Head-End Ignition: A Successful Method
This is a basic description of the method I used for head-end ignition (HEI) on my recent two-stage flight. My method proved successful, I think, but please don’t blame me if you try this and get different results. I present it here purely for your enjoyment and edification.
So, the problem exists with a two-stage flight of how to successfully get the power to the igniter that is in the head of the sustainer motor, for ignition during flight. Given that the driver for this ignition is forward of the igniter, running the wires on the outside of the airframe and around the rear of the rocket is impractical, since some movement in the interstage coupler can short or sever the wire. The obvious answer is HEI, but we must be able to do it reliably, safely and not impair the reliability of the casing which is keeping the not insubstantial pressures and temperatures neatly contained during the motor burn.
After a lot of reading and speaking to a few people I came up with this solution. Thanks to anyone I have discussed this with. Your help was appreciated! I really wanted something that could be used more than once, but truth-be-told it would not bother me if I had to clean it all out and start again. It was not a particularly onerous task.
Firstly, a standard 75mm CTI forward closure part was mounted in the mill, centered, and a hole drilled and tapped to suit the RP-SMA pigtail harness I had purchased off eBay. Then the RP-SMA bulkhead connector was carefully threaded from the inside of the housing into the hole and nipped into place. A small amount of West System epoxy sealed around the base of the connector to the closure. Note that I also had to check that the pigtail would be capable of taking the igniter current and not acting like a fuse
Next up, a couple of disks were cut from some 1.6mm G10 sheet using my lathe. Only one of these ended up being used. Three holes were drilled: two for the 2mm banana receptacles and one to let any excess RTV move up from below the disk. These disks are a close clearance fit to the bore of the closure. The receptacles were press-fitted into the G10 and the pigtail coax soldered to the bottom of the them.
Remember when soldering gold, it is usually good practice to remove the gold from where you are soldering unless you know it is only a thin coating. “Why?” you ask. Gold is actually soluble in solder. If there is a significant layer of gold on what you are soldering it will slowly but surely DIFFUSE INTO THE SOLDER, leaving a gap between the solder and the item you had soldered on to. True story. Don’t leave it there because it is easy to solder on to .
Also not visible is a 100nF capacitor soldered across the two receptacles. This was to provide some ESD protection for the assembly. I don’t want any surprise ignitions, thanks very much.
Next up the assembly was held vertical in a vice (softjaws please), checked for level, and then some addition-cured RTV mixed up and poured in. For accuracy measure the RTV components by mass. The addition-cured RTV means it will set the whole way through without having to have moisture from the air (condensation-cured types) diffuse into the assembly. This would be impractical in this situation. Anyhow, it was filled to a depth and then the disk pushed onto the top of the pour. The disk had 2mm plugs in the receptacles, along with some CG153 contact grease. The plugs give a decent handle to help with manoeuvring. I also put some short pieces of tubing over the rear of the plugs to keep the RTV off them. Once the first pour had gone “green” (slightly off) I added the short lengths of tubing around the rear of the plugs and poured a second batch to the requisite level. I wanted the final disk to be just proud of the closure when it was added to the stack.
While the mix was curing, I machined a hollow tube which is a plug cutter. This was pushed over the tubing and cut through the cured RTV nicely. So now I have a nice RTV sandwich with perfect access to the 2mm receptacles.
The igniter for the motor was glued onto the fuel grain from an E28 Blue Thunder motor. The BT propellant is the easiest to light. The grain is easy to cut with a scalpel. The glue used was some HTPB that was left over from an M motor. As well as the standard igniter, a regular eMatch was wired in parallel and added to the motor slug. This is to provide some redundancy in the ignition. I suspect it was overkill .
A shorting plug was made to screw onto the RP-SMA connector and fitted to the assembly. That kept the igniter shorted out for the entire duration of the preparation, up to the point out at the pad where I removed the shorting plug and connected to the sustainer electronics. The ends of the igniters were soldered to some 2mm plugs after being passed through a disk of G10. The plugs were inserted into the receptacles, a little more RTV was mixed to seal around the plugs, and the disk was settled in place on top.
During assembly of the M2020 this augmented igniter was glued in place in the core of the top grain, and the end closure fitted.
This motor was treated very carefully after being assembled .
For details, including video of the flight, have a look here: viewtopic.php?f=6&t=5019&start=105
From the data I have the ignition was almost instantaneous. The motor burned normally after ignition and the rocket coasted nicely to apogee. Since the sustainer is currently MIA I don’t know how the HEI assembly stood up to the flight environment. I suspect it is 100% fine. Time will tell.
Please remember if you are doing anything like this to really think through the safety implications the whole way. Risk does go up because the igniter is onboard from the early stages. I spent months thinking this through and the final solution was something I was relatively happy with as far as risk. It still scared me, but I knew I had done many things to mitigate those risks during preparation and flight.
I should probably also update this with what safety systems I had in the electronics to prevent untimely ignition. I will put something together on this in the near future.
Share and Enjoy!
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