Reliability of Delay Times in RMS?

[JasonF]

I just ordered the parts for my level1 rocket, but have some concerns about the reloadable motors out there. The kit is going to be a modified Tethys, and I had thought to cert on an H123W. A number of threads lately have made me pause. There seems to be a problem with the delays on Aerotech motors for some folks. I think one guy had the same rocket crash on multiple cert attempts due to premature deployment. What has been the most reliable RMS ejection timing in your experience? I will be going to an altimeter for level2, but not yet. I like the Pro38s, but there isn't much variety for the individual motor cases.

Edit: Listed wrong motor

 

[solrules]

What your best bet to do, as far as delays go, is to use electronics. Now, you said you don't want to do that (yet!), I will bypass that. The next best thing is to use rocksim, get a free demo (if you don't have it already) of Rocksim from Apogee Rockets , put your rocket dimentions in, and find the proper delay, and use the closest delay to that, that is available to you.

Now, the premature deployment could be from many things, such as improper assembly, leading to a blow-by of the delay grain, igniting the black powder soon into flight; The person could also have used too short of a delay for the rocket (again, use rocksim to predict the right delay), or the delay grain could have burnned a bit fast, and deploying early (which you can't fix).

EDIT: I ccan't type

 

[Rocketjunkie]

In my experience, Aerotech delays are fairly accurate (within 10%) at least 90% of the time. There are random short and bonus delays, most likely due to how the motor ignites and sometimes beyond the control of the flyer. Follow assembly directions exactly, including any addendum sheets and have an experienced flyer observe your RMS assembly.

Pro38 delays are very reliable and accurate.

 

[Justy]

I have never seen a Pro38 motor ejection delay fail.

I re-wrote this next part no less than four times, because each time it sounded like I was ragging on AT, and I'm not...

I have seen several bonus delays, and sometimes no apparent ejection charge at all, on Aerotech motors that have been sitting around on shelves since before the factory fire. I believe these failures are mostly due to age, and the fact that I live in a pretty humid climate. If your reload is several years old, is $25 for a simple Perfectflite timer really too much? You set it to a couple of seconds beyond the delay on your motor, and if the delay has degraded over time, you've got a safety net. It could save your rocket, motor casing, and certification flight. If everything works right, you get an extra smoke cloud and no big loss. Then you can use it as backup for your altimeter deployment when it comes time to do L2.

I have an old H123W that was given to me as a freebie, and not only is it that old, but it's been stored in a Ziploc instead of its original sealed bag. No WAY would I fire it without backup electronics.

If your reload was made after the fire -- have they started making H123's again yet? -- I'd say no worries, go for it.

 

[n3tjm]

Yes, H123's have gone back in production.

 

[maxq2244]

Jason,
First off I have to agree with rocketjunkie on AT delays being fairly accurate, pro 38/54's are great too with the ajustable delays.

I think the most important part of it is picking the propper delay time for the rocket you are going to fly. PML has the rocksim files on their web site. I have built several PML kits and used their motor suggestion cart as a starting point. Never had a problem yet. I tend to build heavy so I go with a little shorter delay.

What mods are you planning on doing?

 

[JasonF]

Originally posted by maxq2244
Jason,
First off I have to agree with rocketjunkie on AT delays being fairly accurate, pro 38/54's are great too with the ajustable delays.

I think the most important part of it is picking the propper delay time for the rocket you are going to fly. PML has the rocksim files on their web site. I have built several PML kits and used their motor suggestion cart as a starting point. Never had a problem yet. I tend to build heavy so I go with a little shorter delay.

What mods are you planning on doing?

The one for starters, is going zipperless. Cut the airframe at 16.5", measured from aft, and installing a coupler bulkhead with ubolt. I'm also replacing the nylon shock cord with 5 yards of 1/4" kevlar, and adding a kevlar chute protector. When I've certified on this one, I'll order another length of QT. I'll need about 18" for the stretch, and an inch or two for the payload bay. I'll use a 7" coupler tube for altimeter payload, with a section of QT around the middle. If you have ever seen Lance's Tethys, I'm basically cloning it. I might have the bulkhead done a bit differently, but essentially the same thing. It also follows the PML conversion plans pretty close. I'll need a bigger chute after stretching. I was thinking of a 60" main, and an 18" drogue. The altimeter I want is the missileworks RRC2. I have a variety of charge holder options, but I'll probably just use centerfuge tubes.