L1 Cert flight with 4" Mad Cow Little John motor recommendations

[sambatterman]

Recommendations for a 9-pound 4" diameter, 48" long Mad Cow Little John Level 1 Cert flight motor selection. What would you recommend for motor and delay? Thanks. Interested in Aerotech only.

 

[Joekeyo]

I got my level one on the same with an H100W drilled to an 8 sec delay. JLCR band broke, but other than that, excellent flight.

 

[crossfire]

I got my level one on the same with an H100W drilled to an 8 sec delay. JLCR band broke, but other than that, excellent flight.

10 pound rocket on a H100? Must have been no wind. How high 700'?

 

[jmasterj]

His was probably the cardboard version; an H100 isn't nearly enough power for a 10 pound rocket. Doesn't even meet the NFPA's 3:1 thrust requirement.

Part of the level 1 experience should be figuring out your motor. Put your rocket into Openrocket with the measured weight and CG and see what motors get you enough rail speed.

 

[Rocketjunkie]

With the weight, go for a high thrust at least a full H. H999 (38/360) to I600. Recommend electronic deployment. Otherwise go for the highest thrust motor that has delay and ejection. (I357 would be a good choice.)

 

[Budro0]

At 9lbs, you are probably looking at an I, not an H for your flight. Openrocket is going to be your friend here. I have a similarly heavy DX3 that I flew on an I245, but it took some trial and error in OR to figure it all out.

 

[sambatterman]

I'm planning on flying my 4" Mad Cow Little John All fiberglass rocket that weights roughly 9 pounds (including motor and lead in the nose) on an Aerotech I215R - at 215/4.5, I get 47 pounds of thrust/5 = 9.4. Right on the money for 5:1 ratio.
Does anyone see any issues with this?

 

[jmasterj]

Have you run a sim? Even a thrustcurve.org analysis? I bet the rail exit speed is going to be pretty low with that motor.

Edit: yup. Average thrust doesn't tell the whole story. You need more initial thrust, and you need to be able to figure out your rail exit speed.

 

[sambatterman]

Yes - I've run it on OpenRocket numerous times - apogee at 1109 ft. Max velocity at 252 ft/s and Max acceleration at 52.1 m/s2

What number should I be focused on if it's not the 5:1 ratio?

 

[sambatterman]

Here's the thurstcurve data for I215
Manufacturer
AeroTech
Designation
I215R
Common Name
I215
Motor Type
reload
Delays
4,6,8,10
Diameter
54 mm
Length
156 mm
Total Weight
527 g
Prop. Weight
208 g
Avg. Thrust
215.0 N
Initial Thrust
222.2 N
Max. Thrust
—
Total Impulse
399.0 Ns
Burn Time
1.9 s
Motor Case
RMS-54/426

 

[jmasterj]

What's the rail exit velocity? The rule of thumb that works for low wind is 50fps or 15m/s or so.

Thrust curve says 13.5m/s with average drag and 2m of rail.

The 54mm Is have much lower initial and maximum thrust than most 38mm Is. You might want to look at those plus an adapter (maybe an I285R?).

 

[sambatterman]

Thanks - so I also can go with an I599N-P - it's 301 ft/sec and 173 m/s2 apogee at 1269 - better choice?

 

[jmasterj]

Thanks - so I also can go with an I599N-P - it's 301 ft/sec and 173 m/s2 apogee at 1269 - better choice?

It's plugged (that's what the -P means) with no motor ejection possible, so you'd need electronics. That's probably not what you want if you haven't already planned on it.

Do you already have the 54/426 case? If not, I'd suggest considering a 38mm I or even something like an H550 if you want to stay really low.

I apologize if I came off harshly in that first post.

 

[sambatterman]

Yeah - so we're not doing electronics on this. It's not really designed for that.
So an I-229 will use the same casing - and it does have a delay, not plugged.

Thrustcurve has I-229 at
Total Weight
514 g
Prop. Weight
196 g
Avg. Thrust
229.0 N
Initial Thrust
243.3 N
Max. Thrust
—
Total Impulse
407.0 Ns
Burn Time
1.8 s
Motor Case
RMS-54/426
Propellant
Blue Thunder

 

[jmasterj]

That thrust curve link I posted above has the I229 with even less rail speed than the I215.


Based on my experience with my 8.5lb 4" rocket, I think you want to look for initial thrust of at least 350N. Look at the I285R vs the I215R:
https://www.thrustcurve.org/motors/...23100000001e9&motors=5f4294d200023100000000c7
That doesn't tell you what the rail exit speed is going to be, but you can see the much higher initial thrust that's going to get you that higher rail exit speed.

Have you consulted with the person who you'd like to do your cert?

 

[sambatterman]

I'm literally stunned that I cannot use a 54mm Motor to get a level 1 cert on a 4" 9lb rocket. In openrocket, where are you seeing the "slow off guide" warnings?

