Mid-summer rocketry update for those of you with nothing better to do on a lazy Sunday afternoon!
The blog has been quiet, but LPRD Rocketry has not been asleep! We’ve been busy bees, with more than ten of our team still working through the summer pushing our way towards flying a liquid propellant rocket engine. We have a couple of events coming up, starting with a firing on our own next weekend! For the first time ever, we will be attempting to light the engine without the guiding hand of OrbitalATK. Fingers crossed! (Though of course we’re doing much more than just crossing our fingers in preparation and practice)
A bit of a step down from the concrete bunker, but we’ll take it
As for things we’ve already accomplished this summer, as you know, one of the biggest and most experimental designs we’re considering implementing is what we call an “asymmetric thrust rocket” with one motor pointed down the axis and a second motor higher up the body pointed through the center of gravity at an angle.
Well, it’s real, and it’s flown. Ish. Note that the high final angle was due to heavy winds combined with our off-axis thrust and a misapplication of center of gravity.
Unfortunately, we had some issues in discipline getting things 100% done ahead of time, and this led to a last minute rush which caused a mistake leading in the partial loss of our rocket due to an improperly wired parachute ejection charge.
In addition, only one of the motors lit, so we had an unexpected flight, but we still managed to prove the concept. I’ll let you read the notes from the guys at the launch about what caused the failure. We will learn from these mistakes and move on to become better, safer, and more efficient.
The purpose of this launch was to test out an asymmetrically aligned motor as opposed to the traditional vertical motor. For this, we used an F70. As for the normal vertical motor, we used a G33. The objective was to have both of the motors ignite simultaneously.
We arrived on scene with the intention of preparing for launch on site. Chief problem was the avionics bay. The ejection charge was not prepared and the avbay was suffering problems. Due to a foreplanning problem and misunderstanding, launch lugs had to be epoxied on day of. The avionics bay was impossible to secure to the rocket and apply an air hole to, due to the difficulty of inserting the avbay, aligning the avbay to the rocket, and interacting with the avbay in the rocket. There was no room in the avbay to attach rivets to or drill holes in.
There was no good way to turn on the avbay on the rod. There was a switch, but due to lack of internal support in the avbay, the switch could only be turned on before inserting the avbay into the rocket. We replaced the switch then with a long pair of wires, that we would cross & tie outside the wires together to provide power to the rocket.
Weight should have been added to the rocket to center CG at the asymmetric motor, but it was a bit low. The fins needed to be cut to raise the CP to not be over stable. The net effect of not cutting the fins & not adding weight kept the rocket in a good range of stability, ~2. It was very windy. Very very windy. The bourbon sloppy joe was very good. On the stand, the altimeter repeatedly beeped 4 or 5 times.
Only one motor(asymmetric) ignited limiting the rocket’s maximum projected height to a couple of hundred feet. It was over stable and headed directly into the wind. After faltering initially, the rocket aligned itself with an angle of about 30 degrees to the vertical. The estimated burn time is about 1.5 seconds after which the rocket drifted off in a parabolic projectile. The parachute did not eject.
Cause of Failure:
There were two failures: failure to ignite lower motor on stand and failure to eject parachute.
While application, insertion, and connection of the igniters to the battery wires and to the motors are ripe for user error, the igniter failed to ignite on the motor. The igniter was securely connected to the battery by Dan, but it did not light.
The 4 or 5 beeps the altimeter was giving was actually code for ejection charge discontinuity. Due to ignorance of the meaning of those beeps and the necessity of firing due to a highly delayed schedule, we launched anyway thinking all was well.
Other Concerns/things to be addressed:
There was only one person working on the rocket and the rocket was initially half done and so had very little room to be worked around. Because there was only one person working on it, we ended up not considering a few items (launch lugs for instance) which resulted in delays at and before the launch. The design of the rocket, particularly the small 1.5” diameter, was not appropriate for the application, and that original design characteristic consumed many many many hours of time.
One more slight obstacle to get over, but we’re very very happy with the performance of the design. Even with the poor conditions, misconstructed rocket (due to lack of time), and lighting of only one motor, the rocket ended up with less than a 45 degree flight path. With corrections, we can surely reduce this and hope to drop it under 15 degrees with our next attempt. We’ll see how it goes. Hope to see you there with us!
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