Health and Safety

Health and Safety is an extremely important side of our project.

A diagram is shown below to illustrate our safety idea in the design of a hybrid rodnet:

Hybrid motor diagram

Some general precautions that apply:

Risks of paraffin (candle wax):

  • Can cause burns if put on skin when melted
  • Burns steadily on every surface exposed to air if lit when melted
  • Similar to oil fires, if water is added when it is on fire, a fireball can occur; see this video

Ways to prevent these things happening:

  • Wear gloves when handling liquid paraffin
  • Don’t have ignition sources near the paraffin
  • Put out fires with CO2 extinguishers, avoid adding water to liquid paraffin fires

Risks of gaseous oxygen / oxygen enriched environment (see this website, these HSE guidelines):

  • If something is on fire, it will make it burn very vigorously
  • It will make things set alight much easier
  • Many materials that do not burn in normal atmosphere will burn in oxygen
  • Oil and grease react violently with pressurised oxygen.

Ways to prevent these things happening:

  • Avoid oxygen leaks by testing for leaks using leak testing fluid
  • Cut off oxygen supply as soon as a leak is found
  • Keep all oxygen tests outside, so that any leaking oxygen can disperse quickly
  •  Remove grease / oil from all parts, and test parts by running oxygen through them, without lighting the motor.

Risks of oxygen tanks (see this report, these HSE guidelines):

  • A malfunctioning tank can leak oxygen, causing the same risks as above
  • If the tank is heated, the oxygen inside can build up pressure until the tank explodes, although this requires either sustained heating or concentrated heating.
  • Should a leak occur, the plastic handle on the tank will catch fire quite readily

Ways to prevent these things happening:

  • Keep oxygen tanks outside, so any oxygen buildup can quickly disperse
  • Keep all heat sources away from the tanks, and store them in a shaded place outside
  • Turn off oxygen valve if a leak occurs anywhere except on the tank
  • Leave the tank to vent in a safe space but do not turn the valve if the tank is leaking

The following list details all the ways the rocket engine could fail, and their associated risks:

  • Rupture of combustion chamber walls:
    • This would happen on the sides of the combustion chamber, and would likely cause the paraffin core to explode due to the high pressures inside the combustion chamber being held purely by the wax.
    • This could send pieces of paraffin out along the crack (Fig. 1.)in the chamber walls to the left and right of the motor.
    • These chunks would not be burning, as the thin layer of liquid paraffin would have been blown off during the explosion, and the exposure to open air would result in rapid cooling (similar to blowing a candle out), stopping any combustion occurring.
    • The metal containment of the motor would not send shards of metal out, because the mild steel we are using will crack, instead of shattering, under pressure, as shown by Fig. 1.
    • We will prevent the pieces of paraffin that could be launched out from harming us by standing far away or standing behind a blast shield.
    • Shown by Fig. 1.
Hybrid chamber simulation1
Fig. 1: A simulation showing an exaggerated diagram of what happens when the chamber pressure is just over 1000 psi. It shows the area of greatest stress being along a single line.
  • Failure of the rear end cap (injector):
    • This would involve the rear block fracturing, resulting in exhausts going backwards, potentially causing a fire on the oxygen feed line.
    • Prevent adverse effects by spraying water onto the entire combustion chamber section to ensure it is cool, and if any exhaust gases begin firing backwards, they cannot start a fire due to the water.
    • Also, the half meter of pipe leading from the combustion chamber to the cut-off ball valve is made out of metal, so will not burn.
  • Failure of the front end cap (nozzle):
    • This would involve the nozzle walls deteriorating / melting and thus increasing the minimum diameter of the nozzle, resulting in a decrease in chamber pressure and temperature (i.e. combustion rapidly slows).
nozzle grooves
Fig. 2: A diagram of the nozzle section of the motor. The nozzle is made by coring out a nozzle shape from a solid rod of steel. There are grooves on the ends of the parts so that they interlock, forming a seal. The combustion chamber pressure pushes the inner groove against the outer groove, thus making the seal
  • Failure of joints between ends and walls:
    • This would involve the rings of metal that bolt the pieces together developing a gap in-between, which would result in exhaust gases firing out perpendicular to the walls of the rocket. It could also let liquid paraffin drip out underneath the motor.
    • This could cause anything flammable around the rocket to be set alight.
    • Prevent the surroundings from being set alight by spraying the area with water during the test.
    • This is highly unlikely, since the pipes have grooves cut into the ends so that they interlock (Fig. 2). This means that the chamber pressure will cause the seal between parts to improve, meaning no exhaust can escape.
  • Failure of welds between oxygen feed pipe and rear end cap:
    • This would involve the welds developing leaks, causing oxygen to leak into the surroundings.
    • This will cause the air around the motor to become oxygen enriched, increasing the risk of things catching fire.
    •  Prevent oxygen from building up by testing the motor outside, so that oxygen can quickly disperse.
    • We will prevent things catching fire by spraying the motor and its surroundings with water during the test.
    • This is highly unlikely, because we will test the welds for leaks, and if a leak is detected, we will weld over it, therefore stopping any leaks from occurring.
  • Failure of oxygen feed pipe:
    • This would involve a crack forming down the metal feed pipe, causing a lot of oxygen to leak.
    • This would result in similar things as shown in the above scenario.

Below is a diagram of how a hybrid motor tests could be set up:

 

To light a motor, the ball valve will be shut, and all cameras would turned on. Then the oxygen tank valve would slowly opened. After that, an ignitor would lit and placed into the motor through the nozzle. While that is happening, the water tap would be turned on, spraying water over the motor*. All people would be behind the blast shield. Finally, the ball valve would be opened, starting the test.

Should there be any indication of something going wrong, the chain can be pulled, closing the valve and so stopping the oxygen flow.

At the end of the test, the chain would pulled to close the ball valve. Once the motor is only smouldering, the oxygen tank valve can then be slowly closed. A metal cover can be placed over the nozzle to starve the paraffin of oxygen, and so put out the burning paraffin.

*Note: the water will not actually contact a large amount of lit liquid paraffin, so will not cause a fireball as mentioned in the general precautions section. Instead, it will cool the motor and any leaking exhaust gases, preventing a fire from breaking out. 

If there is anything I have missed, please contact me via email: commraid_@hotmail.co.uk