Neil:
BEFORE dealing with the ATF, you need to contact your local fire official and get their permission if you plan on placing this type 4 magazine in a detached garage...
from NFPA 1127:
NFPA 1127: High Power Rocketry
This section deals with motors over 62.5 grams, here is the important stuff on storage requirements:
Section 4.19: Storage of High Power Rocket Motors, Motor Reloading Kits, and Pyrotechnic Modules.
4.19.1: High power rocket motors, motor reloading kits, and pyrotechnic modules shall be stored at least 7.6 m (25 ft) from smoking, open flames and other sources of heat.
4.19.2: Not more than 23 kg (50 pounds) of net propellant weight of high power rocket motors, motor reloading kits, and pyrotechnic modules subject to storage requirements of 27CFR 55 shall be stored in a Type 3 or Type 4 indoor magazine.
4.19.2.1: The indoor magazine shall be painted red and the top shall bear the following words in white letters at least 76mm (3 in) high: EXPLOSIVES KEEP FIRE AWAY
4.19.2.2: The indoor magazine shall not be located in a residence.
4.19.2.3: The indoor magazine shall be permitted to be located in a detached garage or out building.
4.19.2.4: The indoor magazine shall be permitted to be located in an attached garage of a single-family residence where approved by the authority having jurisdiction and the Bureau of Alcohol, Tobacco and Firearms.
https://www.atf.gov/alcohol/info/revrule/rules/2002-3.htm
ATF will approve variances to store explosives in a residence or dwelling only upon certain conditions including, but not limited to, receipt of a certification of compliance with State and local law, and documentation that local fire safety officials have received a copy of the certification
Notice 4.19.2.3 and 4.19.2.4
the "authority having jurisdiction(AHJ)" referenced here is your local Fire department fire chief.... There may be both city and county fire regulations....
Get your local AHJ to OK it and the Feds will also OK it....
Goto the Aerotech website and get a MSDS sheet on AErotech engines so the Fire People will know what you are talking about..
Maine is a NFPA 1 state and NFPA adopts NFPA 1122/1125/1127
You might also need to check to see if Maine consoders APCP propellant an explosive, in which case you may need to also get a Maine State Explosives license/permit.