Starstreak (3D Printed Parts + Plywood Fins + BT-80/BT-5 tubes)

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BigMacDaddy

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I posted about this earlier in the "what I did rocket-wise..." thread but figured this deserves its own thread.

I made some progress on my Starstreak build today and yesterday. Still a fair amount of detail I can incorporate in the design (not to mention everything still needs shoulders and other internal parts). Anyway, below is what I have so far - comes in at just about 60cm so just shy of 24". Using BT-80 booster tube and BT-5 sustainer / mini-rocket tubes.

Considering how many fins I should include for sustainers / mini rockets -- likely going with 2 canards and 3 fins on rear of each (pictures / renders seem to show 3 or 4 fins on rear and 2, 3, or 4 on nose -- with mixes of the configurations as far as I can tell). I suppose I will make the canard fins rotate if I can make that work on these tiny airframes and fins (have done that before but always on much bigger models).

One of my big questions is what is the best way to do gap staging. I have done gap staging with a 2-engine cluster feeding a single sustainer motor. However, I have never fed a sustainer cluster (or in this case multiple separate sustainers). The sustainers will be mini engines (A-10s probably) if that matters. I also need to eject the recovery system for the booster so I was thinking some zero delay ##-0 engines (to ignite multiple sustainers) + standard ##-# engine to eject the main recovery system/nose cone.

Do folks have opinions on which would be more viable:
- 3x C6-0 booster cluster w/ Individual BT-20 tubes extending to enclose sustainer mini engines (each tube vented to outside). Central C6-5 could be used to eject nosecone.
- 2x D12-0 engines fed through a funnel into BT-50 tube that extends up to a small chamber right below the 3x mini engines (w/ a vent or two in this chamber). 3rd D12-5 engine ejects recovery for booster / main rocket.
- Something else?

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I am afraid that the ejection charge of a C6-0 might not be enough to bridge the gap (gap is around 10-11"). I think that 2x D12's would be better. My assumption is that it might be better to carry gasses in one tube to bridge the gap and then open up in a small space exposed to all three engine right at the end (less cooling, if first booster engine does not ignite everything 2nd booster engine would get another chance, one sustainer engine launching might even ignite the others).
 
I am afraid that the ejection charge of a C6-0 might not be enough to bridge the gap (gap is around 10-11"). I think that 2x D12's would be better. My assumption is that it might be better to carry gasses in one tube to bridge the gap and then open up in a small space exposed to all three engine right at the end (less cooling, if first booster engine does not ignite everything 2nd booster engine would get another chance, one sustainer engine launching might even ignite the others).
Going 1-1 booster sustainer (the”normal” ratio!) I have successfully gapped c6-0 motors to 18 mm motors routinely up to 15” without fail.

My favorite booster motor is still the D12-0 (although C11-0 is probably as good, just haven’t used it yet). I have done 53” gap with D to D. 2 for 2. I am 0-1 for 72” in flight (but 1 for 1 on test stand)

I think if you do a good duct manifold, ducting a single midline 24 mm motor mount to the three 13mm mounts, with a good “seal” except for the vents just caudal to the 13mm sustainer nozzles, your probability of booster ignition is very high.

Now idea of chute deployment for booster. Go with two (or three) 13mm short delay motors all ducted to a single baffle then to chute . Not sure if your gonna go rear or forward eject, but if any ONE fires, should be sufficient.

Rationale:

Clusters (in this case 3 or 4, depending on if you go 2 or 3 on the booster 13mm motors) are getting pretty reliable with 12 volt ignition battery and perhaps the new igniters, but “stuff” happens.

Failure possibilities

Central core 24 doesn’t light, any or all the 13 mm motors light. Result is the rocket never leaves the pad, the 13mm motors (use smallest thrust possible) aren’t sufficient to lift the stack off the rod. Booster Chute pops on pad. SUSTAINERS DO NOT LIGHT. Slightly embarrassing, but safe and no damage.

Core lights and at least one booster lights. Good flight with successful booster chute deployment.

Core lights and no 13 booster lights (unlikely, but good reason to go with three boosters.) the stack will lift. The booster will come in ballistic, but given shape (while ballistic is never good) unlikely to hurt anyone or anything.

