Motor ejection charge and SLCF

[loxley]

Hi,

I am new to rocketry and I'm a little confused on how the motor ejection charge triggered (sorry if that's not the correct terminology). To deploy the parachute we are relying on the motor charge, the motors delay is 14 seconds. So if I wanted to deploy the parachute earlier, could a stratologger or timer be used to trigger this motor charge?

 

[Antares JS]

The delay element in a motor is a slow-burning smoke charge that burns its way through to the ejection charge. when the burning in the delay element reaches the ejection charge, it goes off.

I'm guessing you have an Aerotech DMS motor with an adjustable delay if your delay is 14 seconds. If I'm right, about that, you adjust the delay with the drilling tool at the link below. If I'm wrong, then you need to either find the delay drilling tool that goes with your motor or tell us what kind of motor you have so we can point you to the right one. The drilling tool cuts away part of the delay element, effectively making it shorter so that it takes less time to burn through and set off the ejection charge.

Frankly, I would advise not worrying about electronics until you have this kind of stuff down, but they are used to set off their own ejection charges independent of motor ejection charges. A lot of the time, rockets flying with electronic deployment have no motor ejection charges at all.

https://wildmanrocketry.com/products/uddt?_pos=1&_sid=143c5da30&_ss=r

 

[loxley]

The delay element in a motor is a slow-burning smoke charge that burns its way through to the ejection charge. when the burning in the delay element reaches the ejection charge, it goes off.

I'm guessing you have an Aerotech DMS motor with an adjustable delay if your delay is 14 seconds. If I'm right, about that, you adjust the delay with the drilling tool at the link below. If I'm wrong, then you need to either find the delay drilling tool that goes with your motor or tell us what kind of motor you have so we can point you to the right one. The drilling tool cuts away part of the delay element, effectively making it shorter so that it takes less time to burn through and set off the ejection charge.

Frankly, I would advise not worrying about electronics until you have this kind of stuff down, but they are used to set off their own ejection charges independent of motor ejection charges. A lot of the time, rockets flying with electronic deployment have no motor ejection charges at all.

https://wildmanrocketry.com/products/uddt?_pos=1&_sid=143c5da30&_ss=r

Thank you for your advice, we're using a Cesaroni motor and we can't change the delay as the tool is quite expensive to order on our budget (student project at university, self funded). So to get an earlier deployment we would need an electronic deployment mechanism? Could we still keep the motor charge for backup, incase it this electrical deployment fails?

 

[rocketace]

Yes you can use the motor as a back up.
Another option could be to find a local club, go to a launch and ask to see if you can borrow someone's delay adjustment tool.
Also, if you have not already, I would highly recommend finding someone at a local club to help mentor your team. Their experience can be an invaluable asset and save a lot of headache.

 

[loxley]

Yes you can use the motor as a back up.
Another option could be to find a local club, go to a launch and ask to see if you can borrow someone's delay adjustment tool.
Also, if you have not already, I would highly recommend finding someone at a local club to help mentor your team. Their experience can be an invaluable asset and save a lot of headache.

Thank you, that's very helpful.

 

[Antares JS]

Thank you for your advice, we're using a Cesaroni motor and we can't change the delay as the tool is quite expensive to order on our budget (student project at university, self funded). So to get an earlier deployment we would need an electronic deployment mechanism? Could we still keep the motor charge for backup, incase it this electrical deployment fails?

The Cesaroni Pro-38 delay drilling tool is like $20 (see link below). The fact that you can build a rocket and buy a motor case (You did buy a motor case, right? Common newbie mistake with Cesaroni...) and a reload, and are considering buying electronics, but somehow can't afford the delay drilling tool, is a bit eyebrow-raising.

You CAN drill the delay without the delay drilling tool. I think you take off one second per 1/32" you drill the delay, but someone else should confirm this, as I have never tried it myself. The delay drilling tool is much easier and quicker and gets you more consistent results.

You can use motor ejection as a backup to electronic ejection, that's no problem. Depending on your timeline though, I would suggest looking for an altimeter other than the Stratologger CF. They are hard to get.

https://wildmanrocketry.com/products/p38-dat?_pos=1&_sid=646a3b3f8&_ss=r

 

[tim cubbedge]

Yes, 1/32” will remove about 1 second.

 

[Bat-mite]

Thank you for your advice, we're using a Cesaroni motor and we can't change the delay as the tool is quite expensive to order on our budget (student project at university, self funded). So to get an earlier deployment we would need an electronic deployment mechanism? Could we still keep the motor charge for backup, incase it this electrical deployment fails?

You can't afford a $20 delay drill, but you can afford a $65 altimeter?

 

[Kelly]

And, if you're using an altimeter for apogee deployment, but your motor delay is 14s (when the proper delay might be, say, 6s) then it's not really a 'backup'. It's just a way to have your rocket self-destruct in the air rather than on the ground.

 

[prfesser]

To shorten a delay, all that is needed is a drill bit and a piece of masking tape.



Size of the bit isn't terribly important, around 1/8" diameter. Suppose you want an 8 second delay instead of 14 seconds; the delay must be shortened by 6 seconds. At 1/32" per second, the delay must be drilled 6/32" or 3/16" deep. Measure 3/16" from the tip of the bit and wrap a strip of masking tape around the bit as seen here.

Do NOT use a power drill. Propellant is easily drilled by simply twirling the bit in the fingers. Hold the delay grain and bit over a sheet of paper and drill until the strip of tape touches the delay grain. Clean out any shavings onto the paper, and burn paper and shavings safely; for example, use a launch system and a spare motor starter to ignite the shavings.

One other thing that has already been mentioned but should be emphasized. A CTI motor REQUIRES both the reload (which looks like a complete motor, but is NOT) and an aluminum CTI case for that size of reload. The reload is inserted into the case and the end closure screwed on. If you use only the reload, without the case, a ruined, burnt rocket is absolutely guaranteed.

Best -- Terry