(As seen in Metalworking Production & Purchasing, January 2005)
When completed, 57,000 pounds of thrust will shoot the 54-foot long Canadian Arrow - Canada's first manned rocket - into sub-orbit. Just over a minute into its flight, the rocket will reach an altitude of 110,000 feet while traveling at Mach 4. With these forces, Geoff Sheerin, team leader, and Steve Helps, chief welding technician, know that weld quality is key to the structural integrity of the rocket.
In building the Canadian Arrow, Sheerin and Helps are using a combination of AC/DC TIG and spot DC TIG welding to achieve the required weld quality on the rocket's critical stainless steel components. The AC/DC TIG capabilities of Miller Electric's 45-lb. Dynasty® DX 200 has provided this exciting project with the right tools to help this team reach for the stars in a soundly-built rocket. Why? Let's just call it a dream.
|Geoff Sheerin and David Middlebrook, Miller district manager, stand in front of the Canadian Arrow model.
Ansari X-Prize Drives Desire For Space Travel
In the early 1900s, the general public gazed in awe at the sky. The era of modern flight was being ushered in and advances in technology were spurred by contests, often with cash prizes, to see who could build the fastest plane or complete the longest flight. Before long, rockets were being shot into orbit and men were walking on the moon. Since then, the dream of regular flights into sub-orbit with civilian passengers has largely gone unrealized.
The idea got a boost in 1996 when a group of scientists announced the creation of The Ansari X Prize - a U.S. $10 million prize to the first private company who successfully launches a manned vessel into sub-orbit and returns it safely to Earth - twice within a two-week span. The prize was won early in 2004 by the United States-based team of Spaceship One, but that hasn't stopped other contestants from moving forward. For the Canadian Arrow team, it's a matter of national pride and fulfillment of a dream that started out for Sheerin at the age of 14.
"I found out that you got your astronaut wings if you went higher than 50 miles above the earth's surface," he recalls. "You're pretty naïve at 14, so I thought I'd build some of these rockets and start a space business to give people rides into space, charging to make them astronauts."
His dream wasn't so naïve after all, as evidenced by the Canadian Arrow. Using the World War II-era German V2 rocket as a template, Sheerin's team is currently testing the engine and putting the finishing touches on the rocket's structure. The frame and the skin of the Canadian Arrow is constructed from 304 stainless steel, an alloy recognized throughout the aerospace industry for its ability to resist corrosion and handle cryogenic propellants (19,000 lbs. of liquid oxygen and alcohol provide the thrust). It also handles re-entry into the earth's atmosphere better than other metals and is the same material used to build other rockets.
|Here, Steve Helps, lead welding technician, welds a prototype wing spar using the Dynasty 200 DX.
Design Challenges and Solutions
Before building the rocket, the Canadian Arrow design team attempted to find the original drawings(without luck. Fortunately, an old V2 was being refurbished in Hutchison, Kansas, so chief CAD designer Chris Corke flew to Kansas and painstakingly measured all the components by hand, made pencil sketches and recreated the original blueprints.
The original V2 was designed for mass production (some 3,000 rockets were launched. History buffs can visit www.v2rocket.com/start for details). However, mass production equipment and expensive tooling are not feasible within the Canadian Arrow budget, so some design changes have been made. For example the wing spars ("rib components for the rocket's fins" in layman's terms(see photo) were originally stamped from a single piece of metal.
"We created patterns that could be laser cut and bent in several sections on a standard press brake," says Sheerin. "We needed to bend it in sections because the spar flares out to mate against the rocket's tail" (picture a Y-shaped channel).
The wing spars are spaced evenly up the entire tail section and they transmit the aerodynamic load from the air to the base of the rocket's tail, which in turn further distributes the load to the base of the stainless steel alcohol fuel tank.
The wing spars and the horizontal and vertical ribs (see diagram) of the rocket form an elaborate stainless steel latticework; 16-gauge stainless is used for vertical channels and 20-gauge stainless is used for the horizontal channels. The skin is very thin(just 22-ga. 304 stainless(but it ties all of the elements together, so it contributes significant structural integrity. Needless to say, weld quality is critical.
Point and Shoot: Power Source's Preset Parameters Simplify Welding
The original German design team who built the V2 rocket relied on spot welders to adhere the skin, which is a process of joining two steel sheets by holding the pieces between two charged electrodes and applying pressure at the desired location to create the weldment. Most spot welders are designed like a set of tongs with limitation on reach caused by the unit's "throat depth." This type of spot welder becomes inefficient for projects like the Canadian Arrow because designers would have to make pieces of the skin small enough to accommodate the reach of a conventional spot welder. With today's technology, such as the spot TIG welding capabilities of the Dynasty 200 DX, the Canadian Arrow team isn't handcuffed with the limitations of standard spot welders.
