Staging with interstage rods

[mrwalsh85]

Hey guys,

So I have a project that I want to start working on, while I build and fly smaller 2 stage rockets to gain experience. The sustainer is already built, so I just need a booster for it... hence the reason for my inquiry.

My design will require the use of interstage rods, rather than a coupler.

Is there a rule of thumb for the amount of engagement in the interstage rods?

I will need to modify my motor mount - I already have a plan in mind, just need to know how far to have the interstage tubes protrude past the aft motor mount.

Any input would be appreciated.

Thanks in advance!
-Mike

 

[Cameron Anderson]

Hey guys,

So I have a project that I want to start working on, while I build and fly smaller 2 stage rockets to gain experience. The sustainer is already built, so I just need a booster for it... hence the reason for my inquiry.

My design will require the use of interstage rods, rather than a coupler.

Is there a rule of thumb for the amount of engagement in the interstage rods?

I will need to modify my motor mount - I already have a plan in mind, just need to know how far to have the interstage tubes protrude past the aft motor mount.

Any input would be appreciated.

Thanks in advance!
-Mike

What will the rods seat in? I used rods on a 2 stage project last year and it took a tremendous amount of work to get them aligned properly where they were rigid and secure without binding.
I flew a 3" airframe and the rods recessed 2" into the booster and fit another 2" forward into the sustainer and everything held. I much prefer using couplers for ISCs personally but rods can be done.

 

[mikec]

Is there a rule of thumb for the amount of engagement in the interstage rods?

If the rocket comes apart in flight then the rods were too short.

I think a lot depends on how well the rods fit, what kinds of motors are being used, and the flight conditions. I've seen this sort of configuration work but like Cameron I wouldn't prefer it.

Also, when I've seen it the rods were in the booster and fit into the sustainer, sounds like you are thinking the opposite?

How big a rocket are we talking about here?

 

[Wallace]

Rods really need to be done "perfectly" in order to be in any way consistent. Length is not as critical as smoothness/stiffness/toughness. But, if you can get it set up right, there's absolutely no reason it won't work. Just think in terms of potential binding, in all axis, under any circumstances.

 

[BABAR]

Just thinking out loud. Interstage rods are different from launch rods or rails, in that the sustainer is (hopefully) already up to stable velocity at separation. So purpose of interstage rods is to maintain orientation of booster and sustainer long axis from launch to separation. If the upper stage was a cluster the simple ideal may be a single rod going right up the middle between the clustered motors, but I suspect you are planning on a single midline sustainer motor, so that’s out.

On my long gap stage (BT60 to BT60 D12-0 to A8-3) I used three short dowels, each about 6 cm, integrated into three soda straws. A big factor was that I couldn’t put buttons on the sustainer because the rail was only 4 feet long and the gap was 53 inches, so I needed something that would hold together during the 1/2 hour it was on the pad being blown around waiting for my turn to launch.

Three short rods seemed like a good choice, as it resisted bending from winds coming from any direction.

I wish I had thought to take a pic BEFORE I launched it, the sustainer was lost. The pic below shows the first attempt, on the booster had a 2 motor cluster, A8-3 for the booster chute and the D12 as the main. You can guess which one didn’t light. And I guess if one wasn’t going to light, it was the best outcome.

On the next round, got three straight trails, booster came down perfectly under chute, the sustainer is presumably still up there somewhere.

 

[jderimig]

More than one rod in one tube is overconstraint meaning you need perfect alignment to avoid slop or binding. Two stages are hard. I would avoid a concept that makes it harder.

 

[BABAR]

More than one rod in one tube is overconstraint meaning you need perfect alignment to avoid slop or binding. Two stages are hard. I would avoid a concept that makes it harder.

YMMV, three short rods worked perfectly for me.

 

[BABAR]

Rods really need to be done "perfectly" in order to be in any way consistent. Length is not as critical as smoothness/stiffness/toughness. But, if you can get it set up right, there's absolutely no reason it won't work. Just think in terms of potential binding, in all axis, under any circumstances.

There is a trick to getting a perfect fit.

Rods and tubes need to be mounted either inside or outside an identical diameter tube.

Mount the rods first. They need to be as straight relative to the long axis of the section, with a little standoff just a tad thicker than the thickness of the receiver tubes. Glue them down well.

Put the tubes on the premounted rods, use tape at the forward edge to hold them in place.

Now slide the segment with the tubes on the rods over (or inside, if you want to keep things internal and you have the space, won’t work for minimum diameter )your second segment (the one you want the tubes on, you can go either way, rods on sustainer or booster.) Put a tack or fast drying adhesive to hold them in place. Be sparing with the glue so you don’t glue the rods to the tubes or the booster to the sustainer.

This is key—- put a mark on both booster and sustainer to line up so the same rod goes in the same tube. Unless you are really really good, it won’t fit if you try to rotate it 120 or 240 degrees, the same rod needs to go in the same tube. Depending on how critical fin alignment is, you may have to preplan your positioning, like it it is a scale rocket and the booster and sustainer fins need to be aligned.

