"Official Nomenclature" 

There are some "official" terms used to describe the various Jetex motor parts and components.  Here are the basics, based on a Jetex 150-PAA Loader engine.

1:  Main case (in larger rockets, called the thrust chamber 
2:  Spacer (a balsa plug, used when single fuel pellets are burned) 
3:  Insulator disc (used between the single fuel pellet and the spacer) 
4:  Pellet (propellant) 
5:  Jet wick (fuse) 
5A:  Wire gauze disk (serves as a burn rate catalyst) 
6:  Flame shield (protects end cap and nozzle from propellant burn heat) 
7:  Cap sealing washer (this is the over-pressure safety seal) 
8:  Spring safety clip (which keeps the case closed, unless pressure gets too high) 
9:  End cap assembly (in larger rockets, called the "aft closure assembly") 
10:  Jet nozzle (only the exhaust hole; this is not a separate part) 
11:  Jet wick, lead in (represents the exterior part of the wick/fuse)

Jetex Motor Specifications (Comparative)

Data posted in green is from original factory packaging and instruction sheets
Data in black is from our own recent measurements (bold) or other "non-official" sources
Figures in red indicate extrapolations, and must be regarded as approximations at best

Lab Reports

A rather intensive series of scientific tests were made of the Jetex engine series in 1953-1954, by Ron Warring for Aeromodeller magazine.  This is the best performance information we've yet found.  The following remarks are based upon articles titled “Engine Analysis No. 15 (New Series), by Ron Warring, in Aeromodeller (Nov & Dec 1953, and Jan 1954), to which we give credit.

Click here to view "Engine Analysis No. 15 (New Series)"
 

Augmenter Tubes

The makers of Jetex originally offered two sizes of "augmenter tubes," which were formed from thin aluminum sheet; the forward end is bell-mouthed and considerably larger in diameter than the Jetex engine (one tube size is for the two 50 engines, the other for the 150 and 600), and the edge of the bell is placed about even with the rear of the engine case. 

When the jet is operating, cool air is drawn into the tube mouth and is expelled at the tail of the model along with the hot gases.  This air not only cools the gases but contributes to thrust.  Figures show that the augmenter will increase thrust from 20 to 30 per cent. 

Ignition is not difficult with these tubes in place; they prevent the sparkling fuse from burning the model interior, too.  To work successfully, plenty of air must be able to reach the forward end of the augmenter.  This accessory is normally used only inside closed fuselages, as it would have quite a lot of air drag on an open-fuselage Jetex-engine installation. 
 
 

...  ... 

Jetex 50 augmenter tube kit [click B&W images to see full-size color illustrations] 
 
 

Jetex Jetmaster augmenter tube kit, with the extension tube made in two parts that clip together [click image to see full-size color illustration] 
 

...

Jetex PAA-Loader 150 augmenter tube [click images to see full-size color illustrations] 
 
 

Other Jetex Accessories

Among the curious and unusual Jetex accessories we've seen is this Dempster Turb-O-Prop, a Techni-Bilt Product from the Dempster Co. in Kansas City. Billed as "the first practical replacement for rubber-band power", it was spun by the exhaust gases from the 50-size pellets burnt in the unit in front of the propellor.  One of these sold on eBay in November 2002 for $170.50. Does anyone know if it works? 
 

[Click image to see full-size drawing] 
 

[Click image to see full-size color photo] 
 

Rapiers and Other Micro Rockets

There are several other kinds of small rockets on the market today, but none has the features of the Jetex.

Most notable of these is the Rapier, similar to Estes motors in configuration, with Jetex-like performance. Graham Knight reports:

" ... they just look like an Estes rocket, cardboard tube with a nozzle at one end.  Performance,  L1 is less power than a Jetex 50, I guess about 70%, L2 is a bit more, maybe 110%. I haven't seen an L4 yet but hear it is more power than L2 and burns longer. Burn time for L1 & L2 is around 20-25 secs.  Performance does vary a bit from motor to motor.  If you like Jetex you'll love these!" 

