Green Onion - my first high power rocket

[Pipcard]

(Built with the assistance of the UAH Space Hardware Club)

 

[Green Baron]

Most talk, some do :-)

[Zeiss Ikon]

Don't have any video (smart phones weren't around/affordable yet back then) but I certified High Power with NAR in 1998 (then quit flying rockets, mostly due to expense, around 2002).  I see your rocket is a veteran -- bump marks in the white paint, reattachment of fins, etc.  Good to see multiple flights; so many folks build HPR and don't think it's a bad thing when the rocket disassembles sometime during the flight -- and don't even bother to pick up the broken pieces from the field.

[Pipcard]

8 hours ago, Zeiss Ikon said:

Don't have any video (smart phones weren't around/affordable yet back then) but I certified High Power with NAR in 1998 (then quit flying rockets, mostly due to expense, around 2002).  I see your rocket is a veteran -- bump marks in the white paint, reattachment of fins, etc.  Good to see multiple flights; so many folks build HPR and don't think it's a bad thing when the rocket disassembles sometime during the flight -- and don't even bother to pick up the broken pieces from the field.

No, that's its first flight. Really, I spray painted the nose cone, and the paint kept chipping off when I tried to carry it around in my backpack. And I don't have much experience with epoxy so the epoxy fillets on the fins were done sloppily (I had to sandpaper the paint off and reapply the epoxy because there wasn't enough when the other club members came to check it).

Edited March 11, 2018 by Pipcard

[Zeiss Ikon]

Aha!  My first High Power model came a couple years after returning to the hobby (I flew Estes rockets for a couple years in 7th-8th grades, early 1970s).  I was pretty skilled at building by the time I attempted/spent the money for HPR, and I also built the rocket in part to prove a point I'd made at a launch a while before -- that with correct construction, you didn't need the weight of epoxy on rockets in the H-I-J power range.  I selected a kit with plywood fins that went through the body tube to the motor mount tube, sandwiched them between the centering rings (i.e. wood to wood geometric construction), then applied the same techniques for fillets and paint that I'd been using on smaller rockets: double glue joints with yellow carpenter's glue, aerodynamic fillets built up with lightweight wood filler (non-structural), and multiple layers of wet-sanded primer before applying color.

I also had a car to transport the rocket... 

[Slam_Jones]

Wow, that thing screamed off the launch pad!  What was the initial TWR, if you happen to know? 

Edited March 12, 2018 by Slam_Jones

[Zeiss Ikon]

The announcer called out an H219 motor -- that's 219 N average thrust (and Aerotech motors are commonly slot grains, near constant thrust profile), on a rocket that probably massed a bit less than 1 kg on the pad (including the full motor).  Call it 20+ G liftoff acceleration, increasing a bit as the propellant burned off.  That's on the high end of H motor thrust levels, as I recall -- my cert flight was an H128 reloadable, and part of my certification was the supervised assembly of the motor (nozzle, propellant grains, delay grain, both closures, and ejection charge) the day before the launch.

[wumpus]

18 hours ago, Slam_Jones said:

Wow, that thing screamed off the launch pad!  What was the initial TWR, if you happen to know? 

I've seen documentation from high-powered model rocket manufacturers state that you need TWR>6 for stability with their motors.  Even Orbital solid boosters (that resupply ISS) have some pretty high TWR.  According to the above, this was higher than that.

[Zeiss Ikon]

6 hours ago, wumpus said:

I've seen documentation from high-powered model rocket manufacturers state that you need TWR>6 for stability with their motors.  Even Orbital solid boosters (that resupply ISS) have some pretty high TWR.  According to the above, this was higher than that.

That seems reasonable.  The model needs to be moving fast enough for the fins to produce useful levels of stabilizing lift (likely 10 m/s or faster) before the model leaves the launch guide (rod, rail, or tower).  Since the guide is usually much less than 5 m long (the distance traveled to accelerate to 10 m/s at 1 G), the rocket needs to accelerate significantly faster than 1 G to reach the needed speed in the available distance.  Once off the launch guide, the gravity turn is less pronounced the faster the rocket accelerates, which both reduces the walk to retrieve the rocket after landing, and reduces the likelihood of damage due to high speed at the time of recovery system deployment.

FWIW, I've flow a rocket weighing every gram of a pound (the NAR limit for non-HPR models) on a D12-3 (did it for a movie camera, the launch appeared in a PBS special on rocketry back in 1998).  That seems like a roughly 3 G launch, but the Estes black powder D12 is a semi-cored grain, with a pronounced thrust spike just after ignition, then around a 1 second sustain at slightly less than half the average thrust.  It gets the rocket going fast enough to be stable on a three foot (1 m) rod, but there's no margin -- if your guides have excess friction, or your ignition leads don't separate cleanly, you can wind up with a tip-off, where the rocket veers at an angle as soon as it loses the launch guidance -- still with 4/5 of the engine burn to run.

Yes, even when the rocket weighs a few tens of grams, this stuff is rocket science.