Rocketry Planet

fAs a fan of scale sounding rockets, to say I was intrigued when I saw photos of Tom Engelhardt's Level 3 D-Region Tomahawk would be an understatement. Engelhardt, who hails from Goettingen, Germany, had shared his photos with readers of Rocketry Planet's Forums last year and I thought they deserved closer inspection. His nearly full-scale D-Region was a beautiful rendition of the venerable NASA sounding rocket.

Tom and helper, Lotta, pose with the Tom's D-Region Tomahawk.

The D-Region Tomahawk was one of NASA's high altitude work horses designed specifically to explore the D-layer of the ionosphere. According to Engelhardt's research, the rocket had its first and only successful flight on February 5th, 1968, from Wallops Island, Virginia, reaching 118.5km using the Thiokol TE-M-416 motor, delivering 44,500 Newtons of thrust over 9.5 seconds. Despite the success, the Tomahawk has not flown in the single stage mode since then, but it has been used as an upper stage for the Taurus-Tomahawk, Nike-Tomahawk and Sandhawk-Tomahawk variants.

Hobby enthusiasts from the United States can't really appreciate the price our European brothers pay to participate in the sport, not just from the standpoint of the actual cost, but the price they pay for parts availability, the exchange rate differences and finding suitable launch sites to put their creations into the sky. Anyone participating in the hobby outside of America could well challenge our definition of the word commitment.

Tom's model was built as a 1:1.2 scale replica using Public Missiles, Ltd.'s 7.5" phenolic tubing, which was reinforced with two layers of hand-laid R&G hollow glass fiber cloth and 60 minute Aeropoxy resin. The airframe tubes were mated with a PML fiberglass nose cone and composite reinforced 1/2" 5-ply birch plywood fins.

From this pile of parts, the D-Region took shape.

During construction, Tom used special techniques to insure quality results. In lieu of access to vacuum bagging equipment during the lamination process, he worked excess epoxy out of his layups with foam rollers, comparing the final cured weights to the resin company recommendations for the optimal resin-to-fiber ratio.

Tom's D-Region was built with a central 98mm PML 98mm motor mount tube, but with the limited space available for him to fly the Tomahawk, combined with the high cost of large APCP motors in Europe, he decided to make a 75mm convertible motor mount adapter. This would give him additional flexibility with his propulsion choices — a decision that really made sense. The 75mm motor mount adapter uses a Slimline retainer and is secured into the 98mm motor mount with four stainless steel machine screws.

Facing the large expanse of the D-Region's fin area, Engelhardt enlisted the experience of his TAP member, Juerg Thuering, calling on Thuering's knowledge of composites before finally deciding on a final fin construction design.

The fins were laid out on 1/2" 5-ply birch plywood before being cut out and laminated with an Aramid-Kevlar composite reinforcement.

"The fins posed a challenge as they had to be large, yet sturdy and scale," Engelhardt explained in his certification documentation. "After long discussions with TAP member Juerg Thuering and after going through various designs and options, a simple plywood fin was chosen. The due possible high loads during the flight and even higher loads on impact, the fins were then strengthened by a layer of Aramid-Carbon fiber."

Engelhardt's research led him to create a fifth fin, constructed using the same techniques, which he submitted to a 4-point bend test at the University of Goettingen.

The airframe was reinforced with two layers of hand-laid R&G hollow glass fiber cloth.

The electronics bay features lots of devices.

"The failure point for this type of fin was roughly determined at approximately 7500 N with the ultimate failure point being at 9100 N," Tom stated about the fin testing. "However, the data obtained in this test can only suggest this fin will withstand the forces during flight and landing."

Neither vertical load testing or torque stress testing were able to be performed due to Tom's limited time during the build and the cost it would entail.

After the fins were attached to the airframe, an additional layer of carbon fiber and fiberglass cloth were added, before being covered with Auto-K Spritzspachtel grau, a German-brand lightweight spray-on gray primer with "micropore filling capacity," to fill in the small imperfections before final painting could begin.

For recovery electronics, Engelhardt chose three electronic devices:

• a GWiz LC 800 Deluxe for drogue and main parachute deployment,
• a GWiz HCX for back-up main parachute deployment, and
• a blacksky Timer 2N for backup drogue chute deployment.

In addition, he would fly Louis Scheyer’s Altimax, a new altimeter prototype featured last year on Rocketry Planet, to serve as a flight logger and to determine the Altimax's capabilities in larger diameter rockets.

Tom's drogue chute compartment uses a 4" central tube.

At apogee, the LC 800 would eject a Giant Leap TAC 24" drogue parachute, using the Timer 2N as a backup set for 2 seconds past apogee. To reduce opening shock, a Giant Leap Slider would be used on the drogue chute as well. The drogue recovery shock cord nearest to the ejection charge would be a 20 foot 1/2" tubular Kevlar® unit made by Giant Leap, with a 1/2" tubular nylon Giant Leap shock cord for attachment to the drogue chute itself, both featuring sewn-in loops at the ends. 

At 1200 feet AGL, the HCX is programmed to fire the primary main parachute ejection charge with the LC 800 acting as a backup, programmed to fire at 800 feet. The main parachute would be a Giant Leap TAC 9B parachute with its matching deployment bag.

Tom and crew walks the Tomahawk out to the pads. Photo: Andi Kamber.

For ejection charge containment, the task was assigned to SafeEject ejection charge canisters, made and distributed by Juerg Thuering. The drogue and main parachute compartments were made from lengths of PML phenolic tubing and was designed in such a way to reduce the overall compartment's size, using 4" tubing for the drogue compartment and 6" tubing for the main compartment, both held in place with 4" and 6" to 7.5" PML birch plywood centering rings.

Turning his attention to finishing his rocket in time for an upcoming launch at Val de Ruz, Switzerland, Engelhardt went to work on the scale details of his Tomahawk. The external fluting on the booster fin canister were uniquely made of 1mm balsa wood strips. After measuring and cutting each fluted piece to shape, each balsa piece was soaked in water for an hour before being taped to a section of airframe tubing to dry. The result was a nicely curved detail piece that after drying was epoxied into place using 15 minute hobby epoxy. Afterwards, the balsa was treated with wood filler and sanded to a smooth finish.

After lifting the D-Region Tomahawk into place, you can click the image and see it in all its liftoff glory. Photos: Andi Kamber.

After spending another 48 hours of, in Tom's words, "gluing, filling, sanding, swearing and priming," the rocket was ready for painting. And yes, Tom added the 96 bolts to the PML nose cone and payload section as well, adding important scale details to an already beautiful rocket. Tom painted the model in the original paint scheme, which included three black fins and one red fin, a white airframe, and a red and brown payload section with a silver nose tip.

The beautiful Tomahawk was deserving of a beautiful flight, but that was not to be, as gremlins made their way into Tom's Level 3 flight plan. According to Tom, the M1315 launched the Tomahawk into the Swiss sky on a beautiful flight to 3458 feet.

But at apogee the 2.3 gram drogue ejection charge blew a hole through the side of the 4" phenolic drogue compartment, creating a loss of pressure. The culprit was speculated to be that the additional force required for the 4" drogue chamber to eject the 7.5" airframe was too great for the unreinforced phenolic tube, which ruptured because of the overpressurization.

In the end, the HCX got the main chute out, but by the time it did the rocket was coming in ballistic and suffered a separation. The booster experienced one broken fin while Engelhardt experienced a bruised ego. But fear not, Tom will be back and already has his sights set on another D-Region or perhaps a Sandia Tomahawk. I wish him all the luck.

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