First mortar deployment test successful

While the record breaking Stratos II+ launch of October 2015 was a huge success for DARE, there was also one disappointment: not all parts of the rocket came back safely to Earth. The capsule, with the important data, landed safely by a parachute but the oxidizer tank and engine did not. Later it was found that the lines that connect the drogue parachute to the tank were snapped at drogue deployment, resulting in the tank going into a ballistic trajectory back to Earth and hitting the ocean with nearly the speed of sound.

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Figure 1: Broken parachute lines

While it cannot be said with 100% certainty, it is highly likely that when the drogue was pushed out of its storage space, it was cut by a sharp edge of the rocket. To prevent this from happening in the future it was decided to develop a drogue mortar deployment system. This system will shoot the drogue away from the rocket to get it on a safe distance from the rocket, with its sharp edges.

During the last few months the drogue mortar was designed and built and this week the first mortar test was performed. In the end a flight version will be tested on the Aether rocket that is currently under development. This blog post will elaborate on the drogue mortar project.

Working Principle

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Figure 2: A sketch of the mortar

A sketch of the mortar assembly can be seen in the figure above. As stated already, the goal of the mortar deployment system is to shoot the packed drogue parachute away. The working principle is a bit like a canon. The drogue itself is stored in a sort of cup, called a sabot, which is then put in the mortar tube. When the mortar fires, high pressure gas flows into the breech volume and this forces the sabot against the lid. When the pressure is high enough, the nylon bolts that hold the lid in place will shear out and the complete package flies away. For tests the drogue is replaced by a piece of MDF wood which has almost the same density as a packed parachute.

In the last months the mortar deployment system was produced. As can be seen in figure 3, it consists of an aluminum tube welded to an aluminum plate. The mortar tube itself was anodized and covered with a teflon coating by ATM Heerhugowaard. This made the mortar not only really smooth, but also more durable, so multiple tests can be done without damaging the metal. A feed system will guide high pressure nitrogen gas into the mortar. Nitrogen is used for tests, but later this might change to CO2 cartridges.

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Figure 3: The mortar in its test stand with sabot, drogue dummy, lid and nylon bolts

Test

For the first tests the main goal is to find out which exit velocities the parachute will reach at certain burst pressures. Therefore a striped board is used in combination with a high speed camera to determine the exit velocity. Furthermore the pressure inside the mortar and the shock load were measured.

The first test resulted in a misfire. With an inlet pressure of 10 bar, the pressure in the mortar was only 2.5 bar. It was decided to increase the pressure to 20 bar, but even that was not enough. As a last attempt it was decided to increase the pressure to 40 bar, and this worked!

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In figure 4, the data obtained from the test can be seen. Red indicates the pressure inside the mortar tube and blue the reaction loads measured by a load cell. When the valve opens at 0 seconds, it is clearly visible that the pressure starts to increase until it reaches the pressure at which the bolts shear out. At that moment the drogue is shot out and the pressure drops quickly. This also gives a reaction force to the mortar. The bolts broke at 4.3 bar while they were expected to burst at 4.5 bar, so this is a satisfying observation. The reaction loads are almost 200 kg. This order of magnitude was also expected, but is still mind-bogglingly high. In the design for the flight version of the mortar this should definitely be taken into account.

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Figure 4: Test data, red is pressure in bar and blue the shock load kg

Future Improvements

During the test it was found that the sabot leaked too much, this can also be seen in the slow motion footage. Just before ejection, the oil that was used to reduce friction can be seen spurting out. This is most probably also the reason why 40 bar of inlet pressure was needed.

The current plan is to update the sabot by adding  O-rings. This will give a better sealing, so lower inlet pressure is required. This will have two effects on the exit velocity, firstly the O-rings cause more friction (exactly the reason it was initially decided not to use an O-ring) and this will lower the exit velocity. On the other hand there is also less leakage, so this might increase the exit velocity again. It will be seen in the next tests which one has a bigger effect.