T 0377/96 (Rocket propellant/ESA) 13-01-1999

1. The appeal is admissible.

2. Main request

2.1. The Board considers, in agreement with both parties, that D1 represents the closest prior art. This document relates to high energy binder type composite propellants. D1 proposes to replace the known high energy polymeric binder GAP with a modified form of said polymer in order to improve the mechanical properties of the propellant. Such propellants contain an oxidizing agent. Five such agents are mentioned, among which AP and HMX are mentioned in the first place (page 9). The propellants may also contain fuel compounds (combustion improvers) such as Al and B (page 10). Propellants containing modified GAP have been compared with propellants containing unmodified GAP. Specifically disclosed are comparative examples 12 and 13, comprising GAP as an energetic binder, a mixture of AP and HMX as oxidizer and Al as fuel compound. Of the comparative examples, example 12 has the highest specific thrust (239 kgf.s/kg; Table 3). The specific thrust was measured on a standard small-size rocket motor with an adjusted pressure of about 5 MPa and a nozzle opening ratio of 6 (page 15; the cited pressure of 50 kgf/m2 must be a clerical error and should read 50 kgf/cm2).

In respect of this state of the art the problem existed to provide a solid propellant combination for a rocket engine having an improved specific impulse. The patent in suit proposes to solve this problem by the solid propellant combinations according to claim 1. The respondent disputed that this technical problem had in fact thereby been solved, since the appellant had failed to present an exact comparison with the propellant representing the closest state of the art. In the Board's judgment, however, a comparison under exactly the same conditions, which in many cases is not possible without undue burden, is not an absolute requirement for accepting that the stated problem has actually been solved. It is sufficient that the available evidence shows on the balance of probabilities that it is much more likely than not that the stated problem has been solved. In the present case the specific thrust measured under the conditions set out in D1 cannot be directly compared with the calculated max.Isp for equilibrium flow of the propellant combinations indicated in Table 1 of the patent in suit. These calculations were made on the basis of a pressure of 10 MPa and an opening ratio (Ae/At) of 100. However, the appellant has also calculated the theoretical I(vac) for eq. flow at a pressure of 5 MPa and an opening ratio of 60 for comparative example 12 of D1 and has found a value of 3198.9 m/s (pages 2 and 5 of the technical report filed with the letter dated 12 September 1995). Taking into consideration the same reduction of 92% as used in the patent in suit to get a realistic value for the specific impulse this amounts to a max.Isp at eq. flow of 2943.0 m/s. This value is 84.4 m/s lower than the corresponding value for the AP/Al/GAP propellant given in Table 1 of the patent in suit. Since the conditions for the calculations of the comparative examples of D1 (pressure 5 MPa and opening ratio 60) were again not exactly the same as those used in the examples of the patent in suit (pressure 10 MPa and opening ratio 100) the comparison is still not exact. Nevertheless, taking into account that the calculated difference between the two I(max) values is not negligible, as has been pointed out by the appellant, the Board finds it reasonable to assume, on the balance of probabilities, that a calculation on the basis of identical pressure and opening ratio would also have resulted in a higher specific impulse for the composition according to the patent in suit. In this situation it is not sufficient for the respondent simply to state that the comparison was not made on exactly the same basis, without providing evidence or at least sound arguments to the effect that for the comparative examples of D1 a substantially different value would have been obtained if the calculations had been made under the conditions mentioned in the patent in suit. The respondent has, however, not provided such evidence. With the comparative examples comprising boron as fuel compound, filed with the statement of grounds of the appeal, the appellant has shown that boron containing propellants according to the patent in suit have a higher specific impulse than boron containing propellants with a HTPB binder known in the art. The Board therefore accepts that propellants according to present claim 1 have a higher specific impulse than comparable propellants, based on the same fuel compound, according to D1 and is thus satisfied that the stated problem underlying the invention has actually been solved.

2.2. It remains to be decided whether it was obvious to a person skilled in the art to solve the said problem by providing a propellant combination according to claim 1. In this respect, the Board considers that, although in D1 AP and HMX were treated as equivalent oxidizing agents, it was known in the art that they were in fact not completely equivalent. D2, related to solid composite propellants comprising Polytaz (condensated triaminoguanidinium azide) as binder, not only discloses that AP and HMX are suitable oxidizing agents in combination with Al but also that AP together with NP and HNF has been found to be particularly effective oxidizers for use in the formulation of high energy compositions (column 2, lines 5 to 22). Polytaz is, like GAP, a high energy, azide containing polymer.

On the basis of this disclosure a skilled person would therefore have had good reasons to expect that AP, NP and HNF would be more effective oxidizers than HMX also in combination with GAP and Al. It was therefore obvious to a skilled person trying to improve the specific impulse of compositions based on Al and GAP to calculate the specific impulse of such compositions with any oxidizer chosen from said preferred group of AP, NP and HNF, since, as indicated in the description of the patent in suit and confirmed during oral proceedings, a computer programm enabling the skilled person to calculate the specific impulse of any propellant composition in dependence on the rocket geometry and the combustion conditions belonged to the state of the art. More particularly, the skilled person trying to improve the performance of the propellant according to comparative example 12 of D1, which already contained AP as an oxidizer, was provided with a clear incentive to calculate the specific impulse for an analogous composition wherein the HMX was completely replaced by AP. The improved specific impulse thereby found by the appellant is therefore no more than a confirmation of the skilled person's reasonable expectation by a routine calculation.

2.3. In this context, the Board is unable to agree with the appellant's argument that a skilled person would have been prevented from performing this calculation by the fact that comparative example 12 of D1 had the highest specific thrust, so that he would not have expected that a further improvement could be obtained by modifying the composition. Said example is only a comparative example which happens to have the highest specific thrust of the comparative examples tested in D1. It cannot be inferred from this fact that the composition of comparative example 12 of D1 was disclosed as having the highest possible thrust of compositions comprising Al and GAP. Therefore, D1 did not suggest that the calculations mentioned in the preceding paragraph would be useless, or that they were unlikely to give the desired result.

The subject matter of claim 1 lacks therefore an inventive step within the meaning of Article 56 EPC.

3. Auxiliary request

Claim 1 of the auxiliary request differs from claim 1 of the main request by limiting the propellant combination to more specific compositions. One of these compositions comprises 57-67% AP, 23% Al and 10-20% GAP. For the reasons given above, it was obvious to the skilled person trying to improve the specific impulse of propellant combinations to investigate in detail compositions of AP, Al and GAP, and to calculate the specific impulse of such compositions. There is no evidence, nor did the appellant argue, that it required more than routine work to select, after having selected the desired components, the optimal amounts indicated in claim 1 as a result of performing routine calculations. Furthermore, in the calculations made in D2 for similar compositions comprising Al and AP, these compounds were taken in the range of 8 to 30% and 38 to 65% respectively (Table VI, runs 29 to 38), thereby providing the skilled person with additional guidance in respect of suitable concentration ranges. Thus at least one of the compositions according to claim 1 lacks an inventive step.

4. It follows from the Board's considerations set out in point 2.1 above that it was accepted that an improved specific impulse was in fact obtained. It was therefore not necessary to give the appellant an opportunity to provide further evidence, more convincingly demonstrating such an improvement. The request for the opportunity to provide further evidence is therefore refused.