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THE GRAVITY OF THE SITUATION

March 1, 2017

 

This week we are looking at the mid-stage fairing and seeing a mid-life crisis.  (OK, I am WAY past mid-life!)  We began the work last week looking at titanium to handle the structure joining two stages…bad idea!  This week we add cylinders to push the vehicles apart for staging, and add skins to this fairing stage.  Staging now separates the mid and second stage, then sheds the cover and the structure.  Those are expendable unless we find a way to parachute them back to earth.

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Here we see the first and second stage pistons driven from their cylinders and the fairing cover ejected.

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Now we really have to reduce the titanium structure mass, and add skins and covers of carbon fiber.  I tend to build on the heavy side, expecting analysis to point out where to reduce mass.  I can deliver illustrations, but validation comes from the high paid partners.  Since we have no other mechanical parts here, this is a good place to start tuning my mass estimations.  And I really need a tune up!

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With my typical bulkhead and skin thickness being quickly carved, this will be a very heavy piece of airframe.  It produces details that are visible in renderings, but overweight in the flesh.  Too strong is not always bad though.  Our model was plywood and fiberglass.  The glass was so strong I had to cut a lot of structures out  to reduce weight.  It completed a good flight, and then two less good flights.  The second crash was a power dive from 300 feet while still attached to the carrier aircraft.  The fiberglass never cracked, and it is taped up and on display today…we thank Performance One Aviation of Mesa Arizona for super duty body work!

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ON DISPLAY IN OUR SHOWROOM

This study did generate some numbers, and we can be sure it needs some mass reduction work.  We know that the big space plane projects are being done mostly of composites now.  So these lessons can tune our mass estimates more accurately.  This is not a one material target though.  We see carbon fiber and titanium in the model.  There are also some thermal protection material, wiring harnesses, fasteners and other materials mixed in.  that means we can factor the material densities a bit in either direction when the total assembly is considered.  It certainly seems to need some work.  The initial mass figures indicate a lead barge…36,435 lbs!

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Now my thick carbon fiber skins are a bit much, but the shuttle thermal tiles were very light.  Their density was only .005-.007 lb./cu. in. so we can factor that in.  Other TPS may not offer that low density so we can’t get carried away if we want the most economical solutions.  Another area for reduction may be the titanium structure.  (gee, it’s only 23,032 lbs!)  It can likely be a mix with carbon fiber parts.  Titanium is now available with new manufacturing methods too.  It is possible to use 3D printing, or additive manufacturing.  As much as I chopped this model up to reduce weight, we can better target stress areas and cut down low stress areas.  How much would this help here?  Take a look at the potential of these parts:

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I believe that there are smart people out there who can deliver strong parts with low mass.  My next run at cutting mass divided the structure up with a lot less titanium and a lot more thin carbon fiber.  Oh, gee, we carved it down to “only” 15,427 lbs!

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Even this requires drastic measures, as even small units of titanium are a lot heavier than carbon fiber.  the new images show a lot more nice dark carbon and less metal…not so shiny.  Possibly not too bright either though.

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This is why Dan Raymer warns us not to fall in love with your CAD models and drawings.  They are so pretty, but the engineers will always be changing them…usually for good reason.  Other problems with this fairing include eight servos and four big gas cylinders.  This mid-stage may be just a bridge too far.  I am inspired to revert to some older ideas that worked better.

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If I refer to our suborbital version, we had no mid-bridge and used part of the tail structure as hard points.  We can eliminate four servos and two cylinders along with a lot of messy mass.  Now I can look forward to a redesign of the second stage and a lot more weirdness to come.  But perhaps not as much as we see in this image!

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So now you may be wondering how a one-man design can look so much like a committee creation.  Just remember, the boss always has the right to be wrong.  This is a real-time design so we can get used to going back to the drawing board at times.  Bad ideas are the seeds of tomorrow’s better ideas.  Stay tuned for more of those…?

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