Jolly Logic Altimeter 2 question

Any idea what AP2E (apogee to ejection time) of -0.1 means? I suspect it is because I flew it without vent holes, though the other measurements seem reasonable for the flight, this is the only anomalous one.

I would think it means that the rocket is still climbing at ejection. With no vent holes the internal pressure may have still been dropping at ejection and the sensor thinks that the rocket is climbing even if it has started to drop. Altitude, air pressure, is the only data point that needs vent holes. Any calculation that uses altitude may be off a bit.

Mike

According to User's Manual




Therefore, ejection charge was 0.1 seconds prior to apogee on that flight.

Yeah, I read that too, but the part I failed to mention is that I have only ever gotten -0.1. Three flights, three completely different rockets on three different engines, and AP2E is always -0.1. Thought it might be a known bug.

dward said:

Yeah, I read that too, but the part I failed to mention is that I have only ever gotten -0.1. Three flights, three completely different rockets on three different engines, and AP2E is always -0.1. Thought it might be a known bug.

Click to expand...

Ooohh. That paints it an entirely different color.

Sorry, I don't have that specific knowledge; but John Beans, who founded Jolly Logic, posts on here from time to time. Maybe he, or someone else with knowledge of the specific workings of the altimeter can provide some insight into your issue.

Thanks, I'll look him up.

I checked my flight logs and I've gotten a variety of results from -.3s to 2.6s. So mine seems to be working as I would expect.

Those numbers sound reasonable. It could just be I had 0.1 seconds of early deployment on each flight. Not likely, but possible. Thanks for checking, though.

Do you have the proper vent holes in the other rockets?

Without the vent holes the altimeter cannot sense the air pressure until ejection. Early deployment because the internal pressure didn't change until deployment. Or you have a really loose fitting nose cone.

Think about it, a rocket with the motor loaded and a properly fitted nose cone is basically a sealed tube. The pressure inside the rocket won't change fast enough during the launch until the ejection charge pushes the nose cone out and opens the tube. Vent holes are used in high altitude flights to prevent tube separation from the difference in pressures. High pressure inside versus low pressure outside. High pressure wins and pushes tubes apart.

Simple to fix by adding the proper size vent holes. The altimeter instructions should explain what size the vent holes should be.

I also have the JL Alt2 and haven't seen any odd readings.

See ya,
Rod

Sorry for not weighing in here earlier (sometimes days go by without me logging in when I'm heads-down designing).

The AP2E time is measured between the end of the ejection shock and apogee, or vice-versa. Much of the time when you eject on the way up, the rocket kind of "grinds to a halt". What the altimeter senses is the ejection shock and then some buffeting, spring-back, and even tumbling-- then the apogee. It usually happens rather close together, and times are rounded to the nearest tenth in the display (even though they are sampled every 1/244 of a second). Typically, you don't see very big numbers here, because if you've got a healthy ejection shock, stuff is going to stop rising pretty quickly. Often, not always.

If you're worried it's not working correctly, you can simulate a launch a by shaking (launch), sucking (in cupped hands or a bottle), shaking(ejection), set on floor (landing lower down). The speed values will be wacky, but the timing ones should be reasonable.

Help any?

Thanks John, it was helpful. The simulated test should put my mind at ease. Great product, by the way. Can't wait to see what you come up with next.