135 mm gel-monoprop rocket
 

Hello All,

Pressure (piston)-fed, gel-form *monopropellant* rockets have been developed and even flight-tested (see: https://www.google.com/#q=gel+solid+rocket+propellant - this link leads to numerous others), and they--unlike liquid monopropellant rockets--have proved very safe. They are being studied for everything from artillery rockets to sounding rockets to restart-able launch vehicle upper stages, because they share the trait of burn duration controllability with liquid rocket engines (maybe even gel-propellant model rockets will become available eventually). ALSO:

An existing gel-monopropellant rocket that is covered in one of the above-linked-to websites is a German 135 mm diameter test vehicle. With its "forward-offset," clipped-delta fins (this would reduce drag due to turbulence induced by the fins, giving the flow the opportunity to settle down before leaving the rocket), a scale model of this rocket should also have excellent low-drag characteristics, likely making it a good Scale Altitude contender.

I hope this information will be helpful.