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AeroTech D9 Thrust Curves - RockSim Crash

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FatBoy

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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 acurate? I would tend to believe the AeroTech curve more than the other curve.

D9 Thrust Curve.jpg
 

MarkH

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The one on the NAR site is correct. IMO the D9 curve in the catalog is mixed with the D15 curve. It appears (in the catalog) where the two intersect, the D9 turns into what is the D15, and the D15 turns into what is actually the D9. Look at the curve that comes with the reload and it should be pretty close to the NAR curve.

Also the D15 has a relative short burn compared to the D9, which is not what is shown in the catalog. Since they are both 20 N-s motors, their areas should be about the same, but the catalog shows the D15 having a much greater area than the D9.

I posted this in the catalog thread a few days ago.

... a D15T would work well with the Big Daddy. The D9W may not as you indicated.
 

FatBoy

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Thanks for your reply! The curve that comes with the reload is much closer to the catalog curve than the NAR curve.
 

MarkH

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Thanks for your reply! The curve that comes with the reload is much closer to the catalog curve than the NAR curve.
Ok, then I'm not sure if the NAR one is more accurate, but there is still an error in the catalog. If you look at the Curve that comes with the motor and compare it with the catalog, the error is more obvious.

I'd go with the max liftoff weight printed on the reload pack. The D9-7W pack that I have says the max liftoff weight is 4.2 oz. I don't have a D9-4W, but its max liftoff weight is probably a couple ounces higher.
 
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bobkrech

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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 acurate? I would tend to believe the AeroTech curve more than the other curve.
The first thrust curve is published in the AT D9 instruction sheet. https://www.aerotech-rocketry.com/c...Instructions/RMS-24_40/rms_24_d9-e28_4-04.pdf

The second thrust curve is an actual AT D9 thrust curve measured by NAR S&T in 1998 during certification testing. http://www.nar.org/SandT/pdf/Aerotech/D9.pdf

Both are accurate.

Personally, I don't know why you would want to launch a Big Daddy on a D9 because it will be underpowered.

The Estes D-12 http://www.nar.org/SandT/pdf/Estes/D12.pdf has a peak thrust of 30 N providing a 14 G acceleration off the rod, and is cheaper than a reloadable D.

On the other hand, IMO the Estes E9 http://www.nar.org/SandT/pdf/Estes/E9.pdf is a marginal motor for a Big Daddy while the AT E15 http://www.nar.org/SandT/pdf/Aerotech/E15.pdf and E28 http://www.nar.org/SandT/pdf/Aerotech/E28.pdf reloads rock.

Bob
 

MarkH

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The Big Daddy flies just fine on an E9. Check EMRR. Maybe some weather cocking if its windy. Use an Estes E pad and 1/4 " rod and no problems. Rocsim is a bit conservatibe wrt the E9. What's even more funny is that Apogee warns against using it in anything weighing more than 5.0 oz. The maxi V-2 may be marginal on an E9 but I've not seen any problems with the Big Daddy E on an E.
 

bobkrech

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It really depends on the wind speed. The wind velocity in mph where severe weathercocking occurs due to cross wind fin stall is approximately the rod velocity in fps divided by 6.

If the Big Daddy is built at stock weight of 5.3 oz w/o engine, the rod velocity with an Estes E9 and a 4' launch rod is 40 fps and severe weathercocking occurs when the wind speed reaches 7 mph.

With an Estes D12, the rod velocity is 48 fps, severe weathercocking occurs when the wind reaches 8 mph.

With an AT E18 the rod velocity is 54 fps, severe weathercocking occurs when the wind reaches 9 mph.

With an AT E28 the rod velocity is 72 fps, severe weathercocking occurs when the wind reaches 12 mph.

Bob
 

bobkrech

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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
 
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