IGNITION and OPERATION

	Discussion of propellant ignition has been reserved until this point since 
it is really a test stand function and is required only for actual operation 
of the engine. The propellants used in amateur rocket engines require a 
separate source for ignition. Because the engines are small, the use of an 
engine-mounted spark plug is not generally feasible. Even if it were, the 
ignition of incoming propellants in the combustion chamber by a small spark 
plug is dangerous and unreliable. Propellant timing is extremely important 
in a bi-propellant liquid rocket engine. An excess of either propellant 
(if both are liquid) in the combustion chamber can lead to severe 
over-pressure upon ignition (known as "hard" start) and possible fracture 
of the combustion chamber. The amateur engine using gaseous oxygen is not 
nearly as sensitive to hard starts as if the oxidizer were a liquid.

	Hundreds of tests with small liquid-fuel rocket engines employing gaseous 
oxygen as the oxidizer have indicated that hot-source ignition provides 
excellent propellant ignition characteristics, and drastically reduces 
hard starts. Hot-source ignition works as follows: two lengths of insulated 
#16 or #18 solid wire are taped together and their exposed ends are bent to 
form a spark gap of about 3/32-inch. A small amount of cotton is wrapped 
around, or attached to, thc wires very near the spark gap but not obstructing 
it. This ignition assembly is pushed through the nozzle into the combustion 
chamber of the rocket engine so that the spark gap is in the lower end of 
the combustion chamber but not blocking the nozzle throat. The wires outside 
the engine are bent or taped to hold the ignition assembly in position 
during the ignition phase. The free ends of the two wires are attached to 
the spark source (a Ford Model-T spark coil is ideal for this purpose). 
Figure 13 details this hot-source igniter. The ignition procedure, after 
the test stand is prepared for firing is:

Figure 13 Hot-source igniter for small liquid fuel rocket engines using 
gaseous oxygen oxidizer. Ignitor is consumed during each use and must be 
replaced.

l. The operator ascertains that the area is clear and ready for firing.

2. The operator checks operation of the spark coil and then disconnects the 
coil from the battery for safety. The battery should be at the operator's 
remote station.

3. The ignitor cotton is soaked in gasoline or kerosene .

4. The ignitor is pushed through the nozzle into the combustion chamber and 
secured.

5. Gas cylinder valves are opened, the fuel tank is pressurized, and all 
gas pressures adjusted to operating values.

6. Cooling water is allowed to flow through the engine at the proper rate.

7. The firing bell or horn is sounded. The spark coil is reconnected to 
its battery.

8. The oxygen flow needle valve is opened very slightly to allow a very 
small flow of gaseous oxygen to pass over the ignitor and out the combustion 
chamber .

9. The spark coil is energized. Inside the combustion chamber the cotton 
igitor should immediately burst into flame in the oxygen atmosphere. The 
operator may have difficulty ascertaining that the cotton is actually 
burning although small flaming bits of material may be ejected from the nozzle.

l0. The fuel control needle valve is now opened very slightly to allow fuel 
to flow into the combustion chamber. A flame should immediately appear at the 
nozzle exit and a low whistling sound should be heard.

11. The oxygen and fuel flow rates should now be rapidly and 
simultaneously increased by opening the control needle valves until tie 
combustion chamber pressure gauge indicates that desired conditions 
Exist inside the chamber.

12. The operator will need to judge whether more or less oxygen is required 
for desired O/F ratio operation. More oxygen is required if the exhaust is 
bright yellow or smoky. (this is an indication of unburned carbon in the 
exhaust); if the exhaust is transparent or bluish the oxygen flow should he 
decreased slightly. The correct mixture ratio is achieved when the exhaust 
gases are transparent (or nearly so) but the supersonic standing shocks 
(Mach diamonds) in the exhaust are clearly seen. Remember that as you 
vary the fuel and oxidizer flows you are changing not only the amount 
of material passing through the engine but are also affecting the 
temperature of the burning gases. Both of these effects will affect the 
combustion chamber pressure.

13. The noise from the engine will he quite high, but it is a good indicator
of engine operation.  It may be necessary to wear ear protection because of 
this high noise level.

14. The operator should have a timer or have someone time the engine run. 
It is quite safe to simply let the engine run out of liquid fuel. The 
gaseous nitrogen pressurizing the fuel tank then purges the fuel supply 
system automatically. The engine will abruptly stop operation and the 
operator can then turn off the flow of gaseous oxygen. If the engine is to 
be stopped prior to fuel depletion the fuel flow control valve should 
be quickly turned off, followed by opening of the nitrogen purge valve. 
After the engine has stopped operation (thus assuring that the nitrogen 
purge has forced all fuel from the engine) the gaseous oxygen valve may 
be turned off. The
nitrogen purge valve is closed, the cylinder valves are closcd, and the 
fuel tank vent valve opened. The oxygen line is vented by briefly  
opening the oxygen flow need1e valve. Water should be allowed to 
flow through the engine cooling jacket for several minutes after run 
termination.

l5. In the event of engine failure, the shutdown sequence detailed in 
(14), above, should be followed. Always shut-off the liquid fuel first. 
If engine metal parts are burning, also immediately shut-off the flow of 
gaseous oxygen (metal will burn vigorously in an oxygen environment).

16. A new ignitor will be required for each ignition attempt or firing. 
The ignitor assembly is partially consumed during the ignition process 
and residue is quickly blown from the combustion chamber upon ignition of 
the liquid fuel.

17. Always inspect the engine and other components for damage, apparent 
overheating or hot spots prior to another firing.

18. Some engine designs may exhibit combustion instability (chugging, 
chuffing, erratic combustion, etc.) at low chamber pressures or low fuel 
injection velocities. To avoid this problem, the operator should rapidly 
increase the chamber pressure after initial introduction of the 
liquid fuel.

	Ignition and operation of small liquid-fuel rocket engines in the manner 
described offers the amateur a relatively safe and interesting activity. 
The operator will quickly discover and use many procedures to improve 
engine and test stand operation.

	After achieving initial operation of the engine and test stand, 
the amateur can begin to consider methods of measuring engine thrust,
determining the heat transfer to the cooling water, and noting 
the characteristics of the rocket engine exhaust. Photography 
of this exhaust is a definite challenge.  As these additional 
features are added to the experimertal set-up, the amateur 
should always keep safety and safe operating procedures foremost in mind.
