VACUUM INSULATED BLAST TUBE

Abstract

A vacuum insulated blast tube includes an insulating layer of a burn resistant material such as phenolic resin formed on an interior surface of the blast tube to provide the necessary erosion and thermal insulation properties to protect the blast tube and a vacuum insulated layer formed in the walls of the blast tube itself to provide the additional thermal insulation properties to protect any non-propulsive sub-systems positioned in the void space around the blast tube. A void space in the walls of the blast tube is held under vacuum with a pressure of less than 25 Torr and a thermal conductivity Tcond_vac of less than one-third of the thermal conductivity of air Tcond_air to form the vacuum insulation layer.

Claims

1. A vacuum insulated blast tube, comprising: a hollow blast tube shell defining a void space therein along a length of the shell; and an insulating layer formed of a burn resistant material on an interior surface of the shell, wherein the void space is held under vacuum with a pressure of less than 25 Torr and a thermal conductivity Tcond_vac of less than one-third of the thermal conductivity of air Tcond_air to form a vacuum insulation layer.

2. The vacuum insulated blast tube of claim 1, wherein the pressure in the void space is a medium vacuum between 25 Torr and 10.sup.3 Torr.

3. The vacuum insulated blast tube of claim 1, wherein the pressure in the void space is a high vacuum between 10.sup.3 Torr and 10.sup.6 Torr.

4. The vacuum insulated blast tube of claim 1, wherein Tcond_vac is less than one-fifth Tcond_air.

5. The vacuum insulated blast tube of claim 1, wherein Tcond_vac is approximately 1/6.6 that of Tcond_air.

6. The vacuum insulated blast tube of claim 1, wherein Tcond_vac is less than one one-hundredth the thermal conductivity of the burn resistant material.

7. The vacuum insulated blast tube of claim 1, wherein the burn resistant material is just thick enough to withstand erosion and elevated temperatures of hot gasses passing therethrough due to the combustion of propellant to protect the blast tube itself, wherein the vacuum insulation layer provides additional thermal insulation to achieve a total thermal insulation.

8. The vacuum insulated blast tube of claim 7, wherein the burn resistant material provides at most 40% of the total thermal insulation and the vacuum insulation layer provides at least 60% of the total thermal insulation.

9. The vacuum insulated blast tube of claim 8, wherein the burn resistant material comprises a phenolic resin.

10. A vacuum insulated blast tube, comprising: a hollow blast tube shell defining a void space therein along a length of the shell; and an insulating layer of phenolic resin having a thermal conductivity Tcond_phenolic on an interior surface of the shell, wherein the void space is held under vacuum with a pressure of less than 10.sup.3 Torr and a thermal conductivity Tcond_vac of less than one-fifth of the thermal conductivity of air Tcond_air to form a vacuum insulation layer, wherein Tcond_vac is less than one-two hundredth the thermal conductivity of the phenolic resin Tcond_phenolic.

11. The vacuum insulated blast tube of claim 10, wherein the insulating layer is just thick enough to withstand erosion and elevated temperatures of hot gasses passing therethrough due to the combustion of propellant to protect the blast tube, wherein the vacuum insulation layer provides additional thermal insulation to achieve a total thermal insulation.

12. The vacuum insulated blast tube of claim 1, wherein the insulating layer provides at most 40% of the total thermal insulation and the vacuum insulation layer provides at least 60% of the total thermal insulation.

13. A rocket motor, comprising: a cylindrical rocket motor casing having a diameter D1; a combustion chamber including a propellant positioned in the cylindrical rocket motor casing; a nozzle positioned aft of the combustion chamber to expel hot gases resulting from combustion of the propellant; a vacuum-insulated blast tube that couples the combustion chamber to the nozzle, said blast tube having a diameter D2<D1 that defines a first void space between the blast tube and the rocket motor casing; one or more sub-systems positioned in the first void space, wherein the vacuum-insulated blast tube comprises, a hollow blast tube shell defining a second void space therein along a length of the shell; and an insulating layer formed of a burn resistant material on an interior surface of the shell that erodes as hot gases pass through the blast tube and protects the blast tube from elevated temperatures, wherein the second void space is held under vacuum with a pressure of less than 25 Torr and a thermal conductivity Tcond_vac of less than one-third of the thermal conductivity of air Tcond_air to form a vacuum insulation layer to provide additional thermal insulation to thermally insulate the one or more sub-systems.

