Jetex Rocket 50-HT

Note that photos of the Jetex 50-HT engine are shown on the Jetex 50 series page.  The Jetex 50-HT fuel pellets have the same composition as other Jetex 50 propellants.  A 1/8” central hole through the longitudinal axis of the grain provides increased burning surface, and consequently greater chamber temperature and pressure, and thrust.  (Hollow-core propellant grains are the norm in solid fuel engines for both tactical and space missions.)

The standard Jetex 60 (single-spring) motor should not be used with the more potent 50-HT fuel.  Especially designed for 50-HT fuel is the double-spring chamber-sealer, combined with a steel-barreled chamber.  According to older Jetex literature: “It is interesting to note that the new fuel, despite its greater thrust and burning heat, burns cleaner and actually reduces maintenance.”

Early Jetex literature also states:  “Quick to spot such possibilities as combining a 50-HT charge with one of the lower-thrust 50 charges, one for high acceleration and the other for sustained duration, resourceful model designers can work out many combinations tailored to individual requirements.  Additional variations can be achieved by the different wick arrangements now possible with the drilled charges.”

When using the recommended method of loading (where the igniter wick is doubled in the pellets and retained with gauze), a maximum thrust of 5 ounces [142 gm] can be expected with a burning time of 2 to 3 seconds.  Due to propellant differences, firings may run as long as 3.5 or 4 seconds.  If the gauze is left out, both thrust and burning time will vary widely, with peak thrust being reached more slowly and the run lasting longer (but with reduced thrust).

To attain more thrust at the cost of reduced burning time, the wick can be doubled and coiled on the face of the top pellet, with a gauze fitted.  Peak thrust in this case is reached in a nearly constant time of two seconds.

By combining one cored and one plain pellet, with the igniter wick doubled and retained with gauze, the peak thrust is reached in about 1.5 seconds, with the sustainer effect of the plain charge extending the total burning time or motor run to about 7 seconds.

The following is reprinted from the original instruction sheet provided with the first 50-HT engines sold by American Telasco Ltd.  [Notes in brackets are ours.]

Jetex Operation

50 “HT” Reaction Motor:  The only motor engineered to take the new Jetex Super “HT” fuel.

The Jetex 50-HT high thrust jet engine has been designed specifically to handle the higher temperatures and pressures developed by the Jetex 50-HT cored fuel pellets.  Although its fully loaded weight of half an ounce is no more than that of the familiar 50-B engine which has been the standard model propulsion unit for many years, the 50-HT consistently produces better than five ounces of thrust with a duration of up to four seconds, using the cored 50-HT pellets. This engine may the used also with the standard 50 solid fuel pellets for superior performance and reliability, particularly in contest flying.  (The 50-HT pellets, however, MUST NOT be used in the standard 50-B engine.)

The 50-HT engine may be loaded in several ways to obtain different thrust patterns.  When loaded with the standard No.  50 solid fuel pellets it will produce slightly higher and more consistent thrust than will the 50-B, due to the tighter sealing action of the double spring clips.  This is very helpful in contest flying; and is highly important in the testing and trimming of any model.

For testing of vertical take-off designs, or for operating such models from small flight areas, a short-duration, maximum-thrust pattern may be desired.  In this case the setup shown in Figure 1 would be used.  Thrust and duration will be similar to the left-hand peak of the graph in Figure 2.  Alternatively, by removing the spacer and insulator disk and using a stand-ard solid pellet, a low sustaining thrust will be added to the thrust pattern.  This is shown in the right-hand section of the graph in Figure 2.  This pattern will be found useful far gaining maximum altitude and duration with swept-wing or delta designs--particularly under windy or gusty conditions.

For maximum thrust AND duration the fuel charge shown in the graph of Figure 3 will be used.  This is an extremely high performance pattern which will greatly magnify any maladjustment of the model.  It should not be used until after careful trimming, balancing, and testing.

Many multi-engine combinations may also be worked out, using the 50-HT as a booster unit for high angle launching or to gain maximum altitude.  Because of its very light weight and high thrust/weight ratio, its effect on the wing loading of models designed for larger engines will be of small consequence.  Due to its rapid thrust buildup and short duration, the 50-HT engine should be ignited after the main engine to eliminate any possibility of lapse in effective thrust.

