New Year, new build thread - Mach 1 Karman, US Rockets Trident, K&S Orion Starfighter

Sorry for the long silence - a multitude of issues

Trident
Jerry kindly sent me a kit that will allow me to make the rocket dual deploy. But I've already ducted two of the tubes. I need to make a decision....

On the one hand, the ducted ejection is what a Trident is all about, right?
But.....let's do some math.

What is the ejection charge going to do to the motor pod? For the first look, I'm going to ignore the ports.
If I consider the diameter of the tube (2.25) and the length between the top CR for the actual motor tube and the nose cone (4.5), and the 1.2g of ejection charge ...... the calculations show there will be about 130psi in the motor pod.... that seems high - this is only cardboard!

But the pressure will try to vent through the ports. Just how big are they? The width (0.125) and length (2.5) for two ports is a whopping 0.625 sq in. This is 2% of the surface area of the tube, and about 15% of the surface area of the nose cone.. Not a lot of open space. Unlike a plain tube where the ejection charge has the full diameter to push the cone off here the pressure just has the narrow slots to pass through. I get the feeling the motor pod is going to "rapidly vent to the atmosphere" (blow up) instead of pushing a chute out of the passenger compartment.

What if I removed the supplied powder and replace it with something less. Then I am back into the total volume of the motor pod, side pods, and passenger pod. Anything less may get too wimpy to actually get the chute to deploy.

It appears that the safest bet is going to make use of the dual deploy.

Comments/thoughts? Please! Thanks

How did the trident deploy originally? People got it to work with ducting, so I cant imagine it was a killer problem

The *original* Trident was ducted through the three tubes. I did a good job building mine (way back when) but sadly never got to fly it... that of course would have told me if I indeed *had* done a good job.

Nytrunner said:

How did the trident deploy originally? People got it to work with ducting, so I cant imagine it was a killer problem

Click to expand...

The problem is some things don't scale up. For true cylinders, whether they be 1" diameter or 10' in diameter, they meet tangentially with only "one point" (or maybe I should say 1 line) touching, This makes it difficult to widen the slot without creating a gap. I made a mock up with a 0.75" slot. It consumes a lot of the tube and creates a significant gap.



I guess dowels could be glued to the tubes to cover the gap. Just need to make sure everything is air tight. Below is a sketch of adding dowels (shown red). The one on the left is 1/8" and the one on the right is 1/4"



The TOTAL volume of the motor pod, 3 ducting pods, and passenger pod is equivalent to a 2.25" diameter tube x 48" length. With 1.2g ejection charge (standard for 29mm CTI motors), this would create a little less than 12psi to eject the chute - a little light (I typically plan on 15) but if the cone is not too tight it would probably be OK. The pressure will probably be reduced some due to the impact of the static pressure having to feed through the ports. Hence there may only be 10psi or less to actually move the laundry and cone. Now I get a bit worried....

We have the 1.2g in the 2.25 tube motor pod but it is only 4" long (between the top CR and the nose cone) instead of the equivalent 48". If I ignore the ports, it would have to hold 130psi!
For a typical rocket with a 2.25 diameter, the ejection charge would push against the 3.8 sq in of nose cone to push it out. With a 48" equivalent length the pressure wave would expand throughout the entire volume. But here the gas is constricted and has to fit through the ports which are smaller. And the ports don't actually move like a cone would, where the movement helps to reduce the pressure. The cone moves up enlarging the volume (probably insignificant effect) until it is off the tube allowing the gasses to expand into the atmosphere (very significant). The rocket may not see the full pressure because the cone came off before full pressure was achieved.

Back to the motor pod vs the 3 duct tubes and passenger pod. I calculated the total volume of all the pieces at around 190 cubic inches. The motor pod alone is only a bit over 15 cubic inches. This is less than 8% of the total volume, yet the ejection charge occurs here and then through the ports has to feed the other 92% of the volume.

Bottom line - Will sufficient pressure be relieved through the ports before the ejection pressure spike causes a rupture of the motor pod. THAT I CAN"T ANSWER. I don't have the knowledge to calculate the dynamics of the gasses vs the structural integrity of the cardboard.

I am open to suggestions or inputs from others who can help figure this out.

The easy way out is to use a dual deploy option located in the passenger pod and bypass the ducted ejection.
I admit I would RATHER use the ducted ejection to be a true Trident, just.......I also don't want it to be raining down in pieces........

But on a positive note...
I got my vinyl today from Stickershock!

Am I missing something or does a bulkhead of some sort get installed above the ducting hole? If no, I would think that would help a bit and take up a bit of the volume of the tube so you could use a bit less powder. It would also force the pressure to exit into the ducting hole more effectively instead of filling the entire tube.

