Discussion on SRM slag requirement and proposed solution

1
Discussion on SRM slag requirementand proposed
solution

• ISO/TC20/S14/WG3 ODCWG
• 2008.10.22

2
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 Japan _at_ September 2008.
• It is not feasible to comply with the
  requirement. Unfeasible restriction shall be
  transferred to a Technical Report.
• (2) Comments from US _at_ 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 6.1.5 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)

3
2. Views on the feasibility of requirements

• 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

4
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.

5
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

6
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 dont 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 dont 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 dont 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 Table 1 Degradation of performance in the cases that eliminate Aluminum Table 1 Degradation of performance in the cases that eliminate Aluminum Table 1 Degradation of performance in the cases that eliminate Aluminum
Propellant Composition Temperature Isp
Ordinal propellant with Al 20Al 3528.5 331
Case1 Propellant without Al 12HTPB, 88AP 3005.9 304.1
Case2 Replace Al by RDX 12HTPB, 68AP, 20RDX 3061 308.6
Case3 Replace Al by XDX 12HTPB, 68AP, 20HDX 3049.1 307.7
Remark Combustion Pressure 5MPa, Area Ratio 100 Remark Combustion Pressure 5MPa, Area Ratio 100 Remark Combustion Pressure 5MPa, Area Ratio 100 Remark Combustion Pressure 5MPa, Area Ratio 100
7
3. What is the practical threshold to limit using
solid motor? Summary of views
US France Japan
Causes of slag Slag is produced in the cavity of the SRM Slag is produced in the cavity of the SRM Adding 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
8
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)
9
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.

10
4.2 Proposed change Combined description among
US, UK and JPN proposals

• 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)

Country Decision (comment) Country Decision (comment)
Brazil Japan Agree
Canada Russia
China Ukraine
France United Kingdom
Germany United States
Italy