TNT Logo Polo Shirts – place your order!
| May 30, 2016 | 1:16 pm | TNT | Comments closed


We are going to do a run of TNT Logo Polo shirts. Unless we have a group order of more than 36 shirts the cost per shirt will be $30.99 + tax. (The club is not marking these up – this is actual cost.) Attached is a flyer that has all the details.

If interested please send your name, color choice(s), and size(s) to the email address listed on the attached PDF. Orders will need to be prepaid and there will be a wide variety of pickup options available. Final details will be available once we know how many shirts we need to order.

Please place your order by June 12th, 2016.

Thank you,

Tony Huet, Prefect




Thanks to all who helped Estes at the Family AdventureFest!
| August 26, 2015 | 10:06 pm | Special Projects, TNT | Comments closed

Don Magness of Squirrel Works Model Rocketry asked Tripoli North Texas to help recruit volunteers for the Estes Make-it-Take-it rocket build event at the recent Coleman Family AdventureFest in Irving. Don had been contacted by Mike Fisher at Estes to help find volunteers so Don asked me to contact TNT membership and ask for help. Sam Barone also helped recruit volunteers as well. On Saturday alone attendees built nearly 400 rockets which was more than expected, causing a quick trip back to the hotel for more rockets. Overall, nearly 800 rockets were built in just 3 days! Mike thought the event was a huge sucess.

Thanks again to all who volunteered: Sam Barone, Tony Huet, Ted Macklin, Don Magness, Buzz McDermott, Harry Spears, Jason Unwin, and Kirk Wood.

– Tony Huet

Rail guide placement and binding
| January 20, 2014 | 10:45 pm | TNT | Comments closed

In response to post 13426 by Dave Schultz on DARS-General, I thought I’d see if moving the rail buttons as he suggested would have any affect on how smoothly a rocket might move on a rail. I devised a simple experiment that anyone should be able to try on their own. I’m sure someone can compute the math, but I’m more of an empirical kind of guy.

I made a simple rocket using a 4 foot long PVC pipe and attached rail buttons as usual and added a 340 gram weight (a c-clamp that was handy) to the aft end. I  slid the rocket on a rail held horizontally with the buttons at a 90 degree angle to the ground to side load the buttons as in a cross wind. I tilted the rail towards vertical and measured the angle at which the rocket started to slide freely down the rail. On average, it would start sliding at 35 degrees from vertical. I repeated this experiment about 10 times and got consistent results. (I used a new rail free from exhaust residue.)

I then did the same experiment with the rail buttons in the location Dave Schultz suggested in the post referenced above. I had to tilt the rod much higher – to only 20 degrees from vertical before the rocket started to slide freely. I also repeated this experiment about 10 times.

To me this shows that the new button arrangement is more likely to bind as the rocket moves along the rail. Without compelling evidence that moving the buttons mitigates some other safety concern, it seems like a better idea to leave the buttons in their typical locations.

How much does a rocket rotate due to wind…
| January 13, 2014 | 9:25 pm | Tips and Tricks | Comments closed

… while the rear button is still on the rail but after the fore button has released?

Recently there were several posts on DARS-General regarding a thought experiment by David Schultz about moving rail buttons to minimize perceived rotation caused by wind once the front rail button was clear of the rail. The proponents of the experiment surmised that the rocket was capable of significant rotation on the rail around the rear button and that moving the rear button forward closer to the CP would reduce or eliminate such rotation. A counter argument was made that no such rotation had been observed ‘in the wild’ by a group of experienced fliers and was likely not a real issue.

I was contacted by Jack Sprague of DARS off list to discuss this issue. I could not see the problem he and Dave Schultz described in spite of watching video of many take-offs in windy conditions. In all cases, rotation (weathercocking) did not occur until well after the rocket cleared the rail. I even pointed out Dave’s webpage at:

which shows his rocket leaving the rail vertically and then making a sharp turn into the wind. Dave’s own proposed solution to the severe weathercocking is to raise launch speed and avoid high winds.

It became clear that I was not going to make any headway against the thought experiment with empirical observations from photos or videos. So I decided to see if I could calculate how much a rocket would rotate while on the rail. I enlisted the aid of my son who is an Aerospace Engineering major at UT Austin. The results below are my interpretation of his suggested solution.

Jack had used an example rocket 1.6m long and had specified the CG, CP, and rail guide button locations. I added some additional information to allow me to do the computations involved.

