Ducted Rear Ejection - A Build Thread

So about three months ago, I started to look around for something exciting I could do with my daughter on weekends this summer. I can't recall what it was that made me start searching for nearby model rocket clubs, as I hadn't built/launched one since I was in elementary school. Regardless, I found CMASS launches practically down the street in Acton and quickly started building.

Fast forward...

So after building some kits, I quickly ventured into scratch building. After a couple 3-4FNC's I started looking at my options for variety. The ducted rockets looked interesting, and so did rear ejection. So I started searching. I found the plans for an old (1970) Estes "Design of the Month", the Black Knight (www.plans.rocketshoppe.com/mrn/designs/Black_Knight.pdf)

The concept of a dual chute rocket, rear ejection rocket appealed, and the ducted ejection hit was there too. Perfect. But, I don't want to just copy somebody's plan. So I threw the plan out the window and just kept the concept of dual, ducted, rear ejection. From there, I got concerned. Two parachutes from separate locations, but right next to each other? Seemed like a formula for tangled lines and plummeting rockets. Better move them further apart rather than having them right next to the main BT. Parachutes in pods then. OK. I'd seen that most of the ducted ejection designs used body tubes to redirect the ejection gasses. Running body tubes, even BT-5 outwards from the main BT and into the airstream seemed like it add a huge drag factor. Alright then, pods usually go out on the ends of fins, lets leave them there and just put the tubes into the fins. Easy. Eventually, threw together a design in Solidworks just to get an idea of proportion, shape, etc. but the majority of the project came together a bit less orderly.

Disclaimer: I have no idea if anything like this has been done before. I did a bunch of searches here and around the web for construction like this and didn't find much of anything. If there is any resemblance to some other project / kit, it was unintentional and completely without my knowledge. However, if there was, I'd really like to hear about it, so please let me know if this reminds you of anything! Onwards to pictures!

First, motor mount. I knew I wanted fins (with ducts) near the back of the rocket. To get there, the ejection gasses needed to do a 180, and travel back down between the BT and the motor mount. I had some BT-55 standard mounts, and began to modify one to allow gasses to pass, while still performing it's centering function.


The blue portion was retained, and still centered a standard (BT-20) tube inside of my main body tube.

I chose to make an extended motor mount tube, with the hope that a long enough flow path would reduce or eliminate the need for wadding. This was important as the wing pods would be a bit tight. (or so I thought). So the extended tube had the "gas by-pass" CR near the top and two others (one balsa) near the bottom. It was about now that I started getting concerned about trapped hot particles and the potential to cause an internal fire. I choose to coat the motor tube, and CRs that would face the ejection charge chamber with aluminum duct tape. It went on very smoothly, but looking back, contributed to a great deal to the overall weight.

The main BT got two duct holes on each side, an internal coating of aluminum tape (partial, crooked coverage. Much harder than it looks to apply, it's REALLY sticky), and a thick balsa bulkhead to hold in the pressure.

The next step was the primary fins that would have the ducts for the ejection gas. All the parts were cut and laid out. 1/8" balsa for the core, 1/32 birch plywood for the outer surfaces, and a bit more of the foil tape to assure reduced trapped hot particles.

Constructed the two fins, basically just gluing the cut out balsa on the plywood in the marked locations.

Foil tape applied to both halves of each wing, retaining a small strip of exposed wood on the center rib to assure glue penetration.

Glued on the other side of each wing and trimmed the edges down.

Then used a belt sander to round the leading and trailing edges.

Some clever fabrication going on here, nice work ! I'm watching op: .

How did your daughter like the club launches ?

Having gotten a start on the main body / motor mount and the primary fins, the pods were next. I stuck with BT-20, as I had some nose cones. I cut two gas ports in each, and added a toothpick parachute shelf just aft of the gas ports, to make a space for the ejection gasses and ensure that the wadding and/or chutes wouldn't block them. The toothpicks were cut flush, reglued (in one case) and sanded. Nose cones are Estes and were glued into the front of each tube.

Rear 'plugs' were needed to hold the chutes in the pods during acceleration. A yellow tube coupler and the center cut out portion of the centering rings used on the motor mount, were superglued together and they made a nice flush cap. I wanted something a bit more decorative, so I mocked up a couple engine expansion bells out of cardstock and glued them on too. (they were eventually painted black)

It was about now that I realized that just having fins on the pods was going to be a bit boring. (gasp!) At least for me. I decided to continue the string of 'firsts' and add ring fins to each of the pods. To do so, I added 4 small fins offset 45 degrees from the primary fin in preparation to receive a ring fin. I then glued the pods to the primary fin tips.

