What's the primary reason for cutting propellant grains?

The last two, mainly... if you build a core-burning motor with one long grain then the thrust curve is very progressive, as the increase in diameter of the core increases the burning surface area.

The short segments (known as "BATES" grains) burn on both ends as well as the sides, which reduces the length of each one as the core diameter increases, and thereby produces a thrust level that is more consistent throughout the burn time.

It also does allow one manufactured grain to be used in several different motor sizes, as in many of the Aerotech "RMS High Power" types.

-dave w

The other BIG reason is to keep 38 and 29 MM motors exempt from having to have a LEUP to buy them. As long as each grain in a motor is less than 62.5 grams, they are exempt.

Les.

Cutting them into segments creates a Bates grain geometry. Relatively constant thrust profile due to a constant burn area - they burn from the ends inward as well as from the core inward. Without the segments the motor thrust would be progressive, increasing as the motor burns and peaking at the end of the burn.

There is a great animation at

to the Movies area and look at the motor burning in a cutaway view. The Supreme Goddess of the Sky is one beautiful bird!

Originally, back when even the AFT said rocket motors were pads, that was done so that huge shipping bills via Flying tigers were not needed to ship Class "B" motors, as the less then 62.5 grains made them Class "C" grains. Heck, I think only four or five major airports were pickup spots for class "b" motors.

It was a huge market growth oppurtunity, since any motor manufacture that could ship H and I (and later, small J) engines without the expensive courier chrages would/did get quite a market share of the business.

One strange method, was done by Syner-Jet. they had an H motor that had the first 62.5 grain and nozzle all glued together in the single use motor case. When you recieved the motor, you put the other grain(s) in, and then glued the delay end closure on.

Kosden, Revenna, the market favorite AT, and a few others were doing the reloads strong by end of 90, and early 91.

Then a few years later, all the LEUP started stuff down hill.

Yep. They also had them in larger sizes. I bought a couple of the J motors, still have one.

Actually, Ravenna folded and was reborn as Syner-Jet. IIRC that was when they started marketing reloadables and later the "kit" motors -- don't recall any being sold under the Ravenna brand.

Thanks for the info! As a followup question, couldn't you cast a single, long segment with a high surface area, star core configuration that burns/erodes away towards a circular burning core, thus providing a more stable thrust curve?

Also, with segments, what's to keep them stationary when they burn (not smacking each other around)? Are they generally adhered to the motor case somehow?

Thanks again! Dave

Yes, I think your right about R3 becoming Syner-jet. Plasma-jet was in Cleveland proper as well.

two old school motor companies, both in Les's neck of the woods even.

R3 did have a few mini kosden like reloadables at the late 1990 launch I went to. They were prototypes. But they had a very bad batch of Single use motors, due to AP gain size differences from their AP supplier, and lots of motors catoed.

In fact, I heard a person say while looking at the R3 reloadable, "who the F@4) is going to pay for this", as he was holding his 7 motor cluster rocket that was all blown out at the bottom.

ACS had a few motors cato as well. people then stayed away from both companies motors.market forces at work.

Later that night at the bar, is when I heard clamoring for motor certification.

they probably changed their name to get away from the bad batch blues.

There's more core geometries than nose cone shapes. Different core geometries help tailor different thrust profiles for different propellants. I'm not sure what you mean by a "stable" thrust curve, other than rapid motor pressurization to prevent chuffing. It can be done, but complex core shapes are harder to cast/machine and thin propellant sections are more susceptible to mechanical damage. Good ignition methods can bring a motor with simple geometey up to pressure just as rapidly

All the Bates grain motors I've built are free-standing, not case bonded. I've used grain spacers before, but they were meant to ensure rapid grain face ignition, not physical support during burn. Case bonding grains brings up issues with propellant mechanical properties and radial pressure differentials.

Yep. I had some of those too. :(

I've not done any high power stuff, though I've certainly 'kibitzed' here often enough. This is piqued my interest, though.

So, what is apparently being said (and re-affirmed by the animation), is that the grains are initially held in place at their faces, but once burning commences, they are then held in place by the gas pressure of the burning grains? Is this a correct summation?

David Erbas-White

Yes.

Equilaterally.

I don't think "held in place by the gas pressure" is the correct interpretation. I'm certain there is grain movement within a burning motor due to acceleration, the longitudinal (fore - aft) pressure gradient within the motor case, and flow friction. The evolving gases at the burning grain surfaces may prevent direct contact, but I don't think segmented, free-standing grains are "stationary" within a burning and/or accelerating motor. They would certainly tend to move towards the nozzle end, I would think. I haven't really thought about it too much. My AR experiments are based upon other's already successful motor designs.

I'm sure one of the manufacturers can make a better comment on segmented grain motion in an operating motor.

From a manufacturing standpoint, star grains are hard to make compared to the relative ease of bates grains.

Normal operating pressure, generally keeps the grains in place.

Yes.

Held in place by physical expansion, caused by gas pressure.

Yeah, but in order for them to be in static equilibrium, the pressures on both sides of the segment would have to be exactly equal. There's significant delta-P's in the chamber (although small in relation to the overall chamber pressure), and if one side of the segment were to burn a "tiny bit more" propellant over a given time, it would experience a pressure differential and thus a force.

Regardless, even IF the segment had the exact same pressure of both sides, they'd cancel each other out and you'd still be left with acceleration, vibration, and shear forces from the gas flow that could cause segments to move.

Dave

Yeah, but in order for them to be in static equilibrium, the pressures on both sides of the segment would have to be exactly equal. There's significant delta-P's in the chamber (although small in relation to the overall chamber pressure), and if one side of the segment were to burn a "tiny bit more" propellant over a given time, it would experience a pressure differential and thus a force.

Regardless, even IF the segment had the exact same pressure of both sides, they'd cancel each other out and you'd still be left with acceleration, vibration, and shear forces from the gas flow that could cause segments to move.

Dave

Seems like the most probable behavior (if the segments were free to slide in the liner) is that they would stack at the rear of their possible travel against the aft closure assembly (carried there by the acceleration, and the lengthwise pressure difference in the chamber), with a space between them maintained by the "air bearing" effect of the gas generated from the end surfaces. Whether this latter effect would be dominant would depend on factors including the length of the motor and the acceleration of the rocket... I've heard that where both are extreme (i.e., a J570 machbuster), there can be problems with acceleration squeezing the stack of grains backward so hard that the propellant is distorted, constricting the core... epoxying the grains into the liner to keep them in fixed positions has reportedly improved reliability in such applications.

-dave w

Unless the pressure on the outside of the grains is the same as within the grain cores, as in uninhibited, free standing grains burning on the outside. Free standing grains, I thought, are used to mitigate propellant pressure and expansion issues allowing propellants and cases of different modulus of elasticity to be used together.

I don't know, I'm just trying to keep what little I think I know making sense.