Problem: using the following dimensions and information, compute rotation due to wind while the rear button is on the rail by itself.

* rocket of 1.6m length
* buttons at 1.1m and 1.6m from the nose
* CG of the rocket is at 1.1m
* CP is at 1.25m from the nose
* mass of 3.175kg (7lbs.), diameter of 7.62cm (3″)
* take off speed of 8.94m/s (20mph)
* cross wind of 6.7m/s (15mph)

from that:

* determine the amount of force the wind exerts on the rocket
* find the moment of inertia about the bottom rail button
* determine angular acceleration
* calculate the amount of rotation for the time supplied

Obviously a take-off speed of 20mph is well below NAR safety guidelines but was chosen to exaggerate the effect of wind. If rotation is an issue, this example should clearly show it. At 20MPH, the rear rail button is ‘unguided’ for .056 seconds.

A few simplifying assumptions were made. The surface area was determined as a rectangle, and a CD of .82 of the side plan of the rocket was chosen based on reference values on the internet. (A short cylinder with a aspect ratio of <7.) Force was calculated as F = A x P x Cd.

The moment of inertia was calculated as a rod using I= 1/3 ML^2. (Using the formula for a cylinder gives a similar result.) This is a rough approximation, but good enough for illustration.

The moment was computed using Force x rear rail button distance to CP. (As described by Dave Schultz in post 13426.)

Angular Acceleration = Moment / Moment of Inertia

Angular Rotation = .5 * Acceleration * time^2

finally, convert the answer in Radians to degrees.

For the example above, if the first button is located at CG, the rocket rotates .032 degrees while the rear button on alone on the rail. If you move the buttons to just forward of CG and the rear at CP, the time on rail is shortened, the amount of rotation drops to .008 degrees.

So in this worst case scenario the maximum you can reduce rotation is .032 degrees, assuming you can get it to zero. Moving the button closer to the the CP does reduce rotation, but primarily due to the shorter time the button is on the rail. As the rear button moves closer to CG or the center of the rocket, the moment of inertia actually drops – the rocket is EASIER to rotate on the rear button. But this is mostly offset by the shorter time on rail.

The force from windspeed rises as windspeed^2, so the amount of rotation in a 10MPH wind is only .014 degrees in this example.

Rotation is a result of time^2, so increasing launch speed from 20MPH to 30MPH to 40 MPH reduces rotation from .032 to .014 to .003 degrees respectively.

Under NAR safe launch speed guidelines, a rocket launched in a 10MPH wind should have a launch speed of 40MPH.     (page 4)

This rocket would rotate .003 degrees with the rear button located at the rear of the rocket in those conditions.

The conclusion I draw from these calculations is that wind induced rotation about the rear button as the rocket leaves the rail is negligible even under extreme launch conditions. A take off speed of 20MPH in a 15MPH wind is a scenario not likely to be encountered in actual conditions, yet calculated rotation was only .032 degrees. Any increase in launch speed reduces the amount of rotation even closer to zero.

Dave Schultz can claim that he is correct – rotation is reduced due to the shorter time the rear button is on the rail.  But if the fore button is moved further foreword of the CG the rail must be extended to maintain launch speed. However the amount of rotation is so small that the practical effect is nil. As Dave clearly describes on his webpage linked above, the most reliable solution is to use a launch speed that ensures stability off the rail to counter the effects of the wind.

Of course it’s possible I’ve made an error along the way or used the wrong terminology. I’m sure someone will check my work and let me know if I’ve missed something. But it sure looks like leaving the rear button where it is normally placed is not going to cause any kind of issue related to wind.

BALLS 2013
| November 27, 2013 | 6:29 pm | TNT | Comments closed

Here are a few photos from the Tripoli North Texas trip to BALLS 2013. A more complete post to come later.

L3 Rocket at the pad

L3 Rocket at the pad

Robert's L3 flight.

Robert’s L3 flight.

Boost of Ken's 2-stage rocket

Boost of Ken’s 2-stage rocket

Arming the electronics.

Arming the electronics.

How to get a 2 stage out to the pad.

How to get a 2 stage out to the pad.

Dave Schaefer's X-2 safely out of the wind.

Dave Schaefer’s X-2 safely out of the wind.

Michael, Tony, Ken, Dave, and Robert

Michael, Tony, Ken, Dave, and Robert