14. The rocket motor insulated blast tube of claim 13, wherein the pressure in the void space is a medium vacuum between 25 Torr and 10.sup.3 Torr.

15. The rocket motor insulated blast tube of claim 13, wherein the pressure in the void space is a high vacuum between 10.sup.3 Torr and 10.sup.6 Torr.

16. The rocket motor insulated blast tube of claim 13, wherein Tcond_vac is less than one-fifth Tcond_air.

17. The rocket motor insulated blast tube of claim 13, the burn resistant material comprises a phenolic resin.

18. The rocket motor insulated blast tube of claim 13, wherein the burn resistant material is just thick enough to withstand erosion and elevated temperatures of hot gasses passing therethrough, to protect the blast tube wherein the vacuum insulation layer provides additional thermal insulation to achieve a total thermal insulation required to protect the one or more sub-systems.

19. The rocket motor insulated blast tube of claim 18, wherein the burn resistant material provides at most 40% of the total thermal insulation and the vacuum insulation layer provides at least 60% of the total thermal insulation.

20. The rocket motor insulated blast tube of claim 13, wherein the burn resistant material comprises a phenolic resin.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG. 1, as described above, is an embodiment of a known insulated blast tube design in which a layer of phenolic resin is formed on an inner surface of the blast tube of sufficient thickness to protect both the blast tube and any non-propulsive sub-systems are positioned in a void space around the blast tube;

[0018] FIG. 2 is an embodiment of a vacuum insulated blast tube in which a layer of burn resistant material such as phenolic resin is formed on an inner surface of the blast tube of sufficient thickness to protect the blast tube and a vacuum insulating layer is formed within the blast tube to provide thermal insulation to protect any non-propulsive sub-systems positioned in a void space around the blast tube; and

[0019] FIGS. 3A and 3B are a Table and a plot comparing the thermal conductivity of a vacuum to air and phenolic resin.

DETAILED DESCRIPTION

[0020] Referring back to FIG. 1, the insulated blast tube uses a layer of phenolic resin to provide both the erosion and high temperature protection for the blast tube itself and the additional thermal insulation required to protect non-propulsion sub-systems such as CAS or TVC positioned in the void space around the blast tube inside rocket motor casing.

[0021] Phenolic resin is selected for its resistance to erosion or corrosion in the presence of very hot combustion gases (e.g., its low char rates). If the insulating material is quickly degraded and removed it is of no value. However, phenolic resin is not a very good insulating material. Its thermal conductivity is approximately 1 to 1.5 W/mK (Watts per Meter-Kelvin) whereas air is approximately 0.026 W/mK. The thermal conductivity of the resin is at least 38 that of air.

[0022] As a result, to provide the additional thermal insulation required to protect the non-propulsive sub-systems the insulating layer of phenolic resin must be much thicker e.g., 4-5, what it would otherwise be to just protect the blast tube. This additional thickness has two drawbacks. First, it adds weight to the blast tube and overall rocket motor. The additional weight can affect the position of the Cg in flight and may require additional weight forward on the rocket motor as counterbalance. Second, the increased thickness necessarily increases the diameter D2 of the blast tube, which in turn reduces the volume of void space available to accommodate non-propulsive sub-systems.

[0023] The present disclosure provides an insulated blast tube in which an insulating layer of a burn resistant material such as phenolic resin is formed on an interior surface of the blast tube to provide the necessary erosion and thermal insulation properties to protect the blast tube and a vacuum insulated layer is formed in the walls of the blast tube itself to provide the additional thermal insulation properties to protect any non-propulsive sub-systems positioned in the void space around the blast tube. The vacuum insulated layer being a far better thermal insulator than any burn resistant material or air.

