Heavenly Hobbies - Backdraft
Contributed by Dick Stafford
Brief: The glossy sheet provided with the kit says it is s/n number 17. This kit will be fun! Construction: The 23 pages of photo-illustrated instructions are provided in Microsoft Word format and are quite detailed. I decided not to print them given the number of photos. However, the instructions have some minor errors and are a bit confusing in places. (Once again, make sure you read the other reviews, I will not repeat every "gotcha" they point out.)
The parachute tube subassembly includes a BT-50 and a handful of other parts. The main features are a cap that protects the parachute from the booster motor's ejection charge and a balsa piston, which Heavenly Hobbies calls the EZject. The cap is attached to a centering ring with a Kevlar strap and is coated with high temperature epoxy. In my case I used JB Weld. The piston has a small hole pre-drilled in the middle and you merely center it between two knots in another Kevlar strap. This makes me wonder why more kits don't use this type of piston. On this section, I dry fit and sanded the coupler so it fit properly, I also used epoxy vs wood glue so it wouldn't seize up. (Now you know where I had problems with the aft section.) Finally, I installed the top fins at the very end of the build rather than in sequence as directed in the instructions. Upper body section - The upper body includes another BT-50, two more rings, a motor hook, Kevlar twine, and a short piece of a larger tube. Other than installing a motor at the wrong end, there is only one tricky part. That is lassoing the Kevlar twine around a centering ring. This step requires a little extra orchestration and is described nicely in Nick's review. The short ~3/8" piece of larger tubing is cut down an attached to reinforce the top of the body tube. The instructions didn't say where to place the ¼" launch lug. I placed it on the upper tube near the loaded CG.
Due to the confusion factors in the instructions and the fit of the couplers, I rate this build a '3' since it's a non-standard configuration and even minor errors in the instructions threaten to affect the build. I also didn't ding the rating due to the problem I had due to the aft coupler fit. That's my problem--I should have dry fit this like all the other parts. However, the poor fit is worth a half point deduction. Finishing: Construction Rating: 3 out of 5 Flight and Recovery: The Simit spreadsheet allows the user to select motor and delay combinations and provides a graph of altitude, velocity, and acceleration. John Smolley's review describes the software, how to interpret the graphs, and includes several screen snaps. I fiddled around with various combinations but decided the default E9-4/C11-3 combo would be good for the inaugural flight and I have plenty of both those motors. The fuse is cut to provided the desired delay and burns at ~0.57 sec per inch. I thought 12 seconds would be about right so on the field we cut a 6" section just to be conservative. The fuse is placed in the retro motor and is held in with pieces of a tooth pick. It routes to through the hole in side of the cone. An Estes igniter is placed in the end of the sheath of the fuse and secured with tape. I cut a piece of scrap wire the length of the rocket to extend the fuse igniter down to the base. This and the booster's igniter were connected with a clip whip, although the leads could have been twisted. The boost was nice and high. The booster separated and tumbled down safely. The rocket had barely arced over when the retro fired. This resulted in a long walk. The rocket was recovered less nose cone--the piano wire attachment had evidently failed. Flight Video (although not great) Captures the Success! (Video provided by Nick) We wanted to try again so Nick acquired another cone and carefully bored it out by hand. We attached the shock cord to the tip with a woodscrew, washer, and a dab of 5-minute epoxy. He also coated the inside with the epoxy. Flight 2 used the same E9-4/C11-3 motor combination and an 8" section of fuse. The boost was the same with the booster section recovering close by. I lost sight of the booster--until it went off at about 50' AGL. The rocket lifted about 25' and ejected on cue. A perfect flight! Flight Video 2 - Missed it but listen to the excitement! (Video provided by Nick) As we were recovering it, we heard the PA announce that we were not to fly another rocket until we discussed the flight with El Presidente (a.k.a. Trip Barber). It was hard not to take note of the recovery and nobody warned the contest range. Anyway, Trip reviewed the rocket and the flight and merely told us not to fly it again. He was mostly worried about the retro motor igniting near or on the ground due to the possibility of a grass fire. This made sense. It appears a 7" section of fuse would be more prudent. Even though this flight worked perfectly, there wasn't much safety margin. The post flight inspection revealed that the inside of the cone and the un-covered shoulder held up fine. There was some scorching in the fuse hole and below it on the outside. I will clean this up and cover the area with aluminum tape and/or JB Weld. Also, the Kevlar is the worst quality I've seen. It is coming unraveled and the individual strands are breaking. This may eventually need to be replaced. Due to the model's construction, this may prove difficult. Despite my feeling that there is a high probability of failure, I have to give this kit a 4 for flight and recovery. After all, it performed well twice for me. The deductions were the nose cone loss and fraying Kevlar. Flight Rating: 4 out of 5 I have sinced painted the Backdraft. It survived two flights naked. Now that it's painted, has the probability of failure increased? Luckily, the paint job ain't that good ;)
Summary: As I mentioned, I built off of the soft copy instructions. I personally hated this and won't do it again. I like having the instructions on the workbench and don't want my laptop anywhere near CA, epoxy, or sanding particles. On another subject, H.H. Simit can be modified by the user to include additional motors and other rocket designs. You can even omit the retro motor to use it on 'ordinary' rockets. I don't need it for general designs but others might find this useful. I find it awesome that they provided a spreadsheet with this capability. Thanks, Nick, for giving me the opportunity to build, fly and review this interesting kit! Overall Rating: 3 out of 5 Flight Log
What You Can Do
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Aft airframe (booster) section - This section consists of a pre-slotted body tube, three fins, a 24mm motor tube, 2 rings, a motor hook and a coupler. The confusion factor started early. The instructions refer to a long 3.75" motor hook but the photos show what is clearly a shorter hook. In addition, some of the following dimensions (e.g., where to spread glue for the upper centering ring does not match the actual position of the ring.) Nevertheless, the booster fin can was straightforward and assembles like a typical rocket with TTW fins. If you build one, read the entire set of instructions before starting and dry fit things to make sure you see how they should go together. I found out about the coupler fit the hard way but, since you won't make this mistake, I won't go into the ugly details.
Middle airframe section - The middle section consists of a body tube, three fins, 2 rings, a coupler, and a parachute tube subassembly. The body tube is pre-marked for locating the surface mount fins.
Nose cone - The nose cone is hollowed and has two fuse ports pre-drilled. The surface is not as nicely finished as, say, a Semroc or FlisKits cone. Aside from the extra prep, there are several things you need to do. This includes coating the inside with high temp epoxy (J- Weld), cutting a channel for the shock tether, attaching a wire near the tip as an attachment point for the tether, and covering the shoulder with aluminum foil. The shoulder took a lot of sanding to get it to fit when covered with foil (I actually used metal duct tape). I had my doubts about the short wire as an attachment point for the shock cord, but decided to try building it stock. As it turned out, it wasn't in fact sufficient.
C.S. (June 25, 2008)