An effective heat shield to shield to protect the deck fromthe turbo’s radiative heat can be made from a single layer of Inconel 718 sheetor foil. Inconel 718 is widely used forhigh temperature thermal insulation and particularly on commercial and militaryjet aircraft engines. Heat shields can be a single layer of .012 thick Inconel718 foil, or any thickness of Inconel718 sheet can be used.
The basic approach is to place a conditioned sheet ofInconel 718 between the hot turbo and the deck lid. Simply placing anunconditioned sheet of any metal - with the same shiny surfaces on both sides -between the turbo and the deck lid will help some, but not a lot. The trick isto condition both surfaces of the Inconel sheet in a way that results in mostof the heat absorbed by the heat shield to be radiated back toward the turboand very little heat radiated toward the deck lid.
Pardon a small bit of theory: Different surfaces willradiate heat differently. A rough black cast iron burner on the stove will radiateheat much better than the shiny chrome surface of a clothes iron. If you placeyour hand over a cast iron burner and place the other hand over a clothes iron(with both at the same temperature) the cast iron burner will heat your handvery quickly compared to the clothes iron, because the cast iron is emitting orradiating more heat. The measure of a surface’s ability to radiate or emit heatis called “emissivity”. The value of emissivity is between 0 and 1 with 0 beingthe worst, and 1 being the best or the ideal radiator. In short – the heatshield surface facing the turbo should have a high emissivity (close to 1) soit can radiate bheat back out toward the turbo. Conversely, the surface facingthe deck should have a low emissivity (close to zero) so that it radiates verylittle heat toward the deck.
Here is a description of the process used to create Inconel718 heat shield surfaces:
A sheet of inconel 718 can be suspended in a common gasfired ceramics kiln and fired to a temperature of about 1800-2300 F with the naturalgas burners turned a bit rich to create an oxidizing atmosphere. (I used thekiln at my local junior college art/ceramics class.) The heating in anoxidizing atmosphere will create a permanent oxide layer on both surfaces ofthe inconel sheet much like anodize creates a permanent oxide layer on thesurfaces of an aluminum sheet. The emissivity of both oxidized surfaces will bethe same at a very high .95, but because both sides are the same emissivity notmuch has been gained relative to protecting the deck.
The heat shield surface facing the deck can be conditionedby coating it with a thin layer of gold. Gold is a lousy emitter of heat with avery low emissivity of .05. The ceramic industry has a material called “LiquidBright Gold” that is a gloss black liquid that contains flakes of gold metal. (Itis available at any ceramic art store.) Liquid Bright Gold is used to createthe gold on ceramic substrates (like the gold on the rim of a ceramic/chinadinner plate), and it is simply painted on the substrate, allowed to dry, andthen fired in a kiln. At elevated temperature, the gloss black liquid acts as aflux and also promotes adhesion of the gold to the substrate. The gloss blackcarrier eventually burns off and a thin layer of pure gold with a matt finishis left on the surface.
If the surface of the oxidized inconel that faces the deck iscovered with gold (.05 emissivity), then very little heat will radiate fromthat surface. If the opposite surface that faces the turbo is inconel oxide(.95 emissivity), then the sheet of inconel will heat up, but it will radiatealmost all of it’s heat back toward the turbo. In rough terms, the heat shieldwill radiate 95% of it’s heat back at the turbo, and 5% of it’s heat toward thedeck.
Because the oxide layer is actually part of the parentmaterial, and the gold is fused to the oxide, both surfaces are very robust andwon’t peel, flake, or rub off when cleaned.
In the early to mid 1970’s, this exact technique was used todevelop the flight heat shields for the Voyager spacecraft. There were two heatshields that shielded the science instruments (they were covered in gold surfacedkapton) and to shield the beryllium encased radioactive thermal generators fromthe 5,000 F exhaust plume of the onboard solid rocket motor. Almost all of the processdevelopment was done at a local junior college ceramic art class where I coulduse their kiln. All the artists were more than happy to let me/NASA hog thekiln to fire heat shield test samples.
A spacecraft is not a Pantera, but I’m quite confident thisconfiguration will work very well. Incidentally – if one shield doesn’t quitework, then multiple layers can be added. The “thermal drop” from multiplelayers goes by the 4th power so a 2 layer shield is 16 times (2 tothe 4th) better than a one layer shield.
