Safety, aspects of testing and flying rockets, and other practical advice.

First safety; Rockets are not safe! They explode, often when you least expect it, and they always operate at high pressures and high temperaturers. Working with rockets is playing with fire.

The first thing you should invest in is a cheap hydrotest pump. Water is mostly noncompressable, so if you fill your tanks with water and then use a small displacement pump you can test your tanks to a proof pressure above the operational pressure you are going to use in the tanks. Most commercial plumbing has a safety factor of four, rockets have safety factors ranging from 1.25 for manned vehicles, to 1.0 for unmanned. I try to hydrotest every component used on the rocket, and even then things sometimes rupture.

It is also a good idea to put some type of barrier between you and the rocket. For hydro testing, this can be a peice of plywood, just to catch fittings that might pop off. Tanks stretch and spring back with considerable energy. When firing a rocket it should be remembered that TNT has only 1/2 the ISP of most rocket propellants. Heavy bunkers are required equipment. This rapidly gets into military fortification construction teniques. About the only web site with overpressure information is: FAS. Heavy lexan or mirrors or TV cameras should be used for viewing. For static testing you want most of the mass between you and the rocket. I put some near the rocket, and some near me. You can almost guarantee that at some point in static testing the engine will explode, and do not expect to duck fast enough, have the shielding in place. For flights, the old addage "what goes up will come back down" is the rule. Fortunatelly rocket tanks are designed to be light, but still it's a good idea to have several layers of concrete and steel above your head when launching rockets, the more the better. Alternating layers of gravel and hard material have been found to breakup the projectile and absorb the energy better.

Even loading the propellants onto the rocket can be dangerous. Often pressures can be high, so a bunker may be needed just for loading. Frostbite can also be a problem so keep several buckets of water available, and some gloves.

N2O is better for small engines because it is available and storable. It is also a very well behaved oxidizer, most of the time. Actually, it isn't an oxidizer until it is heated to about 1000 degrees, which means that you don't have to worry about materials compatibility. Since it we operate our tanks at about 50 deg F, we can use common steel. With liquid oxygen you have to use stainless or aluminum tanks because of the low temperatures. LOX also has lots of compatibility problems, actually it's the oxygen gas that causes small grease spots or organic gaskets to explode, the liquid is too cold. Dispite the problems, for larger vehicles and well run launch organizations LOX is the better choice, but N2O is a good first step. Like H2O2, N2O is somewhat of a monopropellant. It contributes about 50% of the energy. H2O2 has instability and extreme compatibility problems, while N2O only has a minor detonation problem in it's warm high pressure gas state which even then requires an ignition source.

The mix ratio for N2O and C3H8is about 8/1 by weight for best ISP which is easy to figure because both N2O and C3H8 have Mol weight of 44. Most of the energy comes from the N2O which is almost a monopropellant, the C3H8 is almost a catalyst. N2O is a very forgiving oxidizer, because it isn't an oxidizer until it is heated to about 1000 d F, so your injector should give it a little room to vaporize before you mix in the C3H8. One note of caution; To avoid hard starts, use a neon transformer, or 2, for a hot ignition spark near the injector face, feed a little C3H8 into the chamber first, then a little N2O, then when a fire is started, turn the main valves. A chamber full of cold N2O vapor will explode if the C3H8 is late on the scene. for N2O/C3H8, fuel first then N2O into the chamber for ignition, otherwise BANG. Right now we're running hybrids and you can figure on about a mix ratio of 10 / 1. Hybrids are very hard to start. We use a flow of O2 to start a fire inside the combustion chamber. The chamber is lined with that plastic batting material they sell in sewing store for quilts, it forms sticky burning droplets on the fuel surface. After the chamber is saturated with O2, for field starts we use an inch of 28 guage steel wire across 12 voltsDC, and if 120AC is available, we use a neon transformer with electrical zip cord... IT BURNS! with smoke out the nozzle, the main valve is turned. The N2O/C3H8 is the system we started for a regeneratively cooled chamber system, which is now on hold until we find a better test site (one out of range of window glass). Liquid bipropellants have a possibility of exploding, although N2O is still very well behaved even there.

We use the blowdown system of pressurization for N2O, with either CO2 or heat being used to increase the C3H8 pressure. 50 d F works ok on a winter day. A question for you, one that I ask everyone who asks me a question; On blowdown presurization systems there is one question I have yet to find an answer to; the pressure drop inside the tank during discharge. I have looked through many texts and references and have asked many people, but no one has given me a specific equation yet. I even wrote a simple Basic program to model the process with all the thermodynamic data built in, which gave some very interesting results,(several types of liquids gave nearly the same pressure history), which leads me to believe that we're dealing with a basic thermodynamic principal here, just one no one has written down where I can find it. Do you have a reference for blowdown pressure drop? I'll send you a copy of my program, (shareware, but really cheap).

I have found no logic to the price of N2O. I use two sources. For small quantities, <10#, I buy it for $3/# at the local auto racing store. It's mixed with SO2. For 50# lots I buy it from a local industrial gas supplier for $1.80/#. When I tried to buy 400# in a cryo cylinder the price went up to $2/#? I have been in touch with the manufacturer, and they have a price of about $0.50/# in 20,000# lots.