Wednesday, January 26, 2011

Review: Heavenly Hobbies - Backdraft

The Heavenly Hobbies Backdraft is a BT-60-based, 24mm-powered rocket that on face value looks like a standard 2-stager. If you've read the previous reviews, you'll know that there is one major difference--the upper motor is used as a retro rocket. Heavenly Hobbies calls this the TailWind delayed deployment system. The upper stage is ignited with a slow-burning fuse ('green cannon fuse'), so the flight prep will be out of the experience base of most rocketeers. Heavenly Hobbies includes an electronic spreadsheet, H.H. Simit, to help with the motor/delay selection.

The glossy sheet provided with the kit says it is s/n number 17. This kit was fun!



Construction:
The kit includes 40 parts, including lots of tubes, laser cut rings and fins, and a pre-hollowed balsa cone. The laser cut parts all fit well, however, the BT-60 couplers and nosecone shoulder required sanding.

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.)

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.




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.

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.


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.


Flight and Recovery:
Nick and I have flown this model twice at the time of writing on the last day of NARAM-50. The use of a retro motor and a fuse raised some eyebrows, but in the end the flights were approved.

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.




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!



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.

The third flight At ESL-122 was on the same pair of motors.  The retro motor was ignited with a slightly longer 7.25" fuse. I lost sight near apogee until the retro motor went off at about 400'. This seemed like a good altitude and everyone thought it was cool! Again, there was some damage and missing parts. The Kevlar® shock cord didn't hold up and the parachute cap tore off and was lost.

The fourth and final flight was at ESL-147 and, once gain, used on the same motors and same 7.25" fuse. Why tempt fate? This model guarantee's high pucker-factor flights and it didn't disappoint. It was a great flight and the retro-ejection was on cue. However, the parachute cap was again lost and the main Kevlar® shock tether needs to be replaced. It is now retired.

Summary:
This was an interesting kit to build and, with the benefit of two reviews, was not bad at all. Needless to say, the retro motor set-up is quite unique and the flights provide quite an adrenaline rush! You might check with your local RSO before buying one.

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!