We've also heard that there are no mounting provisions accompanying the Rapiers.  However, the instruction sheet suggests several options, among them the use of Velcro; gluing the motor to the model; metal clips; and slipping the motor into a pre-formed paper tube attached to the airplane. 

"The units are about the same diameter as schoolroom chalk, 1-1/2" long for the small units, and 2 inches for the larger size.  Each motor consists of a cardboard outer casing (about 1/16" wall thickness) filled with molded propellant.  Unlike the old Jetex motors, Rapiers have no nozzle.  However, like Jetex they are ignited via a separate fuse, which fits into a hole molded in the "output end" of each motor."   

The Rapier motors are made by: 

Dr.-Ing. Jan Zigmund 
Jirankova 2281 
530 02 Pardubice 
CZECH REPUBLIC 

We received this technical information from Dr. Zigmund: 

Model jet propulsion units Rapier are designed for recreation and sport flying to power balsa or paper aircraft models. The units are manufactured of a card tube that is filled with pyrotechnic propellant. On one end the unit is closed with a refractory nozzle, on the other end with a refractory cover. Provided the directions for use are followed, the construction of the units provides the user with a sufficient safety when handling the unit. No dangerous fragments of material fly away from the card tube on a prospective rupture of a unit. If a rupture of a unit occurs, the wall of the card burning chamber usually burns through and the propellant burns up quietly. Thickness of the tube is chosen to allow the tube surface to be heated approximately only to 150°C after the burn up of the propellant. Interior of the card tube is charred in so far that its prospective filling with a home made propellant leads to a crush of the chamber. 

 Model jet propulsion units Rapier are marked by a low thrust and long burning time. Dependent on chemical composition of the propellant it is possible to obtain burning time of about 20 seconds and thrust ~50 mN (5ponds) at Rapier L -1 units, and burning time of about 25 seconds and thrust ~75 mN with Rapier L -2 units. L - 1 and L - 2 units are designed to power chuck - gliders and aircraft models with wing span 250 - 400 mm and weight up to 25 g. I recommend that the weight of models should be from 2x up to 3x of thrust of jet unit in ponds (see performance of jet unit at the side of a box). The achieved flight times are 40 - 300 seconds. 

 Model jet propulsion units Rapier are analogous to widely spread JET-X motors that could have been used repeatedly. Jet units Rapier are designed, contrary to JET-X, for one flight only. The possibility of repeated utilization has been replaced by more comfortable handling and safer function. The unit is simply inserted by its front part into the container fitted to the aircraft model. With this arrangement the unit is cooled during the flight and after burnout it is easy to remove from the container without any danger of burning. Burning chamber is thrown off after burnout. 

 My aim is to offer the modellers romance and excitement of flight of jet unit propelled model again. I would like to let the modellers have a possibility to experiment and find their own way to the best construction of an aircraft model. 

My intention is to produce jet units of L line (series) i.e. for chuck gliders and aircraft models. The units will be packed in green boxes and marked with L letter. 

 According to customers’s interest I would like to produce also modellers rocket motors in future. They will be packed in red boxes and marked with R letter to avoid mistaking them for L units. 

 I would like to express my thanks to all modellers and friends who encouraged me to forward with development of Rapiers. 

 

	Rapier L-1	Rapier L-2	Rapier L-4*
Length (mm)	40	50	50-60
OD (mm)	10	11	14.6
ID (mm)	6.5	7.5	10
Weight, loaded (g)	5.0	7.3	11.6
Weight, dry (g)	2.1	2.8	5.0
Weight, propellant (g)	2.9	4.5	6.5
Propellant mass fraction (Wp/Wl)	0.58	0.62	0.56
Duration (sec)	~20	~20	14-20
Thrust (avg, mN)	~50	~75	~150-170
Total impulse (total, Ns)	~1	~1.75	~2.5

* in development 

Dr. Zigmund's letter also refers to the Czech certification of the motors and fuse (Decision No. 214 - 99 - 0167/V 06317 was rendered on August 12, 1999 by the Authorized State Testing Laboratory No. 214 VVUÚ a.s., Pikartská 7 716 07 Ostrava - Radvanice, Czech Republic:  to certify "Model jet propulsion units  RAPIER"  At the same time decision No. 214 - 99 - 0168/V 06318 was rendered to certify "Fuse type 07" designed for Rapier units.) 

