The Rocksim/Openrocket base drag trick

[RobVG]

In Apogee's Peak of Flight newsletter, #154, Bruce Levison shows us a way to compensate for stability calculation inaccuracies for short fat rockets.

He tells us precisely how to draw and add a zero mass cone to the back, but he says it's for rockets with less than a 10:1 length to diameter ratio. He goes on to say, "much less".

How much less? Is there a more concise mathematical definition of a "short stubby"?

 

[Banzai88]

10:1 or less is the accepted norm. Less would be all the way down to below 1:1, and that would make for an interesting odroc

 

[RobVG]

10:1 or less is the accepted norm. Less would be all the way down to below 1:1, and that would make for an interesting odroc

Thanks for that. Guess it's kinda like the accepted static margin.

 

[neil_w]

At 1:1 you’ve entered saucer territory.

 

[DarthMuffin]

FYI, from my and a friend's experience using the base drag cone trick, it works. Actual flight results were slightly higher than with the drag cone, but nowhere near the estimate without (like maybe 20-25% of the difference between the two).

 

[RobVG]

FYI, from my and a friend's experience using the base drag cone trick, it works. Actual flight results were slightly higher than with the drag cone, but nowhere near the estimate without (like maybe 20-25% of the difference between the two).

The

Thanks for your input!

I was mainly concerned about stability. I've heard you need to remove the cone before running sims but you've had better results leaving it on?

 

[Banzai88]

The

Thanks for your input!

I was mainly concerned about stability. I've heard you need to remove the cone before running sims but you've had better results leaving it on?

I always remove the cone for altitude and delay predictions and have been spot on with launches.

 

[neil_w]

The base drag hack is intended solely to account for base drag in the CP calculation, that’s all. It should be removed for flight sim.

If some particular sim comes out more accurate with it included, then great I guess, but I would always remove it.

 

[DarthMuffin]

I left the cone on for sims, but the rocket had enough nose weight to be borderline stable without the base drag cone. Like I said, actual flight for both me and my friend was way closer to the sim with the base drag cone than without. But still a bit higher than the sim in each case. Mine was the 4" Gizmo in my profile pic, friends was both a Pemberton tech 3FNC and a 5in (?) Jart. My guess at the discrepancy is because the back of the rockets isn't just a flat plane, each instance has a motor mount sticking out some which gives a tiny bit of tailcone effect.

My gut instinct is that at launch rail speeds it's not really going fast enough for base drag to help with stability much. I don't have the know-how to verify that though. So I designed for barely stable without the base drag and picked a high thrust motor for quick takeoff.

 

[BigMacDaddy]

So this means this would be appropriate for this oddroc? Does a flying nose cone also have the same base drag as a short stubby rocket?

 

[neil_w]

I would say yes, use it there.

 

[lakeroadster]

What's your folks opinions on what the diameter of the zero mass cone should be for the following simulations?

Use:

A) the largest diameter found on the rocket,​

or​

B) the diameter at the rear of the rocket?​

 

[neil_w]

What's your folks opinions on what the diameter of the zero mass cone should be for the following simulations?

Use:

A) the largest diameter found on the rocket,​

or​

B) the diameter at the rear of the rocket?​

View attachment 505829 View attachment 505830

Base drag is inherently due to the abrupt end of the rocket, in the rear. So the cone should be equal to the diameter at the rear of the rocket. However, if the tail cone is abrupt enough, then it probably stops behaving like a transition. IMHO the tail cone on the Cygnus does not quality, but who knows.

Therefore, for the Cygnus, I think I would apply it to the diameter at the very back (note that you will need to adjust the length of the cone). For the Fat Man, I would not use it at all.

 

[DarthMuffin]

You just need more plutonium as nose weight

 

[rharshberger]

You just need more plutonium as nose weight

Only $4000 per gram, why not! Oh and a visit from one of those pesky alphabet agencies....

 

[Rocket Guy in Milford]

The base drag hack is intended solely to account for base drag in the CP calculation, that’s all. It should be removed for flight sim.

If some particular sim comes out more accurate with it included, then great I guess, but I would always remove it.

How LONG should the Base Drag Hack be?

 

[neil_w]

How LONG should the Base Drag Hack be?

The starting point is Pi times the diameter, according to the original article.

There are potentially some modifiers based on the relative size of the motor vs the tail of the rocket, or whether there is a tail cone.