Drastic change in apogee due to rail buttons.

[aykaqs]

I am designing a rocket for an apogee of around 30,000 feet. I have primarily used OpenRocket and RasAeroII for design and simulation. Adding rail buttons didn't cause a significant change in the apogee on OpenRocket but when I added them on RasAeroII, the apogee dropped down by a couple of thousand feet. I am not aware of how significant the changes are due to the addition of the rail buttons.

I have used 1515 Rail Buttons which protrude 0.43 inches (1.1 cm) out of the body tube and I have used two of them. Can someone please share their insights on whether this kind of dip in apogee is normal or is there something wrong with my simulation?​

 

[JoePfeiffer]

Which OR version?

 

[OzHybrid]

I am designing a rocket for an apogee of around 30,000 feet. I have primarily used OpenRocket and RasAeroII for design and simulation. Adding rail buttons didn't cause a significant change in the apogee on OpenRocket but when I added them on RasAeroII, the apogee dropped down by a couple of thousand feet. I am not aware of how significant the changes are due to the addition of the rail buttons.

I have used 1515 Rail Buttons which protrude 0.43 inches (1.1 cm) out of the body tube and I have used two of them. Can someone please share their insights on whether this kind of dip in apogee is normal or is there something wrong with my simulation?​

If you need assistance with simulation issues, post both the simulations and provide version details.

 

[Blast it Tom!]

I can't speak to any simulation (they are all guilty until proven innocent), but in general I would expect a rocket going that far, that fast, to have drag issues with rail buttons protruding on one side. It could lead to a turning moment that would cause the rocket to arc, which can really trim off altitude.

Always know what your simulation is actually doing. And welcome to the forum!

 

[FredA]

Railbuttons add a lot of drag - this is why people use towers.

 

[Neutronium95]

I am designing a rocket for an apogee of around 30,000 feet. I have primarily used OpenRocket and RasAeroII for design and simulation. Adding rail buttons didn't cause a significant change in the apogee on OpenRocket but when I added them on RasAeroII, the apogee dropped down by a couple of thousand feet. I am not aware of how significant the changes are due to the addition of the rail buttons.

I have used 1515 Rail Buttons which protrude 0.43 inches (1.1 cm) out of the body tube and I have used two of them. Can someone please share their insights on whether this kind of dip in apogee is normal or is there something wrong with my simulation?​

To the best of my knowledge, Openrocket doesn't simulate the drag from rail buttons. You can approximate with a similarly sized launch lug, but I don't know how accurate that would be. For a flight to 30,000 feet, you'll probably be in a flight regime where you'll want to be using RASAero anyway.

Edit: I just tried it with 22.02, and it does seem to simulate the drag from them. At least when I added a massless 1515 rail button to a 29mm min diameter design I had and reduced the apogee by a few thousand feet.

 

[SolarYellow]

Most people don't put any effort into making rail buttons not totally suck. Squared off extrusions, stacks of round cylinders, etc. Round is one of the worst shapes for aero drag.

 

[Hobie1dog]

to the forum

 

[aykaqs]

Which OR version?

Version 22.02

 

[Buckeye]

I am designing a rocket for an apogee of around 30,000 feet. I have primarily used OpenRocket and RasAeroII for design and simulation. Adding rail buttons didn't cause a significant change in the apogee on OpenRocket but when I added them on RasAeroII, the apogee dropped down by a couple of thousand feet. I am not aware of how significant the changes are due to the addition of the rail buttons.

Well, depends on the diameter of the rocket. Buttons are a big impact on a 29mm tube, not so much on a 7.5" tube.

RASAero is pretty rigorous with tuning the drag models with other data and flight observations. I am not sure what OR is doing with rail buttons.

Round is one of the worst shapes for aero drag.

Indeed. Side note: A very prominent automotive wind tunnel uses round cross-section struts (stings) to hold the vehicle, then tries to subtract out the large aero effect of those struts (maybe). I never understood that. A streamlined/airfoiled sting is obviously less intrusive.

 

[JoePfeiffer]

Version 22.02

In that case I'm curious to see your .ork file, if you wouldn't mind sharing it.

