"Especially after the space shuttle Columbia disaster we know that there's interest in learning how these things function," Davis told SPACE.com.
Model rocket engines are classified using letters, with an A-engine being the least powerful. A B-engine is twice as powerful as an A-engine, a C-engine is twice as powerful as a B-engine and so on, with the most powerful engine classification up around the N level. The term engine and motor is used interchangeably.
Engines in the A through D range are the most common, made using black powder and most popularly manufactured for decades by Estes Industries of Penrose, Colo. These engines are well below the 62.5-gram plateau at the heart of the new ATF rules, said Estes owner Barry Tunick.
"The hobby of Estes model rocketry remains totally secure at this point," Tunick said. "There are no problems getting our rockets and it still remains one of the safest hobbies."
Rocket motors at the E level begin to make a transition into a higher powered form of the hobby, with F- and G-engines used by much more experienced rocketeers. H-engines and above are generally the province of experienced adult hobbyists who fly from fields measured in square miles and launch with clearance from the Federal Aviation Administration.
The 62.5-gram level for a single engine is reached around the mid-G-motor level, said Trip Barber, vice president of the National Association of Rocketry (NAR), the hobby's recognized leader in promoting model rocketry.
Depending on the manufacturer, motors starting in the medium range around the F-classification and above use an ammonium perchlorate composite propellant (APCP). Both black powder and APCP appear on the ATF's current list of explosives that are subject to the regulations of the Safe Explosives Act of 1970.
That inclusion of APCP and black powder on the explosives list has been a point of contention by the rocketeers toward the government from the very beginning, a fact underscored by U.S. Sen. Michael Enzi, R-Wyo., who is planning to propose a "technical correction" to the rules regarding model rocketry.
"Congress defined an explosive as any chemical mixture or device whose primary or common purpose is to function by explosion. The ATF claims that the primary or common purpose of a rocket propellant (i.e. ammonium perchlorate composite propellant) is to explode. I am told that if the ATF's interpretation is true, a hobbyist's rocket motor would explode upon ignition. A rocket propellant is not designed or intended to explode," Enzi wrote Feb. 3 to the Senate Majority Leader.
Davis at the ATF said she is aware of the objection some have about including certain chemicals on the explosives list.
"There are a number of propellants on our list of explosive materials. They don't necessarily explode, but they deflagrate. That would classify them to be put on our list for purposes of oversight and control on their distribution, manufacture and use," Davis said.
Deflagrate means a rapid burning of a chemical with intense heat.
Many model rocket enthusiasts have e-mailed SPACE.com since a Feb. 25 story first raised concerns about the new rules to say that gasoline, for example, is not included on the list and is much more dangerous than either APCP or black powder.
Davis' response was that other federal laws specifically exempt gasoline from the same treatment that other explosives receive.
As a result of the new rules adopted by the ATF, hobbyists that want to continue to handle and fly rocket motors with propellant amounts greater than 62.5 grams will be able to do so by applying for a permit, undergoing a background check, paying a $25 fee and getting fingerprinted.
"There's a process for that and it's not an impossible process," Davis said, noting that application forms are available on Internet.
Nevertheless, high-powered rocket enthusiasts are not happy.
"A rule that anyone firing a rocket larger than an Estes must get a $25 permit, undergo a background search and give tacit consent to a full-body cavity search would probably discourage a fair number of people from the sport," rocketeer Dave Klingler wrote SPACE.com.
In addition to putting regulatory constraints on their weekend fun, the high-powered rocket group is concerned that in many cases it's adults flying the bigger rockets who mentor younger students just learning about model rocketry.
"If adult participation is adversely impacted by new regulations, as I believe it will be, then we will be missing a critical element in supporting youth rocketry of all forms," said NAR president Mark Bundick. "We need adults involved in rocketry activities to show the young people interested in technology the way to go."
It was that very type of inspiration that motivated astronauts such as Jerry Ross and Jay Apt when they were younger to pursue careers in aerospace.
"There will always be a small group of highly motivated adults to help these kids, but the growth of the past 15 years of our hobby with a large, active adult membership will be lost if these regulations come to pass," Bundick said.
The key logistical problem created by the ATF regulations involves shipping the higher-powered engines with more than 62.5 grams of propellant.
Under the new rules shippers will have to employ a number of costly security measures to continue to ship the larger engines or be forbidden outright from shipping the bigger motors.
"If there is a material out there that is forbidden to be shipped, carried, transported as stated by the AFT -- setting aside exemptions -- we are not carrying it," said David Bolger, a United Parcel Service spokesman in Washington D.C.
Bolger said UPS is closely following the discussions on Capitol Hill about shipping potentially hazardous materials. However, given the number of federal agencies that have a say in shipping material such as model rocket engines, there still is some confusion about what is and isn't allowed to be shipped under various rules.
"We have 14.1 million packages we deliver every day and every one is important. People who ship eggs, or CD-ROMs or even model rocket engines, are very important to us. If there is a product that is unable to be shipped for a variety of reasons, we want to understand why and provide our customers with all the rules and regulations that exist," Bolger said.
Smaller engines, such as those made by Estes, can continue to be shipped following the rules already in place, Davis said.
"Sometimes if there's confusion people just err on the side of being conservative and decide not to ship anything, but that's a business decision on their part," Davis said.
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