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Lightweight solid rocket motors

A rocket motor burns propellants under high pressure and expells the burned gases through a nozzle. The ideal rocket motor today uses liquid propellants. This allows to store the propellants in lightweight reservoirs, under low pressure. The rocket motor uses pumps to inject the propellants under high pressure. The pumps and the motor form a little and lightweight device. The propellants are commonly made of two liquids: an oxidizer and a reductor. The best common oxidizer propellant is liquid oxygen. The best reductor is hydrogen, though other reductors like petrol are excellent. Inside the rocket motor the oxidizer and the reductor are mixed and ignited.

A solid rocket motor trades in two characteristics mentioned above. First of all, it is a giant rocket motor: it contains all the propellant it will burn and must keep the whole under high pressure. Hence the weight of that motor/reservoir is much more important. Second, a solid rocket motor contains no liquid oxygen. Instead it uses solid powders or plastics that can release oxygen. This is a far less efficient way to use oxygen. On the other hand, a solid rocket motor is much simpler and reliable than a liquid rocket motor. It contains no moving parts. It doesn't need to mix the propellants since the propellants are factory-mixed in the solid propellant block. One more advantage of solid rocket motors is they can burn up quickly, releasing all their push force in a short time. This is useful when trying to escape Earth gravitation.

A common way to build space-bound rockets is to use a big first stage using low-cost and heavy techniques. The last stage on the contrary uses expensive high technology and is lightweight and very efficient. Best example is the Space Shuttle. It takes off relying mostly on its two giant solid rocket motors as a first stage, then ends the flight relying on its liquid hydrogen-oxygen motors.

So, a solid rocket motor is made of a block of solid propellant surrounded by a heavy reservoir.

Maybe a way to make the motor/reservoir more lightweight would be to rely on the solid propellant itself to hold the pressure. The reservoir would be a light steel skin, like the reservoirs of the Atlas rocket. At the beginning of the flight the center part of the block burns at high speed and creates a high pressure. That pressure is contained by the outer layers of the block. Steadily, as the center of the block burns away, the block burns slower and creates less pressure. The outer layers of the block would burn slowly and create no more pressure than the steel reservoir can hold.

The fact the pressure decreases is compatible with a travel towards Space. First, a high motor inside pressure is especially needed when the outside pressure is high, that is at sea level. When the rocket travels towards space and the outside pressure decreases, a lower motor pressure can be used. Second, the optimal nozzle end diameter depends on the outside pressure. The lower the pressure, the wider the optimal nozzle output diameter. The fact the burn rate and pressure decreases will decrease the amount of gases expelled. Hence a same nozzle output diameter can be optimal at all altitudes.

When the center chimney of a solid propellant block burns, its surface increases steadily, hence the amount of gases produced. A common trick to circumvent this is to give a star section to the chimney. This makes the chimney has a wide surface from start on. For the current purpose this may not be enough, as the chimney must produce much more gasses at the start than at the end. Making the outer layers burn slower is quite easy. On the other side, very quick burning speed at the inside may be difficult. More precisely, it may be difficult to get a reliable high speed burn speed together with a high energy yield. A simply solution is to carve chimneys inside the block. Another solution is to put fast burning fuzes through slow burning high yield solid propellant.

Liquid oxygen can be used inside a solid rocket. That is a hybrid rocket motor. The solid propellant is depleted in oxygen-rich compounds. Liquid oxygen is contained in a reservoir above and injected inside the burning motor. In this case the oxygen flow should decrease with the time. I don't know if it is better to inject the liquid oxygen at the top of the motor, to spray it near the nozzle convergent or to spray it all along the chimney. An interesting point with this solution is the oxygen-depleted solid propellant will burn at lower temperature, hence sparing the lightweight steel envelope at the end of the flight. Also, the oxygen-depleted propellant is less dangerous to store. And an explosion of the rocket during take off would be less destructive since the solid propellant will yield few destructive energy.

What about the weight of the liquid oxygen reservoir? It can be lightweight and under low pressure, with a pump to inject it under high pressure inside the motor. This would be a little sorry since it compromises the advantages of solid rocket motors. Another way is to use a heavy reservoir under high pressure. This is interesting since the volume of liquid oxygen needed is not very high. Anyway, are there ways to improve on this? A first way can be to use two reservoirs. The main reservoir would be heavy, yet it would be dropped at high altitude and the motor would switch to the second oxygen reservoir, at lower pressure and more lightweight. A second way is to place the oxygen reservoir inside the motor. Either in the center of the chimney or above the chimney and surrounded by slow burning solid propellant that is lit at the end of the flight. This approach seems risky to me but it may be sound anyway. A third approach could be to put the liquid oxygen in chimneys or little burnable resrevoirs inside the solid propellant. This is straightforward. A fourth solution is to make the liquid oxygen reservoir be heavy by being made of solid propellant surrounded by a lightweight sheet of steel, just like the motor itself. Once the motor and the oxygen are burned up, the reservoir would become a second stage solid rocket motor, possibly with a second little liquid oxygen reservoir above.

A problem with liquid propellants rocket motors is the propellants must be mixed and lit very quickly inside the motor and yield a stable flame. That's one reason why such motors need a high internal pressure and are very expensive. Inside the hybrid motor mentioned above I expect things to be simpler. The oxygen is gradualy mixed with partly burned hot gases. It should ignite immediately, with no instabilities. Maybe this can be used to burn hydrogen too. The rocket motor can transport a lightweight reservoir of hydrogen and spray that hydrogen during the end of the motor flight to gain some specific impulse.

Eric Brasseur  -  March 31 2005