Each pod/fin was then glued to the main BT, and all fin joints were filleted. Primary fin joints were epoxied for strength to assure that the ejection charge wouldn't cause them to separate from the main body.

The rocket finally got the finishing touches.

• 2 Ring fins, custom made from two layers of cardstock, 3" dia x 2.75" length.
• Balsa transition from BT-55 to BT-50.
• Forward airframe and nose cone.

Final weight, 112.85g. Right on the edge of flyable with a B or C engine. Unfortunately, it will probably never see paint unless I decide to fly it on composite engines with a bit more thrust.



This bird FLEW in Acton this last Saturday at the CMASS launch. Went up beautifully, slowly rotating around the long axis, on a C6-3.
(I didn't have any B6-2's and was worried a B6-4 would have too long a delay)
Parachutes (2 6" Estes plastic) ejected, though only one opened. Came down moderately slowly, and stuck point first into the soft grass field about 40' away.
The rocket was beaten to the ground only by the motor mount, which had also ejected and landed nearby. I don't think it was sufficiently glued in the first time, as there was nearly no damage to it or the body tube. Should be repairable, but I'll have to look into adding some pressure relief holes in the BT. Not sure how I'll find the right balance between them and the fin ducts though.

Sadly, I neglected to take any pictures of the flight. I had my hands full keeping an eye on my daughter (3 years old) while also watching. There was a gentlemen, possibly from the club, that I saw getting video on his cell phone. Perhaps he will see this thread and post the video for me.

Lastly, this rocket needs a name! I'm not really that good with that kind of creative stuff and I'm open to suggestions. Any ideas? (PG please)

samb said:

Some clever fabrication going on here, nice work ! I'm watching op: .

How did your daughter like the club launches ?

Click to expand...

She really liked them, and we attended all three in Acton this year. Not sure if we will make it out to Amesbury, but it's possible. After the first, we invited along some of her young friends and their parents, and made it a bit of a play-date, setting up a ways back from the pads and bringing snacks. Running around was severely limited for safety (of course) but playing together on the blankets and watching when the LCO started each countdown really seemed to keep them engaged. Only down side, trying to prep rockets and keep an eye on her was a bit tough. My wife attended with out newborn the second time, which was much appreciated and worked well. We'll try to do it that way in the future.

I think her favorite part is picking the rocket (for me to prep) before watching it go up. Still a bit young to help with much of the construction, especially sanding and paint, but still picks colors for paint, and between design choices for my scratch builds. She seems to be having fun with it so far. Time will tell.

MCriscione said:

She really liked them, and we attended all three in Acton this year. Not sure if we will make it out to Amesbury, but it's possible. After the first, we invited along some of her young friends and their parents, and made it a bit of a play-date, setting up a ways back from the pads and bringing snacks. Running around was severely limited for safety (of course) but playing together on the blankets and watching when the LCO started each countdown really seemed to keep them engaged. Only down side, trying to prep rockets and keep an eye on her was a bit tough. My wife attended with out newborn the second time, which was much appreciated and worked well. We'll try to do it that way in the future.

I think her favorite part is picking the rocket (for me to prep) before watching it go up. Still a bit young to help with much of the construction, especially sanding and paint, but still picks colors for paint, and between design choices for my scratch builds. She seems to be having fun with it so far. Time will tell.

Click to expand...

Kudos to you and your wife for starting them out young. Time will tell, but I'll bet they already have some great stories about going to their first rocket launch that they wont forget.

Great project! I might try a couple of small pressure relief holes up around the transition, no more than 1/16th inch. As you've discovered, the ejection charge is pretty energetic. Some have been described as "shotgun".

very neat. Flying with the young ones is fun but challenging. I let my boys paint their rockets with some tempera paint, then spray krloyn fixatif on top for protection. I definitely recommend the Fliskit Whatchamacallit as a first rocket, she'll need a guidance from you but my youngest and I built his when he was about 3 and a half. https://fliskits.com/products/01prod_fs.htm

This is awesome!

I got back into rocketry a couple years ago when my younger daughter was 4. We've had a lot of fun since then.

Your ducted fins are a work of art, and are the answer to a problem that I have been thinking about for almost a year. Been trying to figure out how to do something similar, but with the engines in the outboard pods and the chute in the main body. If I ever get it built will be sure to credit your build thread.

