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Solid and Hybrid Rocket Motors

I shoot anything I can get to burn. (I even shot some pine rosin/PP once...) Right now I am working with a "sensitized"-AN/Al/Sucrose (sucrose being both binder and fuel). I am talking to Mr. Yawn about using a variation of his recrystilization method along with compressing it with an 80-ton press. I am going to try an alternate of vacuum dehydration shortly.

The plug will be a ceramic/metallic composite riding in a housing so that only the lowest tip is exposed. Right now I am trying to develop a way to actuate it at 2500 psi and 5500-6500 deg. F. so that it will act as a pintle valve to regulate the chamber pressure.

The casing will be steel and be insulated with a ceramic/ceramic composite and the fuel will be cast in place. The nozzle will be a variation of a spike configuration and will probably be ceramic/metallic composite. It should survive a sustained environment of 6000 deg. F with no time limits. I already have it to the point that I can control 4000 deg. F. and the only damages will be from erosion. I run completely balanced burn profiles...no fuel-rich blends for cooling. No regenerative cooling. It can cool down after firing, but often cracks from thermal shock after the burn. The nature of the composite should cure this if all goes well.

One aside: during and immediately after a burn the nozzle will often beso hot that it radiates a brilliant pure-white light so bright that you cannot look directly at it without welder's goggles. Then the outside cools and turns dull to black and you can see a bright red glow below the surface as it cools to dull red and then flat again. Very peculiar "pyro" there. It Would make some relatively spectacular color pictures.

Here are a couple of different propellants that Bill uses:

1) HFG (Hybrid Fuel Grain)/Hydrogen Peroxide (90% Liquid):

The fuel grain is based on: 150 units of Al; 0.02 units of red iron oxide; 13 units of R45M; 8 units of Boron. This 171.02 units of fuel is run with 210 units of H2O2 (or a ratio of 1.2279:1.00 oxidizer/fuel ratio by weight ) at 2,500 psi and a temp. of about 6,700 deg F. Frozen Isp = appx. 295-300 or so and shifting Isp = appx 315-320. (Requires a steel casing, ceramic insulation and ceramic nozzle insert. Exhaust velocity if I remember correctly, is over 8.000'/sec. )

2) Solid fuel/ Ammonium Nitrate based:

425 units of AN; 182 units of Al; 68.79 units of R45M. @ 2,500 psi & about 5,400 deg F. yields frozen Isp = appx. 277-279 and shifting Isp = 285 or so. Others using AP/fuel and Potassium Perchlorate/fuel. For small motors I use Pyrodex-based and Goex-based solids instead of BP. In every one of these a deviation of just 1-2% in mixture (and sometimes as little as 0.005% of a catalyst) and/or a pressure variation of just 5-10 pounds might drop the Isp by as much as 20-30 sec. So one of the basic demands is maintenance of a predictable operating pressure. Sometimes a slight decrease/increase in pressure will produce a corresponding drop of several seconds and if you continue to increase/decrease the pressure there will be another high plateau or a striking increase in Isp. Beats me why… I never claimed to be a rocket scientist. I think it must have to do with flame stability in the chamber at different pressures.

Some data regarding a H2O2/Al/FeO2/R45M/Boron hybrid:

Then it remains at 298-299 through 3100#. (These seven are from a series of 16 on just this question on this formula alone.)

As you can see there is a transition peak at approximately 2450-2500# and a dramatic drop between there and 2600#. This is the narrow band of optimum conditions I shoot for. Then there is a resumption of the 297-299 Isp range which takes place within a 1-psi shift between 2978# and 2979#. This is the most dramatic and drastic transition I have seen so far, but is not uncommon over a broader pressure band. The H2O2 hybrid gets about 9,600+ ft./sec. exhaust velocity at max pressure. Others please check me out on this and let me know if I am missing something here.

As for Bills question about size, I sincerely believe that one can develop a motor of any size with enough work on it. Under the right conditions just about anything will combust. In the HP stuff I work with

anything from 1 1/2" on up. The largest motor I ever built was a 4" in 1963. But I mostly like 2 1/2" because I have access to chrome-lined steel tubes certified at 10,000# in that size. I also have about 18' of 2 1/4 I.D.

Stainless in the shop right now. I can get quality material up to 3 7/8" in stainless...all with precision-ground chrome lining. In the smaller stuff I go as low as 1/4" I.D. I use milled and re-combined Pyrodex or the new Goex BP substitute. I have just over 22# of this and others in the storage cabinet right now.

I really hesitate to talk about a lot of stuff because it is illegal where I live.

In my younger days, before I sort of gave up on liquids, I used regenerative cooling with grooves cut in the casing/nozzle walls to circulate LOX through. It worked pretty well, as pointed out by Larry. I also tried enriched mixtures and it works but decreases impulse. It also makes a very good cutting torch. (A couple months ago I was at a research center and saw a new cutting torch made in Russia which is a small kerosene/LOX rocket motor with an oxygen injector...it will cut 9" of reinforced concrete.) A problem I always had was that before thermal transfer could take place the metals in the casing/nozzle would melt or vaporize just as Larry stated in his post. Boundary-layer cooling with excess fuel or oxygen injected just ahead of the nozzle works very well but there is a problem getting complete coverage (a.k.a. "protection") plus you have to have an additional high-pressure injection system to contend with unless you want to run the risk of unbalancing your fuel/oxidizer injection system by slaving off of it. It also cools your exhaust where you need high temperatures/velocities the most (just before they accelerate through the nozzle and throat). In essence liquid-fueled rockets are supremely complicated but tremendously satisfying if you can get them to work. If you try them and then decide to go to something else you might consider a stab at high-intensity hybrids. Or, since you are apparently another gun-nut like me you might try another of my long-ago projects...a small-diameter rocket launched from a 12 ga. shotgun: you accelerate it to about 900-1,100 ft./sec. with the gun and the rocket motor continues after leaving the barrel.

One thing I finally accepted is that there are no absolutes here. Everything is a trade-off of some sort: you get higher inputs with higher pressures but then you have to contend with higher temperatures, heavier casings, thermal transfer/control problems, etc... In a liquid-fueled motor you also have to consider that if you are running at 2,500# chamber pressure you need at least a 3,000# fuel system to get the fuel/oxidizer into the chamber. They would really scream, but would make a real mess if something goes wrong.

Best free advice I can give, which is worth exactly what I charge for it, is that none of us are really ever going to the moon. We are in this to have some fun and satisfaction. If you build one and blow it up, build another and try again (provided the first one doesn't kill you). If nothing else you will learn more from failures than successes. You get the additional education and some really spectacular video footage to share with us all. At least you will have tried, which is more than most people do about their dreams.