Where do you recommend I buy a 38mm to 54mm motor adapter? I guess I'm going to have to reinvest in casings...

 

[jmasterj]

I'm not sure there is a warning like that in OpenRocket. There are warnings on thrust curve and a default minimum speed. I'm not saying you can't do it, but you'd need a long rail and low wind to have a good chance at success. I would not personally endorse a cert flight with a rail speed below 50fps, but I'm not the one who will be certifying you.

If you look at the geometry of a 38/480 motor vs a 54/426 motor, you'll see that there's a lot more initial surface area to burn on the former, while the latter will burn longer (partly) because the distance between the edge of the core and outside of the grain is longer. You can see that in those two thrust curves above.

(Note that the y-axes are not the same height.)

 

[boatgeek]

I'm literally stunned that I cannot use a 54mm Motor to get a level 1 cert on a 4" 9lb rocket. In openrocket, where are you seeing the "slow off guide" warnings?

Where do you recommend I buy a 38mm to 54mm motor adapter? I guess I'm going to have to reinvest in casings...

Larger diameter, lower impulse motors tend to be longer burning, unless they're a really fast propellant like the Warp 9 that requires electronics.

Motor adapters can be bought at most of the usual suspect vendors, or you can make your own with a few centering rings and a piece of motor mount tube. Instead of investing in casings, you could use a 38mm DMS H550 or I500.

 

[jmasterj]

I like the Aeropack adapters, which you can get at most of the usual vendors.

 

[loopy]

I'm literally stunned that I cannot use a 54mm Motor to get a level 1 cert on a 4" 9lb rocket. In openrocket, where are you seeing the "slow off guide" warnings?

Where do you recommend I buy a 38mm to 54mm motor adapter? I guess I'm going to have to reinvest in casings...

You'll want 38mm casings anyway...trust. The loads available in 38mm are staggering...the J500G is an incredible motor (my L2 cert motor). Look into a 38/720 case and a set of 38mm spacers, and that should get you what you want for this rocket and many others. Assuming you have an aeropack retainer on your rocket, look into a 54/38 adapter from Aeropack as well.

 

[sambatterman]

Yep - I do have an Aeropack retainer and I just ordered the Aeropack 38-54mm adapter. Thanks for the suggestions for the cases. The DMS I500 was quite expensive ($85), so thinking I will go with casings. Just bummed I did a bunch of sims and did not understand the rail speed, so I wasted money on 54mm casings. My son and I are both getting certified - so everything is more expensive.

Here's my plan - I'm going to use a 38 to 54mm adapter from Aeropack and fly an Aerotech I300 in a 38/480 casing - this motor goes to 400 N in 0.1 seconds. Apogee at 1400 ft.

Thanks for the help - if you see any issues with this plan, please let me know...

 

[jmasterj]

That should get you to a good rail exit speed.

Perhaps you could resell the 54mm casings? You could also share a casing for the cert flights; you have to build the motor in front of the witness anyway.

 

[Tractionengines]

you have to build the motor in front of the witness anyway.

YES, DMS motors are more expensive and take a different delay drilling tool, but they are " plug-n-play". For the cert flight, no nerves building your motor in front of a witness, it's less to worry about.
My vote would be the DMS I500. It's Blue Thunder, so lights easy. Lots of thrust off the rail. ~$ 75 Then the issue is hazmat shipping if you can't pick it up.

But to each their own, it's your cert flight. Have your sims ready and show your witness, then pick your motor. Have fun.

 

[mtnmanak]

First, I will echo the recommendation to use a single use (DMS) motor on your Level 1 cert flights. It takes a huge amount of stress out of the day and removes a possible failure point.

I'm literally stunned that I cannot use a 54mm Motor to get a level 1 cert on a 4" 9lb rocket. In openrocket, where are you seeing the "slow off guide" warnings?

Where do you recommend I buy a 38mm to 54mm motor adapter? I guess I'm going to have to reinvest in casings...

In OpenRocket, first, you must set the correct length of the "rod" (rail) you will be using. If you don't know what length rails your club uses, I would set the sim to 6 feet - that is about the minimum I would expect, so you know if your rocket is good off a 6' rail, you should be good at the club if their rails are longer. Do remember that the sim assumes the rail is giving you positive control for the entire length of your rocket, but in real life, once the top button leaves the rail, your rocket is no longer getting positive flight control. You set the length in the "Edit Simulation" dialogue:



Once you have run your sim, there should be a column in the Simulations tab that says "Velocity off rod". This is the key number you are looking for. In 0-5 MPH winds, you should be shooting for about 30 MPH off the rod. For higher winds, you should increase the allowed rail velocity accordingly.

In the Sim results below, I sim'ed an I500 DMS motor in a Wildman 4" Darkstar - similar in size and weight to the rockets you are discussing in this thread:



You can see that, off a 6 foot rail, the sim is predicting about 40 MPH off the rod. I would be happy with that and feel comfortable flying this motor.