Nice project and good design and fabrication. Looks great! Ducting ejecting gases is not easy. There can be leaks and managing pressure can sometimes be difficult. I think the larger 24 mm BP motors like the D12-3 can have ejection charges that are sooty and hot enough that they may do internal damage especially when the gases are ducted.

That is an awesome piece of work.

You my admiration for the foil tape inside the BT. I think I would have flung the thing across the room before I could have gotten a result that good (or just settled for much worse results).

Thank you all for the compliments, I was quite pleased with how it's turned out. The 'wings' especially, but the overall shape I find really appealing. More and more I'm thinking I may go ahead and paint it up anyway and just run it on composites from here on out. Maybe Aerotech D-10s. It's just too bad they are so expensive compared to Estes BP equivalents. Any idea if the ejection charge sizes are comparable or are Aerotech's larger?

mjennings said:

...I let my boys paint their rockets with some tempera paint, then spray krloyn fixatif on top for protection. I definitely recommend the Fliskit Whatchamacallit as a first rocket...

Click to expand...

Those are both great ideas! No sanding really required, strong fin attachment without needing fillets and paint she can use safely.

samb said:

...I might try a couple of small pressure relief holes up around the transition, no more than 1/16th inch...

Click to expand...

That's about what I was thinking. I'll have to do a quick calc for pressure increase. I found a post here that might be helpful: https://www.rocketshoppe.com/forums/printthread.php?t=13377&pp=40 I'll have to see if I can find a way to measure the resistance of the gas ducts.

75Grandville said:

...Your ducted fins are a work of art, and are the answer to a problem that I have been thinking about for almost a year. Been trying to figure out how to do something similar, but with the engines in the outboard pods and the chute in the main body. If I ever get it built will be sure to credit your build thread.

Click to expand...

I think this type of ducted fin could work great for that, just be aware that the ducts need to fit in above the motors, which may mean you need longer pods. Of course, mine fit chutes, and have about 2.75" clearance, just enough for a motor. Humm, maybe if I do paint it, I'll mod it to have 1 rear ejection parachute from the middle and two standard engines, one in each pod.

aerostadt said:

Nice project and good design and fabrication. Looks great! Ducting ejecting gases is not easy. There can be leaks and managing pressure can sometimes be difficult. I think the larger 24 mm BP motors like the D12-3 can have ejection charges that are sooty and hot enough that they may do internal damage especially when the gases are ducted.

Click to expand...

When I started to realize I was quickly approaching the weight limit of standard BP engines, I started to think hard about moving up to a 24mm mount. I had the same fear, that the ejection charge would just be too big and blow the thing apart. I'm glad I stuck with 18mm.

This is a very cool rocket,and well designed. I was lucky enough to see it fly in Acton this past Saturday. When it was announced that a " ducted, rear eject with pods and ring fins" was about to launch I had to stop and watch.

MCriscione, I have a bunch of 18mm composite D's I would be more than happy to let you have one to try, I should be at most of the CMASS launches for the rest of the season.

As far as composite D motors go, if you invest in an 18/20 casing, the reloads get cheaper. Like $4 each, which is roughly comparable to BP motors. The nice thing about the reloadables is that they supply some black powder for the ejection charge, and you pour it into the charge well. That lets you control how much BP you use, and how energetic the ejection charge will be.

I can't speak to the 18mm single use motors, but the AT 24mm SU motors that I have used seemed to have very energetic ejection charges. Like, blew my ejection baffle out of the body tube and watched my nose cone and parachute drift away energetic. YMMV.

MALBAR 70 said:

This is a very cool rocket,and well designed. I was lucky enough to see it fly in Acton this past Saturday. When it was announced that a " ducted, rear eject with pods and ring fins" was about to launch I had to stop and watch.

MCriscione, I have a bunch of 18mm composite D's I would be more than happy to let you have one to try, I should be at most of the CMASS launches for the rest of the season.

Click to expand...

I'm glad you got to see it. I'm going to have to contact the club and find out if there's any way to check who was on the adjacent pad that got the video footage. I'd love to have it, but will have to find out who to contact. I'm pretty sure it was a club member, but I may be wrong. Maybe they can check the flight card stack for me. Your offer of a motor is greatly appreciated, I'll have to consider it in conjuction with the post below though regarding energetic ejection charges. Do you or anyone else have experience with the 18mm charges?