Dick Chandler - 1969 Mangusta 8MA708
An effective heat shield to shield to protect the deck from the turboas
radiative heat can be made from a single layer of Inconel 718 sheet or
foil. Inconel 718 is widely used for high temperature thermal
insulation and particularly on commercial and military jet aircraft
engines. Heat shields can be a single layer of .012 thick Inconel 718
foil, or any thickness of Inconel 718 sheet can be used.
The basic approach is to place a conditioned sheet of Inconel 718
between the hot turbo and the deck lid. Simply placing an unconditioned
sheet of any metal - with the same shiny surfaces on both sides -
between the turbo and the deck lid will help some, but not a lot. The
trick is to condition both surfaces of the Inconel sheet in a way that
results in most of the heat absorbed by the heat shield to be radiated
back toward the turbo and very little heat radiated toward the deck
lid.
Pardon a small bit of theory: Different surfaces will radiate heat
differently. A rough black cast iron burner on the stove will radiate
heat much better than the shiny chrome surface of a clothes iron. If
you place your hand over a cast iron burner and place the other hand
over a clothes iron (with both at the same temperature) the cast iron
burner will heat your hand very quickly compared to the clothes iron,
because the cast iron is emitting or radiating more heat. The measure
of a surfaceas ability to radiate or emit heat is called aemissivitya.
The value of emissivity is between 0 and 1 with 0 being the worst, and
1 being the best or the ideal radiator. In short a the heat shield
surface facing the turbo should have a high emissivity (close to 1) so
it can radiate bheat back out toward the turbo. Conversely, the surface
facing the deck should have a low emissivity (close to zero) so that it
radiates very little heat toward the deck.
Here is a description of the process used to create Inconel 718 heat
shield surfaces:
A sheet of inconel 718 can be suspended in a common gas fired ceramics
kiln and fired to a temperature of about 1800-2300 F with the natural
gas burners turned a bit rich to create an oxidizing atmosphere. (I
used the kiln at my local junior college art/ceramics class.) The
heating in an oxidizing atmosphere will create a permanent oxide layer
on both surfaces of the inconel sheet much like anodize creates a
permanent oxide layer on the surfaces of an aluminum sheet. The
emissivity of both oxidized surfaces will be the same at a very high
.95, but because both sides are the same emissivity not much has been
gained relative to protecting the deck.
The heat shield surface facing the deck can be conditioned by coating
it with a thin layer of gold. Gold is a lousy emitter of heat with a
very low emissivity of .05. The ceramic industry has a material called
aLiquid Bright Golda that is a gloss black liquid that contains flakes
of gold metal. (It is available at any ceramic art store.) Liquid
Bright Gold is used to create the gold on ceramic substrates (like the
gold on the rim of a ceramic/china dinner plate), and it is simply
painted on the substrate, allowed to dry, and then fired in a kiln. At
elevated temperature, the gloss black liquid acts as a flux and also
promotes adhesion of the gold to the substrate. The gloss black carrier
eventually burns off and a thin layer of pure gold with a matt finish
is left on the surface.
If the surface of the oxidized inconel that faces the deck is covered
with gold (.05 emissivity), then very little heat will radiate from
that surface. If the opposite surface that faces the turbo is inconel
oxide (.95 emissivity), then the sheet of inconel will heat up, but it
will radiate almost all of itas heat back toward the turbo. In rough
terms, the heat shield will radiate 95% of itas heat back at the turbo,
and 5% of itas heat toward the deck.
Because the oxide layer is actually part of the parent material, and
the gold is fused to the oxide, both surfaces are very robust and wonat
peel, flake, or rub off when cleaned.
In the early to mid 1970as, this exact technique was used to develop
the flight heat shields for the Voyager spacecraft. There were two heat
shields that shielded the science instruments (they were covered in
gold surfaced kapton) and to shield the beryllium encased radioactive
thermal generators from the 5,000 F exhaust plume of the onboard solid
rocket motor. Almost all of the process development was done at a local
junior college ceramic art class where I could use their kiln. All the
artists were more than happy to let me/NASA hog the kiln to fire heat
shield test samples.
A spacecraft is not a Pantera, but Iam quite confident this
configuration will work very well. Incidentally a if one shield doesnat
quite work, then multiple layers can be added. The athermal dropa from
multiple layers goes by the 4^th power so a 2 layer shield is 16 times
(2 to the 4^th) better than a one layer shield.
Dick Chandler - 1969 Mangusta 8MA708
