My First Two Stager - Boosted Patriot Build

At the beginning of February I was flying my rocket fleet at a nearby park. I made the mistake of flying my Estes M104 Patriot on a B6-6 motor; the recover devices never deployed and the rocket was forced to lithobrake. The ejection charge on the motor blew about a second after impact. The force was strong enough that the charge ejected the entire MMT, centering rings and all! But it's alright because we can rebuild and improve.





I had been thinking about adding a booster to the Patriot for a while, but there was a motor mount clip in the way and not enough room for a coupler of adequate length. With the MMT now cleanly removed, I could take a shot at my first two stage rocket.

Easily do-able. Either terminate your booster motor mount just about 1/8” aft of the sustainer motor nozzle, or extend it around the nozzle and punch two hole punch size holes which will be just aft of the sustainer casing. You are gap staging and in either case you will need to duct your burn through booster gases to the nozzle of the sustainer motor.

Whichever method you choose, you ALSO will need to vent in the outer rocket tube body of the booster. The most aesthetic is to cut sections out of the both forward and rear centering rings of the booster motor mount, 1/8 of the circumference should be sufficient. Thus the cool unheated gas will flow forward from the inside of the booster, across the 1/8” gap or through the holes, and back along the space between the mount tube and outer body tube

After reading the Handbook of Model Rocketry by G. Harry Stine a few times over, I've chosen his gap staged design for this mission. Visually, I prefer this method because it looks more like a sounding rocket than taping the motors together. Here's the booster design in OpenRocket:



The body of the booster is a 6.5" section of BT60 tubing. The coupler is 1.5" long and protrudes 0.75" into the rear of the sustainer. The MMT runs the full length of the booster and will abut the sustainer motor casing when fully assembled. According to Stein's book and research, the most reliable way to ignite the sustainer motor is to funnel the burning ceramic shrapnel from the booster ejection into the nozzle of the sustainer. There's a 30 millisecond delay between burnout/blow-through of the lower stage and ignition of the upper stage. To avoid the booster separating before upper stage ignition, the hot gases from the booster motor need to be vented. I used a drill press to make a 1/4" hole through the body tube and inner tube; just behind the coupler. In the future, I will follow @BABAR suggestion in the above post because the large holes in the booster body are not attractive. Finally, I used the same fins from the Estes M104 Patriot on the booster.

The final product (before clean up and paint) came out like this:

I am not experienced with open rocket. You may want to check to see if the booster by itself (i.e., after successful sustainer ignition and separation) with an EXPENDED engine casing will be stable. This will be bad, as the result will resemble the namesake of the Officer's Club at Randolph Air Force Base, which IIRC was referred to as the Auger Inn.

BABAR said:

I am not experienced with open rocket. You may want to check to see if the booster by itself (i.e., after successful sustainer ignition and separation) with an EXPENDED engine casing will be stable.

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I took a look and the booster alone with an expended motor casting is not stable. Barrowman Equation comes up as -0.82

With the lower stage finished and the MMT reinserted into the upper stage (moved back an extra 0.5"), I could call it complete here. But I won't.

The standard Patriot fins are quite large and the more I looked at real world sounding rockets and multistage model designs, I noticed a trend towards smaller fins on the upper stage. My understanding is that since the rocket is already traveling at high speeds when the booster separates, smaller fins provide more stability than they would at takeoff speeds and have less drag. I tinkered around in OpenRocket for a while looking a different fin shapes and configurations for the upper stage. An important mission requirement for this build is that it needs to be flyable both as a single stage and two stage rocket. I found that if I used the fins left over from my massive Viking fleet build that I could add an 100' to the apogee in two-stage mode and the upper stage would still be stable on its own if flown on B motors (perfect since I fly at a baseball field more often than not).



I set to work removing the existing fins from the upper stage:

Completed construction of the Boosted Patriot. Now on to paint and decals.

The issue that I seem to be having with this build is my "off the rod" speed. My best speed (according to OpenRocket) is 38 fps on a C6-0. I've been told that rockets should leave the rod in excess of 40 fps to ensure stable flight. Does anyone have tips on how I can increase my launch speed?

Extra info: my simulations are set to use a 6 foot rod, 3/16 in diameter.

What about an 18mm composite engines? That might give it the extra thrust you need off the line.

CalebJ said:

What about an 18mm composite engines? That might give it the extra thrust you need off the line.

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The problem with 18mm QJets and AT composite motors is there are no booster motors. (i.e. they all have a delay and an ejection charge).With BP booster motors the blowby gasses burn through at the top of the propellant core igniting the sustainer motor.
Attaining sufficient speed off the launch guide is why many larger low power two stage rockets have a 24mm mount in the booster stage. In lieu of that, I would go with a longer rod/rail.

Thanks - I wasn't sure what was available for boosters. Haven't done a two stage since starting back into rocketry a couple of months ago, and composite engines are still new to me.

Assuming this is no longer a "scale" build and just a sport model, you can add a side pod or two to the booster and cluster launch it. Gives you the added advantage of having a chute or streamer deploy from the booster.

https://www.rocketryforum.com/threa...taged-2-successful-flights.47731/#post-463606

BABAR said:

Assuming this is no longer a "scale" build and just a sport model, you can add a side pod or two to the booster and cluster launch it.

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Yes, this is not a scale build, just for fun and to expand my knowledge. Thanks for the link Babar!

...and it's finished!

Love the artwork. Hope the tree doesn't have any portents for landing locations!

Recommend go with the smallest possible SUSTAINER motor for first flight.

Hoping for two straight trails, a soft landing, and a short walk! (also hoping given current situation you can find a place to fly her!)

Put rail buttons on it and use a 10 foot rail.

Gut says those sustainer fins look small. Is Open Rocket always right about the CP?

With simple fins, its ok.

But they look small to me, too. I don't think you need as much stability margin for a sustainer. You don't have the crosswind at the tip of the rod issue. Hypothetically, once the rocket is clear of the rod, it should quickly accelerate to move laterally with the wind and the angle of attack should drop to zero. But that doesn't mean that the air above you is all moving at the same speed. So as it enters a new wind speed/direction layer, it can shift again. It's most obvious in the smoke trail. The angle of attack should jump briefly in the shear zone and the rocket should adjust.

The problem with tiny fins is low control authority. I think it might be prone to coning. Or fly straight, turn, fly straight. The cone fin rocket I tried yesterday seemed to do that.

In OpenRocket, try setting a reasonable crosswind and then add stability calibers to the graph. That's an eye opener. The static stability (in the picture) is about 2 for my cone fin, but drops to 1.35 at the start of flight in a 15mph crosswind.

Thanks for the advice @charles.garnes and @Mike Haberer

Joekeyo said:

Gut says those sustainer fins look small. Is Open Rocket always right about the CP?

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I'm nervous about it as well. The simulations keep coming out well so it's down to a real world flight at this point.

By the way, I built a booster Patriot too. The Improved Malemute and the Patriot share a motor.