75Grandville said:

As far as composite D motors go, if you invest in an 18/20 casing, the reloads get cheaper. Like $4 each, which is roughly comparable to BP motors. The nice thing about the reloadables is that they supply some black powder for the ejection charge, and you pour it into the charge well. That lets you control how much BP you use, and how energetic the ejection charge will be.

I can't speak to the 18mm single use motors, but the AT 24mm SU motors that I have used seemed to have very energetic ejection charges. Like, blew my ejection baffle out of the body tube and watched my nose cone and parachute drift away energetic. YMMV.

Click to expand...

I think the idea of a 18/20 case is a great one, especially as I'd be able to use it for other rockets in the future. The only down side is the limited number of reloads for it. It's only really good for flying D impulse in standard engine mounts. On the other hand, I would be able to fly D impulse motors in standard mounts! I'll think about it, and see if I can justify the $35 to myself. Anybody that has one of these want to chime in regarding their usefulness across your LPR fleet? I'm sure it would send a number of my lighter rockets clear out of sight.

The 18mm AT SU D composite motors have very hot ejection charges. A lot of unburned propellant burns with the ejection charge and fries thin LPR body tubes above the motor mpint. Compressing it with ducting or a tight chute pack will compound the problem. Use a reload case and adjust the BP as needed. Stay away from the valuerockets SU motors on this one. Cost is no object because you are flying high end stuff with the CMASS boys. With Pods, rear eject and ring fins only the best will do. BP motors have somewhat variable ejection charges, most are strong, a few are weak and a few are shotgun. I assume the flight was successful and no one was seriously injured. That is all you can ask for when flying with pods. Good luck!

Daddyisabar said:

The 18mm AT SU D composite motors have very hot ejection charges. A lot of unburned propellant burns with the ejection charge and fries thin LPR body tubes above the motor mpint. Compressing it with ducting or a tight chute pack will compound the problem. Use a reload case and adjust the BP as needed. Stay away from the valuerockets SU motors on this one. Cost is no object because you are flying high end stuff with the CMASS boys. With Pods, rear eject and ring fins only the best will do. BP motors have somewhat variable ejection charges, most are strong, a few are weak and a few are shotgun. I assume the flight was successful and no one was seriously injured. That is all you can ask for when flying with pods. Good luck!

Click to expand...

Thanks, that's great information about the 18mm ejection charges. I won't say that cost is no object for me, and I wouldn't really call this 'high end', but your right that I'd like to keep this rocket flying around for a while, so taking the time and doing it right is the way to go. I'm thinking the 18/20 case is the best choice for me so far. An yes, it was a successful flight, although I am still a bit wary, as the rocket comes down nosecone first and could hit someone point-on (however low the speed). The recovery on it's first flight was on only one 6" chute, and it came in fast enough to stick into the soft dirt/grass and stay standing (maybe an inch and a half penetration). In the future I'll add a warning on the flight card comments for a "heads-up on return if near the flight line", as a reminder to the LCO, even if just for my own comfort.

Also, it's funny, I'm in Littleton, MA. We're practically neighbors.

If there's an award for "Most Ambitious Rocket Builder," I think we've found the winner.

Seriously, ducting the ejection charge through the fins--that's a stroke of genius!

Anything with ducts and pods is "high end" Low Power. When the Hi Po guys make an effort to come watch an LPR launch you know you are doing something right. That little 18mm case is a bit pricey but it will make up for the cost in reload savings in a short while, especially if you are hooked and fly every couple of weeks, I need to break down and buy them for my P 61, the SU 18mm D's ejection temps are just too much when clustered.

The 24mm and 29mm reload cases are an even better value when you can get them on sale. Rear ejecting a 29mm case out the back of a pyramid rocket is always fun.

Check out the ducted SR 71 builds here on the forum. They duct from the pods to the main body tube through the wings. Clustered Estes SR 71s are fun and cheap and you can go ducted or nonducted.

Keep flying at your club and you will be safe. A little stick in the mud is OK. When the big boys crash they get out the shovel or even the backhoe. That coming down part is always the hardest.

