SRBs

[Shaun]

I'm just wondering what the relation of the length and width of an SRB effects power and duration of burning. Is length power, and width endurance? Is there some sort of equation to this?

[RoboRay]

In the real world, length generally influences power and width duration, because they don't burn from the nozzle end up to the nose... there is a hollow shaft through the center of the SRB which all ignites, and it burns from the core outward to the sides.

The exact performance can be adjusted or even varied through the burn by the complexity of the shaft's shape.

In KSP, it's just numbers in a config file so it's arbitrary.

Edited August 21, 2013 by RoboRay

[Tiron]

In the real world, length generally governs power and width governs duration, because they don't burn from the nozzle end up to the nose... there is a hollow shaft through the center of the SRB which all ignites, and it burns from the core outward to the sides.

In KSP, it's just numbers in a config file so it's arbitrary.

And as I recall, the surface area governs the thrust, so they like to use fancy high-surface area patterns for the shaft...

[RoboRay]

And as I recall, the surface area governs the thrust, so they like to use fancy high-surface area patterns for the shaft...

Yeah, I was looking for some good example pictures to add...

[Tiron]

Yeah, I was looking for some good example pictures to add...

That 'Rod and Tube' design seems like it's probably a theoretical one... Seems like if you tried to actually build it, the rod would probably just fall out. On fire.

And 'dual composition' refers to having an outer layer of a different kind of fuel, which you can see in the diagram.

[RoboRay]

This, by the way, is why you should be very gentle with SRBs... cracks in the fuel will expose more surface area, leading to higher than expected burn rates. Or explosions.

I lost a model rocket when I was a kid after using a second-stage motor that had been dropped (and presumably cracked). First stage went well, but the thing just blew up when the second stage ignited.

[Motokid600]

How exactly is solid fuel packed so that there's no cracks or bubbles to begin with?

[Tiron]

How exactly is solid fuel packed so that there's no cracks or bubbles to begin with?

I suspect the same way bombs are made: It's poured in as a molten slurry and allowed to set. Or somesuch.

[Anton P. Nym]

That 'Rod and Tube' design seems like it's probably a theoretical one... Seems like if you tried to actually build it, the rod would probably just fall out. On fire.

I'd guess there'd be struts inside of some inert and very heat-resistant material to anchor the rod in place, but I'm not a rocket scientist. (I just play one on my computer.)

-- Steve

[ManaaniWanderer]

It could be that the central rod in anchored to the top of the SRB

[Tiron]

It could be that the central rod in anchored to the top of the SRB

Well I suppose if you ran some kind of inert rod all the way down maybe, otherwise you're anchoring it with just fuel and that seems like a Bad Idea.

[RoboRay]

Yeah, it would need some kind of support, like a rod down the center and a bottom plate. I have no idea if anyone ever tried to build one like that, or if it's just theoretical musing like you suggested.

[lazarus1024]

It could be that the central rod in anchored to the top of the SRB

It is. It basically looks like a large stalagtite inside of the SRB. The binders and what not make the SRB fuel pretty darned tough. The thrust from the burning fuel compresses things pretty evenly inside of the booster and push it back upwards...so unless you messed up the patterning, there is even force all the way around the rod. It has been used on SRBs before, but it is not the profile that the SST used; an 11 point star with two truncated cones near the bottom. It caused high initial thrust for 50s with gradually reducing thrust till burn out.