Please Explain "Cable Cutter".

[jcato]

<snip>
The only concern I have with that is adding nose weight to a model that's already adequately stable. I try to keep the margin on my rockets comfortably over 1:1, but no higher than that in order to minimize weathercocking. Adding an altimeter sled to the nose would obviously work against that, possibly pushing well into overstable. Maybe I'm overthinking that too; I suppose flying upwind minimizes the distance down range to floats...

Angle your rod/rail slightly *down*wind. That way, the vehicle will weathercock to vertical. I did this once in a demonstration flight for a Boy Scouts 'Aviation Camporee' when the altitude wasn't going to be too high and good downrange drift distance was available. Worked fine.

-- john.

 

[rharshberger]

One more tangential question - if I did this, and used the altimeter really just as a means of signaling the cable cutter, can the vent holes for the altimeter be through the AV bay mounting bulkhead at the back of the nose cone, rather than through the side of the cone? I can't think of a reason that it would be a problem, but I don't yet know what I don't know...

The static ports for the altimeter nood to be exposed to the airflow, by putting them in the AvBay bulkhead it may work but you will be exposing the alt to ejection gasses and residue, the residue is corrosive.

 

[EXPjawa]

The static ports for the altimeter nood to be exposed to the airflow, by putting them in the AvBay bulkhead it may work but you will be exposing the alt to ejection gasses and residue, the residue is corrosive.

Ah, well, that's the didn't know that I knew I didn't know... Thanks. If that's the case, holes in the sides of the shoulder ought to work, right? They'd be closed up at launch, but don't really need to open until decent. Unless, that is, they need to be open on the pad so the altimeter can get a good initial calibration. I could see where that could be an issue...

 

[Bat-mite]

Ah, well, that's the didn't know that I knew I didn't know... Thanks. If that's the case, holes in the sides of the shoulder ought to work, right? They'd be closed up at launch, but don't really need to open until decent. Unless, that is, they need to be open on the pad so the altimeter can get a good initial calibration. I could see where that could be an issue...

You nailed it. The way altimeters work, they look for a sudden decrease in atmospheric pressure. This lets them know they are going up, and they calculate altitude based on the rate of change and the pressure readings. When they start reading the same pressure for a period of time, they know they are at apogee. Then, as pressure increases they know they are descending, and they calculate the altitude until it hits 300' or 500' or wherever your charge is set.

The point is that barometric altimeters do not actually read altitude. They read pressure changes and calculate altitude.

If there are no sensor readings until after apogee, the altimeter has no way of knowing altitude, and thus cannot calculate the 300' (or whatever) for the mains.

Three small holes 120 degrees apart on your nose cone will do the trick. You could go with a single, larger hole, but as I have experienced, this is ineffective in high winds, like how you can't breathe when you stick your head out of the window of a fast-moving car.

 

[EXPjawa]

OK, that's what I'd suspected. I've been looking for ways around drilling exterior holes - strictly for cosmetic reasons - but obviously they're key to functioning. So, a few small holes probably won't degrade appearance much. Maybe I can disguise them as attitude control thrusters. That begs the next question - does it make a difference where along the length of the cone they should be placed? My gut tells me they should as far back as practical, since air pressure would be higher at the front, but smoothing out around the side toward the shoulder. Or at least would with a non-conical shape.

 

[Bat-mite]

OK, that's what I'd suspected. I've been looking for ways around drilling exterior holes - strictly for cosmetic reasons - but obviously they're key to functioning. So, a few small holes probably won't degrade appearance much. Maybe I can disguise them as attitude control thrusters. That begs the next question - does it make a difference where along the length of the cone they should be placed? My gut tells me they should as far back as practical, since air pressure would be higher at the front, but smoothing out around the side toward the shoulder. Or at least would with a non-conical shape.

Probably not, but I'm guessing here. The altimeter needs to be mounted above the sled, so that air can pass underneath it. As long as air can into and out of the cone, and the holes are the right size, then I think you're okay. But others should chime in.