Bottle Rocket B

[SOCoach]

In the past I've worked all the calculations with my students, but it is really difficult for them (not always easy for me either). We typically through trial and error find a design that works, and by keeping careful notes on what everything weighs we can create a backslider fairly reliably. It is always a fine line however, and too many times it would fly great on flight one and crash on flight two.

I noticed your specs included an overall weight of your rocket is around 100 grams. We used many different fins and bottles but have had a real hard time finding a strong, but light enough nose cone that we could make a few feet in length. We've tried paper and covering it with paper mache but usually resort to cardboard tubes because they are easy to find and use. Using the tubes our rockets end up 130 or 140 grams. What type of nose cones are you using (if I can ask).

[joeyjoejoe]

There are several designs we've used. For light and easy:
1. Use thin Lexan fins (3 is all I typically use) cut from Lexan sheets available from most hobby stores (I want to say .05" to .07"). I usually cut them about 2.5-3 inches tall by 4 inches long and then fold 1/2 inch of the height over 90 degrees to allow the fins to be attached to the pressure vessel. You will need some really good scissors to cut Lexan. Another thing that makes things easier is to attach the fins to the side of a bottle and then cut completely around the bottle just above and below the fins to produce a "fin skirt". This skirt can then be attached to another pressure vessel rather quickly (adds weight but simplifies the build). It also can be attached slightly lower on the pressure vessel moving the CP back a bit more - something that is often crucial when you don't want to add weight to the rockets nosecone (which moves the CG forward). Remember the CG must be around a rocket diameter ahead of the CP. You can lower the skirt even lower but you will need to read up on a technique called "heat curling" to strengthen the part of the skirt that hangs below the bottle. Furthermore, you need to be very careful about doing this since it may not fit on launchers that are provided for you. Once you have built a skirt, it can be removed from that destroyed lawn dart and put on the next rocket rather quickly.
2. Instead of a coney, I prefer to build the longer rockets. I use the smaller FTC tubing (the one that a bottle cap snuggly fits inside of)-available at Lowes. Then I glue a bottle cap to the top of the rocket (bottom of the bottle) insert my ftc on top (14"-22" is what I usually use) and tape or glue a 5 or 6 inch hand-made lexan cone (.03" to .04" thick) to the top of the pressure vessel and to the FTC to help keep the FTC in place.
3. There isn't enough time in the day to go over the nosecone options. I've used the aforementioned "guppied" 16 and 20 oz soft drink bottles as well as two Lexan cones with their bottoms taped together. I've also used a combination of the two: the top half of a 16 oz Dr. Pepper bottle filed down to fit into the top of the FTC with a Lexan cone as the actual nose cone.
I could provide a picture but I'll bet we'll all benefit if I don't.
Good luck!


To fasten things to the pressure vessel, I use "Goop" but I'll bet that this is prohibited. If so, you may luck out and find "Stretchy" packing tape (yes, that is the brand name). No idea why but it sticks OK to boxes, walls etc. but is almost un-removable from plastic.

[SOCoach]

My students have tried the FTC tubing before but it usually comes out way too heavy. Are there different thicknesses of tubing?

[dragonzord0130]

Does anybody know if the event supervisors are allowed to prevent you from pressurizing and/or pulling the trigger to launch the rocket? I was at an invitational last Saturday, and the event supervisors, who were parent volunteers from one of the schools competing (I think), did not allow my team to go anywhere past the tent to fill our rockets with the desired amount of water. I know that it's not that big of a deal, but I still would like to know because it would be nice to make some final adjustments to the fins and position it the way I want to before launch.

[chalker]

Does anybody know if the event supervisors are allowed to prevent you from pressurizing and/or pulling the trigger to launch the rocket? I was at an invitational last Saturday, and the event supervisors, who were parent volunteers from one of the schools competing (I think), did not allow my team to go anywhere past the tent to fill our rockets with the desired amount of water. I know that it's not that big of a deal, but I still would like to know because it would be nice to make some final adjustments to the fins and position it the way I want to before launch.

In all events, event supervisors generally have final say when it comes to safety, and this sounds like it was potentially a safety precaution. That said, the general guidance to event supervisors is they should never touch a competitors device. So if they we loading the rockets on the launcher for you, I would definitely have issues with that were I to witness it.

[SOCoach]

Can anyone give me an easy (or fairly easy) explanation of how to do the math to determine a backsliding rocket? I've worked it out before but as I recall it was complicated. I have a couple of bright 7th graders that are willing to learn, but I don't want to frustrate them. Finding the center of gravity or balance point is easy enough . . . Then isn't it a matter of finding area? I can't remember.

[joeyjoejoe]

The Barrowman equations are the easiest way to calculate the Center of Pressure without resorting to calculus but it would still be a bit of a challenge for most 7th graders.
Honestly, the easiest way is to use a program like RockSim or Open Rocket to design your rocket (and even launch it!) before cutting the first part out.
I use RockSim (licensed years ago) and it is awesome! It isn't free though. You can make tiny tweaks to your design right on the screen and then do a test launch to see how it is affected. I've heard great things about OpenRocket too but I haven't used it.

[SOCoach]

Can you simulate water rockets or strictly motor driven rockets?

[joeyjoejoe]

Yep. Sorta.
I can't speak for OpenRocket but RockSim even calculates the weight of the rocket fairly accurately given the materials you select for the various sections of the rocket. Once you've done that you have to select an engine. Even though "water at 60PSI" isn't an option, it doesn't really matter. I am primarily concerned with how changes affected the flight so I would select an engine and stick with it. Then I would make changes, simulate a launch and see how it compared to the previous launches. It even simulated the fall down to earth realistically, complete with backslide if designed correctly.
I can't remember which engine I selected, but it was one that took the rocket to "water rocket" heights (around 120m).

[SOCoach]

I downloaded Open Rocket and if nothing else it does calculate CG and CP which would help us determine if it will backslide or not. That alone will save us a tremendous amount of time in not having to repair rockets (as long as it is accurate). Plus it seems like a program I can teach my 7th graders to use. Refresh my memory . . . . to backslide does the CP have to be in front or behind the CG?