Thank you, Dr. Zigmund.  We're now converting his Standard Units to a more familiar British Units, for ready comparison to Jetex specifications. 

There are several other kinds of small rockets on the market today, but none have the features of the Jetex.  All of the other reloadables are bigger--much bigger.  And all the rest of the small rockets are really high-impulse boosters, offering nothing in the way of sustained power. 
 

This illustration shows both the Apogee 1/4-A and the Estes B sized conventional (high thrust, short duration) model rocket engines.  The Apogee 1/4-A motors, propelled by black powder, are quite light and effective when a very short burn time can be applied.  The B size motors, also with black powder, are the next step up in physical size and performance.  These two motors represent the main alternatives to Jetex propulsion in micro rocket sizes.  Great for their own special (ballistic) applications, neither work well for freeflight aeroplanes. 

[click image to see full-size graphic] 

This graph shows the thrust curves for several common 1/2-A through D size conventional small rocket motors.  Note the thrust is at best one or two seconds.  1/2-A motors produce a total impulse of about 0.28 pound-seconds (1.25 Newton-seconds).  The larger D motors produce a total impulse of about 4.5 lb-sec (20 N-sec), and reach nearly 7 pounds at peak thrust.  The largest Jetex engines produce perhaps 6 ounces of thrust, for perhaps 15 seconds. 

NAR Standard Model Rocket Classifications

Larger rocket motors are customarily rated by their thrust and burn duration, the product of which is called their "total impulse," and is often rated in pound-seconds.  This refers to the equivalent amount of forward thrust the motor produces, in pounds, if all the propellant burned for one second.  Thus a motor with 500 lb-sec total impulse might burn at 100 lbs thrust for 5 seconds, or 1 lb thrust for 500 seconds.  The important thing to remember is that "total impulse" refers to total thrust yield over the burning duration; this is not the same as average thrust (unless the motor happens to burn exactly one second). 

Total impulse is the starting point for determining a rocket propellant's efficiency, which is called "specific impulse," or Isp.  Specific impulse is the total impulse (power generated) of the propellant divided by its weight.  This number is measured in "seconds." 

Thus, a motor which produces 500 lbs thrust for 1 second has 500 lb-sec total impulse.  If this motor burns 5 lbs of propellant to produce this thrust, then 

500 lb-sec / 5 lb = 100 lb-sec/lb, or 100 sec 

Model rocket motors have long been classified by a standard adopted by the (US) National Association of Rocketry.  These apparently arbitrary classifications have proven useful for both standardization of motor sizes and performance, and for safety classifications by legislators. 

For many years the only model rockets produced were of the A, B, C, and D classifications, which produce less than 20 newton-seconds (or about 4.5 lb-seconds) of total impulse.  More recently, smaller and larger motors have reached the market, including the new Micro-Maxx (by Quest) which is rated at 1/8A, and produces less than 1 ounce-second of total impulse.  The largest commercial "model" rockets today go into the M and N classes, and produce hundreds of pound-seconds of total impulse. 

Jetex motors fall within the A through D classes shown in the table below, based upon their total impulse.  Jetex 50 is an A class motor, for instance, producing about 0.5 ounce thrust for about 15 seconds, or a total of 7.5 ounce-seconds total impulse.  The "big" Jetex 600A Spacemaster produces over 200 ounce-seconds of total impulse. 

Note that the NAR motor classifications are based only upon the total propellant impulse, as measured in newton-seconds.  A newton-second is the thrust force of one newton operating over a one second period of time.   (1 lb = 4.448 newtons; 1 oz = 0.278 n; 1 gm = 0.0098 n)