nosecone mounted altimeter?

Can an altimeter be installed inside a nosecone (not for any type of ejection, just to know "how high")

If it is possible, how would it work. I dont have any questions about how to install the control....just how the control would have to see the pressure readings through the cone.

I would think to have a vent hole in the bottom of the cone and one in the side of the rocket. What I don't know is how "motor ejection pressure" would effect the altimeter.

I've seen Jim Jarvis do it.

GRIFFIN said:

Can an altimeter be installed inside a nosecone (not for any type of ejection, just to know "how high")

Click to expand...

Sure! I'll use my PR Little Dog as an example. It's a single-deployment fiberglass rocket. The nose cone is also fiberglass. Originally, the folks that gave me the kit had a wooden plug that had a fiberglass sled expoxied onto it for the altimeter (a PerfectFlite MAWD) and a 1/8" hole just as the bottom of the nose cone, just above the shoulder, for the air to get into it. The wooden plug was force fit into the nosecone so that none of the ejection gasses or particles could make it up to the altimeter.

I lost the nosecone on its first flight, but the shock cord was still securely attached to the wooden plug, and it read something like 1800 feet AGL on the first flight.

On the relacement nose cone, I epoxied in a 1/4x20 threaded rod into the tip of the cone. It comes all the way out the back end to the tip of the shoulder. I've got a fiberglass bulkhead that fits onto the rod, and an eyenut that screws on top of it to keep it secure inside the shoulder. Inside the nose cone I've got a different sled now, that mounts both an MAWD altimeter and a BigRedBee transmitter.

Everything inside the cone is protected not only by the bulkhead, but by the 6"X6" kevlar that's threaded down onto the shock cord, which is also protected by a kevlar wrap. Haven't had any problems with ejection charge powder getting onto the chute or into the nosecone, and the altimeter seems to read accurately.

n5wd said:

Sure! I'll use my PR Little Dog as an example. It's a single-deployment fiberglass rocket. The nose cone is also fiberglass. Originally, the folks that gave me the kit had a wooden plug that had a fiberglass sled expoxied onto it for the altimeter (a PerfectFlite MAWD) and a 1/8" hole just as the bottom of the nose cone, just above the shoulder, for the air to get into it. The wooden plug was force fit into the nosecone so that none of the ejection gasses or particles could make it up to the altimeter.

I lost the nosecone on its first flight, but the shock cord was still securely attached to the wooden plug, and it read something like 1800 feet AGL on the first flight.

On the relacement nose cone, I epoxied in a 1/4x20 threaded rod into the tip of the cone. It comes all the way out the back end to the tip of the shoulder. I've got a fiberglass bulkhead that fits onto the rod, and an eyenut that screws on top of it to keep it secure inside the shoulder. Inside the nose cone I've got a different sled now, that mounts both an MAWD altimeter and a BigRedBee transmitter.

Everything inside the cone is protected not only by the bulkhead, but by the 6"X6" kevlar that's threaded down onto the shock cord, which is also protected by a kevlar wrap. Haven't had any problems with ejection charge powder getting onto the chute or into the nosecone, and the altimeter seems to read accurately.

Click to expand...

So I can put the vent hole in the side of the nose cone. I wasn't sure if that would work or not. Thanks for the quick reply.

GRIFFIN said:

So I can put the vent hole in the side of the nose cone. I wasn't sure if that would work or not. Thanks for the quick reply.

Click to expand...

Yes, you CAN put the vent hold in the side of the nose cone, but you may suffer some turbulence at that point, so the altimeter reading may or may not be 100% accurate. I've heard of folks running a tube from the nosecone-mounted altimeter down into the body, one or more diamters of the body tube, and venting the altimeter at that point, the theory being that there's going to be less turbulence at that point on the rocket than on the nosecone.

Once the rocket stops accelerating and reaches apogee, that reading should be as accurate as the altimeter can make it, regardless of where the vent hold is (on the nosecone or down a little bit into the airframe).

quickburst said:

I've seen Jim Jarvis do it.

Click to expand...

And, you've seen me do it, you just didn't know it! :y:

GRIFFIN said:

Can an altimeter be installed inside a nosecone (not for any type of ejection, just to know "how high")

If it is possible, how would it work. I dont have any questions about how to install the control....just how the control would have to see the pressure readings through the cone.

I would think to have a vent hole in the bottom of the cone and one in the side of the rocket. What I don't know is how "motor ejection pressure" would effect the altimeter.

Click to expand...

I've used altimeters in the nose cone for deployment many, many times. It seems to work fine. I use a bulkhead in the shoulder of the cone to separate the electronics from the main chute section. The bulkhead has a 1/4" hole in it, and the wires to the ematches pass through this hole. These is a vent hole in the side of the airframe a few inches below the cone that is the pathway for the pressure to adjust into the airframe and then into the cone.

In this position, the altimeter will see something of a pressure spike when the main charge goes off. Best as I can tell, this is not a problem either for the altimeter itself (exposure to gases or to a pressure spike) or for the peak altitude reading (which is not affected by the pressure spike). I have done this with MAWD's, HA45's, several of the Missileworks altimeters and the G-wiz LCX.

Jim

I watch Ben Russell fly his Loc Mini-Mag with the altimeter in the nose cone three times this week. He has two 3/16" holes about 4 inched down from the tip of the nose cone. Every flight was perfect and ejection right at apogee.

From a theoretical point of view (and assuming the altimeter assumes the air pressure at power-up means "start here") it makes sense that for detecting apogee for recovery deployment should work just fine with the static port(s) on the nose cone base. I would think that deployment would happen when the altimeter realizes the air pressure has stopped changing as it would when the rocket stops moving.

Perhaps some of you who are intimately familiar with the workings of made-for-rocketry altimeters that trigger a deployment and know what the software is looking for can confirm (or deny) this.

Getting an accurate altitude reading, on the other hand, is a different matter since that requires airflow over the static ports to never be such that the pressure is lower than true static pressure or you'd get readings that would err on the high side. Getting the static port away from any changes in cross section (and bumps/steps) for the airflow to be steady and also having a smooth surface around it is necessary for that.

This is why locating static ports on the surface of an airplane is something that requires great care, so that both the altitude and especially the airspeed data is correct.


Added: I just skimmed through the instructions for the MissileWorks RRC-2 and it detects apogee by sensing a positive pressure slope (starting to fall again). That would work with static ports in the nose cone. Those instructions go on to say

RRC2 instructions said:

Equally as important as sealing the electronics bay or payload section is the proper location, sizing, quality, and quantity of static pressure ports. Always try to locate a static port on the airframe where it is not obstructed by any object that may cause turbulence upstream of the airflow over the port. Also try to locate the static port as far away as possible from the nose cone or body transition sections.

Click to expand...