Using CTI I55 as sustainer motors

[Tim51]

This is a supplemental post relating to my build thread for my first HPR clustered airstart here Whilst waiting for the weather to warm up enough to begin painting the airframe, I'm thinking about motor choices. I'm thinking for a first flight I'll use a CTI J449 Blue Streak to kick things off, and then airstart two CTI I55 Mellows. Since the J449 burnout is at 2.8 seconds, I'm thinking to ignite the I55's at 3 seconds after launch. This combination sims to just over 6K apogee, and is a relatively slow climb at Mach 0.43 max. Mellow is a new propellant to me, so I'm asking if anyone with experience of using it in sustainers has any comments / advice ? Does airstarting at T+3 seem reasonable for this propellant? Does it take a long time to come up to pressure, or is it easy to light? Are there any other factors to consider?
.ork file attached + pic of dummy run igniter attachment using spent motor lining.

 

[djs]

I've seen mellow loads burn fine, and sometimes chuff in bad ways (ask @jd2cylman). If you've got two airstarting, I'd be concerned that they may light at two different times and cause some off axis thrust.

 

[Tim51]

I've seen mellow loads burn fine, and sometimes chuff in bad ways (ask @jd2cylman). If you've got two airstarting, I'd be concerned that they may light at two different times and cause some off axis thrust.

Thanks - good to know.

 

[mikec]

I've flown a two-stage with an I55 in the second stage without any issues.

A group at TRA-SD flew a two-stage last year with four L265 Mellows in the upper stage and it worked fine, no obvious sign of ignition delays.

 

[Tim51]

I've flown a two-stage with an I55 in the second stage without any issues.

A group at TRA-SD flew a two-stage last year with four L265 Mellows in the upper stage and it worked fine, no obvious sign of ignition delays.

Thanks. I'd be interested to know what (if any) delay(s) do you recommend between booster burn out and sustainer ignition?

 

[mikec]

Thanks. I'd be interested to know what (if any) delay(s) do you recommend between booster burn out and sustainer ignition?

I used a Raven with altitude lockout for my I55 flight, and a nominal 1-second delay between first-stage burnout and second-stage ignition. The onboard video shows that this is about what happened, no real delay between ignition and thrust. I'd say your delay was OK, I might go a few tenths of a second longer.

 

[Tim51]

I used a Raven with altitude lockout for my I55 flight, and a nominal 1-second delay between first-stage burnout and second-stage ignition. The onboard video shows that this is about what happened, no real delay between ignition and thrust. I'd say your delay was OK, I might go a few tenths of a second longer.

Thanks very much.

 

[mikec]

Two more notes: 1) make sure you have the igniters all the way up in the motors; when I've had problems with CTIs lighting it's usually been because the igniter wasn't in good contact with the BP pellet. 2) You should at least consider the possibility that one motor won't start and how stable your rocket will be as a result. Unfortunately I don't know of a sim program that can model asymmetric thrust. You don't have a really large off-axis moment and the I55s are low-thrust so it probably wouldn't be too bad in your case.

BTW, I'm not convinced you need the aluminum tape, I've had good results just folding the leads a few times at the nozzle end so they won't slip out through the plastic cap. Ideally you'd like enough slack that the igniter is completely blown out of the motor, if not with small motors you can get unintended thrust vectoring (probably not a concern with 38mm). It's probably OK with the tape.

 

[Tim51]

Two more notes: 1) make sure you have the igniters all the way up in the motors; when I've had problems with CTIs lighting it's usually been because the igniter wasn't in good contact with the BP pellet. 2) You should at least consider the possibility that one motor won't start and how stable your rocket will be as a result. Unfortunately I don't know of a sim program that can model asymmetric thrust. You don't have a really large off-axis moment and the I55s are low-thrust so it probably wouldn't be too bad in your case.

BTW, I'm not convinced you need the aluminum tape, I've had good results just folding the leads a few times at the nozzle end so they won't slip out through the plastic cap. Ideally you'd like enough slack that the igniter is completely blown out of the motor, if not with small motors you can get unintended thrust vectoring (probably not a concern with 38mm). It's probably OK with the tape.

Thanks Mike - much appreciated! Part of my prep & pad 'ritual' involves measuring the straightened length of the igniter against the length of the grain to ensure it's going to be well seated, then insert once, pull out, check it's straight then re-insert - just to 'feel' it's properly there before I loop the wire and fit the cap. As you observe, my thinking in using the I55's was that an asymmetrical staging wouldn't create too much of a 'diversion'.. The alu tape arose from concerns that heat from the first stage could damage the wiring (it was also something that I recalled from my first reading of Mark Canepa's MHPR 2 - I suppose with loads that 'cook' a little on the pad such as Reds that might be issue (..?) but either way I appreciate your input.)

 

[AndrewW]

I am not sure if it is directly related to Mellow propellant but I just had a two stage flight with a J94 5G Mellow in the sustainer that failed to light. The flight was in January with an ambient temp in the low 30s so that may have had something to do with it. I had originally thought that the starter may have slipped during boost but the BP pellet had completely burned away without igniting the motor.

 

[Rocketjunkie]

Mellows have BP pellets built into the top grain. If the e-match is in contact with the pellet, they will light instantly.