Staging with interstage rods

[jderimig]

Correct. While everyone is harping on precision alignment, my thought process was to utilize slop to my advantage. For instance, a 1/2" rod into a 9/16" interstage tube. It'd allow for some misalignment, but it would still have stability.

Decisions, decisions...

Just don't go too fast.

 

[mikec]

Please just remember that solutions that work fine for LPR or small HPR rockets may fail completely at larger scales.

I'd be surprised if that Tiny Tim picture is the whole story of how that system worked, but it is interesting.

 

[BABAR]

How big and heavy is your

OK Folks... I guess it's time to let the cat out of the bag.

I know I'm reaching way ahead here, but I figure I can build while I work on other, smaller 2-stage rockets to gain more experience.

When I built my L3 WAC Corporal, I knew I should have considered eventual staging... Alas I did not. So now I have a single stage rocket that I need to retrofit.

I have thrown around a couple of ideas, but still working to determine best practice. Being in the steam locomotive restoration industry has really exposed me to some older pieces of equipment - that you wouldn't even think existed, since today's technology can "do everything". Some of these pieces of equipment seem like they'd be just the ticket for boring interstage rod holes.

The WAC Corporal has a Tiny Tim booster, which has an open air interstage using three rods connecting the booster to the sustainer. At the actual contact surface, I believe there is a metal ring that supports the weight of the sustainer. I've attached a photo that came from the White Sands Missile Range Museum (via John Coker's website) that shows the interstage clearly.

The WAC flies very well, and very stable. I have been considering doing the appropriate Tiny Tim booster for the WAC and just need to figure out the interstage. I have centering rings that are approximately 4" apart - so I could do (3) interstage rods that engage by 4-5 inches... Since the interstage plate will support the sustainer also.

Again, this is just in the brainstorm stage. I do want to pull the trigger and buy tubing and fiberglass to start that portion of the build... Even if it doesn't turn into a Tiny Tim, it can be used for another project eventually.View attachment 396493 View attachment 396497

Okay, depending on how much room you have between your motor mount tube and your outer body tube, this may work.

Scale model already has three rods, which in my opinion is perfect, allows three attachments which can combined resist lateral shear forces from any direction.

The position of the three existing rods is, I believe from the drawings, just INSIDE the diameter of your sustainer body tube.
Build your Tiny Tim booster to scale, except make the rods say 1.5 body tube diameter longer than scale forward, and add 1.5 or 2 times longer BACKWARD to serve as the attachment points INSIDE THE BOOSTER (won’t be visible from outside.) The extra forward part is the segment that is going to insert into sustainer.

It will be critical that these rods are perfectly straight relative to the long axis of the booster.

Once built, you need a TUBE size that fits around the FORWARD extended length of the rod, but also fits BETWEEN your sustainer motor mount and outer body of sustainer. Length needs to long enough to reach from say, 2 cm beyond the forward centering ring of your sustainer motor mount to say 2 cm aft of the rear end of the sustainer (you’re gonna trim this off later.)

Line up your booster and sustainer EXACTLY at the orientation (rotationally, axially) you want the finished combined stack to be when you place your showroom model on the shelf, and mark the sites the rods hit you rear exposed centering ring of sustainer. PUT A TICK or some sort of marker so Rod #1 will always be in Tube #1, etc, or in this case Hole #1 with Tube #1.

Drill holes (make sure they are as perfectly axially aligned as you can) from the rear end through the rear and forward centering ring (if you have a MIDDLE centering ring, even better, just have to make sure you get through at least two rings.). IF you have the space, won’t hurt to go a bit bigger than your tube, this gives you a bit of flexibility in the next step. These holes don’t HAVE TO BE PERFECT, but they need to allow enough slop if they ARE NOT perfect to allow the placement TO BE perfect.

Slide your tubes over the rods on the booster. They should be a LITTLE oversized, sort of like a standard launch lug on a 1/8” rod, you don’t need or want much slop here. In fact, if you have slop, put some tape around the rods so the tubes are PERFECTLY centered on the rods for a snug fit, but not so tight it is hard to slip the tubes off.

Okay, now take the tubes OFF the booster.

Dry fit Slide the tubes into the pre-drilled holes, leaving about 2 cm sticking out. You may want to put a pointy plug in the forward end of the tubes, will make it easier to “find” the forward centering ring hole, since you will be probing blind. (Or possible if you have no baffle you can put a flashlight in the forward end of the sustainer section and sight through the tube as you seek for that forward hole,) They will NOT be perfect YET, but there should be enough slop to shift the into perfect position.

Now try dry fitting the booster into the tubes, again line up Rod#1 with Tube#1. With the slop of the slightly over drilled holes, you should be able to get a perfect alignment, where all three rods slip easily into and out of the tubes.

Use three or four dowels on the outside of the booster and sustainer (looks like they have same body tube diameter) held with rubber bands or blue tape (don’t mess up your finish!) to get the length spacing right.

Now check it out from every angle to make sure everything is copacetic. Mark your external dowel positions.

Take it all apart, repeat with glue on the tubes. If you don’t already had pointy plugs on the forward ends of the tubes, Plug the forward ends of the tubes with something to make sure no glue gets inside the tubes (or you will end up with a hanger queen, booster glued permanently into sustainer! Sorta like when I test fit a single use G motor in a rocket too soon after using CA glue. Glued the motor in permanently. I did get ONE FLIGHT. at least I had definite motor retention!)

Not sure how to get a firm glue fix on the tubes BETWEEN the centering ring, this might be a good place to drill small holes in the rear ring and maybe use a syringe or small tube to drop some gorilla glue into it. I like polyurethaning glue in hidden places as it is strong and expands and fills gaps.

Once the tubes are in solid, trim off the excess aft of the tail of the sustainer.

Anyway, my 2 cents, or maybe 4 as I am long winded.

Best of luck with your project.

 

[wrad]

My Skylark model used rods for the interstage coupler.

https://www.rocketryforum.com/threads/skylark-sounding-rocket.145395/

The 3 rods were 2mm carbon and slotted into 3mm brass tubes (2.1mm id). I slotted them in by about the diameter of the main body tube (effectively 1c) I think this is way overkill as there is literally no slop in the connection at all. As for the alignment that everyone is so worried about, my centring rings and the coupler were 3D printed with the proper alignment so no issues whatsoever, the two parts slide together and apart very smoothly.

 

[rharshberger]

The WAC Corporal with TinyTim booster may have been like the boosted Iris where both booster and sustainer were lit at the same time and the booster burned out before exiting the tower, hence the reason for a very simple ISC, as the two parts were fully supported by a tower until separation. I have been considering this type of intestage (using 3 rods and brass sockets) for an upcoming 2 stage HPR build of my own.

 

[BABAR]

The WAC Corporal with TinyTim booster may have been like the boosted Iris where both booster and sustainer were lit at the same time and the booster burned out before exiting the tower, hence the reason for a very simple ISC, as the two parts were fully supported by a tower until separation. I have been considering this type of intestage (using 3 rods and brass sockets) for an upcoming 2 stage HPR build of my own.

I have no knowledge to the contrary, but why would you put such huge fins on the booster if it never left the tower?

 

[rharshberger]

I have no knowledge to the contrary, but why would you put such huge fins on the booster if it never left the tower?

It was just a supposition on my part, but you are probably correct. Depending on source the Tiny Tim motor had a burn between .6 and 1 second.