Ascent Motor Ejection Testing (Patreon)

Hey Patrons! More high speed footage for ya :)

In order to fire an ascent and landing motor for a full launch/landing, we either need to drop a stage, or somehow get them in the same TVC mount. I went with the second option.

Now you can't just leave the second motor recessed into the sleeve/tube/airframe, and it's a lesson I've learned a few times now. Not just because of heat though, also because of the Krushnic effect: https://www.nar.org/pdf/TCR1.pdf Basically, you lose motor performance when the exit plane is within a larger tube(this could actually be used for pretty effective throttling if someone wants a fun project!). 

So! I've got the motors in a stack, and when the ascent motor has burned out, we can discard it. It's lightweight enough, with a low enough apogee that I feel safe letting it come in ballistic without a chute. As for the mechanism itself, I'm extremely proud of it. I've been working the design for 2-3 months now and it's in a place I like. You can figure it out from the video, but here's a little TLDR.

The ascent motor fires against a locking pin, which is how the thrust force travels into the vehicle. There's positive retention on the ascent motor from a lip near the nozzle. I need to characterize how this changes or is affected by the heat of a motor burn, but one thing at a time. That's an easy fix if it doesn't hold up. When it's time to eject, we gimbal hard past our normal limited range on one axis. This slams the ascent motor into an ejection pin within the vehicle, which pushes the ascent motor past the lip. 

The spring loaded landing motor takes its place, forcing the locking pin out for a second, sliding down to the lip, then locking out when the pin snaps back. The landing motor cannot slide out of the mount(under normal circumstances). It has a cardboard sleeve around it which is JUST a bit too large to allow for that angular deflection to move past the lip. This was one of the main things to dial in, really tight tolerance there.

The ascent motor can flip out when it's hit with the ejection pin, and I hear you asking... "hey joe, how do you know the ascent motor won't just flip out on it's own? What's to stop that from happening during a burn?" Wow. What an excellent question thank you for asking. The locking pin is on the opposite side from the lip, which means that when we fire the ascent motor, it's actively pushing itself up against the half-sleeve! The pin acts as a fulcrum when a motor fires, so we'd need a really anomalous side force for the motor to eject itself during firing.

Okay and finally, as you can see, we don't quite get the motor out every time, which is not acceptable performance for this type of system. Just like chute ejection, reliability needs to be 💯💯💯💯💯. So more testing is coming soon. When I'm at 100% reliability, I'll do a hot fire test. We'll strap everything down(probably to the thrust measuring test stand), fire the ascent motor, eject it, and fire the landing motor. This'll let us know if the mount can tolerate the heat it will see during flight. Pending the success of all of that, we'll be ready to do an all up launch/landing flight.

Wait and also finally finally finally, I keep forgetting to mention the ignitor. Haven't settled on how to route that into the landing motor, but I have confidence I can either cut out a slot for it to run up the cardboard sleeve while maintaining diameter, or have it just slip past the ascent motor. Either way, I've had success with some carefully placed epoxy to hold stuff like that in place, even during high shock/vibe events like we'll go through here.

I've got lots more testing to do(fins, software, simulation, state estimation, etc), so stay tuned! That said, everything is progressing right along, I'd expect a first landing test in September for sure.

Blue skies,

Joe