I found two different thrust curves published for the AeroTech 24mm D9 Reload Kit. The one on the left is from the AeroTech catalog (I highlighted the D9 in yellow), and the one on the right is from the NAR Standards and Testing info found on thrustcurve.org. As you can see, the two curves are very different. In fact, the curve on the right doesn't even peak until right around 0.8 seconds into the burn - which certainly sounds like it would spell disaster.
I started looking for this engine's thrust curve when simulations in RockSim 9 of a Big Daddy on the D9 crashed and never left the pad. Could RockSim possibly have the wrong thrust data for this engine?
Does anyone have a feel for which curve is accurate? I would tend to believe the AeroTech curve more than the other curve.
Both curves are accurate.
NAR S&T had in impromptu test session last night to investigate the observed differences in the AT D9W thrust curves. We fired several AT D9W reload left over from a previous recertification test.
The AT 24/40 casing has a case capacity of approximately 20 grams of APCP so that fully loaded you can obtain from a 100% E to a 25% F total impulse depending on the APCP formulation employed. The AT D9W reload is a nominal 100% D reload, resulting in an unfilled motor casing where ~50% of the volume is unfilled with propellant. For the motor to develop full thrust this volume must be completely pressurized, and if the Kn is suboptimal, you can get longer than expected ignition delays and case pressurization times.
We fired 2 AT D9W reloads built according to the AT directions which has the void volume forward of the propellant grain, and intentionally misassembled a third AT D9W with the void volume aft of the propellant grain. The 2 properly assembled reloads took ~0.4 and ~0.55 seconds to reach peak thrust and the incorrectly assembled AT D9W required 0.5 second so reach peak thrust. These times were shorter than the 0.8 seconds recorded in the 1998 NAR recertification testing, but significantly longer than the 0.2 second time to peak thrust indicated on the AT instruction sheet.
A critical concern with any rocket motor is the start-up time. You want your motor to come up to pressure quickly so that it smoothly and rapidly accelerates off the pad. This happens when the motor casing has a minimum dead volume. If the propellant grain is older and slightly oxidized, the ignition delay can be significantly longer than that of a fresh motor. If a propellant grain does not completely fill the casing, then the void must also be fully pressurized before the motor develops full thrust. This delay is also exacerbated by a physically small propellant grain and slow burning propellants which is that the AT D9W reload uses.
So to answer you initial question, both thrust curves are accurate representations of the AT D9W performance. The AT D9W thrust curve on the AT instruction sheet was probably obtained from a just manufactured reload kit, which the 1998 NAR S&T thrust curve was likely obtained from a reload kit that was at least several months old. Both curves yield equivalent total impulse and average thrust, however the time to peak thrust varies from 0.2 seconds for the newly manufactured motor to as much as 0.8 seconds in a older motor.
Motors in certification testing are assembled and operated exactly according to the manufacturers assembly instructions, and the c-slot is not roughened with sand paper during assembly to remove surface oxidation. For this motor, it probably would be a good idea to remove any surface oxidation prior to assembly to promote rapid flame propagation and minimize ignition delay. An AT D9W with a 0.2 second time to peak thrust will perform as intended, however any model rocket motor with a 0.8 second time to peak thrust is likely to perform poorly.
Bob Krech, TRF Propulsion Moderator
Member NAR S&T
