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NERO Rocket motors   Star Grain Motor

Star Grain Motor

1 Design
2 Simulation
3 Static test
4 Conclusion
The SG grain shape was developed to fulfill the needs for a powerful booster motor for larger projects. A typical booster has a burn time of less then one second and therefore a long thin motor was needed with a large initial propellant area.



Design [Top] [Contents]

These specific requirements became attached to a rather new area for the NERO which was the casting of grains by means of a mold. The SG propellant has a grain core in the shape of a five point star with the points rounded. Once the design was made and the test-motor was decided to be a 38mm custom made motor, the mold and the caster could be modeled. Making the mold and the caster is not much work, but it had to be done very precise since there was no CNC equipment available to make the parts automatically. The final product of mold and caster was completely 'tricked' together but it worked nevertheless fine.

The caster is used for several reasons. It gives a platform to work from, it enables you to efficiently degas the propellant and the incorporated jack facilitates extraction of the mold. The heat cord wrapped around the bottom part also gives you the opportunity to preserve a good working temperature which would work fine with a shaker or vibrator to facilitate the degassing of the propellant. Once the grain is properly casted, the vacuum is released until the binder in cured.




Simulation [Top] [Contents]

After the first feeble steps the grain design was send to a Dutch research institute to run a simulation on the grain growth during the burn. Apart from this great picture they send us, they also send a large table with surface numbers for each step of the burn. Each step was then scaled up to the proper dimensions and further processed.


SuperGrain" grow behavior



Static test [Top] [Contents]

The first static test was done before we got the simulation data. A rather crude model was used to predict pressure and thrust. It was intended to keep the test in the SAFE range.

The test was done on the NERO test bench. This computerised testbench is usually fitted with the C* motor, but it readily accepts other motors with standard thread.

Readings from the static test were then iterated to pressure readings, since the relatively small test motor had no pressure sensor slot.

Graph with simulation and measured data (please note that the time scale is not linear). The graph shows two lines:
  1. Simulation is the blue line. It's derived from the computed surface of the grain at each step in the simulation. The surface is then fed into the nozzle ratio equation and finally into the pressure equation.
  2. Measurement data is the red line. It's a pressure curve, iterated from the thrust curve of the first static test of the SG38 motor.

By changing the n and a propellant parameters, both curves can be aligned and the found values for a and n will be used for simulations and computations of other models with the same propellant.

If you feel offended by the name "SuperGrain", please don't. This name was adopted after the first static test and we were all very excited about the result. It will be changed appropriately when time comes.



Conclusion [Top] [Contents]

The I400 SG38 ended up as a fine motor very suitable as a booster. It's high start thrust ensures a good liftoff characteristic and once the motor is burned up, it can be ejected from the main vessel like the Ariane and Space shuttle do.

At this moment the I400 QSG38 is fitted with the Quickstart(tm) ignition system which ignites the SG38 in 400 milliseconds to nearly 90% of its maximum thrust.

The initial "weak" propellant formulation will be substituted for a more powerful one on the next static tests, both to enhance performance and to test material strength.

Various pictures of the SG38 test motor (with thanks to Eadrik Wildeboer for the pictures).

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