not much else. Perhaps more importantly, if you are thinking of a dual purpose rocket, “sometimes I will fly with tubes open and functioning as fins, sometimes I will plug them with motors”, you got a problem. If the rocket flown WITHOUT motors is dependent on the tube fins for stability, you have squared your skywriting probability when you 1. Plug those tube fins with motors and simultaneously 2. Add tail weight with those same motors. Put another way, the second configuration moves Cp forward and Cg backward. Think either more NON-tube fin area or adding nose weight.

next and likely more controversial, assuming you are going with 18 mm outboards and depending on your launch site rules, the easiest and most efficient way of dealing with ejection charges (or burn through forces of zero delay motors, which are less than ejection charges but NOT negligible) is to use zero delay motors, loosely friction fitted, and eject them. This is perfectly legal by NAR rules, don’t know about Tripoli, but may NOT be allowed by local clubs or landowners (the motors are biodegradable, but I am told they can mess with farm equipment, like sod farms.). I say most efficient because with zero delay motors they eject as soon as they burn out, reducing the mass of the model (which assuming the main is still burning should give you more altitude) AND the force of ejection itself according to Newton actually provides a kiss of thrust as the motor flies out the backside.

hope you get three straight trails and a short recovery walk!

I swing tested with 1 and 3 engines. It passed. So I am not dependant on the tube fins.
I designed it to eject to atmosphere and not eject the motors.

Yesterday, I went with 3 A motors but it sounds like A B A is a better idea?

 

[jqavins]

That's a great idea. I could do some ohm tests and use that to double check.

You'd have to perform that test at the pad if you're using a regular ohm meter. And for that matter, it will have to be one with a sub-ohm range. That's why what I'm doing is a project.

 

[BABAR]

I swing tested with 1 and 3 engines. It passed. So I am not dependant on the tube fins.
I designed it to eject to atmosphere and not eject the motors.

Yesterday, I went with 3 A motors but it sounds like A B A is a better idea?

Great decision to swing test it in both configurations. I am kicking myself for not suggesting that.

IMO A B A is as good as if not better than A A A, IF the rocket is stable with the additional mass of the B motor. The difference in mass is less than 2 grams, so it should be

https://www.rocketreviews.com/compare-estes-a8-to-estes-b6.html

Would like to see a picture of the rocket itself, a little more detail than the ones already provided!

In general, I think it is always safest to make sure your central motor is AT LEAST as powerful as the outboards. Safest configuration is where the central motor is extremely dominant, and in fact if the central motor doesn’t light, the outboards don’t have the thrust to get the rocket off the pad. That way, if core doesn’t light, rocket sits merrily on the pad blowing smoke and sparks but not a danger to anyone.

In non-electronic deployment black powder clustering, which includes outBoards, Unless the outboards are ducted to the central rocket (in other words, unless the outboards can deploy a chute, sounds definitely NOT the case here), successful liftoff based on thrust from the lit outboards in the event of UNSUCCESSFUL ignition of the core motor means the rocket has no way of deploying the chute. This is considered poor form.

 

[BABAR]

Here are curves for A B A

https://www.rocketreviews.com/combined-motors-9966.html
And A A A


https://www.rocketreviews.com/combined-motors-181118015106.html

 

[cbwho]

It's a modified Crossbow SST. I added a 70mm payload bay and changed the fins etc. This shifts the CG forward. I swing tested on a C and a A C A setup.

The "wings" got singed on the side pod ejection charges so I plan to add aluminum tape on the trailing wing surface.

 

[jqavins]

In general, I think it is always safest to make sure your central motor is AT LEAST as powerful as the outboards. Safest configuration is where the central motor is extremely dominant, and in fact if the central motor doesn’t light, the outboards don’t have the thrust to get the rocket off the pad. That way, if core doesn’t light, rocket sits merrily on the pad blowing smoke and sparks but not a danger to anyone.

I think in this case the criterion that 2×A8 won't get it off the launch rod but 2×A8 + B6 will give sufficient thrust to weight ratio is not possible. Consider. If the rocket's weight is less than the sustained thrust of 2×A8. that means it weights more than 8 N (about 800 g mass). To not move at all would need a lot higher mass because of the thrust peaks, but it needs 800 g at least, or 7.9 N

If one A8 lights, with peak thrust of about 9.5 N, it will lift off the pad and may or may not clear the rod (then fall down as the initial thrust spike ends). If the 2 A8s light it will surely clear the rod, but not with safe speed.

In short, the minimum mass to stay on the rod with 2×A8 is more than the maximum mass for a safe launch on 2×A8 + 1×B6. (And a C6 doesn't help the situation, since it's peak thrust is the same as the B6.)

So, this is going to be a heads up launch every time.

EDIT:
Confirmed by thrustCurve.org. Even if the outboards are 13 mm ¼A3s, to keep the apogee at 4 ft the mass needs to be 330 g. At 330 g, the C5 gives you 19.9 ft/s off the rod, and all other 18 mm BP engines are worse.

(Thrust curve doesn't support more complex clusters, like 2×¼A3 + C5, but that can't possibly make enough difference to be safe when everything lights.)

It's a modified Crossbow SST. I added a 70mm payload bay and changed the fins etc. This shifts the CG forward. I swing tested on a C and a A C A setup.

The "wings" got singed on the side pod ejection charges so I plan to add aluminum tape on the trailing wing surface.

By the way, nice rocket.

 

[boomtube-mk2]

This is a clustered, tube-fin rocket using a canted triple engine motor mount.

T
The motor tubes a buried inside of three of the tube fins.

T

 

[jqavins]

Nice. I'd like to see a CFD model showing how the exhaust plumes through tubes affect the stability, and the effectiveness of the boat tail, but only for academic curiosity. As long as if flies straight it's very nifty.

 

[boomtube-mk2]

It is very stable with a unusual smoke trail.

It attempts to create the typical canted motor spreading smoke pattern but the air flowing through the tube fins causes the three smoke trails to recombine below the lower nosecone.

 

[jqavins]

Are you sure that's due to the tube fins? I bet the same thing would happen with that boat tail and regular fins.