The goal of this design was to design an HEI system that would directly ignite a second stage propellant integrate a pressure transducer. However, after careful thought and consideration, the team decided to move forward with a different design. This was because it was difficult to ensure that the IHEI would actually light the entire motor. It was significantly more complex to cast the design. Also, strong risk factors to consider with this design include delimitation and erosive burning if machined incorrectly. For example, the choke would have to be severely tapered.
Head-End Ignition (HEI) Design Considerations
The purpose of the head-end ignition is to ignite the second stage (sustainer) motor. In the past, our rocket team has used COTS propellant inside the HEI in order to light the rest of the sustainer grain and the design did not allow for a pressure transducer to collect sustainer pressure data. But we wanted to know if there was a way to ignite the second stage propellant directly while integrating a pressure transducer so we came up with an initial design that does both:
This has a center hole for the pressure transducer to collect data, and uses threaded bolts with igniters attached to ignite the rest of the grain directly. This initial design used a retention cylinder and phenolic to line and hold the igniter bolts and pressure transducer in place, but we revised this design to look more like our forward closure.
Here is the first revision of our design. The purposes and basic components are the same, there is just more of a similarity to our forward closure for the booster motor for ease of integration.
The materials for this design are Aluminum 6061-T6 for the forward closure part of the integrated HEI, Canvas Phenolic for the liner, and our propellant Angry Goat. The igniter bolts are ⅛ NPT hex plugs that have a hole drilled through the center for the e-match to be fed through. With these NPT plugs, we hope to mitigate the chance of leaks, but we will also be using high temperature epoxy and teflon tape to further seal the bolts.
Integrated Head-End Ignition Testing:
The integrated head-end ignition is different from our past designs so we must test this to see if it is feasible to incorporate it into our final rocket. Here is an assembly with our design for testing:
To test if this will light in low pressure, we will be testing whether or not the propellant ignites in near-vacuum conditions. We will prime our HEI by inserting the igniter bolts and pressure transducer into their designated holes. We will then set up the IHEI into our test stand and connect the eye bolt on the end cap to the ground to secure it. After, we will connect a vacuum pump to a vacuum valve which is connected to the side of the test case. We will decrease the pressure slowly to check for leaks in the system until near-vacuum conditions are reached. Then, we will ignite the HEI and check pressure data and video to see if our test was successful. This success will be determined if our propellant lights burn for more than 0.5s and the pre-choke pressure remains above 250 psi for that amount of time.
Failure Analysis and Risk Mitigation:
In order to prevent potentially catastrophic events from happening with the ignition of the sustainer motor, the team made sure to complete a thorough risk analysis and came up with several risk mitigation techniques. The possible failure modes that we analyzed were the igniters failing, the IHEI not lighting, the choke point facing erosive burning, and leaks of the igniter bolts. The probability, severity, and mitigation techniques used are listed in greater detail in the risk assessment matrix below.
