Gap staging - looking for recommendations on gap length

[Zeus-cat]

I am planning on designing and building a 2 stage rocket that uses gap staging. I have several Semroc heavy duty 1.15 inch outer diameter tubing with 4 sets of slots for fins. The tubes are 22 inches long. Obviously the booster tube will need to be cut down, but what is a good length?

I have G. Harry Stine's book and know that 12 inches will work fine, but I don't need that much. I am thinking maybe 8 inches. Now is that 8 inches between the top of the booster motor and bottom of the upper stage or 8 inches for the overall booster length? Any recommendations?

 

[Intruder]

I think that it's 8" from the top of the booster motor to the nozzle of the sustainer motor. Just be sure to use a stuffer tube.

 

[Handeman]

I agree with Intruder, especially about the stuffer tube. If you are going with an 8" gap, I would keep the burning particles and heat from the booster motor contained as much as possible.

Just a thought, but instead of vent holes in the top of the stuffer tube, you might want to leave the stuffer tube about 1/16" short of the sustainer motor and then use vent holes at the top of the booster BT or leave the tube open through both ends so it can blow by out the aft end of the booster.

 

[Intruder]

I agree with Intruder, especially about the stuffer tube. If you are going with an 8" gap, I would keep the burning particles and heat from the booster motor contained as much as possible.

Just a thought, but instead of vent holes in the top of the stuffer tube, you might want to leave the stuffer tube about 1/16" short of the sustainer motor and then use vent holes at the top of the booster BT or leave the tube open through both ends so it can blow by out the aft end of the booster.

By that do you mean to use the TTW fins to center the motor mount and leave the centering rings off. If so, that would be the "venting method" to use along with venting the stuffer tube because it wouldn't be visible.

Just out of curiosity, what type of motors do you plan to use.

 

[Zeus-cat]

I was planning on designing it for D12's in both upper and lower stages. If it is light enough I thought it could also fly on C's in both stages. Of course, gap staging gives you the flexibility to use a 24mm motor in the booster and an 18mm in the upper stage too.

The inner diameter of the body tube is 1.14 inches, or almost 29mm. If I build the booster for 24mm motors the stuffer tube doesn''t really help much. I'm think the vent holes are the better way to go for this design.

Thanks for the help guys. I doubt this will hit the build queue for at least a few months, but I like to ponder things like this for a while.

 

[MarkII]

In every gap-staged design that I have built so far, I have vented it back down the booster and out via perforated centering rings. I have always made the motor tube long enough to extend to just below the nozzle of the sustainer's motor when it is in place. I punch a few holes in the top of that extended motor tube, usually with a handheld hole punch, just a little bit below the top. If the space between the outside of the motor tube and the inner wall of the booster's body tube is small, I take the hole punch and use it to take "bites" out of the edges of the centering rings after I have glued them onto the motor tube.

In your design, the space between the ST-9 motor tube and the inner wall of the LT-115 is too small to permit even doing that, though, plus the couplers are an inch long. You could try cutting channels into the sides of the CR-9115's and chiseling them out with your hobby knife. I am not sure if that will give you openings that are large enough (about 1.8mm deep plus however wide you want to make them) to be effective, so you might end up having to cut vent holes in the booster's body tube just below the top. (If you used BT-50 instead of ST-7 for the motor tube, you could make the spaces in the centering rings 2mm deep.)

At 1.14", the inner diameter of Semroc's LT-115 isn't just close to 29mm, it actually is 29mm. LT-115 is 29mm motor mount tubing.

MarkII

 

[Zeus-cat]

I am thinking I will go with the vent holes.

I think it would make more sense to just cut the couplers before installation if I were to go that route. Cut one coupler into six 60 degree pieces. Stagger them 60 degrees apart so half of the space betwen the motor mount is open and the other half is filled with coupler. I'll have to do some cipherin' to see if it even makes sense to do that.

 

[powderburner]

I have G. Harry Stine's book and know that 12 inches will work fine, but I don't need that much. I am thinking maybe 8 inches.

Regardless of the length you choose, I believe you can improve your chances of upper stage ignition by keeping the hot blow-through gases (from the lower motor) more tightly contained, and thereby hotter.

Of course, the gases are going to expand as they move forward through the stuffer tube (or if you're not using a stuffer, through the lower stage rocket body tube). Expansion due to forward travel is kind of unavoidable, but you can at least minimize the expansion by not letting the hot gas expand in a radial direction any more than absolutely necessary.

The more the hot gas (from the lower motor) is allowed to expand, the more it will cool off (this is basic thermodynamics & gas flow). If you provide a forward gas path that is close to the same diameter as the inside of the lower motor, the blow-through hot gas will still expand a little but not nearly as much as if the blow-through gas was vented into a larger body tube.

For an 18mm motor in the lower mount, you could just extend the motor mount tube past the thrust ring to contain the hot gas forward to the upper motor nozzle. For a 24mm motor in the lower mount, you could still use an 18mm stuffer tube by gluing it inside a CR2050 thrust ring for the lower motor. The 18mm stuffer would be about the same ID as the lower 24mm motor and would do a good job of containing and focusing the hot blow-through gas until it reaches the upper motor nozzle.