Your alignment now is perfect. Carefully take the rod segment off, so you don’t shift the tubes.

Now you should be able to put solid fillets on the tubes.

Voila

I am thinking between 1 and 2 body diameters should be long enough for the FREE portion of the rods (the part that sticks out)

 

[cerving]

It would be nice if somebody made a "reverse coupler"... that fits OUTSIDE the body tube. That would make things like this easy, along with MD staging (assuming you can machine down the rear closure to match the OD of the body tube).

 

[BABAR]

It would be nice if somebody made a "reverse coupler"... that fits OUTSIDE the body tube. That would make things like this easy, along with MD staging (assuming you can machine down the rear closure to match the OD of the body tube).

Doesn’t seem like it would be hard to do, but while you could “blend” the attachment to either the sustainer or more likely the booster, you are going to have an “edge” or rim that would give you a lot of drag on the second component, which kind of messes up the “minimum diameter” point.

 

[OverTheTop]

When designing optical component mounts for spectrometers we use a pin, a slot and a plane to ensure parts are not over-constrained.

What about considering something like one long rod but a couple of shorter locating features? Would have to be careful to avoid binding...

What about hanging the sustainer motor out the back for the coupling? That was a successful staging technique I used on my first two-stage flight earlier in the year. O3400-M2020. I had a non-locking taper on the interface between the two airframes.

Food for thought!

 

[cerving]

Doesn’t seem like it would be hard to do, but while you could “blend” the attachment to either the sustainer or more likely the booster, you are going to have an “edge” or rim that would give you a lot of drag on the second component, which kind of messes up the “minimum diameter” point.

The "outside" coupler would be attached to the booster and slide over the sustainer. You'd need HEI too, which puts this into EX territory, unless you have some kind of outside tunnel for the wires (which would also add some drag...).

 

[K'Tesh]

Hmmm... How to sim this...

 

[jderimig]

You'd need HEI too, which puts this into EX territory, unless you have some kind of outside tunnel for the wires (which would also add some drag...).

I have seen very thin flat wire running along the airframe for this purpose to avoid HEI.

 

[Wallace]

There is a trick to getting a perfect fit.

Rods and tubes need to be mounted either inside or outside an identical diameter tube.

Mount the rods first. They need to be as straight relative to the long axis of the section, with a little standoff just a tad thicker than the thickness of the receiver tubes. Glue them down well.

Put the tubes on the premounted rods, use tape at the forward edge to hold them in place.

Now slide the segment with the tubes on the rods over (or inside, if you want to keep things internal and you have the space, won’t work for minimum diameter )your second segment (the one you want the tubes on, you can go either way, rods on sustainer or booster.) Put a tack or fast drying adhesive to hold them in place. Be sparing with the glue so you don’t glue the rods to the tubes or the booster to the sustainer.

This is key—- put a mark on both booster and sustainer to line up so the same rod goes in the same tube. Unless you are really really good, it won’t fit if you try to rotate it 120 or 240 degrees, the same rod needs to go in the same tube. Depending on how critical fin alignment is, you may have to preplan your positioning, like it it is a scale rocket and the booster and sustainer fins need to be aligned.

Your alignment now is perfect. Carefully take the rod segment off, so you don’t shift the tubes.

Now you should be able to put solid fillets on the tubes.

Voila

I am thinking between 1 and 2 body diameters should be long enough for the FREE portion of the rods (the part that sticks out)

He makes it sound easy. Practice makes plerfect..

 

[jderimig]

When designing optical component mounts for spectrometers we use a pin, a slot and a plane to ensure parts are not over-constrained.

What about considering something like one long rod but a couple of shorter locating features? Would have to be careful to avoid binding...

What about hanging the sustainer motor out the back for the coupling? That was a successful staging technique I used on my first two-stage flight earlier in the year. O3400-M2020. I had a non-locking taper on the interface between the two airframes.

Food for thought!

^This. Its harder to get an overconstrained system to have robust fit, Exactly constrained is easy. One tight sliding fit to locate position. 2 features to locate rotational alignment (only). Done.

 

[BABAR]

He makes it sound easy. Practice makes plerfect..

It really wasn’t that hard to do this on the OUTSIDE of the rocket tube. Makes for bad aerodynamics, but I was just trying to keep the pieces together. Would be tougher to glue the tubes to the Inside of the outer body tube, but not a major difficulty,

For black powder minimum diameter staging the easiest coupler is just let the sustainer motor stick out the back, nested in the sustainer. You friction fit the upper stage motor, and can usually get by with extra security with a piece of cellophane tape on the OUTSIDE, with just a single piece on the outside the motor is still usually able to slide into the booster. I needed the extra security on my way rocket because the booster was so long that I didn’t have enough rail to reach buttons on the sustainer.

