Great Lakes Rocket Society

In the mid 1990s, several rocket enthusiasts in the Great Lakes region met and started the Great Lakes chapter of the Reaction Research Society, otherwise known as the Great Lakes Rocket Society. The "rocket club" has been building a 22-foot long, 1-foot diameter, 1000-pound-thrust liquid-propellant rocket. The rocket reposes in a tool & die shop in Toledo, Ohio, in its final phase of construction. (See Fig. 1.) We are presently completing assembly of its three main subsystems: structure, propulsion, and electronics. The story of the development of these subsystems is told here, from the perspective of the rocket’s design and construction history.

Fig. 1. The Huron rocket, with two tanks and one parachute shown. Rick Wills (l) designed propulsion; Mike Jacobs (r) handled structure.

GLRS president Ken Weidaw III is an unlikely person to own an Atlas missile vernier engine. When we first met, he pulled his lawyer’s satchel out of the trunk of his car. Instead of removing legal papers, out came a new, surplus Rocketdyne LR-101 engine. And he intended to use it.

Ken not only has an engine and expertise in space law. While researching the history of the German V-2, in the NASA archives in Huntsville, he recently discovered the original blueprints of the V-2. The archivist was unaware the library had them.

Over in Toledo, mechanical engineer and tool & die-class machinist Mike Jacobs, had been getting tips from Reaction Research Society (RRS) president David Crisalli about amateur rockets, and was intent on building one. Mike’s machining skills and mechanical design know-how have proven crucial to the airframe design of our prototype rocket.

Another core member, Rick Wills, is the working rocket engineer of the group. He has recently been involved in hypersonic scramjet engine development in the Experimental Research Branch of the Advanced Propulsion Division of Wright-Patterson Air Force Base in Dayton, Ohio – in Building 18, the former "Hanger 18" of UFO notoriety. Red-glowing pipes driven by the compressor downstairs deliver hypersonic air to the intakes of engines under test. It is unlikely that any alien bodies could be stored "on ice" in the building and withstand such heat.

As the fourth member of the group, my contribution to rocket development has been an electrically actuated (computer-controlled) on/off ball-valve controller and a remote data acquisition and control computer that can be used on test stands for static firings, or as a flight controller in rockets. I put enough work into these subsystems to commercialize them through my company, Innovatia. They are presently being field-tested by other rocket developers.

One of our earlier contributors, Robert Wolf, provided project guidance. Rob has a Ph.D. in aerospace engineering from MIT, taught orbital mechanics at the Air Force Academy, and worked on the largest hybrid rocket ever built (at Amroc). He is presently at Pioneer Rocketplane.

Slightly dazed by the loss of Rob to California, the remaining group members proceeded to develop a concept for a prototype rocket, dubbed "Spirit in the Sky" by Rick. It is the first (and probably only) rocket of the "Huron" design, the first of our Great Lakes series. Beginning with Ken’s engine, Mike went to work on design of the structure while Rick worked out the propulsion subsystem, calculating tank sizes and flow rates.

Mike’s structural design required machining of the engine, which was made to be mounted on swivel bearings. These were cut off, along with the redirected fuel line feeding into the bottom of the double-jacketed, regeneratively-cooled thrust chamber. (See Fig. 2.) Mike rebuilt the top of the engine to attach to the thrust ring through several steel rods. The airframe, made of 12-inch dia. Public Missiles Ltd. phenolic tubes, were cut into sections to accommodate bulkheads made of a new plastic, Nylatron GS. But the phenolic tubes cannot bear the engine thrust. Three aluminum (Al) 6061-T6 U-channel longerons run the length of the body tube and attach to the bulkheads. These load-bearing members result in a stiff vehicle structure, and are capable of taking the half ton of upward push from the engine.

Fig. 2. Modified LR-101 engine produces 1000 lbf of thrust, burning kerosene and LOX. Note taped fuel inlet near the bottom end of the regeneratively cooled nozzle.

Both ends of the airframe are Public Missiles Ltd. nose cones. The engine shroud (or "boat tail") is a truncated nose cone with an end ring mounted by three threaded rods extending down from the thrust ring. (Fig. 3) On the end ring is attached the electrical umbilical connector, a commonly available 9-pin Molex pin-and-shell connector that supplies ground power, computer communications and ignition power to the on-board flight controller.

