Limits of the base drag cone?

[LithosphereRocketry]

I'm working on some drafts of a large-scale Saturn I model and have been running into some conflicting info on my simulations. Without help, the CP on the Saturn I Block 2 is fairly far forward:

However, it's a fairly short rocket at 7:1 fineness ratio, so I thought the base drag hack might help:

That seems REALLY optimistic to me though. Can someone with a bit more experience with this technique weigh in on which of these numbers (or neither) is more accurate?

 

[Cape Byron]

How does it look with a motor selected and recovery gear accounted for, or are they the mass objects we can see on the drawing?

 

[LithosphereRocketry]

How does it look with a motor selected and recovery gear accounted for, or are they the mass objects we can see on the drawing?

Recovery is very roughly accounted for, no motor yet - just trying to get a sense for where the Cp will be before I flesh out the mass.

This is what it would look like with the largest configuration I'd consider flying:

 

[Cape Byron]

When in doubt, swing test. If you have a stable OR sim and a stable swing test you're pretty much good to go. I don't swing everything I design, just the odd one that didn't model too accurately in the older version of OR.

 

[lakeroadster]

I'm working on some drafts of a large-scale Saturn I model and have been running into some conflicting info on my simulations. Without help, the CP on the Saturn I Block 2 is fairly far forward:
View attachment 538721
However, it's a fairly short rocket at 7:1 fineness ratio, so I thought the base drag hack might help:
View attachment 538722
That seems REALLY optimistic to me though. Can someone with a bit more experience with this technique weigh in on which of these numbers (or neither) is more accurate?

It certainly meets the specifications as define by Bruce Levison in Issue 154 of the Apogee Peak of Flight Magazine.

FWIW....

I've designed and built a number of oddrocs and what I've found is as long as the base of the rocket doesn't have a tailcone design, and it is flat, that the base drag hack works for rockets that also don't exceed the 10:1 length to dia. ratio.​

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The one rocket that I have had a problem with stability, my Cygnus Probe Ship, has a slight tailcone. I assumed the tailcone wasn't enough to override the use of the base drag hack. I was wrong. @neil_w questioned my use of the hack for this rocket. Sometimes I have the bad characteristic of not heeding good advice.​

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I always swing test my rockets as @Cape Byron mentions above. However, my Cygnus Probe ship seemed stable (marginally) during the swing test. The rocket did not fly stable.​

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Sometimes The Gods of Stability are fickle.​

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[LithosphereRocketry]

When in doubt, swing test. If you have a stable OR sim and a stable swing test you're pretty much good to go. I don't swing everything I design, just the odd one that didn't model too accurately in the older version of OR.

It certainly meets the specifications as define by Bruce Levison in Issue 154 of the Apogee Peak of Flight Magazine.

FWIW....

I've designed and built a number of oddrocs and what I've found is as long as the base of the rocket doesn't have a tailcone design, and it is flat, that the base drag hack works for rockets that also don't exceed the 10:1 length to dia. ratio.​

​

The one rocket that I have had a problem with stability, my Cygnus Probe Ship, has a slight tailcone. I assumed the tailcone wasn't enough to override the use of the base drag hack. I was wrong. @neil_w questioned my use of the hack for this rocket. Sometimes I have the bad characteristic of not heeding good advice.​

​

I always swing test my rockets as @Cape Byron mentions above. However, my Cygnus Probe ship seemed stable (marginally) during the swing test. The rocket did not fly stable.​

​

Sometimes The Gods of Stability are fickle.​

​

​

​

View attachment 538862 View attachment 538863​

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Swing test would be a great option if it weren't 5ft tall and 20lb

 

[Cape Byron]

Swing test would be a great option if it weren't 5ft tall and 20lb

Fair point. I didn’t look at the weight.

 

[LithosphereRocketry]

Fair point. I didn’t look at the weight.

That's fair - it wasn't mentioned explicitly. I might try 3D printing a scale model and swing testing that - of course aerodynamics doesn't always scale, but it should provide a good guess.

 

[lakeroadster]

Swing test would be a great option if it weren't 5ft tall and 20lb

See if you can rent a shot-putter from the local high school...

 

[jmmome]

When in doubt, just add nose weight. That's what I did for my 6+ feet tall Saturn V & Skylab Saturn V (same rocket- different nosecones).

 

[LithosphereRocketry]

When in doubt, just add nose weight. That's what I did for my 6+ feet tall Saturn V & Skylab Saturn V (same rocket- different nosecones).

I've looked at that, but I'd need to more than double the weight of the rocket to get a stability margin I'd be comfortable with. This is the Saturn I Block 2, not the Saturn 1B or Saturn V, so it's a lot shorter and has a lot less leverage.

 

[jmmome]

Yikes- please post a video of the flight when you decide how to handle this.

My only thought- put as much weight in the nosecone section as you can (I use BB's & epoxy), launch from the far pad & make it a heads-up flight. As long as you are throwing a LOT of initial thrust at the rocket & not trying to simulate a (relatively) slow lift-off, I suspect you'll be pleasantly surprised. But I applaud your concern for the safety of the crowd, as well as the protection of all the hard work you will put into construction of this rocket.

I get your Monty Python reference.