SORAC and the CATS

by Bob Fortune, LUNAR #660

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SORAC is a peppy acronym which stands for SORe AChing Back because the dang thing weighs so much. Actually it stands for Sub Orbital Rocket, Amateur Class. It is designed to achieve 200 kilometers and claim the CATS prize. Let's see, that prize was 250 thousand semolis last time I heard. Regardless, it's not an easy task. We all know how tough it is to get something to go a mile, 2 miles is really a challenge and 3 miles takes some real doing or a lot of money or both. This rocket is scheduled to hit 120 miles, look around, release it's payload to descend by parachute, and then fall into the tundra way up north in the ice and snow. Just like the big boys at NASA do it only cheaper.

The vehicle is 15 feet long and 12.4 inches in diameter. It weighs over 500 pounds at liftoff. It has a boost phase of 5400 pounds thrust for 4 seconds, and a sustainer phase of 2000 pounds thrust for 37 seconds. To say it gets up and goes once lit is a bit of an understatement. Most of the airframe is composed of off-the-shelf materials and parts though the design and construction is the result of years of experimentation on the part of Bill Colburn. Bill is the proprieter of Aerocon, inventor of the candy motor (KNO3 and sugar),semi-retired systems engineer who designed 5 mission critical Apollo electro-explosive systems, retired theatrical magician, and walking font of knowledge. An interesting character indeed, if anything has been done before with a rocket propelled device Bill has been there and done that.

The nitrous tank alone without bulkheads weighs about 80 pounds. Motor unit including fins will weigh 150 pounds and the nose cone, electronics, and recovery system will weigh about 70 pounds. The main airframe is 3/16" thick filament wound fiberglass which happily happens to have a working pressure of 1000 psi and burst pressure of 2000 psi. This allows the fiberglass airframe to contain the nitrous necessary for flight at 750 psi and still have a reasonable safety margin. Epoxied into place (with the addition of a generous thrust ring and hundreds of roll pins) are steel bulkheads to form the 10 foot tall by 12 inch diameter nitrous tank. This tank is connected to the motor section with a coupler of similar material and the nitrous feeds into the head end of the motor via 1 1/2" piping.

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The motor section head end has a fitting to match the 1 1/2" nitrous pipe and is also made of steel. An additional port in the head end contains the ignitor assembly which is similar to an AeroTech I211 motor. This head end is mated to 3 feet of similar fiberglass tubing which contains the fuel grain and motor aft end using epoxy and roll pins. The aft end is comprised of a solid aluminum billet turned to match the tube inside diameter and nozzle outside diameter and again is epoxied and roll pinned in place. A solid plate of turned teflon forms the after seal between grain and aft closure. The nozzle is turned from graphite on a lathe and is coupled to a phenolic and mica exhaust bell which is thermally isolated from the aluminum closure using Pour-Stone, a common building material. Simple, inexpensive, and robust features have been designed in from the start of this program which is 30 years in the making.

Fins are triangular shaped like an LTV Scout and are a lamination of 90 thousands thick aluminum with a sandwich of half inch endgrain balsa epoxied in between each fin surface. The leading and trailing edges are cast from silica filled epoxy using a mold that some ingenious inventor type came up with to solve the problem. The fins are then riveted to the airframe which is also the motor section of the rocket.

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Originally the vehicle was designed to fly on an Ammonium Nitrate motor back in the 70's changing to Ammonium Perchlorate in the 80's. However with the increase in regulations surrounding manufacture and transportation of such motors becoming increasingly strict and problematic a hybrid system was utilized. A motor for the 90's and beyond. The cool thing about nitrous motors is that one can purchase the denatured variety almost anywhere and the fuel grain can be anything from salami to cardboard.

The rocket will be propelled using liquid nitrous oxide as an oxidizer burning a combiniation of HTPB (which lots of rubber things are made out of including solid propellant motors), ground polyethelene (take a gallon milk jug and turn it into sand) and carbon black. Flow an oxidizer, nitrous oxide, over an appropriate fuel with an adequate ignition source and you have a hybrid rocket motor. (Motor testing is currently going on at Rocket Ranch which is just east of Livermore over the pass.)

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It's a really simple system not even the BATF can gripe about, only hard part is getting the thing lit and flying, therein lies the rub. It is currently illegal for a U.S. citizen to launch a vehicle into space. So how does one then attempt the CATS prize? Find a friendly foreign country to launch your bird for you, hook up with a learning institution with sufficient resources, or pay an incredible sum of money to NASA and have them launch your project. If NASA does it, I don't believe you can indicate on the flight card that you want to push the button yourself and the cost is significantly more than a buck. Stay tuned for further updates.