Solid Rocket Motors

Solid propellant rocket motors (SRM) offer some unique capacities, that are either hard or impractical to archive with other means of rocket propulsion.

CS uses solid propellants in a few highly specialized applications where instant firing, high thrust to weight ratio and compact design makes SRM the ideal means of propulsion. The most noticeable of these uses is the LES or Launch Escape System motor.

SRM technology dates back to the middle ages, but modern day high energy composite propellants is a relatively new invention.

Ancient SRM propellants were always compressed powders, like black powder. This has a very low specific impulse ( 70 – 110 sec ) and, most significantly, cannot be scaled up due to the very poor mechanical properties of compressed powders.

Amateurs have made great use of sugar / potassium nitrate propellants. This form of cast solid propellant offers far better mechanical performance over the powders, but still, when upscaling above a certain size, the brittle nature of molten sugar makes such motors unreliable. Also, the ISP of potasium nitrate propellants are in the 80 – 140 sec range.

The first type of true composite propellant to see extensive use was the potassium perchlorate / oil / asphalt type fuels. In the 1940ties and 1950ties this simple type of composite propellant saw extensive use in Jet Assisted Take Off rockets, or JATOs. These were used to increase the payload of heavy bombers and transport aeroplanes. The propellant was known as Galcit, and hundreds of tonnes was produced and consumed in JATOs of all sizes.

Boeing_B-47B_jato

One of Strategic Air Command´s B-47 nuclear bombers during JATO assisted take off.

 

One of the most powerfull galcite JATOs ever designed allowed the US Air Force to launch a Super Sabre jet fighter with no runway, right off the back of a truck.

North American F-100D

Jet fighter in zero length take off on the worlds most powerful Galcite SRM.

 

The use of JATOs required the propellant to be sturdily reliable, as it was a manned application, and a single JATO exploding would likely take the plane it assisted with it. Within a certain temperature range, asphalt oil type fuels provide exactly that. However, by replacing the potasium perchlorate with ammoniumperchlorate, and the asphalt oil
fuel/binder with polymers, the rocket engineers could manufacture better propellant grains. These new propellants could be used from the hot deserts of California to the freezing cold of the South pole. So Galcite went out of use in the mid 1950ties.

To the amateur however, it is another story. Galcite can be cast with just about the same equipment as potassium nitrate / sugar propellants. Unlike modern day propellants, it can be done with no vacuum, and the molten fuel can be poured, much like cement, into a mold and left to cool. The mechanical properties are excellent in the range form +5C to +35 C, although Galcite JATOs was used in a much larger temperature range.

In Europe ammonium perchlorate is very scarce, as it has practically no commercial use. This makes Galcite even more interesting, because potasium perchlorate can be obtained with much less trouple.

CS has investigated Galcite, and found it to be both reliable and fairly easy to work with:

Further tests have confirmed our belief in the revival of this simple propellant. During these tests, a very high level of reproducibility has been measured, and ISP in the 185 sec range is common. Given the high density of Galcite compared to ammonium perchlorate propellants, its performance is almost comparable.

As of now, a large mixer is under construction and a 70 kg propellant LES SRM is planned.