We can all probably agree with the relative levels of cleanliness. Around the bottom of the scale, there’s “I Am Comfortable Living In This.” A little more clean, you probably find “Company is Coming,” closely followed by “My Parents Are In Town.” Near the top of your list, you probably find “Apartment On The Market.”

“Clean for Oxygen Use” may top the charts. It’s certainly not a household standard.

This cleaning method is the entire reason we constructed our clean room. Much of our rocket will come into contact with high-purity oxygen, whether in liquid or gas form. Gaseous oxygen loves lighting things on fire, and liquid oxygen freezes most substances solid — so we need to be sure everything is as clean as humanly possible. To do so, we have a six-step cleaning process.

Step One: Alcohol Bath

After we identify a subassembly to clean, we remove each piece from storage. We bathe each individual fitting, pipe, adapter, and valve in isopropyl alcohol. (That’s the same alcohol you put on wounds to clean them.) For 12 minutes, they rattle around inside an ultrasonic chamber. By vibrating them very, very quickly, the machine dislodges defects, dust, and other gunk that is clinging to them.

Isopropanol is also a dehydrant. This agitation bath ensures every out-of-the-way nook and cranny is water-free. Any water left in the system would freeze in contact with cryogenic liquids, decreasing functionality and making the rocket explosion-prone.

Step Two: Nitrogen Purge

After they’re removed from the bath, each part is individually inspected for remaining debris.

Then, every part is dried with a pressurized jet of filtered nitrogen. Not only does this ensure no isopropyl alcohol is left on the part, it blows away any remaining foreign materials.

Step Three: Alcohol Rinse

As if Step One weren’t enough, we then subject each component to yet another round of alcohol. This time, the isopropanol is targeted in a stream. The entire part is washed beneath a squeeze bottle before moving on to Step Four.

Step Four: Nitrogen Purge

More drying! Like most alcohols, isopropanol is flammable so we need to make sure each part is bone-dry before assembly. This last round of nitrogen is usually enough to get the last bits of stubborn junk off of our fittings.

Step Five: Critical Inspection

Once the second nitrogen blow-down is complete, we are fairly certain nothing remains. But just to be sure, though, we inspect each piece from every angle for leftovers. Inside and outside, nothing is allowed to escape our prying eyes. And on the off-chance we still find refuse holding on? We restart the whole process from scratch. We bought smaller ultrasonic bath just for that purpose.

Step Six: Assembly

Finally, we are sure that our parts are Clean for Oxygen Use. We bubbled, tossed, dried, washed, and dried most everything (and even brushed some with a high-grade pipe cleaner), and it’s time to put the pieces together. One by one, being sure not to stir the air or drop anything, the rocket starts taking shape. We have 24 subassemblies ranging in size from one component to thirteen.

Each über-clean subassembly is then given a new home on the high shelf in our clean room. Small subassemblies are bagged and given a unique name so they don’t get confused down the road.

And that’s how you make a rocket Clean for Oxygen Use!