The common wisdom is that solid rcket motors cannot be throttled, cannot be stopped. This is in fact wrong… with effort, the level of thrust can be altered at will, and the combustion even stopped. The best way to do this is to change the port area. A common approach to doing that has been to put a plug in the throat; by moving the plug fore or aft, the effective throat area can be increased or decreased. To shut the motor down, the plug needs to be moved very quickly, rapidly increasing throat area. A dP/dT (change in pressure over time) of around 100,000 psi per second is generally required to extinguish a solid rocket.
Several American solid rocket companies – Aerojet, Thiokol, Hercules, United Tech – built demo versions in the 1960’s. The goal was to build an upper stage for the Minuteman ICBM… the restartable motor would be able to make the trajectory of the warhead almost completely unpredictable, and thus extremely difficult to intercept.
I’ve made copies of several articles on these motors over the years… articles I keep misplacing. So when I stumbled across this one, I scanned it in. Enjoy.
5 Responses to “Restartable solid rocket motor”
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That’s one mighty impressive ignition system filling the bottom of the grain bore inside the nozzle, isn’t it? 🙂
I went past that concept within _days_ of trying out my first high-thrust solid-fueled homemade rocket engine.
No, you light it from the _top_ of the central bore and keep the whole nozzle throat clear from anything that might cause overpressure as it’s ejected via its mass and inertia.
Pat,
That’s the variable throat actuator. The ignightors are at top of the unit, the ‘bore’ is a ring in the fuel to take into account hydrolics.
This just adds to the list of fundimental design faults that are part of the STS.
Well, female wisdom would have you deliver the chemicals separately like in a liquid fuel rocket motor. Powder, yes, but it is then stoppable and throttleable. (sp) I have a friend who makes these for computerized remote controlled model rockets that have quite the range and with a gimble, – maneuverability.
> female wisdom would have you deliver the chemicals separately like in a liquid fuel rocket motor. Powder, yes…
That’s been tried and abandoned. Spraying in just the dry powder (such as ammonium perchlorate oxidizer and aluminum fuel), gives you all the performance of a solid (translation: meh), with all the complexity of a liquid, and then some. Dry powder spraying is complex, heavy and unreliable compared to liquid injection.
The alternative is to create a slurry… a mix of the powdered propellants with a liquid. Somethign like aluminum powder in oil. Perfornace can be quite impressive… the theoretically best-performaing chemical rockets are slurries. But these also have problems; high performance turbopumps *really* don’t like grit.
Very cool. Though how I got through a 400-level rocket propulsion class without hearing about these I will never know.
“> female wisdom would have you deliver the chemicals separately like in a liquid fuel rocket motor. Powder, yes…
That’s been tried and abandoned. Spraying in just the dry powder (such as ammonium perchlorate oxidizer and aluminum fuel), gives you all the performance of a solid (translation: meh), with all the complexity of a liquid, and then some. Dry powder spraying is complex, heavy and unreliable compared to liquid injection.
The alternative is to create a slurry… a mix of the powdered propellants with a liquid. Somethign like aluminum powder in oil. Perfornace can be quite impressive… the theoretically best-performaing chemical rockets are slurries. But these also have problems; high performance turbopumps *really* don’t like grit.”
I have wondered about both of these concepts in the past. Thanks for discussing the results 🙂