Going with multiple C6-0s adds complexity which is never good.

Also , three C6 motors IF they all light get you to 339 grams max take off (and this puppy’s full stack is gonna be heavy) and cost you 37.5 grams of tail weight, compared with 396 grams max take off for single D12 at about 25 grams tail weight.

Only one or two C6 motors doesn’t light, the remaining one or two LIKELY will get it off the rod, but also likely with insufficient velocity, which will not endear you with the RSO.



As for boosters, if you are going 13mm I like the 1/2A6-2.

First, it’s got the shortest delay, you will want streamer recovery (you reeeeeaaaallly don’t want to be chasing four parachutes all over the place, you want these down fast and close). So even though will Likely deploy during coast, streamers especially if rolled up a bit are more forgiving on shock cord stress and zippers.

Second, while not the weakest, the 1/2A6-2 is pretty low thrust, so if staging occurs off vertical you want as LITTLE power as you can achieve. The bad thing about black powder non-electronic staging is there are NO lockouts, once that core zero delay ignites it is highly likely to stage, at whatever speed and orientation it finds itself in when propellant burn through occurs. You want to do everything possible to avoid fecal-turbine interaction.

You definitely don’t want long burn long delay or high thrust sustainer motors for first flight. Just successfully stage three sustainers gets you all the “oh wow, that was COOL!” On the first flight you are gonna get. Any extra sustainer motor beyond that is simply more search time, as they will go higher and drift farther.

I will now step off the soap box. Sorry for length.
 
Going 1-1 booster sustainer (the”normal” ratio!) I have successfully gapped c6-0 motors to 18 mm motors routinely up to 15” without fail.

My favorite booster motor is still the D12-0 (although C11-0 is probably as good, just haven’t used it yet). I have done 53” gap with D to D. 2 for 2. I am 0-1 for 72” in flight (but 1 for 1 on test stand)

I think if you do a good duct manifold, ducting a single midline 24 mm motor mount to the three 13mm mounts, with a good “seal” except for the vents just caudal to the 13mm sustainer nozzles, your probability of booster ignition is very high.

Now idea of chute deployment for booster. Go with two (or three) 13mm short delay motors all ducted to a single baffle then to chute . Not sure if your gonna go rear or forward eject, but if any ONE fires, should be sufficient.

Rationale:

Clusters (in this case 3 or 4, depending on if you go 2 or 3 on the booster 13mm motors) are getting pretty reliable with 12 volt ignition battery and perhaps the new igniters, but “stuff” happens.

Failure possibilities

Central core 24 doesn’t light, any or all the 13 mm motors light. Result is the rocket never leaves the pad, the 13mm motors (use smallest thrust possible) aren’t sufficient to lift the stack off the rod. Booster Chute pops on pad. SUSTAINERS DO NOT LIGHT. Slightly embarrassing, but safe and no damage.

Core lights and at least one booster lights. Good flight with successful booster chute deployment.

Core lights and no 13 booster lights (unlikely, but good reason to go with three boosters.) the stack will lift. The booster will come in ballistic, but given shape (while ballistic is never good) unlikely to hurt anyone or anything.

Going with multiple C6-0s adds complexity which is never good.

Also , three C6 motors IF they all light get you to 339 grams max take off (and this puppy’s full stack is gonna be heavy) and cost you 37.5 grams of tail weight, compared with 396 grams max take off for single D12 at about 25 grams tail weight.

Only one or two C6 motors doesn’t light, the remaining one or two LIKELY will get it off the rod, but also likely with insufficient velocity, which will not endear you with the RSO.



As for boosters, if you are going 13mm I like the 1/2A6-2.

First, it’s got the shortest delay, you will want streamer recovery (you reeeeeaaaallly don’t want to be chasing four parachutes all over the place, you want these down fast and close). So even though will Likely deploy during coast, streamers especially if rolled up a bit are more forgiving on shock cord stress and zippers.

