Discussion on SRM slag requirement and proposed solution ISO/TC20/S14/WG3 & ODCWG

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. (1) Comments from September –It is not feasible to comply with the requirement. Unfeasible restriction shall be transferred to a Technical Report. (2) Comments from September 2008 –There is great controversy over whether 1 mm is feasible or practical or even necessary. Recommend making this requirement more generic and that ultimate resolution be achieved when the supporting standard becomes a work item. (US.DF) –Section is unacceptable to the U.S. since none of our solid rocket motors can meet this constraint. Note that Reference 13 here is a NASA document and is inappropriate to be used in this context and should be removed.(US.WA)

2. Views on the feasibility of requirements (1)View of Japan 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]

2.1 French view in detail 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.

2.2 Japanese view on nozzle design in detail 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 ]

2.3 Japanese view on propellant in detail 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 agent effective 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. Table – 1 Degradation of performance in the cases that eliminate Aluminum PropellantCompositionTemperatureIsp Ordinal propellant with Al 20 ％ Al Case1: Propellant without Al 12 ％ HTPB, 88 ％ AP Case2: Replace Al by RDX12%HTPB, 68%AP, 20%RDX Case3: Replace Al by XDX12%HTPB, 68%AP, 20%HDX Remark: Combustion Pressure: 5MPa, Area Ratio: 100

3. What is the practical threshold to limit using solid motor? Summary of views USFranceJapan Causes of slag Slag is produced in the cavity of the SRMAdding to the left, At the end of motor operation, unburn Al particles will be accumulated at the burning surface and form large unburnt particles. Effective preventive measures The nozzle throat should be external. Metal (Aluminum) should be replaced by Octogen, Hexogen, HMX or similar. The proposed nozzle design can not eliminate slag. Performance of Almi-less propellant will not satisfy us, or any bonding agent is not effective. Practical actions （ Current practice) Making this requirement more generic and that ultimate resolution be achieved when the supporting standard becomes a work item More practical requirement. Detail technical Report to be developed to encourage the awareness of solid motor issue. Proposed change for description in STD Accretions of solid combustion products be avoided and particle size controlled to avoid further contaminating the near Earth space environment. See “Conclusion”

3. What is the practical threshold to limit using solid motor? Summary of views UK other delegations Causes of slag Effective preventive measures Practical actions （ Current practice) Proposed change for description in STD Solid rocket motors shall avoid releasing any products larger than 1 mm in their largest dimension that will remain in the protected regions for more than 25 years. (to be proposed)

4. Conclusion 4.1 Summarized situations To limit the generation of slag completely is difficult under the current technology level. Some improvement could be possible in future to some extent. There are demands to launch payloads by low cost solid launch vehicles, and some nations may select development of such vehicles in their national strategy, or find business chances to provide such launching service. ISO shall find the balance point between the preservation of the orbital environment and still promoting space business without imposing excessive burden. But some operational constrains may be necessary. GEO protected region, where natural force to clean up the environment can not be expected, shall be respected. The risk in LEO is relatively small, because small slag tends to re- enter within not so long future (ex. 1 cm slag in 800 km altitude will re-enter within 25 years), and not all the world solid motor stages will not operated in such high altitude. It will be agreed for ISO to notify the risk of solid motor, and encourage to improve it in design or operation manner.

4.2 Proposed change Combined description among US, UK and JPN proposals 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) CountryDecision (comment)CountryDecision (comment) BrazilJapanAgree CanadaRussia ChinaUkraine FranceUnited Kingdom GermanyUnited States Italy