Thursday, September 18, 2008

Coney Backslider Rocket

I finished working with the "Coney" rocket a week or so ago and found the "sweet spot" for balance. The "Coney" is a rocket designed by Robert Youen. It is a backslider or backglider rocket. You have to be able to calculate the CLA (center of lateral area), CG (center of gravity) and the CP (center of pressure) using the Barrowman equations or better yet the RockSim equations which put the CP even farther aft. Theoretically, a backslider or backglider is a very long skinny rocket with a length to diameter ratio of 30 to 50 and for backsliding the CG should be midway between the CLA and CP according to the research and report done by Bob and Peter Alway. The coney has a ratio of 12:1. After experimentation, I found the "sweet spot" to be farther aft than midway using the Barrowman equations and even a little aft using the better RockSim equations (just using the software and letting it do the caluclating.)

I went through four nosecones using each one through multiple crashes and repairing until they were unusable finding what works. It is a sweet thing to watch a properly balanced backslider return home, just slowly drifting perfectly horizontally to a slow sideways soft landing.

I got a great video of one launch that happened to drift right back to me within 20 feet of the launcher after an estimated 220 foot apogee. I was only using about 70 or 80 psi and could go up to 120 with the one liter bottle I was using with no problem at all. The video is in the gallery page of the website.

The term backslider or backglider is a little misleading because they don't usually glide backwards except right after apogee until they turn over 90 degrees. Theoretically, they could set up a backward glide with a little angle but this is not really even desirable because then they start "flying" in some undetermined direction and at a higher speed so the landing will be harder and farther away. A perfectly flat descent is the best and is usually what you get anyway. The more you set it up for a backglide, the more apt to fishtail on the ascent (and so reduce the apogee) because of the reduced stability.

I estimated from the last frames of the video going down beside my 4' chain link fence that it was traveling at about 15 feet per second (10 mph) compared to a normal lawn dart velocity of 50+mph besides spreading the impact over the whole side of the rocket compared to the nose. No broken fins, no crumpled nosecones or bottles -- just a perfectly soft, no damage landing with no complicated ejection systems or parachutes to get fouled.

Later I will build an easier to build backslider with a tubular extension rather than the very long cone which is harder to make. It should look better, too.

The details are in chapter 5 of the manual.

Tuesday, August 26, 2008

Bug fixes released in version 1.1 of free instructions

I found a really irritating bug in the free instructions eBook. When the contents link in chapter 1, "Ian Clark Cable Tie Release" was clicked, then that link and every other link in the eBook would result in a pop-up registration form.

A previous bug that was fixed without rolling the version level was the link to the Plutoid 1B fin templates to a PDF file that didn't work. That one was caused by the software creating the PDF file automatically putting the extension "PDF" in capitals and the actual file name extension was in lower case which is standard formatting.

The release dates were also corrected from August (vol 1) and September (vol 2) to September and November or December respectively. The earlier dates were a projection before the project was even started. The current dates have not changed since the project actually got started. Don't worry, you should not expect the dates to keep sliding. I am still on track.

These bugs plus some other typographical errors and some format corrections and improvements were uploaded today as version 1.1.

If anyone runs into any other mistakes or problems, please leave a post here or send an email to me at or both.

Saturday, August 23, 2008

Weekend With Grandkids, Stomp Rockets and Water Rockets

I had some of my grandkids visiting yesterday afternoon and again today. Yesterday my little 2+ year old grandson came to me and said "Papa, Papa...rocket, rocket," and kept repeating it until I relented and went out with him and my 6-year-old granddaughter and we did stomp rockets until we wore the rockets out. I made a mental note that my next batch of stomp rockets needed to be made more robust and more of them. They both just love them. We shot them in the air, into a tree in the yard several times, across the road into an empty field, at a fence, at my teenage "grandson" (not really a blood relative--nephew of my son-in-law) and trying to get them to land in a plastic tub (got close but never quite succeeded).

Then today, we were doing water rockets. We had just been doing bounce or "squash" recovery but we did some parachute recoveries and they loved that, at least until a slight wisp of a breeze came up and they drifted half a block away. That is one advantage of bounce recovery -- you can get a little more accurate with the recovery location.

When my teenage "grandson" and I retired to play Age of Empires III on two computers and my home wireless network, it wasn't long before my granddaughter was bugging me to go fly rockets some more. Unfortunately for her, I felt we should finish the video game first and then she finally got entertained by Nanna until they went home.

What a great couple of days! Back to work now...

Water Bottle Rocket Profiles for Fins

I have been working the last few days on a page that will be added to the web site. The page will be "Bottle Profiles for Fins." This may not seem like a very interesting topics but it actually is quite a time saver. I have prepared "exact" profiles for every bottle I have in my several cubic feet of supplies, put hem onto a gridwork and turned them into pdf files for downloading, viewing in a browser, and printing. You can then layout your fins for your water bottle rocket so that they will fit the curved sides of the top of the bottle (bottom of the rocket) that transitions to the nozzle (bottle neck and mouth). It saves tons of time in trying to fit them by trial and error. This page is almost ready to go so you should check for it now here: Then it will be back to work on the main manual so it will be ready before the end of September.

Wednesday, August 13, 2008


Welcome to the blog for Gary's The Complete Water Rocket Manual and related publications. I will be posting the progress towards releasing volume one and two here as well as other interesting information about water rockets and launchers. This is an open Blog and all are welcome to comment or ask questions. You can also contact me personally at

Currently, I am working on chapter 5 and am on track to complete volume one sometime in September.

This is brand new so don't be afraid to be the first to post.