Bottle Rocket B

[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?

[SOCoach]

I think I found it . . . going from the tip of the nosecone back (to the bottom of the bottle), the CG needs to come before the CP in order to backslide. Is that correct?

[Toms_42]

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).

I'm not sure how accurate RockSim is for bottle rockets. I've used it for solid fuel rockets, but never for water. If you're trying to get an accurate idea of launch height, using a solid fuel engine in the sim will be very different, as water rockets tend to be one quick burst vs solid fuel is more prolonged at lower thrust.

EDIT: tip for rocksim: ALWAYS put moderate wind in. without it, rockets that are overstable (Cg too far ahead of cp) will perform just as well as a properly balanced rocket.

[SOCoach]

I am looking to buy a new rocket launcher. Has anyone purchased the Bigfoot Water Launchers? Any good? Any other brands any would recommend?

[Toms_42]

I am looking to buy a new rocket launcher. Has anyone purchased the Bigfoot Water Launchers? Any good? Any other brands any would recommend?

I have a friend who bought the bigfoot launcher and thought it was great. Back in 2012, we used this:
http://www.pitsco.com/Aquaport_II_Water_Rocket_Launcher
It's expensive, but it worked well. Of course, you can easily make a water rocket launcher yourself, with a guide such as this one:
http://www.uswaterrockets.com/construct ... torial.htm
The money for an aquaport would probably be more useful if spent on an air compressor, as pumping the bottle rockets can be very tedious especially if you have a lot.

[SOCoach]

Our team competed at an invitational this weekend and one of the rockets had a long nosecone of clear plastic material . . . but had no cap on the end. The nose was open approximately an inch at the end of the rocket. I assumed when it was launched air would rush down the nosecone and knock it down, not so. The rocket got great height and backslid beautifully. Has anyone seen or had any success with an open ended nosecone?