Throttle Your Solid Rocket Motors With This One Simple Trick!

For decades, mankind was content to launch payloads into orbit and then watch hundreds of thousands of hours of blood sweat and tears just crash into the ocean. Then, partially because of huge advancements in being able to throttle rocket engines, we started landing our first stage boosters. [Joe] over at the YouTube channel is tired of watching SpaceX have all the booster landing fun, but he’s not quite at the throttled liquid engine stage yet. So in the video below the break he asked the question: Can you throttle solid rocket motors? Yes. No. Sort of.

Throttling liquid rocket engines is actually not that different from throttling any other engine- by limiting the amount of fuel and oxidizer. This is challenging all on its own because well… it’s rocket science. With liquid rocket engines though, the concept is at least straightforward. But model rocketry hobbyists only use liquid fueled engines on the extreme high end. The vast majority instead use solid fueled rockets where the fuel is pre-mixed and isn’t variable at all.

These obvious hurdles didn’t stop [Joe] from trying. And trying again. Then, again. And once more for good measure. And then again for repeatability. There are definitely some failures along the way, and we applaud [Joe] for even admitting that he didn’t know how to use a drill properly. Hackers of any age can relate to the time when the didn’t know how to do something, although we also tend to not talk about that part too much.

We won’t spoil the ending except to say that the video is definitely worth a watch to see how [Joe] essentially solves the problem of limiting the effective thrust of a solid rocket engine without actually throttling the engine, and learns about a new issue he’d never seen before.

Of course you can also make rocket engines at home out of a plethora of ingredients, just be sure to do it in somebody else’s kitchen!

14 thoughts on “Throttle Your Solid Rocket Motors With This One Simple Trick!

  1. Not to be that critical guy, but I kinda read this site because I enjoy interisting technical stuff, but this article is completely deviod of that. I’m not completely opposed to posting youtube videos on here, but like, at least talk about the hack, rather than just effectively say “here’s a cool video, but I won’t tell you what’s cool about it”.

    Like from the thumbnail it looks like a thrust deflection system? Idk exactly, but that’s more than the article tells me.

    1. Hi Nathan. I can see where you’re coming from. When I wrote this, I decided to leave it out because 1) it’s in the picture and 2) it really *is* worth watching to find out how it went. Even if you just jump to the end.

      Another aspect of it is that for me, Hackaday is a celebration of the hacker as much as the hack. You’ll notice that I highlighted the process the hacker himself went through instead of the technical hack. Sometimes it swings the other way. But, I appreciate your input and will keep it in mind in my future articles.

      Thanks for taking the time to articulate your thoughts without going into insult-and-destroy mode :-)

  2. Explain-like-I’m-five time. Question for the rocket scientists: is it possible to throttle a solid-fuel rocket by selectively introducing a retardant or suppressive agent into the process?

    Very wasteful of course, when every gram of weight really needs to be put towards thrust, but if you needed to reduce thrust for a short period, could you inject something non-combustible to achieve that?

    I assume that fuel/oxidiser ratios are very carefully calculated to just burn, baby, burn, but lightly interfering with combustion for a few seconds might achieve it.

    1. no. You have no practical way of introducing it on demand, as you’d have to fight the combustion pressure. Also adding retardant (and its delivery mechanism) is adding a lot of dead weight.

    2. Yes, you could introduce something to reduce the burn rate, but it would almost certainly be either a liquid or a gas. Reproducibly introducing a solid into the combustion chamber would be challenging to say the least. And as others have pointed out, the mechanism for introducing anything into the combustion chamber would be heavy; the game is unlikely to be worth the candle, to use an old phrase.

      FWIW solid propellant (“fuel” is what burns, “oxidizer” supports that combustion, and both are referred to as propellants) can have a very wide range of composition. I have made solid propellant that was 40% oxidizer (ammonium perchlorate is the oxidizer of choice) and two days ago I made some that was 82% oxidizer. Both work fine, though the latter is significantly more powerful—higher specific impulse—than the former.

      When I first started making propellant, as a chemistry prof I found it difficult to accept the fact that an exact composition—specific percentages of oxidizer and fuel—wasn’t necessary for a successful solid propellant. There is a more-or-less optimum composition, but other percentages will work as well.

    3. The key thing here is that he needs to adaptively control the thrust in a closed feedback loop according to how the rocket is moving, with very quick response time. He’s trying to land the rocket SpaceX-style.

  3. I told this guy long ago when he first began his attempts to land propulsively like the SpaceX Falcon that unpredictable variations in the commercial motors he was using would make that effort nigh impossible. He found that to be the case.

    I also mentioned the Krushnic effect as a possible throttling method which he apparently experimented with as described in this video although for reasons I can’t determine, since he didn’t provide sufficient details, his results didn’t match the much better results (much larger throttling range) obtained by a study performed and documented in a tech report by a National Association of Rocketry member.

  4. He didn’t really dig into the combustion surface cast into the propellant grain and how that can be tailored to create a custom thrust profile. Granted, that thrust profile is not adjustable in flight, but it is a form of variable thrust.

    Some SRBs create differential thrust by fluid injection into the exhaust flow. SRBs also use cosign losses for precise guidance when used as upper stages.

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