Starfire's OpenRocket Files

[DaHabes]

Estes Boosted Bertha #1946. I included a stock version and my own modified version (24 mm booster).

If you haven't built it yet, I highly recommend the 24 mm MOD for the booster. The C11-0, and D12-0 move it real nice off the pad, but in my opinion an 18 mm booster is way under powered. If you build the booster with an 18 mm MMT, you have a very high risk of it going horizontal off the pad.

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STOCK - 18 mm Booster
View attachment 569400

MOD - 24 mm Booster
View attachment 569401

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I built mine with both the booster and sustainer as 24mm. Flies great with C11's. Retrievable on a big field with D12's. I haven't had the balls to stick E's in it yet...

 

[Starfire73]

Note however that the tail cone shape *greatly* reduces base drag, so the standard base drag CP correction requires modification in such instances. However, I don't know exactly what the modification should be (would love to know if someone has a formula).

As such, I suspect your sim is a bit optimistic.

Now that you mention it Neil. I only use the base vortex cone to calculate CP and determine stability/need for nose weight. Once established, I temporarily delete the cone and re-run my sims for altitude, velocity, delay, etc.

Sometimes the stability margin is too close without the cone, but in many cases the sims run fine without the rocket going unstable.

 

[neil_w]

Now that you mention it Neil. I only use the base vortex cone to calculate CP and determine stability/need for nose weight. Once established, I temporarily delete the cone and re-run my sims for altitude, velocity, delay, etc.

Sometimes the stability margin is too close without the cone, but in many cases the sims run fine without the rocket going unstable.

Keep in mind that sims will generally run successfully at much lower stability margin than you would ever want to fly in real life. I wouldn’t use that as your criterion.

 

[Starfire73]

Keep in mind that sims will generally run successfully at much lower stability margin than you would ever want to fly in real life. I wouldn’t use that as your criterion.

I agree. I don’t use the fact the sim will run properly as a measure for stability.

 

[DaHabes]

Now that you mention it Neil. I only use the base vortex cone to calculate CP and determine stability/need for nose weight. Once established, I temporarily delete the cone and re-run my sims for altitude, velocity, delay, etc.

Sometimes the stability margin is too close without the cone, but in many cases the sims run fine without the rocket going unstable.

A word of caution is required here. If the model is unstable without the cone and stable with the cone, then you cannot re-run the sim without the cone to get altitude; it tumbles from the get-go. You have to leave the cone on in that case. I generally leave the cone on for my sims as a result. There are two ways to handle this regarding altitude. One, go with a small motor for the first flight to observe the flights behavior making it a heads up flight. That way if the altitude is way off and the ejection not at apogee, everyone is on the alert for an errant recovery. This is primarily for LPR models.

For MPR and HPR, use electronics for apogee ejection. For MPR, the Eggtimer Apogee deployment altimeter is the perfect choice with motor ejection as backup with a longish delay; Ive used it for this in multiple instances. If the Apogee falters and motor eject is late, you may zipper the model but it won't come in ballistic. This is the easiest altimeter to build from Eggtimer - all through the hole soldering, very easy.

For HPR, you can use the same altimeter or, for more redundancy, use an Eggtimer Quantum in a nose cone AV-Bay. The Quantum can be configured for apogee deployment on the Main channel and a set delay on the Drogue channel. Use motor eject as a tertiary redundancy (unless it's a plugged motor). Same logic as above, it won't come in ballistic.

I like short, fat rockets - easy to transport, easier to see at altitude. The deployment methods above allow them to be flown with a solid safety profile.

 

[Starfire73]

Thanks.

I usually make the base vortex cone zero mass, and zero coefficient of drag, and see what the stability is. If I have to I will add nose weight at that point. Then I run the sim in OR like that. In my experience the results are pretty close to real life.

I think my comment about removing the cone and re-running sims (those with marginal stability I should add - not totally unstable) threw everyone a curve. I guess having thought more about it, I do that just for the heck of it. Some rockets you can't do that in OR because of course, they are completely unstable.

 

[Starfire73]

Estes, Black Brant XII
(9723)

 

[Starfire73]

Estes, Antar
(7310)





Please use Starfire's OpenRocket Files Feedback Thread for feedback or questions regarding these renders.

To see this and my other OpenRocket files, I would also encourage you to stop by and view my blog: Starfire's OpenRocket Files.