WE CLAIM:
1. A monolithic tube-in matrix heat exchanger 40, comprising:
a monolithic body 50 having a first fluid inlet 46 and a first fluid outlet 47;
said monolithic body having a second fluid inlet 48 and a second fluid outlet 49;
a first plurality of first fluid tubes 52 formed in said monolithic body and extending between said first fluid inlet and said first fluid outlet;
a second plurality of second fluid tubes 54 formed in said monolithic body and extending between said second fluid inlet and said second fluid outlet;
said monolithic body being formed of a highly conductive metal material which is coated by an oxidation-resistant coating;
at least one diffusion barrier coat on an exterior of said first plurality of first fluid tubes and an exterior of said second plurality of second fluid tubes;
wherein said first plurality of first fluid tubes form a first array which is transverse to said second plurality of second fluid tubes which forms a second array.
2. The monolithic tube-in matrix heat exchanger of Claim 1, wherein said first and second fluid inlets and said first and second fluid outlets are at opposed faces of said monolithic body.
3. The monolithic tube-in matrix heat exchanger of Claim 2, wherein said first and second fluid inlets and said first and second fluid outlets are at non-opposed faces of said monolithic body.
4. The monolithic tube-in matrix heat exchanger of Claim 1, wherein said monolithic body is a polygonal body of at least six sides.
5. The monolithic tube-in matrix heat exchanger of Claim 1, wherein said highly conductive metal is a casting alloy.
6. The monolithic tube-in matrix heat exchanger of Claim 5, wherein said highly conductive metal is one of a copper casting alloy or an aluminum casting alloy.
7. The monolithic tube-in matrix heat exchanger of Claim 1, wherein said highly conductive material is high temperature-resistant alloy.

8. The monolithic tube-in matrix heat exchanger of Claim 7, wherein said first and second plurality of fluid tubes are one of an incoloy alloy, an Inconel alloy, titanium aluminide alloy, stainless steel alloy or refractory metals.
9. The monolithic tube-in matrix heat exchanger of Claim 1, wherein said first and second plurality of fluid tubes are circular in cross-sectional shape.
10. The monolithic tube-in matrix heat exchanger of Claim 1, wherein said first and second plurality of fluid tubes are non-circular in cross-sectional shape.
11. The monolithic tube-in matrix heat exchanger of Claim 10, wherein said first and second plurality of fluid tubes are one of flat oval or lobed star shaped cross-section.
12. The monolithic tube-in matrix heat exchanger of Claim 11, wherein at least one tube of lobed star shaped cross-section is helically twisted along a portion of its length.
13. The monolithic tube-in matrix heat exchanger of Claim 11 further comprising at least one internal fluid turbulation feature 1082.
14. The monolithic tube-in matrix heat exchanger of Claim 1, in which at least one tube of flat oval shape cross-section is helically twisted along a portion of its length.
15. The monolithic tube-in matrix heat exchanger of Claim 1 in which at least one tube cross-section has a non-circular exterior perimeter with a circular interior perimeter.
16. The monolithic tube-in matrix heat exchanger of Claim 10 in which at least one fluid tube cross-section has a non-circular exterior perimeter with a multi-lobed interior perimeter.
17. The monolithic tube-in matrix heat exchanger of Claim 16 containing at least one internal fluid turbulation feature.
18. The monolithic tube-in matrix heat exchanger of Claim 1, wherein at least one of said first and second plurality of fluid tubes have at least one internal flow turbulation feature to increase convection heat transfer.

19. The monolithic tube-in matrix heat exchanger of Claim 18, wherein said at least one internal turbulation feature includes lobes to increase wetted surface area and reduce matrix weight.
20. The monolithic tube-in matrix heat exchanger of Claim 19, further comprising at least one helical internal flow turbulation feature.
21. The monolithic tube-in matrix heat exchanger of Claim 20, wherein said lobes and said at least one helical internal flow turbulation feature turn in the same direction or different directions.
22. The monolithic tube-in matrix heat exchanger of Claim 1, further comprising a plurality of plenums 43, 56 extending from a face of said body.
23. The monolithic tube-in matrix heat exchanger of Claim 22, wherein said plurality of plenums are cantilevered from a face of said body.
24. The monolithic tube-in matrix heat exchanger of Claim 1, further comprising at least one plenum physically separated and cantilevered from four faces of said body.
25. The monolithic tube-in matrix heat exchanger of Claim 1, further comprising at least one plenum extending from four faces of said body.
26. The monolithic tube-in matrix heat exchanger of Claim 1, wherein a single row of said first fluid tubes are disposed between transverse rows of second fluid tubes.
27. The monolithic tube-in matrix heat exchanger of Claim 1, wherein at least two rows of said first fluid tubes are disposed between transverse rows of said second fluid tubes.
28. The monolithic tube-in matrix heat exchanger of Claim 1, wherein an electrical circuit is used to detect at least one of a crack 57formation or growth in said matrix heat exchanger.
29. The monolithic tube-in matrix heat exchanger of Claim 28, wherein said circuit is a wheatstone resistance bridge circuit.
30. The monolithic tube-in matrix heat exchanger of Claim 28, wherein said circuit is a wheatstone capacitive impedance bridge circuit.

31. The monolithic tube-in matrix heat exchanger of Claim 1, wherein an electrical circuit is used to detect fluid leakage from one of a said first plurality of tubes and said second plurality of fluid tubes to the other of said first plurality of fluid tubes and said second plurality of fluid tubes.