Gap Staging Different Motor Tubes

[Dredger]

I'm planning to gap stage a couple of rockets with dissimilar motor tubes. The first stage will use a D motor (24mm/B-50), the second stage will use a C motor (18mm/B-20). Both will be housed in a B-60 sized body tube.

Because they are dissimilar sizes, the B-50 will fit over the B-20, with a slight gap around the circumference of the connection.

Any ideas how to close that gap, or if it is even needed. I know that vent holes are needed for the hot gasses of the first stage to reach the second stage. So I'm wondering if that circumference gap will actually serve, of if it will be problematic by venting vertically around the second stage, as opposed to perpendicular.

One solution I thought might work is to make both motor tubes B-50, then for the second stage use an adapter for an 18mm motor. This way the two motor tubes will align.

Any thoughts?

 

[aerostadt]

I have done this, but I am thinking about it. I have a model that has flown many times with a 24 mm motor in both stages with gap staging over a short distance. However, the model goes way too high for a small field, especially the second stage. I have bought Estes plastic holders that allow an 18 mm motor to fit in the 24 mm body tube. Obviously, I can now mix things up. I can put an 18 mm motor in the second stage and either a 24 mm or 18 mm motor in the first stage. I plan to do this within the next few months.

 

[K'Tesh]

I asked Carl McLawhorn about gap staging in the Semroc Omega (the vents in the centering rings for the booster), and he wrote that you want a way to vent the booster's gasses so it doesn't pop off because of pressurization before the engine can be ignited. However, this issue is more of a problem with large gaps between motors. I'd say don't worry about it.

Tim Van Milligan has an article about it over on Apogee's website.

 

[caveduck]

Hi Dredger, welcome to TRF!

You didn't say how big the gap was but there's no need to have the motor tubes be the same size. You do need a vent so that pressure from the forward end of the booster tube is relieved to the outside of the model. Gap staging works by more or less straight line propagation of burning fueld particles from the booster onto the propellant of the upper stage motor...if there's no venting the sudden pressure will definitely pop the stages apart before that can happen. Even with correct venting it gets dodgy at longer distances. At last NARAM I saw a nice scale model with maybe an 8" gap fail to stage on both flight attempts.

 

[Dredger]

For my first try, I'm planning a gap stage distance of about 6"-8".

My plan is to take my homemade big bertha and add a first stage to it just for grins. Both body tubes will be B-60. I'm going to design both motor tubes with B-50 (24mm) to handle D motors, and use an adapter for the C motors.

For the first stage I'm planning some small trapezoidal/rectangular fins, maybe 2" long by 1" height. I figure the super sized big bertha fins will provide enough stability.

 

[BABAR]

Couple of ways to transition to different size engines.
With all due respect to the those rocketeers who are owned by their felines, there are lots of ways to skin this cat.
When I do longer gap staging (up to 3 feet), I tend to cheat. I don't know if it is necessary, but I use a chimney to "duct" my lower stage ejection products (okay Shread, FLAME) to the upper stage nozzle. I do this even if I am staging the 24 to 24 or 18 to 18, by downsizing one body tube. I think this actually works out in your favor if you are going 24 to 18. The second diagram above has some ideas for this.

I do smear some epoxy on the INSIDE of the chimney just above where the lower stage engine butt up against it. This is because for a few seconds even AFTER staging there is a flame coming out of the lower stage, and it will fry the inside of the the smaller chimney.

Heads up, one of the PROBLEMS with longer gap staging is the recovery of the booster. Larger and long boosters have two issues, they are heavier and if you aren't careful the will come in ballistic because they are stable. Options include

making sure the booster by itself is NOT stable (can probably eyeball this, and for 6-8 inches really isn't likely to be an issue. Start getting into 12-14 inches though.......)

OR

adding a recovery device to the booster. Lot of ways to do it, I like adding an auxillary engine with it's own nose cone and streamer or chute. example here
https://www.rocketryforum.com/showt...rt-in-the-paint&highlight=hyperspace+injector

You need vent holes to dump the initial "cold" gas between the two engines. These should be just below the upper stage engine. I think most people just do one. I do two, one on each side, just for theoretical concern that asymmetric gas venting might deviate the rocket just prior to upper stage ignition.