Second, while not the weakest, the 1/2A6-2 is pretty low thrust, so if staging occurs off vertical you want as LITTLE power as you can achieve. The bad thing about black powder non-electronic staging is there are NO lockouts, once that core zero delay ignites it is highly likely to stage, at whatever speed and orientation it finds itself in when propellant burn through occurs. You want to do everything possible to avoid fecal-turbine interaction.

You definitely don’t want long burn long delay or high thrust sustainer motors for first flight. Just successfully stage three sustainers gets you all the “oh wow, that was COOL!” On the first flight you are gonna get. Any extra sustainer motor beyond that is simply more search time, as they will go higher and drift farther.

I will now step off the soap box. Sorry for length.
Thank you so much for thoughtful and detail reply.

I would also prefer to go with a D12-0 in the booster. I think I can design a good manifold and was thinking it would be best to include channels directly across each engine opening with a vent at the end of each channel.

I think all the 13mm motors that I know of have much less burn time than a D12 so not sure I could use them for the booster recovery deployment (booster would deploy recovery before sustainer ignite - unless I am misunderstanding). I would prefer to use a paired engine with a delay (i.e., D120 + D12-3 or D12-5) for ejecting the booster recovery - easier to keep things in sync. Also pair of D12 engines provides a ton of lift.

I think I might be able to set this up so that when the booster deploys its nosecone this mechanism also pushes off the nosecones of any sustainers that have not launched. I think that would avoid them coming in ballistic. Need to play around with that idea (biggest issue is that the central column in this model is only a BT-50 tube so ideally would rather have a larger area to store / eject chute).
 
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1. I think all the 13mm motors that I know of have much less burn time than a D12 so not sure I could use them for the booster recovery deployment (booster would deploy recovery before sustainer ignite - unless I am missunderstanding).
not so fast, Here’s the burn data on D12

https://www.rocketreviews.com/estes-d12-5112.html
zero delay motors “burn through” at the end of propellant burn. I suspect they may actually burn through FASTER than the thrust curve, as the forward propellant wall breaks down at burn through, rather than a progression from propellant to delay grain in a standard motor.

in any case, the graph shows a
Thrust Duration: 1.6500 s
so even for “Delays have a tolerance of plus or minus 10% or 1 second, whichever is greater” quoted from the Estes motor chart. So you are guaranteed to be good with a 3 second delay motor, probably okay with a 2 second delay. I use 18mm motors for chute deployment on my long gap stagers with a D12-0, so usually an A8-3 (big part is usually I buy the 24 motor blast off pack so I end up with a bunch of A8-3s)
I would prefer to use a paired engine with a delay (i.e., D12-3 or D12-5) for ejecting the booster recovery - easier to keep things in sync. Also pair of D12 engines provides a ton of lift.
you maaaaay need two D12s if pad weight is over 396 grams, I might go conservative and say 300-350 as thpuppy will be draggy as heck, and those booster fins aren’t that big (how critical is scale here? BTW). I’d also consider mini rail buttons as this is a complex bird and the less rocking on the pad the better.
I think I might be able to setup this so that when the booster deploys its nosecone this mechanism also pushes off the nosecones of any sustainers that have not launched. I think that would avoid them coming in ballistic.
that would be a neat trick, I have no idea how to do it. Another option would be some mechanism that would RETAIN the sustainers on the booster if they don’t light, but that’s tricky too.
Need to play around with that idea (biggest issue is that the central column in this model is only a BT-50 tube so ideally would rather have a larger area to store / eject chute).
Got an idea for this. Booster has central core BT-50 for the D12-0. Imagine six BT-5 tubes arranged radially around the core. We are going to cut away parts of three of them in a moment. Those three are ducted to the boosters. BETWEEN each of these ducted three tubes is a BT-5 that runs the length of the booster, the forward holes are hidden by the sustainers on the ducted tubes. Each of THESE a three has a 1/4A3-3T, some wadding, short shock cord, and a streamer on it. you can try chutes but tough not to tangle them. Or maybe one with a chute and other two streamers. Triple redundancy and extreme simplicity. Of course, if sustainers don’t ignite you’re hosed, as the sustainers will block the streamer tubes.

