Altimeter or Motor ejection ???

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

ThreeJsDad

Well-Known Member
Joined
Nov 26, 2009
Messages
659
Reaction score
256
Location
Greensboro, NC
If you are flying an altimeter for DD how many of you use the motor for separation and the altimeter for the main? How many use the altimeter for both events?

These seems like a double edged sword. If we use the altimeter for apogee separation we have less chance of a late or early separation but if we don't get everything right with the charge we risk damaging the rocket.

By using the motor we are pretty sure we will get separation but it could be early or late if we get the delay wrong.

What are your thoughts?
 
I rarely if ever use the motor for apogee event with 38mm motors unless the delay is much longer than apogee. On any 54mm motors, if they even have a delay, I always pull the BP charge and use redundant electronics.
 
I rarely if ever use the motor for apogee event with 38mm motors unless the delay is much longer than apogee. On any 54mm motors, if they even have a delay, I always pull the BP charge and use redundant electronics.

I am thinking more about 24mm to 29mm motors and that is why I posted it in the MPR section. What do you do with 24mm and 29mm motors?
 
Sorry - missed the Midpower section. I only have one 29mm motor and it only flies in rockets with no electronics so delay must be trimmed accordingly. If I were flying electronics, I would still use delay for backup, just in case!
 
Regardless if my rocket sports a 18mm motor or a 75mm motor, if I have electronics on board that can control the events, then I let it handle them. If the motor has an ejection, I always time it so that it is past apogee don't use it for any event. Electronics are more accurate than any motor delay.
 
For the most part, If I've got a dual deployment altimeter, I'll use both charges. For 24/29mm, I just use a motor with a delay that's longer than predicted coast time. That way it won't blow early, and may even save a flight where the apogee misfires.
 
These seems like a double edged sword. If we use the altimeter for apogee separation we have less chance of a late or early separation but if we don't get everything right with the charge we risk damaging the rocket.

Even with midpower rockets, it takes a pretty excessive charge to damage a rocket. This is where ground testing comes in. Most of the time, I end up with a smaller charge with the DD altimeter, than is included in the motor eject charge. Actually, my first DD rocket uses low power (thin wall) tubes, and it survives the DD charges just fine.

Most of the time I keep the motor eject as a backup, with the exception of the rare case when the rocket might need a longer delay than the motor can provide.
 
Sorry - missed the Midpower section. I only have one 29mm motor and it only flies in rockets with no electronics so delay must be trimmed accordingly. If I were flying electronics, I would still use delay for backup, just in case!

No need to apologize especially when yiou make a valid point about using the motor as a back up. I could leave the motor delay on the longer side and it becomes my back up. Good idea !!!
 
Altimeters get the chutes out every single time exactly when you want them to. They aren't subject to wind, weathercocking, chuffing, or anything. Motor deployment works when the delay time selected matches the actual time it takes to get to apogee, which may or may not be consistent for the same rocket/motor combination on any number of flights.
 
Altimeters get the chutes out every single time exactly when you want them to. They aren't subject to wind, weathercocking, chuffing, or anything. Motor deployment works when the delay time selected matches the actual time it takes to get to apogee, which may or may not be consistent for the same rocket/motor combination on any number of flights.

Valid point, they really do remove any guess work.

Does anyone have charge size formulas for Pyrodex. I have a bunch and it is more readily available for me.
 
Altimeter deployments are nearly 100% accurate if you do them properly. However, it's always nice to have a backup, particularly on the drogue chute. Run a simulation on your flight using OpenRocket, and see what the time-to-apogee will be. If it's 20% or more less than than maximum delay time of your motor's eject, then you should be able to drill the delay down and use it as a backup (set it for 2 secs or so past your expected apogee). If not, it's going to pop early, so you might as well just dump the BP from the motor because it's probably going to do more harm than good leaving it in.
 