Further information regarding the design and operation of all types of Jetex-powered model will be found in the booklet “All About Jetex,” on sale at your hobby store.

ENGINE INSTALLATION

1. The mounting clip should be securely attached to the model, using both screws and cement.

2. The engine itself must be held firmly in the mounting clip.  Any looseness should be taken up by bending the ends of the clip inward to insure a tight grip.

3. When the engine I externally mounted, at least 1/4" [6 mm] clearance must be allowed between the engine and any combustible material.  As an additional safeguard the adjacent structure may also be protected with metal foil or thin asbestos sheet, attached with a non-flammable adhesive such as water glass [sodium silicate].

4. Internal installations should allow 1/2" [13 mm] clearance all around, plus protective insulation.  Also air venting of the engine compartment is highly desirable.

5. Behind the nozzle some heat protection should be provided for and inch or two.  Beyond this point the rapidly expanding gases are cool enough to be safe.

LOADING THE ENGINE

1.  Before operating the 50-HT engine, study Figure I to become fam-iliar with all the parts.  Open the engine by unseating the two spring safety clips (9). This may be done conveniently by using the small Jetex screwdriver as a lever [a few of the early 50-HT engines came with a cute yellow-plastic handle screwdriver, marked “Jetex” in red].  Then make sure that the mounting clip stud (14) and its lock nut are pulled up tight, as these parts form the seal at the front end of the main case (1).  Examine the nozzle assembly to see that the flame shield (7) and the cap sealing washer (8) are firmly and evenly seated in the end cap (10).  Finally examine the jet nozzle (11) and if necessary ream it out with the wire paper clip provided, to remove any machining burrs [careful here; DO NOT enlarge the nozzle hole!].  The engine is now ready to be loaded and operated.  (Note that certain replacement parts such as the spacer insulator disks, wire gauze disk and cap sealing washer are included in the 50-HT Fuel and wick package.)

2. Insert the fuel charge according to the desired loading pattern.  See Figures 1, 2, and 3.

3.  Take about 3" [8 cm] of igniter wick and insert the doubled end into the fuel pellets an illustrated.

4.  Press the wire gauze disk firmly in place, with the wick passing through the edge notch provided for it. 

5. Coil the reminder of the wick on top of the wire gauze disk.

6. Place the end cap on the main case, seating it firmly and even; then close the spring safety clips.  The screwdriver may be inserted into the loops In the springs and used as a lever to help do this.

7. Now snap the loaded engine into the mounting clip, with the jet nozzle to the rear of the model.

8. When ready for firing, take another 1" [2.5 cm] of igniter wick and in-sert it into the jet nozzle so that the inner end will contact the inside coil of the wick.  Not more than 1/2" [13 mm] should protrude for lighting.

FIRING THE ENGINE

1. Be sure that the engine is held securely in the mounting clip.

2. Push the lead-in wick in so that it surely contacts the inner coil of wick.

3. Light the wick, using a match or any glowing igniter.

4. Hand launching should be delayed about two or three seconds, or until the fuel charge is burning strongly. A little practice will be helpful in determining the precise timing at this point. In static launching, as from a ramp or pad, support should be provided for at least double the total length of the model.

5. In multi-engine booster operation, restrain the takeoff until both engines are producing thrust.

ENGINE CARE AND MAINTENANCE

1. After firing, one minute or more should be allowed for the engine to cool before handling it.

2. Open the engine as soon as it is cool and clean all loose ash from the case, using a wooden stick or dull knife blade.

3. Clean out the jet nozzle with the straight end of a paper clip [gently and carefully!], and scrape all loose ash from the cap and nozzle assembly.

4. Examine the wire gauze disk.  If it is not burned out, it may be put aside for reuse.  This part should be replaced whenever it become so burned or brittle that it ill no longer fit firmly into the main case.

5. If the cap sealing washer is in good condition, the engine is now ready for reuse.  This part should be replaced whenever it becomes soft or fuzzy, or when there are signs of power leakage between the cap and the main case.

6. It is not necessary to clean the inside of the engine to a completely bright surface after use.  So long as the fuel pellets fit smoothly into the case, sufficient cleaning has been done.  The LOOSE ash, however, should never be left in the engine overnight; it may absorb moisture and result in destructive corrosion.