Outfitted with a unique Tec® TIG Spot Gun from Tec Torch Company (see photo), the Dynasty 200 DX can create a spot weld using a single electrode (tungsten) to weld each piece of 22-ga. skin to the 16- or 20-ga. rails, eliminating any access issues.
Prior to making the spot TIG welds, Helps uses a second Dynasty with a traditional TIG torch to weld the gaps that exist in the channels to create a flush surface for the skin to butt up against. This will ensure a sound spot weld between the rail and the skin. Helps first cleans the stainless steel channel with a stainless steel wire wheel on the tip of a pencil grinder to prevent porosity and imperfections in the weld (see photo). He lays a bead using 1/16 in. or 3/32 in. ER308L stainless steel filler rod to join the gap, welding at 70 to 125 amps. ER308L is suggested for use with 304 stainless, as the slight difference in alloys can help prevent cracking. Helps sets the maximum amperage on the inverter's control panel to 125 amps, which provides him with better resolution on the foot control.
That new bead is polished, then sections of the skin are clamped to the frame and Helps begins the spot TIG welds. Weld consistency is extremely important, both aesthetically and functionally; the skin must be assembled perfectly and smoothly to ensure good aerodynamics. The Dynasty 200 DX helps the Canadian Arrow team achieve this kind of structural integrity by controlling the weld parameters through its programmable functions.
"The programmable feature is very nice, and the parameter setup is very straightforward," reports Helps. "Especially with this process here, where no filler metal is required, you can really speed production once it's all laid out and parameters are set. In a high productivity shop, this would be ideal."
For spot welding, Helps programs the Dynasty 200 DX for a high frequency arc start, an output of 100 amps using direct current polarity, an arc-on time of 1 second for joining the skin to 20 ga. material or 1.5 seconds for 16 ga. material and three seconds of 100 percent argon shielding gas post-flow to ensure weld quality. A 2 percent thoriated tungsten recessed 1/16 in. into the nozzle of the TIG gun produces enough heat to create a strong weld without any unacceptable ripples or penetration on the back side of the weld. Spot welds are made every 2 in. along both horizontal and vertical rails of the rocket's frame to ensure the skin's strength.
Using the Dynasty 200 and a special gun, Steve Helps spot TIG welds the skin to the rails of the wing.
"An inverter helps with this spot welding application because it starts the arc more positively and consistently than conventional technology, which is important because of the short arc-on time. If the arc stumbled during the start, the weld would time out before the arc became established and delivered the required amount of energy," says Helps.
Because there are no standards that cover this type of welding, Helps conducted extensive destructive weld tests on the TIG spot welds (bend tests and macro etching(see photo) to ensure weld integrity. For Helps, who volunteers his time and works for one of the world's largest defense contractors and has 20 years of experience, ensuring weld quality is second nature.
Low-line Power Is No Problem
The Dynasty 200 also includes a number of features with universal appeal to the metalworking industry, not just rocket builders. Miller's exclusive Auto-Line(tm) technology allows the welder to maintain a consistent output (and weld quality) even if the primary power fluctuates between 120 to 460 volts. Since the old sports complex that serves as the Canadian Arrow's production has low-line 230, single-phase power, the ability to manage primary power is critical. Further, the facility only has a 60-amp breaker(and a conventional TIG unit without power factor correction draws more than 90 amps at full output. Fortunately, the Dynasty 200 draws less than 16 amps at full output on 230V single-phase power.
"We chose the Dynasty 200 DX to answer two challenges," says Helps. "The traditional TIG welding of stainless steels and the critical application of spot TIG welding. This power source with its timing system addressed both of those issues. The AC abilities also address future opportunities for welding aluminum. With the spot TIG welding there are three factors - current, time and pressure - that go into a strong weld. Outside of pressure, which is left to the operator, we can preset time and current. This provides a level of quality control, and both cosmetically and structurally, this was the best possible way to do it."
The Canadian Arrow has a caring team with a vested interest in making this rocket a reliable, reusable vessel. Because of this, more time is spent in perfecting design, including welds. Much of the welding has been completed in shops in the London, Ontario area. Now, with the skin coming into place and the rocket taking final shape, a machine that symbolizes the pride of a nation and the realization of a naïve young boy's great idea will race for the sky on the strength of welded stainless steel and dreams.
For more information on the Canadian Arrow, visit planetspace.org.