First HPR two stage question

[JohnnyComboLoader]

Hello. I'm in the planning step of my first HPR two stage and I just want to double check something. Is the sustainer motor alone good enough for separation from the booster and breaking of two sheer pins, or is my only choice to break any sheer pins used a black powder charge? The top section is going to be minimum diameter so right now I'm looking at situating the electronics for sustainer ignition in the interstage coupler(which doubles as a nose cone for the booster). I've put a lot of thought into safety and everything will be good if the sustainer motor alone(38mm 'I' or baby 'J') will break the two sheer pins sperating it from the booster. And if it will do so safely. For example, a potential concern of mine is that the sheer pins will break unevenly causing the rocket to veer off at an angle. Also, I have a follow up question. I was wondering if a significant amount of thrust, say of a 38 mm motor, would be squandered to break sheer pins that sheer under about 40lbs of pressure.

 

[tim cubbedge]

In a perfet world I would expect the sustainer to "drag separate" at booster burnout but not always the case so I use a 1/2 gram BP separation charge. I also throw in some baby powder to keep is slippery. You really don't want to use the sustainer motor for separation if you can help it. It usually leaves a mess in the ISC. Some people do use it though. Also, shear pins are not needed at this location. You want a loose fit at the sustainer/ISC.

 

[Titan II]

I always use a single shear pin at the ISC/sustainer connection. It works fine. Regarding your other issue....

https://www.rocketryforum.com/threads/separation-of-stages.162323/
If you do a search, you will find other threads that are likely to help you.

 

[cerving]

I always recommend lighting the sustainer motor from the sustainer itself, it removes a lot of complications trying to do it from the booster. Uneven shear pin separation is one of them. If you light it from the sustainer's electronics, it's an easy slip-fit, which makes putting the stack together at the pad easier (and I think safer), plus it allows for drag separation. Even if it didn't drag separate, with no shear pins then the sustainer ignition would easily separate the stages.

 

[Jmhepworth]

I was trying to understand what the shear pin is for. Like Cris, I light the sustainer from the sustainer av bay. I want the connection between the interstage coupler and the sustainer to be loose, enhancing the likelihood of drag separation. I use a separation charge (fired from the interstage coupler, but in the one instance where that did not fire, drag separation must have done the trick because there’s no way the separation charge would have survived exhaust from the motor. I do use shear pins to keep the interstage coupler attached to the booster, mostly because I have so little room for a parachute that it takes shear pins to keep it together.

 

[JohnnyComboLoader]

Thanks for the replies. The reason I wasn't looking at igniting the sustainer using the sustainer electronics is it is going to be minimum diameter and I'm not sure how to have an igniter reach the motor in that case without running the igniter outside of the rocket. And considering the location, I can't think of a way to ditch the igniter after ignition of the booster if I opted for this option. Thus, why I'm looking at housing the electronics for sustainer ignition in the ISC. The sheer pins would be to prevent drag separation from occurring before ignition of the sustainer. And, ideally, the sustainer would easily break them after ignition. I definitely have more thinking to do. Maybe running the igniter outside of the top stage covered by a panel with the electronics for ignition located in this stage would be a better idea at this point. Thanks again for the wisdom.

 

[Titan II]

Flat ribbon wire has been used in MD rockets to light the sustainer.

Regarding the single shear pin. It is to keep the fins aligned.

 

[Cameron Anderson]

Thanks for the replies. The reason I wasn't looking at igniting the sustainer using the sustainer electronics is it is going to be minimum diameter and I'm not sure how to have an igniter reach the motor in that case without running the igniter outside of the rocket. And considering the location, I can't think of a way to ditch the igniter after ignition of the booster if I opted for this option. Thus, why I'm looking at housing the electronics for sustainer ignition in the ISC. The sheer pins would be to prevent drag separation from occurring before ignition of the sustainer. And, ideally, the sustainer would easily break them after ignition. I definitely have more thinking to do. Maybe running the igniter outside of the top stage covered by a panel with the electronics for ignition located in this stage would be a better idea at this point. Thanks again for the wisdom.

I used tape to cover my first ignitor that I ran down the side of my rocket. Easier than a panel. You just don't want to position your ignitor and electronics where premature stage separation removes the ignitor before it can fill fire.

 

[JimJarvis50]

Like others that have posted, I prefer to light the sustainer motor from the sustainer and not the IC. I also use a separation charge so that the motor doesn't light while the motor is in the IC. Due to these things, I don't pin the sustainer to the IC.

The way that I have wired the motor ignitor and the separation charge is with the ribbon cable mentioned previously. It's called either Gecko wire or Tapper wire. This wouldn't be ideal for a 38mm motor, but it could be done.

Just curious, what motor are you using for the sustainer? Doing MD staging is difficult unless you have a motor with no thrust ring. If you have a thrust ring, then you can use a sleeve over the part of the motor going into the IC (like in the fourth pic). How are you doing it?

Jim

 

[JohnnyComboLoader]

Just curious, what motor are you using for the sustainer? Doing MD staging is difficult unless you have a motor with no thrust ring. If you have a thrust ring, then you can use a sleeve over the part of the motor going into the IC (like in the fourth pic). How are you doing it?

I'll probably use a CTI motor as they're easiest to light. Including the fin can, the bottom part of the sustainer will be a slightly larger diameter than the thrust ring. And the interstage coupler will slide over the bottom section of the sustainer(Including a given thrust ring) with cut out spots for the fins.

Ribbon wire looks promising, could you inform me where you bought your flat wire? I've tried looking under the names gecko wire and tapper wire to no avail. I found something that may work but I'd rather get something more similar to what you've used. Thanks again

 

[Finicky]

Like others that have posted, I prefer to light the sustainer motor from the sustainer and not the IC. I also use a separation charge so that the motor doesn't light while the motor is in the IC. Due to these things, I don't pin the sustainer to the IC.

The way that I have wired the motor ignitor and the separation charge is with the ribbon cable mentioned previously. It's called either Gecko wire or Tapper wire. This wouldn't be ideal for a 38mm motor, but it could be done.

Just curious, what motor are you using for the sustainer? Doing MD staging is difficult unless you have a motor with no thrust ring. If you have a thrust ring, then you can use a sleeve over the part of the motor going into the IC (like in the fourth pic). How are you doing it?

Jim

I'd love to use that flat ribbon. I Googled both Gecko wire and Tapper wire and could not find it. Just all kinds of ribbon cables. Do you have a website or particular vendor ?

 

[Titan II]

Search ghost wire or flat speaker wire. Numerus hits in Amazon. Frankever and Sewell are two brands.

 

[JimJarvis50]

I'll probably use a CTI motor as they're easiest to light. Including the fin can, the bottom part of the sustainer will be a slightly larger diameter than the thrust ring. And the interstage coupler will slide over the bottom section of the sustainer(Including a given thrust ring) with cut out spots for the fins.

Ribbon wire looks promising, could you inform me where you bought your flat wire? I've tried looking under the names gecko wire and tapper wire to no avail. I found something that may work but I'd rather get something more similar to what you've used. Thanks again

Sorry, it's Taperwire. I use the 24 gauge telephone wire for 4 conductors (igniter and separation charge). If you're careful, you can remove the top wear layer and get a really thin wire. I remove the wear layer and then cover each "half" of the wire with a line of scotch tape (thinner than the wear layer).

https://taperwire.com/telephonewire.htm
I would recommend that you have at least one caliber (fin free) of your lower air frame available to overlap into your interstage coupler. If you don't want the fins to be that high, you can use a section of air frame as a sleeve.

Jim