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perhaps the most important part of a

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space voyage is coming home safely

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the spacecraft must survive the extreme

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heat and friction of re-entry into the

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earth's atmosphere

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scientists and engineers create heat

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shields to protect the spacecraft

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but how do they know it will really work

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the answer is the arc jet facility

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for decades this laboratory recreated

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the heat gases and chemistry experienced

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during re-entry giving us the confidence

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to fly explore

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the arc jet combines heat the proper mix

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of gases and the chemistry that occurs

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at extreme temperatures to simulate the

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flow experienced by spacecraft during

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their fiery re-entry through earth's

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atmosphere so when you're coming back

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from low earth orbit or even the moon

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you're coming back at very high

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velocities

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and so from coming back from lower thor

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but you're on the order of seven

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kilometers a second

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uh from the moon you're coming back

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about 11 kilometers per second that's

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really fast and so when those vehicles

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actually

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encounter the atmosphere then they're

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just essentially slamming right into

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that air and so that creates a shock

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wave around the vehicle

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and right behind that shock that kinetic

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energy associated with the reentry is

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converted to chemical energy you know so

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you take the the basic air it's mostly

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nitrogen about 77 nitrogen about 23

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oxygen and so what happens is when you

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go through that conversion process going

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to kinetic to chemical energy what

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you're doing is you're breaking apart

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the molecules and so at first first is

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going to go as the oxygen then it's

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going to go the nitrogen so as as

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opposed to an oxygen molecule or

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nitrogen molecule now you'll have atomic

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oxygen or atomic nitrogen and atomic

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oxygen can be very i'll call it

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corrosive

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because it really just eats up material

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so all that gas on the other side of the

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shock on the vehicle side then

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encounters the thermal protection system

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and so that's how you have to know

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well how does that gas affect that

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thermal protection system is it going to

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protect the vehicle and of course the

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only way we know how to simulate that on

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the ground is through these arctic

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because we do not know how a material

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will behave until it gets in these

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facilities there's no

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analytical equivalent to an arc jet we

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have math models and we have tools but

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they require arc jet testing to

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benchmark them and to provide decor

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properties for those tools so there's a

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lot of materials that that might perform

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well

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in a strictly thermal environment such

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as blowtorch

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but will not perform the same in an arc

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jet or in flight because of this

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chemistry of the flow field

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so the basic operation of the arc jet is

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to simulate the re-entry environment

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the nuts and bolts of it is you want to

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take gas and just take it to a very high

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energy level

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and

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the heater is the workhorse of the arc

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jet facility so all the components come

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together uh to make the test work with

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high pressure water cooling for the

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cooling of segments to the

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test gas the nitrogen oxygen to the

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electrical power and then into the

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vacuum system for testing material

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okay so when we

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receive a model typically the models

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come

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with thermocouples already attached and

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then in the test chamber we have two

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hydraulically actuated sting arms so we

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would establish our flow field

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and then the model is on a sting arm b

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and then we rotate it out out of the

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flow field so on that sting arm is where

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all the data hookups are so we would run

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that to

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to our data system

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during the test we get all kinds of

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additional data

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we would test these types of ablators

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typically up to three thousand four

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thousand

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degrees fahrenheit even higher

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we would test them at very high

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temperatures for

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different types of trajectories

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primarily for the lunar is what gave the

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highest types of heating rates and

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correspondingly the highest temperatures

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of the surface of the model

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for a

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iss type of re-entry

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the heating rates are a lot lower so

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we're talking about two thousand three

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thousand

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degrees but for a lunar type very

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ballistic type of re-entry

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you're about four thousand degrees even

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higher than that

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the use of the arc jet began in the

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1960s for the apollo program

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scientists and engineers needed to test

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the heat shield material for the apollo

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spacecraft which would be returning to

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earth at unprecedented speeds as you

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know we had to develop an apollo heat

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shield material

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and to do that we actually tested

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several different ablator materials

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from different companies well we did

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learn several things that there are

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certain ingredients that you can put in

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the material that do not enhance its

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performance

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in fact one of the materials that we

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actually tested

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a big claim by the company

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was the ingredient that they put in it

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was what really made it great

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we tested that material with their

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so-called

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magic ingredient

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and we didn't get very good results

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however we took the ingredient out of

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the material

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retested it and it was much better

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so that's the facility allows you to

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test those things and make judgments on

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the material itself

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absolutely

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the apollo material because of the arc

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jet testing

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went from a high density material of

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around 60 pounds per cubic foot

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all the way down to around 32 pounds per