If you DO have a "tube in a tube" like the second example above, you need holes in both the inner and OUTER tubes. OR (and this looks better and is probably more aerodynamic) you can cut slots in the inferior centering ring so the gas will vent out the back of the rocket.

I have no scientific knowledge of the size of the holes. I just use a standard hole punch like for notebook paper.

One question, why use a C motor on the upper stage? If you are just doing it for the fun of staging, go with an A8-5 or A8-3. It will stage just as well (actually MORE reliable as it has a much larger nozzle) and it won't go as far. If indeed you are going for altitude, then go ahead with the C and be ready for a long walk!

I am far from an expert on staging, but using this technique (knock on wood) I have yet to have a staging failure.

Straight Trails!

 

[Turd]

I have gap staged 24 to 24 and 24 to 18 (mm) motors at up to 28 inches apart with no failures, there are no special considerations or preparations needed.

Just put 2 or 3 vent holes within an inch of the second motor, and make sure you use a small bit or something to expose fresh propellant on the second motor.

Edit: I should say that I always used tube equal to the motor size... Not like a BT50 connected to a BT80 then leading to the second motor mount.

 

[aerostadt]

The gap staging that I have done is trivial compared to the previous 2 posts. I have used about a 1 and 5/8" OD tube with 24 mm motors over a distance of about 2" and it has worked fine. I would like to try an 18 mm in the second stage with Estes engine holder in order to hold the altitude down.

 

[Astro-Baby]

Thanks guys from me on that - I am thinking of a two stager where that could be very useful - I was thinking of a chimney system for this but maybe its not necessary with a gap of no more than 3-4" Mine would be D sized engines but its likely to be a very heavy and draggy bird so wont go too high anyway.

 

[BABAR]

Thanks guys from me on that - I am thinking of a two stager where that could be very useful - I was thinking of a chimney system for this but maybe its not necessary with a gap of no more than 3-4" Mine would be D sized engines but its likely to be a very heavy and draggy bird so wont go too high anyway.

Nope, no chimney needed for 3-4" (and some would argue may never need it, I just know it has worked for me on the longer gaps.)

Straight Trails!

 

[Verna]

I'm planning to gap stage a couple of rockets with dissimilar motor tubes. The first stage will use a D motor (24mm/B-50), the second stage will use a C motor (18mm/B-20). Both will be housed in a B-60 sized body tube.

Any ideas how to close that gap, or if it is even needed. I know that vent holes are needed for the hot gasses of the first stage to reach the second stage. So I'm wondering if that circumference gap will actually serve, of if it will be problematic by venting vertically around the second stage, as opposed to perpendicular.

Regardless of the configuration you still need to make sure you vent enough gas. Going 1 to 1 you may be ok with the current gap but that's hard to say without seeing it. You may need a couple of vents. My rule of thumb is a total vent area equal to the nozzle diameter of the largest engine(s) and on opposite sides. If and when you get to cluster staging, obviously you'd need more.

Go here https://www.sears572.com/supersaturnv.htm and find the build article. You may find some of it helpful.

Verna
www.vernarockets.com
https://www.facebook.com/RocketBabeDustStorm

 

[adrian]

Here's what I did for recovery of a long booster:



This is the joint between the 2nd and 3rd stages of my 4-stage Rheinbote model. (The rocket itself is the one in my avatar.) The coupler is a piece of BT-20 expanded and sealed with a strip of paper so that it fits easily over the regular BT-20 of the 3rd stage motor mount. The streamer is Nomex, not long enough to count as a proper recovery streamer but enough to destabilise the booster so that it tumbles rather than plummets. The motor mount is centred by thin strips of balsa rather than rings so that the gaps between the strips act as vents.

My SA-2 has a D12-0 staging to a C6-5:



There's a tube running through the interstage gap to ensure that burning particles from the D12-0 get to where they need to be. Both the tube and its vent holes are almost impossible to see unless you're a lot closer than you want to be when a D12 is about to ignite.