But unlikely That sustainers won’t light. all my one to threes (Estes MIRV kit and two scratchers, Three Dog Night and Cerberus) were 100% successful (9 for 3!) Properly ducted and vented, a D12-0 can easily light three sustainers.

just brain storming here.
 
Will follow your thread with much anticipation as I'm known to like a Starstreak! ;)

Why not go for four aft fins on the Hitiles (the darts)? It matches the real thing and is in keeping with the scale/high quality standard of the rest of your design. The darts will need to be stable at the expected separation speed, so play with their design in OR to see whether a 3 or 4 fin configuration best suits your stability and nose weight requirements.

+1 on the smallest size streamer/chute recovery system that your darts can contend with - bring them down fast and close!
 
so even for “Delays have a tolerance of plus or minus 10% or 1 second, whichever is greater” quoted from the Estes motor chart. So you are guaranteed to be good with a 3 second delay motor, probably okay with a 2 second delay. I use 18mm motors for chute deployment on my long gap stagers with a D12-0, so usually an A8-3 (big part is usually I buy the 24 motor blast off pack so I end up with a bunch of A8-3s)

Ah, yes... silly bad mental addition... for some reason I thought D12 had more than 1.7sec of thrust. So even with .3sec of thrust for the 1/2A6 with a 2 second delay that would be ok (an A10 w/ .9s of thrust and 3sec delay would also probably be good).

Need to re-read other ideas when I am fully awake...
 
Will follow your thread with much anticipation as I'm known to like a Starstreak! ;)

Why not go for four aft fins on the Hitiles (the darts)? It matches the real thing and is in keeping with the scale/high quality standard of the rest of your design. The darts will need to be stable at the expected separation speed, so play with their design in OR to see whether a 3 or 4 fin configuration best suits your stability and nose weight requirements.

+1 on the smallest size streamer/chute recovery system that your darts can contend with - bring them down fast and close!

Thanks for feedback / following!

I was trying to figure out how many fins are on the Hitiles (by the way I just figured out how to read / pronounce that as the manufacturer intended - Hit-iles, like "projectiles" that always "hit" << at least that is my assumption).

I saw some versions seem to have 3 in rear and 3 in front but then others seemed to have 4 in rear and 2 in front. I can do this as 4 and 2. Probably a bit more stable if I cannot make the Canards move.
 
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Definitely four aft fins on the Starstreak darts and only two canards. In the the second generation of it, the dart aft fins changed shape from triangular to the larger trapezoidal set you have in your initial post. Some images on-line are CAD/graphic renderings rather than actual photos, plus some images posted as Starstreak are actually of other, later Thales missiles.

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Are you considering 3D printing the dart fin cans and/or a combined nose cone/canard assembly?
Doing that might take some of the fiddly work out of the construction for you plus it leaves the door open to make spare parts for lost/broken darts. I lost my first dart in its test air-start launch off the booster, mainly because the chute was too big so it drifted too far, so I had to rush a fourth one into production for the all-up launch.
 
Definitely four aft fins on the Starstreak darts and only two canards. In the the second generation of it, the dart aft fins changed shape from triangular to the larger trapezoidal set you have in your initial post. Some images on-line are CAD/graphic renderings rather than actual photos, plus some images posted as Starstreak are actually of other, later Thales missiles.

View attachment 581102


Are you considering 3D printing the dart fin cans and/or a combined nose cone/canard assembly?
Doing that might take some of the fiddly work out of the construction for you plus it leaves the door open to make spare parts for lost/broken darts. I lost my first dart in its test air-start launch off the booster, mainly because the chute was too big so it drifted too far, so I had to rush a fourth one into production for the all-up launch.
Thank you for that. Your larger build definitely encouraged me to think this is possible - hope I can make it work in a smaller scale.

I am trying to catch important scale details that I see in the pictures. I need to do at least one more pass through / revision to add smaller details, launch lugs, etc... If you see any other missing / incorrect details please let me know (once my eyes overlook something they tend to keep overlooking it).

I did actually make the rear booster fins a bit longer (extended them towards the rear).

