3 ms·

So the SRBs don't burn from the bottom up?

SAI_Peregrinus 6y ago
Nope, inside out. They're hollow, and the top section has a star cross section rather than cylindrical. The goal is to keep an overall constant pressure for the entire burn time.
- bfieidhbrjr 6y ago
  TIL. That's amazing, thanks.
  - wondringaloud 6y ago
    Which piece of it is amazing?
- matt-attack 6y ago
  If you ever used a toy rocket as a kid, the engines had a long hole down the middle. Apparently the Space Shuttle uses the same idea: https://www.siriusrocketry.biz/ishop/images/EstesStandardEngine.jpg https://www.siriusrocketry.biz/ishop/images/EstesStandardEng...
  - dTal 6y ago
    Estes black powder motors are in fact end-burners: https://farm8.staticflickr.com/7457/12472921013_aa92200267_k.jpg https://farm8.staticflickr.com/7457/12472921013_aa92200267_k...
Sharlin 6y ago
Nope, they burn from the inside out! The propellant does not constitute a solid cylinder but has a hole in the center going through the whole thing. Ignition happens at the top and the burning (very rapidly) spreads from there to the whole inner surface.
- m4rtink 6y ago
  You can get even fancier things with the inner hole to inflence your burn profile. For example, it can look like a star - this will give you high initial thrust as the burn area is big. As the inner spokes burn off, the burn front will star to approximate a circle, reducing thrust. This can be important to get you rocket the initial acceleration it needs to lift off but to avoid excessive acceleration destroying the rocket later on (as it it burning the rocket get lighter but the solid motor thrust would nornally stay the same or even grom slightly, due to the circular burn front being a bigger and bigger circle).
clarkmoody 6y ago
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!
205guy 6y ago
Other comments have already mentioned the various patterns. What hasn't been mentioned is that end-burners (as they're called) are possible, but have the downside of heat on the casing and change in center of gravity. Coming from model rocketry myself, I had the same question, so I went straight to wikipedia to find out: https://en.wikipedia.org/wiki/Solid-propellant_rocket https://en.wikipedia.org/wiki/Solid-propellant_rocket Somehow I thought that the combustion of the propellant had to be inline with the thrust, but TIL there is a combustion chamber, and it's the ejection of the combusted gases through the nozzle causes the thrust. I like hearing other people's experience with model rocketry. It's interesting because I experienced it more as model building (assembling and decorating) with a fun day of launching and running after them. I never got into the physics of the flights (other than lighter==higher) or even thought of investigating propellant properties--geometry and chemistry--as amateur rocketry does. I guess nobody told me "you can make your own engines if you want," but even then I'm not sure I would've pursued it.

Nope, inside out. They're hollow, and the top section has a star cross section rather than cylindrical. The goal is to keep an overall constant pressure for the entire burn time.

bfieidhbrjr 6y ago
TIL. That's amazing, thanks.
- wondringaloud 6y ago
  Which piece of it is amazing?
matt-attack 6y ago
If you ever used a toy rocket as a kid, the engines had a long hole down the middle. Apparently the Space Shuttle uses the same idea: https://www.siriusrocketry.biz/ishop/images/EstesStandardEngine.jpg https://www.siriusrocketry.biz/ishop/images/EstesStandardEng...
- dTal 6y ago
  Estes black powder motors are in fact end-burners: https://farm8.staticflickr.com/7457/12472921013_aa92200267_k.jpg https://farm8.staticflickr.com/7457/12472921013_aa92200267_k...

Sharlin 6y ago

Nope, they burn from the inside out! The propellant does not constitute a solid cylinder but has a hole in the center going through the whole thing. Ignition happens at the top and the burning (very rapidly) spreads from there to the whole inner surface.

m4rtink 6y ago
You can get even fancier things with the inner hole to inflence your burn profile. For example, it can look like a star - this will give you high initial thrust as the burn area is big. As the inner spokes burn off, the burn front will star to approximate a circle, reducing thrust. This can be important to get you rocket the initial acceleration it needs to lift off but to avoid excessive acceleration destroying the rocket later on (as it it burning the rocket get lighter but the solid motor thrust would nornally stay the same or even grom slightly, due to the circular burn front being a bigger and bigger circle).

clarkmoody 6y ago

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!

205guy 6y ago

Other comments have already mentioned the various patterns. What hasn't been mentioned is that end-burners (as they're called) are possible, but have the downside of heat on the casing and change in center of gravity. Coming from model rocketry myself, I had the same question, so I went straight to wikipedia to find out: https://en.wikipedia.org/wiki/Solid-propellant_rocket https://en.wikipedia.org/wiki/Solid-propellant_rocket Somehow I thought that the combustion of the propellant had to be inline with the thrust, but TIL there is a combustion chamber, and it's the ejection of the combusted gases through the nozzle causes the thrust. I like hearing other people's experience with model rocketry. It's interesting because I experienced it more as model building (assembling and decorating) with a fun day of launching and running after them. I never got into the physics of the flights (other than lighter==higher) or even thought of investigating propellant properties--geometry and chemistry--as amateur rocketry does. I guess nobody told me "you can make your own engines if you want," but even then I'm not sure I would've pursued it.