 

[SolarYellow]

I am designing a rocket for an apogee of around 30,000 feet.

If I was going for that apogee, I'd forget rail buttons and invest in a tower.

 

[Blast it Tom!]

I'm just curious about the two sims. If RasAero is knocking off a few thousand feet, is it because the off-axis drag of the rail buttons causes an (assumed) perfectly vertical flight to arc, or is it simply adding to the drag? I.e. is it a 1D or 2D trajectory calc? How many degrees of freedom for the rocket itself?

For the O.P., these are the things you need to know when using simulation software, at least in my opinion.

 

[watheyak]

I'm just curious about the two sims. If RasAero is knocking off a few thousand feet, is it because the off-axis drag of the rail buttons causes an (assumed) perfectly vertical flight to arc, or is it simply adding to the drag? I.e. is it a 1D or 2D trajectory calc? How many degrees of freedom for the rocket itself?

For the O.P., these are the things you need to know when using simulation software, at least in my opinion.

I don't think rail buttons cause a change in trajectory, either in sims or reality. Otherwise we'd see most flights arc in the direction of the tower. And we don't. But that'd be a nice feature as that's usually away from the crowd.

And 1515 seem pretty monstrous for the usual size of rocket that reaches 30,000'. If this is say a 4 or 5 inch rocket, then yes, I believe 1515 rail buttons could certainly sap 2000'.

 

[Chuck Rogers]

RASAero is pretty rigorous with tuning the drag models with other data and flight observations.

That is correct. The RASAero II Rail Guide and Launch Shoe models have been calibrated against flight data from multiple rockets, large and small, with large and small (relative to the size of the rocket) Rail Guides and Launch Shoes, at both Subsonic and Supersonic Mach numbers. In particular the Interference Drag Factor (the drag of the rocket with the Rail Guides installed is higher than the drag of the rocket by itself and the Rail Guides by themselves, added together) has been refined using flight data.


Charles E. (Chuck) Rogers
Rogers Aeroscience

 

[Chuck Rogers]

I'm just curious about the two sims. If RasAero is knocking off a few thousand feet, is it because the off-axis drag of the rail buttons causes an (assumed) perfectly vertical flight to arc, or is it simply adding to the drag? I.e. is it a 1D or 2D trajectory calc? How many degrees of freedom for the rocket itself?

In RASAero II there is no re-trimmed increase in angle of attack due to the off-center drag from the Rail Guides or Launch Shoes. In RASAero II the Rail Guide drag and Launch Shoe drag are just added as an increase in the axial force acting on the rocket. Same for the Launch Lug drag.


Charles E (Chuck) Rogers
Rogers Aeroscience

 

[FredA]

Otherwise we'd see most flights arc in the direction of the tower. And we don't.

Only if there is no spin.....

 

[watheyak]

Only if there is no spin.....

Good point!

But I stand by my assertion that rockets with rail buttons still fly straight.

 

[OzHybrid]

Good point!

But I stand by my assertion that rockets with rail buttons still fly straight.

Sounds like it needs to be tested with guidance, but just roll guidance appplied. Launched out of the same tower.

 

[JoePfeiffer]

Off center drag has a lot less of an effect than people expect. I imagine it actually is causing an effect on the trajectory, but rather than a marked change to come back over the crowd it's just a slight difference in the arc.

OR also treats the drag from launch lugs and rail buttons as just axial.

 

[OzHybrid]

Are all drag elements modeled centrally axially? What about a thick single fin, which is what I would use to model an external camera housing?
@JoePfeiffer

 

[JoePfeiffer]

Are all drag elements modeled centrally axially? What about a thick single fin, which is what I would use to model an external camera housing?
@JoePfeiffer

Yes. It would have been nice if, when pods were being added, drag had been refined to include torque from off-center components, but it wasn't. On the other hand, it was already enough of a rewrite that it nearly killed the project, so trying to do off-center drag might have been the final nail. And as people who have flown rockets with various random pods mimicking jet engines will attest, it's not as big a deal as it seems like it ought to be.