Daddyisbar is right the ducted SR-71's are impressive I think MicroMeister did one. May be wrong on that I couldn't find it with a quick search but I did find one of his other ducted fun
https://www.rocketryforum.com/showt...-70-Valkyrie-Micro-6x-Cluster!&highlight=duct

Well I did bite the bullet and bought an 18/20 case from lawndartman here on the forum, and a couple D-13 reloads to give the ejection charge adjustment a try. A BIG thank you to lawndartman who I PMed about a years old sales thread, and he still followed up and offered me a casing. It's old stock, but it's brand new in package and in great condition. Regarding the rocket, I've reinstalled the engine mount that self-ejected on the first flight, this time with quite a bit more glue to hopefully hold it in place. I've also put a coat of primer on it, but it doesn't look like it's going to get paint before the CMASS launch tomorrow. There's just no time to let it dry. That won't stop it from flying though.

I still need to decide if I should add some vent holes or if adjusting the ejection charge alone is sufficient to prevent a major blow-out, but I'm not sure what to do. I don't know how wide apart the lines between no-ejection/lawndart and blowing out the airframe (or mm again), but I'm worried they may be closer together than convenient. Thoughts and opinions welcomed and encouraged here guys.

Finally, I think I've decided on a name. The first flight was _nearly_ perfect and the rocket had a very graceful twisting rotation nearly the whole way up. To that end, I'm going to name it the "Double Helix" after the path that the two pods trace out through the air. It may need a helical paint job too, we'll see.

Great name!

You could do a bit of ground testing - add a bit of BP into the ejection well, run an ignitor up through the empty casing, seal the bottom, see what happens.

75Grandville said:

You could do a bit of ground testing - add a bit of BP into the ejection well, run an ignitor up through the empty casing, seal the bottom, see what happens.

Click to expand...

Just stick an M-80 in there, should be interesting.

75Grandville said:

You could do a bit of ground testing - add a bit of BP into the ejection well, run an ignitor up through the empty casing, seal the bottom, see what happens.

Click to expand...

I could probably do that if I had a bit of black powder around, but at least for now, I don't. And it certainly won't be ready for tomorrow's launch in that case. I think I'm just going to go with a reduced charge, say maybe 70% and see where that gets me. It really doesn't take much force to get the chutes to pop, and can be done by blowing 'up the tailpipe', just like you'd hope for with a normal LPR that pops the nose cone.

neil_w said:

Just stick an M-80 in there, should be interesting.

Click to expand...

:y:

Maybe someday, I'm not certified for that yet. Also, didn't think they made motor ejecting M-anythings. No fun doing all that duct work and not using it. Also no space for electronics.

I have two rockets with the 6" chute and in my experience, no matter how I wrap it, it only opens half the time. I replaced the chute in my Fletcher with a 9" and have had much better luck with that. The 9" fits a BT-20 tube just fine.

Just a thought,
Why not just let the engine mount eject out the back? Have it tied off so it does not fall free. As the pressure builds in the main tube the gasses push past the open front centering ring, through the fins into the pods, as pressure builds, it pops the motor mount, releasing the pressure.
You have already proven this will work, why secure the motor mount when the pods deployed just fine the first time??
Danomite!

mjennings said:

Daddyisbar is right the ducted SR-71's are impressive I think MicroMeister did one. May be wrong on that I couldn't find it with a quick search but I did find one of his other ducted fun
https://www.rocketryforum.com/showt...-70-Valkyrie-Micro-6x-Cluster!&highlight=duct

Click to expand...

Yep! I've done a number of ducted ejection clustered models. Estes SR-71, Geo-Sat HLV, Deep Space Transport, I've also downscaled all those models to Micro Maxx clustered motor powered along with the Trident, Micro PMCs of the SR-71, TR-4, F14 Tomcat and others.

Have to say the OP has done an outstanding job with his ducted ejection system, I've done a couple thru Fins with Rectangular brass tubing to extend the usable flying time. I've also used lots of adhesive backed Stainless Steel tape that outlasts aluminum or chrome mylar tapes about 20 to 1.

Danomite45 said:

Just a thought,
Why not just let the engine mount eject out the back? Have it tied off so it does not fall free. As the pressure builds in the main tube the gasses push past the open front centering ring, through the fins into the pods, as pressure builds, it pops the motor mount, releasing the pressure.
You have already proven this will work, why secure the motor mount when the pods deployed just fine the first time??
Danomite!

Click to expand...

I certainly could have but I have to admit, the whole point of this particular build was to experiment a bit with in-fin ducts. I could certainly have achieved the same external look without them, but would have lost the dual chute descent. Which admittedly, I'm still working on. The next time this flies, it will have some nylon 9" chutes so hopefully deployment will go a bit smoother. If I was to fully rebuild, or build another, I'd probably look into the stainless steel tape recommended my Micromeister.