I love innovation in rocketry. A couple of months ago I had read on fellow blogger Dick Stafford’s Rocket Dungeon about an upcoming release of the Backdraft rocket. After reading the info on the Heavenly Hobbies page I was intrigued. The information was sketchy but I inquired with Jose M. Andrade-Cora at Heavenly Hobbies to provide more information. He sent me this press release on the Backdraft rocket.
BACKDRAFT: Blast from the Past or Window to the Future?
Back in 2002, as our family vacation loomed near, I placed an order for some NARTS publications to provided some relief from the “packaged” distractions found in the big Florida theme parks. Among the assorted reports and technical papers, there was one that seriously caught my attention. It was a NARAM-34 report by Bruce Markielewski about construction of what he termed “retro-rockets”. These models were rockets that bypassed ejection at apogee for a delayed deployment of their recovery system. To counteract the gravity force during re-entry, a second engine was fired. This second engine would slow down the rocket sufficiently for the recovery system to take over. I thought that “retro-rockets” were a great idea, as they added complexity and interest to the usual rocket flight patterns that, frankly, were getting old.
Markielewski’s stated objective was to “ …design and build a reliable, functioning model rocket using a “retro-rocket” technique to slow the descent of the model rocket before deploying a parachute or other recovery device.” This objective, however interesting, failed to focus on the possibility of using the “retro-rocket” concept to reduce the drift of the model from the launching pad. Common Dual Deployment® techniques use electronic actuators to force the ejection of a streamer or small “drogue” parachute to cause a fast, but controlled, descent. A larger parachute is subsequently deployed by similar means. The idea is to reduce the time the rocket is subject to crosswind drag, therefore reducing the drift.
However, a rocket using the “retro” techniques does not “break up” to deploy a recovery device in its coasting phase after apogee. It, therefore, presents a smaller frontal area to crosswinds. Less frontal area means less drag and less drift. Also, crosswinds acting on parachutes create lift forces, so not having a parachute in this phase of the descent also helps. Moreover, a compact rocket presents a much more aerodynamic profile in the vertical direction, allowing higher (more negative) terminal velocity limits. This leads to much reduced periods during which the model rocket is subject to crosswinds, and again, less drift. In simple words, the “retro-rocket” technique is superior to the common Dual Deployment® technique in reducing crosswind drift.
To put Markielewski’s work in perspective, one must realize that to achieve his objectives, a fair amount of mathematical calculations were required, perhaps too many or too intensive for the average modeler. However, I realized right then and there that the limitations that were real in 1992 were no longer there in 2004. The personal computer had removed them from the picture! In other words, I had found an alternative for Dual Deployment® which, provided the rocketeer had a computer at home, didn’t require expensive electronics or demanding mathematical calculations, rendering it attainable to many more rocketeers. This was the birth of the TailWind® delayed-deployment system.
Soon after my vacation was over, I started working on the first prototype for my version of the “retro-rocket”. It was a single-stage, large diameter rocket with ducted ejection gases for rear ejection. That design never took to the skies. To my knowledge, the available software was not able to model the unusual shapes and flight paths that I was conceiving. I had realized that the controlling software was even more essential than the rocket itself. My top priority then became to develop the software that would take the place of the deployment electronics. The software was essentially a simulation program, optimized for the BACKDRAFT® model. It obviously had to take into consideration gravity, thrust and drag, but it also had to give due weight to the unusual changes in physical characteristics that would occur during flight. This work eventually became the H.H. Simit® simulation software.
H.H. Simit® (v1) is included in all new BACKDRAFT® kits. It provides simulated altitude, velocity and acceleration information for the model’s flight. It also allows the flyer to easily choose primary and secondary engines (and their respective delays) using the provided data. These choices are critical for the safe flying of the model. Future rocket or engine modifications will not be a problem as the information database is user-editable. You don’t need to buy a new version of H.H. Simit® to fly a new version of BACKDRAFT® or to accommodate the thrust characteristics of a new engine! As development continues, H.H. Simit® will be made available as a separate product.
Subsequent prototypes abandoned the large-diameter airframe for a more energy-efficient BT-60 based design. The final design offers a booster-style detachable propulsion module, straight-through ejection gas ducting and EZject® piston-actuated system for rear ejection. With the EZject® system, flameproof wadding and / or parachute protectors are completely unnecessary, making field preparation a breeze! EZject® is different from other piston-based ejection systems in that its simplicity and low manufacturing cost makes it available to all rocket designs, even the most simple and basic (see the Heavenly Hobbies’ Stratos-13 ELM® and Stratos-18 ELM® models).
The essential physical difference between the BACKDRAFT® and a common 2-stage rocket is that the second-stage engine has exchanged places with the recovery system. The secondary engine been moved forward, behind the nosecone, while the parachute has moved back. This secondary engine has also been rotated 180°, so that the nozzle points to the sky while the rocket sits at the launch pad. As I said before, when lit, this engine will provide thrust to counteract the force of gravity and slow down the rocket sufficiently, so that the parachute can open without tearing to shreds. Since the parachute can open as close to the ground as desired, drift is reduced considerably. The reduction in drift allows the hobbyist to fly higher and/or in windier conditions. This is the magic of the TailWind® delayed deployment system.
Since the secondary engine is far removed from the primary engine, the primary engine cannot be used to directly ignite the secondary one. This problem would normally be remedied with an electronic timer or other similar artifact. However, it was a design objective for the BACKDRAFT® to “kick it old school” and avoid any electronic control artifacts inside the rocket. As a result, a good old visco fuse (“cannon fuse”) is used to delay the firing of the secondary engine. This fuse is lighted at the same time as the igniter for the secondary engine, so a good 12-volt launch system is needed. You also need to time your fuse accurately, so a stopwatch will come in handy also.
It is critical to follow the instructions in building and flying this model. I have gone through several prototypes in perfecting a safe and reliable flying procedure, so you don’t have to! Use H.H. Simit® to your advantage, and remember to allow yourself a margin of error in your calculations.
Jose M. Andrade-Cora
Heavenly Hobbies LLC
Dual Deployment® is a trademark of Adept Instruments, Inc.