Indeed former experiments have strongly indicated the large influence of At/Ae on the specific impulse and of eroding nozzle which create changing At/Ae values during burning. At this stage there is no model available which allows us to predict how optimal situations can be reached. Hence a number of experiments are necessary to find out the optimal situations.

The first picture shows the set up of the motor. The construction of the motor is made in such a way that it is possible to remove and replace all parts easily. For instance, the nozzle is composed of 3 individual parts, the convergent or inlet part, the middle or throat part and the outlet or divergent part. By changing one, for instance the nozzle part, it is possible to change the throat diameter or throat material.

Figure 1: view of ZAS5P-9.

The propellant grain is fixed onto the motor case with very fluid polyurethane (Protoflex190-15N). First the exact quantity of polyurethane was pored into the empty motor and then the grain was slowly released downwards. Before inserting the grain, it was covered with a number of silicon "dots" at the side and the bottom, in order to assure that no contact between motor case and grain could exist and that the polymer would cover the entire outer surface.

Both experiments used the same propellant: 64,5% Zn, 25,5% S and 10% Al. Because of a shorter outlet diameter, the Ae/At ratio in ZAS5P-8 was initially 26.4 while in ZAS5P-9 this was 39.5.The free space between the propellant grain and the throat was filled up with zinc-sulfur powder to assure sufficient pressure at the start.

The motor has the following characteristics:

The following chart shows the thrust measurement of both motors. The measurements yielded much noise which was partly filtered out afterwards.

The main observations are:

• Slow take up (dip during the first 0.1s): the slow take up may be due to the fact that
  • in both cases the igniter was not positioned behind the throat, in the middle of the ignition powder, but slightly before the throat. Reason for this is that the diameter of the igniter is slightly larger than the nozzle throat diameter (7 mm)
  • still some part of the initial burning surface may have been covered with a thin polyurethane layer, as it was difficult to have it completely cleaned in the motor.
  • The fact that a rather smaller amount (65 gr) of Zn-S powder was used than usual is probably not the cause. Also in several other motors which functioned very well small amounts, even as low as 53 gr were used.
• Clock shape of the thrust curve: this is due to the fact that the throat erodes during burning. Hence the smallest throat diameter is continuously increasing. At more or less constant pressure this yields higher thrust values.
• Higher top thrust for ZAS5P-9. This is due to the fact that the throat in this motor was more eroded and hence a larger throat area "At" (around 20%, which corresponds with the different in final At area of both motors).
• Slightly longer burning time of ZAS5P-8: this is the consequence of the fact that the throat area was smaller than of the ZAS5P-9 and thus that less material could be expelled at the same pressure. The difference is 0.1s or about 8% of the total time.

The Isp for both motors is surprisingly low and the same: 52s calculated on the total propellant mass and 55s based on the expelled mass.

As already mentioned, the throat of ZAS5P-9 eroded from 7 mm in diameter to 10 mm, while in ZASP-8 the erosion is from an initial throat diameter of 7 mm to 11 mm. Why there is a difference is not clear at all. Both motors had the same nozzle throat material, same propellant, same amount of ignition powder, same igniter. ZAS5P-7, which was an almost identical motor, except for the throat diameter which was 8 mm, erode till 10.7 mm. The thrust curve, except for the start, is very similar to the two tested motors.

Conclusions:

1. erosion of the throat provides thrust shapes which deviate strongly from the common triangular shapes with higher thrust at the start and continuously decreasing;
2. ZAS5P motors with eroding throats have a slightly smaller working time than with a non eroding throat (1.2s to 1,3s compared to 1.7s);
3. All motors (except ZAS5P-2) with eroded throat have their maximum thrust between 0.7 and 0.8s. This may indicate the real burning time, which would lead to a burning rate between 37 cm/s and 42 cm/s , or to the end of the erosion process (much more difficult to accept).
4. the erosion which we face now is probably too large. Somewhat less erosion will lead to a more flat thrust shape. This could probably be realised with a longer cylindrical throat, or with less erosive throat material. A more flat curve will make it mare easy to find the optimal ratio of Ae/At, since the motor will burn most of the time at a more constant mass rate.

With thanks to the whole GEA team and the VRO member Bert Kimpe for their assistance.