Altitude loss from camera

[Swissyhawk]

I'm getting ready to put a Mobius camera on my Formula 75. It will be the first time I've put a camera on a rocket. It will be mounted externally in an Additive Aerospace camera shroud. What kind of loss of altitude should I expect due to the camera and shroud?

 

[tim cubbedge]

I recently launched a rocket on same motor with & without Mobius. Similar weather/wind conditions.
Flight #1 = 5399' no camera (wind ~10 MPH)
Flight #2 = 4416' with camera (no wind)
Flight #3 = 4746' no camera (wind ~10 MPH)

Each flight I used a Loki J525. Not sure why the big defference in flight 1 & flight 3 other than variations in propellant or higher winds at altitude.

 

[mccordmw]

I recently launched a rocket on same motor with & without Mobius. Similar weather/wind conditions.
Flight #1 = 5399' no camera (wind ~10 MPH)
Flight #2 = 4416' with camera (no wind)
Flight #3 = 4746' no camera (wind ~10 MPH)

Each flight I used a Loki J525. Not sure why the big defference in flight 1 & flight 3 other than variations in propellant or higher winds at altitude.

The acceptable variance between motors is 20%, so I'd suspect your flights are all within those tolerances.

 

[Cabernut]

I've had about 10% less, but I would guess that really depends on how much drag it creates. In a shroud vs. exposed could make a big difference.

 

[Handeman]

It also depends on the motor and flight profile. If you are using a low and slow motor, you might not see much altitude loss at all since you are using low speeds. Put a camera on a rocket that's running near Mach and you get a significantly higher drag profile.

 

[DavidMcCann]

I recently launched a rocket on same motor with & without Mobius. Similar weather/wind conditions.
Flight #1 = 5399' no camera (wind ~10 MPH)
Flight #2 = 4416' with camera (no wind)
Flight #3 = 4746' no camera (wind ~10 MPH)

Each flight I used a Loki J525. Not sure why the big defference in flight 1 & flight 3 other than variations in propellant or higher winds at altitude.

The acceptable variance between motors is 20%, so I'd suspect your flights are all within those tolerances.

I'm pretty sure Scott counts each grain of AP by hand for his motors. where the flights off vertical?

 

[dhbarr]

Same breed of igniter all three times?

 

[DavidMcCann]

It also depends on the motor and flight profile. If you are using a low and slow motor, you might not see much altitude loss at all since you are using low speeds. Put a camera on a rocket that's running near Mach and you get a significantly higher drag profile.

All four of my 2.6" screech flights were just over M1 and about 9,000'. I'm curious what it'd hit without the camera.

 

[markkoelsch]

Start with a basic question. What is the frontal area of the shroud? It has an open bottom, so a bunch of base drag.

No sim here just an educated guess. If you take the frontal area of the shroud and compare it to the total area frontal area of the clean rocket you can get a percentage the area the shroud adds.

In the Screech case the tube frontal area is about 5.3 square inches ( excluding fins). I am making a guess on the shroud area of a square inch. So we and up with the ratio of 6.3/5.3 or about 1.18. So, it you multiply this by 9000' you get about 10600'. This is an off the top of my head swag, does not take Mach effects into consideration etc. I am sure if the shroud was gone it would for certainly go at least 10 k.

 

[Incongruent]

Skin friction drag is a large contributor to drag as well.

 

[Buckeye]

The acceptable variance between motors is 20%, so I'd suspect your flights are all within those tolerances.

Kind of. Average thrust acceptable variation is 20%. Impulse acceptable variation is 6.7% standard deviation, which is probably the more relevant parameter for the example above from timbucktoo. Still, the two no-camera flights are probably within 2 standard deviations of motor performance, which is about 95% confidence. So, it is still hard to quantify the camera effect on drag from this observation.

Better aero analysis is needed, like wind tunnel or CFD!

 

[markkoelsch]

Skin friction drag is a large contributor to drag as well.

You are not wrong. That said, in this case the it would likely be a small contributor. Same rocket plus or minus the shroud. Same motor.

 

[DavidMcCann]

Kind of. Average thrust acceptable variation is 20%. Impulse acceptable variation is 6.7% standard deviation, which is probably the more relevant parameter for the example above from timbucktoo. Still, the two no-camera flights are probably within 2 standard deviations of motor performance, which is about 95% confidence. So, it is still hard to quantify the camera effect on drag from this observation.

Better aero analysis is needed, like wind tunnel or CFD!

Wonder if Calspan would let me into their tunnel for a couple hours Our best bet would probably to ask one of the SEDS teams to approach someone about some testing.

 

[EeebeeE]

I think it depends on a number of variables.

1. Diameter of airframe in relation to the size of the camera. A mobius attached to the side of a 7.5" airframe isn't going to cause as much proportionate reduction in altitude as one attached to a 54mm airframe.
2. Shape of the camera. The keychain cameras are more aerodynamic than others are.
3. Location of the camera. I try to mount mine about 2-5" (depending in size of the rocket and nc shoulder) below the forward edge of the airframe where the nose cone attaches. I have been told that the air moves away from the airframe at that point and that anything in that area is subject to a little less wind. Don't know if it is true or not, but the added bonus is that I get a nice shot of separation.
4. Size of the camera mount. I have seen the goiters that some people are 3D printing to hold cameras. For what they provide in wind resistance in the forward end of the mount, they more than lose as base drag at the aft end. The closer the camera is to the rocket, and the less the entire assembly stands away from the airframe, the less wind resistance.
   

I have found that for keychain cameras, a piece of velcro attaching the bottom front ot the camera to the airframe, along with a #4 screw through the keychain hole into the airframe with a small piece of electrical tape wrapped around the camera except for the end with the camera hole to shroud off the gap between the camera and the airframe, will hold in place at speeds well past Mach 1. The profile is a lot lower, and the loss of altitude is minimal to the point that on my 5.5" rocket, there is no difference.

 

[tim cubbedge]

I'm pretty sure Scott counts each grain of AP by hand for his motors. where the flights off vertical?

The flight with camera was straight up. Zero wind. Landed ~200 from pad.
The other 2 flights with no camera were maybe 1-2 degrees into wind. Minimal weather cocking.

All three flights used quick burst fat boy igniters.