[0024] For equivalent rocket motor designs and thermal insulation requirements, the vacuum insulated blast tube is lighter weight and has a smaller diameter D2. Less weight is generally desirable and specifically in a blast tube to ensure that the center of gravity (Cg) of the rocket in flight is properly positioned relative to the center of pressure (Cp). A smaller diameter produces a larger volume of void space to accommodate non-propulsive sub-systems. For like designs, the vacuum insulated blast tube will be at least 10% lighter and have a diameter D2 that is at least 20% smaller.

[0025] As shown in FIG. 2, an embodiment of a vacuum insulated blast tube 200 includes a hollow blast tube shell 202 defining a void space 204 therein along a length of the shell. An insulating layer 206 formed of a burn resistant material (e.g., phenolic resin) is formed on an interior surface of the shell 202. The thickness 207 of insulating layer 206 is selected to provide the required erosion protection and thermal insulation to protect the blast tube shell 202. The void space is sealed and held under vacuum with a pressure of less than 25 Torr (assuming operation of the rocket at or near sea level and room temperature of 25 C) and a thermal conductivity Tcond_vac of less than one-third of the thermal conductivity of air Tcond_air to form a vacuum insulation layer 208 to provide the additional thermal insulation required to protect non-propulsive sub-systems positioned in the void space around the blast tube. Typically, the thickness 207 is only sufficient to protect the blast tube shell 202, plus some safety margin, and does not appreciably contribute to the additional thermal insulation required for the sub-systems. To compare, thickness 207 of insulating layer 206 may be less than or even the total thickness of the insulating layer 120 shown in FIG. 1 in which the phenolic resin provided the insulation for both the blast tube and the sub-systems.

[0026] In different embodiments, the vacuum may be a medium vacuum between 25 Torr and 10.sup.3 Torr or a high vacuum between 10.sup.3 Torr and 10.sup.6 Torr. A higher vacuum (lower pressure) corresponds to less particles (contaminants) in the vacuum and thus a lower thermal conductivity. Tcond_vac may be less than one-fifth the Tcond_air and reach a limit of approximately 6.6 less than air. Tcond_vac is about one two-hundred and fiftieth ( 1/250) that of phenolic resin and thus a much better thermal insulator. As a result, the thickness of the vacuum insulation layer can be significantly less than that of the additional phenolic resin that would otherwise be required to provide the additional thermal insulation.

[0027] In an embodiment, the burn resistant material is just thick enough to withstand erosion and elevated temperatures of hot gasses passing therethrough due to the combustion of propellant to protect the blast tube. The vacuum insulation layer provides additional thermal insulation to achieve a total thermal insulation to protect the non-propulsion subsystems. For example, in all blast tube designs the burn resistant material will provide at most 40% of the total thermal insulation and the vacuum insulation layer provides at least 60% of the total thermal insulation. In many blast tube designs the burn resistant material will provide at most 25% and the vacuum insulation layer at least 75% of the total thermal insulation.

[0028] Referring now to Table 300 of FIG. 3A and a plot 302 of the relative thermal conductivity of a vacuum to air in FIG. 3B, at sea level and a temperature of 25 C, air has a thermal conductivity of approximately 0.026 W/mK, phenolic resin between 1 and 1.5 W/mK and a high vacuum of approximately 0.004 W/mK. As defined herein, a low vacuum is between 760 Torr (1 atm) and 25 (Torr), medium vacuum between 25 Torr and 10.sup.3 Torr or a high vacuum between 10.sup.3 Torr and 10.sup.6 Torr. Under low vacuum the difference in thermal conductivity with air is insufficient to provide the requisite thermal insulating benefits given the requirement to form and hold a vacuum. At medium and high vacuum, the thermal conductivity of the vacuum is less than , preferably less than and ideally approximately 1/6.6 that of air. This provides a substantial thermal insulating benefit over air, and a very substantial thermal insulting improvement over phenolic resin.

[0029] While several illustrative embodiments of the disclosure have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the disclosure as defined in the appended claims.