Frequently Asked Questions
February 23, 2013
Isn't Amateur Rocketry today the same as when we had the Basement Bombers of the 1950's and 60's?
NO! The major difference is the propellant used by amateurs today. In the old days, all amateurs had to work with was zinc/sulfur packed powder, packed black powder or potassium nitrate/sugar. The packed powders were extremely dangerous and accounted for most of the accidents reported in the media. There was also "Caramel Candy" (potassium nitrate/sugar) propellant, which was much safer. However, it was a hard, brittle propellant that would often crack upon ignition. This resulted in the motors over pressurizing and failing very dramatically on the launch pad.
Today, most amateurs use the same type of propellant used in the solid rocket industry, composite solid rocket propellant.
What is a composite solid rocket propellant?
Basically, composite propellants are solid particles of oxidizer and fuel suspended in a binder. The binder is liquid when cast into the rocket chamber and sets up or cures to form a rubbery compound. The two most common oxidizers are ammonium perchlorate (AP) and ammonium nitrate (AN). A typical fuel for AP is aluminum powder while magnesium powder works best for AN.
Regardless of what composite propellant is used, they burn in a very predictable way with respect to pressure, will not ignite from a spark, do not require working with molten ingredients and burn very slowly when not confined inside a rocket chamber.
If composite propellants are so much better, how come guys in the 1950's and 60's didn't use them?
Simple. They did not know about them as the composite propellant formulations were classified by the U.S. Government. During that time period, the United States was locked into a "Cold War" with the Soviet Union. A race was on to develop ICBM's on both sides with the United States using composite solid rocket propellants. In the 1970's and 1980's, information about formulating composite propellants was declassified and published in various textbooks. Once amateurs had access to this knowledge, they switched to the composite propellants for safety and higher performance.
You talk about ammonium nitrate propellants in "How To Make Amateur Rockets, but aren't they almost impossible to ignite?
That used to be true, but Dr. Adolf Oberth invented a new ammonium nitrate Space booster propellant for the Air Force when he was a member of Wickman Spacecraft & Propulsion Company. We've simplified the formula for use by amateurs by removing any need for vacuum casting or oven curing. The propellant will light easily with a match.
I don't need to mix or cast in a vacuum? Doesn't that make a pretty lousy propellant with lots of voids?
No. Ammonium nitrate propellants can be made to a very high quality without vacuum mixing or casting. If you want to mix and cast in a vacuum, that's fine. It's just not necessary.
For ammonium perchlorate propellants, a lot of amateurs are putting a bonding agent in the mix for a better bond between the ammonium perchlorate particles and binder. This does generate gas that has to be removed, but why generate the gas in the first place. A bonding agent is not necessary for amateur rockets as the structural strength of the propellant is rarely critical in amateur motors. Just leave it out as we do and you will have no gas generated to remove.
I hear the specific impulse for ammonium nitate propellants is lower than for ammonium perchlorate propellants. Is that true?
Yes, the specific impulse for ammonium perchlorate propellants can be 14 to 20 seconds higher than ammonium nitrate propellants. But, what real impact does that have on the average amateur rocket motor. In truth, not much. Let's look at an example.
The thrust level divided by the Isp gives the propellant weight flow in lbs per second. So the lower Isp propellant, ammonium nitrate, requires a higher weight flow of propellant to give the same thrust as an ammonium perchlorate motor. If we want 50 lbs of thrust for 10 seconds, an ammonium perchlorate propellant motor would require 2.025 lbs of propellant (Isp=246.9). The ammonium nitrate propellant motor would require 2.182 lbs (Isp=229.1). You would need an extra 0.157 lbs of propellant with the ammonium nitrate motor or 2.5 ounces more propellant. That's not very much of a difference.
I keep hearing that ammonium nitrate propellants have a much lower burn rate than ammonium perchlorate propellants. Is that true? And, what does burn rate mean anyway?
Let's answer the last part of the question first. The burn rate is how fast the propellant burns at a given pressure. The higher the pressure, the faster the propellant burns. Ammonium perchlorate propellants burn a lot faster than ammonium nitrate propellants. Consequently, ammonium nitrate motors tend to be longer burning motors than ammonium perchlorate motors. So, if you want long burning motors, go with ammonium nitrate. If you want fast burning motors, go with ammonium perchlorate.
What confuses people is that they think you can not get as much thrust with the slower burning propellant. That's not true. It just means that for a given nozzle throat diameter, you need a little more propellant surface area to get the same thrust as the faster burning propellant. When you are designing your own motors, this is not a problem.
So which propellant is better, ammonium nitrate or ammonium perchlorate?
Both and neither. We feel it is safer for the complete novice to begin with ammonium nitrate propellants since ammonium nitrate is less reactive than ammonium perchlorate. Which is better? Depends on what you want. If you want long duration motors, we recommend ammonium nitrate. If you want shorter duration motors, use ammonium perchlorate. Either type of propellant can produce whatever thrust level or impulse you are seeking. So really, just select the one that is best for your flight.
Does your bookset "How To Make Amateur Rockets" explain about ammonium perchlorate propellants, too?
Yes. We carry ammonium nitrate propellants through as an example, but the same mixing, casting and design techniques work for ammonium perchlorate as well. You have all the tools, software and knowledge to make ammonium perchlorate propellants in the bookset.
What about using PVC pipe and fittings for the motors. I keep hearing that it is dangerous to use PVC. That it fractures which will produce tiny pieces of flying plastic if a motor explodes?
