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SS1 Reduced Scale Test Glider
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Aerodynamic stability is a concern for this unusual design, both for the rocket powered ascent and the radio controlled glide recovery. The most prudent course is to build some smaller scale versions of the SpaceShipOne and the SS1 in order to learn as much as we can about balance and flight characteristics.
After we had tested the 4" rocket version and successfully flown several different copies on motors sized from G through I (see photo left), the next obvious step was to get our test mule to glide. So forget everything we already learned about CG, CP, and nose weight; this next craft -- a glider -- needs to be as light as possible.
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Since I had made the molds already(these molds are the exact same proportions as the larger SS1), the sensible path seemed to be to use those molds to create lightweight copies of the fiberglass cones and airfoil that we had been using on the 4-inch test mules. This took some trial and error but I eventually ended up with a set of fairly decent lightweight parts. To make them, I first laid in a skin of 2-ounce fiberglas cloth and epoxy resin, and then filled the molds with 4-pound expanding foam.
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To make the stabilizers I used an extruded polystyrene product called CELLFOAM 88™ made by an Australian insulation company, Solartex. It has a pretty wide variety of uses, from making concrete forms to pallets -- but I bought my Cellfoam 88 at the hobby store. It is lighter and stronger than balsa (and is far lighter than the honeycomb nomex composite we used on the 4" rockets - but not as tough...).
Notice that I made more than one set. Based on our history, I don't predict this to go well the first time out.
The tube in the photo is a standard 4" tube -- but I didn't use it because I decided it was too heavy.
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Here is how I further modified the parts to make them lighter:
- I trimmed the shoulders of the cones down to one inch and hollowed out most of the foam centers.
- After trimiming the airfoils that I had made with the mold and expanding foam, I guessed that I could make the parts lighter if I cut them straight from styrofoam with the hot wire cutter. I was right.
- I also made a styrofoam replacement for the 4-inch tube. The foam core is a 4" diameter cylinder six-inches long, and it is wrapped in a tube rolled from vellum paper 8-inches long (to extend over the shoulders of the cones).
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Above are photos with all the pieces assembled. Unlike assembling a high-power rocket, I gently tacked the parts togther so they would actually come apart. It is mostly held together with stick-pins and scotch tape.
I feel better about the carved styrofoam airfoils and tube. These parts are much lighter. All that's left is to take it outside and throw it....
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HOLY COW!! IT FLIES!! This thing is actually a really decent glider!
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OK, big deal. We were able to make a glider. Why is that worth publishing on the Internet?
The answer, quite simply, is science.
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For example, we learned one very important item with this simple glider. We know from demonstration where the Center of Gravity (CG) needs to be when the SS1 is in glider mode so it can be airworthy and returned home via remote control. The CG of the glider is shown on the illustration above with the black CG symbol.
The green CG symbol is at the location that we proved, through trial and error, stabilizes the rocket while under thrust. (When we fly the full-sized SS1 we will likely be able to use a CG somewhere between those two points; we will have the ability to correct the flight path if needed under boost).
As we defined in our analysis of Test Flight One, a marginally stable design will exhibit the tendency to reverse directions when under thrust. The CG at the time this occured was very close to the CG location of the glider.
So we know that we need to balance the rocket with the CG forward for the boost phase, then move the CG backward for the glider phase. Simple.
The orange CG symbol, by the way, is the center of gravity of the large SS1 without any balancing weight or radio control electronics installed. There will be some additional balancing weight required in order to trim the big pig for flight.
INDEX
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