Do model rocket engines provide a constant thrust level of thrush or does it vary?

Answer 1

In short, yes and no. Model Rockets use solid fuel (propellant) in their engines (more commonly referred to as motors). This fuel burns, it doesn't explode. Therefore, like anything that burns quickly, there will be irregularities and flaws where it burns more quickly (or slower) than other areas of the propellant grain. These flaws and irregularities come from the process of making the propellant grain itself (it's an incredibly difficult task), and they're inherent when you have any rocket motor.

Interestingly enough, these irregularities are rather consistent and can be represented well in thrust curves (they'll be provided by the manufacturer of the motor). Most of these thrust curves provide constant thrust, with a few bumps here and there. Again, that's just the price you pay for solid burning motors.

So, to answer your question, yes, rocket motors are designed to provide constant thrust. However, there will be irregularities in the thrust performance as the propellant grain burns. It won't be enough to affect the rocket's flight, though.

On a side note, if you DO want constant thrust, use liquid propellant. Good luck doing it on the cheap, though. Or not dying while doing it, for that matter.

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Actually some model rocket engines are designed to produce a burst of thrust and quit, others are designed to produce a long nearly steady burn, It all depends on the shape that the fuel grain is molded in. You will have to consult the thrust graph in the data sheet.

Because model rockets must get off the pad quick to fly stably even the steady burn engines are designed to provide a burst of thrust initially before dropping back to the lower sustaining level.

For what it is worth use of liquid fuel engines on model rockets is illegal. If you want to try liquid fueled engines someday you will have to join an Amateur Rocketry group and apply for permits to launch with the FAA so that aircraft will avoid the area.

Answer 2

It is constant.