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Thursday, December 28, 2017

Altitude compensation attachments for standard rocket engines, and applications, Page 6: space shuttle tiles and other ceramics for nozzles.

Copyright 2017 Robert Clark


Some possibilities for altitude compensating nozzles include actual adjustable-sized nozzles but using non-flexible materials such as ceramics. In such cases the high expansion ratios needed for optimized expansion at high altitude would give nozzles of impractical size, for instance they wouldn't fit within the width of the rocket stage.

A couple of possibilities for dealing with this eventuality:



And:



Vacuum optimized nozzles, whether altitude compensating or not can take a great deal of mass of an engine or stage. See for example the specifications of the Star 48B here:



You see the weight of the nozzle assembly is nearly that of the case assembly. This would become even more of a weight problem with extreme expansion ratios of hundreds to one. 

 The lightweight space shuttle underside tiles may provide a solution. Their volume density is only 0.144 gm/cm^2. And according to this report their areal density is 1.19 gm/cm^2:

TPS Materials and Costs for Future Reusable Launch Vehicles.

 The AETB-8/TUFI listed is a toughened tile material that has higher impact resistance while maintaining the same temperature resistance.

 Judging from the size of the size of the Star 48B nozzle, the nozzle weight might be reduced from 90 lbs to ca. 20 lbs using the AETB-8/TUFI tiles.

 A possibly even more lightweight material was developed by aerospace engineer/mathematian GW Johnson. He described it in this video presentation at the 16th Mars Society conference in 2013:

Reusable Ceramic Heat Shields - GW Johnson - 16th Mars Society Convention.

 Johnson estimated the volume density as only 0.03 gm/cm^3, a third that of the shuttle tiles. Also interesting is that Johnson originally developed the material to act as insulation for ramjet combustion chambers. 

 Note that even the insulation in the Star 48B solid motor is a sizable weight at 60 lbs, compared to the 129 lbs casing weight. Then Johnson's ceramic might also be able to be used as a lightweight replacement for solid motor insulation.

  Bob Clark 

5 comments:

  1. Hi Bob:

    If you glom onto the fiber reinforcing correctly, my low density ceramic composite stuff has promise as an external heat shield for atmospheric entry, but only if it is black enough to radiate efficiently (something the raw materials manufacturer says is possible).

    As a ramjet insulator, what I took advantage of was a very low thermal conductivity that associates with the low density. The ramjet application was essentially restricted to 3200-3300 F gas temperatures, something feasible only for lean mixtures or very limited Mach number, if full strength mixtures (subsonic).

    But, that low density also associates with low strength and high porosity, something incompatible with exposures to high pressures, especially high pressure increase rates. I would never expose it to a rocket booster environment. Too much too fast; I think that would crush the material. Pressures are far lower way out in a nozzle bell, though.

    Just things to be aware of. -- GW

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    Replies
    1. In your tests for ramjet combustion chamber insulation how much pressure was your material exposed to, both in reusable and ablative scenarios?

      Bob Clark

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  2. What requirements other than temperature resistance would be needed for nozzle materials?

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  3. Such a light material... surely has low strength? How does that factor into the mass of the nozzle required?
    Have there been previous examples of TPS material used in nozzles?

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