Well, the old HPR rule of thumb used to be to restrict your motors to 5 BATES
grains and utilise a core area that’s at least 3 times the throat area of your
nozzle, but that’s going back to the 90s and since then enthusiasts &
manufacturers have been producing some long motors with loads of grains
although some/many with tapered cores.
Thing is, it’s hard to define a rule of thumb for erosive burning coz there
are so many influences that affect the magnitude of it other than the obvious
gas flow velocity down the core eg. Propellant type, oxidizer particle size,
core geometry (eg non cylindrical core profiles are generally more influenced),
chamber pressure the motor’s running at, initial propellant temperature, the
(non erosive) burn rate of the propellant, and there can be a strong influence
with the type of binder utilised etc etc...
What’s more, its contribution can often be a positive one for many flight
profiles.
Troy
From: arocket-bounce@xxxxxxxxxxxxx [mailto:arocket-bounce@xxxxxxxxxxxxx] On ;
Behalf Of William Claybaugh
Sent: Thursday, 13 April 2017 6:53 AM
To: arocket@xxxxxxxxxxxxx
Subject: [AR] Erosive burning rule of thumb?
Is there any experience-based rule for erosive burning in cylinderical-type
grains (Bates, Finocyl, etc.)? I am particularly looking for experience as to
how small the core diameter can be as a percentage of grain diameter: 50%
appears to assure no erosion at L/D of 10:1 or less, what about 33%?
When does overall length begin to play? I know that around 15:1 will lead to
erosion with a 50% core diameter, for example. What is the "critical" length at
33%? What about other core diameters?
Bill
