Barometric deployment in a purely conical rocket

Thanks, but the co-pilot is what I have to hand. No spare cash for new altimeters right now!

Darren, here's what I think.

First, you need to avoid picking your pressure sample from the forward conical skin of your airframe because:

Your altimeter will be looking for an initial drop in pressure indicating a launch, it will then monitor the subsequent pressure drop, filtered via algorithm, until the pressure begins to rise(descent). This is where your apogee charge will fire. The program will then monitor the increasing pressure until xx pressure is reached and fire your main charge.

With a pressure port on the front of your air frame, a ram air effect will cause a rise in pressure at launch, which will put your altimeter in a confused state for the remainder of the flight.

Perhaps it would not sense 'launch' until deceleration(burn out), which for many cones occurs very near apogee.

If you did recover your cone, the altimeter would contain no useful data.

Soooo-

I suggest you need to pick your signal from a vertical surface.

Here's food for thought:

One or more vertical tubes are built into your airframe in such a manner that they pass all the way from front to back. two or three holes dispersed radially around the midpoint of one or more of these tubes should provide a more realistic barometric signal.

Here's what I would do...

Use a timer, single deployment.

-Bruce OBrien

If you put the vent on the base it will be subject to massive pressure fluctuations from turbulence. Won't work.

If you put the vent on the conical surface, it will be subject to ram air pressure effect. May overload and damage the sensor.

If you put the vent at the tip of the nosecone and vent through to the base you might be able to get a usable reading from a port on the airflow going through the straight portion of the inside tube but it's probably going to interfere with the placement of your recovery system.

If you build the cone rocket with two or more large diameter holes around the periphery and try to sample the air flowing through these it's going to be subject to turbulence from the changing direction of flow.

Better use a MAD or accelerometer.

Hi Darren:

If you look on my rocket page listed below, you'll see an M-powered rocket called "Pinhead" that was similar in design to what you describe.

Placing the altimeter vents on the surface of the cone is probably a recipe for disaster. The faster your rocket travels, the more inaccurate your barometric data will be.

What I did on Pinhead was use a small diameter carbon fiber tube (an arrow shaft) as a "static probe" (a barometric pressure probe) sticking out of the front of the cone. This static probe had its own little nose cone to seal off the front, but it had a series of tiny vents on the side that carried ambient pressure air back to a sealed altimeter compartment in the main body of the rocket. In effect, the static probe was like a miniature, cylindrical rocket with conventional altimeter ports along its flanks.

The only caveat with this approach is that the mini vents need to be far enough ahead of the main cone to avoid "bow wake" pressurization effects. I had an aerodynamics friend look at the situation. He recommended that for my Mach 0.95 flight, the probe needed to be fairly long, about 1.5 times the base diameter of the conical rocket, and the vents in the probe needed to be about 1.0 base diameters ahead of the front of the cone. If you're not travelling so fast, your probe could probably be shorter.

Despite this fancy hardware, I did what others have mentioned, and I used an altimeter that used accelerometer data for the apogee deployment and barometric data for the main deployment. But when we looked at the data after the flight, we were happy to see that the barometric data was pretty accurate using this static probe technique.

How about using a pitot tube at the tip of the cone? Seal the end, and drill a vent hole through the side part way down, but high enough off the cone to avoid any pressure build-up from the cone.

I've never tried this myself, but it sounds logical.

Ah, "static probe", not pitot tube. I'll have to remember that.

Yeah, a "pitot tube" is a port that points forward to sense dynamic pressure (to read air speed). In many cases a pitot tube and static port are combined into a unit called a "pitot-static probe" which has a pitot orifice on its nose and static ports on its side. (The whole assembly may sometimes get called a "pitot tube"...)

-dave w

Yup. Here's a contact who would probably make a straight one for you. Standard units are about $100 or so.

Brad Hitch

Thanks for the idea, but that would spoil the scale outline I'me trying to achieve.

This is the sort of thing i was worried about.

Not actually bothered with getting any data out of it, just putting out a drogue at apogee, and a main sometime later.

Neat idea, but I think I'd want to test that before using it for real.

The drogue deployment is a scale feature I wanted to model.

Don't currently own either. Might have to put this off for a while!

Yeah, Andy suggested similar...

The reason I wanted electronic deployment was because I wasn't confident of my sim, that and the fact that arranging for motor ejection is a bit awkward.

Hmmmm, who do I know with a g-wiz ;-)