Mark’s Advice

If you're thinking about getting into liquid rocketry for the first time, and don't know where to start, here's some straightforward advice from Mark Holthaus of FAR. If you're wondering who this guy is, he's worked on experimental rockets and spaceplanes at Boeing and appeared on Mythbusters:

1. Start as simple as possible. Don't start with metal printing.

2. Use denatured alcohol (from Lowes, Home Depot, or Ace Hardware) and LOX as propellants. These are the easiest to get and burns cooler than LOX and Kerosene. The denatured alcohol is 50% ethanol and 50% methanol. (NOTE: Its sale was recently banned in a few states including California, so you may need to purchase regular ethanol.)

3. Go blowdown. Fill tanks half way and pressurize the ullage. No pumps or pressurization systems. The thrust will start at maximum (500-lbf) and go to half (250-lbf) if you use half propellant and half ullage.

4. The 2.5-gallon stainless steel water fire extinguishers make great propellant tanks. McMaster-Carr 6423T3. They burst around 1000-psi. You have to replace the valve and weld a fitting on and drill and weld a pressure fitting on the bottom of the tank for pressurization.

5. Make your rocket engine ablatively cooled. Use silica cloth and phenolic resin to make the ablative.

6. Use a chamber pressure of 300-psi.

7. Make your injector from aluminum and design it for 100-psi pressure drop.

8. Use simple single-impingement injector holes.

9. Stay under 500-lbf thrust. (or smaller)

10. Buy a pre-made tube for your rocket body. Home Depot SAKRETE 8 in. x 48 in. Concrete Form Tube is one option.

11. Use full-port stainless steel ball valves from McMaster-Carr 46495K21. Be sure you take them apart and clean them. They may have oil behind the ball.

12. Use seamless stainless steel tubing from McMaster-Carr 89895K742.

13. Clean all LOX lines, hoses, tanks, fittings, and valves with the following:

       a. 25% Simple Green and 75% water to remove the oil.

       b. Rinse with distilled or deionized water to remove the Simple Green

       c. Rinse with Isopropyl Alcohol to remove the water.

       d. Dry with warm air to remove the isopropyl alcohol.  When you don't smell alcohol it is dry.

14. Use double-compression stainless steel fittings. They are easy to get from McMaster-Carr or Titan Fittings. Making 37-degree flares on tubing is too difficult to get right the first time for AN or JIC fittings. Avoid NPT fittings as much as possible. NPT when tight and not leaking will end up pointing in the wrong direction.

15. Insulate all tanks, lines, and fittings that touch LOX.

16. Add relief valves to LOX tanks set at 125% operating pressure. Don't set it too close or it'll keep popping off.

17. Have fill and drain valves located between the tank and the motor.

18. Add remote operated solenoid valves at the top of tanks used for emergency venting only. Continuously venting LOX as GOX through a solenoid can freeze it in place.

19. Add remote readable pressure transducers for each tank.

20. Paint your rocket white and not silver or black. It will make it easier to load LOX on a hot day. (Rick Maschek also suggests white because it will be the easiest color to find out in the desert)

21. A LOX vent should be at least 3/8" for most rockets. 1/4" is too small to vent quickly enough.

22. Avoid using solenoid valves for your main valves. They are too heavy and have too much of a flow restriction. Use full port ball valves like those listed above for this purpose.

23. Plastic flex lines are not compatible with LOX, They become brittle and break when they reach LOX temperature. PTFE is an exception, but is always found in steel braided flex lines.