I'll qualify this by saying I've never built a ducted setup, and could be way off in how this would actually work.

Actually, the nose cone is just above the ports, glued to the body tube.
Rough calculations - the motor pod has 15.2 sq in. Each duct pod has 38.4 sq in - for 3 pods that is 115.3 sq in. And the passenger pod has 60.8 sq in.
So the concern is the pressure in the motor pod (only 15.2), having to pass through the ports to pressurize the other 176 sq in of rocket.
Will the pod rupture before enough pressure/gas goes through the ports?

Below is a very rough sketch - not to scale or proportion. The air flow to eject the top cone for the chute is shown in red



Noticed I didn't show a bottom cone for the top section, but you get the idea

I "may" have a potential solution (unfortunately too late to incorporate into my rocket)

The big concern is the build up of the pressure in the motor pod vs the strength of cardboard.

Especially since a cylinder will fail on its length if there is a weakness, such as a cut port...

In addition, sharp corners create stress points - curves are better

Based on the 3 points above I would suggest we do the following:

1) Install a coupler in the motor pod between the Motor tube centering ring and the nose cone. Ideally, we may actually take a third layer (cut a section of coupler or body tube length wise to make a "coupler" for inside the coupler.

2) Make 2 ports per tube, each 0.5 x 1" long separated by 0.375". This strip in between the two ports should provide some strength. Dowels will be bonded (I show 1/4" diameter) along the pod to close the gaps and add strength.

3) Using a dremel or a dowel with sandpaper wrapped around it, round the ends of the ports.
This would only need to be done for the actual 2.25 motor tube. I am not really concerned with the 1.25 pod tubes as there is the big opening heading up to the passenger pod for the pressure to relieve itself. The two ports with rounded corners is shown at the bottom of the pix below.

Will this work? I'm not sure - I would feel better with 3 layers of cardboard. And even that solution is a definite "maybe".
Of course, one could fiberglass this section to add strength as well (Again - too late for my rocket as the motor and nosecone have already been glued in)

Fortunately, Jerry has offered to send me a replacement kit!!
I will try this idea on that one. But for now on this one I am going to abandon the ducted ejection.
Actually, since I have not glued the pod cones on yet I did try to go in and hack the ports bigger - not a lot of success trying to work with epoxy hardened cardboard from inside a 1.2" tube and having to work to 2.5 ~ 3" down. And since I don't have a way to add a coupler inside the motor pod to increase its strength I would still be concerned with it failing

Well - Decided to try....

Using a long drill bit and files, I now have 2 ports that are 3/8" wide x 2.5" long (see pix).


Along with my concept of "PPE" - glove with masking tape to support the drill bit


The third pod I'm just going to cut the port 1/2" x 2.5". That gives me a total of 3.125. That is an equivalent of a 1.995" diameter tube.

I may decide to a expand the first 2 ports to a full 1/2", although I am concerned with wrecking things. That will grow the port area to 3.75", or the equivalent of a 2.18" diameter tube.
Of course, so long as I maintain basic structural integrity I can still do electronic deployment.

After the pods are installed I plan to perform a tethered ejection test. I will first use an adapter to install a 24mm CTI motor with its 0.6g of ejection to see if it can get the laundry out.

After that? Jump to a 38mm with 1.2g???

I got to do some work this weekend

First, I cut the 1/2" x 2.5" slot in the third pod and motor section.



Epoxied the tubes together. You can see the gap with just a 1/2" wide port vs. the 3/4" called out. I used a 1/8" dowel epoxied between the tubes to seal the gap. (need to grab a picture of it)


Started cutting ports on the tops of the pods and passenger pod. I've done them all 1/2" x 3"

Modified the tail cone for the passenger section,

Clearance is needed for the ports. A few views attached.
Basically, the cut section clears 2 of the ports and the angled portion will be against the 3rd port (but with the angle it will allow the gasses to flow)

I plan to fill the cone with 2 part foam to reduce the volume

missed a pix

Apogee has a video regarding a ducted rocket. He used thick cardstock to cover the gaps. (Possibly the cluster duck or Dual 18 )

Or you could use the piece of the tube that you cut out, flip it over, and trim that to cover the gap, kind of like a fillet. Or a cut a strip from a smaller dia tube if that fits better.

Frustrated - have run into a "life situation" that is now limiting me to work on rockets barely an hour or two each weekend. But here is what I've done...

Filled the cone with 2 part foam to remove the empty volume.

Installed a T nut for a rail button.


The cone is very loose in the body tube. I decided to glue some strips inside the tube to build it up. It took 3 layers of regular copy paper. Might have been able to get a partial 4th but it was good enough for me.


The kit comes with an oversized Estes style "tea bag" style shock cord mount.
I instead drilled 2 holes in the cone and passed some kevlar through the holes for an anchor. I didn't want to trust the loop built into the nose cone....


