Design

38mm COTS Blue Thunder grain installed in forward closure ~80Ns

Exhausts to chamber via canted nozzles

1 sec burn time at ~500 psi

Troubleshooting

Throughout the summer and fall of 2022, the HEI was uncooperative. We struggled with pressure loss, difficulty igniting the grain, and troubles with assembly. Listed below are some more specifics of the difficulties encountered. 

Pressure Loss:

Our first test in 2022 successfully lit the grain, but the phenolic on the front end of the HEI was ejected. This is because we did not put high temperature RTV in the holes that the electrodes screw through. While there were nuts tightened down both sides of each electrode, this did not provide the proper seal. We adjusted our process for constructing the front end of the HEI to accommodate for this. We first fill both of the electrode holes fully with RTV. Then, a light coating of RTV is applied to the threads on each of the electrodes before they are inserted into the holes. RTV is both a liquid gasket and an adhesive, meaning it works best if a layer is applied to both materials being bonded together. After inserting the electrodes into the phenolic, the nuts are added on and tightened. They can also have a light layer of RTV applied to their threads. After the nuts are tightened, a coating of RTV is added around all of the nuts and a short ways up the threads of the electrodes to provide an extra layer of seal. 

Our Static Fire in Summer 2022 went horribly wrong. The HEI again lost pressure, but this time it was on a full motor. There are a number of reasons it could have failed and we are still not sure exactly which it was, though we have taken steps to address them all.

1) the reason previously listed.

2) Machining error on the inner diameter of the aluminum casing. It may have been machined slightly out of tolerance for the O-rings (tolerance is +.002 in) because measuring inner diameters precisely is hard. We re-machined this part and had more people take measurements more frequently to make sure we had the right measurement.

3) Gaps between the phenolic nozzle and the aluminum case. In a full motor, once the HEI ignites the grain, the gas fills every available space. This includes the slight gap between the aluminum casing of the HEI and the HEI nozzle/casing. This puts hot gas right next to aluminum, a metal that does not respond well to the temperatures reached in our motors, and o rings, which do melt when directly exposed to the this heat. To fix this, we now apply RTV at the start of the taper that the HEI nozzle sits on. This prevents any gas from sitting next to the aluminum or directly next to the o rings. 

Failure to ignite:

There needs to be an e match or igniter across the electrodes. Yes, this does sound ridiculous using an igniter to light a small grain to light the big grain, but it works. We experimented with a couple types of e matches to light our HEI grain. Estes igniters were not enough. Many of our struggles with ignition were because the Estes igniter would light but the HEI wouldn't. Currently we use firework starters that have been beefed up with QuickDip Pyrogen or are in a disposable glove tip that has been filled with propellant shavings. It is also important to make sure that there is good contact between the igniters and the electrodes. We tightly wrap the igniter leads two or three times around each electrode. 

When using QuickDip, we've had successes with a 9V LiPo battery, though there is a delay that depends on how much QuickDip was applied. With the glovetips taped onto the firework starters, we've used a car battery and it lit instantaneously. 

Assembly:

This system was placed on a 3 in motor. The casing for the HEI (the unlabeled black thing outside the HEI grain) is 1/8th inch phenolic. It is easy to crack. In this case, the o ring grooves cannot be even slightly over the diameter listed in the Parker O ring guide. This will result in snapping the casing. It is also difficult to cut. We use a miter saw and found that the best method is to go slowly and have lots of extra stock because it cannot handle shear stresses well. 

Pictures

Tests