 

[Chuck Rogers]

RASAero II is the same.


In addition to Rail Guides/Launch Shoes/Launch Lugs, RASAero II also has Protuberance Drag inputs.


For the three types of Protuberance Drag:

1) Streamlined - No Base Drag

2) Streamlined - With Base Drag

3) Inclined Flat Plate


All are modeled as an increase in the axial force acting on the rocket. There is no re-trimming of the rocket at a new angle of attack.


I haven't done a trim analysis for adding Rail Guides, i.e., use the Rail Guide locations and actually calculate the effect of the Rail Guide axial force where the rocket increases angle of attack until it is trimmed (zero moment about the CG) at a new angle of attack, but my assumption is that for Rail Guides this trimmed angle of attack is very small. One can imagine how little angle of attack on the Fins is needed to counteract the axial force on the Rail Guides being applied on their moment arm, one-half the rocket diameter.


It is true that for large camera shrouds, this trimmed angle of attack may be small, but it is definitely not very small. One would have to do trimmed angle of attack calculations to see just how large this trimmed angle of attack is.


Again, in RASAero II, using the Protuberance Drag inputs above to model a camera shroud, RASAero II just models it as an increase in the axial force on the rocket.


Charles E. (Chuck) Rogers
Rogers Aeroscience

 

[OzHybrid]

RASAero II is the same.


In addition to Rail Guides/Launch Shoes/Launch Lugs, RASAero II also has Protuberance Drag inputs.


For the three types of Protuberance Drag:

1) Streamlined - No Base Drag

2) Streamlined - With Base Drag

3) Inclined Flat Plate


All are modeled as an increase in the axial force acting on the rocket. There is no re-trimming of the rocket at a new angle of attack.


I haven't done a trim analysis for adding Rail Guides, i.e., use the Rail Guide locations and actually calculate the effect of the Rail Guide axial force where the rocket increases angle of attack until it is trimmed (zero moment about the CG) at a new angle of attack, but my assumption is that for Rail Guides this trimmed angle of attack is very small. One can imagine how little angle of attack on the Fins is needed to counteract the axial force on the Rail Guides being applied on their moment arm, one-half the rocket diameter.


It is true that for large camera shrouds, this trimmed angle of attack may be small, but it is definitely not very small. One would have to do trimmed angle of attack calculations to see just how large this trimmed angle of attack is.


Again, in RASAero II, using the Protuberance Drag inputs above to model a camera shroud, RASAero II just models it as an increase in the axial force on the rocket.


Charles E. (Chuck) Rogers
Rogers Aeroscience

And @JoePfeiffer How difficult would it be to add up the off center drag elements as a vector sum and if the total was above a threshold in any direction give a warning? Rather than doing a full flight analysis.
Similar to the OR discontinuity and jagged edge warnings.

 

[kjhambrick]

I am designing a rocket for an apogee of around 30,000 feet. I have primarily used OpenRocket and RasAeroII for design and simulation. Adding rail buttons didn't cause a significant change in the apogee on OpenRocket but when I added them on RasAeroII, the apogee dropped down by a couple of thousand feet. I am not aware of how significant the changes are due to the addition of the rail buttons.

I have used 1515 Rail Buttons which protrude 0.43 inches (1.1 cm) out of the body tube and I have used two of them. Can someone please share their insights on whether this kind of dip in apogee is normal or is there something wrong with my simulation?​

Very interesting, @aykaqs

Please post your RASAero and ORK files and if that's not possible, please give us:

Rocket dry mass, diameter, length, fin shape, fin span, fin thickness and nose shape and length.

Motor Common Name or total motor mass, propellant mass, total impulse and burn time.

Launch Site altitude, temperature, rail length.

Thanks !

-- kjh

 

[JoePfeiffer]

And @JoePfeiffer How difficult would it be to add up the off center drag elements as a vector sum and if the total was above a threshold in any direction give a warning? Rather than doing a full flight analysis.
Similar to the OR discontinuity and jagged edge warnings.

I suspect (without actually looking) it would be very close to the level of effort required to just model off center drag. There's a bunch of stuff to do before that...