Scored the plastic for grip, and used epoxy to glue in the cone.

And now, to put the passenger pod on. A bit of epoxy, my foam guides, and some rubber bands...

I included a view looking into the pod and the ducts



On the Karman, applied some fillet using some spare epoxy from the Trident...

Trident -
And now, to put the passenger pod on. A bit of epoxy, my foam guides, and some rubber bands...



I included a view looking into the pod and the ducts



Cut and angled some 1/8" dowels.
Mixed up some Rocketpoxy and started reinforcing the joints and sealing the ducts!
Also glued the pod nose comes on using Titebond II




Karman - used spare epoxy from Trident to fillet other side

Trident - you may noticed I've favored this build....

Jerry - I owe you an apology

I took the rocket (without fins) and set up a static test with a 30" thin mil chute. Had to do some jury rigging to install a 24mm load (CTI E31). CTI states their 24mm loads have 0.6g of BP for the ejection charge. I strapped the rocket to some benches, and 3 - 2 - 1 - 0 and nothing. Bad ignitor. Replaced it and tried again.



So what happened? I expected the charge to go off but the cone to stay on, or even if it came off the chute to stay in the tube.

What really happened? The chute and cone came off quite energetically!

Now the cone is a bit on the loose side, but I think good enough to satisfy a safety inspection.

I did not try a 29mm (which comes with 1.2g of BP). I guess my options are to fly with 24mm or to use a 29mm and remove some powder. I can also run a test case with a 29mm - I ran out of time today....

Karman - I had a few "bubbles" in the fillet, so added more epoxy...



Orion - They say "third times the charm". First cockpit I used 0.040" styrene. I felt it was too thick so made one from 0.020". But I glued one side on upside down! Duh. Third one came out OK. I added some internal epoxy fillet and temporarily attached it to the tube (protected with wax paper) to get the right shape. I used the top section for this.


And I filleted the lower section. Lots of different fins/wings for that....

Glad your ground test was successful!

I had a hunch it would work out fine, the trident design has been reliable in the past.

But when in doubt, EXperiment!

Orion - Decided to shape the dowels to blend with the fins/wings



And then a shot of primer




Trident - Since the ejection test worked, time to install the fins..........

Any updates, specifically looking for such about the Trident you were working on.
Allen

Yes, some. Unfortunately I'm now in a situation where I basically get an hour or two a week to work on my rockets.

And I also confess, I normally put on an annual display for a one day Rocket Fest out local observatory runs. I really wanted to have my Saturn V ready for this (and the situation that limits me a an hour or so was not expected) so I prioritized its build over these.

But here is where I stand

Trident - as shown previously, fins were attached. I have since filleted the fins and started cleaning up some of the joints



more to come...

Looks great! Haven't started mine yet, but understand I need to do s bit of measuring to get the tube lengths correct for a more consistent 2.25x upscale.
I've built a ton of the original Trident (kits and clones), and never had an issue with ejections.
Your testing verified vent accuracy for me.
Thanks for your build posts!
Allen

BTW - Not sure if you caught it, but due to the weight, I also changed the lugs to rail guides. See posts 30 and 45
Also, you need to change some of the nose cones to be more scale....

it's alive....It's Alive....IT'S ALIVE!!!!

Time to resurrect this thread

Again, I apologize - the last year and half has been "difficult".

I did make progress, but had limited time to download pix and update this thread.

When we last left the builds, we had the following status:

Karman - fixing some bubbles in the fillet
Orion - third try on the cockpit and started some fillets
Trident - attaching fins and starting fillets

Catching up....
Karman: I realized I almost forgot the Launch Lug for the Karman. It comes with both micro-buttons and a launch lug. But I misplaced them. So I substituted a long lug that I ran along the 2 body tube joints. And then time for primer.




Orion: More fillet and installed the cockpit with primer


Trident: Forgot to show the application of the fillet



Showing how the tubes are tied together with screws

Karman: Painted white and applied vinyl decals



Orion: - Painted Auxiliary Motor Tube, main body, and top


Trident: Cleaned up primer plus some more F&F


The Trident would not fit in my normal paint area, so I built a special stand from PVC to allow me to paint it as one assembly

Trident: Final prime then white



And after decals applied


Good thing I made the rocket to separate - otherwise this is how it would fit in my car...


Still needed to paint the motor tube rings....

Orion: Mounted the Aux Motor Tube to the Main Body


Everything together

I got to fly the Karman and Trident.
First, here is a pix of a Trident clone, the Hawk's Hobby Trident, and this Trident (the biggest one). I flew all three.


This Trident on the pad


Launch


And successful return!


Karman on the pad and launch