Throttle Control with Solid Motors? (Patreon)

Yes! Well... sorta...

For the last few weeks (and a few weeks in the fall too) I've been trying to get throttle control with these Estes F15 rocket motors that Scout(the landing rocket) uses. That's the last step toward closing in on landing this thing after a billion years of trying.

First I tried messing with something called the Krushnic effect: https://www.nar.org/pdf/TCR1.pdf

This is where you push a sleeve past the exit plane of the nozzle, and as you start to mess with the airflow surrounding the exhaust, you reduce the effective thrust of the motor. Equations surrounding how this works don't really exist, and measured data for it is sparse. It's a chaotic and turbulent process so the relationship between sleeve diameter, length, thrust reduced, etc is going to vary for each different class of motor. In the fall of 2021 I conducted several static fire tests on the stand at fixed sleeve lengths and compared the results (shown in a plot).

In simulations on the full Scout flight profile, I found that to effectively deal with the variation between different F15 motors and and ignition come-up times, I needed at least 20% total throttle range. That would put our nominal throttle at 90% which would give us up or down 10% in range. One of the things I'm trying to do more often is actually *listen* to the data and let that drive design decisions. This is relevant because during testing I was only able to achieve roughly 10% total throttle range.

Could an actuated sleeve increase landing reliability? Sure! Does it close all the edge cases? Nope :( On to the next attempt at throttling!

This one is fairly nutty, but it looked good in simulations, so hear me out. In the absence of tight tolerances in your build, what if you could just turn the rocket motor on and off really fast? This is how kinetic kill vehicles for ICBM interception work: https://youtu.be/KBMU6l6GsdM

So the idea was that we'd quickly actuate a paddle or deflector in front of the nozzle to block all axial exhaust flow and divert out in equal directions to the the side. In theory, this was a cool idea and again, it did work in sims! In practice, I don't have a CNC machine that can cut anything harder than aluminum, and in hot fire tests the rocket motor chewed right through that aluminum blocker. I also had trouble getting tolerances with the 3d printed part tight enough that we didn't divert a majority of the exhaust in one direction, which literally changes the vector of the thrust. Basically, we'd introduce a bunch of side force when the blocker is engaged, which is no good for this application.

Finally, I came across this post on Reddit: https://www.reddit.com/r/rocketry/comments/kvhxfd/solid_rocket_motor_throttle_opinions_if_you_want/

I had actually commented on this a while back, but didn't think much of it at the time. This person used ceramic blockers to variably impinge the exhaust, which gives some degree of control on *how much* throttle you want. So I checked with them to see if they were alright with me trying a version of their design(they were!) then I got to work on my own spin on it.

I used ultra high temp easy-to-machine ceramic from McMaster for the blockers, a servo on either side, and spun up a little variable throttle program to run during firing and IT WORKS. We need a few more firings before flight, but here's what I've found. Exhaust diverts pretty evenly out to the side, so undesired side-forces seem minimal, though it goes everywhere so this setup needs to stick out the aft of the rocket a bit. 

Axial thrust seems to correlate pretty well with throttle commands, though we still need more firings to dial in the relationship between the two. Erosion on the ceramic is much lower than I thought! We fired with a hairline fracture that I accidentally created during assembly, and it spread during firing, but other than that I'm surprised how well the material held up. Without that fracture I'd be fine firing it a few more times with these ceramic blockers.

One thing to note is that exhaust from black powder motors like this won't be as hot as exhaust from APCP motors. I really want to make a little Aerotech G8 long burn hopper this year, and I'm skeptical of how well the ceramic will deal with an 18 second burn, but that's a problem for future me.

I figured I'd put all this info in a Patreon post along with some photos since I need a few weeks to get a video about it together. I'm excited to be experimenting with this stuff and it gives me more confidence that maybe we can stick the landing this year? Every time I say that it doesn't happen but maybe *this* time it will???

Blue skies, and happy 2022!

Joe