Overview

This simple static thrust measurement stand consists of a balance with the rocket mounted on one side with a kitchen scale on the other.

Test Stand

The kitchen scale measures 0 - 500grams. Attached to the lever arm is a dampner that prevents the spring in the scale from oscillating as the rocket induces vibration into the rig.

The scale dial is video taped at 60 frames per second which gives us a stable time reference for the measurement samples. These readings are then manually transferred to a spreadsheet by replaying the video frame by frame. Ouch!

The balance can swing all the way around on the pivot allowing us to permanently mount the rocket on it, while giving us the ability to easily fill it with water.

In this setup the nozzle is 85mm from the pivot point, while the scale is at 1415mm from the pivot point.

	The static thrust measurement test stand.
	Detail view of the dampner next to the kitchen scale.
	Setting up the camera to video tape the kitchen scale.

We chose a restricted nozzle to help stretch out the data so it could be more accurately measured and visualised.

The thrust measurement stand is balanced so that it reads zero Newtons with the rocket empty. This was done so that the data obtained would reflect the thrust (force) that contributes only to rocket acceleration, as it already incorporates the force needed to overcome the weight of the water in the rocket. Hence the thrust measurement does not represent the absolute thrust. Ultimately we are only interested in how much force is contributing to the acceleration of the rocket upwards anyway.

If we had balanced the stand while the rocket is loaded with water, by the time the thrust ended and all the water was out, the scale would read the weight of the water since it would now be unbalanced by that amount. This would have introduced a bias to the data. Since we do not know at what rate the water empties from the rocket, we later cannot remove this bias from the thrust data.