Eric Brasseur Home    |    Links    |    Contact    |   

The Pencil Rocket

The disadvantage of a solid fuel rocket is the whole block of solid fuel must be located inside a though rocket motor. This implies an excess of weight and of volume. Perhaps there is a way to avoid this drawback.

The basic way a modern rocket motor works is the following. Hot gasses with a high pressure are produced inside a chamber by a block of solid rocket fuel (or by liquid fuels injected inside the chamber). The gasses move at subsonic speed towards the outlet were they are accelerated by a converging nozzle up to the speed of sound. At that speed they arrive inside a diverging nozzle where they accelerate above the speed of sound up to the maximum supersonic speed their heat and the outside pressure allow.

One could smear a little bit of solid fuel inside the supersonic diverging nozzle (or inject some liquid fuel). The gasses produced by that additional fuel will be driven to supersonic speed by the gasses coming from the inside of the motor and they will be allowed to expand and accelerate inside the diverging nozzle just like the other gasses. (I believe so, which is not a proof.)

This makes the whole diverging nozzle could be made of solid rocket fuel. That fuel will burn and add the propulsive power of its gasses to that of the gasses coming from the inside of the motor.

This implies the whole rocket motor can be made of solid rocket fuel. Provided the burning speed of the different zones of the powder block are tuned the right way. What matters is that while the rod of solid rocket fuel burns up its burning end keeps the shape of a rocket motor. In the following drawing the light green fuel is supposed to burn very fast, the yellow medium fast and the dark green slowly. The whole block burns from left to right and just leaves the end mantel.

I made some little rocket motors "that way" that lift of even though the gasses were subsonic and I used only one burning speed. So this system works for sure. But I don't know if it would have a better yield than a normal modern rocket motor. If it has a good yield, then it has some advantages upon normal rocket motors, since it needs no outer container and it burns up gradually leaving no useless weight. It could allow a single stage solid fuel rocket to go into orbit.

Another question is whether the carving out by burning of the rocket motor shape will keep symmetrical enough. This can perhaps be attained by using circles of very high speed rocket fuel to force the symmetry.

I don't know what a possible outer mantel should be made of. Perhaps a lightweight porous and rigid material.

I don't know either if such a rocket would be easier to manufacture than a normal solid fuel rocket motor. Anyway, it would be an interesting alternative to consumable rocket. (Solid fuel rocket motors are often used to boost the first few minutes of a space rocket. For such purposes, the idea depicted in this text would be quite useless. It doesn't matter much that such first-stage boosters be heavier. 'Just add more of them to compensate... Lightweightness (for a same content of energy) becomes essential only for the upper stages of the rocket. This is best demonstrated by the Ares I rocket, which has a sole giant solid fuel booster as a low-efficiency first stage and a very high efficiency hydrogen-oxygen liquid fuel second stage. By the way, it will propel 25 metric tons to orbit using only two rocket engines!)

To guide the rocket maybe the outer mantel can be pulled by ropes to bend a little bit. Another solution would be to use little hydrazine motors at the top of the rocket.

Perhaps an aerospike version of this motor can be build. A layer of fast fuel would be placed just beneath the outer mantel. It would then really have a pencil shape while burning.

At hypersonic speed yet still inside the atmosphere, maybe the outer side of the rocket motor can be shaped to be lit and act as a ramjet, partially mixing its fuel-rich flames with air to increase the yield.

Eric Brasseur  -  May 4 2001  till  December 23 2008