Info

1. When a flight vehicle has reached the desired flight velocity (for a ballistic missile to attain a predetermined velocity or for a satellite to achieve an accurate orbit), or a precise total impulse cutoff is needed.

2. As a safety measure, when it appears that a flight test vehicle will unexpectedly fly out of the safe boundaries of a flight test range facility.

3. To avoid collisions of stages during a stage separation maneuver (requiring a thrust reversal) for multistage flight vehicles.

4. During research and development testing, when one wants to examine a partially burned motor.

The common mechanisms for achieving extinction are listed below and described in Chapters 2, 5, and 6 of Ref. 13-1.

1. Very rapid depressurization, usually by a sudden, large increase of the nozzle throat area or by fast opening of additional gas escape areas or ports. The most common technique neutralizes the thrust or reverses the net thrust direction by suddenly opening exhaust ports in the forward end of the motor case. Such a thrust reversal using ports located on the forward bulkhead of the case is achieved in the upper stages of Minuteman and Poseidon missiles. This is done by highly predictable and reproducible explosive devices which suddenly open additional gas escape areas (thus causing pressure reduction) and neutralize the thrust by exhausting gases in a direction opposite to that of the motor nozzle. To balance side forces, the thrust termination blow-out devices and their ducts are always designed in symmetrically opposed sets (two or more). In Fig. 1-5 there are four symmetrically placed openings that are blown into the forward dome of the case by circular explosive cords. Two of the sheathed circular cord assemblies are sketched on the outside of the forward dome wall. The ducts that lead the hot gas from these openings to the outside of the vehicle are not shown in this figure. The forward flow of gas occurs only for a very brief period of time, during which the thrust is actually reversed. The rapid depressurization causes a sudden stopping of the combustion at the propellant burning surface. With proper design the explosive cords do not cause a detonation or explosion of the remaining unburned propellant.

2. During some motor development projects it can be helpful to see a partially consumed grain. The motor operation is stopped when the flames are quenched by injecting an inhibiting liquid such as water. Reference 13-12 shows that adding a detergent to the water allows a better contact with the burning surface and reduces the amount of water needed for quenching.

3. Lowering the combustion pressure below the pressure deflagration limit. Compared to item 1, this depressurization occurs quite slowly. Many solid propellants have a low-pressure combustion limit of 0.05 to 0.15

MPa. This means that some propellants will not extinguish when vented during a static sea-level test at 1 atm (0.1 MPa) but will stop burning if vented at high altitude.

A sudden depressurization is effective because the primary combustion zone at the propellant surface has a time lag compared to the gaseous combustion zone which, at the lower pressure, quickly adjusts to a lower reaction rate and moves farther away from the burning surface. The gases created by the vaporization and pyrolysis of the hot solid propellant cannot all be consumed in a gas reaction close to the surface, and some will not burn completely. As a result, the heat transfer to the propellant surface will be quickly reduced by several orders of magnitude, and the reaction at the propellant surface will diminish and stop. Experimental results (Chapter 12 of Ref. 13-1) show that a higher initial combustion pressure requires a faster depressurization rate (dp/dt) to achieve extinction.

Was this article helpful?

0 0
Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

Get My Free Ebook


Post a comment