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Hobbyists
are known for not leaving well enough alone. Usually that's a good thing
because many innovations and improvements in existing technology can be
attributed to them. That goes for aerodynamics, electronics, biology (gardening),
chemistry (paints and adhesives), automobiles, and many other realms. In
this case, the venerable Jetex 50B has had its thrust level amped by a factor
of 6 simply by drilling a hole through the center of the fuel pellets and
using variations on how to arrange the wick and screen inside the engine.
Then, the thrust increase was so great that it required replacing the single-spring
retainer for the nozzle with a double spring version. It was also necessary
to replace the original aluminum body with a steel body to handle the increased
heat levels. Other than size and shape, the 50-HT (High Thrust) is really
a whole new engine. Here is more information on the
Jetex series of motors. 50-HT Hopped-Up
Jetex
  Look
what they did to the small, once lady-like Jetex engine! It's been souped
up into a real Tiger-type go-go-go-man miniature rocket plant. More than six times as powerful as the standard 50B, the new Jetex "50-
HT" high-thrust reaction engine makes possible vastly improved model air-planes,
missiles and even rocket-type boats and cars. The earlier
50B developed a 3/4 ounce thrust; the 50-HT by American Telasco offers five
ounces. The latter can lift vertically a 3-ounce machine with enough power
surplus for eye-opening acceleration. The lighter the model, of course,
the more spectacular the acceler-ation and over-all performance. Competition model aircraft should show tremendous improvement in duration.
Scale jobs such as the Lockheed F-104 and the X-15 are feasible. Engine and fuel have undergone some changes, A 1/8-inch diameter hole drilled
through the fuel pellet is a "trick" borrowed from full-scale solid-fuel
makers. They found hollow charges greatly improved thrust figures. The 50B single-spring motor should not be used with the potent 50-HT fuel.
For the new fuel a double-spring chamber-sealer is combined with a steel-barreled
chamber. Despite greater thrust and heat, the new pellet bums cleaner and
reduces maintenance. By combining a high acceleration 50-HT
charge with one of the lower-thrust 50B charges for sustained duration,
resourceful designers can work out many combinations. Additional variations
can come from different fusing arrangements now possible with the drilled
charges. To evaluate basic combinations in terms of performance
we have prepared a series of graphs which reveal the thrust in ounces as
related to time. "Time is in seconds at the top of each graph; thrust in
ounces is at the left. Labeled A, B, C, and D, these graphs show test readings
for successive trials under varying conditions. When using the recommended
method of loading-see Graph C-wick doubled in the pellets and retained with
gauze, a maximum thrust of five ounces can be anticipated with a burning
time of two to three seconds (although two firings lasted four seconds and
one went 3-1/2 seconds). What happens if the gauze is left out?
Both thrust and burning time vary widely, with peak thrust being reached
more slowly and the run lasting longer but with thrust falling off (Graph
A). If you want more thrust at the cost of reduced burning time, the wick
can be doubled and coiled on the face of the top pellet, with gauze fitted.
Peak thrust in this case is reached in a nearly constant two seconds (Graph
B). By combining one cord and one plain pellet (Graph D) with wick
doubled and retained with gauze, the peak thrust is reached in about 1-1/2
seconds with the sustainer effect of the plain charge extending the total
burning time or motor run to about seven seconds. What effect will
the 50-HT have on model design? To handle the high thrust, it is probable
that longer moment arms and higher thrust lines will be employed, just as,
they were in free flight gas. Wing sections will be thinner and more double-surfaced
wings should be seen. Structures will be sturdier. Who really can
predict what ingenious modelers will do with a 600% increase in thrust?
American Telasc6's 50-HT should stir things up a bit.
Articles About Engines and Motors for Model Airplanes, Boats, and Cars:
Posted June 23, 2012
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