3 ms·

Fun fact: the shape of the propellant determines the thrust curve as it burns, as thrust is proportional to the surface area burning. And as material burns away

Fun fact: the shape of the propellant determines the thrust curve as it burns, as thrust is proportional to the surface area burning. And as material burns away, the surface area changes according to the pre-burn geometry. StackExchange pictures and discussion: https://space.stackexchange.com/questions/4153/could-3d-printing-be-used-to-achieve-perfect-grain-geometry-of-solid-and-hybrid https://space.stackexchange.com/questions/4153/could-3d-prin...

EvanAnderson 6y ago
I love recalling my feelings from the day somebody told me that thrust profiles related to surface area of the burn in solid fuel rocket motors. (That's kind of an odd sentence...) It's a silly thing little, perhaps, but it helped shape my personality. When I got involved with high power model rocketry I'd been flying little Estes kits for years. I had rudimentary understanding of aerodynamic forces and testing for stability, and had hacked around with flight simulation code cribbed from G. Harry Stine's "Handbook of Model Rocketry". I was young and thought I "knew stuff". Somebody at a high power launch remarked offhandedly about the difference thrust profiles of an end-burner vs. a core-burner. After I digested it I began to reflect on the visceral sense of how little I really knew about rocketry, and the world in general, that I felt. It was a seemingly simple piece of trivia about a topic that's not particularly important in my day-to-day life. It was, however, one of those deeply humbling "unknown unknown" moments. I've grown to absolutely adore the feeling of learning about new, uncharted depths in my knowledge because it means there's something new to learn about. The lesson about false confidence and humility was also a really good one. I hope it was a lesson that changed me for the better.
- dTal 6y ago
  That's funny, my experience was almost exactly the reverse of yours. When I first got into model rocketry, I was inundated with technical material. NASA had a huge variety of short technical briefs on things like stability, drag coefficients, etc - I think they gave them out for free to anyone who asked - and I had a fair collection. Unfortunately the mathematics in in them was far too much for 8 year old me, and I wasn't really able to use them in earnest for anything. Rocket science - proper science - seemed unapproachably complex. But a few years later I was given a copy of G Harry Stine's book, and it was like the doors were thrown open. Everything in it was hugely practical! And perfectly readable to 11 year old me. One great big book that said "you can totally do all this, and here's how". I credit the BASIC code in the appendices with helping get me into programming. I spent a long time painstakingly translating one of them into TI-83+ BASIC. So the lesson that I learned was the opposite as yours - it's easier than it looks!
  - EvanAnderson 6y ago
    My revelation wasn't that it was difficult or easy-- it was that unknowns lurk, in vast numbers, in places you don't even know exist, let alone don't know to look. It sounds easy, with hindsight, to say "Of course the burn profile of the fuel grain would determine the the thrust curve". The idea of fuel grain burn profile just simply hadn't occurred to me. Sure-- I knew motors had total and specific impulse but, I grew up on Estes motors as "black boxes" and never thought about motor manufacture or that performance could be engineered. I never thought about the motor as anything more than "put an ignition source in, get energy out". I know I've had a lot more of these moments. This was just a memory evoked by the parent post. I'm having trouble coming up with another of these "deep holes that look shallow, or don't even look like a hole at all" moments immediately to relate a second anecdote. These shallow-looking deep holes clearly exist in large numbers. By their very nature we can't know how many there are. They're fractal, too. Inside each one are more similarly-appearing (or not appearing) holes to go down. That idea might trouble some people, but it's very exciting to me. The only troubling bit is knowing I have a finite period explore the space of knowledge and a limited capacity to understand. That doesn't discourage me, though.

EvanAnderson 6y ago

I love recalling my feelings from the day somebody told me that thrust profiles related to surface area of the burn in solid fuel rocket motors. (That's kind of an odd sentence...) It's a silly thing little, perhaps, but it helped shape my personality. When I got involved with high power model rocketry I'd been flying little Estes kits for years. I had rudimentary understanding of aerodynamic forces and testing for stability, and had hacked around with flight simulation code cribbed from G. Harry Stine's "Handbook of Model Rocketry". I was young and thought I "knew stuff". Somebody at a high power launch remarked offhandedly about the difference thrust profiles of an end-burner vs. a core-burner. After I digested it I began to reflect on the visceral sense of how little I really knew about rocketry, and the world in general, that I felt. It was a seemingly simple piece of trivia about a topic that's not particularly important in my day-to-day life. It was, however, one of those deeply humbling "unknown unknown" moments. I've grown to absolutely adore the feeling of learning about new, uncharted depths in my knowledge because it means there's something new to learn about. The lesson about false confidence and humility was also a really good one. I hope it was a lesson that changed me for the better.

dTal 6y ago
That's funny, my experience was almost exactly the reverse of yours. When I first got into model rocketry, I was inundated with technical material. NASA had a huge variety of short technical briefs on things like stability, drag coefficients, etc - I think they gave them out for free to anyone who asked - and I had a fair collection. Unfortunately the mathematics in in them was far too much for 8 year old me, and I wasn't really able to use them in earnest for anything. Rocket science - proper science - seemed unapproachably complex. But a few years later I was given a copy of G Harry Stine's book, and it was like the doors were thrown open. Everything in it was hugely practical! And perfectly readable to 11 year old me. One great big book that said "you can totally do all this, and here's how". I credit the BASIC code in the appendices with helping get me into programming. I spent a long time painstakingly translating one of them into TI-83+ BASIC. So the lesson that I learned was the opposite as yours - it's easier than it looks!
- EvanAnderson 6y ago
  My revelation wasn't that it was difficult or easy-- it was that unknowns lurk, in vast numbers, in places you don't even know exist, let alone don't know to look. It sounds easy, with hindsight, to say "Of course the burn profile of the fuel grain would determine the the thrust curve". The idea of fuel grain burn profile just simply hadn't occurred to me. Sure-- I knew motors had total and specific impulse but, I grew up on Estes motors as "black boxes" and never thought about motor manufacture or that performance could be engineered. I never thought about the motor as anything more than "put an ignition source in, get energy out". I know I've had a lot more of these moments. This was just a memory evoked by the parent post. I'm having trouble coming up with another of these "deep holes that look shallow, or don't even look like a hole at all" moments immediately to relate a second anecdote. These shallow-looking deep holes clearly exist in large numbers. By their very nature we can't know how many there are. They're fractal, too. Inside each one are more similarly-appearing (or not appearing) holes to go down. That idea might trouble some people, but it's very exciting to me. The only troubling bit is knowing I have a finite period explore the space of knowledge and a limited capacity to understand. That doesn't discourage me, though.