One condition for a standard powder rocket motor to work properly is that, when the inside pressure increases, the gas output must increase quicker than the combustion rate.

Otherwise the combustion will be divergent: the pressure will increase continuously until the motor explodes or the combustion strangles itself.

A lot of good powder rocket propelants are basically of the diverging sort. When the pressure increases, the combustion speed increases more than the output flux, so the pressure increases even more... till the explosion.

To avoid that behaviour, chemical products are mixed with the propelant. Their purpose is to make the combustion rate increase less than the output flux. So whenever the pressure increases, the increase in combustion rate will be compensated by a greater increase in output flux. The pressure will not continue to increase.

Such improved propelants make the rocket motor operate at one given pressure, automatically regulated.

The purpose of this text is to propose to use an active mechanical system instead of the chemical additives.

One way to do it would be to allow the diameter of the output to varry. Before the pressure increases the output would be slightly opened, before the pressure decreases the output would be slightly closed:

Another way would be to put little output nozzles around the main output. Those little outputs would be opened and closed by valves when the pressure increases or decreases:

That way no additives would be necessary, allowing to save some weight and increase the motor's yield.

Other advantages are:

• The fact the pressure is regulated and is thus guaranteed not to rise above a given limit allows to decrease the security factor of the rocket motor body. It can be build lightweighter.

• The motor can be used on a broader range of start temperatures. the difference in combustion rate will be compensated by the active regulation system.

• The propelant shape inside the motor can be more simple and the central hole can be littler, because here too differences can be compensated by the active regulation system. So more propelant can be put inside the motor and it will be easier to manufacture.

• The pressure regulation device can be used to steer the rocket.

• When valves are opened and closed some power is lost due to flux irregularities. Being given that the opening and closing can take a hunderth of a second and will occur every few seconds, the amount of energy lost will be very little. Besides the regulating valve will only output a minute part of the gasses.

• Powder rocket motors are prefered because of their simplicity and reliability. A system using valves is a complication. I think that complication is worth its advantages, especially about security. Besides, if the main output is surrounded by a crown of valves, some of those valves can fail and remain open or closed without disturbing the motor's functioning.