Discussion on SRM slag requirement and proposed solution. ISO/TC20/S14/WG3 & ODCWG 2008.10.22. 1. Requirement and Comments from the world. Two major comments have came on the following requirement. Solid rocket motor shall be designed to avoid into the release into Earth.
ISO/TC20/S14/WG3 & ODCWG
Two major comments have came on the following requirement.
(1) Comments from Japan @ September 2008.
(2) Comments from US @ September 2008
There is no practical methods to avoid by design, or verify the phenomena on the ground.
(2) View of United States
There is great controversy over whether 1 mm is feasible or practical or even necessary.
None of our solid rocket motors can meet this constraint
(2) View of France
There are two way to avoid the slag. [see detail in 2.1]
(3) Again, view of Japan
Still the slag can not be stopped generating. [see detail in 2.2]
The technology for proposed combination of propellants have not been matured. [see detain in 2.3]
Two solutions allow to avoid slag.
(1)The nozzle throat should be external, not internal (other wording: emerged vs. submerged).
Slag is produced in the cavity of the SRM when the throat is inside.
(2)Metal (Aluminum) should be replaced by Octogen, Hexogen, HMX or similar; by definition of these changes, there can be no residual of significant size; this is very well known and qualified. Remember that we talk only of particles larger than 1 mm.
Adding to the phenomena introduced in NASA report, another factor exists.
At the end of motor operation, internal pressure drops and the combustion of solid propellant goes very inactive. In such a region, it is very difficult to burn Al particles fully and most of Al particles are accumulated at the burning surface and form large unburnt particles. We always observe many large unburnt Al particles inside the rocket motor after the static firing tests. They can be ejected outside the motor as well as the slag in the cavity you mentioned. It is inevitable, and does not depend upon the nozzle type.
In the space, pressure drops further, and even below the PDL (pressure deflagration limit) of the propellant, remaining propellant* continues to decompose mainly with the radiative heat from the nozzle throat whose temperature is over 3000K. Al particles may not burn and form unburnt particles, which would be ejected outside the motor.
The amount of the slag mentioned above is not large, however, such a possibility cannot be denied and must be considered.
[*depends on the grain design ]
To replace the current propellant with non-metalized solid propellant is possible, however, it is not realistic now.
AP/HMX/HTPB may be one of the candidates, however, if we don’t want compromise the specific impulse of the propellant (see table -1), the concentration of HMX should be high. We can measure the burning rate with a strand burner, we can burn small motor, however, the application to the full-scale motor is another problem.
We don’t have a good bonding agenteffective to both of AP and HMX. We have to conduct aging test of the propellant itself and the bond line between the propellant and insulation with full time scale (we don’t believe accelerated aging test), and so on. We agree with France we should continue the discussion of new propellants, however, it is not the solution now.
6.1.5 Accretions of solid combustion products shall be avoided and particle size shall be controlled to avoid further contaminating the near Earth space environment.
Particularly solid rocket motors shall avoid releasing products remain in the GEO protected regions.
[Technical report may be developed by experts.]
Table-4 Decision of each member countries(expected to be filled in by members)