low power modular rocket concept - built & flown

[g3d]

I wanted to start seeing how effective my build strategies were when implementing the 24 mm diameter modular rocket concept I specified here.

I used builders supplies obtained from Apogee to construct the modular rocket concepts - stage 1 and stage 2 - where I ended up with a booster, a sustainer with airfoil fins, a payload area to hold & protect the Jolly Logic Altimeter II, and finally a parachute area with a weighted nose cone, as shown here:

Note that all components where glued with quick-setting lightweight epoxy.

(Note the post-flight masking-tape from field repairs)

Here is a shot of the airfoil fins, which I built by using standard Avion Balsa Fins from Apogee, cutting 2 pieces of new 1/2 scale fin section from 1/16 balsa, and gluing them with the grain going the opposite direction from the Avion fin. This allowed me to use the angle between the leading edge of the fin and the glued pieces as a sanding guide to rough out the shape using 100 weight sandpaper. Final shaping and balancing was done using 220 weight sandpaper, combined with several passes with quick-setting modeler's epoxy putty.

Both the airfoil and booster fin cans were first glued to the 18 mm motor mount on top of centering rings using cutouts to maximize connectivity surface area. The rings were then taped off, and the rocket was painted with 2000 degree heat-resistant Rustoleum paint (I think this stuff is typically used for BBQs, it created a fairly gritty finish). Which when dry, I lightly sanded, and inserted into the sustainer and booster airframes, glued, and filleted with lightweight epoxy.

For kicks, I used masking tape to reinforce the booster fin-can fins by cutting small strips, which I soaked in epoxy, and place over the root of the fin against the motor mount, in an experiment to mimic composite and glass fabric reinforcement used on higher-powered builds. (This is not shown in this picture - the tape you see here is from field repairs).

Finally, I used a couple of centering rings, a 24 mm coupler, and an engine stop, to construct a payload module with foam inserts for the Jolly Logic Altimeter II.

Finally, today I conducted a number of flight tests, as follows:

Flight 01, single stage, B6-6 motor. Results: stable flight, good parachute, safe recovery, no damage. It did come down a little fast, but I did cut a large hole in the center of the chute to ensure recovery.

Flight 02, dual-stage, B6-6 in the sustainer, B6-0 in the booster. Results: stable flight, good parachute, safe recovery, no damage from landing. Found damage to the base of the sustainer airframe from blow-back gasses from the stage-separation (note masking tape at the base of the fins on the sustainer). Due to increase in observed altitude, I was thankful for the rapid descent on this launch. Booster popped and landed within 6' of the launch pad.

Flight 03, single stage, C6-7 motor. Results: stable flight, good parachute, safe recovery, no damage.

Flight 04, dual-stage, C6-7 in the sustainer, C6-0 in the booster. Results: stable flight, good parachute, safe recovery, no damage.

At this time, I began using the altimeter.

Flight 05, single stage, C6-7 motor. Results, stable flight, parachute failed to open (foam rubber surrounding the altimeter blocked pressurized airflow to the chute compartment), tumbled down, no visible damage other than nicks & scratches.

Altimeter reading: 525 ft apogee, 107 mph

Flight 06, reset parachute for easier opening, C6-7 motor in sustainer, B6-0 motor in the booster. Results, again, stable flight, parachute failed to open, tumbled down, some slight visible damage on the sustainer airframe (slight buckling).

Altimeter reading: 792 ft apogee, 150 mph

Flight 07, removed chute, and used smaller, lighter Estes 12" plastic chute, placed in the sustainer, C6-7 motor in the sustainer, C6-0 motor in the booster. Results: stable flight till booster motor burnout and sustainer motor ignition, rocket visibly buckled, and switched to a mostly parallel flight pattern. Good chute ejection, and wind blew the rocket safely back onto the field.

Here is a shot of the buckled airframe:

Altimeter reading: 603 ft apogee, 223 mph

Lessons learned:

1) Making airfoil fins by hand is tedious, painful, and time-consuming.
2) Measuring where to cut in the airframe for fins glued to the motor-mount is critical for a rocket with clean aerodynamic properties.
3) Design of how to construct the end of the booster where it meets the sustainer motor-mount needs to be improved.
4) Don't fly a rocket with visible buckling evident on the airframe, especially after repeated parachute deployment failures.
5) Even though the fins were thick, and relatively "strong" from the multiple layers of balsa, glue, modeling putty, and paint - a new construction methodology - such as reenforcing with paper - seems key, due to the inherent soft nature of the balsa.

Build notes:

I definitely would like to obtain some better shock cord material (e.g. small-diameter Kevlar) to replace the quick & dirty solution I used for this build - 1/8" cotton-poly woven string.

Placing the shock-cord mount deeper into the sustainer (or attaching it to the motor mount) would save a lot of room, allowing for optimal parachute location upon separation.

Placing too much weight in the nose (3 pennies) for optimizing the center-of-gravity has a dramatic affect on altitude performance.

 

[Montaro]

I liked the way you explained your project, i have 2 question

1.- what was the 2 stage final weight ?
2.- what was the parachute size ?

 

[msjohnso]

As to the lesson learned about the nose weight - if the model is borderline stable, *not* adding some nose weight can cause what Harry Stine used to refer to as "an amazing stunt pattern in the dirt."

MJ

 

[g3d]

I liked the way you explained your project, i have 2 question

1.- what was the 2 stage final weight ?
2.- what was the parachute size ?

1) Currently, I'm not sure, and as I have mostly dissembled the rocket at this point, there is no way to accurately get this information. The Open Rocket design anticipates it weighing in at 48.6 grams without engines, however I ended up building a) additional extensions above the sustainer, and b) heavier motor mounts than anticipated in the design.

2) I used a 12" hexagonal yellow nylon chute from Apogee for flights 1-6, and a plastic 12" Estes chute for flight 7.

https://www.apogeerockets.com/parachutes.asp#Low-Power_chutes

As to the lesson learned about the nose weight - if the model is borderline stable, *not* adding some nose weight can cause what Harry Stine used to refer to as "an amazing stunt pattern in the dirt."

I ended up adding extra weight to the nose due to the heavier-than-anticipated motor mount and airfoil fin construction given the anticipated 0.52 caliber stability with the two-stage configuration.

 

[g3d]

I wanted to get a few more flights out of this system before retiring this design, so I currently intend to -

1) Rebuild the sustainer
2) Redesign the booster

...here is a shot of the motor-mount with the airfoil fins attached:

...here is one showing the curvature of the airfoil:

...and one showing the original design & fit of the booster to the sustainer motor mount:

...via a modified 24 mm coupler.

 

[g3d]

So far, I have cleaned up the booster stage, shown here, and changed the staging coupling design so that the motor-mount for the booster fits into a centering ring extending below the sustainer, which will be covered by the booster stage airframe.

I also cleaned up the airfoil-finned sustainer motor-mount and changed it to conform with the new coupler design.

Here's what they look like when stacked:

Next steps will be to measure, cut, slot, fit and glue the new airframes, then put it together and see how it flies.