luke strawwalker
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An end-burner burns from one end to the other, just like a cigar-- or our Estes rocket motors... you light them on one end at the center, they burn forward and outward through the propellant grain til it reaches the casing wall, and then burns forward until the propellant is all consumed.
A core burner burns from the center outward... it burns along the full LENGTH of the grain, from the very forward end of the motor grains all the way to the nozzle, but burns from the center outward--- sorta like the way the map burned up at the beginning of every episode of "Bonanza"... Remember that or am I dating myself here?? LOL
The shuttle SRB's work this way-- burning from the center outward, like sucking on a Lifesavers candy until its just a thin ring of material right before it's gone... the center hole gets bigger and bigger until the propellant is consumed. A moon-burner has an off-center port or core in the grain or a C-slot or other such core... what this does is, by putting the core hole off-center, it creates a thicker "wall" of propellant to burn through before it gets to the casing wall and burns out, since at that point all the propellant would be consumed... When its lit, it starts burning straight outwards, just like a regular core-burner, but it soon reaches the wall of the casing where the propellant is thinnest-- nearest the core hole. Then it starts burning in an ever wider "C" shape until finally there's just a little crescent shaped piece of propellant left on the other side of the casing, opposite where the core hole was... this "crescent moon" shaped bit of propellant burns thinner and thinner until it burns off the casing liner wall, at which point the motor burns out since all the propellant has been burned.
It's basically a "cheat". The only way to increase the burn duration of a core burner with a given propellant, (assuming a basic propellant with a constant burn rate for all motors here-- modifying the burn rate will give a longer burn, but we don't want to confuse the issue-- for sake of argument, we'll stick with a single type of propellant with a given burn rate of "X" inches per second). Increasing the burn duration of a core burner with a given propellant requires a thicker propellant grain wall-thickness cross-section width... IE, thicker propellant. To do this with a core burner, means you have to increase the diameter of the casing so the propellant is thicker around the central core hole, or you have to reduce the diameter of the core hole while keeping the same casing diameter (which can have flow problems and also reduce the surface area of the grain at ignition to the point the motor has a long thrust buildup problem). Say the propellant "web" or propellant cross-section is 1 inch thick, and the propellant burns at 1 inch per second-- the motor will burn out in 1 second, since it will take 1 second to burn through that 1 inch thick piece of propellant...
Now, if you move the core hole off-center, you get a much thicker propellant "wall" on one side of the hole, while keeping the same casing diameter, and the same core hole diameter... in exchange you've created a much thinner propellant "wall" on the side of the core hole nearest the casing... but it's the thicker one we're interested in, and the total amount of propellant... Say your hypothetical motor, burning the same propellant that burns at a rate of 1 inch per second, is arranged as a moonburner, and say if you moved the same size core hole as the coreburner off-center to get a 2 inch thickness between the core hole inside wall and the casing wall directly opposite the off-center core hole-- the same motor, same propellant, but now it has TWO inches of propellant to burn through, not 1... at 1 inch of burn per second, it will have a burn duration of 2 seconds now...
Ares I basically faced the same problem... the shuttle SRB's were great at producing massive amounts of thrust, but they were only designed to burn for about 2 minutes. Adding segments to a core burner would be about like adding grains to a reload-- the extra length will increase thrust, sure... but then thrust isn't the problem-- the motor needs more DURATION to get the vehicle higher and faster before burnout. The only way to increase the DURATION of an SRB (assuming the propellant burn rate remains the same) is to increase the thickness of the burning propellant, which basically means, increasing the DIAMETER of the SRB, NOT increasing the LENGTH of the SRB... Of course the problem is, the SRB's had to use the same size casing... which was the already made and used casings for the shuttle, which were 146 inches in diameter...
Wonder if ATK ever thought about making a moon-burning SRB?? LOL
Probably not, as casing erosion would be SEVERE (probably a show-stopper) for a motor that large with that long of a burn duration... As for the single vs. multiple nozzles-- good information there... the thing is, when you have multiple nozzles firing in close proximity to each other, you have to take careful accounting of flow interactions and plume expansion and plume recirculation effects... that's some bigtime number crunching and CFD modeling... and basically it's NOT going to be as efficient as a single large nozzle, but then again, most things in rocketry are tradeoffs... for instance, the SSME's are under-expanded in vacuum, but overexpanded at sea-level. When they were designed, they had to accept some tradeoffs and loss of efficiency to make them work... design them for maximum efficiency in vacuum, they'll be SO overexpanded at sea-level you get flow seperation and all sorts of bad things start to happen... design them for maximum efficiency at sea-level, and they're SO underexpanded in vacuum the efficiency and performance goes down the drain...
The multiple nozzles aren't the most efficient means of converting pressurized hot gas into thrust, but it's the most efficient way to MANUFACTURE the nozzles in question... much like the RD-107 engines on the Soyuz rocket-- multiple chambers are less efficient than a single large chamber, but it's easier to control and overcome combustion instability in a cluster of smaller nozzles, and easier to manufactur smaller nozzles than a single large one...
later! OL JR
PS... I'd bet that gluing the grains together is more about keeping the off-center core hole aligned on a moonburner than it is about "sealing off" the ends of the grain to prevent them from burning off between the grains in flight... if the holes aren't aligned and don't stay aligned, the motor is going to be hard or impossible to install an ignitor into, get lit correctly, or burn right during the thrust phase...
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