Had to look up HEI, Head End Ignition. Ignorance on my part, but why the reluctance in high power to put the electronics to light the sustainer in the booster, rather than run it down from the Av Bay on the sustainer? Then use the motor itself as the coupler? I guess you need a secure method of sustainer motor retention holding the motor from the front rather than from the back..... can you put electronics on the sustainer so if it doesn't light it will still dual deploy? I would assume you have that anyway, whether you use the booster electronics to light the sustainer OR the sustainer electronics.

This may sound crazy, but could you put a small blue tooth controlled igniter in the sustainer motor? I mean, small enough that it sits next to the nozzle, outside the jet and air stream, but communicates with the AV BAY electronics via blue tooth? That way you don’t have to run any wires down your minimum diameter bird.

 

[K'Tesh]

Here's something I've been thinking of. Glue your internal tube's launch lugs to the outside of a single length of body tube, while a launch rod is threaded inside them (making sure not to glue it accidentally to the body tube). Allow to dry, rotate, and repeat until you have all your interstage rod's locations in place. Then cut the body tube to the length of your two motor tubes.

 

[Wallace]

It really wasn’t that hard to do this on the OUTSIDE of the rocket tube. Makes for bad aerodynamics, but I was just trying to keep the pieces together. Would be tougher to glue the tubes to the Inside of the outer body tube, but not a major difficulty,

For black powder minimum diameter staging the easiest coupler is just let the sustainer motor stick out the back, nested in the sustainer. You friction fit the upper stage motor, and can usually get by with extra security with a piece of cellophane tape on the OUTSIDE, with just a single piece on the outside the motor is still usually able to slide into the booster. I needed the extra security on my way rocket because the booster was so long that I didn’t have enough rail to reach buttons on the sustainer.

Had to look up HEI, Head End Ignition. Ignorance on my part, but why the reluctance in high power to put the electronics to light the sustainer in the booster, rather than run it down from the Av Bay on the sustainer? Then use the motor itself as the coupler? I guess you need a secure method of sustainer motor retention holding the motor from the front rather than from the back..... can you put electronics on the sustainer so if it doesn't light it will still dual deploy? I would assume you have that anyway, whether you use the booster electronics to light the sustainer OR the sustainer electronics.

This may sound crazy, but could you put a small blue tooth controlled igniter in the sustainer motor? I mean, small enough that it sits next to the nozzle, outside the jet and air stream, but communicates with the AV BAY electronics via blue tooth? That way you don’t have to run any wires down your minimum diameter bird.

I like your train of thought but, unfortunately, no, you can't. Safety first..

 

[Wallace]

https://www.marsasystems.com/index.php/gadgets/marsanet-gadgets

 

[Wallace]

While that may be "allowed" it's been vetted. So...

 

[Wallace]

And, unfortunately, Bluetooth just does not have the required range. Good, if not great ideas, but I doubt anyone will allow them at a sanctioned launch.

 

[Wallace]

Here's something I've been thinking of. Glue your internal tube's launch lugs to the outside of a single length of body tube, while a launch rod is threaded inside them (making sure not to glue it accidentally to the body tube). Allow to dry, rotate, and repeat until you have all your interstage rod's locations in place. Then cut the body tube to the length of your two motor tubes.

Reminds me of the 'ol double cut method. Commonly used in sheet vinyl flooring. If you overlap 2 pieces and cut through both at the same time, doesn't matter if the cut is straight or not, the edges will always match. That's why the seams always look "perfect". Armstrong, class of '81..

 

[BABAR]

And, unfortunately, Bluetooth just does not have the required range. .

I was thinking of a Bluetooth connection between the sustainer AVBAY and a small battery and igniter and Bluetooth relay on the nozzle of the sustainer, so they would be a fixed distance, say, three to five feet apart.

Concur however safety is always first priority, and adding a Bluetooth connection is simply another point of potential failure (although apparently wired connections fail too!)

 

[Nytrunner]

but why the reluctance in high power to put the electronics to light the sustainer in the booster,

Several folks prefer to separate their booster by means Other than a high-power exhaust flame. And some flight profiles benefit by separation prior to sustainer ignition

 

[K'Tesh]

Reminds me of the 'ol double cut method. Commonly used in sheet vinyl flooring. If you overlap 2 pieces and cut through both at the same time, doesn't matter if the cut is straight or not, the edges will always match. That's why the seams always look "perfect". Armstrong, class of '81..

A variation of the idea would be to use a tube coupler to temporarily link two tubes and then continue like I suggested using one tube that would be cut apart.

 

[BABAR]

Several folks prefer to separate their booster by means Other than a high-power exhaust flame. And some flight profiles benefit by separation prior to sustainer ignition

Okay, that makes much more sense to me. Thanks!

 

[mrwalsh85]

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.

 

[Wallace]

You do realize metal would be "allowed" in that particular use case right? I picked up some titanium shafts on Amazon pretty cheap, shipping just took a few weeks. Think they were inteded for rc truck suspension but were real close to what I needed. Super strong,tough,straight and lightweight. Maybe think along those lines in lieu of composites?

 

[mrwalsh85]

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...