I was planning to make the Hitile nosecones as a single piece with the canards (possibly with the canards able to pivot -- maybe together for simplicity / strength -- but I have not printed this so I may be overestimating my ability to make something this small and complex).

Since the Hitiles are minimum diameter and since the rear area does not have much detail I was thinking of surface glued fins without a fin can. I could certainly do fin cans if it helped.
 
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Ok, updated with the 4 fins on rear of Hitiles. It is a tight fit -- I think I will need to modify the bracket details to avoid the fins getting hung up on them.

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Was this what you were considering for 24mm central and 13mm outer? Fits in BT80 tube.View attachment 581114
Thanks Ken -- if I did cluster to cluster then I would do something like that... Adds quite a bit of weight and really blocks up the BT-80 tube (i.e., not much room for a recovery system for the booster).

I am going to build some of the components to see how heavy this gets. It actually does not need to be very heavy and much of the weight will be in the front so may not be too difficult to make this work with a pair of C11 or D12 engines in the rear.
 
Thanks Ken -- if I did cluster to cluster then I would do something like that... Adds quite a bit of weight and really blocks up the BT-80 tube (i.e., not much room for a recovery system for the booster).

I am going to build some of the components to see how heavy this gets. It actually does not need to be very heavy and much of the weight will be in the front so may not be too difficult to make this work with a pair of C11 or D12 engines in the rear.
Correct on the "not much room for a recovery system" part; however this configuration really simplifies the staging in that each 18mm booster (corrected from earlier "13mm") will have ejection directly up to a Hitile.
 
Made v1 (v1.1 in some cases) of several of the parts... Decided to try a small 2mm hook to hold the NC in front of Hitile to central structure. This can also be used to eject undeployed Hitile nosecones if I can eject that central nosecone (still need to work out the ejection flow to allow that so this is all just a mock-up to help me think about this.

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Was this what you were considering for 24mm central and 18mm* outer? Fits in BT80 tube.View attachment 581114
(*corrected)
Coyote’s pic gave me an idea that may greatly simplify recovery.

you are going with BT-5s for the sustainers, the pic is with BT-20s, although BT-5s would obviously also fit.

i would still duct the central D12-0 to the three sustainers.

use the 18mm outboards for A8-3s or maybe B4-2s. There is a reason. We are going bat-crazy rear eject (@Daddyisabar is gonna love this.)

I usually try to attribute ideas to their source, but I can’t remember where I found this. How to do minimum diameter rear eject.

in order to do this in a NAR legal fashion, you can’t modify the motor. But if you have a black powder motor that has only 1/2 (maybe 2/3?) of the casing filled with propellant, you CAN as I read it insert something non-obstructive in the rear half of the casing as an anchor. The item I saw in this article (please if someone can reference it do so) is a cut safety pin, so you have the circular spring end and two short rods, cut with angled tips. Bend it out so it is just a bit larger than the lumen of the casing, attach a medium length of descent Kevlar that will withstand at least one ejection Charge.

here’s my personal mod idea.

use A8-3 in BT-20, you said you have 10 inch length of booster.

make a bulkhead say from two piece of MAT board, or thin plywood, not too thick, definitely not more than 1/4 inch.

need about, say, 6-9 inches of Kevlar (needs to be flexible/coil-able, as the motor MUST be able to push away from the bulkhead at least 6 inches to generate KINETIC FORCE) between anchor and the bulkhead. The bulkhead is NOT glued to the tube, it is push up against engine block to protect contents from ejection charge.

take another BT-20 tube 6.5-7 inches long, cut a lengthwise strip maybe 1/8 inch wide out of it, enough so it will roll up inside the outer tube. This will serve as a motor block for the motor AND the Kevlar AND the bulkhead, specifically you cannot allow the motor ejection charge to push the bulkhead further up the tube.

“But Babar, why such a long engine block?”

good question. The long block section will contain the shock cord (continuing through the bulkhead and the length of BT-20, it will secure in a screw eye or other attachment at the forward end. The INSIDE must be as smooth as a baby’s heinie, which is why you can’t just use a short motor block, the whole thing (shock cord from bulkhead end, streamer or maybe chute, more shock cord) needs to slide in and out smooth as silk.

operation:

Booster Motors (D12-0, three A8-3) ignite on pad

D12-0 hits burn through at about 1.65 seconds and ignites sustainers, which fly away.

