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Comment by hoorayimhelping | Hacker News Reader

Red is kerosene, blue is liquid oxygen, orange is liquid hydrogen, the fuel that looks like fire is solid fuel (think compressed gunpowder).

Left to right: Saturn V, Space Shuttle, Falcon 9 Heavy, SLS (still under construction).

2:22, you hear Neil Armstrong say "inboard cutout," that is the center engine shutting off to limit the thrust, and therefore G-forces the astronauts felt. As fuel ran out, the rocket stack would weigh less and so the rocket would push harder. By the time the main engine cutoff, the Saturn V was pulling 4 and a half Gs.

Also, just noticed the SLS keeps its launch escape tower (the big needle on the top of the rocket - see this: https://www.youtube.com/watch?v=AqeJzItldSQ) - this wouldn't happen in a real flight. The flight plan calls for ditching than when it's not longer useful - you can hear the Apollo 11 call saying, "tower jettisoned."

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In the video SLS keeps its launch escape tower until 8:13, then jettisons it. I do think that's too long, I wonder where they got that. If it does stay for so long, I wonder why.

It was likely kept longer than needed/useful because they didn't want to run into it. If the rocket is still accellerating it could theoretically catch up to the ejected tower. The probably kept it until they knew its velocity after release would be greater than the capsule's orbital velocity.

The Apollo escape tower thrusts at an angle specifically to prevent that.

The lateral componant was to pull the capsule out of the flightpath of the rocket. The total accleration of the tower+capsule was miniscule compared to the rocket overall. If it didn't move left/right a bit then the rocket would easily catch up.

The thinking at the time was that, without the capsule attached, the tower might not fly as planned. It could be unstable in the thin up atmosphere. It might spin. That lateral thrust, withough the mass of the capsule, might cause it to spiral, leaving it floating out in front of the accelerating rocket.

Isn't the escape tower designed to have a slight radial thrust component? The problem with avoiding an oncoming booster is the same when triggering the escape sequence for human passengers. I mean, there is an automatic self-destruct, but that must not be the only contingency.

Yes but without the mass of the capsule that 'slight' radial thrust isn't so slight And the CG is totally different. The worry is that an ejected tower might spin/spiral or even take a pinwheel motion rather than fly properly out of the way.

Without the weight of the capsule the acceleration of the escape tower is also much higher. So with even a bit of radial acceleration it should get away from a possible collision course pretty quickly.

It "should". That's not good enough.

Well, the whole point of the escape tower is to accelerate the capsule away from the rest of the vehicle. So the tower accelerating by itself should have no issue getting out of the way.

I don't know how SLS's escape tower works and I expect your right with respect to it. However that's not true of all escape towers. The escape tower used for the Mercury-Redstone missions at least was meant to only work when the rocket below it was no longer accelerating.

This became a factor during the (unmanned) Mercury-Restone 1 incident, where the launch tower flew off when the rocket was still on the launchpad, leaving the capsule behind on the rocket because the capsule was only designed to detach when the rocket was in freefall (it wasn't in freefall, because it was still on the launchpad.)

We don't need to guess about these things. They keep it until after second stage separation because they might need to actually use it for an abort up through that point. Here's a link confirming that the Saturn V launch abort system was retained until 30 seconds after the second stage separation for exactly that reason: https://www.hq.nasa.gov/alsj/CSM15_Launch_Escape_Subsystem_p... I couldn't find such detailed information on SLS, but it's a similar rocket with the same design philosophy and this logic likely applies equally.

The acceleration of the launch abort system on its own, when it's not dragging the entire capsule with it, is double digit gees. The main rocket is not going to catch up to it and run into it.

I think they're retaining it until just after the final stage separation, i.e. it will be used if that stage separation goes poorly.

Wasn't sure why the SLS escape tower stayed on while the S5 was shown jetissoning it

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

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.

TIL. That's amazing, thanks.

Which piece of it is amazing?

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/EstesStandardEng...

Estes black powder motors are in fact end-burners:

https://farm8.staticflickr.com/7457/12472921013_aa92200267_k...

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.

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).

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-prin...

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.

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!

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.

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

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.

Great info, thanks for adding, you are helping ;)

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