We have been testing PVC pipe motors for almost 20 years and have seen them fail on the test stand. What happens is the end cap or nozzle lets go while the PVC pipe portion remains intact. We have deliberately put in pressure spikes to chambers and can only fragment them with explosive over-pressurization. We have had motors go from 0 to 650 psi in 200 milliseconds. Way beyond the failure pressure of the motor and guess what happens? No fragments, just the end caps or nozzles blow off. The failure point is at the glue bond line.
We can fragment PVC motors when we over pressurize them to levels of 1,500 psi in about 50 milliseconds. But, if your motor is overpressurizing at these levels, there is a lot more wrong with your motors than using PVC pipe and fittings. You have made a serious error in the design of your motor or have a propellant that detonates. The propellants in our bookset "How To Make Amateur Rockets" do not detonate and you should not get pressure rises coming anywhere close to dangerous levels if you follow the procedures in the bookset and use the included software.
Wouldn't reusable aluminum cases be better and safer than PVC pipe and fittings?
Aluminum permits higher pressures which means higher specific impulse or better propellant efficiency. That's certainly better. Being able to reuse the chambers, bulkhead and nozzle would save money, too. If you want to go this route, we recommend buying the reloadable motor hardware set. You can make your own reloads for it using the methods in the bookset.
From a safety standpoint, aluminum is not better than PVC. If the propellant detonates or overpressurizes at almost detonation rates, aluminum will fracture just like PVC. It is a myth that aluminum only splits and does not fragment. We have seen aluminum chambers fragment.
If a case is going to fragment, the PVC plastic is actually better since the plastic fragments will not go as far or penetrate skin as well as metal fragments. To see for yourself, take a small piece of plastic and throw it. Now, take a small piece of metal the same size and throw it. The difference in speed and range is dramatic. The metal fragment is much more dangerous.
Do I need a chemistry laboratory and machine shop to make these motors and rockets?
No. To produce the propellant you will need a simple triple beam laboratory balance capable of measuring to a tenth of a gram. This is necessary to measure the exact amount of curing agent and propellant ingredients. They can be purchased new for around $125 or you may be able to find one at a pawn shop for around $50. For multiple pound batches of propellant, you should have a Kitchen Aid mixer. These cost around $175 to $200. However, you can get by with an electric "egg beater" type mixer which is cheaper. These sell for around $50. The chambers, nozzles and bulkheads can be made using an electric saw and electric hand drill. However, the work is a lot easier if you have a bandsaw, table saw and drill press. The balance of the tools are simple pliers, files and screwdrivers.
If you make the motors out of PVC pipe and fittings you already know how cheap that is. For the larger PVC pipe motors such as the three and four inch motors, you will need a phenolic or graphite insert for the nozzle. These cost a dollar or less per motor firing.
Do I need an engineering or chemistry degree to make my own amateur rockets? Will I be able to understand what is in the book?
The bookset "How To Make Amateur Rockets" is written in simple plain English with every step explained and accompanied by illustrations, photographs and examples. The book was written for someone with no technical background. The video covers the material in the book. You see someone actually do the steps in the book such as mix the propellant and cast it, make a chamber, insulate it, make a nozzle and bulkhead and assemble the motor and test fire it. You also see someone go through the process of making a rocket for the motor and flying it.
Can I get all the chemicals and materials I need to make my own motors and rockets?
Yes, the materials are widely available from a variety of companies. The chemicals needed to make AN propellant are ammonium nitrate, magnesium powder, binder and curing agent. For AP propellants, you will need ammonium perchlorate, aluminum powder, binder and curing agent. None of the chemicals used in making the propellant are on the ATF explosives list.
After September 11th, is it legal to make my own motors and fly rockets?
Yes. The federal government does not regulate the production of composite propellants for your own use. They are not on the ATF Explosives List.
To fly most of your rockets, you will need a waiver from the FAA. It is an easy form to fill out and costs nothing to get a waiver. In the book, we explain the federal law and how to fill out an FAA waiver form and where to send it so you can legally fly your rockets.
You should also check your state and local laws regarding solid rocket motor making. The best place to find out is to call your local fire marshal. If you live in a city, you may have a city fire marshal. If you live outside a city, then contact your county fire marshal. Explain to them you want to know the state and local laws, if any, regarding individuals making their own rocket motors for hobby rockets with parachute recovery. Make it clear to them you are not talking about making fireworks. In most cases, it will be legal. They may ask you to let them know what chemicals, in what quantity and where you will be storing them in your house, basement or garage. In case of fire at your residence, the local fire department wants to know what may pose a danger to fire fighters.
Do I need to be a member of Tripoli or NAR to make and fly my own rocket motors or rockets? Do I need to be certified by either organization?
You do not need to be a member of Tripoli or the National Association of Rocketry (NAR), nor do you have to be certified by them to make your own rocket motors or fly amateur rockets.
NAR does not permit amateur rocket motors at its launches. They are opposed to amateur rocketry activities and still consider all amateurs "basement bombers". Tripoli does permit members to fly their own rocket motors. Tripoli amateurs must also be certified to level two to be permitted to fly their own motors at Tripoli launches.
Ok, I'm going to take the plunge, but what if I have a problem with my motors or rockets? Can you help?
Absolutely! When you buy our "How To Make Amateur Rockets" book, software and video set, you join a growing international community of fellow amateur rocketeers who have bought our books. We are just a phone call, letter or E-mail away. Tell us what you don't understand or what problem you are experiencing and we will give you the answer to get you back on track.