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cubic foot by the addition of some

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fillers we call them

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uh within the material and we still got

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really good performance we got

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good temperature performance we got good

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installation performance and we got the

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good surface performance that we wanted

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from the ablator itself

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so absolutely you can test a material

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with various compositions

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and make good judgments on how that

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composition changes that material

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under these conditions of pressure and

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temperature

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as the apollo program ended the arc jet

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was called into service once again to

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test the thermal protection system for

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the space shuttle

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unlike the apollo spacecraft which was a

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one-use vehicle the shuttle spacecraft

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would be used again and again for

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multiple flights

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its thermal protection consisted of new

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lightweight tiles

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and reinforced carbon-carbon shielding

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for the nose and wing leading edges

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i was a manager on the carbon system

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which was a high temperature portion of

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the orbiter thermal protection system

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and because we needed to go to high

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temperatures which was the hottest spot

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on the vehicle we could test

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temperatures in excess of

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3000 degrees fahrenheit on the surface

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of the material

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which allowed us to develop

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performance curves performance analysis

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limits of the materials

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performance just about anything you want

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to know

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early on in the carbon program we

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uncovered along with some testing done

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at ames research center

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a phenomenon that we called

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oxidation between the coating of the

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carbon which was a silicon carbide

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coating and the carbon substrate which

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is basically carbon

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we had what we call pinhole formations

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between the interface

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which

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was not satisfactory because we could

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lose the carbon coating system

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due to lack of strength between the

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coating and the carbon cell and carbide

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coating

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that was a very important finding

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between

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ames and johnson space center in arctic

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testing as a result of that we developed

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a system called the teo system

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a tetraethylene silicate which we could

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infiltrate into the porosity of the

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carbon system

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which then protected

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this formation of these so-called

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pinholes

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for decades work continued with the arc

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jet in service of the space shuttle both

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testing for improvements of the thermal

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protection system as well as for

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unexpected events

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yeah very memorable tests uh for me was

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after the columbia accident

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obviously

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it all hit us very hard

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and we immediately came into this

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facility to help

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uh understand what happened during that

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accident at that point in time we knew

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that we actually had a hole in the

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leading edge of the orbiter and so what

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we did is we we put a hole in some

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aluminum plates and we put some

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big thick cables

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behind that and we were trying to do is

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trying to understand how the flow could

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be ingested through a hole

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and then destroy some of the cabling

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that controls the vehicle itself

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and so we actually set up that process

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and actually did the those arctic tests

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here

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and of cour it helped us get data to

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correlate to our models so we could

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better describe what happened during the

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accident itself

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so those are some very important tests

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that we did and very quickly and

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very important to understanding and all

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that is is documented in the cave report

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sts-117

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where we had a

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blanket that was uh

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detached partially from uh from the

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ohm's pod which is back the back on the

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back towards the tail of the orbiter and

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we could see it

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we were concerned about how to repair it

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and so we some innovative folks said hey

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we got these

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medical staples on board we sent an

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astronaut out there and used this staple

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gun to stitch this thing back together

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but we didn't know is what would happen

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with the staples during re-entry would

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they just disintegrate and then

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and then the repair would be worthless

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did that test and that repair worked

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beautifully we had camera on it steve

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did a good job setting up a camera and

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we can see the staples glowing we can

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see the pins glowing and they decide yes

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do the eva

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and it worked great

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another mission was sts-118

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we saw some ice damage to a tile and so

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we said hey we're really concerned about

289
00:10:50,550 --> 00:10:48,079
this is this going to cause a problem

290
00:10:52,710 --> 00:10:50,560
during re-entry so we actually machined

291
00:10:55,590 --> 00:10:52,720
a damage into a tile we did several

292
00:10:56,949 --> 00:10:55,600
tests so we did those tests convinced

293
00:10:59,269 --> 00:10:56,959
ourselves that we'd be okay during

294
00:11:01,590 --> 00:10:59,279
re-entry and lo and behold the the

295
00:11:04,230 --> 00:11:01,600
vehicle did come back and without any

296
00:11:07,110 --> 00:11:04,240
problems whatsoever

297
00:11:09,590 --> 00:11:07,120
the arc jet proved time and again the

298
00:11:12,710 --> 00:11:09,600
importance of testing materials to see

299
00:11:14,870 --> 00:11:12,720
how they would behave during re-entry

300
00:11:17,750 --> 00:11:14,880
the data gathered and the lessons

301
00:11:20,710 --> 00:11:17,760
learned from the arc jet live on