Axial expandable exhaust duct

1. An exhaust duct for an engine, the exhaust duct comprising:

an outer exhaust duct;

a nested exhaust duct capable of having at least two configurations, wherein the nested exhaust duct is circumferentially surrounded by the outer exhaust duct for a first length of the nested exhaust duct in a first configuration and wherein the nested exhaust duct is circumferentially surrounded by the outer exhaust duct for a second length of the nested exhaust duct in a second configuration, wherein the second length is less than the first length;

an integrally formed key; and

an integrally formed keyway;

wherein the integrally formed key is located away from the integrally formed keyway in the first configuration, the integrally formed key is located within the integrally formed keyway in the second configuration, and one of the integrally formed key and the integrally formed keyway is on an interior surface of the outer exhaust duct.

2. The exhaust duct of claim 1 , wherein the integrally formed key is on the interior surface of the outer exhaust duct.

3. The exhaust duct of claim 2 , wherein the integrally formed keyway is on an exterior surface of the nested exhaust duct.

4. The exhaust duct of claim 1 and further comprising an integrally formed nested lip on an exterior surface of the nested exhaust duct.

5. The exhaust duct of claim 4 and further comprising a spring attached to the nested lip and circumferentially conformal with the nested exhaust duct.

6. The exhaust duct of claim 5 , wherein the spring is uncompressed in the first configuration and wherein the spring is compressed in the second configuration.

7. The exhaust duct of claim 6 , wherein the compressed spring is configured to exert a compressive force in order to retain the key within the keyway during operation of the engine.

8. The exhaust duct of claim 7 and further comprising an integrally formed arresting face on the interior surface of the outer exhaust duct.

9. The exhaust duct of claim 8 , wherein the integrally formed arresting face is located away from the spring in the first configuration and wherein the integrally formed arresting face is in contact with the spring in the second configuration.

10. The exhaust duct of claim 1 , wherein the first length is equal to an axial length of the nested exhaust duct.

11. A method of manufacturing an axially extendable exhaust duct for an engine, the method comprising:

manufacturing an outer exhaust duct;

manufacturing a nested exhaust duct capable of having at least two configurations, wherein the nested exhaust duct is circumferentially surrounded by the outer exhaust duct for a first length of the nested exhaust duct in a first configuration and wherein the nested exhaust duct is circumferentially surrounded by the outer exhaust duct for a second length of the nested exhaust duct in a second configuration, wherein the second length is less than the first length; and

manufacturing an integrally formed key and an integrally formed keyway;

wherein the integrally formed key is located away from the integrally formed keyway in the first configuration, the integrally formed key is located within the integrally formed keyway in the second configuration, and one of the integrally formed key and the integrally formed keyway is on an interior surface of the outer exhaust duct.

12. The method of claim 11 , wherein the integrally formed key is on the interior surface of the outer exhaust duct and the integrally formed keyway is on an exterior surface of the nested exhaust duct.

13. The method of claim 11 and further comprising manufacturing an integrally formed nested lip on an exterior surface of the nested exhaust duct, a spring attached to the nested lip and circumferentially conformal with the nested exhaust duct, and an integrally formed arresting face on the interior surface of the outer exhaust duct.

14. The method of claim 13 , wherein the spring is uncompressed and the integrally formed arresting face is located away from the spring in the first configuration and wherein the spring is compressed and the integrally formed arresting face is in contact with the spring in the second configuration.

15. The method of claim 14 , wherein the compressed spring is configured to exert a compressive force in order to retain the key within the keyway during operation of the engine.

16. The method of claim 15 and further comprising:

extending the nested exhaust duct in the first configuration in an axial direction;

rotating the nested exhaust duct relative to the outer exhaust duct; and

allowing the spring to exert the compressive force to retain the key within the keyway.

17. An axially extendable exhaust duct for an engine, the exhaust duct comprising:

an outer exhaust duct having an integrally formed key on an interior surface of the outer exhaust duct;

a nested exhaust duct with an integrally formed keyway on an exterior surface of the nested exhaust duct, which is capable of having at least two configurations, wherein the nested exhaust duct is circumferentially surrounded by the outer exhaust duct for a first length of the nested exhaust duct in a first configuration and wherein the nested exhaust duct is circumferentially surrounded by the outer exhaust duct for a second length of the nested exhaust duct in a second configuration, wherein the second length is less than the first length; and

a spring attached to and circumferentially conformal with the nested exhaust duct, wherein the spring is uncompressed in the first configuration and wherein the spring is compressed in the second configuration and configured to exert a compressive force in order to retain the key within the keyway during operation of the engine.