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Fixed boosters. 29 mm mount. Booster engine help.

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ChrisLentz

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I am currently building a cluster that has 3 X 29mm motor mounts. The boosters are fixed. Other than using a reloadable engine with no ejection charge, how do I do it?
Cluster.jpg
My question is...
Do I just take the black powder out of the booster engines? Or just not put it in if it comes separate? Single use preferably.
 

rstaff3

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That's what I have done. You will get various opinions about what to do after the powder is out. On SU motors my go-to is to fill the well with 5-min epoxy. On reloads, I fill the well with either grease or dog barf and tape it off.
 

ChrisLentz

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I kind of thought the same idea. Epoxy the top without the bp charge inside.

Is there such a thing as a single use plugged booster above an E?
 

EXPjawa

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Are you relying on motor deployment rather than some form of altimeter? I'm guessing that's the case, based on the lack of an alt bay. If it were me - and this is what I've done - I'd port the outer tubes into the main body and run charges in all three motors (like the old Estes Trident). Since clustering SU motors can be tricky getting all to light and you have no back up charge, consider what would happen if the outer motors lit, but the center did not. It would boost nice and straight, probably with enough thrust to get a bit of altitude. But it would not deploy, because you were counting on the center motor and it wasn't burning. But - if the outer ones are ported into the center, when they fire their ejection charges, they'll serve as a built-in back up.
 

ChrisLentz

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That is a really good idea. Having all three light is a concern to me. Thankfully the launch will eventually take place, once I have the certification to fly an I motor or clustered equivalent, in a very remote area so if they don't all light, at least the public will be safe.

Now...If I vent the boosters to the main...when I fly with just a single G would the nose still pop off or would the gasses get ported into the boosters and have an ejection failure on my main?

I had one come in ballistic from 800 feet due to my nose cone venting in the shoulder (I should have put a coupler on it and was warned by a RF user it might happen).
[video=youtube;07YeQDpE-QM]https://www.youtube.com/watch?v=07YeQDpE-QM[/video]

I have concern that it may do the same. A single G80 will get this to 900ft but 2 X G125 and a G80 will push this to 3700 feet according to Rocksim. Typically my actuals are about 100 feet lower than Rocksim, that being said my rocket will fair pretty badly from 800 feet with an ejection malfunction.
 

rstaff3

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Are you relying on motor deployment rather than some form of altimeter? I'm guessing that's the case, based on the lack of an alt bay. If it were me - and this is what I've done - I'd port the outer tubes into the main body and run charges in all three motors (like the old Estes Trident). Since clustering SU motors can be tricky getting all to light and you have no back up charge, consider what would happen if the outer motors lit, but the center did not. It would boost nice and straight, probably with enough thrust to get a bit of altitude. But it would not deploy, because you were counting on the center motor and it wasn't burning. But - if the outer ones are ported into the center, when they fire their ejection charges, they'll serve as a built-in back up.
This makes too much sense.
 

EXPjawa

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This makes too much sense.
LOL.

Chris, if you do it right, any individual charge will pressurize the entire, combined, internal cavity. That means both SRB bodies and the main tube. So, that only works if all three motors are properly retained. If one falls out somehow, then the whole system is compromised. But I don't see that as an issue unless you're friction fitting. Here is an example of one I built that has a similar need:

http://www.rocketryforum.com/showthread.php?122619-Binary-Star&highlight=binary+star

In this case, there are only two motors, but they're in parallel body tubes. There's only one recovery device on only one side (originally was intended to have a chute on both sides). So, to make sure that either motor could blow the cone, I put a port between the sides just above the baffles (which I'd probably eliminate next time). I cut rectangular holes in the tubes, fitted a bridge piece of light ply, and then fitted in triangular balsa bits to close it out. After that, it was filled/blended/smoothed/etc to make it look good and minimize drag. You can sort of see it here:

 

rstaff3

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LOL.

Chris, if you do it right, any individual charge will pressurize the entire, combined, internal cavity. That means both SRB bodies and the main tube. So, that only works if all three motors are properly retained. If one falls out somehow, then the whole system is compromised. But I don't see that as an issue unless you're friction fitting. Here is an example of one I built that has a similar need:

http://www.rocketryforum.com/showthread.php?122619-Binary-Star&highlight=binary+star

In this case, there are only two motors, but they're in parallel body tubes. There's only one recovery device on only one side (originally was intended to have a chute on both sides). So, to make sure that either motor could blow the cone, I put a port between the sides just above the baffles (which I'd probably eliminate next time). I cut rectangular holes in the tubes, fitted a bridge piece of light ply, and then fitted in triangular balsa bits to close it out. After that, it was filled/blended/smoothed/etc to make it look good and minimize drag. You can sort of see it here:

I have only clustered APCP when the motors were airstarted and haven't though to do this with BP outboards, although on my bigger clusters I always have a few motors eject the laundry.

If you duct them, and decide to fly on one motor, remember to plug the extra leaks :D

Nice job blending the duct on that one...it looks great.
 

EXPjawa

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I have only clustered APCP when the motors were airstarted and haven't though to do this with BP outboards, although on my bigger clusters I always have a few motors eject the laundry.

If you duct them, and decide to fly on one motor, remember to plug the extra leaks :D

Nice job blending the duct on that one...it looks great.
Thanks. If I'd have done it during the early construction (rather than an after thought), it would be much more integrated. You can make the two ports directly face each other, and have just a little bridging on both sides of the rocket. In this case, I couldn't readily cut through at the tube joint, so its all on one side.

But you make an excellent point Dick, one which I only eluded too. If you fly a single center motor, leaving the other two mounts empty will allow the ejection charge to vent out rather than build pressure. I'd consider making some sort of easy-to-remove plug for that, maybe a balsa bulkhead with a eyelet in it so you have something to grab onto.
 

75Grandville

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Or even two spent motors, which is what I plugged my 4-29 SS with to fly on only two motors. Making sure they all light, that's a different story!
 

rstaff3

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+1 When necessary, I generally just use spent motors too.
 

rharshberger

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Are you relying on motor deployment rather than some form of altimeter? I'm guessing that's the case, based on the lack of an alt bay. If it were me - and this is what I've done - I'd port the outer tubes into the main body and run charges in all three motors (like the old Estes Trident). Since clustering SU motors can be tricky getting all to light and you have no back up charge, consider what would happen if the outer motors lit, but the center did not. It would boost nice and straight, probably with enough thrust to get a bit of altitude. But it would not deploy, because you were counting on the center motor and it wasn't burning. But - if the outer ones are ported into the center, when they fire their ejection charges, they'll serve as a built-in back up.
To this excellent recommendation I can add, use different length delays so that all three motors ejection charges don't go off at once, use the correct/simmed delay in the central motor, and +1 and +2 second delays in the booster or longer if desired, then there are three chances to get the laundry out without over pressurizing and blowing out a cardboard airframe tube (not much of a issue with FG tubes), I would say don't ask me how I know but I've had that experience already once.
 
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