MEANWHILE, there is a good chance the booster MAY continue to ascend, in an case even if it stops cold, it will only have 2-3 seconds to start to fall downward before the A8-3s start firing.

each A8-3 ejects from tube and accel away from the booster UNTIL

motor reaches end of ”tether”( shock cord length between anchor and bulkhead), at this point the kinetic energy obtained “yanks” the shock cord, streamer or chute and remaining shock cord from the ”sleeve-like” long motor block.

you can probably get a pretty good length of crepe paper streamer rolled up in each of your tubes.

the remaining forward end of the booster is gonna need to be tough, as the booster is gonna come down nose first, but at least the fins should have a lesser impact.

I thought of trying this with the central 24 mm motor, but the D12-0 is fully packed and has no room for the anchor. I think a C11 would, but c11 max lift of is 170 grams, compared to 396 for the D12.

you COULD also go with a 7 motor cluster (d12-0 middle, three 13 mm A10-0T’s individually ducted to sustainers, and three 18 mm A8-3s as above), but that’s going from 4 motors to 7 on the cluster, still might be easier. I dunno,
 
Finally got back to this design.

I think I simplified things by having a central 24mm motor and 3x 18mm motors (rather than 6x). Plan is to have the 24mm motor ignite the mini engines in the dart sustainers through BT-50 motor mount tube and for 18mm motors to eject the nosecone of the booster along with chute(s) that bring down booster.

Really pleased with how the fins on the Hitiles / darts and all the parts worked out around the BT-55 front tube. Need to modify the front bracket a bit so that it does not interfere with the darts launching.

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Looks awesome. I might suggest three additional vents on the front facing portion of the booster body right between each sustainer, if that allows some more of the booster ignition charge to escape. Not sure if that is a feasible place for a vent. How far does the 24mm motor tube extend into the body towards the sustainers? Is that like a through tube all the way up to the vent holes?
 
Looks awesome. I might suggest three additional vents on the front facing portion of the booster body right between each sustainer, if that allows some more of the booster ignition charge to escape. Not sure if that is a feasible place for a vent. How far does the 24mm motor tube extend into the body towards the sustainers? Is that like a through tube all the way up to the vent holes?
Thanks.

Not sure how easy it is to see here but the BT-50 motor mount tube goes right up into the part the sustainer darts are plugged into. Each dart has a channel and an exhaust vent at the end of that channel. This whole yellow part is 16mm thick so the exhaust gases should pass very close to the mini-engine nozzles.

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Thanks.

Not sure how easy it is to see here but the BT-50 motor mount tube goes right up into the part the sustainer darts are plugged into. Each dart has a channel and an exhaust vent at the end of that channel. This whole yellow part is 16mm thick so the exhaust gases should pass very close to the mini-engine nozzles.

View attachment 598826

Great to see more progress.

Worth considering a ground test of the 24mm motor ejection charge igniting the 18mm motors in the Darts. I'm not talking from any experience of this type of staging ignition, but my initial impression is that the vents risk losing too much of the ignition hot gas/sparks after already sub-dividing 3 ways.

My other thought was why have the vents, what is their benefit? From my experience with my own Starstreak, the exhaust from the 18mm BP motors in the Darts is going to cause blast damage to the upper transition no matter what you do. I've now added removable S/Steel blast shields to stop my fibreglass transition from being damaged further by the air-start of my 18mm motored Darts. Yet to be tested in flight.



Dart exhaust damage 13Feb22.jpg Blast Shield pic2.jpg
 
Great to see more progress.

Worth considering a ground test of the 24mm motor ejection charge igniting the 18mm motors in the Darts. I'm not talking from any experience of this type of staging ignition, but my initial impression is that the vents risk losing too much of the ignition hot gas/sparks after already sub-dividing 3 ways.

