Opinions on upscale construction

[six-o-one]

While waiting for my Navaho AGM kit to come in the mail ( to replace my one gone astray), I have begun my project of an upscale version of the same rocket to a BT-55 version (staying with 18mm motors). I have done the upscale math, cut the fins, booster, BT, etc. Here is where I am looking for opinions. The original kit uses gap staging. The original booster length is 3". Upon upscale, to maintain original, though upscaled margins, the booster is just over 4", basically 4". So, what that leaves me with is gap staging which is obviously a bigger gap. If built just like the original in reference to motor mount placement in the BT and booster, that leaves a 1 3/4" gap. Best case scenario if I move the motor mounts, making the motor nozzle flush on the booster (like the Harpoon, or many others I am sure), and extend the motor mount 1/4" from the upperstage, and thus the motor extends 1/2", giving me a gap of 3/4". Just looking for opinions and input from you more experienced builders on this one. Is 1 3/4" gap too large? Is the 3/4 gap still too large also due to larger BT? Which, if either, do you guys think is OK? Thanks in advance for any thoughts.

[CPMcGraw]

While waiting for my Navaho AGM kit to come in the mail ( to replace my one gone astray), I have begun my project of an upscale version of the same rocket to a BT-55 version (staying with 18mm motors). I have done the upscale math, cut the fins, booster, BT, etc. Here is where I am looking for opinions. The original kit uses gap staging. The original booster length is 3". Upon upscale, to maintain original, though upscaled margins, the booster is just over 4", basically 4". So, what that leaves me with is gap staging which is obviously a bigger gap. If built just like the original in reference to motor mount placement in the BT and booster, that leaves a 1 3/4" gap. Best case scenario if I move the motor mounts, making the motor nozzle flush on the booster (like the Harpoon, or many others I am sure), and extend the motor mount 1/4" from the upperstage, and thus the motor extends 1/2", giving me a gap of 3/4". Just looking for opinions and input from you more experienced builders on this one. Is 1 3/4" gap too large? Is the 3/4 gap still too large also due to larger BT? Which, if either, do you guys think is OK? Thanks in advance for any thoughts.

Stine indicated gaps up to 12" are reasonable. See his commentary on pages 168 and 169 of the Seventh Edition. A good example of having a more-than-1" gap is the old Estes Omega. The gap in that model is 2.25" between the top of the booster motor casing and the bottom of the sustainer motor casing. One thing Estes did with that model was to increase the length of the booster motor tube to 4", leaving 0.75" of open space.

Keep in mind something we've discussed here and on OldRockets many times in the past -- it's not the gasses that ignite the sustainer, nor the heat of the gasses, but burning particles from the booster making their way up the nozzle, which essentially continue the burning process into the sustainer motor by a mechanical transfer of a chemical reaction.

What this boils down to is, as long as there is a clear, unobstructed path between the top of the booster and the nozzle of the sustainer, and as long as the gap doesn't stretch beyond about 12", then you should achieve staging.

Stine also recommended opening some 1/4" diameter vent holes near the top of the booster to allow the pressure wave to be relieved before it has the chance to blow the stage off prematurely. The pressure that builds up from the ignition of the sustainer will be far greater than these ports can vent out, and that is what should blow off the stage.

[CPMcGraw]

While waiting for my Navaho AGM kit to come in the mail ( to replace my one gone astray), I have begun my project of an upscale version of the same rocket to a BT-55 version (staying with 18mm motors).

One thing you could do is study the Cherokee Double-D booster configuration, since the stages will be essentially the same...

[six-o-one]

Craig, thanks as always for the info. The 12" allowable gap margin is reassuring. I have read the handbook, but don't own a copy. Occasionally run by the library to check on things. If that's the case, I am not sure which way I will go. Will sit down and figure it out today. I had considered extending the motor tube to keep the gasses/particles focused on the upper stage motor, so it was interesting to hear you present that as an option.

I think I have asked here before about the vent holes. On my Navaho, I did not use vent holes and it performed perfectly. I have seen several reports of others with no vent holes and no problems, but have seen a few who used them. Not sure what I will do there. My inclination is to not use them. If I was going up to 24mm motors, I think I would definitely do them, but since I am staying with the 18mm, guess I am kinda figuring that if they didn't build up enough pressure to blow the stages apart on the Navaho with its T25 BT, it would probably be safe with the extra space inside the tubes with the BT-55. Craig, anyone, if there is a flaw in my logic here, LMK.

Craig, as far as the Cherokee DD goes, maybe I need to look at your plans and booster configuration. On mine, my booster is 3" long and I use traditional Estes-style butt-and-tape staging. I have the upper stage motor mount extended slightly from the BT, allowing for it to butt up against the booster motor. This is where it is different from the Navaho if I stick with upscaling the booster also, as that makes the booster 4" and thus brings on the gap issue. One perfect flight so far, so hopefully I am not flawed there, maybe just different from yours.

Thanks

[six-o-one]

Well, so much for looking at the Cherokee DD plans. Im on dial-up here at home. No chance. Have to wait until I go back to work Jan. 3 to check that out.

[tbzep]

The Estes Omega (Cineroc Carrier) uses gap staging. I forgot the actual distance, but just eyeballing my Omega, it looks like it might be a good 2 inch gap.

The Omega has been just as reliable in my and my friends' experiences (and from what I've read over the years) as any other method that puts the two motors together.

[six-o-one]

Thanks for the input. Sounds like I may be OK whichever way I go.