The top end of the body tube contains twin parachute silos, with smaller-diameter tubes through which a steel cable runs, holding the nose cone onto the body. A pyrotechnic device produces gas which pushes two sharpened buttons sideways, cutting the cable and opening vents into the chute silos. The gas used to cut the cables also pushes out the chutes.

The fins (not shown) are a carbon-Kevlar layup with a 7075-T6 Al alloy implant, which allows mechanical connection to the rocket body. The four fins are trapezoidal in shape and are foam filled, with titanium leading edges.

The launch rail is of Unistrut construction, a kind of U-channel with ends bent inward. Bulkheads support two spring-loaded steel pins. Their outer ends fit into the rail, and are held out by washers sandwiched between the rail and the rocket body. When the guide pins leave the upper end of the rail, the washers fall off, allowing the pins to retract flush with the side of the body tube. (See Fig. 3.)

Fig. 3. The engine shroud (middle); (r) a stack of nylatron GS bulkheads; (l) parts for retractable launch-rail guide pins.

Mike did much of the machining of these structural components, including "hard points" for mounting the bulkheads to the longerons. The place of assembly was Tim Reinhart’s shop, Timon Tool & Die, in Toledo. (Fig. 4) Tim, the good-natured, cigar-puffing owner, has graciously rented us use of his machines and a corner of his shop, where the rocket lies. He has taken an interest in our unusual activity, attending our meetings and contributing reasonable design suggestions, and shop time and space.

A structure more imposing than the rocket itself is its cradle and erection system, also built by Mike. (See Fig. 5.) The 150-pound (empty weight) rocket is raised hydraulically and has wheels for transport on the flat bed of a truck.

Fig. 4. GLRS president Ken Weidaw (l) and shop owner Tim Reinhart (r), beneath opened chute. Note modified engine mount to bottom of body tube.

As our estimates stand, the initial mass of the rocket is 100 kg (220 lbf). The propellant is 34 kg, weighing 75 lbf. Consequently the burnout mass is 66 kg and the mass ratio,

Mass ratio is a structural figure-of-merit. Ideally, the mass of the structure is minimal, yet can support large amounts of propellant. Use of high strength-to-weight-ratio materials, such as carbon composites yield a high MR. The mass ratio is effectively increased by staging, but our rocket is single stage and somewhat heavy (of mediocre MR). The tradeoff is lower cost, a significant design factor in this project.

The Rocketdyne engine burns kerosene (Jet A aviation fuel) and liquid oxygen (LOX) in a mixture ratio of 1.8 pounds LOX per pound of kerosene. That’s a mass flow rate of 1.2 kg (2.6 lbf) per second of kerosene and 0.66 kg/s (1.5 lbf/s) of LOX. In all, 22 lbf of fuel and 40 lbf of oxidizer are burnt in about 18 seconds. The burn time has recently been extended to about a half minute.

Rick designed the tanks (Fig. 6) using steel end domes that were welded to pipe sections. The plumbing is typical of pressure-fed liquid-propellant rockets. A pressurized tank is filled from an external connection through an electrically actuated (remotely controlled) fill valve. This pressure source drives a self-referenced regulator that sets the maximum propellant tank pressure at several hundred psi. The regulator output feeds fuel and oxidizer tanks through check valves. Each tank is vented and has its own fill valve. The two tank outlets feed into the main engine valves, which then feed the intake manifolds of the engine.

The LOX plumbing is SS316 stainless steel. The LOX tank and lines are insulated to minimize their heating and the cooling of external components (such as electrical cable) that might otherwise be in contact with them. The three tanks and engine combustion chamber are instrumented with pressure and temperature sensors. For static testing of the engine, turbine flow sensors measure fuel and oxidizer volumetric flow rates. All sensors connect directly to the Huron flight computer, which doubles as a data acquisition and control system for ground testing.

The Huron flight controller (see Fig. 6 and below) is a 7.15 ´  4.15 inch circuit board with 25 input/output (I/O) channels, solid-state data recorder, telemetry, data processing and mission clock. It is designed to include all the essential data measurement and control functions of a sounding rocket, and can be configured and programmed according to mission requirements.

Its one thermocouple channel is intended for engine temperature measurement. Ambient pressure, temperature and acceleration are used to calculate when to deploy the chutes, close tank valves, measure distance and speed, and trigger payload events. Eight differential analog inputs can be configured for load cell (thrust), tank pressures, and RTD temperature measurements. Bridge-based sensors can be powered from the on-board bridge voltage supply.