My other thought was why have the vents, what is their benefit? From my experience with my own Starstreak, the exhaust from the 18mm BP motors in the Darts is going to cause blast damage to the upper transition no matter what you do. I've now added removable S/Steel blast shields to stop my fibreglass transition from being damaged further by the air-start of my 18mm motored Darts. Yet to be tested in flight.



View attachment 598881 View attachment 598883
Agree about the value in testing this -- the vents are to let the cold air out of the motor mount tube since this is gap staging. I definitely worry about splitting those gases up so plan to test this with the booster held in place just to see if the sustainers ignite. Should be a good bit of chaos is all 3 of the darts ignite...

Also no idea about how much damage the parts will take due to this. Could be a major issue or could just require paint touch up after each launch...
 
Agree about the value in testing this -- the vents are to let the cold air out of the motor mount tube since this is gap staging. I definitely worry about splitting those gases up so plan to test this with the booster held in place just to see if the sustainers ignite. Should be a good bit of chaos is all 3 of the darts ignite...

Also no idea about how much damage the parts will take due to this. Could be a major issue or could just require paint touch up after each launch...
I understand better now, thanks for explaining the vent path being for the cold air to exit as the hot gases pressurize the 24mm MMT.

Testing will be interesting to see what degree of erosion/blast damage occurs. If it proves to be too heavy from just one launch, then perhaps the transition design might be able to be adapted to have some modular/replaceable bits rather than a single piece 3D print. That way the most damaged parts could be pushed out and replaced.

One question I forgot to ask before - will your fins be hinged and spring loaded, like the real thing for tube launching?
 
I understand better now, thanks for explaining the vent path being for the cold air to exit as the hot gases pressurize the 24mm MMT.

Testing will be interesting to see what degree of erosion/blast damage occurs. If it proves to be too heavy from just one launch, then perhaps the transition design might be able to be adapted to have some modular/replaceable bits rather than a single piece 3D print. That way the most damaged parts could be pushed out and replaced.

One question I forgot to ask before - will your fins be hinged and spring loaded, like the real thing for tube launching?
They are hinged right now and I was looking into double torsion springs to open them -- but those springs are so expensive... Not sure why I cannot find cheaper ones -- actually one of the holdups. If I cannot find them I might just CA glue them in the open position.
 
The vents do allow the cold gas to escape, but you could put those anywhere if that’s all you were trying to do.

the location of the vents is critical not for allowing the COLD gas to escape, but by providing a PATHWAY for the hot gas that follows. The hot gas has (or produces) photons that are pretty much the only thing that can travel INTO an exhaust nozzle. Makes sense if you think about it, neither particles nor even the gas itself easily travels into the nozzle because it is a CLOSED space, gas alone would just compress the gas INSIDE the nozzle up against the powder, not helpful.

the flaming gas from the zero delay motors gives off heat in the form of photons. These can travel THROUGH the “trapped gas well” of the nozzle right to the exposed propellant and light it off. So the vents guide the pathway of the gas NOT INTO but simply ADJACENT to the nozzle outlet, from there the Photons travel outward in all directions, some of which “shine” directly into the nozzle and ignite the propellant.

the vents also serve to decompress the intervening space so the pressure doesn’t push the sustainer off the booster before the propellant ignites.
 
Finally did my ground test today and the venting setup I designed worked great. Single D12-0 ignited the 3x A10-3 sustainers without any apparent issues. Also 3D printed parts held up quite well. I neglected launch lugs on this design so just taped it to the rail.

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What was a bit more thrilling than expected was what the little BT-5 dart rockets did after they ignited. I knew they would not be very stable since they were not moving when they launched but I kind of assumed they might fly mostly upwards since they are pretty nose heavy. Anyway, chaos ensued when those three rockets took off. I am glad I sent my kid away and did the video myself (it is not the best video since I was in duck and cover mode once the darts took off). Part of the issue seemed to be that the darts got a bit held up in the booster body (this made them tilt inwards and launch across the body of the booster). I am going to give them a couple of millimeters of clearance from the frame to help out on this. We actually recovered all 3 rockets (although one made it to the access road and got run over before I found it). One rocket did go mostly straight up so I was able to see it recover on streamer as planned.

Here are the relevant frames from the video.


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