You might not need the extra "help" from a stuffer tube, but then again, you never know--
(This is one of those design features that you never notice when they are working well, but if you leave it out and then have problems later, you'll kick yourself for missing the chance to add a quick, cheap ignition aid.)

 

[MarkII]

I am thinking I will go with the vent holes.

I think it would make more sense to just cut the couplers before installation if I were to go that route. Cut one coupler into six 60 degree pieces. Stagger them 60 degrees apart so half of the space betwen the motor mount is open and the other half is filled with coupler. I'll have to do some cipherin' to see if it even makes sense to do that.

I did that with my Upscaled Midget. I used LT-115 for the booster tube, just like you want to do for your project, but I wanted to give it an 18mm motor mount. At the time, Semroc didn't have an BT-20 or ST-7 to LT-115 centering ring, and I'm not sure if they even do now. To adapt the 18mm motor tube to the body tube, I planned to adapt the BT-20 to ST-9, and then adapt the ST-9 to the LT-115. First I printed up a fin marking wrapper in VCP. I printed up a wrapper for 12 fins on a 0.736" diameter tube. I used it to mark 12 equally spaced lines around my BT-20 motor tube. Then I slipped an AR-2050 centering ring onto the tube and used the marked lines as guides to marking the CR into 12 sections. (AR-2050's work because the ID of ST-9 is the same as the ID of BT-50.) Then I cut the centering ring into 12 sections. I took 6 of the sections and glued them between every other line near the upper end of the motor tube, and then glued the other 6 sections between the same lines around the lower end. I was pleasantly surprised (and relieved) to find that the cut-up centering rings worked fine to center the motor tube. My Upscaled Midget stages fine every time by venting down through the booster tube in between the sections of centering rings. Granted, I did have a little bit more space to work with between the outer wall of my motor tube and the inner wall of the tube that contained it (about 0.1" vs. your 0.07"), but anyway, that's how I did it.

MarkII

 

[JRThro]

For an 18mm motor in the lower mount, you could just extend the motor mount tube past the thrust ring to contain the hot gas forward to the upper motor nozzle.

That is what I plan to do when I build the Twin Shark 2-stager that my son drew and I refined. It'll have the booster motor tube be about 6" long, with the extra 3+ inches extending past the thrust ring and almost to the nozzle of the sustainer motor.

I like the idea of having the vent holes be invisible, so I'm going to punch holes in the motor mount centering rings rather than in the walls of the booster body tube. Since the extended motor tube, or stuffer tube, won't extend all the way to the nozzle of the sustainer motor, I don't think I'll need to add vent holes to the stuffer tube.

Here's the thread on the Twin Shark:
https://www.rocketryforum.com/showthread.php?t=5133

 

[Verna]

I am planning on designing and building a 2 stage rocket that uses gap staging...


One thing I discovered is that the nozzle openings can make a huge difference when chad staging over long distance. The larger the upper engine nozzles are the better. We used stffers that allowed us to seat the next stage engines in the top of the tube and vented them and the main bt. Think about engine combinations that will allow for larger nozzels.

Verna
www.vernarockets.com

 

[rosko_racer]

I always wanted to make this a 2-stage rocket and with the help of a friend I got this far. The only permanent glued parts are the fin can with the MM and the upper stage with its MM. The stuffer tube is not glued yet. Since the booster is heavy it will need a parachute and the only place to put it is inside the reducer area. The idea here is that upon separation, the sustainer will take the reducer (which will be ankered to the booster by a 10" kevlar string) with it then separate. The reducer will parachute with the booster.

The booster's end is 5.5" from the sustainer motor; the sustainer motor will be

But as you can see, the ports will vent where the parachute is, most likely melting the parachute.

- What is the max distance the exhaust ports can be from the sustainer for a successful stagging?
- Can the exhaust be vented below the CR (and through holes in the aft CRs)?
- Can I omit the ports and still stage successfuly?

 

[kjohnson]

Your proposed location looks ok to me.

Yes.

Maybe. But why risk it?

 

[rosko_racer]

Your proposed location looks ok to me.

Yes.

Maybe. But why risk it?

Thanks Kevin!

Did new port holes at the proposed location and covered the old ones with a custom coupler inside the BT-20. I also made six 1/8" holes at the top MM CR to vent the gases. Did not have to make holes at the bottom MM CR because those molded side pods are open at the bottom. Hopefully the hot gas is cool enough to go through the MM without melting any of the plastic parts...

It is ready to fly but I am waiting for better weather. Will post follow-up flight report at a later date.

 

[tbzep]

Raul,

That upper stage is pretty short and has very little fin area. You might want to check it for stability, fully loaded with the largest motor you plan to use. You don't want to go through all this work and end up with a sky writer when it stages. It's a really cool looking rocket. I look forward to seeing it completed.

 

[rosko_racer]

tbzep:

Thanks for the heads-up. I SIMed this rocket using Rocksim and overrid the weight and CG values for both the sustainer alone and the sustainer+booster to match the actual values. Looks like it will be having stable flights even when the margin for the sustainer alone is .25.. SIMs well on all Estes engine combinations. I can add some extra weight to the NC if needed for good measure.