Fig. 6. Pressure, fuel and oxidizer tanks (front to rear) behind flight controller demonstration. A ball-valve controller is to its right.

Outputs include 7 quarter-ampere current-sinking lines and an ignition relay. These lines can drive relays, small motors and other actuators with a minimum of additional circuitry. Two PCM channels directly drive model radio-controlled or industrial servomotors for camera positioning or aerodynamic steering of fin controls. Two pulse generators provide controlled-charge pulses for pyro devices, and prevent accidental depletion of the on-board battery by a shorted igniter or welded-shut relay contacts. The pulsers can also be used for staging, but in the single-stage Huron, they blow the chutes.

Eight digital inputs connect directly to grounded switches to detect successful parachute deployment, tilt-switch states or the LOX-tank fill level, using a cryogenic thermistor. Both digital input and output channels can be expanded by another eight lines using the serial I/O bus.

Because the Huron rocket uses passive ballistic guidance only, no guidance computations are required. With mission programming, an acceleration sensor channel can be used to compute speed and distance. Along with an ambient pressure sensor, the flight computer can make decisions about when various events, such as parachute deployment, should occur.

A stream of telemetry data during flight is provided by configuring channels to be telemetered. The telemetry data is sequenced in a telemetry frame, consisting of 200 subframes of data values. Designing the frame is simplified by use of scheduling commands. Multiple levels of channel multiplexing are possible so that the telemetry frame is optimal for the mission.

Communication with the flight controller is via a 3-wire RS-232 or RS-485 serial port. Telemetry data is output to the transmitter from this port (up to a 19,200 baud rate). Not all acquired data can be output in processed form for most missions. Instead, the data recorder stores the raw data in battery-backed on-board memory so that it can be processed after the flight. The playback command outputs processed data that can be viewed directly or captured into a spreadsheet or MathCAD file. Its subframes include clock time, channel, processed value, and unit. The recorder, when turned on by mission programming, will turn itself off when full.

Calibration of channels is simplified by applying one or two arbitrary but known amounts of input from a given sensor (or its emulated output) and typing in the known value. The Huron relates the raw measured data value to the known value it is supposed to be, and calculates the correction parameters. These can be saved in a text file for archiving, and downloaded before flight.

Because the flight controller has all the functions required of a remote data acquisition and control system for static test-firing, it can be placed near the test engine at the stand and instrumented with short wiring. Then, using the RS-485 serial port, a 3-wire cable of over 1 km can be run back to the blockhouse, where it is attached to the serial port of the mission control computer. For flight, the on-board Huron controller interfaces to the ground through the electrical umbilical. At liftoff, the non-locking connector simply disconnects. The power source is transferred prior to launch from a ground-based supply to the on-board battery. Hold time on the ground does not thereby endanger the ability of the battery to perform due to excessive discharge.

The Huron flight controller prototype was built in 1996, and several etched-circuit-board units are fully operational. Two are being field tested as static test-firing controllers, one by Mark Holthaus of the RRS in California. The unit is well-suited for testing and use on "high-power" solid rockets, and a successful test flight will confirm full operational status.

The main engine propellant valves, pressure valve and vent valve are remotely operated. They consist of an electronics control board mounted on a Grainger 2L008 gearmotor. (See below.) The valve controller actuates on/off ball valves. Magnets are placed on a collet which is mounted on the output shaft of the gearbox. The magnets can be moved for precise location of the on and off states of the valve, and then bonded into place or recessed into the collar.

The valve stem is turned bidirectionally for on and off actuation. This is necessary for valves controlling cryogenic fluids such as LOX. A drain channel is drilled at right angles to the main ball channel so that LOX is not trapped in the ball of a closed valve. To keep the drain channel on the exit-port side, the valve must be rotated in the reverse direction when closing.

The VC1 also has electrical outputs that indicate when the valve is in opened and closed states. Lights can be attached to these output connectors for visual indication, or they can be run into a computer. For the Huron, they are run into digital inputs of the flight controller.

So much effort has been put into this first design that we intend to take our time in both displaying and testing the Huron rocket before flying it. Besides static test-firing the engine, the chute deployment system and electronics will be tested on the ground.

Display of the rocket is intended to encourage others to learn astronautics and become "ignited" with interest to pursue the dream that motivates RRS members in our quest toward the sky.