Rockets

(End burners)

Making an end-burner requires some hot fuel and careful packing. The parameters are all pretty critical so making lots of them as work-horse motors is probably not in the cards.  In my case, and lots of CATOs later, I managed to get some respectable altitudes using 75/15/10 pulverone and sand reinforced kitty litter w/toilet wax nozzles (see movie below).  They are very much more cool than the core burners - mainly because they thrust for a longer time and therefore look and sound neater in-flight. Notice that these have a 1/8" hole (as opposed to 3/16 hole usually found in a 1/2" rocket). A smaller hole means more thrust. The size gets very critical when they get this small. 1/64" smaller and it explodes. 1/64" bigger and it can't lift itself off the launch pad. When they do work, the thrust is spread over a much longer time, so the weight of the rocket can be critical. A light rocket will go hundreds of feet high while a heavier rocket will struggle to get off the pad.

Note the tooling for the above rocket. If you look carefully, you can see the rub marks on the core where the nozzle material usually ends. That leaves about 3/16" (or less) penetration into the BP grain. This penetration creates an initially larger burn area to get a good pressure and thrust going. BP burns faster when the pressure is greater so the penetration (dimple?) sets the tone for the rest of the burn. That makes it somewhat critical although nozzle opening and depth, case size, and the BP formula also are important. 

Here is a test stand run of a short (5" total, 4" of composition) 5/8" end burning driver that uses Paulownia BP. This thing goes to the moon if launched on a stick. The thrust is about .8 pounds for four seconds.

Click on the above image to see movie

Click on the image below to see and hear an end burner launch. Note that both launches get washed out by the bright sky but compare the burn time of the end burner with the burn time of the core burner (4 seconds vs. about .5 seconds). The second rocket was the 3/8" and it can carry a bigger load than the 1/2" but it can't go nearly as high. Look for the loud roar and echo (and slight chuffing) from the 3/8" and compare it to the  1/2" sound. 

Altitude for the first rocket (1/2 inch end burner) is unknown but probably greater than 800 feet (wild guess since it went out of sight and then I saw a small smoke puff from the header popping (1/2 gram of BP)). I lost it on the way down so it wasn't recovered. The altitude for the second rocket is whatever height it takes 7 seconds to fall from (;-}).  By pure math, that would be over 700 feet but, with wind resistance,  it is more likely about 350 or so.

Rockets - Quarter Inch

The above picture shows some cheaper spiral wound tubes from cannonfuse.com that I experimented with to make 1/4 inch rockets. These work great! Note the short core and also note that the opening is tapered up to 1/8". With these you can use the hottest fuel. I load them to the top of the core, add 1/4 " of fuel, 1/4" of sugar fuel as a delay and then put a small payload in the top (there is actually plenty of room since only about 2/3 of the body is used). Flash or small balls from cheap Roman Candles work great as payload (only one ball will fit comfortably). I even put BP in it to just make it 'huff' when it gets to altitude. These will go 150 feet high (or more - much higher than a bottle rocket).

Modified Rockets for Reliability and Ease of Manufacture

Here is a 1/2 inch scheme that uses simple construction and lifts a film can to 100-150 feet. The 3/8" version (same length) will put a film can to 80 feet -- which is a nice low break height (sort of a strong hobby mortar height). This motor was developed to eliminate having a different fuel for each size rocket and to simplify the delay mechanism. 

On small star bundles (like the rocket below), you should put in a clay bulkhead and a 3/32 pass fire else the delay charge will push through the payload and might cause some stars to blow blind. The pass fire hole lets the powder in the payload get going and makes for better star ignition.  A 1 inch delay is just about right for a 3/8 rocket with a small payload like the following 28 gram example - which uses a 3/4" salute shell cut in half for a nose cone. You can get several small stars into the cavity - fill it with powder and you are ready to rock:

 When you put a film can on a 3/8 version of this, cut the delay grain in half so the rocket doesn't nose over before the payload ignites. You probably won't need a pass fire hole - just let the delay charge belch into the film can. Make sure the lid is glued on tightly. The maximum weight for a 3/8" rocket/payload/guide stick is about 35 grams which will get you 80 feet or more. For a 1/2 inch rocket/payload/guide stick the weight should be 45 grams or less. Streamlined and weight reduced payloads will dramatically affect the altitudes you get. Conservatively, you can expect a streamlined 30 gram (total) 1/2 inch version to get to 200-250 feet. A titanium mix in the delay material (RP with titanium instead of aluminum) makes a great sparkling launch. 

Click on the above image to play the .wmv movie (500k)

See the movie above for a 3/8" titanium RP launch. The payload was a film can with 8 grams of micro-stars (Davis white antimony tri-sulfide) in it and six 3/8" pressed stars of various colors (Bleser stars with added PVC, AFN Bright Green, and Skylighter Blue - see compositions.html). Total weight was a little over 35 grams - so it was a bit on the heavy side but still good. The colors in the movie are washed out - the reds, greens, and blues were fairly good at the scene. This rocket is every bit as good as the 1.4g stuff - We were firing them along side1.4g rockets and the only rockets that had more class were the Phantom Blue Streak and the Phantom Lunar Beam Rockets.  The rest of the small commercial rockets we shot, including a wide variety with much larger payloads, were no better and mostly poorer in quality.