Altimeter deployments are nearly 100% accurate if you do them properly. However, it's always nice to have a backup, particularly on the drogue chute. Run a simulation on your flight using OpenRocket, and see what the time-to-apogee will be. If it's 20% or more less than than maximum delay time of your motor's eject, then you should be able to drill the delay down and use it as a backup (set it for 2 secs or so past your expected apogee). If not, it's going to pop early, so you might as well just dump the BP from the motor because it's probably going to do more harm than good leaving it in.
I agree.

I have several rockets that use both motor eject and altimeter eject. For me, it comes down to the flight profile. If the sim indicates that I can use motor eject with a long delay, then I do, with altimeter as primary. I have actually had a flight where the motor eject saved the rocket.
 
Altimeter deployments are nearly 100% accurate if you do them properly. However, it's always nice to have a backup, particularly on the drogue chute. Run a simulation on your flight using OpenRocket, and see what the time-to-apogee will be. If it's 20% or more less than than maximum delay time of your motor's eject, then you should be able to drill the delay down and use it as a backup (set it for 2 secs or so past your expected apogee). If not, it's going to pop early, so you might as well just dump the BP from the motor because it's probably going to do more harm than good leaving it in.

Another excellent point. I need to get Open Rocket up and running.
 
If you are flying an altimeter for DD how many of you use the motor for separation and the altimeter for the main? How many use the altimeter for both events?

These seems like a double edged sword. If we use the altimeter for apogee separation we have less chance of a late or early separation but if we don't get everything right with the charge we risk damaging the rocket.

By using the motor we are pretty sure we will get separation but it could be early or late if we get the delay wrong.

What are your thoughts?

Sim programs (Open Rocket is free) are pretty much spot on for predicting the length of delay needed for a rockets flight. With 24 and 29mm motors even small HPR I always use the motor delay, seeing the delay is long enough. For me DD is for bigger rockets, motor delay means more time flying and less time prepping. In my 23 years as a bar I've had 2 incidents with motor delay 1 my fault ( drilled to much off) zipper result and 1 super big delay bonus most likely due to old motor, no damage to rocket. I've seen more incidents with DD than with motor ejection, DD is not fail proof! We have a Rockets for Schools program here seen close to a thousand flights through the years, all have motor ejection using I,J,K motors, no problems!
 
Last edited:
I did a couple test flights on a rocket that carries a couple of eggs for a local high school STEM project. I have 2 altimeters with 3 apogee charges..plus motor back up. The rocket breaks into 2 pieces at apogee..nose cone w/electronics and egg payload...and booster has it's own chute. Here is a photo from one of the flights....see anything wrong?

egg2.jpg


2 secs later the motor ejection goes off and finally pushes out the chute for the booster.

egg3.jpg

The question is why not use the motors ejection too..if the delay is long enough?

Tony
 
Altimeter deployments are nearly 100% accurate if you do them properly. However, it's always nice to have a backup, particularly on the drogue chute. Run a simulation on your flight using OpenRocket, and see what the time-to-apogee will be. If it's 20% or more less than than maximum delay time of your motor's eject, then you should be able to drill the delay down and use it as a backup (set it for 2 secs or so past your expected apogee). If not, it's going to pop early, so you might as well just dump the BP from the motor because it's probably going to do more harm than good leaving it in.
Yep, that's my thoughts too. If I can't adjust the motor ejection delay to something close to the simmed delay plus a second or two, then I ditch it and go 100% electronic. If I can adjust it, then it's used as an apogee backup. Typically, these days I can't get the longer delays for some motors due to availability.
 
For me:

Small / light rockets, typically in the 'MPR' Range: motor eject with a chute release

larger rockets in the HPR range: it depends. Big motors where I'm expecting 2500' or more feet, electronic DD, wth motor back up (if applicable). For flights under 2500' Motor eject & chute release (When applicable)

I must admit, I am still relatively new at L2 & DD.. I've done a bunch, but still want more practice.. (Isn't every flight a practice flight?!)
 
Back
Top